Comparison of the effects of neurokinin-3 receptor blockade on two forms of slow synaptic transmission in myenteric AH neurons

AH neurons are intrinsic sensory neurons of the intestine that exhibit two types of slow synaptic event: slow excitatory postsynaptic potentials which increase their excitability for about 2-4 min, and sustained slow postsynaptic excitation which can persist for several hours, and may be involved in long-term changes in the sensitivity of the intestine to sensory stimuli. The effects of the neurokinin-3 tachykinin receptor antagonist, SR142801, on these two types of synaptic event in AH neurons of the myenteric ganglia of guinea-pig small intestine were compared. Slow excitatory postsynaptic potentials were evoked by stimulation of synaptic inputs at 10-20 Hz for 1s, and sustained slow postsynaptic excitation was evoked by stimulation of inputs at 1Hz for 4 min. SR142801 (1microM) reduced the amplitude of the slow excitatory postsynaptic potential to 26% of control, and also reduced the increase in input resistance and the extent of anode break excitation associated with the slow excitatory postsynaptic potential. In contrast, SR142801 did not reduce the increase in excitability, the increase in input resistance or the depolarisation that occur during the sustained slow postsynaptic excitation. SR142801 did not change the resting membrane potential or the resting input resistance.We conclude that tachykinins, acting through neurokinin-3 receptors, are involved in the generation of the slow excitatory postsynaptic potential, but not in the sustained slow postsynaptic excitation, and that the release of transmitters from synaptic inputs to AH neurons is frequency coded.     

Presynaptic GABA(B) receptors modulate organum vasculosum lamina terminalis-evoked postsynaptic currents in rat hypothalamic supraoptic neurons

Whole-cell patch-clamp recordings obtained from 36 hypothalamic supraoptic nucleus neurons in explant preparations evaluated a role for GABA(B) receptors in modulating postsynaptic inhibitory and excitatory currents evoked by electrical stimulation in the organum vasculosum of the lamina terminalis. At a holding current of -65 mV, application of baclofen (1-10 microM) induced a dose-dependent reduction in the amplitude of pharmacologically isolated inhibitory and excitatory postsynaptic currents, converted paired-pulse depression in inhibitory postsynaptic currents to paired-pulse facilitation, and enhanced paired-pulse ratios for excitatory postsynaptic currents. In media containing 2-hydroxysaclofen (200-400 microM), baclofen-associated events were blocked and paired-pulse depression in evoked inhibitory postsynaptic currents was abolished. In addition, a progressive increase in the amplitude of inhibitory postsynaptic currents implied that GABA was endogenously active at presynaptic GABA(B) receptors. In contrast, no paired-pulse depression was observed for inhibitory postsynaptic currents evoked in six non-magnocellular neurons. Neither baclofen nor 2-hydroxysaclofen altered holding currents or input resistances in supraoptic neurons, or altered the kinetics of the evoked responses.These observations imply that the terminals of both inhibitory (GABAergic) and excitatory (glutamatergic) afferents to supraoptic nucleus neurons from organum vasculosum lamina terminalis neurons are subject to modulation by presynaptic GABA(B) receptors, and that this modulation is preferentially directed to the inhibitory inputs.     

Role of protein kinase C in modulation of excitability of CA1 pyramidal neurons in the rat

Biochemical and in situ hybridization studies demonstrated that the levels of protein kinase C variants were significantly increased in the hippocampus of the experimental models of epilepsy in rats. In addition it has been demonstrated that protein kinase C plays an important role in modulating synaptic transmission in the hippocampus. We examined the effects of activating of protein kinase C on the excitability of CA1 pyramidal neurons and synaptic transmission, using whole-cell current-clamp and extracellular field potential recording techniques. Indolactam V (1 microM) a novel protein kinase C activator, increased the excitability of CA1 neurons acting at both pre- and post-synaptic sites. Indolactam V, acting postsynaptically, significantly reduced the threshold for initiation of action potential from -42+/-3.8 mV to -51+/-3.1 mV and selectively inhibited the slow afterhyperpolarizing potential. Indolactam V also altered the neuronal firing properties in response to prolonged depolarizing pulse by eliminating the spike frequency accommodation. Our data indicate that indolactam V potentiated both amplitudes of Shaffer-collateral stimulation evoked excitatory postsynaptic currents and disynaptically evoked inhibitory evoked postsynaptic currents. However, the potentiation of inhibitory evoked postsynaptic currents amplitudes was not observed after blockade of NMDA and AMPA/kainate currents suggesting it was due to excitatory activity driving inhibitory neurons. The results indicate that the potentiation of pharmacologically isolated excitatory postsynaptic currents (215% of control) and amplitudes of population spikes (290% of control) was due to action of indolactam V presynaptically since the agonist reduced the paired-pulse ratio and the potentiating effect was not blocked by dialyzing the postsynaptic neuron through the recording electrode with a specific protein kinase C inactivator calphostin C. These findings suggest that protein kinase C increases the amplitude of epileptiform activity by causing potentiation of excitatory synaptic transmission, increasing the excitability of postsynaptic neurons and reducing negative feed back provided by slow afterhyperpolarizing potential.     

