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Membrane potential changes in rostral thalamic neurons during spontaneous and triggered spindles
SHORTCOMMUNICATIONS
479
Membrane potential changes in rostral lhalamic neurons during spontaneous and triggered' spindles The effect of low-frequency stimulation in medial and intralaminar thalamic regions on the membrane potential of cells in the ventrolateral (VL) and ventroanterior (VA) nuclei of the thalamus has been studied extensively 5,6. The typical response thus induced consists of an EPSP followed by a prolonged IPSP during the course of which transmission in the cerebello-thalamo-cortical pathway is frequently blocked in VL neurons7. Since the recruiting responses thus evoked in the cortical EEG by intralaminar stimulation bear some resemblance to spontaneously occurring spindles, the inference has been drawn that periods of spontaneous EEG synchronization, as they might occur during drowsiness or the transition from wakefulness to sleep, might be accompanied by EPSP-IPSP sequences in rostral thalamic neurons similar to those evoked by repetitive electrical stimulation7,s, and this interpretation appears to be supported by some indirect evidence (ref. 8, Figs. 4, 17). However, while changes in membrane potential occurring concomitantly with the development of spontaneous cortical spindles have been investigated for cells in the ventrobasal complex1, 4 (where these changes are much more pronounced under this condition than during the build-up of recruiting responses4), comparable observations for VAVL neurons have not been published and will therefore be reported here. Glass micropipettes filled with 2 M potassium citrate were used for intracellular recording from VA-VL in paralyzed enc6phale isol~ cats. Spindle 'tripping' was induced by single square pulses of approximately 200 #A and 0.1-0.8 msec duration applied through concentric stainless steel electrodes to different areas in thalamus and head of caudate nucleus. Cortical EEG was monitored with silver ball electrodes from anterior sigmoid gyrus. Periods of EEG desynchronization, which were most common in the unanesthetized animal, were accompanied by a relatively steady membrane potential level in these neurons without any long intervals of hyl~rpolarization or interruption in firing. The spontaneous development of spindles, in contrast, was accompanied (Fig. 1A) and sometimes preceded (Fig. 1C) by a hyperpolarizing shift. The duration of this membrane polarization was closely related to that of the EEG synchronization, lasting from 0.5 to over 3 sec; amplitudes of 5-15 mV were most common, although values in excess of 20 mV were sometimes obtained (Fig. 1D). In the unanesthetized preparation, no exception to the parallel development of EEG synchronization and membrane hyperpolarization was observed during recording periods of up to 60 min. Following the spindle and membrane hyperpolarization the cells gradually returned to their 'resting' membrane potential level; no postinhibitory rebound was observed after spontaneous spindles. Small oscillations were found to be superimposed on the hyperpolarizing shift in most cases (Fig. 1A, C). Whether these are indicative of successive EPSP-IPSP sequences s or whether they represent synaptic activity which is additional to one sustained membrane polarization period cannot be unequivocally determined in the absence of continuous monitoring of membrane resistance a. However, on visual inBrain Research, 41 (1972) 479-481
480
SHORT COMMUNICATIONS
Fig. 1. Membrane potential changes in VA-VL units during spindles. A and B, Same cell, presumably in VA, during development of spontaneous spindle (A) and following single shock to intralaminar region (B); prolonged hyperpolarization with superimposed small oscillations is similar in both cases. C and D, Identified VL unit during spontaneous (C) and caudate-triggered (D) spindle. Time calibration: 400 msec in A-C; 100 msec in D. Spikes retouched. Dashed lines indicate approximate 'firing level'.
