Inward rectifier and low-threshold calcium currents contribute to the spontaneous firing mechanism in neurons of the rat suprachiasmatic nucleus

Intracellular and voltage-clamp studies were carried out to clarify the mechanism for spontaneous firing activity in neurons of the suprachiasmatic nucleus (SCN) of rat hypothalamic brain slices in vitro. SCN neurons displayed spontaneously firing action potentials that were preceded by a depolarizing pre-potential and followed by a short spike after-hyperpolarization (AHP). Injection of inward current with a duration longer than 50 ms resulted in a depolarizing voltage sag on hyperpolarizing electrotonic potentials. The inward rectification was depressed by bath application of caesium (1 mM) but not by barium (500 M). SCN neurons also showed a rebound depolarization associated with spike discharge (anodal break) in response to relaxation of hyper polarizing current injection. The rebound depolarization was reduced by nominally zero calcium. Cadmium (500 M), cobalt (1 mM) or caesium (1 mM) but not nicardipine also depressed the rebound depolarization. Under voltage-clamp conditions, hyperpolarizing steps to membrane potentials negative to approximately –60 mV caused an inward rectifier current, probably H current (I _H), which showed no inactivation with time. Bath application of caesium (1–2 mM) suppressed I _H. Caesium (2 mM) depressed the slope of the depolarizing spike pre-potential, resulting in a prolongation of the interspike interval of tonic firing neurons. We conclude that both the inward rectifier current, I _H, and the low-threshold calcium current contribute to the spike prepotential of spontaneous action potentials in firing neurons of the rat SCN.     

Dendrotoxin blocks accommodation in frog myelinated axons

1. Intracellular microelectrode recordings from large sensory and motor myelinated axons in spinal roots of Rana pipiens were used to study the effects of dendrotoxin (DTX), a specific blocker of a fast activating potassium current (GKf1). 2. Dendrotoxin reduced the ability of myelinated sensory and motor axons to accommodate to a constant stimulus. A depolarizing current step, which normally evoked only one action potential, after dendrotoxin treatment (200-500 nM) produced a train of action potentials. These spike trains lasted 29 +/- 2.8 (SE) ms on average in sensory fibers (n = 18) and 40.2 +/- 4.5 ms in motor fibers (n = 9). 3. After dendrotoxin treatment, in addition to a reduction in the ability to accommodate to a constant stimulus, a slowing in the rate of action potential generation was evident (spike frequency adaptation). 4. Dendrotoxin had no effect on the rising phase of conducted action potentials evoked by peripheral stimulation. Together with a lack of effect on the absolute refractory period, these results indicate that dendrotoxin does not affect sodium channel activity. 5. The steady-state voltage/current relationship was unchanged in response to hyperpolarizing current pulses; however, there was a significant increase in cord resistance in response to depolarizing current steps, demonstrating that DTX decreases outward rectification. 6. A computer model based on Hodgkin and Huxley equations was developed, which included the three voltage-dependent potassium conductances described by Dubois. The model reproduced major experimental results: removal of the conductance, termed GKf1, reduced the accommodation in the early phase of a continuous stimulus, indicating that this current could be responsible for the early accommodation. The hypothesis that the slow potassium conductance GKs regulates late accommodation and action potential frequency adaptation is also supported by the computer model. 7. In summary, these results suggest that in amphibian myelinated sensory and motor axons, the activity of potassium conductances can account for accommodation and adaptation without involvement of sodium conductance activity.     

Recording of action potentials and polarization of a single crayfish motor axon through a sucrose-gap capillary suction electrode

Summary A capillary suction electrode was developed which contains in its tip a sucrose-gap. The nerve fiber is pulled by negative pressure through the closely fitting openings of an outer and an inner capillary. The space between these openings is o.1 mm wide and can be perfused with sucrose solution. Because of the small physical size of the electrode tip the electrode can be used to record from single unmyelinated nerve fibers in small preparations in combination with other electrodes or transducers.Action potentials were recorded from branches of the motor axon of the opener muscle of the crayfish claw. The amplitude of the recorded action potentials was 7–22 mV, and all records were monophasic positive. The action potential showed depolarizing but not hyperpolarizing afterpotentials.Hyperpolarization of the nerve fiber by –0.05 to –0.2 A current injected through the recording electrode led to about 50% increase in the amplitude of the recorded action potentials and to a marked prolongation of the depolarizing afterpotentials. Depolarization of the fiber decreased the amplitude of the action potential.This work was supported by the Deutsche Forschungsgemeinschaft.     

