Excitatory transmission in the basolateral amygdala.

1. Intracellular current-clamp recordings obtained from neurons of the basolateral nucleus of the amygdala (BLA) were used to characterize postsynaptic potentials elicited through stimulation of the stria terminalis (ST) or the lateral amygdala (LA). The contribution of glutamatergic receptor subtypes to excitatory postsynaptic potentials (EPSPs) were analyzed by the use of the non N-methyl-D-aspartate (non-NMDA) antagonist, 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX), and the NMDA antagonist, (DL)-2-amino-5-phosphonovaleric acid (APV). 2. Basic membrane properties of BLA neurons determined from membrane responses to transient current injection showed that at the mean resting membrane potential (RMP; -67.2 mV) the input resistance (RN) and time constant for membrane charging (tau) were near maximal, and that both values were reduced with membrane hyperpolarization, suggesting an intrinsic regulation of synaptic efficacy. 3. Responses to stimulation of the ST or LA consisted of an EPSP followed by either a fast inhibitory postsynaptic potential (f-IPSP) only, or by a fast- and subsequent slow-IPSP (s-IPSP). The EPSP was graded in nature, increasing in amplitude with increased stimulus intensity, and with membrane hyperpolarization after DC current injection. Spontaneous EPSPs were also observed either as discrete events or as EPSP/IPSP waveforms. 4. In physiological Mg2+ concentrations (1.2 mM), at the mean RMP, the EPSP consisted of dual, fast and slow, glutamatergic components. The fast-EPSP (f-EPSP) possessed characteristics of kainate/quisqualate receptor activation, namely, the EPSP increased in amplitude with membrane hyperpolarization, was insensitive to the NMDA receptor antagonist, APV (50 microM), and was blocked by the non-NMDA receptor antagonist, CNQX (10 microM). In contrast, the slow-EPSP (s-EPSP) decreased in amplitude with membrane hyperpolarization, was insensitive to CNQX (10 microM), and was blocked by APV (50 microM), indicating mediation by NMDA receptor activation. 5. In the presence of CNQX (10 microM), ST stimulation evoked an APV-sensitive s-EPSP. In contrast, LA stimulation evoked a f-IPSP, which when blocked by subsequent addition of bicuculline methiodide (BMI; 30 microM) revealed a temporally overlapping APV-sensitive s-EPSP. These data suggest that EPSP amplitude and duration are determined, in part, by the shunting of membrane conductance caused by a concomitant IPSP. 6. Superfusion of either CNQX or APV in BLA neurons caused membrane hyperpolarization and blockade of spontaneous EPSPs and IPSPs, suggesting that these compounds may act to block tonic excitatory amino acid (EAA) release within the nucleus, and that a degree of feed-forward inhibition occurs within the nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)     

Low-threshold Ca2+ channels mediate induction of long-term potentiation in kitten visual cortex.

1. The induction mechanism of long-term potentiation (LTP) in developing visual cortex was studied by recording intracellular responses from layer III-IV cells in slice preparations of kitten visual cortex at 30-40 days after birth. 2. Strong stimulation of white matter produced a late depolarizing response after an orthodromic action potential. This depolarizing response was abolished by membrane depolarization or hyperpolarization caused by current injection through the recording electrode. In addition, this response was reduced by bath application of a low concentration (100 microM) of Ni2+ without any changes in the rising slope of the excitatory postsynaptic potential (EPSP) or orthodromic action potential. This suggests that this response is mediated by low-threshold Ca2+ channels (LTCs). 3. The involvement of LTCs in the induction of LTP was tested. White matter was stimulated at 2 Hz for 15 min as a conditioning stimulus to induce LTP, and the resultant changes in EPSPs were tested by low-frequency (0.1 Hz) stimulation of white matter. Conditioning stimulation produced a large N-methyl-D-aspartate (NMDA) receptor-mediated depolarizing response in these cells, which obscured the presence of the late depoliarzation. Therefore the test was conducted in a solution containing an NMDA antagonist 2-amino-5-phosphonovalerate (APV). 4. Weak conditioning stimulation, which evoked no LTC responses, never induced LTP; whereas strong conditioning stimulation, which evoked LTC responses, always induced LTP. Strong conditioning stimulation failed to induce LTP when LTC responses were prevented either by membrane depolarization or hyperpolarization or by a bath application of 100 microM Ni2+. 5. In a solution without APV, the application of Ni2+ also prevented the induction of LTP. 6. When cells were impaled by an electrode containing a Ca2+ chelator 1,2-bis-(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), LTP was never induced, even though LTC responses were evoked by conditioning stimulation. These results indicate that Ca2+ influx into postsynaptic cells through LTCs induces the LTP. 7. The responses mediated by LTCs, which were evoked by the injection of current pulses into the cells, were maximum at the critical period of visual cortical plasticity, suggesting that LTCs in postsynaptic cells regulate the plastic changes in developing visual cortex.     

