ANATOMICAL EVIDENCE FOR A MECHANISM OF LATERAL INHIBITION IN THE RAT THALAMUS

The aim of this study was to determine whether or not thalamic reticul ar nucleus (Rt) neurons form synaptic connections with the thalamocort ical (TC) neurons from which they receive synaptic contacts. Therefore , we examined, in adult rats, the relationships between single TC and Rt neurons, which had been marked simultaneously with an anterograde/r etrograde tracer (biocytin or Neurobiotin), using the extracellular or juxtacellular technique. (i) From 30 successful extracellular microap plications of marker into the Rt, 22 gave retrogradely marked TC somat odendritic arbors at the fringe of or clear outside the anterogradely darkly stained Rt axon terminal fields. Following biocytin application into the thalamus, few cells were retrogradely stained in the Rt at t he periphery of the anterogradely labelled axon terminal field. (ii) T he juxtacellular filling of a single Rt cell was accompanied by the ba ck-filling of a single TC neuron (n = 4 pairs), which presumably forme d synaptic contacts with the former cell. The somatodendritic complex of the back-filled TC neuron was located outside the Rt cell's axonal arbor. These anatomical data provide clear evidence that Rt and thalam ic neurons predominantly form between themselves open rather than clos ed loop connections. Because TC neurons make glutamatergic synapses on to Rt cells, which are GABAergic, and are the first elements synaptica lly activated by prethalamic afferents into the TC-Rt network, the pre sent results strongly support the hypothesis that Rt neurons principal ly generate a mechanism of lateral inhibition in the thalamus.