Local synaptic circuits and epileptiform activity in slices of neocortex from children with intractable epilepsy.

1. Single and dual intracellular recordings were performed in neocortical slices obtained from tissue samples surgically removed from children (8 mo to 15 yr) for the treatment of intractable epilepsy. Electrical stimulation and glutamate microapplication were used to study local synaptic inputs to pyramidal cells. 2. In recordings with potassium-acetate electrodes, activation of presynaptic neocortical neurons with glutamate microdrops did not elicit a clear increase in postsynaptic potentials (PSPs) but did suppress current-evoked repetitive spike firing in recorded neurons. Bicuculline (10 microM) blocked this effect, suggesting it was caused by the activation of presynaptic gamma-aminobutyric acid (GABA) cells. In recordings with KCl electrodes, glutamate microdrops elicited an increase in the frequency and amplitude of depolarizing PSPs. Bicuculline (5-10 microM) blocked the glutamate-evoked PSPs, suggesting they were reversed GABAA-receptor-mediated inhibitory postsynaptic potentials (IPSPs). In one cell recorded with a KCl electrode (total n = 8), current-evoked spike trains elicited afterdischarges of reversed IPSPs, thus revealing a recurrent inhibitory circuit. Therefore local inhibitory synaptic circuits were robust and could be observed in tissue from patients as young as 11 mo. 3. In addition to short-latency (10-25 ms), monosynaptic excitatory postsynaptic potentials (EPSPs), electrical stimulation at low intensities sometimes elicited delayed EPSPs (20-60 ms). When GABAA-receptor-mediated synaptic inhibition was partially reduced in bicuculline (5-10 microM), electrical stimulation evoked large EPSPs at long and variable latencies (100-300 ms). Glutamate microapplication caused an increase in the frequency and amplitude of EPSPs; preliminary results suggest that glutamate microdrops were less likely to evoke EPSPs in tissue from younger patients (8-12 mo) than in slices from patients greater than 4 yr. Evidence for local excitatory synaptic circuits was thus found when synaptic inhibition was partially reduced. 4. After further reduction of inhibition in bicuculline (5-50 microM), electrical stimulation elicited epileptiform bursts. In pairs of simultaneously recorded neurons, bursts were generated synchronously from long-latency EPSPs (100-300 ms) in slices from patients as young as 8 mo. Reflected EPSPs at very long and variable latencies (500-1,100 ms) and repetitive epileptiform bursts could be evoked synchronously in pairs of cells. Glutamate activation of local presynaptic neurons elicited robust epileptiform events in recorded cells. This was seen in slices from patients as young as 16 mo. 5. These data provide physiological evidence for the presence of local inhibitory and excitatory synaptic circuits in human neocortex at least as early as 11 and 8 mo, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)     

Synaptic activity in chronically injured,epileptogenic sensory-motor neocortex

We recorded spontaneous and evoked synaptic currents in pyramidal neurons of layer V in chronically injured, epileptogenic neocortex to assess changes in the efficacy of excitatory and inhibitory neurotransmission that might promote cortical hyperexcitability. Partial sensory-motor neocortical isolations with intact blood supply ("undercuts") were made in 20 rats on postnatal day 21-25 and examined 2-6 wk later in standard brain slice preparations using whole cell patch-clamp techniques. Age-matched, uninjured naive rats (n = 20) were used as controls. Spontaneous and miniature excitatory and inhibitory postsynaptic currents (s- and mEPSCs; s- and mIPSCs) were recorded using patch-clamp techniques. The average frequency of s- and mEPSCs was significantly higher, while that of s- and mIPSCs was significantly lower in neurons of undercuts versus controls. The increased frequency of excitatory events was due to an increase in both s- and mEPSC frequency, suggesting an increased number of excitatory contacts and/or increased release probability at excitatory terminals. No significant difference was observed in 10-90% rise time of these events. The input-output slopes of fast, short-latency, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate (AMPA/KA) receptor-mediated components of evoked EPSCs were steeper in undercuts than in controls. The peak amplitude of the AMPA/KA component of EPSCs evoked by supra-threshold stimuli was significantly greater in the partially isolated neocortex. In contrast, the N-methyl-D-aspartate receptor-mediated component of evoked EPSCs was not significantly different in neurons of injured versus control cortex, suggesting that the increased AMPA/KA component was due to postsynaptic alterations. Results support the conclusion that layer V pyramidal neurons receive increased AMPA/KA receptor-mediated excitatory synaptic drive and decreased GABA(A) receptor-mediated inhibition in this chronically injured, epileptogenic cortex. This shift in the balance of excitatory and inhibitory synaptic activation of layer V pyramidal cells toward excitation might be maladaptive and play a critical role in epileptogenesis.     