Properties of subthreshold response and action potential recorded in layer V neurons from cat sensorimotor cortex in vitro

Properties of the action potential and subthreshold response were studied in large layer V neurons in in vitro slices of cat sensorimotor cortex using intracellular recording and stimulation, application of agents that block active conductances, and a single-microelectrode voltage clamp (SEVC). A variety of measured parameters, including action-potential duration, afterpotentials, input resistance, rheobase, and membrane time constant, were similar to the same parameters reported for large neurons from this region of cortex in vivo. Action-potential amplitudes and resting potentials were greater in vitro. Most measured parameters were distributed unimodally, suggesting that these parameters are similar in all large layer V neurons irrespective of their axonal termination. The voltage response to subthreshold constant-current pulses exhibited both time and voltage dependence in the great majority of cells. Current pulses in either the hyperpolarizing or subthreshold depolarizing direction cause the membrane potential to attain an early peak and then decay (sag) to a steady level. On termination of the pulse, the membrane response transiently overshoots resting potential. Plots of current-voltage relations demonstrate inward rectification during polarization on either side of resting potential. Subthreshold inward rectification in the depolarizing direction is abolished by tetrodotoxin (TTX). The ionic currents responsible for subthreshold rectification and sag were examined using the SEVC. Steady inward rectification in the depolarizing direction is caused by a persistent, subthreshold sodium current (INaP) (54). Sag observed in response to a depolarizing current pulse is due to activation of a slow outward current, which superimposes on and partially counters the persistent sodium current. Both sag in response to hyperpolarizing current pulses and rectification in the hyperpolarizing direction are caused by a slow inward "sag current" that is activated by hyperpolarizing voltage steps. The sag current is unaltered by TTX, tetraethylammonium, (TEA), Co2+, Ba2+, or 4-aminopyridine. Fast-rising, short-duration action potentials can be elicited by an intracellular current pulse or by orthodromic or antidromic stimulation. Spikes are blocked by TTX. The form of the afterpotential following a directly evoked spike varies among cells with similar resting potentials. Biphasic afterhyperpolarizations (AHPs) with fast and slow components were most frequently seen. About 30% of the cells displayed a depolarizing afterpotential (DAP), which was often followed by an AHP. Other cells displayed a purely monophasic AHP.(ABSTRACT TRUNCATED AT 400 WORDS)     

Adenosine enhances afterhyperpolarization and accommodation in hippocampal pyramidal cells

Adenosine added to the perfusion fluid of rat hippocampal slices at 10 mol · l^–1 enhanced long lasting afterhyperpolarizations after single action potentials, bursts of action potentials or calcium spikes. Accommodation of firing during a depolarizing pulse was potentiated. An increase in calcium dependent potassium conductance is likely to mediate these effects. Adenosine at 50 mol·l^–1 induced a hyperpolarization accompanied by a reduction in input resistance. The hyperpolarization could be reversed at –85 mV. In TTX and TTX-barium treated slices the amplitude of the slow spike was decreased. This may result from a shunting of inward current in the dendrites due to an adenosine induced increase in potassium conductance. It is suggested that adenosine reduces pre- and postsynaptic exicatory signals principally by enhancing one or more potassium conductances. This effect is a powerful means for modulation of neuronal excitability and synaptic efficacy and can explain the antiepileptic activity of adenosine.     

Monosynaptic excitatory connexions of reticulospinal neurones in the nucleus reticularis pontis caudalis with dorsal neck motoneurones in the cat

