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Repetitive burst-firing neurons in the deep layers of mouse somatosensory cortex
Neuroscience Letters, 99 (1989) 137 141
137
Elsevier Scientific Publishers Ireland Ltd. NSL 06001
Repetitive burst-firing neurons in the deep layers of mouse somatosensory cortex A. A g m o n 1 a n d B.W. C o n n o r s 2 ZDepartment of Neurology, StanJbrd University School of Medicine, Stanford, CA 94305 (U.S.A.) and :Section of Neurobiology, Division of Biology and Medicine, Brown University, Providence, R1 02912 (u.s.A.) (Received 20 July 1988; Revised version received 14 December 1988; Accepted 14 December 1988)
Key words: Mouse; Somatosensory cortex; Bursting neuron; Action potential; Pyramidal neuron Intracellular recordings were made from neurons of the mouse somatosensory cortex isolated in vitro. Two physiologically distinct classes of pyramidal cells were observed: regular-spiking cells were the majority, and generated accommodating trains of single spikes; bursting cells generated clusters of 2 5 action potentials, and clusters were separated by prolonged afterhyperpolarizations. The bursting cells of the mouse neocortex were unusual in producing repetitive bursting during sustained current pulses, and in being localized to a laminar zone straddling layers V and VI.
The intrinsic firing properties of different neurons in the neocortex are not uniform. For example, pyramidal cells can be distinguished from aspiny and sparsely spiny cells by their action potential shapes and repetitive firing patterns [14, 19]. There are also distinctive physiological subclasses of pyramidal cells. Stow and fast pyramidal tract neurons in the cat differ in both passive membrane properties and action potentials [6, 16, 23]. Another striking example is the dichotomy between 'regular-spiking' neurons and intrinsically 'bursting' neurons of the guinea pig [9, 19]. In response to a step current stimulus just above threshold, regular-spiking cells generate single action potentials. In contrast, at threshold bursting cells generate a cluster of 2-5 spikes riding upon a slow depolarizing envelope. It is important to stress that the intrinsic behavior of a neuron derives from its complement of voltagedependent membrane channels, and is independent of synaptic input. Intrinsically bursting neurons in guinea pig neocortex have a specific laminar distribution. Whereas regular-spiking cells are found in all layers from II to VI, bursting cells have so far been observed only in layers IV and the upper aspect of layer V [9, 19]. Very little is known about the incidence of bursting cells among various other species and areas of neocortex. In this study we present observations on intrinsically Correspondence: B.W. Connors, Section of Neurobiology, Box G, Division of Biology and Medicine, Brown University, Providence, RI 02912, U.S.A. 0304-3940/89/$ 03.50 ~(-';1989 Elsevier Scientific Publishers Ireland Ltd.
138 bursting neurons in the primary somatosensory cortex o f the mouse. Unlike previously described bursting cells of guinea pig cortex, those of the mouse had a deeper laminar position, and m a n y could generate repetitive bursts during a prolonged stimulus. The general methods for preparing, maintaining and recording from cortical slices have been described [8, 19], however the present study employed several significant modifications [1 3]. Mice (C57 BL/6) 2 5 weeks old were anesthetized with halothane, decapitated and the brains removed into ice-cold physiological solution. Slices, 400/tin thick, were cut on a vibratome, either in a coronal plane or in an oblique plane o f section designed to preserve thalamocortical connectivity [1]. Under transillumination layers IV (the barrels) and the region straddling layers V and VI were visualized as dark bands [2]: the slice and its laminar borders were traced on a camera lucida, and recordings were subsequently made in an interface-type c h a m b e r kept at 32' C. Intracellular recordings were made with microelectrodes filled with 4 M potassium acetate and bevelled to about 200 MQ. Cells were activated by step pulses o f current. D a t a were acquired using a computer-based data acquisition system. The laminar position of each cell was determined by measuring its fractional position between the pia and the border o f the gray and white matter, and comparing this measurement with the camera lucida drawing. An example o f an intracellular recording from a cell that fits the criteria for regular-spiking neurons [19] is illustrated in Fig. IA. This cell fired trains of individual action potentials whose frequency a c c o m m o d a t e d during prolonged current pulses. Spike frequency increased smoothly with current intensity (not shown).
G
II/111 j
....
