Intracellular recordings from neurons in dorsolateral thalamic reticular nucleus during capsular, basal ganglia and midline thalamic stimulation

BRAIN RESEARCH

157

I N T R A C E L L U L A R RECORDINGS FROM NEURONS IN DORSOLATERAL T H A L A M I C RETICULAR NUCLEUS D U R I N G CAPSULAR, BASAL G A N G L I A AND MIDLINE THALAMIC STIMULATION

TAMAS L. FRIGYESI* Department of Neurological Surgery, Kantonsspital, University of Zurich, Zurich (Switzerland)

(Accepted July 5th, 1972)

INTRODUCTION The nucleus reticularis thalami (nR) is a narrow band of scattered cells ('Gitterschicht') intercalated between the internal capsule (IC) and the external medullary lamina; it borders the entire lateral aspect and the rostral pole of the dorsal thalamus. According to cytoarchitectonic and input characteristics, nR can be divided into 6 segments and 18 subnuclei (ref. 12, which is in substantial agreement with ref. 28). In his earliest studies on the synchronization of thalamic and cortical activities, Jasper recognized that intimate functional relations exist between nR and the nuclear groups of the medial and midline thalamus 14; he postulated the existence of parallel projections from nR and adjacent ventrolateral nuclear groups to the sensorimotor cortex (Fig. 12 in ref. 14); and he believed that the final neurons of the polysynaptic thalamic reticular chains are in nR. It has long been known that patterns of extracellularly recorded unit discharges in nR differ conspicuously from those in the ventrolateral thalamus (VL) zl and lateral geniculate body al. In fact, reciprocal relationships exist between unitary discharges in n R and VL 15,16,29: burst discharges were recorded from nR coincident with 'spontaneous' or evoked neuronal silence in VL. Purpura and Cohen first demonstrated that repetitive discharges in nR units which were evoked by 6-8/sec stimulation of medial thalamus exhibit durations similar to inhibitory postsynaptic potentials (IPSPs) in VL neurons elicited by the same stimuli (Fig. 6 in ref. 21). These electrophysiological observations corroborate in general the hypothesis advanced by the Scheibels that nR is in an especially favored position to regulate all thalamocortical transactions 25-2s. They based this hypothesis on anatomical data which show that most axonal systems to and from the dorsal thalamus as well as medial thalamic nuclear masses (MET) send axon collaterals or project onto nR * These studies were carried out in the Parkinson's Disease Research Center, College of Physicians and Surgeons, Columbia University, New York City, U.S.A. Brain Research, 48 (1972) 157-172

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which, in turn, strongly radiates back upon the principal nuclei of the dorsal thalamus. They proposed, in particular, that the prolonged IPSPs, which either phase rhythmic or gait relay activities in VL neurons during sensory, cerebellar or intralaminar stimulation2,7,9-11,19-22,31, are functions of cells in nR. Evoked IPSPs in neurons of the VA-VL (n. ventralis anterior-n, ventralis lateralis) complex exhibit close temporal relations and parallel developmental patterns with surface negative waves evoked by the same stimuli in the motor cortex (MC) (stimulation of midline thalamuslg,20, of IC and basal gangliag-11). Consequently, it is a logical extension of the Scheibels' hypothesis that if IPSPs in VA-VL cells are dependent on activities arising in nR neurons, the synchronously occurring negative waves in MC are also functions of nR neurons. This assumption calls for the operation of a relatively simple synaptic pathway from nR to MC. But recent anatomical reports have emphasized that there is 'absolutely no evidence suggesting cortical terminations' of nR axons25, and that only 4 ~ of nR axons could be traced to the level of the striatumZL Moreover, Minderhoud is, unlike Mettler 17, could not find disappearance of nR cells after hemidecortication. The present intracellular study of nR neurons focused on the dual problem of functional relationships between nR and VA-VL complex and nR and MC. The data in this report substantiate the Scheibels' hypothesis. However, the results also indicate that nR neurons not only project back upon the dorsal thalamus but also have neocortical linkages. A preliminary note has appeared elsewhere s. METHODS