Nature of inhibitory postsynaptic activity in developing relay cells of the lateral geniculate nucleus

Using intracellular recordings in an isolated (in vitro) brain stem preparation, we examined the inhibitory postsynaptic responses of developing neurons in the dorsal lateral geniculate nucleus (LGN) of the rat. As early as postnatal day (P) 1-2, 31% of all excitatory postsynaptic (EPSP) activity evoked by electrical stimulation of the optic tract was followed by inhibitory postsynaptic potentials (IPSPs). By P5, 98% of all retinally evoked EPSPs were followed by IPSP activity. During the first postnatal week, IPSPs were mediated largely by GABA(A) receptors. Additional GABA(B)-mediated IPSPs emerged at P3-4 but were not prevalent until after the first postnatal week. Experiments involving the separate stimulation of each optic nerve indicated that developing LGN cells were binocularly innervated. At P11-14, it was common to evoke EPSP/IPSP pairs by stimulating either the contralateral or ipsilateral optic nerve. During the third postnatal week, binocular excitatory responses were encountered far less frequently. However, a number of cells still maintained a binocular inhibitory response. These results provide insight about the ontogeny and nature of postsynaptic inhibitory activity in the LGN during the period of retinogeniculate axon segregation.     

An N-methyl-D-aspartate receptor mediated excitatory postsynaptic potential evoked in subthalamic neurons in an in vitro slice preparation of the rat

Subthalamic (STH) neurons with slow EPSPs mediated by an N-methyl-D-aspartate (NMDA) receptor were studied in rat brain slice preparation. When STH neurons were intracellularly recorded with KCl-filled electrodes, stimulation of the internal capsule (IC) evoked a short duration depolarization followed by a slow excitatory postsynaptic potential (EPSP) lasting 100-200 ms. The amplitude of the slow EPSP was increased when the neuron was hyperpolarized by a low intensity current injection but was blocked when it was hyperpolarized with strong current. The slow EPSP was reversibly suppressed by application of 30-50 microM DL-2-amino-5-phosphonovareric acid (APV). STH neurons also were recorded, with potassium methylsulfate filled electrodes, in the slice preparation obtained from rats that received chronic knife cuts of the IC at the level of the entopeduncular nucleus. Stimulation of the IC immediately rostral to the STH evoked a fast EPSP followed by a slow EPSP, and IPSPs were largely eliminated in this preparation. The slow EPSP was augmented in MG-free medium and suppressed by 50 microM APV. These results suggest that NMDA receptor mediating slow EPSPs may regulate activities of STH neurons.     

Inhibitory transmission in the basolateral amygdala.

1. Intracellular recording techniques were used to characterize synaptic inhibitory postsynaptic potentials (IPSPs) recorded from neurons of the basolateral nucleus of the amygdala (BLA). Bipolar electrodes positioned in the stria terminalis (ST) or lateral amygdala (LA) were used to evoke synaptic responses at a frequency of 0.25 Hz. 2. Two synaptic waveforms having IPSP components could be evoked by electrical stimulation of either pathway: a biphasic, excitatory postsynaptic potential (EPSP), fast-IPSP (f-IPSP) waveform, and a multiphasic, EPSP, f-IPSP, and subsequent slow-IPSP (s-IPSP) waveform. Expression of either waveform was dependent on the site of stimulation. ST stimulation evoked a similar number of biphasic (45%) and multiphasic (50%) synaptic responses. In contrast, stimulation of the LA pathway evoked mainly (80%) multiphasic synaptic responses. 3. Both the f- and s-IPSP elicited by ST stimulation could be reduced in amplitude in the presence of the glutamatergic, N-methyl-D-aspartate (NMDA) antagonist, (DL)-2-amino-5-phosphonovaleric acid (APV, 50 microM), and were abolished by the glutamatergic, non-NMDA antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM). In contrast, a CNQX-resistant f-IPSP was evoked with LA stimulation and abolished by subsequent addition of bicuculline methiodide (BMI), a gamma-aminobutyric acid (GABAA) receptor antagonist, suggesting direct inhibition of BLA neurons by GABAergic LA interneurons. The sensitivity of the s-IPSPs and the f-IPSPs to glutamatergic antagonists suggests the presence of feed-forward inhibition onto BLA neurons. 4. The f-IPSP possessed characteristics of potentials mediated by GABAA receptors linked to Cl- channels, namely, a reversal potential of -70 mV, a decrease in membrane resistance (13.5 M omega) recorded at -60 mV, a block by BMI, and potentiation by sodium pentobarbital (NaPB). 5. The s-IPSP was associated with a resistance decrease of 4.5 M omega, a reversal potential of -95 mV, and was reversibly depressed (approximately 66%) by 2-hydroxy-saclofen (100 microM), suggesting activation of GABAB receptors. 6. The large resistance change associated with the f-IPSP, its temporal overlap with evoked EPSPs, and the development of both spontaneous and evoked burst firing in the presence of BMI suggests that the f-IPSP determines the primary state of excitability in BLA neurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Intracellular correlates of spatial memory acquisition in hippocampal slices: long-term disinhibition of CA1 pyramidal cells