Summary 1. Projections of reticulospinal neurones (RSNs) in the nucleus reticularis pontis caudalis (N.r.p.c.) to dorsal neck motoneurones supplying splenius (SPL, lateral head flexor) and biventer cervicis and complexus (BCC, head elevator) muscles were studied in the cat anaesthetized with pentobarbiturate or -chloralose. 2. Threshold mapping for evoking antidromic spikes revealed that most of RSNs tested projecting down to brachial segments but not to lumbar segments (C-RSNs) gave off collaterals to the gray matter of the upper spinal cord in C2–C3 segments. 3. Spike triggered averaging showed that negative field potentials were evoked after firing of a single C-RSN (single fibre focal synaptic potentials, FSPs) in the region of C2–C3 where large antidromic field potentials from nerves supplying SPL or BCC muscles were evoked. The single fibre FSPs ranged between 1 and 10 V in amplitude and had latencies between 0.7 and 1.2 ms from the onset of the triggering spike. In most cases, a presynaptic spike preceded the negative potential by 0.3 ms. These results indicated that C-RSNs project to the SPL or BCC motor nucleus. 4. Spike triggered averaging of postsynaptic potentials revealed EPSPs (single fibre EPSPs) in 36 dorsal neck motoneurones, predominantly in SPL (25) and less in BCC (11) motoneurones, evoked from 15 C-RSNs. The amplitude of the single fibre EPSPs ranged from 5 to 310 V, and had latencies of 0.8–2.0 ms from the onset of the triggering spikes of C-RSNs, or 0.3–0.5 ms from the presynaptic spike when recorded. The results indicated monosynaptic excitatory connexions of C-RSNs to dorsal neck motoneurones. 5. Single fibre EPSPs from a C-RSN were usually recorded from either BCC or SPL motoneurones but not from both types of motoneurones, when tested in many motoneurones. This showed that connexions of C-RSNs with dorsal neck motoneurones were muscle specific. 6. RSNs projecting down to the lumbar segment (L-RSN) also showed branching in C2–C3 segments. Excitatory monosynaptic connexion of L-RSNs with neck motoneurones were demonstrated by recording single fibre postsynaptic population potentials (p.s.p.p.s.) from the C2 ventral root perfused with sucrose. The probability of evoking monosynaptic single fibre p.s.p.p.s. was less (19%) than for C-RSNs (59%).     

Intrinsic cholinergic excitation in the rat neostriatum: Nicotinic and muscarinic receptors

Summary An in vitro slice technique was employed to study the receptors involved in intrinsic cholinergic excitation in the rat neostriatum. The locally evoked synaptic potentials were suppressed by antinicotinic agents, mecamylamine (10 M), d-tubocurarine (3 M) or hexamethonium (100 M), but not by the antimuscarinic agent atropine (100 M). If the slices were exposed to an acetylcholinesterase (AChE)-inhibitor (paraoxon 1–20 M, physostigmine 0.1–0.5 M), the synaptic potentials were potentiated. The amplitude of the orthodromic population spike increased, and it was further facilitated when the stimulus frequencies were raised from 1–3 Hz to 10–30 Hz. The frequency facilitation following exposure to an AChE-inhibitor was blocked by atropine (1–100 M). Intracellular recording indicated that a slow depolarizing potential caused the frequency potentiation of the orthodromic discharges. Apparently rat neostriatum is similar to cholinergic systems in sympathetic ganglia and spinal Renshaw cells, in that nicotinic receptors mediate fast excitation and muscarinic receptors mediate slow excitation.     

Intracellular study of electrical activity of Auerbach's plexus in guinea-pig small intestine

Intracellular recording revealed two general categories of ganglion cells in Auerbach's plexus. The characteristics of one category were relatively low resting potentials, high input resistance, discharge of spikes throughout a depolarizing current pulse, stimulus-evoked synaptic potentials and spontaneous electrical activity. Characteristics of the second category were high resting potentials, low input resistance, spikes only at the onset of a depolarizing current pulse and long duration hyperpolarizing after-potentials. Responses to extracellular electrical stimulation of the ganglia and interganglionic fiber tracts consisted of electrotonic spread of spikes from the processes to the cell soma, somal action potentials and depolarizing and hyperpolarizing responses that were probably EPSPs and IPSPs. Some of the neurons which received excitatory synaptic input responded with a prolonged train of spikes that outlasted by many seconds the duration of the stimulus to the fiber tract. Spontaneous electrical activity consisted of single EPSPs, patterned bursts of spikes that originated in the cell processes and spread electrotonically to, the recording site, IPSPs and action potentials. The burst-type activity showed periodic conversions from a burst pattern to a trainlike pattern of continuous discharge. Spontaneous discharge of single action potentials was superimposed upon a background of continuous synaptic input to the cell. Spontaneously occurring hyperpolarizing potentials were converted to depolarizing potentials when the membrane was hyperpolarized by current injected through the recording electrode. This work was supported by BMVg In San and National Institutes of Health AM 16813     