Experiments were performed on locally anesthetized, succinylcholine paralyzed, enc6phale isol6 cats prepared in a manner similar to that described previously in reports of intracellular studies of diencephalic neurons9. Concentrically bipolar electrodes were employed for stimulating the ipsilateral head of the caudate nucleus (Cd), internal capsule (IC), midline thalamus (MT), substantia nigra (SN), entopeduncular nucleus, putamen and dorsal claustrum, and the contralateral brachium conjunctivum (BC). Criteria for determining experimental positions of stimulating electrodes were described elsewhereL Intracellular recordings were obtained by 2 M potassium citrate filled glass micropipettes from neurons in the dorsolateral nR (dlnR) in the vicinity of the VA-VL complex. The micropipettes were either guided into dlnR perpendicularly in lateral 7.5-10.5, between anterior 8.5-11, stereotaxic planes (Fig. 1A) or driven through the body of the caudate nucleus (which was exposed by suction-ablation of the overlying telencephalon) in the same anterior planes and at an angle to penetrate both nR and the VA-VL complex (Fig. IB). Identification of neurons in dlnR was based on: (1) determination of positions of electrode tracts in histological sections; (2) depth measurements projected from the scale of the micromanipulator to the identified electrode tracts; and (3) characteristics of responsiveness of neurons to electrical stimulation of subcortical structures. In actuality, however, recordings from dlnR neurons were readily identifiable by visual inspection of the oscilloscopic traces during the experiments because patterns of evoked synaptic Brain Research, 48 (1972) 157-172

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Fig. 1. Photomicrographs illustrate glass micropipettes which penetrate dlnR. A, Micropipette driven perpendicularly into dlnR. B, Micropipette penetrates through dlnR, external medullary lamina and VL.

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activities in neurons of dlnR were strikingly different from those in the VA-VL complex (vide infi'a). Surface activities from the MC (anterior sigmoid gyrus) were recorded by Ag-AgCI ball electrodes, monopolarly, against a reference electrode embedded in the temporal musculature. Other details of stimulating, recording and histological techniques were identical with those employed and described previously 9. RESULTS

General remarks The narrow width (less than 300 #m) and the relative low cell density (15-20 cells/100 cu, #m) z5 of dlnR were the dominant factors contributing to difficulties encountered in this study in successfully impaling the constituent neurons by a recording micropipette. In those experiments in which the micropipettes were slanted (vide supra, and Fig. 1B), more than 200 neurons were successfully impaled in the VA-VL complex; the same micropipettes, penetrating through dlnR, provided satisfactory intracellular recordings from only 30 dlnR neurons. An additional 6 successful intracellular recordings were obtained from dlnR neurons in 20 experiments with perpendicular electrode placements (Fig. 1A). Fourteen dlnR neurons were also studied extracellularty prior to impalement. Evoked firing patterns in these neurons were similar in the extra- and intracellular recordings, which suggests that the evoked intracellularly recorded activities were not attributable to the traumatic effects of impalement. Stable intracellular recordings were obtained from each cell included in this report for a minimum of 5 min, and from 6 cells for 35-50 min. Responsiveness. A remarkably high proportion of dlnR neurons exhibited synaptic potentials to the afferent volleys applied. Out of the total of 36 neurons, 13 responded to MT, 9 to IC, 6 to CD, and 3 to SN stimulation at 8/sec. Only 5 dlnR neurons were completely unresponsive in this study. None of the neurons explored responded to 8/sec stimulation of BC, entopeduncular nucleus, putamen or dorsal claustrum. Characteristics of evoked responses of dlnR neurons. Low-frequency (8/sec) stimulation of 1C, Cd, MT, or SN elicited essentially similar responses in neurons of dlnR. Under these conditions, evoked burst discharges were the most prominent synaptic events in these neurons (Figs. 2~,). The durations of evoked trains of spike discharges varied among units and according to the source of the afferent volley. Excitatory postsynaptic potentials (EPSPs) underlie these burst discharges. Postsynaptic inhibitory effects of afferent volleys in dlnR neurons were not encountered under the specified conditions. The major differences in evoked activities in these cells were related to the developmental patterns of the evoked prolonged burst discharges. During 8/sec stimulation of medial third of Cd, the first stimulus of a repetitive train generally elicited no detectable response; the second and third stimuli elicited little response; and overt bursts were commonly demonstrable following the fourth stimulus (Fig. 2A). During IC stimulation, the first stimulus may have elicited no detectable response, but the second stimulus commonly generated a burst response, and the

