Burst discharges of thalamic reticular neurons: an intracellular analysis in anesthetized rats.

In order to analyze the mechanism of burst discharges intracellular recordings were made from 27 somatosensory thalamic reticular (S-TR) neurons in urethane-anesthetized rats. Burst discharges, composed of 2-7 spikes, were always superposed on a slow depolarization (SD) lasting for 40-60 ms, which appeared only when the membrane was hyperpolarized. The number of spikes superposed on an SD varied depending upon the amplitude of the SD. A single shock stimulation of the lemniscus medialis elicited a series of SDs, each without being preceded by a phasic hyperpolarizing potential. The SDs were repeated with spindle rhythms. Evidence has been provided that EPSPs contribute to the mechanism for triggering SDs. In spontaneous rhythmic SDs occurring with the rhythm of EEG spindles, steps representing EPSPs were recordable on the rising phase of each SD. It is suggested that excitatory synaptic inputs to S-TR neurons with the spindle rhythm are responsible for the rhythmic generation of SDs. Ventrobasal relay neurons are presumed as the source of the inputs.

Burst discharges associated with phasic hyperpolarizing oscillations of rat ventrobasal relay neurons.

Intracellular recordings were made from ventrobasal relay neurons in urethane-anesthetized rats. A series of phasic hyperpolarizations repeated with the spindle rhythm appeared in response to single shocks to the medial lemniscus or spontaneously. On the recovery slope of some phasic hyperpolarizations slow depolarizations (SDs) lasting for 30-50 ms with burst discharges were generated as rebound excitation. The voltage dependency of SDs was proved by changing the membrane potential by current injection. The number of spikes triggered by the SD increased as the SD became larger in amplitude and faster in rising speed.

Two types of thalamic reticular cells in relation to the two visual thalamocortical systems in the rat.

We found in urethane-anesthetized rats that thalamic reticular (TR) cells responding to an electrical stimulus of the optic tract (OT) can be further subdivided into two types, viz. S- and L-type cells. S-type cells, which were selectively excited from area 17 of the visual cortex, were characterized by short latency responses (2.3-6.1 ms) to OT stimulation. TR cells activated antidromically from the dorsal lateral geniculate nucleus were all classified as S-type. Long OT latencies (5.2-15.3 ms) and selective excitation from area 18a were peculiar to L-type cells, which showed antidromic responses to the lateral posterior nucleus stimulation. Mapping studies documented that cells belonging to each type were segregated in the thalamic reticular nucleus; L-type cells were located in the most posterior part. It is suggested that S- and L-type cells are inhibitory interneurons modulating activity of geniculocortical and extrageniculocortical projection cells, respectively.

Neuronal organization of rat thalamus for processing information of vibrissal movements.

Vibrissa-responding neurons were searched for in the somatosensory part of the thalamic reticular nucleus (S-TR) and in the ventrobasal nucleus (VB) in urethane-anesthetized rats. More than 90% of the recorded neurons of both species had receptive fields (RFs) on single vibrissae. Movements of RF-vibrissae produced a burst of multiple discharges in S-TR neurons and single spike discharges followed by a prominent suppression of spontaneous discharges in VB neurons. Antidromic invasion from stimulation of the somatosensory cortex in VB neurons was suppressed after RF-vibrissae were stimulated. A possible functional organization comprising VB and S-TR neurons for processing impulses of vibrissal movements was suggested.

Retinal inputs to the geniculate relay cells in the eastern chipmunk (Tamias sibiricus asiaticus): a comparison between color and non-color sensitive cells.

Single unit recordings were made from the relay cells of the lateral geniculate nucleus in the eastern chipmunk. Of 362 relay cells, 47 cells (13%) were classified as color sensitive and the rest as non-color sensitive cells. Non-color sensitive cells were further classified into 5 subclasses: off-phasic, on-phasic, on-off-phasic, on-tonic and uncommon types. Within the color sensitive cells there were 3 subclasses; blue excited and green inhibited (+B-G), blue inhibited and green excited (-B+G), and blue excited (+B) cells. Retinal afferents to color sensitive relay cells had the following characteristics: ganglion cells of their origin were distributed in the central high density areas of the retina and axonal conduction velocities were in the intermediate range, though they were somewhat slow in +B cells.