--'C--'--;_
IV V o •
B
I
i,
II
•
•
e'''
Vl
I
wm
S
L
....] 20
40 msec
0.3 nA
Fig. 1. Different firing patterns of mouse neocortical neurons during injected current pulses. A: recording from a representative regular-spiking neuron of layer V. B: recording from a repetitively bursting neuron located near the border of layers V and VI. Calibration bars apply to both A and B. C: laminar location of 33 neurons from 6 experiments, superimposed on a map traced from a photograph of a living slice. Site of regular-spiking cells identified by dots, bursting cells by asterisks. The horizontal position of each symbol is arbitrary. Boundaries of layer IV and the layer V/VI border marked with dashed lines.
139 In contrast, many neurons exhibited repetitive burst-firing behavior. Fig. 1B shows a recording from one such cell. During a positive current pulse 330 ms long, the cell generated 3 bursts of 3-5 action potentials each. The mean burst frequency during the pulse was about 9 Hz. Action potential bursts always rode upon a slow depolarizing envelope of about 20 mV, and each burst was followed by a trough of relative hyperpolarization. The mean resting potentials of regular-spiking ( - 7 2 mV) and bursting cells ( - 74 mV) were not significantly different. The laminar location of bursting neurons was not random. Fig. IC plots the recording site of all presumed pyramidal neurons encountered during 6 experiments. Dots represent regular-spiking cells, asterisks represent bursting cells. Of the 33 neurons, 4 were classified as bursting cells; each of the 4 was located in the vicinity of the border of layers V and VI. Out of a total sample of 92 presumed pyramidal cells recorded in slices from 33 animals, 13 of 37 (35%) in layers V/VI were bursting cells while none of the 45 cells in layers II/III and IV exhibited bursting properties as defined here. The pattern of bursting was stimulus-dependent and varied from cell-to-cell. Fig. 2A illustrates repetitive bursting at 2 different current intensities in another cell. Mean burst frequency was 9 Hz at the low, and 16 Hz at the high intensity. Burst size was 2 or 3 spikes, with smaller bursts predominating at higher currents. Some neurons could not sustain repetitive bursting beyond 2 or 3 bursts, and reverted to single-spike firing either following a bursting episode or during it (Fig. 2B). The individual spikes in these cases were followed by prominent depolarizing afterpotentials (Fig. 2B, arrows). Two regular-spiking neurons of layers V/VI and one repetitive bursting neuron
B
40 msec
Fig. 2. Firing patterns of 2 different bursting neurons from layers V/VI. A: repetitivebursts generated by step current stimuli of 0.44 nA (bottom) and 0.66 nA (top). B: bursts and single-spikefiringin response to 0.50 nA (bottom) and 0.80 nA (top). Arrowheads identifyexamples of depolarizing afterpotentialsfollowing single spikes. Calibrations apply to all traces.
140
were successfully filled with the fluorescent dye Lucifer yellow CH. All 3 had the classical morphological characteristics of neocortical pyramidal cells [10]. The repetitive bursting behavior of neurons in mouse neocortex is reminiscent of the endogenously generated bursting activity of neurons in many invertebrate ganglia (e.g. ref. 4) and some nuclei of the mammalian central nervous system (e.g. refs. 5, 15, 18, 24). The ionic mechanisms of repetitive bursting vary greatly among different cell types, although many of the mammalian systems seem to depend in part upon a low threshold calcium current to sustain the burst. Such a current apparently exists in neocortical pyramidal cells [11], however the mechanism(s) of intrinsic bursting in neocortex remain to be explored. This study describes two functional classes of neurons in the mouse primary somatosensory cortex: regular-spiking and bursting cells. A similar distinction was previously made for pyramidal cells in the neocortex of guinea pigs [9, 19]. However, in the present work, bursting cells were different in 2 respects: a) they often fired repetitive bursts during a sustained current stimulus, and b) they were found in the zone on both sides of the border of layers V and VI. In contrast, the bursting cells so far described in the guinea pig generate only one burst per step-current stimulus, followed by single-spike firing, and their soma positions are in layer IV and upper layer V. Whether these are true species differences or reflect different methodologies remains