Despite many advances in our understanding of synaptic models of memory such as long-term potentiation and depression, cellular mechanisms that correlate with and may underlie behavioral learning and memory have not yet been conclusively determined. We used multiple intracellular recordings to study learning-specific modifications of intrinsic membrane and synaptic responses of the CA1 pyramidal cells (PCs) in slices of the rat dorsal hippocampus prepared at different stages of the Morris water maze (WM) task acquisition. Schaffer collateral stimulation evoked complex postsynaptic potentials (PSP) consisting of the excitatory and inhibitory postsynaptic potentials (EPSP and IPSP, respectively). After rats had learned the WM task, our major learning-specific findings included reduction of the mean peak amplitude of the IPSPs, delays in the mean peak latencies of the EPSPs and IPSPs, and correlation of the depolarizing-shifted IPSP reversal potentials and reduced IPSP-evoked membrane conductance. In addition, detailed isochronal analyses revealed that amplitudes of both early and late IPSP phases were reduced in a subset of the CA1 PCs after WM training was completed. These reduced IPSPs were significantly correlated with decreased IPSP conductance and with depolarizing-shifted IPSP reversal potentials. Input-output relations and initial rising slopes of the EPSP phase did not indicate learning-related facilitation as compared with the swim and na?ve controls. Another subset of WM-trained CA1 PCs had enhanced amplitudes of action potentials but no learning-specific synaptic changes. There were no WM training-specific modifications of other intrinsic membrane properties. These data suggest that long-term disinhibition in a subset of CA1 PCs may facilitate cell discharges that represent and record the spatial location of a hidden platform in a Morris WM.     

Actions of excitatory amino acid antagonists on synaptic potentials of layer II/III neurons of the cat's visual cortex

Summary Actions of excitatory amino acid (EAA) antagonists on the responses of cells in layers II/III and IV of the cat's visual cortex to stimulation of layer VI and the underlying white matter were studied in slice preparations. Antagonists used were 2-amino-5-phosphonovalerate (APV), a selective antagonist for the N-methyl-D-aspartate (NMDA) type of EAA receptors, and kynurenate, a broadspectrum antagonist for the three types of EAA receptors. In extracellular recordings it was demonstrated that most of the layer II/III cells were sensitive to APV, while the great majority of the layer IV cells were not, By contrast, kynurenate suppressed the responses completely in both layers. Excitatory post-synaptic potentials (EPSPs) evoked by stimulation of layer VI and the while matter were recorded intracellularly from layer II/III neurons. To determine whether the EPSPs were elicited mono- or polysynaptically, the synaptic delay for each EPSP was calculated from a pair of onset latencies of EPSPs evoked by stimulation of the two sites. Forty-two percent of the layer II/III cells were classified as having monosynaptic EPSPs. In 60% of these monosynaptic cells, the rising slope of the EPSPs was reduced by APV while in the other 40%, it was not. In the former (APV-sensitive cells), subtraction of the APV-sensitive component from the total EPSP indicated that the onset latency of the NMDA receptor-mediated component was roughly equal to that of the non-NMDA component. In the latter (APV-resistant cells), only the slowly-decaying component was in part mediated by NMDA receptors. The conduction velocities of the afferent fibers innervating APV-resistant cells were slower than those of the APV-sensitive cells, suggesting that both types of cells are innervated by different types of afferents. The polysynaptic EPSPs of almost all layer II/III cells were sensitive to APV. The subtraction method indicated that the NMDA component had about the same magnitude as the non-NMDA components. When the slices were superfused by a Mg²⁺-free solution, the EPSPs were potentiated dramatically, but this potentiation was reduced to the control level during the administration of APV. Similarly, APV-sensitive components were potentiated during the administration of bicuculline, a selective antagonist for gamma-aminobutyric acid receptors of A type. These results suggest that NMDA receptors participate, at varying degrees, in excitatory synaptic transmission at most layer II/III cells in the cat's visual cortex, and their actions appear to be regulated by intracortical inhibition.     

Excitatory amino acid receptors involved in primary afferent-evoked polysynaptic EPSPs of substantia gelatinosa neurons in the adult rat spinal cord slice.