Long-term effects of synaptic activation at low frequency on excitability of myenteric AH neurons

Intracellular microelectrodes were used to record the effects of extended periods (1-30 min) of synaptic activation on AH neurons in the myenteric ganglia of the guinea-pig ileum. Low-frequency (1 Hz) stimulation gave rise to a slowly developing, sustained increase in excitability of the neurons associated with depolarization and increased input resistance. The increased excitability lasted for up to 3.5 h following the stimulus period. Successive stimulus trains (1-4 min) elicited successively greater increases in excitability. The neurons went through stages of excitation. Before stimulation, 500-ms depolarizing pulses evoked up to three action potentials (phasic response) and anode break action potentials were not observed. As excitability increased, more action potentials were evoked by depolarization (the responses became tonic), anode break action potentials were observed, prolonged after hyperpolarizing potentials that follow multiple action potentials were diminished and, with substantial depolarization of the neurons, invasion by antidromic action potentials was suppressed. It is concluded that a state of elevated excitability is induced in myenteric AH neurons by synaptic activation at low frequency and that changes in excitability can outlast stimulation by several hours.     

Granulosa cells have calcium-dependent action potentials and a calcium-dependent chloride conductance

We have found chicken granulosa cells to be excitable. Experiments using the whole-cell patch-clamp technique showed that they had membrane resting potentials of –62±3 mV (n=8) and generated action potentials, either in response to 10-ms depolarizing current pulses or, on occasion, spontaneously. The action potentials persisted in a Na⁺-free bath and were reversibly blocked by 4 mM Co²⁺. They lasted 0.9–3.0 s with 64 mM Cl^– in the pipette, were shortened 67±8 % by the Cl^– channel blocker 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB; 20 M), and lengthened to 8.7±2.2 when the Cl^– equilibrium potential (V _Cl) was changed from –20 mV to –2 mV by using 134 mM Cl^–in the pipette. With conventional whole-cell voltageclamp, slowly activating and inactivating currents, which reached maximum amplitude after 0.35–1.40 s, were evoked by depolarizing voltage steps. These slow currents activated between voltage steps of –60 mV and –50 mV and reached a maximum inward amplitude at about –40 mV. Changing the Cl^– concentration in the pipette (V _Cl of –2 mV or –20 mV) or bath (V _Cl of –2 mV or + 18 mV) shifted their reversal potential in a direction consistent with a Cl^– electrode. They were inhibited by the Cl^– channel antagonists 4,4-diisothiocyanatostilbene-2,2-disulfonic acid (DIDS; 0.5 mM), NPPB (20 M), and 4-acetamido-4-isothiocyanatostilbene-2,2-disulfonic acid (SITS; 0.5 mM). The slow currents were blocked by Ca²⁺ deprivation, or by Co²⁺ (4 mM), or by replacing external Ca²⁺ with Ba²⁺. They showed pronounced inward rectification when a weakly buffered 0.2 M Ca²⁺ pipette solution was used, but this rectification was much reduced when pipette solutions contained 5 M Ca²⁺. The function of this Ca²⁺-dependent Cl^– current and the physiological trigger(s) of the action potentials and their role(s) in granulosa cell function remain to be determined.     

Voltage clamp of a small muscle membrane area by means of a circular sucrose gap arrangement

Summary A new method is described which permits the measurement of membrane currents of thick muscle fibres (diameter 300 m or more) ofAstacus fluviatilis orBalanus balanus under voltage clamp conditions.The potential difference across a small patch of membrane (60–100 m in diameter) is controlled by connecting a voltage source across it with two external electrodes. One of them is connected to the fluid bathing the muscle fibre. The other, tubular one is in touch with the test area. The current flowing through the electrodes represents the sum of the membrane current flowing across the test area and the leak current flowing in the external fluid between the electrodes. In the first version of the method the leak current is limited by a circular sucrose gap around the test area. In the second, more elaborate method, the leak current is eliminated by a system of two concentric sucrose rings with a guard ring electrode between them. This method permits in addition the measurement of full sized action potentials in the test area.This work has been briefly reported in Cs. fysiol.17, 48 (1968).     