spection of these records, especially Fig. 1C, D, the second interpretation appears to be more plausible. Indeed, prolonged hyperpolarizations with superimposed EPSPs ineffective in initiating spike discharges have also been observed in some ventrobasal neurons during spontaneous spindle development 4. Oscillations of more pronounced amplitude than those illustrated in Fig. 1 were frequently observed following intravenous injection of a short-acting barbiturate or after traumatic membrane depolarization. Spindles triggered by a single shock to an intralaminar thalamic nucleus (Fig. 1B) or to the head of the caudate nucleus 2 (Fig. ID) were of very similar configuration as those occurring spontaneously, as were the shifts in membrane potential hereby induced in VA-VL neurons. Thus, the long-term effect on membrane potential in VA-VL neurons evoked by a single pulse may be as good a model for changes occurring during spontaneous EEG synchronization as is the repetitive low-frequency stimulation which has been commonly employed in such investigations 4-6. Stimulation in the brachium conjunctivum at intensities sufficient to evoke a spike in VL cells during periods of E E G desynchronization failed to elicit a discharge at any time during the hyperpolarization associated with the spontaneous or triggered cortical spindles. These findings corroborate earlier interpretations7, s that spontaneous EEG synchronization is accompanied in VA-VL neurons by a membrane hyperpolarization of similar magnitude as that induced by repetitive thalamic stimulation. The main difference was that the depolarizations superimposed on the long hyperpolarization were much weaker than those electrically evoked by successive stimuli, reaching firing level only rarely. As a consequence, spontaneous as well as BC-evoked discharges were not 'phased' into separate bursts at rhythmic intervals in these cells in the unanesthetized and paralyzed enc6phale isol6 preparation, but were instead abolished for the total duration of the spindles which could last for many seconds. A strong depression in postsynaptic components of the evoked potential in VL during drowsiness and slow-wave sleep in the behaving animal has also been reported 9. Brain Research, 41 (1972) 479-481
SHORT COMMUNICATIONS
481
S u p p o r t e d in p a r t by G e n e r a l R e s e a r c h S u p p o r t G r a n t R R 5402 f r o m the G e n e r a l R e s e a r c h S u p p o r t Branch, Division o f R e s e a r c h Resources, N I H , a n d by V A G e n e r a l R e s e a r c h Service. I t h a n k Mr. G r e g o r y M c C o n n e l l t b r technical assistance.
VA Hospital, Syracuse, N.Y. 13210, and SUNY, Upstate Medical Center, Department of Neurosurgery, Syracuse, N.Y. (U.S.A.)
M. WASZAK
1 ANDERSEN,P., ANDANDERSSON,S. A., Physiological Basis of the Alpha Rhythm, Appleton-CenturyCrofts, New York, 1968, pp. 1-235. 2 BUCHWALD,N. A., WYERS, E. J., OKUMA,T., AND HEUSER,G., The 'caudate spindle'. I. Electrophysiological properties, Electroenceph. clin. Neurophysiol., 13 (1964) 355-365. 3 FELDMAN, M. H., AND PURPURA, D. P., Prolonged conductance increase in thalamic neurons during synchronizing inhibition, Brain Research, 24 (1970) 329-332. 4 MAEKAWA,K., AND PURPURA, D. P., Intracellular study of lemniscal and non-specific synaptic interactions in thalamic ventrobasal neurons, Brain Research, 4 (1967) 308-323. 5 PURPURA, D. P., AND COHEN, B., Intracellular recordings from thalamic neurons during recruiting responses, J. NeurophysioL, 25 (1962) 621-635. 6 PURPURA,O. P., AND SHOEER,R. J., Intracellular recordings from thalamic neurons during reticulocortical activation, J. Neurophysiol., 26 (1963) 494-505. 7 PURPURA,D. P., SCARFF,T., AND MCMURTRY, J. G., Intracellular study of internuclear inhibition in ventrolateral thalamic neurons, J. Neurophysiol., 28 (1965) 487-496. 8 PURPURA, D. P., FI~IGYESI,T. L., McMURTRY, J. G., AND SCARFF,T., Synaptic mechanisms in thalamic regulation of cerebello-cortical projection activity. In D. P. PURPURAAND M. D. YAHR (Eds.), The Thalamus, Columbia Univ. Press, New York, 1966, pp. 153-170. 9 STERIADE,M., IOStF, G., AND APOSTOL,V., Responsiveness of thalamic and cortical motor relays during arousal and various stages of sleep, J. Neurophysiol., 32 (1969) 251-265. (Accepted March 21st, 1972)