to be explored. While neocortical neurons from a variety of intracellular studies in vivo and in vitro meet the criteria for the regular-spiking designation [6, 9, 16, 20, 22, 23], there have been very few unambiguous reports of neocortical neurons with the ability to generate intrinsic bursts. Following the initial descriptions in parietal and cingulate neocortex of guinea pigs in vitro [9, 12, 19], bursting cells were observed in rat motor cortex in vivo [17] and epileptic temporal neocortex of humans maintained in vitro (D.A. McCormick, personal communication). These limited data, plus the present results, suggest that intrinsically bursting neurons may be a common feature of different areas of neocortex in a variety of species from mouse to man. The functions of intrinsically bursting cells in cerebral cortex are unknown. Several lines of evidence indicate that they receive markedly less inhibitory input than regular-spiking cells, whether l¥om intracortical stimulation [7, 21] or from thalamusevoked inputs [I]. Agmon [1], using a novel slice preparation, has also shown that intrinsically bursting cells receive much less monosynaptic input from the ventrobasal nucleus of the thalamus than do regular-spiking cells. Bursting neurons have also been implicated in the initiation of synchronized epileptiform discharges [7, 8, 12, 13], and their behavior suggests that they are highly interconnected with each other. Finally, their restricted laminar location in the infragranular layers suggests that all bursting cells may project to the same subcortical target(s). Thus, the intrinsic membrane properties of pyramidal neurons may correlate with both their excitatory and inhibitory synaptic connections. Further elucidation of bursting cell morphology and connections, both intrinsic and extrinsic, should help to illuminate their functions.
141 We thank M.J. Gutnick for helpful comments
on the manuscript. These studies
were supported by MH17047, NS 01271, NS 25983 and the Klingenstein Fund. 1 Agrnon, A., Intrinsic Properties and Synaptic Connectivity of Mouse Barrel Cortex Neurons: Correlation between Firing Patterns and Thalamocortical Inputs, Ph.D. dissertation, Stanford University, 1988, 173 pp. 2 Agmon, A. and Connors, B.W., Visualization of the barrel field in living rat neocortical slices, Soc. Neurosci. Abstr., 10 (1984) 493. 3 Agmon, A. and Connors, B.W., The thalamocortical and transcallosal barrel field slice preparations: preserving extrinsic connections in vitro, Soc. Neurosci. Abstr., 11 (1985) 750. 4 Alving, B.O., Spontaneous activity in isolated somata of Aplysia pacemaker neurons, J. Gen. Physiol., 51 (1968) 29-45. 5 Andrew, R.D. and Dudek, F.E., Burst discharge in mammalian neuroendocrine cells involves an intrinsic regenerative mechanism, Science, 221 (1983) 1050-1052. 6 Calvin, W.H. and Sypert, G.W., Fast and slow pyramidal tract neurons: an intracellular analysis of their contrasting repetitive firing properties in the cat, J. Neurophysiol., 39 (1976) 420~34. 7 Chagnac-Amitai, Y. and Connors, B.W., Intrinsic excitability and synaptic connectivity of neocortical pyramidal neurons are correlated: implications for epileptogenesis, Epilepsia, 29 (1988) 7 I0. 8 Connors, B.W., Initiation of synchronized neuronal bursting in neocortex, Nature (Lond.), 310 (1984) 685 687. 9 Connors, B.W., Gutnick, M.J. and Prince, D.A., Electrophysiological properties of neocortical neurons in vitro, J. Neurophysiol., 48 (1982) 1302 1320. 10 Feldman, M.L., Morphology of the neocortical pyramidal neuron. In A. Peters and E.G. Jones (Eds.), Cerebral Cortex, Vol. 1, Cellular Components of the Cerebral Cortex, Plenum, New York, 1984, pp. 123-200. l 1 Friedman, A. and Gutnick, M.J., Low-threshold calcium electrogenesis in neocortical neurons, Neurosci. Lett., 81 (1987) 117 122. 12 Gutnick, M.J. and Friedman, A., Synaptic and intrinsic mechanisms of synchronization and epileptogenesis in the neocortex. In U. Heinemann, M. Klee, E. Neher and W. Singer (Eds.), Calcium Electrogenesis and Neuronal Functioning, Exp. Brain Res. Ser., Vol. 14, 1986, pp. 327-335. 13 Gutnick, M.J., Connors, B.W. and Prince, D.A., Mechanisms of neocortical epileptogenesis in vitro, J. Neurophysiol., 48 (1982) 1321-1335. 14 Huettner, J.E. and Baughman, R.W., The pharmacology of synapses formed by identified corticocollicular neurons in primary cultures of rat visual cortex, J. Neurosci., 8 (1988) 160-175. 