Brain Research, 48 (1972) 157-172

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Fig. 2. Effectsof low-frequencystimulation of Cd in dlnR and in MC. The upper traces in this and all subsequent figures are surface recordings from MC (negativity upwards.) The lower traces in Figs. 2-6 are intracellular recordings from neurons in dlnR (negativity downwards). A-D, Continuous recordings. The closed triangles indicate the shock artifacts of 8/sec stimulation of Cd. A, Following the initial 4 stimuli of the repetitive train to Cd, gradual buildup of long-latency clustered firing of dlnR neuron is seen. B, Burst alternation is noticeable during continued stimulation of Cd. C, Sustained high-frequencydischarges of dlnR neuron are demonstrable; compare with discharge rate prior to, in A, and posterior to, in D, stimulation of Cd. D, After 2 sec of 8/sec stimulation of CD, evoked clustered firing is no longer seen in the dlnR neuron. The evoked, long-latency negative wave in MC exhibits gradual buildup, in A, during early phases of Cd stimulation. Coincident with the failure of Cd stimulation to elicit clearly identifiable clustered firing in the dlnR neuron during sustained 8/sec stimulation (in C and D), the Cd evoked long-latency negative wave gradually attenuates, in C, thereafter is no longer seen, in D.

third stimulus was followed by maximal development of the burst responses in all dlnR neurons responsive to capsular stimulation (Fig. 3A). Although considerable variations were observed during MT stimulation, burst discharges following the first stimulus were frequently encountered (Fig. 4B). The 3 dlnR neurons driven by SN stimuli exhibited 3 different modes of burst development. During prolonged 8/sec stimulation, the evoked burst discharges either remained stable (Fig. 4C and D), exhibited gradual (Fig. 3B and C), or rapid (Fig. 2D) attenuation, or showed alternating patterns (Fig. 2B, cf. the first and third with the second and fourth stimuli). Other prominent differences were related to the latencies of evoked burst discharges. Although the data here do not permit the exact determination of their latencies, it appears that the longest latencies followed the Cd and SN stimulation (Figs. 2 and 6), while IC and MT stimulation generated bursts with conspicuously shorter (6-10 msec) latencies (Figs. 3 and 4). Durations of the prolonged bursts varied between 20 and 60 msec. These variations were less dependent on the site of stimulation than on the position of the triggering stimulus within a repetitive train. Fully developed burst discharges exhibited rather uniform durations in the various dlnR neurons (Figs. 2 4 , 7 and 8). Brain Research, 48 (1972) 157-172

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Fig. 3. Characteristics of synaptic effects in dlnR neuron elicited by low-frequency capsular stimulation. Open triangles denote the shock artifacts of 8/sec stimuli to IC. A-C, Continuous recordings. The background discharge of the neuron is seen in A. Gradual buildup of short-latency, prolonged EPSPs with superimposed high-frequency discharges are seen on the intracellular trace, in A, during early phases of capsular stimulation. B and C, Gradual attenuation of capsular evoked synaptic effects are seen in the dlnR neuron. Note parallels in development and attenuation between capsular evoked burst discharges in the dlnR neuron and short-latency negative waves in MC.

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Fig. 4. Characteristic absence of convergence between synaptic pathways arising in MC, MT and SN and terminating in dlnR. A-E, Continuous recordings. Eight/sec stimulation of SN (at lozenge-shaped symbols), MT (at dots) and 1C (at open triangles). Neuron in dlnR is unresponsive to nigral stimuli (in A and after first stimulus in B.) MT stimulation, which effectively generates recruiting responses in MC, elicits prolonged EPSPs with superimposed high-frequency discharges in the dlnR neuron, in B-D. In D, stimulation is abruptly switched from MT to IC. The capsular stimuli are ineffective in driving this dlnR neuron (D and E). E v o k e d EPSPs underlie the burst discharges elicited by either m o d e o f stimulation applied in this study. M a g n i t u d e s and d u r at i o n s o f the e v o k e d EPSPs were largely p r o p o r t i o n a l to the durations and discharge frequencies o f e v o k e d burst discharges. A m p l i t u d e s o f action potentials arising f r o m the crest o f e v o k e d EPSPs were smaller

Brain Research, 48 (1972) 157-172

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than those arising from the rising or declining phase of such prolonged EPSPs or those seen during spontaneous firing of these neurons. Corresponding variations in spike heights were observed in extracellular studies of nR neurons (Figs. 6 and 8 in ref. 29). Background discharge frequencies of dlnR neurons were most commonly in the range of 60-80/see (Fig. 4A and E, Fig. 6A), but occasionally as low as 40/sec 'spontaneous' discharges were observed (Fig. 2A and D). Intraburst firing frequencies of dlnR neurons attained levels as high as 250-300/sec (Figs. 3 and 4). Two types of essential variations from the above described pattern of evoked burst discharges were encountered. Fig. 2C illustrates one of these variations. During prolonged repetitive stimulation of the medial Cd, sustained development of highfrequency discharges was demonstrable in this neuron (cfi frequency of discharge in Fig. 2C with those prior to, at beginning of Fig. 2A, and following cessation of, at end of Fig. 2D, the reptitive stimulation of Cd). The other variation is shown in Fig. 5. Low-frequency stimulation of MT elicited depolarizing inactivation responses in this dlnR neuron; the depolarizing PSPs fused into a sustained potential and only the first spike developed fully under these conditions. It is noted that the extracellular counterpart of this type of responsiveness of dlnR neurons to repetitive stimulation of MT was already observed by Schlag and Waszak (Fig. 4 in ref. 29) during 5 and 10/sec stimulation of the centrum medianum complex. In both the extra- and present intracellular recordings, full spike generation is immediately resumed after:the:cessation of