Brain Research, 41 (1972) 479-481
479
Membrane potential changes in rostral lhalamic neurons during spontaneous and triggered' spindles The effect of low-frequency stimulation in medial and intralaminar thalamic regions on the membrane potential of cells in the ventrolateral (VL) and ventroanterior (VA) nuclei of the thalamus has been studied extensively 5,6. The typical response thus induced consists of an EPSP followed by a prolonged IPSP during the course of which transmission in the cerebello-thalamo-cortical pathway is frequently blocked in VL neurons7. Since the recruiting responses thus evoked in the cortical EEG by intralaminar stimulation bear some resemblance to spontaneously occurring spindles, the inference has been drawn that periods of spontaneous EEG synchronization, as they might occur during drowsiness or the transition from wakefulness to sleep, might be accompanied by EPSP-IPSP sequences in rostral thalamic neurons similar to those evoked by repetitive electrical stimulation7,s, and this interpretation appears to be supported by some indirect evidence (ref. 8, Figs. 4, 17). However, while changes in membrane potential occurring concomitantly with the development of spontaneous cortical spindles have been investigated for cells in the ventrobasal complex1, 4 (where these changes are much more pronounced under this condition than during the build-up of recruiting responses4), comparable observations for VAVL neurons have not been published and will therefore be reported here. Glass micropipettes filled with 2 M potassium citrate were used for intracellular recording from VA-VL in paralyzed enc6phale isol~ cats. Spindle 'tripping' was induced by single square pulses of approximately 200 #A and 0.1-0.8 msec duration applied through concentric stainless steel electrodes to different areas in thalamus and head of caudate nucleus. Cortical EEG was monitored with silver ball electrodes from anterior sigmoid gyrus. Periods of EEG desynchronization, which were most common in the unanesthetized animal, were accompanied by a relatively steady membrane potential level in these neurons without any long intervals of hyl~rpolarization or interruption in firing. The spontaneous development of spindles, in contrast, was accompanied (Fig. 1A) and sometimes preceded (Fig. 1C) by a hyperpolarizing shift. The duration of this membrane polarization was closely related to that of the EEG synchronization, lasting from 0.5 to over 3 sec; amplitudes of 5-15 mV were most common, although values in excess of 20 mV were sometimes obtained (Fig. 1D). In the unanesthetized preparation, no exception to the parallel development of EEG synchronization and membrane hyperpolarization was observed during recording periods of up to 60 min. Following the spindle and membrane hyperpolarization the cells gradually returned to their 'resting' membrane potential level; no postinhibitory rebound was observed after spontaneous spindles. Small oscillations were found to be superimposed on the hyperpolarizing shift in most cases (Fig. 1A, C). Whether these are indicative of successive EPSP-IPSP sequences s or whether they represent synaptic activity which is additional to one sustained membrane polarization period cannot be unequivocally determined in the absence of continuous monitoring of membrane resistance a. However, on visual inBrain Research, 41 (1972) 479-481
480
SHORT COMMUNICATIONS
Fig. 1. Membrane potential changes in VA-VL units during spindles. A and B, Same cell, presumably in VA, during development of spontaneous spindle (A) and following single shock to intralaminar region (B); prolonged hyperpolarization with superimposed small oscillations is similar in both cases. C and D, Identified VL unit during spontaneous (C) and caudate-triggered (D) spindle. Time calibration: 400 msec in A-C; 100 msec in D. Spikes retouched. Dashed lines indicate approximate 'firing level'.