Intracellular recordings were made from substantia gelatinosa (SG) neurons in spinal cord slices to determine a subclass of excitatory amino acid receptors involved in polysynaptic excitatory postsynaptic potentials (EPSPs). In the majority of neurons, polysynaptic EPSPs evoked by A delta fiber were not affected by 2-amino-5-phosphonovaleric acid (APV), while all EPSPs including monosynaptic EPSPs were depressed by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). All spontaneous EPSPs were blocked by CNQX, while spontaneous EPSPs in a few SG neurons were attenuated by APV. These observations suggest that polysynaptic EPSPs evoked through A delta fibers are predominantly mediated by activation of the non-N-methyl-D-aspartate (non-NMDA) receptor subclass.     

Postsynaptic potentials recorded in neurons of the cat's lateral geniculate nucleus following electrical stimulation of the optic chiasm

1. We recorded intracellularly from X and Y cells of the cat's lateral geniculate nucleus and measured the postsynaptic potentials (PSPs) evoked from electrical stimulation of the optic chiasm. We used an in vivo preparation and computer averaged the PSPs to enhance their signal-to-noise ratio. 2. The vast majority (46 of 50) of our sample of X and Y cells responded to stimulation of the optic chiasm with an excitatory postsynaptic potential (EPSP) followed by an inhibitory postsynaptic potential (IPSP); these were tentatively identified as relay cells. We quantified several parameters of these PSPs, including amplitude, latency, time to peak (i.e., rise time), and duration. 3. Among the relay cells, the latencies of both the EPSP and action potential evoked by optic chiasm stimulation were shorter in Y cells than in X cells. Furthermore, the difference between the latencies of the EPSP and action potential was shorter for Y cells than for X cells. This means that the EPSPs generated in Y cells reached threshold for generation of action potentials faster than did those in X cells. The EPSPs of Y cells also displayed larger amplitudes and faster rise times than did those in X cells, but neither of these distinctions was sufficient to explain the shorter latency difference between the EPSP and action potential for Y cells. 4. The EPSPs recorded in relay Y cells had longer durations than did those in relay X cells. Our data suggest that the subsequent IPSP actively terminates the EPSP, which, in turn, suggests that the time interval between EPSP and IPSP onsets is longer in Y cells than in X cells. Furthermore, we found that, for individual Y cells, the latency and duration of the evoked EPSP were inversely related. These observations lead to the conclusion that the latency of IPSPs activated from the optic chiasm is relatively constant among Y cells and thus independent of the EPSP latencies. Thus the excitation and inhibition produced in individual geniculate Y cells may originate from different populations of retinogeniculate axons. 5. The IPSPs recorded in geniculate relay cells following optic chiasm stimulation could be divided into three groups based on their durations. The majority of both X and Y cells showed short-duration IPSPs, whereas the remainder of Y cells displayed medium-duration IPSPs, and the remaining X cells displayed long-duration IPSPs. A positive correlation was seen between the time to peak and duration of these IPSPs.(ABSTRACT TRUNCATED AT 400 WORDS)     

Synaptic responses of neurons controlling the parotid and von Ebner salivary glands in rats to stimulation of the solitary nucleus and tract

Salivary secretion results from reflex stimulation of autonomic neurons via afferent sensory information relayed to neurons in the rostral nucleus of the solitary tract (rNST), which synapse with autonomic neurons of the salivatory nuclei. We investigated the synaptic properties of the afferent sensory connection to neurons in the inferior salivatory nucleus (ISN) controlling the parotid and von Ebner salivary glands. Mean synaptic latency recorded from parotid gland neurons was significantly shorter than von Ebner gland neurons. Superfusion of GABA and glycine resulted in a concentration-dependent membrane hyperpolarization. Use of glutamate receptor antagonists indicated that both AMPA and N-methyl-D-aspartate (NMDA) receptors are involved in the evoked excitatory postsynaptic potentials (EPSPs). Inhibitory postsynaptic potential (IPSP) amplitude increased with higher intensity ST stimulation. Addition of the glycine antagonist strychnine did not affect the amplitude of the IPSPs significantly. The GABA(A) receptor antagonist, bicuculline (BMI) or mixture of strychnine and BMI abolished the IPSPs in all neurons. IPSP latency was longer than EPSP latency, suggesting that more than one synapse is involved in the inhibitory pathway. Results show that ISN neurons receive both excitatory and inhibitory afferent input mediated by glutamate and GABA respectively. The ISN neuron response to glycine probably derives from descending connections. Difference in the synaptic characteristics of ISN neurons controlling the parotid and von Ebner glands may relate to the different function of these two glands.     

Synaptic responses of guinea pig cingulate cortical neurons in vitro.

