Slices of sensorimotor and anterior cingulate cortex from guinea pigs were maintained in vitro and bathed in a normal physiological medium. Electrophysiological properties of neurons were assessed with intracellular recording techniques. Some neurons were identified morphologically by intracellular injection of the fluorescent dye Lucifer yellow CH. Three distinct neuronal classes of electrophysiological behavior were observed; these were termed regular spiking, bursting, and fast spiking. The physiological properties of neurons from sensorimotor and anterior cingulate areas did not differ significantly. Regular-spiking cells were characterized by action potentials with a mean duration of 0.80 ms at one-half amplitude, a ratio of maximum rate of spike rise to maximum rate of fall of 4.12, and a prominent afterhyperpolarization following a train of spikes. The primary slope of initial spike frequency versus injected current intensity was 241 Hz/nA. During prolonged suprathreshold current pulses the frequency of firing adapted strongly. When local synaptic pathways were activated, all cells were transiently excited and then strongly inhibited. Bursting cells were distinguished by their ability to generate endogenous, all-or-none bursts of three to five action potentials. Their properties were otherwise very similar to regular-spiking cells. The ability to generate a burst was eliminated when the membrane was depolarized to near the firing threshold with tonic current. By contrast, hyperpolarization of regular-spiking (i.e., nonbursting) cells did not uncover latent bursting tendencies. The action potentials of fast-spiking cells were much briefer (mean of 0.32 ms) than those of the other cell types.(ABSTRACT TRUNCATED AT 250 WORDS)

1. Intracellular recordings were obtained from neurons of the guinea pig sensorimotor cortical slice maintained in vitro. Under control recording conditions input resistances, time constants, and spiking characteristics of slice neurons were well within the ranges reported by other investigators for neocortical neurons in situ. However, resting potentials (mean of -75 mV) and spike amplitudes (mean of 93.5 mV) were 10-25 mV greater than has been observed in intact preparations. 2. Current-voltage relationships obtained under current clamp revealed a spectrum of membrane-rectifying properties at potentials that were subthreshold for spike generation. Ionic and pharmacologic analyses suggest that subthreshold membrane behavior is dominated by voltage-sensitive, very slowly inactivating conductances to K+ and Na+. 3. Action potentials were predominantly Na+ dependent under normal conditions but when outward K+ currents were reduced pharmacologically, it was possible, in most cells, to evoke a non-Na+-dependent, tetrodotoxin-(TTX) insensitive spike, which was followed by a prominent depolarizing after-potential. Both of these events were blocked by the Ca2+ current antagonists, Co2+ and Mn2+. 4. A small population of neurons generated intrinsic, all-or-none burst potentials when depolarized with current pulses or by synaptic activation. These cells were located at a narrow range of depths comprising layer IV and the more superficial parts of layer V. 5. Spontaneous excitatory synaptic potentials appeared in all neurons. Spontaneous inhibitory events were visible in only about 10% of the cells, and in those cases apparently reversed polarity at a level slightly positive to resting potential. Stimulation of the surface of the slice at low intensities evoked robust and usually concurrent excitatory and inhibitory synaptic potentials. Unitary inhibitory postsynaptic potentials (IPSPs) reversed at levels positive to rest. Stronger stimulation produced a labile, long-duration, hyperpolarizing IPSP with a reversal potential 15-20 mV negative to the resting level. 6. Neocortical neurons in vitro retain the basic membrane and synaptic properties ascribed to them in situ. However, the array of passive and active membrane behavior observed in the slice suggests that cortical neurons may be differentiated by specific functional properties as well as by their extensive morphological diversity.