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Fig. 5. Depolarizing inactivation responses in a dlnR neuron elicited by 8/see MT stimulation. A - D , Continuous recordings. Neuron is uninfluenced by 8/see stimulation of IC; the last stimulus of a lowfrequency train to IC is shown at the beginning of A. Abrupt switch of stimulation from ]C to MT results in the development of summating EPSPs with high-frequency spike attenuation following the second stimulus to MT. A - D , Prolonged, powerful excitatory synaptic drives are associated with inactivation phases during the entire period of 8/sec stimulation of MT. Another abrupt switch of stimulation, now from MT to SN (at end of D), results in the immediate resumption of full-spike generation. The neuron is uninfluenced by low-frequency stimulation of SN.

Brain Research, 48 (1972) 157-172

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Fig. 6. Burst discharges in a dlnR neuron coincident with a triggered spindle burst in MC. A-C, Continuous recordings. A, Background activity of the dlnR neuron ranges between 80 to 100/sec. B, 8/sec stimulation of Cd results in gradual buildup of long-latency burst discharges in the dlnR neuron and incrementing negative waves in MC. C, After cessation of Cd stimulation, triggered spindle burst is seen in MC. Concomitantly, 3 bursts are seen in the dlnR neuron. Note variations in firing frequencies, durations, and magnitudes of underlying EPSPs of these triggered clustered firings of the dlnR neuron.

the 8/sec stimulation of M T (Fig. 5D). The durations of depolarizing inactivation responses were longer than the evoked burst discharges; they ranged from 80 to 100 msec. Triggered spindle bursts in the motor cortical surface recordings are frequently encountered in enc6phale isol6 cats following the cessation of 8/sec stimulation of medial Cd4,10,z0,3z. An example of this is shown in Fig. 6C. Coincident with the Cd triggered neocortical spindle burst, burst discharges of various durations and discharge frequencies were demonstrable in the dlnR neuron (Fig. 6C). Close temporal relations between the triggered negativities or any other component of the spindle burst in MC and the triggered burst discharges in the dlnR neuron were not observed. Convergence. Responsive units in the para-VA-VL region of dlnR exhibited burst discharges to IC, Cd, MT or SN stimuli. Convergence was not encountered among these synaptic pathways on neurons in this area of dlnR (Fig. 4A and D, Fig. 5A and D, and Fig. 7A). Neurons in this region of dlnR were securely driven by only one mode of stimulation applied here without detectable degree of cross modality dispersion or evidence of aftereffects. Figs. 4, 5 and 7 illustrate abrupt changes of stimulation from one subcortical structure to another without altering the timing of the stimulation (stimuli were timed to occur 125 msec apart). Such changes in the target structure of stimulation were always associated with development of response characteristics in dlnR neurons of the ongoing stimulation immediately after the first stimulus (Fig. 4B, Fig. 5A and Fig. 7A). Alternatively, if the neuron was unresposive to the novel stimulus train, the effects of the previously applied stimulation subsided immediately after the cessation of such stimulation (Fig. 4D). An additional, prominent example of this is shown in Fig. 5D: immediate full-spike generation is demonstrable within 2 msec after the first stimulus to SN in a dlnR neuron which exhibited depolarizing inactivation response during preceding 8/sec M T stimulation. The lack of convergence of IC, Cd, M T and SN synaptic pathways on nR Brain Research, 48 (1972) 157-172