1. Intracellular recordings were made from layer V/VI neurons of the guinea pig anterior cingulate cortex to investigate postsynaptic potentials (PSPs) evoked by electrical stimulation of the subcortical white matter (forceps minor). 2. Four distinct types of PSPs were recorded (at the resting potential) under normal physiological conditions; 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-sensitive excitatory postsynaptic potentials (EPSPs) were followed by bicuculline- or picrotoxin-sensitive depolarizing or hyperpolarizing inhibitory postsynaptic potentials (IPSPs), which were further followed by phaclofen-sensitive, long-lasting hyperpolarizing postsynaptic potentials (LPSPs). The average times-to-peak for the EPSP, depolarizing and hyperpolarizing IPSPs, and LPSP were 10, 22, 28, and 146 ms, respectively. 3. In the presence of CNQX and bicuculline, high-intensity electrical stimulation elicited a longer lasting EPSP with a time-to-peak of 21 ms. The amplitude and duration of the EPSP decreased with membrane hyperpolarization and increased with membrane depolarization. The EPSP was reversibly abolished by D,L-2-amino-5-phosphonovaleric acid (D,L-APV). 4. The bicuculline- or picrotoxin-sensitive depolarizing and hyperpolarizing IPSPs and the phaclofen-sensitive LPSP were markedly suppressed by CNQX, suggesting that glutamate (Glu) and/or aspartate nerve terminals project to GABAergic interneurons, and that the GABAergic interneurons are activated mainly by non-N-methyl-D-aspartate (non-NMDA) receptors. 5. In the presence of picrotoxin, the average reversal potential for the compound EPSP was 0 mV, which was similar to that (-6 mV) for the Glu-induced depolarization. In a solution containing D,L-APV at low concentrations, the average reversal potentials for the depolarizing and hyperpolarizing IPSPs and for the early and late components of the gamma-aminobutyric acid (GABA)-induced responses were -62, -72, -70, and -61 mV, respectively. Thus the value for the depolarizing IPSP was similar to that for the late response to GABA, whereas the value for the hyperpolarizing IPSP was almost the same as that for the early response to GABA. The average reversal potential of -90 mV for the LPSP was similar to -93 mV for the baclofen-induced hyperpolarization and to -94 mV for the spike afterhyperpolarization. 6. Application of phaclofen decreased the interspike interval of the spontaneous firing and reversed the increase in the interspike interval after subcortical stimulation. This result indicates that, even in a slice preparation, the anterior cingulate neurons are under tonic inhibitory control exerted by spontaneously active GABAergic interneurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Physiology and pharmacology of epileptiform activity induced by 4-aminopyridine in rat hippocampal slices.

1. Conventional intracellular and extracellular recording techniques were used to investigate the physiology and pharmacology of epileptiform bursts induced by 4-aminopyridine (4-AP, 50 microM) in the CA3 area of rat hippocampal slices maintained in vitro. 2. 4-AP-induced epileptiform bursts, consisting of a 25-to 80-ms depolarizing shift of the neuronal membrane associated with three to six fast action potentials, occurred at the frequency of 0.61 +/- 0.29 (SD)/s. The bursts were generated synchronously by CA3 neurons and were triggered by giant excitatory postsynaptic potentials (EPSPs). A second type of spontaneous activity consisting of a slow depolarization also occurred but at a lower rate (0.04 +/- 0.2/s). 3. The effects of 4-AP on EPSPs and inhibitory postsynaptic potentials (IPSPs) evoked by mossy fiber stimulation were studied on neurons impaled with a mixture of K acetate and 2(triethyl-amino)-N-(2,6-dimethylphenyl) acetamide (QX-314)-filled microelectrodes. After the addition of 4-AP, the EPSP became potentiated and was followed by the appearance of a giant EPSP. This giant EPSP completely obscured the early IPSP recorded under control conditions and inverted at -32 +/- 3.9 mV (n = 4), suggesting that both inhibitory and excitatory conductances were involved in its generation. IPSPs evoked by Schaffer collateral stimulation increased in amplitude and duration after 4-AP application. 4. The spontaneous field bursts and the stimulus-induced giant EPSP induced by 4-AP were not affected by N-methyl-D-aspartate (NMDA) receptor antagonists 3-3 (2-carboxy piperazine-4-yl) propyl-1-phosphonate (CPP) and DL-2-amino-5-phosphonovalerate (APV) but were blocked by quisqualate/kainate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and 6,7-dinitroquinoxaline-2,3-dione (DNQX). CNQX also abolished the presence of small spontaneously occurring EPSPs, thereby disclosing the presence of bicuculline-sensitive (BMI, 20 microM) IPSPs. 5. Small, nonsynchronous EPSPs played an important role in the generation of 4-AP-induced epileptiform activity. 1) After the addition of 4-AP, small EPSPs appeared randomly on the baseline and then became clustered to produce a depolarizing envelope of irregular shape that progressively formed an epileptiform burst, 2) These small EPSPs were more numerous in the 100 ms period that preceded burst onset. 3) The frequency of occurrence of small EPSPs was positively correlated with the frequency of occurrence of synchronous bursts. 4) Small EPSPs and bursts were similarly decreased after the addition of different concentrations of CNQX (IC50 in both cases of approximately 1.2 microM).(ABSTRACT TRUNCATED AT 400 WORDS)     