Does the ascending cholinergic projection inhibit or excite neurons in the rat thalamic reticular nucleus?

In rats anesthetized with urethan, neuronal activity was recorded in those portions of the thalamic reticular nucleus (TR) excitable by visual, somatosensory, or auditory input. Observations were made on changes in rate and pattern of discharge of these neurons during repetitive stimulation of the laterodorsal tegmental nucleus (LDT), which is composed of cholinergic neurons projecting to the thalamus. In general, TR neurons showed spontaneous activity consisting of sporadic bursts of several spikes and responded to sensory stimulation with bursts of spikes which repeated several times. Weak LDT stimulation depressed or eliminated the occurrence of both spontaneous and evoked burst discharges. When LDT stimulation was sufficiently strong, however, the majority of TR neurons resumed their tonic discharges. In some animals the cortical EEG was recorded simultaneously with unit recording in TR. Suppression of burst discharges in TR was obtained even with LDT stimulation weaker than the threshold for desynchronizing the EEG. The induction of tonic discharge, on the other hand, required stimulation strong enough to produce desynchronization. The effects of LDT stimulation, such as the suppression of bursts and the induction of tonic discharge, were mimicked by acetylcholine and were antagonized by scopolamine, both drugs being applied ionophoretically. Cooling of the visual cortex abolished LDT-induced tonic discharges of visual TR neurons. A recent report and our preliminary observation show that, when the resting potential is relatively hyperpolarized, TR neurons generated a burst of spikes superposed on a low-threshold broad spike, which is inactivated and replaced with tonic firing by depolarization. Supported by these facts, the present results suggest that cholinergic input depolarizes TR neurons directly and that further depolarization occurs indirectly via activated cortex when the LDT stimulation is sufficiently strong to desynchronize EEG.

Somatotopic organization in the rat thalamic reticular nucleus.

Mapping experiments were carried out to establish the somatotopic organization of the somatosensory part of the thalamic reticular nucleus (TR) of the rat. Different parts of the body were found to project somatotopically onto the S-TR. The rostral-to-caudal and the dorsal-to-ventral axes in the body parts were transformed into the ventral-to-dorsal and the caudal-to-rostral axes in the S-TR, respectively. The head and face occupied about two thirds of the S-TR, distributing in the ventral half and in the dorsocaudal part. Particularly a large area of the S-TR was devoted to the vibrissae, nose (rhinarium) and lip. The trunk was projected to a small area of the dorsal part. The projections of the hind- and forelimb were mainly in the dorsal part, the former being placed above the latter.

Auditory neurons in the rat thalamic reticular nucleus.

In the thalamic reticular nucleus (TR) of the rat a cluster of neurons has been located which receives auditory inputs and acts as a source of inhibition for relay neurons of the medial geniculate nucleus (MG). These TR neurons (auditory thalamic reticular neurons; A-TR neurons) showed a repetitive burst of grouped discharge upon electrical stimulation of the inferior colliculus (IC) or of the auditory cortex. Many of them responded to tonal stimuli such as clicks or pips. Adjacent to the cluster of A-TR neurons there were the cluster of TR neurons receiving visual inputs (V-TR neurons) and that receiving somatosensory inputs (S-TR neurons). The cluster of A-TR neurons was situated ventrally to the cluster of V-TR neurons, both extending caudally from the level of the rostral tip of the dorsal lateral geniculate nucleus. The S-TR neurons distributed rostrally to the clusters of A- and V-TR neurons. Some of the sensory TR neurons, usually found around the boundaries between the clusters of different sensory modalities, were activated from stimulation of different central sensory pathways. Single electric shocks directly applied to the cluster of A-TR neurons suppressed discharges of relay neurons of the MG, either spontaneous or evoked by click stimuli or by electric shocks to the IC. The postexcitatory suppression of MG relay neurons was similar in time course to the suppression following electrical stimulation of A-TR neurons. Response latencies of the A-TR neurons to IC shocks were found to be 1.0-1.5 ms longer than those of the MG relay cells with respect to the modal and shortest values. It is suggested that A-TR neurons are intercalated in the axon collateral circuit of the thalamocortical projection arising from relay neurons of the MG.