15 Jahnsen, H. and Llinas, R., Electrophysiological properties of guinea-pig thalamic neurons: an in vitro study. J. Physiol. (Lond.), 349 (1984) 205-226. 16 Koike, H., Okada, Y., Oshima, T. and Takahashi, K., Accommodative behavior of cat pyramidal tract cells investigated with intracellular injection of currents, Exp. Brain Res., 5 (1968) 173-188. 17 Landry, P., Wilson, C.J. and Kitai, S.T., Morphological and electrophysiological characteristics of pyramidal tract neurons in the rat, Exp. Brain Res., 57 (1984) 177-190. 18 Llinas, R. and Yarom, Y., Electrophysiology of mammalian inferior olivary neurones in vitro. Different types of voltage-dependent ionic conductances. J. Physiol. (Lond.), 315 (1981) 549-567. 19 McCormick, D.A., Connors, B.W., Lighthall, J.W. and Prince, D.A., Comparative physiology of pyramidal and sparsely spiny neurons of the neocortex, J. Neurophysiol., 54 (1985) 782-806. 20 Ogawa, T., Ito, S. and Kato, H., Membrane characteristics of visual cortical neurons in in vitro slices, Brain Res., 226 (1981) 315-319, 21 Silva, L.R., Chagnac-Amitai, Y. and Connors, B.W., Inhibitory synaptic input correlates with intrinsic membrane properties in neocortical pyramidal cells, Soc. Neurosci. Abstr., 14 (1988) 883. 22 Stafstrom, C.E., Schwindt, P.C. and Crill, W.E., Repetitive firing in layer V neurons from the cat neocortex in vitro, J. Neurophysiol., 52 (1984) 264-289. 23 Takahashi, K., Slow and fast groups of pyramidal tract cells and their respective membrane properties, J. Neurophysiol., 28 (1965) 908-924. 24 Wilcox, K.S., Gutnick, M.J. and Christoph, G.R., Electrophysiological properties of neurons in the lateral habenula nucleus: an in vitro study, J. Neurophysiol., 59 (1988) 212 225.
137
Elsevier Scientific Publishers Ireland Ltd. NSL 06001
Repetitive burst-firing neurons in the deep layers of mouse somatosensory cortex A. A g m o n 1 a n d B.W. C o n n o r s 2 ZDepartment of Neurology, StanJbrd University School of Medicine, Stanford, CA 94305 (U.S.A.) and :Section of Neurobiology, Division of Biology and Medicine, Brown University, Providence, R1 02912 (u.s.A.) (Received 20 July 1988; Revised version received 14 December 1988; Accepted 14 December 1988)
Key words: Mouse; Somatosensory cortex; Bursting neuron; Action potential; Pyramidal neuron Intracellular recordings were made from neurons of the mouse somatosensory cortex isolated in vitro. Two physiologically distinct classes of pyramidal cells were observed: regular-spiking cells were the majority, and generated accommodating trains of single spikes; bursting cells generated clusters of 2 5 action potentials, and clusters were separated by prolonged afterhyperpolarizations. The bursting cells of the mouse neocortex were unusual in producing repetitive bursting during sustained current pulses, and in being localized to a laminar zone straddling layers V and VI.
The intrinsic firing properties of different neurons in the neocortex are not uniform. For example, pyramidal cells can be distinguished from aspiny and sparsely spiny cells by their action potential shapes and repetitive firing patterns [14, 19]. There are also distinctive physiological subclasses of pyramidal cells. Stow and fast pyramidal tract neurons in the cat differ in both passive membrane properties and action potentials [6, 16, 23]. Another striking example is the dichotomy between 'regular-spiking' neurons and intrinsically 'bursting' neurons of the guinea pig [9, 19]. In response to a step current stimulus just above threshold, regular-spiking cells generate single action potentials. In contrast, at threshold bursting cells generate a cluster of 2-5 spikes riding upon a slow depolarizing envelope. It is important to stress that the intrinsic behavior of a neuron derives from its complement of voltagedependent membrane channels, and is independent of synaptic input. Intrinsically bursting neurons in guinea pig neocortex have a specific laminar distribution. Whereas regular-spiking cells are found in all layers from II to VI, bursting cells have so far been observed only in layers IV and the upper aspect of layer V [9, 19]. Very little is known about the incidence of bursting cells among various other species and areas of neocortex. In this study we present observations on intrinsically Correspondence: B.W. Connors, Section of Neurobiology, Box G, Division of Biology and Medicine, Brown University, Providence, RI 02912, U.S.A. 0304-3940/89/$ 03.50 ~(-';1989 Elsevier Scientific Publishers Ireland Ltd.