Potassium and calcium currents and action potentials in mouse Balb/c 3T3 fibroblasts

The electrical properties of Balb/c 3T3 mouse fibroblasts were studied with the whole-cell patch clamp technique. In current clamp mode a resting potential of —75.5±2.1 mV was recorded. In voltage clamp mode an inward current was also observed at potentials negative toV _m. This current crossed the 0-current axis at a voltage nearV _m, and rectified at more positive potentials; the degree of rectification was dependent on [K⁺]_o. At potentials positive to –30 mV a transient inward current was observed, showing a peak amplitude of –193±36 pA at+10 mV; the current amplitude was dependent on voltage and [Ca²⁺]_o, it was strongly increased by 20 mM BaCl₂ and abolished by 2 M verapamil and 1 M nifedipine. These cells, in response to depolarizing stimuli, develop slow action potentials, probably supported by the Ca²⁺ current.     

A method for electrical measurements on isolated protoplasmic droplets from Nitella

Summary A special recording chamber has been constructed which allows for the perfusion of the fragile protoplasmic droplets from Nitella cells while making electrical measurements on the preparation.Measurements were made of the potential of the interior of the droplet relative to the bathing medium, under resting conditions and during electrical stimulation. In a bathing medium whose cationic concentration resembled the natural vacuolar sap, a small resting potential of between 0 and –30 mV was observed. Stimulation with depolarizing current shocks of 100 s in duration and of up to 0.28 mA/cm² elicited active responses of up to 130 mV which outlasted the stimulus artifacts. The responses were graded but they resembled nerve action potentials in shape and duration.     

Afferent control of human stance and gait: evidence for blocking of group I afferents during gait

Summary The cerebral potentials (c.p.) evoked by electrical stimulation of the tibial nerve during stance and in the various phases of gait of normal subjects were compared with the c.p. and leg muscle e.m.g. responses evoked by perturbations of stance and gait. Over the whole step cycle of gait the c.p. evoked by an electrical stimulus were of smaller amplitude (3 V and 9 V, respectively) than that seen in the stance condition, and appeared with a longer latency (mean times to first positive peak: 63 and 43 ms, respectively). When the electrical stimulus was applied during stance after ischaemic blockade of group I afferents, the c.p. were similar to those evoked during gait. The c.p. evoked by perturbations were larger in amplitude than those produced by the electrical stimulus, but similar in latencies in both gait and stance (mean 26 V and 40 V; 65 ms and 42 ms, respectively) and configurations. The large gastrocnemius e.m.g. responses evoked by the stance and gait perturbations arose with a latency of 65 to 70 ms. Only in the stance condition was a smaller, shorter latency (40 ms) response seen. It is concluded that during gait the signals of group I afferents are blocked at both segmental and supraspinal levels which was tested by tibial nerve stimulation. It is suggested that the e.m.g. responses induced in the leg by gait perturbations are evoked by group II afferents and mediated via a spinal pathway. The c.p. evoked during gait most probably reflect the processing of this group II input by supraspinal motor centres for the coordination of widespread arm and trunk muscle activation, necessary to restablish body equilibrium.     

Effects of ryanodine on the spike after-hyperpolarization in sympathetic neurones of the rat superior cervical ganglion

Effects of ryanodine on sympathetic neurones of the rat superior cervical ganglion were investigated by means of intracellular recording. Ryanodine (1 M) significantly shortened the after-hyperpolarization (AH) following the spike evoked by current injection or pre-ganglionic stimulation without affecting the configuration of the spikes. The shortening of AH caused by ryanodine was dose-dependent at concentrations between 0.1 and 1 M and was slowly recovered by washing the tissue over 1 h. A partial inhibition of the apamin-sensitive slow component of AH was the maximal effect obtained at 1 M. Although the input membrane resistance was not changed, ryanodine evoked repetitive discharges at long intervals in response to long depolarizing current pulses applied across the cell membrane. Ryanodine (5 M) did not depress the Ca-spike but shortened the following AH in a lesser degree than that following the normal spike. Spontaneous small fluctuations of the resting membrane potential were occasionally observed under normal conditions. They were facilitated by caffeine and abolished by ryanodine. Caffeine also enhanced the slow component of the AH but did not affect it in the presence of ryanodine. These results suggest that ryanodine inhibits Ca release from intracellular store sites. The released Ca may contribute to generating the long-lasting AH and to regulating the excitability of rat sympathetic neurones.     