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Fig. 7. Characteristics of evoked synaptic activities from neurons in dlnR and in adjacent VL. The top traces are surface recordings from MC. The second traces are intracellularrecordings from neurons in dlnR. The third trace in A, and the third and fourth traces in B, are intracellular recordings from neurons in VL. The recordings from the dlnR and VL neurons were obtained by the same micropipettes, in A and B, respectively. A, dlnR neuron is uninfluencedby 8/sec stimulation of SN. Repetitive MT stimuli, which elicit recruiting responses in MC, generate prolonged burst discharges in the dlnR neuron, and EPSP-IPSP sequences in the VL neuron. Note the close temporal relations between the surface negativities in MC, bursts in the dlnR and IPSPs in the VL neuron. B, The initial 5 stimuli of an 8/sec train to Cd is indicated by the numbers above the MC trace. The incremental characteristic of the evoked long-latency negative wave is noticeable. In this experiment, maximal development of bursts in dlnR neurons were observed following the fourth stimulus to Cd. Similarly, overt IPSPs in VL neurons were observed following the fourth stimuli to Cd. This particular placement of electrodes within Cd, induced alternation of evoked bursts in the dlnR neuron following the fourth and fifth stimuli. A corresponding IPSP alternation in the adjacent VL neuron is noticeable (third trace in B).

n e u r o n s is characteristic in the dorsolateral, p a r a - V A - V L regions. I n contrast, extensive convergence was observed a m o n g these projections o n n e u r o n s in the rostral

pole of n R . Moreover, in the rostral pole, n e u r o n s were e n c o u n t e r e d which were synaptically driven by s t i m u l a t i o n o f the contralateral BC. C o n d i t i o n i n g IC, Cd, M T or SN stimuli elicited excitatory or i n h i b i t o r y effects o n BC evoked activities in n e u r o n s of the rostral pole o f n R in m u c h the same way as in the V A - V L complex 9-11,19,2°,22. Details of these interactions a m o n g converging pathways o n n e u r o n s in the rostral pole o f n R will be considered in a n o t h e r report. F o r the present purposes, it will suffice to p o i n t out that the data revealed p r o m i n e n t differences in convergence properties of dorsolateral a n d rostral n e u r o n s in n R .

Characteristics o f activities in V A - V L neurons and in the motor cortex coincident with evoked burst in dlnR neurons. Figs. 2-8 illustrate intracellular recordings from d l n R n e u r o n s a n d c o n c o m i t a n t l y recorded surface activities from MC. The impracticability o f simultaneously recording intracellularly from two thalamic n e u r o n s

Brain Research, 48 (1972) 157-172

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Fig. 8. Diagrammatic illustration of input-output characteristics of dlnR. Pathways and neurons here represent electrophysiologically, and not anatomically, identified elements. 1, Depicts a neuron in VL, which can be identified by its monosynaptic responsiveness to stimulation of the ascending cerebellar outflow system (BC). The VL neuron projects to that area of MC which sends axons back to it. This MC-VL pathway sends axon collaterals to that part of dlnR which also radiates onto the same region in VL. Thus a corticofugal impulse may elicit oligosynaptic excitation of a VL neuron (lower trace, inset B) and, via an axon collateral, initiate operation of a closed chain in dlnR with resultant high-frequency activation of a constituent neuron (lower trace, inset A). This final neuron of the intra-dlnR closed chain, engaged by the axon collateral, projects upon the same VL neuron which is also engaged by the main cortico-VL axon. It is suggested that the evoked repetitive discharges in dlnR neurons cause repetitive release of inhibitory transmitter and induce prolonged hyperpolarization of VL neurons. It is suggested that this mechanism accounts, in part, for the EPSP|PSP sequences in VL neurons following corticofugal impulses. The synaptic pathway a, in 2, may represent a collateral from other projections piercing through dlnR or the last link in a polysynaptic chain originating in medial thalamus. The inset, in 2, illustrates that activities initiated in MT gain access to a dlnR neuron via a and generate a burst in this element (second trace). These evoked repetitive discharges from dlnR gain access to a neuron in VL via axon b and induce a prolonged IPSP (fourth trace). Data depicted in the inset show that evoked bursts in dlnR neurons, IPSPs in VL neurons, and surface negativities in MC exhibit close temporal relations. This implies that causal relationships exist between evoked burst discharges in dlnR neurons not only IPSPs in VA-VL neurons but also incrementing negativities in MC. This also indicates that topographically arranged connections operate between restricted regions within dlnR and both ends of the reciprocal thalamocortical circuit. Whether this is achieved via axon collaterals, c, or by separate axons, remains to be elucidated. Further description in text. TPC, thalamocortical projection. Other abbreviations, as in text.