138 bursting neurons in the primary somatosensory cortex o f the mouse. Unlike previously described bursting cells of guinea pig cortex, those of the mouse had a deeper laminar position, and m a n y could generate repetitive bursts during a prolonged stimulus. The general methods for preparing, maintaining and recording from cortical slices have been described [8, 19], however the present study employed several significant modifications [1 3]. Mice (C57 BL/6) 2 5 weeks old were anesthetized with halothane, decapitated and the brains removed into ice-cold physiological solution. Slices, 400/tin thick, were cut on a vibratome, either in a coronal plane or in an oblique plane o f section designed to preserve thalamocortical connectivity [1]. Under transillumination layers IV (the barrels) and the region straddling layers V and VI were visualized as dark bands [2]: the slice and its laminar borders were traced on a camera lucida, and recordings were subsequently made in an interface-type c h a m b e r kept at 32' C. Intracellular recordings were made with microelectrodes filled with 4 M potassium acetate and bevelled to about 200 MQ. Cells were activated by step pulses o f current. D a t a were acquired using a computer-based data acquisition system. The laminar position of each cell was determined by measuring its fractional position between the pia and the border o f the gray and white matter, and comparing this measurement with the camera lucida drawing. An example o f an intracellular recording from a cell that fits the criteria for regular-spiking neurons [19] is illustrated in Fig. IA. This cell fired trains of individual action potentials whose frequency a c c o m m o d a t e d during prolonged current pulses. Spike frequency increased smoothly with current intensity (not shown).
G
II/111 j
....
--'C--'--;_
IV V o •
B
I
i,
II
•
•
e'''
Vl
I
wm
S
L
....] 20
40 msec
0.3 nA
Fig. 1. Different firing patterns of mouse neocortical neurons during injected current pulses. A: recording from a representative regular-spiking neuron of layer V. B: recording from a repetitively bursting neuron located near the border of layers V and VI. Calibration bars apply to both A and B. C: laminar location of 33 neurons from 6 experiments, superimposed on a map traced from a photograph of a living slice. Site of regular-spiking cells identified by dots, bursting cells by asterisks. The horizontal position of each symbol is arbitrary. Boundaries of layer IV and the layer V/VI border marked with dashed lines.
139 In contrast, many neurons exhibited repetitive burst-firing behavior. Fig. 1B shows a recording from one such cell. During a positive current pulse 330 ms long, the cell generated 3 bursts of 3-5 action potentials each. The mean burst frequency during the pulse was about 9 Hz. Action potential bursts always rode upon a slow depolarizing envelope of about 20 mV, and each burst was followed by a trough of relative hyperpolarization. The mean resting potentials of regular-spiking ( - 7 2 mV) and bursting cells ( - 74 mV) were not significantly different. The laminar location of bursting neurons was not random. Fig. IC plots the recording site of all presumed pyramidal neurons encountered during 6 experiments. Dots represent regular-spiking cells, asterisks represent bursting cells. Of the 33 neurons, 4 were classified as bursting cells; each of the 4 was located in the vicinity of the border of layers V and VI. Out of a total sample of 92 presumed pyramidal cells recorded in slices from 33 animals, 13 of 37 (35%) in layers V/VI were bursting cells while none of the 45 cells in layers II/III and IV exhibited bursting properties as defined here. The pattern of bursting was stimulus-dependent and varied from cell-to-cell. Fig. 2A illustrates repetitive bursting at 2 different current intensities in another cell. Mean burst frequency was 9 Hz at the low, and 16 Hz at the high intensity. Burst size was 2 or 3 spikes, with smaller bursts predominating at higher currents. Some neurons could not sustain repetitive bursting beyond 2 or 3 bursts, and reverted to single-spike firing either following a bursting episode or during it (Fig. 2B). The individual spikes in these cases were followed by prominent depolarizing afterpotentials (Fig. 2B, arrows). Two regular-spiking neurons of layers V/VI and one repetitive bursting neuron
B
40 msec
Fig. 2. Firing patterns of 2 different bursting neurons from layers V/VI. A: repetitivebursts generated by step current stimuli of 0.44 nA (bottom) and 0.66 nA (top). B: bursts and single-spikefiringin response to 0.50 nA (bottom) and 0.80 nA (top). Arrowheads identifyexamples of depolarizing afterpotentialsfollowing single spikes. Calibrations apply to all traces.