Synaptic control of pyramidal cell activation in the hippocampal slice preparation in the rat

Recordings were made from CA1 pyramidal neurons in a rat hippocampal slice preparation to compare the effectiveness of orthodromic stimuli when delivered at different distances from the cells under study. A stimulating electrode placed in stratum radiatum was less effective in driving nearby pyramidal cells (within 200 micron) than those farther away (greater than 800 micron). Thus for a given field excitatory postsynaptic potential both the intracellular excitatory postsynaptic potential and the evoked population spike were smaller when evoked from a local stimulating electrode than from one more distant. Laminar mapping experiments indicated that the spatial distribution of activated excitatory synapses over the pyramidal cell dendrites was similar for local and distant stimuli. The firing threshold, and the amplitude of hyperpolarizing inhibitory postsynaptic potentials, were also similar for the two stimuli. Responses evoked by the local stimulating electrode were more sensitive to morphine, penicillin and pentobarbital than responses elicited by the distant stimulus, suggesting that some form of GABAergic inhibition limited the efficacy of the local stimulus. The data suggest that in the CA1 region a vertically oriented synaptic inhibitory system exists that powerfully regulates the ability of an orthodromic stimulus to activate pyramidal cells. These results also illustrate the practical importance of controlling the distance between stimulating and recording electrodes, when performing quantitative pharmacological studies of synaptic transmission in the hippocampus.     

Nitric oxide in the afferent synaptic transmission of the axolotl vestibular system

This study was performed using intracellular and multiunit extracellular recording techniques in order to characterize the role of nitric oxide in the afferent synaptic transmission of the vestibular system of the axolotl (Ambystoma tigrinum). Bath application of nitric oxide synthase inhibitors N(G)-nitro-L-arginine (0.01microM to 10microM) and N-nitro-L-arginine methyl ester hydrochloride (0.1microM to 1000microM) elicited a dose-dependent decrease in the basal discharge of the semicircular canal afferent fibers. N(G)-Nitro-L-arginine also diminished the response to mechanical stimuli. Moreover, N(G)-nitro-L-arginine (1microM) produced a hyperpolarization associated with a decrease in the spike discharge and diminished the frequency of the excitatory postsynaptic potentials on afferent fibers recorded intracellularly. Nitric oxide donors were also tested: (i) S-nitroso-N-acetyl-DL-penicillamine (0.1microM to 100microM) increased the basal discharge and the response to mechanical stimuli. At the maximum effective concentration (100microM) this drug affected neither the amplitude nor the frequency of the excitatory postsynaptic potentials. However, it slightly depolarized the afferent neurons and decreased their input resistance. (ii) 3-Morpholino-sydnonimine hydrochloride did not significantly affect the basal discharge or the mechanically evoked peak response of afferent neurons at any of the concentrations used (1microM to 1000microM). However, after 10min of perfusion in the bath, 1microM and 10microM 3-morpholino-sydnonimine hydrochloride significantly modified the baseline of the mechanically evoked response, producing an increase in the mean spike discharge of the afferent fibers. These results indicate that nitric oxide may have a facilitatory role on the basal discharge and on the response to mechanical stimuli of the vestibular afferent fibers. Thus, nitric oxide probably participates in the sensory coding and adaptative changes of vestibular input in normal and pathological conditions.     

Inhibitory nature of tiagabine-augmented GABAA receptor-mediated depolarizing responses in hippocampal pyramidal cells