The inhibitory GABAergic projection of thalamic nucleus reticularis (nRt) neurons onto thalamocortical relay cells (TCs) is important in generating the normal thalamocortical rhythmicity of slow wave sleep, and may be a key element in the production of abnormal rhythms associated with absence epilepsy. Both TCs and nRt cells can generate prominent Ca(2+)-dependent low-threshold spikes, which evoke bursts of Na(+)-dependent fast spikes, and are influential in rhythm generation. Substantial differences in the pattern of burst firing in TCs versus nRt neurons led us to hypothesize that there are distinct forms of transient Ca2+ current (I(T)) underlying burst discharges in these two cell types. Using whole-cell voltage-clamp recordings, we analyzed I(T) in acutely isolated TCs and nRt neurons and found three key differences in biophysical properties. (1) The transient Ca2+ current in nRt neurons inactivated much more slowly than I(T) in TCs. This slow current is thus termed I(Ts). (2) The rate of inactivation for I(Ts) was nearly voltage independent. (3) Whole-cell I(Ts) amplitude was increased when Ba2+ was substituted for Ca2+ as the charge carrier. In addition, activation kinetics were slower for I(Ts) and the activation range was depolarized compared to that for I(T). Other properties of I(Ts) and I(T) were similar, including steady-state inactivation and sensitivities to blockade by divalent cations, amiloride, and antiepileptic drugs. Our findings demonstrate that subtypes of transient Ca2+ current are present in two different classes of thalamic neurons. The properties of I(Ts) lead to generation of long-duration calcium-dependent spike bursts in nRt cells. The resultant prolonged periods of GABA release onto TCs would play a critical role in maintaining rhythmicity by inducing TC hyperpolarization and promoting generation of low-threshold calcium spikes within relay nuclei.

Thalamocortical oscillations mediate both physiological and pathophysiological behaviors including sleep and generalized absence epilepsy (GA). Reciprocal intrathalamic circuitry and robust burst firing, dependent on underlying transient Ca current (IT) in thalamic neurons, support generation of such rhythms. In order to study the regulation of intrathalamic rhythm generation and the effects of GA anticonvulsants previously shown to reduce IT in acutely isolated thalamic neurons, we developed an in vitro rat thalamic slice preparation that retains sufficient intrathalamic circuitry to support evoked oscillations (range = 2.0-4.6 Hz, average = 2.7, n = 38), associated with burst firing in the thalamic reticular nucleus (nRt) and thalamic relay neurons. Extracellular stimulation of nRt evoked in relay neurons a biphasic inhibitory response with prominent GABAA and GABAB receptor-mediated components. The GABAA component was picrotoxin sensitive, outwardly rectifying and Cl- dependent, with a very negative reversal potential (-94 mV), indicating that an active extrusion mechanism exists in these cells to keep [Cl-]i < 5 mM. The GABAB component had a linear conductance, a reversal potential of -103 mV, and was quite long lasting (about 300 msec) so that rebound bursts often were generated on its decay phase, presumably leading to reexcitation of nRt through known excitatory connections. GABAB-mediated responses thus provide a timing mechanism for promoting slow intrathalamic oscillations. Reduction of IT (30-40%) by succinimides slightly increased the threshold for burst generation in relay and nRt cells, but there was little effect on either number of spikes/burst or intraburst frequency, and there were no other direct effects on other measures of cellular excitability. Intrathalamic oscillations were significantly reduced by these agents through a slight decrease in burst probability of thalamic neurons. We conclude that interactions between the intrinsic properties of thalamic neurons and intrathalamic circuitry lead to generation of slow oscillations. A similar mechanism may underlie the pathophysiological 3 Hz spike and wave EEG activity that characterizes GA. Furthermore, anti-GA drugs such as ethosuximide probably exert their action by reducing the burst-firing probability of neurons within populations of reciprocally interconnected relay and nRt neurons, thus producing a desynchronization of the thalamic circuit that prevents spike/wave generation.