140
were successfully filled with the fluorescent dye Lucifer yellow CH. All 3 had the classical morphological characteristics of neocortical pyramidal cells [10]. The repetitive bursting behavior of neurons in mouse neocortex is reminiscent of the endogenously generated bursting activity of neurons in many invertebrate ganglia (e.g. ref. 4) and some nuclei of the mammalian central nervous system (e.g. refs. 5, 15, 18, 24). The ionic mechanisms of repetitive bursting vary greatly among different cell types, although many of the mammalian systems seem to depend in part upon a low threshold calcium current to sustain the burst. Such a current apparently exists in neocortical pyramidal cells [11], however the mechanism(s) of intrinsic bursting in neocortex remain to be explored. This study describes two functional classes of neurons in the mouse primary somatosensory cortex: regular-spiking and bursting cells. A similar distinction was previously made for pyramidal cells in the neocortex of guinea pigs [9, 19]. However, in the present work, bursting cells were different in 2 respects: a) they often fired repetitive bursts during a sustained current stimulus, and b) they were found in the zone on both sides of the border of layers V and VI. In contrast, the bursting cells so far described in the guinea pig generate only one burst per step-current stimulus, followed by single-spike firing, and their soma positions are in layer IV and upper layer V. Whether these are true species differences or reflect different methodologies remains to be explored. While neocortical neurons from a variety of intracellular studies in vivo and in vitro meet the criteria for the regular-spiking designation [6, 9, 16, 20, 22, 23], there have been very few unambiguous reports of neocortical neurons with the ability to generate intrinsic bursts. Following the initial descriptions in parietal and cingulate neocortex of guinea pigs in vitro [9, 12, 19], bursting cells were observed in rat motor cortex in vivo [17] and epileptic temporal neocortex of humans maintained in vitro (D.A. McCormick, personal communication). These limited data, plus the present results, suggest that intrinsically bursting neurons may be a common feature of different areas of neocortex in a variety of species from mouse to man. The functions of intrinsically bursting cells in cerebral cortex are unknown. Several lines of evidence indicate that they receive markedly less inhibitory input than regular-spiking cells, whether l¥om intracortical stimulation [7, 21] or from thalamusevoked inputs [I]. Agmon [1], using a novel slice preparation, has also shown that intrinsically bursting cells receive much less monosynaptic input from the ventrobasal nucleus of the thalamus than do regular-spiking cells. Bursting neurons have also been implicated in the initiation of synchronized epileptiform discharges [7, 8, 12, 13], and their behavior suggests that they are highly interconnected with each other. Finally, their restricted laminar location in the infragranular layers suggests that all bursting cells may project to the same subcortical target(s). Thus, the intrinsic membrane properties of pyramidal neurons may correlate with both their excitatory and inhibitory synaptic connections. Further elucidation of bursting cell morphology and connections, both intrinsic and extrinsic, should help to illuminate their functions.