Modulation of synaptic transmission in hippocampal CA1 neurons by a novel neurotoxin (beta-pompilidotoxin) derived from wasp venom.

We examined the effects of beta-pompilidotoxin (beta-PMTX), a neurotoxin derived from wasp venom, on synaptic transmission in the mammalian central nervous system (CNS). Using hippocampal slice preparations of rodents, we made both extracellular and intracellular recordings from the CA1 pyramidal neurons in response to stimulation of the Schaffer collateral/commissural fibers. Application of 5-10 microM beta-PMTX enhanced excitatory postsynaptic potentials (EPSPs) but suppressed the fast component of the inhibitory postsynaptic potentials (IPSPs). In the presence of 10 microM bicuculline, beta-PMTX potentiated EPSPs that were composed of both non-NMDA and NMDA receptor-mediated potentials. Potentiation of EPSPs was originated by repetitive firings of the presynaptic axons, causing summation of EPSPs. In the presence of 10 microM CNQX and 50 microM APV, beta-PMTX suppressed GABA(A) receptor-mediated fast IPSPs but retained GABA(B) receptor-mediated slow IPSPs. Our results suggest that beta-PMTX facilitates excitatory synaptic transmission by a presynaptic mechanism and that it causes overexcitation followed by block of the activity of some population of interneurons which regulate the activity of GABA(A) receptors.     

Receptor subtypes involved in callosally-induced postsynaptic potentials in rat frontal agranular cortex in vitro

Summary A slice preparation of rat frontal agranular cortex preserving commissural inputs has been used for intracellular recording from layer V pyramidal cells, in order to characterize the synaptic potentials induced by stimulation of the corpus callosum and to reveal the subtypes of amino acid receptors involved. Stimulation of the corpus callosum induced EPSPs followed by early IPSPs with a peak latency of 30 ± 2 ms and late IPSPs with a peak latency of 185 ± 18 ms. Reversal potentials for early and late IPSPs were –75 ± 5 mV (early) and –96 ± 5 mV (late). Late IPSPs were more dependent on extracellular K⁺ concentration. The early IPSPs were blocked by GABA_A antagonists, bicuculline and picrotoxin, whereas the late IPSPs were reduced by the GABA_B antagonist, phaclofen. CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), an antagonist of non-NMDA (N-methyl-D-aspartate) receptors, suppressed both EPSPs and late IPSPs at 5 µM. Early IPSPs remained at this concentration but were suppressed by 20 µM CNQX. In Mg²⁺-free solution, EPSPs were larger and more prolonged than in control solution. These enhanced EPSPs persisted after 5 to 20 µM CNQX, but were reduced in amplitude, and their onset was delayed by 3.6 ± 0.8 ms. The remaining EPSPs were suppressed by 50 µM APV (DL-2-amino-5-phosphono-valeric acid), an antagonist of NMDA receptors. In Mg²⁺-free solution containing 5 to 20 µM CNQX, the late IPSPs were not diminished. The remaining late IPSPs were suppressed by APV or by phaclofen. By contrast, the amplitude of early IPSPs was not affected by APV in Mg²⁺-free solution containing 5 µM CNQX. These results show that stimulation of the corpus callosum can induce GABA_A and GABA_B dependent IPSPs and NMDA and non-NMDA dependent excitation. It is suggested that these four types of amino acid-based transmission are conveyed by intracortical pathways with different characteristics.     

NMDA receptor antagonists reduce medial,but not lateral,perforant path-evoked EPSPs in dentate gyrus of rat hippocampal slice

Summary NMDA receptor antagonists produced differential effects on medial and lateral perforant path-evoked excitatory postsynaptic potentials (EPSPs) recorded in the dentate gyrus molecular layer of hippocampal slices. D-(-)-2-amino-5-phosphonovaleric acid (D(-)APV) and 3-[(±)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) significantly reduced the peak amplitude and total area, but not the initial negative slope, of the medial perforant path-evoked EPSP. Neither antagonist affected any component of the lateral perforant path-evoked EPSP. In contrast, population spikes evoked by stimulation of either pathway were depressed.     