1. The postnatal maturation of intracortical inhibitory circuitry and the development of responses to applied gamma-aminobutyric acid (GABA) and baclofen were studied in pyramidal and nonpyramidal neurons from layers II and III of the rat primary somatosensory and primary visual cortex, in vitro. 2. Depolarizing spontaneous inhibitory postsynaptic potentials (IPSPs) could be recorded in approximately 70% of the young (postnatal day 4-10; P4-10), juvenile (P11-16), and adult cells (P28-41), respectively, when they were loaded with nitrate. At all ages these spontaneous events could be blocked by application of the GABAA receptor antagonist bicuculline methiodide (BMI), indicating that they were mediated by activation of GABAA receptors. 3. In 122 of the 130 adult cells tested, standardized electrical stimulation of the white matter or layer VI evoked a brief excitatory postsynaptic potential (EPSP), followed by both a fast (f-) and a long-latency (l-)IPSP. Similar stimuli evoked a biphasic IPSP in only 51 of the 98 juvenile and in only 1 of the 56 young neurons studied. The mean peak conductance of the f-IPSP and the l-IPSP increased significantly from 50.2 and 7.5 nS, respectively, in juvenile cells to 84.2 and 18.0 nS, respectively, in adult neurons. 4. Application of the N-methyl-D-aspartate (NMDA) receptor antagonist D-amino-phosphonovaleric acid (D-APV) to juvenile cells induced a significant negative shift in the reversal potential of both the f-IPSP and l-IPSP. This effect was accompanied by a reduction in the peak conductance during these events by 31 and 48%, respectively, indicating that a prominent long-lasting NMDA receptor-mediated EPSP occurs concurrent with the early and late IPSP in immature neurons. In adult neurons, D-APV had no significant effect on the reversal potential of the f- and l-IPSP, although the peak conductance decreased by 20 and 5%, respectively, suggesting that there was a smaller concurrent activation of NMDA receptors in this age group. 5. The functional maturation of GABAA and GABAB receptors was studied using focal applications of GABA to the soma and the apical dendrite. Somatic GABA applications to adult neurons held at depolarized membrane potentials evoked a triphasic response, consisting of 1) a GABAA-mediated hyperpolarizing fast component (GABAhf; reversal potential, -76 mV), 2) a GABAA-mediated depolarizing phase (GABAd; -54 mV), and 3) a hyperpolarizing late response (GABAhl; -80 mV). The GABAd response could be demonstrated at all ages in almost every neuron.(ABSTRACT TRUNCATED AT 400 WORDS)

Dendritic activity in guinea pig hippocampal CA1 and CA3 pyramidal neurons was examined by using an in vitro preparation. Histologically confirmed intradendritic recordings showed that dendrites had an average input resistance of 47.0 Mg and average membrane time constant of 33.3 msec. Active spike responses could be evoked by intracellular injection of outward current or by the activation of synaptic inputs. The predominant activity was burst firing. A typical intracellularly recorded dendritic burst consisted of spikes on a slowly increasing depolarizing potential. The spike components of the burst were of two distinct types: low threshold, fast s ikes; and high threshold, slow spikes. Tetrodotoxin (1 gg/ml) b ocked the fast spikes, but slow spikes could still be evoked with direct intracellular stimulation. In contrast to dendritic responses, direct depolarization of CA, somata did not give rise to burst generation. Orthodromic stimuli evoked large-amplitude excitatory postsynaptic potentials, followed by inhibitory postsynaptic potentials in dendrites of CA1 and CA3 neurons. In two instances, simultaneous recordings were obtained from coupled pairs of elements that were presumed to be soma and dendrite of the same CA3 pyramida neuron. Depolarization of either element led to burst generation at that site, and the underlying slow depolarization appeared to evoke a burst at the other site. This potential postsynaptic amplifying mechanism was not ordinarily functional because even suprathreshold orthodromic activation did not normally evoke bursting in dendrites.Although dendrites in the mammalian central nervous system are of obvious anatomical and physiological importance as areas for signal reception and generation, until recently, little direct information has been available regarding the properties of dendritic membranes. Most studies of dendritic function have, of necessity, relied upon data obtained from intrasomatic or extracellular recordings (1-3). Such experiments have provided evidence suggesting that voltage-dependent active responses can be generated in some nerve cell dendrites. Direct evidence for regenerative responses has recently been obtained from intradendritic recordings in Purkinje cells (4-6). One brief report of recordings from procion-stained neocortical dendrites has also appeared (7).Recordings in alligator Purkinje cell dendrites show that two types of spike responses are generated-namely, short-duration, low-threshold, small-amplitude spikes; and high-threshold larger-amplitude spikes with a longer duration (4). It has been proposed that the long-duration action potentials are a result of electronic summation of small spikes (2, 4). Recent investigations into the ionic mechanisms of dendritic spike generation in pigeon and rat Purkinje cells reveal that at least part of the electroresponsiveness is resistant to tetrodotoxin (TTX) and probably is mediated by Ca2+ (5,6).In hippocampal neurons of the guinea pig, certain types of spike activities have been assumed to be of dendr...

The temperature dependence of intrinsic membrane conductances and synaptic potentials in guinea pig hippocampal CA1 pyramidal neurons were examined in vitro as they were cooled from 37°C to between 33 and 27%. Cooling reversibly increased resting input resistance in a voltage-independent manner (Go = 0.58 to 0.75). The amplitude and duration of orthodromically evoked action potentials were increased by cooling (Cl0 = 0.87 and 0.52 to 0.53, respectively), whereas the maximum rates of rise and fall were reduced (Q,,, =

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