141 We thank M.J. Gutnick for helpful comments
on the manuscript. These studies
were supported by MH17047, NS 01271, NS 25983 and the Klingenstein Fund. 1 Agrnon, A., Intrinsic Properties and Synaptic Connectivity of Mouse Barrel Cortex Neurons: Correlation between Firing Patterns and Thalamocortical Inputs, Ph.D. dissertation, Stanford University, 1988, 173 pp. 2 Agmon, A. and Connors, B.W., Visualization of the barrel field in living rat neocortical slices, Soc. Neurosci. Abstr., 10 (1984) 493. 3 Agmon, A. and Connors, B.W., The thalamocortical and transcallosal barrel field slice preparations: preserving extrinsic connections in vitro, Soc. Neurosci. Abstr., 11 (1985) 750. 4 Alving, B.O., Spontaneous activity in isolated somata of Aplysia pacemaker neurons, J. Gen. Physiol., 51 (1968) 29-45. 5 Andrew, R.D. and Dudek, F.E., Burst discharge in mammalian neuroendocrine cells involves an intrinsic regenerative mechanism, Science, 221 (1983) 1050-1052. 6 Calvin, W.H. and Sypert, G.W., Fast and slow pyramidal tract neurons: an intracellular analysis of their contrasting repetitive firing properties in the cat, J. Neurophysiol., 39 (1976) 420~34. 7 Chagnac-Amitai, Y. and Connors, B.W., Intrinsic excitability and synaptic connectivity of neocortical pyramidal neurons are correlated: implications for epileptogenesis, Epilepsia, 29 (1988) 7 I0. 8 Connors, B.W., Initiation of synchronized neuronal bursting in neocortex, Nature (Lond.), 310 (1984) 685 687. 9 Connors, B.W., Gutnick, M.J. and Prince, D.A., Electrophysiological properties of neocortical neurons in vitro, J. Neurophysiol., 48 (1982) 1302 1320. 10 Feldman, M.L., Morphology of the neocortical pyramidal neuron. In A. Peters and E.G. Jones (Eds.), Cerebral Cortex, Vol. 1, Cellular Components of the Cerebral Cortex, Plenum, New York, 1984, pp. 123-200. l 1 Friedman, A. and Gutnick, M.J., Low-threshold calcium electrogenesis in neocortical neurons, Neurosci. Lett., 81 (1987) 117 122. 12 Gutnick, M.J. and Friedman, A., Synaptic and intrinsic mechanisms of synchronization and epileptogenesis in the neocortex. In U. Heinemann, M. Klee, E. Neher and W. Singer (Eds.), Calcium Electrogenesis and Neuronal Functioning, Exp. Brain Res. Ser., Vol. 14, 1986, pp. 327-335. 13 Gutnick, M.J., Connors, B.W. and Prince, D.A., Mechanisms of neocortical epileptogenesis in vitro, J. Neurophysiol., 48 (1982) 1321-1335. 14 Huettner, J.E. and Baughman, R.W., The pharmacology of synapses formed by identified corticocollicular neurons in primary cultures of rat visual cortex, J. Neurosci., 8 (1988) 160-175. 15 Jahnsen, H. and Llinas, R., Electrophysiological properties of guinea-pig thalamic neurons: an in vitro study. J. Physiol. (Lond.), 349 (1984) 205-226. 16 Koike, H., Okada, Y., Oshima, T. and Takahashi, K., Accommodative behavior of cat pyramidal tract cells investigated with intracellular injection of currents, Exp. Brain Res., 5 (1968) 173-188. 17 Landry, P., Wilson, C.J. and Kitai, S.T., Morphological and electrophysiological characteristics of pyramidal tract neurons in the rat, Exp. Brain Res., 57 (1984) 177-190. 18 Llinas, R. and Yarom, Y., Electrophysiology of mammalian inferior olivary neurones in vitro. Different types of voltage-dependent ionic conductances. J. Physiol. (Lond.), 315 (1981) 549-567. 19 McCormick, D.A., Connors, B.W., Lighthall, J.W. and Prince, D.A., Comparative physiology of pyramidal and sparsely spiny neurons of the neocortex, J. Neurophysiol., 54 (1985) 782-806. 20 Ogawa, T., Ito, S. and Kato, H., Membrane characteristics of visual cortical neurons in in vitro slices, Brain Res., 226 (1981) 315-319, 21 Silva, L.R., Chagnac-Amitai, Y. and Connors, B.W., Inhibitory synaptic input correlates with intrinsic membrane properties in neocortical pyramidal cells, Soc. Neurosci. Abstr., 14 (1988) 883. 22 Stafstrom, C.E., Schwindt, P.C. and Crill, W.E., Repetitive firing in layer V neurons from the cat neocortex in vitro, J. Neurophysiol., 52 (1984) 264-289. 23 Takahashi, K., Slow and fast groups of pyramidal tract cells and their respective membrane properties, J. Neurophysiol., 28 (1965) 908-924. 24 Wilcox, K.S., Gutnick, M.J. and Christoph, G.R., Electrophysiological properties of neurons in the lateral habenula nucleus: an in vitro study, J. Neurophysiol., 59 (1988) 212 225.