Tiagabine is a potent GABA uptake inhibitor with demonstrated anticonvulsant activity. GABA uptake inhibitors are believed to produce their anticonvulsant effects by prolonging the postsynaptic actions of GABA, released during episodes of neuronal hyperexcitability. However, tiagabine has recently been reported to facilitate the depolarizing actions of GABA in the CNS of adult rats following the stimulation of inhibitory pathways at a frequency (100 Hz) intended to mimic interneuronal activation during epileptiform activity. In the present study, we performed extracellular and whole cell recordings from CA1 pyramidal neurons in rat hippocampal slices to examine the functional consequences of tiagabine-augmented GABA-mediated depolarizing responses. Orthodromic population spikes (PSs), elicited from the stratum radiatum, were inhibited following the activation of recurrent inhibitory pathways by antidromic conditioning stimulation of the alveus, which consisted of either a single stimulus or a train of stimuli delivered at high-frequency (100 Hz, 200 ms). The inhibition of orthodromic PSs produced by high-frequency conditioning stimulation (HFS), which was always of much greater strength and duration than that produced by a single conditioning stimulus, was greatly enhanced following the bath application of tiagabine (2-100 microM). Thus, in the presence of tiagabine (20 microM), orthodromic PSs, evoked 200 and 800 ms following HFS, were inhibited to 7.8 +/- 2.6% (mean +/- SE) and 34.4 +/- 18.5% of their unconditioned amplitudes compared with only 35.4 +/- 12.7% and 98.8 +/- 12.4% in control. Whole cell recordings revealed that the bath application of tiagabine (20 microM) either caused the appearance or greatly enhanced the amplitude of GABA-mediated depolarizing responses (DR). Excitatory postsynaptic potentials (EPSPs) evoked from stratum radiatum at time points that coincided with the DR were inhibited to below the threshold for action-potential firing. Independently of the stimulus intensity with which they were evoked, the charge transferred to the soma by excitatory postsynaptic currents (EPSCs), elicited in the presence of tiagabine (20 microM) during the large (1,428 +/- 331 pA) inward currents that underlie the DRs, was decreased on the average by 90.8 +/- 1.7%. Such inhibition occurred despite the presence of the GABAB receptor antagonist, CGP 52 432 (10 microM), indicating that GABAB heteroreceptors, located on glutamatergic terminals, do not mediate the observed reduction in the amplitude of excitatory postsynaptic responses. The present results suggest that despite facilitating the induction of GABA-mediated depolarizations, tiagabine application may nevertheless increase the effectiveness of synaptic inhibition during the synchronous high-frequency activation of inhibitory interneurons by enhanced shunting.     

Linear to supralinear summation of AMPA-mediated EPSPs in neocortical pyramidal neurons

It has been hypothesized that voltage-sensitive conductances present on the dendrites of neurons can influence summation of excitatory postsynaptic potentials (EPSPs) and hence affect how neurons compile information. Greater than linear summation of EPSPs has been postulated to facilitate coincidence detection by cortical neurons. This study examined whether the summation of subthreshold AMPA-mediated EPSPs generated on layer V neocortical pyramidal neurons in vitro was linear and if any nonlinearities could be attributed to dendritic conductances. Evoked EPSPs (1-12 mV) were recorded somatically by means of intracellular sharp electrodes in the presence of 100 microM amino-5-phosphonopentanoic acid (AP-5) and 3 microM bicuculline. Two independent EPSPs were evoked by a stimulating electrode in layer I and another in layers III-V. The areas of stimulation were isolated from each other by a horizontal cut below layer I. By subtracting the algebraic sum of the individual EPSPs from the evoked response when both EPSPs were evoked simultaneously, we determined that they summed linearly to supralinearly. Supralinear summation was more likely when the soma was hyperpolarized by DC current injection. Summation was predominantly linear when postsynaptic conductances (i.e., Na(+) and Ca(2+)) were blocked with intracellular QX-314. The supralinear summation of EPSPs (without QX-314) decreased as the time between inputs was increased from 0 to 30 ms. To determine the role of dendrites in nonlinear summation, we substituted a current pulse (simulated EPSP) delivered at the soma for either or both of the evoked EPSPs. Simulated EPSPs combined with either an evoked EPSP or another simulated EPSP showed significantly less supralinear summation than two evoked EPSPs, indicating that the dendritic conductances were largely responsible for the observed supralinear summation.     

Intrinsic membrane properties of neostriatal neurons can account for their low level of spontaneous activity

Electroresponsiveness of neostriatal neurons was studied by intracellular recording in a rat brain slice preparation maintained in standard solution or in solution containing K-channel blockers. In standard solution, the neurons fired repetitively at increasing frequencies with increasing amplitude of direct depolarization. The firing pattern was independent of the membrane potential from which firing was induced. In the presence of tetraethylammonium (20 mM), long-lasting (300-500 ms) plateau potentials could be elicited by the injection of short (5-10 ms) current pulses. Plateau potentials persisted in Na-free solution, in the presence of tetrodotoxin (1-3 microM) and if Ca in the perifusate was replaced by Ba. The plateau was blocked by Cd (500 microM). The plateaux were followed by depolarizing after-potentials. When the plateau potential failed due to fatigue, a small slow depolarization of short duration (10-30 ms) was elicited in Na-free or tetrodotoxin-containing solution, which increased in amplitude with membrane hyperpolarization. This slow depolarization was blocked by Cd, indicating that it was also mediated by Ca. By intrastriatal stimulation in the presence of 4-aminopyridine a long-lasting, voltage-dependent depolarization was triggered from the enhanced postsynaptic potential. In contrast, in the presence of tetraethylammonium, postsynaptic potentials were only slightly increased if they were compared at sizes subthreshold for the plateau potentials. It is concluded that neostriatal neurons, although being characterized as "silent" and "non-bursting", possess slow conductances for inward currents which they share with other mammalian central neurons. However, in contrast, to other central neurons, their Ca-spikes are suppressed by their K-conductances and, in contrast to oscillating neurons, low-threshold Ca-potentials are not prominent.     

