Tonic changes in excitability of thalamocortical neurons during the sleep-waking cycle

Tonic changes in excitability of thalamocortical neurons during the sleep-waking cycle

354 SHORT COMMUNICATIONS Tonic changes in excitability of thalamocortical neurons during the sleepwaking cycle It is well known that shifts from lig...

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354

SHORT COMMUNICATIONS

Tonic changes in excitability of thalamocortical neurons during the sleepwaking cycle It is well known that shifts from light or synchronized sleep (LS) to desynchronized or deep sleep (DS) or to the waking state (W) are associated with tonic, longlasting modulation of synaptic transmission through both the lateral geniculate nucleus (LG) 2 and the ventro-postero-lateral nucleus (VPL) 4. Moreover, it has been demonstrated that during the DS stage the thalamic transmission of sensory volleys undergoes phasic, increases coincident with rapid eye movements (REMs) and related to sudden and short-lasting enhancements of the excitability of thalamocortical neurons 3. The present investigation was directed to ascertain whether the same mechanism, i.e. changes in the level of excitability of thalamocortical neurons, could be held responsible for long-lasting variations in thalamic transmission occurring throughout the sleep-wakefulness cycle. Twenty cats were chronically studied with implanted electrodes in the optic tract (OT), the L G and the optic radiation (OR); and/or in the medial lemniscus (ML), the VPL and the somesthesic radiation (SR). The electrodes were made of two stainless steel wires, 0.2 mm in diameter, that could be used alternatively for stimulation and recording. Threshold stimuli of 0.05-0.1 msec duration, 0.2-0.3/sec, were used. The stimulus intensity was set at the beginning of the experimental session and maintained throughout the subsequent sleep cycle. Other details as to stimulating and recording techniques and methods for localization of electrodes were similar to those described elsewhere 2 4. To obtain comparable results the thalamic transmission and the thalamocortical neuron excitability were tested in the same animal. Thalamic transmission was evaluated by measuring the second or postsynaptic component of the thalamic mass discharge upon stimulation of second order afferent tracts at the pre-thalamic level (the OT ~ L G and the ML ~ VPL responses) (Fig. 1A and B). This component is due to monosynaptic activation of thalamocortical neurons as shown by its latency 3, and its amplitude changes are directly related to modulation of thalamic transmission. The excitability of thalamocortical neurons was evaluated by measuring: (a) the antidromic thalamic response upon stimulation of the corresponding thalamocortical radiation (the OR ~ L G and the SR ~ VPL responses) (Fig. 1C and D); and (b) the first or alpha component of the response evoked by direct stimulation of the L G or VPL in the corresponding thalamocortical radiation (the L G ~ OR and the VPL -~ SR responses) (Fig. 1E and F). The latter responses are due to asynaptic excitation of the thalamic neurons projecting to the cortex, and their amplitude changes give a reliable estimate of the neuronal population responding to the stimulus 3. The experimental design was as follows: (a) out of a randomized series of sequences of light sleep ~ deep sleep -+ wakefulness, the amplitude of an equal number of responses for each stage was measured; the responses recorded in coincidence with REMs had been previously discarded; (b) whenever a preliminary overall F-test proved to be significant (P ~< 0.05), the differences among individual treatments, Brain Research, 29 (1971) 354-357

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Fig. 1. Synaptic transmission through the lateral geniculate (LG) and ventro-postero-lateral (VPL) nuclei and excitability of thalamocortical neurons during the sleep-waking cycle. In this and in the following figure the mean amplitude changes of the measured component of each response are expressed in conventional units. The vertical bars indicate the confidence limits ('X ± t0.ol s~:). The time base (1 msec) is connoted by the horizontal bar. The diagrams represent the mean amplitude changes of the orthodromic (A and B), antidromic (C and D) and direct (alpha component) (E and F) mass responses of LG and VPL nuclei during the different stages of sleep and wakefulness.