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o b t a i n e d in s u c c e s s i o n by t h e s a m e r e c o r d i n g m i c r o p i p e t t e d u r i n g a p e r i o d w h e n t h e a p p a r e n t c o n d i t i o n o f t h e e x p e r i m e n t a l a n i m a l r e m a i n e d v i r t u a l l y u n c h a n g e d . Posit i o n s o f tips o f s t i m u l a t i n g e l e c t r o d e s a n d p a r a m e t e r s o f s t i m u l a t i o n s w e r e also u n c h a n g e d d u r i n g t h e r e c o r d i n g s f r o m b o t h o f s u c h n e u r o n s . U n d e r these c o n d i t i o n s , t i g h t t e m p o r a l c o u p l i n g was o b s e r v e d b e t w e e n e v o k e d b u r s t s in d l n R a n d e v o k e d Brain Research, 48 (1972) 157-172

DORSOLATERAL THALAMIC RETICULAR NEURONS

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IPSPs in VA-VL neurons elicited by the 8/sec stimulation of the same subcortical structure (Figs. 7 and 8). The latencies and durations of the evoked burst discharges in dlnR neurons were shorter than those of evoked IPSPs in VA-VL neurons following the same stimuli. The depolarizing inactivation responses in dlnR neurons elicited by 8/sec M T stimulation exhibited durations similar to those of evoked IPSPs in VL neurons under similar conditions. Fig. 7 illustrates that developmental patterns of evoked bursts in a dlnR neuron were remarkably similar to those of evoked IPSPs in two adjacent VL neurons during 8/sec stimulation of Cd. Overt burst discharge in the dlnR neuron is demonstrable following the fourth stimulus to Cd; in both VL neurons, overt IPSPs are similarly demonstrable after the fourth stimuli to the Cd. The IPSP alternation in the upper VL neuron in Fig. 7B parallels the burst alternation in the dlnR neuron following the fourth and fifth stimuli to the Cd. Attention is also directed to the fact that all records here show close temporal relations and similarities in buildup of evoked bursts in dlnR neurons and recruiting or incrementing surface negativities in the motor cortex. DISCUSSION

All thalamopetal projections send collaterals to dendrites of neurons in dlnRS,6,1z, 23-z8. Thus dlnR neurons obtain samples of ongoing activities in the main axons which terminate in the principal thalamic nuclei. The results described above indicate that the major synaptic events in dlnR neurons are repetitive discharges following each afferent stimulus within a repetitive, low-frequency (8/sec) train of stimuli to IC, Cd, MT and SN. The first stimulus of such trains may trigger little visible response; the second one may elicit only a small response; the third and subsequent stimuli commonly induce prominent burst discharges in these neurons. Development of peak frequencies within a response train in dlnR neurons depended primarily upon the origin of the afferent volley: maximal development of burst discharges was commonly seen after the second or third stimuli to IC or MT, and after the fourth or fifth stimuli to Cd or SN. Latencies of response trains also tended to vary primarily with the site of stimulation: IC and MT evoked bursts exhibited shorter latencies than those evoked by Cd or SN stimuli. However, durations of the evoked bursts appeared to be rather uniform in this sample following either mode of stimulation. In some instances, evoked excitatory effects summated in dlnR neurons and resulted either in the development of sustained high-frequency discharges of the dlnR neuron or in the development of depolarizing inactivation responses. After the initial maximal development of evoked burst discharges in dlnR neurons, the repetitive discharges either remained stable, waxed and waned or attenuated. These prominent variations in responsiveness following individual stimuli suggest that the repetitive responsiveness of dlnR neurons to the afferent volleys applied here was not dependent upon prolonged synaptic excitation due to sustained transmitter action. Rather, these variations in responsiveness indicate that the origin of the discharge patterns of dlnR neurons demonstrated here lies outside these cells. In dlnR neurons, evoked EPSPs considered to be the result of brief, synchronous, presynaptic action were not observed. The Brain Research, 48 (1972) 157-172