Long-term potentiation in frontal cortex: role of NMDA-modulated polysynaptic excitatory pathways

The present study examined the role of N-methyl-D-aspartic acid (NMDA) receptors in synaptic plasticity in regular-spiking cells of rat frontal cortex. Intracortical stimulation, at levels subthreshold for elicitation of action potentials, evoked a late excitatory postsynaptic potential (EPSP) in layer II-III neurons that was sensitive to the selective NMDA antagonist D-2-amino-5-phosphonovaleric acid (APV). This late EPSP showed marked short-term frequency-dependent depression, suggesting that it is polysynaptic in origin. Polysynaptic late EPSPs were selectively enhanced following high-frequency stimulation. This sustained increase in synaptic efficacy, or long-term potentiation, was expressed in regular spiking cells and appeared to result from activation of NMDA receptors on excitatory interneurons. These data demonstrate the existence of an NMDA-modulated polysynaptic circuit in the neocortex which displays several types of use-dependent plasticity.     

Depression of the fast IPSP underlies paired-pulse facilitation in area CA1 of the rat hippocampus.

1. Intracellular recordings from CA1 pyramidal neurons in the rat hippocampal slice have been used to study synaptic transmission after maximal orthodromic stimulation of the Schaffer collateral-commissural fibers. Paired-pulse stimulation was used to investigate how the first (conditioning) stimulation influenced the response to the second (test) stimulation. 2. When the test stimulation was delivered up to approximately 4 s after the conditioning stimulation, the late phase of the excitatory postsynaptic synaptic potential (EPSP) was increased ("late-phase facilitation") whereas the fast (f-) and the slow (s-) inhibitory postsynaptic potentials (IPSPs) were depressed. 3. In terms of appearance and time course, facilitation of the intracellularly recorded EPSP was similar to that of the extracellularly recorded field EPSP in stratum radiatum. 4. The s-IPSP is not involved in facilitation of the EPSP. To show this, we counteracted the s-IPSP either by repolarizing the membrane potential to the resting level or by intracellularly injecting the quaternary lignocaine derivative QX 314. Facilitation of the late phase of the EPSP was unaffected by either procedure. 5. The conditioned response was modified in two ways when the stimulation was delivered at the equilibrium potential for the f-IPSP (Ef-IPSP) and the s-IPSP had been blocked by intracellular injection of QX 314. The amplitude of the EPSP was increased, and the repolarizing phase was delayed with an apparent depolarizing shift of Ef-IPSP. This effect was present at pulse intervals greater than 20 ms and was maximal after 150 ms. Facilitation could be detected at interpulse intervals of up to 4 s. 6. The gamma-aminobutyric acid-B (GABAB) agonist baclofen (1 microM) reduced late-phase facilitation by preferentially increasing the unconditioned response, such that this came to resemble a conditioned response in control medium. 7. The f-IPSP was isolated pharmacologically to investigate its role in the facilitation of the EPSP. This was done by blocking the s-IPSP with QX314 and the EPSP with a mixture of the N-methyl-D-aspartate (NMDA) receptor blocker, 2-amino-5-phosphonovaleric acid (APV, 50 microM), and the non-NMDA receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM). An f-IPSP was then evoked by stimulating the interneurons directly. This potential could be blocked by the GABAA receptor antagonist bicuculline (20 microM), thereby confirming the successful isolation of GABAAergic transmission. 8. With paired-pulse stimulation, the amplitude of the conditioned f-IPSP was depressed.(ABSTRACT TRUNCATED AT 400 WORDS)     

Interaction between peripheral afferent activity and presynaptic inhibition of ia afferents in the cat

It has been demonstrated in man that the H-reflex is more depressed by presynaptic inhibition than the stretch reflex. Here we investigated this finding further in the alpha-chloralose-anesthetized cat. Soleus monosynaptic reflexes were evoked by electrical stimulation of the tibial nerve or by stretch of the triceps surae muscle. Conditioning stimulation of the posterior biceps and semitendinosus nerve (PBSt) produced a significantly stronger depression of the electrically than the mechanically evoked reflexes. The depression of the reflexes has been shown to be caused by presynaptic inhibition of triceps surae Ia afferents. We investigated the hypothesis that repetitive activation of peripheral afferents may reduce their sensitivity to presynaptic inhibition. In triceps surae motoneurones, we measured the effect of presynaptic inhibition on excitatory postsynaptic potentials (EPSPs) produced by repetitive activation of the peripheral afferents or by fast and slow muscle stretch. EPSPs evoked by single electrical stimulation of the tibial nerve or by fast muscle stretch were significantly depressed by PBSt stimulation. However, the last EPSP in a series of EPSPs evoked by a train of electrical stimuli (5-6 shocks, 150-200 Hz) was significantly less depressed by the conditioning stimulation than the first EPSP. In addition, the last part of the long-lasting EPSPs evoked by a slow muscle stretch was also less depressed than the first part. A single EPSP evoked by stimulation of the medial gastrocnemius nerve was less depressed when preceded by a train of stimuli applied to the same nerve than when the same train of stimuli was applied to a synergistic nerve. The decreased sensitivity of the test EPSP to presynaptic inhibition was maximal when it was evoked within 20 ms after the train of EPSPs. It was not observed at intervals longer than 30 ms. These findings suggest that afferent activity may decrease the efficiency of presynaptic inhibition. We propose that the described interaction between afferent nerve activity and presynaptic inhibition may partly explain why electrically and mechanically evoked reflexes are differently sensitive to presynaptic inhibition.     