An inward calcium current underlying regenerative calcium potentials in rat striatal neurons in vitro enhanced by Bay K 8644

The single electrode voltage clamp technique was used to characterize the currents underlying the calcium potentials in rat caudate neurons in vitro. In current clamp experiments, long depolarizing current pulses evoked repetitive firing of fast somatic action potentials. These were abolished by tetrodotoxin (1 microM) and replaced by slow graded depolarizing potentials. These were preceded by a transient hyperpolarizing notch. Addition of 4-aminopyridine (100 microM) abolished the hyperpolarizing notch, enhanced the slow graded depolarizing response and induced the appearance of a slow all-or-nothing action potential. Both the slow graded response and the all-or-nothing action potential were abolished by cobalt (2 mM), suggesting the involvement of voltage-dependent calcium conductances. When the neurons were loaded intracellularly with caesium the action potential duration increased. Substitution of the extracellular calcium by barium (1-3 mM) or external addition of tetraethylammonium (5 mM) further prolonged spike duration and induced the appearance of long-lasting plateau potentials. These were insensitive to tetrodotoxin and were reversibly blocked by the calcium antagonists cobalt (2 mM), manganese (2 mM) or cadmium (500 microM). The calcium potentials were enhanced by the calcium 'agonist' BAY K 8644 (1-5 microM). In voltage clamp experiments when intracellular caesium was used to reduce outward currents and tetrodotoxin to block fast regenerative sodium currents, depolarizing voltage steps from a holding potential of -50, -40 mV activated an inward current. This current peaked in 50-80 ms and inactivated in two phases: an initial one at 150-200 ms followed by a second one after several hundred ms.(ABSTRACT TRUNCATED AT 250 WORDS)     

Repetitive firing properties of putative dopamine-containing neurons in vitro: regulation by an apamin-sensitive Ca2+-activated K+ conductance

Summary Intracellular recording techniques were used to study the effects of apamin (APA), a selective inhibitor of one type of Ca²⁺-activated K⁺ channel, on the electroresponsive properties of dopamine (DA)-containing neurons within the zona compacta of the substantia nigra (SNc) in rat. Bath application of APA (1 M) blocked the slow component of a complex post-spike afterhyperpolarization (AHP_s) without affecting other characteristics of the action potential. Blockade of AHP_s was accompanied by an increase in the number and frequency of action potentials evoked by depolarizing current pulses. However, APA failed to affect the cellular mechanisms underlying spike frequency adaptation or poststimulus inhibitory period. These data indicate that AHP_s can exert a strong influence on the interspike interval but is probably not involved in regulating slower adaptive neuronal responses.     

Analysis of potentials induced in red nucleus neurones from the somaesthetic pathway stimulated at the bulbar level

A somaesthetic pathway to the magnocellular red nucleus (RNm) via relays other than corticoor cerebello-rubral relays was previously found to exist in the cat. At the brainstem level, the ascending spinorubral fibres follow the medial lemniscus (LM). The present paper aims at describing in detail and evaluating the quantitative importance of the short-latency responses in RNm cells after microstimulation performed in the LM through a monopolar electrode. The RNm cells, tested intracellularly in cats anaesthetized with -choralose and placed in a stereotaxic device, were identified by their antidromic activation to stimulation of the rubrospinal tract in the cervical cord. It was established that single-shock stimulation below 100 A current delivered to the LM induced short-latency postsynaptic potentials (PSPs) in 87% of all the rubrospinal cells tested. The responses were indeed due to activation of LM fibres, as demonstrated by different tests: the location of the electrode tip in the LM was verified by recording, with the same electrode, the potentials evoked by stimulating the dorsal columns of the cord. The site was later confirmed histologically. The absence of stimulus spread from the LM to the underlying pyramidal tract was systematically checked by simultaneously recording the responses evoked in RNm cells and in the motor cortex. Monosynaptic excitatory responses (EPSPs) were evoked in RNm cells with a minimum stimulating current of less than 20 A in the LM and a mean threshold of 42 A. Disynaptic inhibitory potentials (IPSPs) were evoked in 23% of these cells with single-pulse stimulation. These latter responses showed a temporal facilitation with short trains of three pulses, which indicated that they were transmitted through inhibitory interneurones. Recordings were also performed from presumed LM fibre terminals running inside the RNm. The results therefore confirm the existence of strong lemniscal projections to RNm and demonstrate that they transmit both excitatory and inhibitory messages to rubrospinal cells. As the somaesthetic pathway to the RNm was previously found to come from the spinal cord, where it is located in the ventral portion, the present results also confirm that the LM is composed of fibres originating not only from neurones in the dorsal column nuclei, but also from cells placed at the segmental levels of the cord. The presumed sensorimotor function of this ascending pathway is discussed.     

