The organization of the visual field representation within the thalamic reticular nucleus (TRN) of the rabbit was studied. Focal injections of horseradish peroxidase (HRP) and/or [3H]proline were made into visuocortical areas V1 and V2 and the dorsal lateral geniculate nucleus (dLGN). The resultant labelling in the thalamus was analysed. A single injection in V1 or V2 results in a single zone of terminal label within the TRN that is restricted to the dorsocaudal part of the sheet-like nucleus. In comparisons of the zones of label following injections at two different cortical sites in V1, a medial to lateral shift in label across the thickness of the TRN sheet is accompanied by a medial to lateral shift in label in the dLGN; a dorsal to ventral shift in label within the plane of the TRN sheet is accompanied by a dorsal to ventral shift in label in the dLGN. Thus, like the dLGN the TRN receives a precise topographic projection from V1. In reconstructions from horizontal sections the zones of label within the TRN resemble 'slabs', which lie within the plane of the nucleus parallel to its borders. Thus, the slabs of visuocortical terminals and reticular dendrites are similarly oriented. As revealed by the orientation of the slabs, the lines of projection representing points in visual space are represented by the oblique rostrocaudal dimension of the TRN. Injections restricted to V1 produce terminal labelling that is confined to the outer two-thirds of the TRN across its thickness, whilst those involving V2 result in terminal labelling within the inner one-third of the nucleus. Thus, the adjacent cortical areas V1 and V2 project in a continuous fashion across the mediolateral dimension of the TRN. The organization of the map within the TRN, which was revealed by visuocortical injections, was confirmed by the pattern of retrograde labelling within the nucleus following geniculate injections of HRP. On the basis of these findings and those in other mammalian species, two major conclusions can be reached that alter our view of the TRN. First, rather than mapping onto the whole nucleus in a continuous fashion, the cortical projection to the TRN has significant discontinuities. Second, rather than integrating efferents from widespread cortical areas, the reticular dendrites are related to focal areas of cortex.

Transmission through the dorsal thalamus involves nuclei that convey different aspects of sensory or motor information. Cells in the dorsal thalamus are strongly inhibited by the GABAergic cells of the thalamic reticular nucleus (TRN). Here we show that stimulation of cells in specific dorsal thalamic nuclei evokes robust IPSCs or IPSPs in other specific dorsal thalamic nuclei and vice versa. These IPSCs are GABA(A) receptor-mediated currents and are consistent with the activation of disynaptic intrathalamic pathways mediated by TRN. Thus, cells engaged in sensory analyses in the ventrobasal complex or the medial division of the posterior complex can interact with cells responsive to sensory events in the caudal intralaminar nuclei, whereas cells engaged in motor analyses in the ventrolateral nucleus can interact with cells responsive to motor events in the rostral intralaminar nuclei. Furthermore, sensory event-related cells in the caudal intralaminar nuclei can interact with motor event-related cells in the rostral intralaminar nuclei. In addition, single cells in one dorsal thalamic nucleus can receive convergent inhibitory inputs after stimulation of cells in two or more other dorsal thalamic nuclei, and TRN-mediated inhibitory inputs can momentarily switch off tonic firing of action potentials in dorsal thalamic cells. Our findings provide the first direct evidence for a rich network of intrathalamic pathways that allows modality-related and cross-modality inhibitory modulation between dorsal thalamic nuclei. Moreover, TRN-mediated switching between dorsal thalamic nuclei could provide a mechanism for the selection of competing transmissions of sensory and/or motor information through the dorsal thalamus.

Transmission of sensory information through the dorsal thalamus involves two types of modality-related nuclei, first order and higher order, between which there are thought to be no intrathalamic interactions. We now show that within the somatosensory thalamus, cells in one nucleus, the ventrobasal complex, can influence activity in another nucleus, the medial division of the posterior complex. Stimulation of ventrobasal complex cells evoked inhibitory postsynaptic currents in cells of the medial division of the posterior complex. These currents exhibited the reversal potential and pharmacology of a GABAA receptor-mediated chloride conductance, indicating that they result from the activation of a disynaptic pathway involving the GABAergic cells of the thalamic reticular nucleus. These findings provide the first direct evidence for intrathalamic interactions between dorsal thalamic nuclei.

The activity of the GABAergic neurons of the thalamic reticular nucleus (TRN) has long been known to play important roles in modulating the flow of information through the thalamus and in generating changes in thalamic activity during transitions from wakefulness to sleep. Recently, technological advances have considerably expanded our understanding of the functional organization of TRN. These have identified an impressive array of functionally distinct subnetworks in TRN that participate in sensory, motor, and/or cognitive processes through their different functional connections with thalamic projection neurons. Accordingly, “first order” projection neurons receive “driver” inputs from subcortical sources and are usually connected to a densely distributed TRN subnetwork composed of multiple elongated neural clusters that are topographically organized and incorporate spatially corresponding electrically connected neurons—first order projection neurons are also connected to TRN subnetworks exhibiting different state-dependent activity profiles. “Higher order” projection neurons receive driver inputs from cortical layer 5 and are mainly connected to a densely distributed TRN subnetwork composed of multiple broad neural clusters that are non-topographically organized and incorporate spatially corresponding electrically connected neurons. And projection neurons receiving “driver-like” inputs from the superior colliculus or basal ganglia are connected to TRN subnetworks composed of either elongated or broad neural clusters. Furthermore, TRN subnetworks that mediate interactions among neurons within groups of thalamic nuclei are connected to all three types of thalamic projection neurons. In addition, several TRN subnetworks mediate various bottom-up, top-down, and internuclear attentional processes: some bottom-up and top-down attentional mechanisms are specifically related to first order projection neurons whereas internuclear attentional mechanisms engage all three types of projection neurons. The TRN subnetworks formed by elongated and broad neural clusters may act as templates to guide the operations of the TRN subnetworks related to attentional processes. In this review article, the evidence revealing the functional TRN subnetworks will be evaluated and will be discussed in relation to the functions of the various sensory and motor thalamic nuclei with which these subnetworks are connected.