i.e. light sleep, deep sleep and wakefulness, were tested by the 'Q' method and its

sequential variant according to N e u m a n and KeulsT; and (c) if the overall F proved non-significant, the analysis was discontinued. As shown in Fig. 1, the mean amplitude of the postsynaptic component of the L G (Fig. 1A) and VPL (Fig. 1B) responses upon stimulation of second order afferent tracts was noticeably lower during LS with respect to DS and W stages, no significant differences being observed between the last two stages. These results point to a facilitated synaptic transmission during DS and W with respect to LS, thus confirming previous researches2,4; the facilitation attained the same degree in the two nuclei. The behavior of thalamocortical cell excitability followed a strictly similar pattern: both antidromic (Fig. 1C and D) and direct (alpha component) (Fig. 1E and F) mass discharges of L G and VPL neurons were lower in LS than during DS and W, without significant differences between the last two stages. This points to an increased excitability o f thalamic neurons both during DS and W as compared with LS, again the behavior of the two nuclei was strictly similar. The analogy between the two phenomena under investigation strongly suggests Brain Research, 29 (1971) 354-357

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Fig. 2. Excitability of lateral geniculate neurons during the sleep-waking cycle after bilateral section of the optic nerves. The diagrams represent the mean amplitude changes of the antidromic (A) and direct (alpha component) (B) responses of the LG nucleus during the different stages of sleep and wakefulness. a causal relationship; i.e. that the increased thalamic output during DS and W is due to an enhanced postsynaptic responsiveness to incoming volleys. Recent investigation has shown that the spontaneous activity of single OT fibers is not influenced by the level of vigilance; it is therefore unlikely that long-lasting fluctuations of the excitability of L G neurons are due to tonic changes of homosynaptic afferent input. To confirm this assumption, in 2 cats the excitability of LG cells was evaluated after bilateral section of the optic nerves. The behavior of both antidromic (Fig. 2A) and direct (Fig. 2B) L G responses showed no variation with respect to the intact animal. Therefore it can safely be concluded that changes of LG neuron excitability are due to fluctuations of a heterosynaptic input. It is relevant to note that reticular fibers impinging upon the L G neurons have been repeatedly described1, e. As far as the excitability changes of VPL neurons are concerned, the similar behavior of the two nuclei leaves little doubt that the same heterosynaptic mechanism acts at the VPL level, but data regarding the unitary activity of ML fibers during sleep are still lacking. Interruption of ML tracts was not attempted in view of the dramatic effects of brain stem lesions on sleep activity. The work was supported by the Consiglio Nazionale delle Richerche (Grant No. 69.01719 115.2264). Department of Nervous and Mental Diseases, University of Genoa, Genoa (Italy) Brain Research, 29 (1971) 354-357

NICOLA DAGNINO EMILIO FAVALE MARIO MANFREDI ANDREA SEITUN ANTONIO TARTAGLIONE

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1 BOWSHER,D., Reticular projections to lateral geniculate in cat, Brain Research, 23 (1970) 247-249. 2 DAGNINO,N., FAVALE,E., LOEB, C., AND MANFREDI,M., Sensory transmission in the geniculostriate system of the cat during natural sleep and arousal, J. Neurophysiol., 28 (1965) 443-456. 3 DAGNINO, N., FAVALE, E., LOEB, C., MANFREDI, M., AND SEITUN, A., Presynaptic and postsynaptic changes in specific thalamic nuclei during deep sleep, Arch. ital. Biol., 107 (1969) 668-684. 4 FAVALE,E., LOEB,C., MANFREDI,M., AND SACCO,G., Somatic afferent transmission and cortical responsiveness during natural sleep and arousal in the cat, Electroenceph. clin. Neurophysiol., 18 (1965) 354-368. 5 MUKHAMETOV,L. M., RIZZOLATTI,G., AND SEITUN,A., An analysis of the spontaneous activity of lateral geniculate neurons and of optic tract fibers in free moving cats, Arch. itaL BioL, 108 (1970) 325-347. 6 SCHEIBEL,M. E., AND SCHEmEL,A. B., Structural substrates for integrative patterns in the brain stem reticular core. In H. H. JASPER,L. D. PROCTOR, R. S. KNIGHTON, W. C. NOSHAYAND R. T. COSTELLO(Eds.), Reticular Formation of the Brain, Little, Brown and Co., Boston, Mass., 1958, pp. 31-55. 7 SNEDECOR,G. W., AND COCHRAN, W. G., Statistical Methods, Iowa State Univ. Press, Ames, Iowa, 1967, pp. 272-275. (Accepted April 2nd, 1971)

Brain Research, 29 (1971) 354--357