Long-lasting potentiation and depression by novel isoproterenol and cholecystokinin 8-S interactions in the dentate gyrus

In the in vitro hippocampal slice, novel interactions of a -adrenergic agonist (l-isoproterenol) and neuropeptide (cholecystokinin 8-S) differentially produce long-lasting modifications in the dentate gyrus. When co-applied, a low concentration of l-isoproterenol (50–75 nM) and cholecystokinin 8-S (1.0 M) produce long-lasting depression of evoked action potentials (i.e., population spikes). In contrast, the same concentration of l-isoproterenol followed by a 30-min wash with artificial cerebrospinal fluid and application of cholecystokinin 8-S produces long-lasting potentiation of evoked action potentials. In neither condition are there corresponding modifications of excitatory post-synaptic potentials. These results indicate that l-isoproterenol and cholecystokinin 8-S temporally interact to differentially produce depression or potentiation of granule cell activation. In contrast to long-lasting modifications produced by continuous application of 1.0 M l-isoproterenol, in which both evoked action potentials and excitatory post-synaptic potentials are affected, the present novel paradigm may modify an extra-synaptic locus.     

Responses of cortical neurons to stimulation of corpus callosum in vitro

1. An in vitro slice preparation of rat cingulate cortex was used to analyze the responses of layer V neurons to electrical stimulation of the corpus callosum (CC). In addition, synaptic termination of callosal afferents with layer V neurons was evaluated electron microscopically to provide a structural basis for interpreting some of the observed response sequences. 2. Layer V neurons had a resting membrane potential (RMP) of 60 +/- 0.68 (SE) mV, an input resistance of 47 +/- 4.74 M omega, a membrane time constant of 4.37 +/- 0.51 ms, an electrotonic length constant of 1.38 +/- 0.25, and produced spontaneous action potentials that were 50 +/- 0.3 mV in amplitude. Intracellular depolarizing current pulses evoked spikes that were sometimes associated with low-amplitude (2-5 mV) depolarizing (5-10 ms in duration) and hyperpolarizing (10-20 ms in duration) afterpotentials. 3. A single stimulus of increasing intensities to the CC produced one of the following response sequences: a) antidromic spike and an excitatory postsynaptic potential (EPSP), which initiated one or more spikes; b) antidromic spike, EPSP-evoked action potentials, and a hyperpolarization, which may have represented an intrinsic cell property or inhibitory synaptic activity; c) EPSP and evoked spikes only; d) high-amplitude EPSP with an all-or-none burst of action potentials. 4. Antidromically activated (AA) neurons always produced EPSPs in response to CC stimulation. When compared with nonantidromically activated neurons, AA cells had a more negative RMP, greater electrotonic length constant (LN), higher ratio of dendritic to somatic conductance (rho), and formed shorter duration, callosal-evoked EPSPs. 5. Neurons in anterior cingulate cortex produced EPSPs of longer duration than did those in posterior cortex (50 +/- 3.57 versus 26 +/- 1.56 ms, respectively). EPSPs in anterior neurons also had a higher maximum amplitude (20.5 +/- 1.0 versus 11.5 +/- 0.79 mV) and longer time to peak (11.6 +/- 2.2 versus 8.2 +/- 0.8 ms). 6. Electron microscopy of Golgi-impregnated neurons following contralateral lesions demonstrated that both pyramidal and nonpyramidal neurons received direct callosal afferents. Synaptic termination of callosal axons with the apical dendritic trees of anterior pyramidal cells was 6 times greater than it was with posterior pyramidal neurons. 7. EPSP shape differences in anterior and posterior neurons may be partially accounted for by the density and distribution of callosal afferents to these two cortices.