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reflexly evoked synaptic effects failed to exhibit sufficiently short latencies with minimal variability to suggest monosynaptic excitation of dlnR neurons in this sample by stinmlation of pathways arising in MC, Cd, MT, or SN. Such effects, however, elicited by corticofugal volleys were observed in an extracellular study o f n R neurons a0. Therefore, the prolonged synaptic excitation of dlnR neurons probably results from temporally dispersed presynaptic actions impinging on dlnR neurons induced by the operation of complex intrathalamic interneuronal organizations 19-'1 activated by the low-frequency stimulation of IC, Cd, MT and SN. Variations and lability of evoked repetitive discharges in dlnR neurons depended on the differences and number of synaptic junctions serially excited by an incoming volley. It is pertinent to recall here that a recent Golgi study of nR in the mature cat revealed that dlnR is characterized by an almost total absence of a dense presynaptic neuropil plexus and dendritic spinelike appendages and by an abundance of smoothsurfaced dendrites, tightly packed in bundles zs. It has been proposed that the conspicuous absence of convergence between activities evoked by synaptic pathways piercing through dlnR are readily explicable by the former, and the generation of burst discharges in dlnR neurons by these pathways is causally related to the latter structural properties 28. The data here also show that analogues exist between certain characteristics of evoked, intracellularly recorded, clustered repetitive discharges of dlnR neurons and those of IPSPs in VA-VL neurons elicited by the same stimuli. Evoked bursts in dlnR neurons exhibited shorter latencies and durations than the respective evoked IPSPs in VA-VL neurons following each stimulus of an 8/sec train to IC, Cd, MT or SN. However, consistent temporal relations persisted between these evoked responses at the two recording sites during the entire period of repetitive stimulation of each of these structures. Gradual or rapid buildup, attenuation, waxing and waning, and summation were the prominent characteristics of evoked bursts in dlnR neurons during 8/sec stimulation of IC, Cd, MT or SN. Similar developmental patterns characterize evoked IPSPs in VA-VL neurons following the same stimuli in this as well as in previous 9-11 and in other studies 19-22. These parallels of evoked activities in dlnR and VA-VL neurons elicited by the same stimuli, together with findings of anatomical studies which show that (1) all projection systems stimulated here send axon collaterals to dlnR and (2) dlnR neurons radiate back to the VA-VL complexS,6,12,'24,~% provide indirect, but compelling evidence that at least some of the evoked IPSPs in VA-VL neurons and the evoked bursts in dlnR neurons are causally related under these conditions. The evoked firing pattern of dlnR neurons here resembles to the excitation clusters seen in inhibitory neurons elsewhere in the vertebrate nervous system 13. These data, consequently, are consonant with the proposition that evoked repetitive discharges in dlnR neurons cause repetitive release of inhibitory transmitters on VA-VL neurons. That is, present interpretation favors the mechanism of persistent transmitter release, in contrast to the prolonged transmitter action, accounting for the long-duration IPSPs in VA-VL neurons. This interpretation is substantiated by data which show that durations of evoked [PSPs in VA-VL neurons may vary extensively following each stimulus of a repetitive volley9,1°,19-~1 in much the same Brain Research, 48 (1972) 157-172

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way as evoked bursts in dlnR neurons do. Consequently, the present data indicate that the tonic background activity of dlnR neurons contribute to mechanisms which differentiate signals from noise in VA-VL neurons and that dlnR neurons are involved in the dispersion of inhibitory effects7-11 initiated by activities arising in MC, Cd, MT and SN and operating on VA-VL neurons. It is emphasized, however, that data here neither show nor suggest that the entire inhibitory mechanism operating over VA-VL neurons is situated in dlnR. It has been evidenced that parallel processing is a distinguishing characteristic of the operation of the thalamic reticular system from specific activities which relay through the dorsal thalamus z°. The data presented here add further evidence for this concept by showing that activities of dlnR neurons distribute in parallel projection systems. In addition to producing prolonged IPSPs in VA-VL neurons, clustered discharges in dlnR neurons induced by appropriate input volleys exhibited developmental patterns similar to those of recruiting of incrementing negative waves in the motor cortex induced by the same stimuli. These parallels in buildup of evoked bursts in dlnR neurons and incrementing negativities in MC were more prominent following the initial 2-4 stimuli of any repetitive train applied here than during prolonged stimulation. This phenomenon can be most easily appreciated during Cd stimulation which generally triggered slow buildup of such bursts and negative waves (Fig. 7B). Furthermore, a comparison of Fig. 2A and D shows that, when 8/sec stimulation of Cd effectively builds up burst discharges in the dlnR neuron, a coincident buildup of neocortical negativities is demonstrable, whereas when the same Cd stimulation becomes ineffective in synaptically driving the dlnR neuron such neocortical negativities are not observed. The latencies of evoked bursts in dlnR were always shorter than those of cortical negativities elicited by the same stimuli. The close temporal relations between such negative waves in MC and IPSPs in VA-VL neurons following IC, Cd, M T and SN stimulation have already been established1°,19, ~°. Together, these observations show that the evoked synaptic events in dlnR neurons occur prior to the development of evoked inhibitory effects in VA-VL neurons or the development of synaptic effects in MC which are recordable from its surface as incrementing negative waves 19. These observations necessitate a consideration of the possibility that not only evoked IPSPs in VA-VL neurons but also, at least in part, the evoked negative waves in MC are functions of neurons in dlnR during low-frequency stimulation of IC, Cd, MT and SN. The existence of a pathway from dlnR to MC has been a controversial issue in the literature. The Scheibels in their Golgi studies found no evidence of cortical projections of nR neurons zS. Minderhoud in a retrograde degeneration study in the albino rat using short (1-3 months) survival time, could not find disappearance of dlnR cells after hemidecortication is. In contrast, Mettler 17 and Chow 6 found regional degenerations in nR following large localized lesions or hemidecortication after long (up to over one year) survival times in the monkey. In case of localized neocortical lesions, the degenerated zones within nR were observed in sectors adjacent to those principal nuclei which also showed retrograde degenerations6,1v,24. The electrophysiological findings in this study which show parallel processing of reticulofugal Brain Research, 48 (1972) 157-172