Suprachiasmatic nucleus communicates with anterior thalamic paraventricular nucleus neurons via rapid glutamatergic and gabaergic neurotransmission: state-dependent response patterns observed in vitro

The hypothalamic suprachiasmatic nucleus uniquely projects to the midline thalamic paraventricular nucleus. To characterize this projection, patch clamp techniques applied in acute rat brain slice preparations examined responses of anterior thalamic paraventricular nucleus neurons to focal suprachiasmatic nucleus stimulation. Whole cell recordings from slices obtained during daytime (n=40) revealed neurons with a mean membrane potential of -66+/-1.2 mV, input conductance of 1.5+/-0.1 nS and state-dependent tonic or burst firing patterns. Electrical stimulation (one or four pulses) in suprachiasmatic nucleus elicited monosynaptic excitatory postsynaptic potentials (mean latency of 12.6+/-0.6 ms; n=12), featuring both AMPA and N-methyl-D-aspartate-glutamate receptor-mediated components, and monosynaptic bicuculline-sensitive inhibitory postsynaptic potentials (mean latency of 16.6+/-0.6 ms; n=7) reversing polarity at -72+/-2.6 mV, close to the chloride equilibrium potential. Glutamate microstimulation of suprachiasmatic nucleus also elicited transient increases in spontaneous excitatory or inhibitory postsynaptic currents in anterior thalamic paraventricular neurons. Recordings from rats under reverse light/dark conditions (n=22) yielded essentially similar responses to electrical stimulation. At depolarized membrane potentials, suprachiasmatic nucleus-evoked excitatory postsynaptic potentials triggered single action potentials, while evoked inhibitory postsynaptic potentials elicited a silent period in ongoing tonic firing. By contrast, after manual adjustment of membrane potentials to hyperpolarized levels, neuronal response to the same "excitatory" stimulus was a low threshold spike and superimposed burst firing, while responses to "inhibitory" stimuli paradoxically elicited excitatory rebound low threshold spikes and burst firing. These data support the existence of glutamatergic and GABAergic efferents from the suprachiasmatic nucleus to its target neurons. Additionally, in thalamic paraventricular nucleus neurons, responses to activation of their suprachiasmatic afferents may vary in accordance with their membrane potential-dependent intrinsic properties, a characteristic typical of thalamocortical neurons.     

Ionic basis of ON and OFF persistent activity in layer III lateral entorhinal cortical principal neurons

Principal neurons in layer III of the rat lateral entorhinal cortex (LEC) generate a self-sustained plateau potential and persistent spiking following the application of a brief suprathreshold excitatory stimulus delivered in the presence of the muscarinic receptor agonist carbachol. This persistent activity can be terminated by application of a second excitatory stimulus, and these cells can be repeatedly toggled between on and off states by consecutive excitatory stimuli. However, the ionic mechanisms that underlie the production of on and off states in layer III LEC neurons are unknown but seem to involve activity-dependent conductances, since they can be initiated by trains of action potentials evoked by either depolarizing current pulses applied to the cell or by repetitive spiking induced by activation of excitatory synaptic inputs. In this study, we obtained intracellular recordings from rat layer III LEC neurons in vitro, and a series of pharmacological and ionic substitution experiments were performed to identify mechanisms involved in the induction and termination of persistent spiking. Our data indicate that initiation of the on state depends on spike-evoked calcium influx and subsequent activation of calcium-activated nonselective cationic current. Moreover, we show that termination of persistent firing in response to an excitatory stimulus can be blocked by tetraethylammonium or iberiotoxin, suggesting that the activation of calcium-activated potassium current mediated by large conductance calcium-activated K(+) (i.e., BK) channels is required to induce the off state.     