Dopamine presynaptically depresses fast inhibitory synaptic transmission via D4 receptor-protein kinase A pathway in the rat dorsolateral septal nucleus

The lateral septal nucleus receives a diffuse dopaminergic input originating from the ventral tegmental area of the brain stem. We examined whether dopamine (DA) modulates synaptic transmission in the slice preparation of the rat dorsolateral septal nucleus (DLSN). Bath application (10-15 min) of DA (30 muM) markedly depressed the amplitude of fast and slow inhibitory postsynaptic potentials (IPSPs) in DLSN neurons, while it produced only a minor depression of the amplitude of excitatory postsynaptic potentials (EPSPs) obtained in the presence of bicuculline. DA (30 muM) depressed the monosynaptic fast IPSP to approximately 50% of control, but did not depress the inward current (I(GABA)) induced by exogenous gamma-aminobutyric acid (GABA). DA decreased the frequency of miniature fast IPSPs (m-fIPSPs) without significantly changing their amplitude. PD 168077, a selective D4 receptor agonist, depressed the fast and slow IPSPs but not the EPSP and decreased the frequency of m-fIPSPs. Both DA and PD 168077 increased the paired-pulse ratio of the monosynaptic fast IPSP. The inhibitory effect of DA on the fast IPSP was significantly attenuated by L-741,742, an antagonist at D4 receptors, but not by SCH 23390 and sulpiride, a D1-like and a D2-like receptor antagonist, respectively. N-ethylmaleimide, a blocker of pertussis toxin (PTX)-sensitive G protein (G(i/o)), attenuated the DA-induced depression of the fast IPSP. N-[2-((p-bromocinnamyl) amino)ethyl]-5-isoquinoline sulfonamide, a protein kinase A (PKA) inhibitor, attenuated the DA-induced depression of the fast IPSP. These results suggest that DA inhibits spontaneous and evoked release of GABA via the D4 receptor-G(i)-protein-PKA system in DLSN neurons.     

Dorsal column nuclei neurons recorded in a brain stem-spinal cord preparation: characteristics and their responses to dorsal root stimulation

Recordings were obtained from dorsal column nucleus (DCN) neurons in a neonatal rat brain stem-spinal cord preparation to study their basic electrophysiological properties and responses to stimulation of a dorsal root. Whole-cell patch-clamp recordings were made from 21 neurons that responded to dorsal root stimulation with a fast excitatory postsynaptic potential (EPSP). These neurons were located lateral to, but at the level of, the area postrema at depths of 100-268 microm below the dorsal surface of the brain. The neurons could be divided into groups according to the shape of their action potentials or voltage responses to hyperpolarizing current steps; however, the response profiles of the groups of neurons to dorsal root stimulation were not significantly different and all neurons were considered together. Dorsal root stimulation elicited excitatory postsynaptic potentials (EPSPs) in all neurons with a very low variability in onset latency and an ability to follow 100-Hz stimulation, indicating that they were mediated by activation of a monosynaptic pathway. The peak amplitude of the EPSP increased with membrane hyperpolarization, and applications of the non-NMDA receptor antagonists 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2, 3-dione (NBQX) and 6,7-dinitroquinoxaline-2,3-dione (DNQX) decreased the amplitude of the EPSP to 14.2% of the control response (n = 6). The descending phase of the EPSP decreased with membrane hyperpolarization and was reduced by the N-methyl-D-aspartate (NMDA) receptor antagonist AP-5 (n = 2). The EPSPs were also reduced in amplitude by applications of the gamma-aminobutyric acid-B (GABA(B)) receptor agonist baclofen, which had no effect on membrane potential or input resistance. These results show that fast EPSPs in DCN neurons elicited by dorsal root stimulation are mediated by an excitatory amino acid acting at both non-NMDA and, to a lesser extent, NMDA receptors. In addition, GABA acting at presynaptic GABA(B) receptors can inhibit these responses.