Receptive-field properties of cells in the dorsal part of the albino rat's lateral geniculate nucleus.

Receptive-field properties of 273 relay (principal, P-) cells of the dorsal lateral geniculate nucleus (LGd) were studied in urethane-anesthetized albino rats, in an attempt to see if there is some relation between the visual property and the conduction velocity of afferent optic nerve fibers. According to properties of the receptive-field center, P-cells were classified into two types, common (89%) and uncommon (11%). The common type consists of OFF-phasic, ON-phasic, ON-tonic and ON-OFF-phasic cells, while the uncommon type includes ON-inhibited, moving-sensitive, ON-OFF-inhibited, simple-cell-like and complex-cell-like cells. The mean response latency to single optic chiasm shocks increases in the order of OFF-phasic (1.94 msec), ON-phasic (2.35 msec), ON-tonic (2.87 msec), ON-OFF-phasic cells (3.04 msec) and uncommon type (3.18 msec). The mean size of the receptive-field center in each of the four common types was smaller than that in the uncommon type; 6 degrees--7 degrees vs. 11 degrees. From responsiveness to moving light spots with speeds faster than 25 degrees--30 degrees/sec, P-cells of the common type were divided into the fast- and slow-movement-sensitive cells. The ratio of occurrences of fast- to slow-movement-sensitive cells decreases in the order of the OFF-phasic (2.7), ON-phasic (2.4), ON-tonic (1.1) and ON-OFF-phasic types (0.06). The optic chiasm latencies were shorter than 2.5 msec in most of the fast-movement-sensitive cells while the reverse was true for most of the slow-movement-sensitive cells. From these findings discussions were made to point out that the rat LGd mainly consists of Y- and W-like P-cells and that the Y/W dichotomy of P-cells approximately corresponds to the previously established fast/slow classification.

Disinhibition of perigeniculate reticular neurons following chronic ablation of the visual cortex in rats.

Effects of chronic ablation of the visual cortex (VC) were studied in the perigeniculate reticular neurons (PGR neurones) which were located in the thalamic reticular nucleus immediately adjacent to the dorsal nucleus of the lateral geniculate body and identified as the I-cells of Burke and Sefton. In rats with the intact VC the PGR neurons responded to single shock stimulation of the optic tract (OT) with bursts of spike spaced regularly. During the inter-burst period the neurons were inhibited, indicating that except for the primary spike burst, others were postinhibitory rebound excitation. In the VC-ablated PGR neurons there were no changes in the primary spike burst, but the remaining ones were very weak or sometimes missing, suggesting that the inhibition was poorly developed. With double shock stimulation of OT it was established that after showing the primary spike burst, the VC-ablated PGR neurons suffered a less intense inhibition than control. To a diffuse, sustained illumination, the normal PGR neurons showed on- and off- responses, whereas the VC-ablated ones were tonically activated during the presence of illumination. These findings were taken as indicating that the inhibitory mechanism for the PGR neurons were made less active after the VC had been ablated chronically.

Optic nerve innervation of so-called interneurons of the rat lateral geniculate body.

In urethane-anesthetized rats, neurons responding to electrical stimulation of the optic nerve (ON) with bursts of spikes were searched for in the region of the thalamic reticular nucleus close to the dorsal nucleus of the lateral geniculate body (LGBd). These neurons were identical with those which had been presumed to be inhibitory internerons (I-cells) of LGBd. Conduction velocityes of ON fibers innervating I-cells were determined by measuring differences in response latency between stimulations of two separate sites along the contralateral ON. The velocity ranged from 1.9 to 7.3 m/sec with an average of 4.6 m/sec, indicating that among the three groups of ON fibers with different velocities, only the slowest group is involved in activation of I-Cells. Calculation of synaptic delays revealed that one group of I-cells was excited monosynaptically and another, disynaptically. Experiments on rats with the visual cortex chronically ablated provided evidence that the disynaptically excited I-cells received ON impulses via axon collaterals of principal cells of LGBd. The region of the thalamic reticular nucleus containing I-cells was found to receive inputs not only from the contralateral but also from ipsilateral ON.