Effects of paired-pulse and repetitive stimulation on neurons in the rat medial geniculate body

Many behaviorally relevant sounds, including language, are composed of brief, rapid, repetitive acoustic features. Recent studies suggest that abnormalities in producing and understanding spoken language are correlated with abnormal neural responsiveness to such auditory stimuli at higher auditory levels [Tallal et al., Science 271 (1996) 81-84; Wright et al., Nature 387 (1997) 176-178; Nagarajan et al., Proc. Natl. Acad. Sci. USA 96 (1999) 6483-6488] and with abnormal anatomical features in the auditory thalamus [Galaburda et al., Proc. Natl. Acad. Sci. USA 91 (1994) 8010-8013]. To begin to understand potential mechanisms for normal and abnormal transfer of sensory information to the cortex, we recorded the intracellular responses of medial geniculate body thalamocortical neurons in a rat brain slice preparation. Inferior colliculus or corticothalamic axons were excited by pairs or trains of electrical stimuli. Neurons receiving only excitatory collicular input had tufted dendritic morphology and displayed strong paired-pulse depression of their large, short-latency excitatory postsynaptic potentials. In contrast, geniculate neurons receiving excitatory and inhibitory collicular inputs could have stellate or tufted morphology and displayed much weaker depression or even paired-pulse facilitation of their smaller, longer-latency excitatory postsynaptic potentials. Depression was not blocked by ionotropic glutamate, GABA(A) or GABA(B) receptor antagonists. Facilitation was unaffected by GABA(A) receptor antagonists but was diminished by N-methyl-D-aspartate (NMDA) receptor blockade. Similar stimulation of the corticothalamic input always elicited paired-pulse facilitation. The NMDA-independent facilitation of the second cortical excitatory postsynaptic potential lasted longer and was more pronounced than that seen for the excitatory collicular inputs. Paired-pulse stimulation of isolated collicular inhibitory postsynaptic potentials generated little change in the second GABA(A) potential amplitude measured from the resting potential, but the GABA(B) amplitude was sensitive to the interstimulus interval. Train stimuli applied to collicular or cortical inputs generated intra-train responses that were often predicted by their paired-pulse behavior. Long-lasting responses following train stimulation of the collicular inputs were uncommon. In contrast, corticothalamic inputs often generated long-lasting depolarizing responses that were dependent on activation of a metabotropic glutamate receptor. Our results demonstrate that during repetitive afferent firing there are input-specific mechanisms controlling synaptic strength and membrane potential over short and long time scales. Furthermore, they suggest that there may be two classes of excitatory collicular input to medial geniculate neurons and a single class of small-terminal corticothalamic inputs, each of which has distinct features.     

Dopaminergic modulation of short-term synaptic plasticity in fast-spiking interneurons of primate dorsolateral prefrontal cortex

Dopaminergic regulation of primate dorsolateral prefrontal cortex (PFC) activity is essential for cognitive functions such as working memory. However, the cellular mechanisms of dopamine neuromodulation in PFC are not well understood. We have studied the effects of dopamine receptor activation during persistent stimulation of excitatory inputs onto fast-spiking GABAergic interneurons in monkey PFC. Stimulation at 20 Hz induced short-term excitatory postsynaptic potential (EPSP) depression. The D1 receptor agonist SKF81297 (5 microM) significantly reduced the amplitude of the first EPSP but not of subsequent responses in EPSP trains, which still displayed significant depression. Dopamine (DA; 10 microM) effects were similar to those of SKF81297 and were abolished by the D1 antagonist SCH23390 (5 microM), indicating a D1 receptor-mediated effect. DA did not alter miniature excitatory postsynaptic currents, suggesting that its effects were activity dependent and presynaptic action potential dependent. In contrast to previous findings in pyramidal neurons, in fast-spiking cells, contribution of N-methyl-D-aspartate receptors to EPSPs at subthreshold potentials was not significant and fast-spiking cell depolarization decreased EPSP duration. In addition, DA had no significant effects on temporal summation. The selective decrease in the amplitude of the first EPSP in trains delivered every 10 s suggests that in fast-spiking neurons, DA reduces the amplitude of EPSPs evoked at low frequency but not of EPSPs evoked by repetitive stimulation. DA may therefore improve detection of EPSP bursts above background synaptic activity. EPSP bursts displaying short-term depression may transmit spike-timing-dependent temporal codes contained in presynaptic spike trains. Thus DA neuromodulation may increase the signal-to-noise ratio at fast-spiking cell inputs.     