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activities are consonant with Mettler's data and Chow's view: 'The cell population in the reticular nucleus may be heterogeneous: some neurons may project entirely to the cortex, some may send only cortical collaterals. A few may act solely as intrinsic internuncials, and still others may send fibers to other thalamic nuclei'. It has already been firmly established that an intimate relationship exists between mechanisms involved in the generation of prolonged IPSPs in rostral thalamic neurons and 8-12/sec, spontaneous or evoked, rhythmic negativities in the motor cortex 1 4,9-1~, 19,z0,31. No attempt is being made here to argue with the various theories which suggest that 'autorhythmicity' or 'pacemaker' effects account for the striking similarities in the 8-12/sec rhythmicities in the thalamus and in the neocortex 2,3,z0. However, the concept 2° that activities within the thalamocortical reciprocal pathways and impulses circulating within closed chains appended to this circuit control the phasing of rhythmic activities in the neocortex and dorsal thalamus is entirely supported by data in this study. The present data suggest that thalamocortical synchronization of activities is attained not by the operation of parallel neuronal loops with identical very low conduction velocities or with identical number of synapses 1 but, inter alia, by the activation of neurons in dlnR which, in turn, disperse modulatory 9° effects, from intra- and extrathalamic sources, on VA-VL and motor cortical neurons. The results here also indicate that synchronization of thalamocortical activities coincident with spindle bursts in the ME 4,10,19,20,31 is dependent on activities mediated by the parallel projections arising in dlnR and terminating in VA-VL and MC. Concomitant with such spindle bursts, repetitive discharges in dlnR neurons, like IPSPs in VA-VL neurons, may or may not show close temporal coupling to any component of the spontaneous or triggered 8-12/sec burst in MC. In view of the nature of sampling of neurons in this study and the extent of the neocortical area whence such rhythmic activities could be recorded, the observed variations in these temporal relationships are not unexpected. SUMMARY

Intracellular recordings from neurons in the dorsolateral segment of thalamic reticular nucleus have permitted interpretation of input-output functions of these elements during low-frequency (8/sec) stimulation of internal capsule, head of caudate nucleus, midline thalamus and substantia nigra. The tonic background activity of dorsolateral thalamic reticular neurons influences the background excitability of dorsal thalamic VA-VL neurons and therefore the capacity of the latter to respond to afferent volleys. Activities which arise in the motor cortex, basal ganglia or substantia nigra and which generate relatively short latency excitation of VA-VL neurons also gain access to dorsolateral thalamic reticular neurons. Motor cortex, basal ganglia, substantia nigra and midline thalamus synaptic pathways, through temporally dispersed presynaptic action, induce repetitive discharges in dorsolateral thalamic reticular neurons which project back to VA-VL. Low-frequency induced burst discharges in dorsolateral thalamic reticular neurons result in temporally dispersed release of Brain Research, 48 (1972) 157-172

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inhibitory transmitters on V A - V L neurons and, thereby, in the production o f prolonged IPSPs in these elements. Neurons in dorsolateral thalamic reticular nucleus are also linked to the m o t o r cortex t h r o u g h relatively simple synaptic pathways: low-frequency activation o f these pathways results in the generation o f surface negative incrementing-decrementing, recruiting or augmenting responses. These parallel processes to single input volleys to dorsolateral thalamic reticular neurons form, in part, the basis o f intrathalamic gaiting o f cerebellofugal impulses as well as synchronization o f rhythmic activities in the reciprocal thalamocortical circuit during lowfrequency stimulation of internal capsule, head o f caudate nucleus, midline thalamus and substantia nigra. This mechanism readily accounts for not only the present demonstration o f various parallelisms between evoked bursts in dorsolateral thalamic reticular neurons and IPSPs in V A - V L neurons evoked by the same stimuli but also for previous extracellular observations which have revealed that unitary firings in dorsolateral thalamic reticular nucleus are associated with neuronal silence in VL. ACKNOWLEDGEMENT This work was supported by a grant from N I N D S No. 09898-01 and -02.

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