spection of these records, especially Fig. 1C, D, the second interpretation appears to be more plausible. Indeed, prolonged hyperpolarizations with superimposed EPSPs ineffective in initiating spike discharges have also been observed in some ventrobasal neurons during spontaneous spindle development 4. Oscillations of more pronounced amplitude than those illustrated in Fig. 1 were frequently observed following intravenous injection of a short-acting barbiturate or after traumatic membrane depolarization. Spindles triggered by a single shock to an intralaminar thalamic nucleus (Fig. 1B) or to the head of the caudate nucleus 2 (Fig. ID) were of very similar configuration as those occurring spontaneously, as were the shifts in membrane potential hereby induced in VA-VL neurons. Thus, the long-term effect on membrane potential in VA-VL neurons evoked by a single pulse may be as good a model for changes occurring during spontaneous EEG synchronization as is the repetitive low-frequency stimulation which has been commonly employed in such investigations 4-6. Stimulation in the brachium conjunctivum at intensities sufficient to evoke a spike in VL cells during periods of E E G desynchronization failed to elicit a discharge at any time during the hyperpolarization associated with the spontaneous or triggered cortical spindles. These findings corroborate earlier interpretations7, s that spontaneous EEG synchronization is accompanied in VA-VL neurons by a membrane hyperpolarization of similar magnitude as that induced by repetitive thalamic stimulation. The main difference was that the depolarizations superimposed on the long hyperpolarization were much weaker than those electrically evoked by successive stimuli, reaching firing level only rarely. As a consequence, spontaneous as well as BC-evoked discharges were not 'phased' into separate bursts at rhythmic intervals in these cells in the unanesthetized and paralyzed enc6phale isol6 preparation, but were instead abolished for the total duration of the spindles which could last for many seconds. A strong depression in postsynaptic components of the evoked potential in VL during drowsiness and slow-wave sleep in the behaving animal has also been reported 9. Brain Research, 41 (1972) 479-481
SHORT COMMUNICATIONS
481
S u p p o r t e d in p a r t by G e n e r a l R e s e a r c h S u p p o r t G r a n t R R 5402 f r o m the G e n e r a l R e s e a r c h S u p p o r t Branch, Division o f R e s e a r c h Resources, N I H , a n d by V A G e n e r a l R e s e a r c h Service. I t h a n k Mr. G r e g o r y M c C o n n e l l t b r technical assistance.
VA Hospital, Syracuse, N.Y. 13210, and SUNY, Upstate Medical Center, Department of Neurosurgery, Syracuse, N.Y. (U.S.A.)
M. WASZAK
1 ANDERSEN,P., ANDANDERSSON,S. A., Physiological Basis of the Alpha Rhythm, Appleton-CenturyCrofts, New York, 1968, pp. 1-235. 2 BUCHWALD,N. A., WYERS, E. J., OKUMA,T., AND HEUSER,G., The 'caudate spindle'. I. Electrophysiological properties, Electroenceph. clin. Neurophysiol., 13 (1964) 355-365. 3 FELDMAN, M. H., AND PURPURA, D. P., Prolonged conductance increase in thalamic neurons during synchronizing inhibition, Brain Research, 24 (1970) 329-332. 4 MAEKAWA,K., AND PURPURA, D. P., Intracellular study of lemniscal and non-specific synaptic interactions in thalamic ventrobasal neurons, Brain Research, 4 (1967) 308-323. 5 PURPURA, D. P., AND COHEN, B., Intracellular recordings from thalamic neurons during recruiting responses, J. NeurophysioL, 25 (1962) 621-635. 6 PURPURA,O. P., AND SHOEER,R. J., Intracellular recordings from thalamic neurons during reticulocortical activation, J. Neurophysiol., 26 (1963) 494-505. 7 PURPURA,D. P., SCARFF,T., AND MCMURTRY, J. G., Intracellular study of internuclear inhibition in ventrolateral thalamic neurons, J. Neurophysiol., 28 (1965) 487-496. 8 PURPURA, D. P., FI~IGYESI,T. L., McMURTRY, J. G., AND SCARFF,T., Synaptic mechanisms in thalamic regulation of cerebello-cortical projection activity. In D. P. PURPURAAND M. D. YAHR (Eds.), The Thalamus, Columbia Univ. Press, New York, 1966, pp. 153-170. 9 STERIADE,M., IOStF, G., AND APOSTOL,V., Responsiveness of thalamic and cortical motor relays during arousal and various stages of sleep, J. Neurophysiol., 32 (1969) 251-265. (Accepted March 21st, 1972)
Brain Research, 41 (1972) 479-481