Stimulus-evoked modulation of sensorimotor pyramidal neuron EPSPs

Sensory cortical neurons display substantial receptive field dynamics during and after persistent sensory drive. Because a cell's response properties are determined by the inputs it receives, receptive field dynamics are likely to involve changes in the relative efficacy of different inputs to the cell. To test this hypothesis, we have investigated if brief repetitive stimulus drive in vitro alters the efficacy of two types of corticocortical inputs to layer V pyramidal cells. Specifically, we have used whole cell recordings to measure the effect of repetitive electrical stimulation at the layer VI/white matter (WM) border on the synaptic response of layer V pyramidal cells to corticocortical input evoked by electrical stimulation of layer I or layer II/III and emulated by local application of glutamate. Repetitive stimulation (10 Hz for 3 s) at the layer VI/WM border transiently potentiated excitatory postsynaptic potentials (EPSPs) evoked by electrical stimulation of layer II/III by 97 +/- 12% (mean +/- SE). The recovery of EPSP amplitude to its preconditioning value was well-described by a single-term decaying exponential with a time constant of 7.2 s. The same layer VI/WM conditioning train that evoked layer II/III EPSP potentiation frequently caused an attenuation of layer I EPSPs. Similarly, subthreshold postsynaptic responses to local glutamate application in layers II/III and I were potentiated and attenuated, respectively, by the conditioning stimulus. Potentiation and attenuation could be evoked in the same cell by repositioning the glutamate puffer pipette in the appropriate layer. The conditioning stimulus that led to the transient modification of upper layer EPSP efficacy also evoked a slow depolarization in glial cells. The membrane potential of glial cells recovered with a time course similar to the dissipation of the potentiation effect, suggesting that stimulus-evoked changes in extracellular potassium (ECK) play a role in layer II/III EPSP potentiation. Consistent with this proposal, increasing the bath concentration of ECK caused a substantial increase of layer II/III EPSP amplitude. EPSP potentiation was sensitive to postsynaptic membrane potential and, more importantly, was significantly weaker for synaptic currents than for synaptic potentials, suggesting that it involves the recruitment of a postsynaptic voltage-dependent mechanism. Two observations suggest that layer II/III EPSP potentiation may involve the recruitment of postsynaptic sodium channels: EPSP potentiation was strongly reduced by intracellular application of N-(2,6-dimethyl-phenylcarbamoylmethyl) triethylammonium bromide (QX-314) and responses to local glutamate application were potentiated by high ECK in the presence of cadmium but not in the presence of tetrodotoxin. The results demonstrate a novel way in which brief periods of repetitive stimulus drive are accompanied by rapid, transient, and specific alterations in the functional connectivity and information processing characteristics of sensorimotor cortex.     

Calcium-dependent potentials with different sensitivities to calcium agonists and antagonists in guinea-pig hippocampal neurons

Effects of organic Ca channel blockers, Ca channel activators and omega-conotoxin on guinea-pig hippocampal CA1 neurons in vitro preparations were studied with intracellular recording methods. Most of the Ca channel blockers, such as prenylamine, D 600, flunarizine, nifedipine, cinnarizine and nicardipine (0.2-4 microM), raised the threshold for Na-dependent spike generation and decreased the amplitude of the spike afterhyperpolarization. Verapamil (5 microM) and diltiazem (5 microM) did not significantly alter the threshold and amplitude of the Na spike. Action potentials elicited in the presence of either tetrodotoxin (0.5 microM) and tetraethylammonium (20 mM) or tetrodotoxin (0.5 microM) and Ba (1.25 mM) consisted of an initial spike component followed by a long depolarization. Both responses were abolished by addition of Co (2 mM) or Cd (0.25-0.5 mM), or by superfusion with a low Ca (0.25 mM)-high Mg(15 mM) medium, indicating that the potentials resulted from Ca entry. The Ca-dependent slow depolarization was preferentially blocked by most of the organic Ca channel blockers at approximately one-third the concentrations (0.1-2 microM) which were required to shorten the Ca spike. When the cell in a solution containing tetrodotoxin (0.5 microM), Co (2 mM) and 4-aminopyridine (2 mM) was hyperpolarized and then depolarized by passing current pulses across the membrane, a transient depolarizing hump occurred on the decay phase of the electrotonic potential. This transient depolarization was abolished by Co (2 mM), Ni (2 mM) or most of the organic Ca channel blockers (0.2-5 microM). Diltiazem (5 microM) did not significantly change these Ca-dependent potentials. The evoked excitatory postsynaptic potential was very resistant to the Ca channel blockers. Approximately 2-10 times higher concentrations (0.5-3 microM) were necessary to decrease the excitatory postsynaptic potential amplitude than to shorten the Ca spike. On the other hand, the minimal concentrations and order of potencies of the Ca channel blockers for depressing the evoked inhibitory postsynaptic potential and for elevating the threshold for Na spike generation were almost the same. Dihydropyridine Ca channel activators, such as Bay K 8644, CGP 28 392 and YC 170 at low concentrations (0.1-1 microM), decreased the Ca spike, the Ca-dependent slow depolarization and the evoked synaptic potentials, while the substances augmented the Ca-dependent transient depolarization. On the other hand, omega-conotoxin (5 microM) reversibly depressed the Ca spike and slow depolarization to the same degree, without affecting the transient depolarization and the evoked excitatory or inhibitory postsynaptic potentials.(ABSTRACT TRUNCATED AT 400 WORDS)     

Excitatory synaptic transmission mediated by NMDA and non-NMDA receptors in the superficial/middle layers of the epileptogenic human neocortex maintained in vitro.

Conventional intracellular recordings were made from regular-spiking cells located in layers II-IV to examine the involvement of excitatory amino acid receptors in synaptic transmission in epileptogenic human neocortical slices maintained in vitro. Extracellular stimuli that were below the threshold for generating action potentials evoked an excitatory postsynaptic potential (EPSP) with short latency to onset (0.8-4 ms). When suprathreshold stimuli were delivered, 95% of the neurons fired a single action potential. In 5% of the population, however, an all-or-none bursting discharge was observed. The EPSP and the bursting discharge were tested with the N-methyl-D-aspartate (NMDA) antagonist 3-((+/-)-2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP, 5 microM) or the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 4 microM). In the presence of CNQX the peak amplitude of the EPSP was reduced by 85% and the bursting discharge was abolished completely. By contrast, CPP reduced the peak amplitude of the EPSP by 52%, attenuated the late phase of the bursting discharge and increased its threshold. These results indicate that excitatory amino acids function as excitatory transmitters in the human brain. While the involvement of non-NMDA receptors in the EPSP is in line with data from normal neocortical slices of other mammals, the participation of NMDA-mediated conductances to the EPSP appears peculiar to the epileptogenic human neocortex. This evidence, together with the contribution of NMDA and non-NMDA receptors to the all-or-none bursting discharge suggests that excitatory amino acid-mediated transmission might be modified in the epileptogenic human neocortex.     

Enhanced excitatory synaptic transmission in spiny neurons of rat striatum after unilateral dopamine denervation

The synaptic transmission and intrinsic membrane properties of spiny neurons in rat neostriatum were studied after unilateral dopamine depletion using in vivo intracellular recording and staining techniques. Two to four weeks after dopamine denervation, the spontaneous firing rate of spiny neurons increased and the spontaneous membrane potential fluctuation stayed at a more depolarized state for longer periods of time. The amplitude of cortically evoked initial excitatory postsynaptic potentials increased and a late excitatory postsynaptic potential that was occasionally found in control neurons was elicited from 23% of spiny neurons after dopamine denervation. No significant changes in intrinsic membrane properties of spiny neurons were observed after dopamine denervation. These results suggest that dopamine inhibits excitatory synaptic transmission of spiny neurons in na?ve animals.     

Slow excitatory amino acid receptor-mediated synaptic transmission in turtle cerebellar Purkinje cells

1. The excitatory synaptic responses of turtle Purkinje cells to climbing and parallel fiber (CF and PF) stimulation have been studied by the use of intrasomatic and intradendritic recordings in intact cerebellum and brain stem-cerebellum preparations in vitro. 2. Activation of CF inputs from the cerebellar peduncle or the region of the inferior olive evoked complex spikes followed by slow excitatory postsynaptic potentials (EPSPs), both of which were evoked in an all-or-none fashion. 3. Single stimuli applied to the cerebellar molecular layer activated fast PF-mediated EPSPs; brief trains of PF stimuli (2-5 stimuli, 50-100 Hz) evoked volleys of fast EPSPs followed by a slow, long-lasting EPSP. The amplitude of the fast and slow PF-mediated EPSPs were both graded with stimulus intensity. 4. Slow EPSPs evoked both by CF and PF stimulation were associated with an increase in membrane conductance and were increased in amplitude by hyperpolarization. 5. The CF-evoked slow EPSP was profoundly attenuated by repetitive activation at interstimulus intervals of less than 15-20 s, whereas the PF-evoked slow EPSP was not reduced by repetitive activation. 6. The PF-evoked slow EPSP readily triggered dendritic pacemaker discharges when activated at or near resting membrane potential. The activation of this potential by phasic PF volleys may, therefore, provide an appropriate synaptic drive to cerebellar Purkinje cells to entrain the intrinsic pacemaker properties of these cells to cycles of motor activity. 7. Both slow synaptic potentials were blocked by the excitatory amino acid antagonists kynurenate and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), but not by DL-2-amino-5-phosphonovalerate (DL-AP5) or L-serine-O-phosphate (L-SOP). The PF-evoked slow EPSP was selectively antagonized by L-2-amino-4-phosphonobutyrate (L-AP4; 20-100 microM). 8. It is suggested that the CF- and PF-evoked slow EPSPs observed in this study represent a novel class of excitatory amino acid receptor-mediated slow synaptic potentials activated by Purkinje cell afferents, which may play a role in synaptic integration and motor pattern generation in the cerebellum.