Distribution and binding parameters of GABA and benzodiazepine receptors in the cat motor thalamus and adjacent nuclear groups.

Quantitative receptor binding autoradiography technique was utilized to study GABA and benzodiazepine receptors in the cat motor thalamus (ventral anterior, ventral medial and ventral lateral nuclei) and adjacent thalamic subdivisions. Binding parameters (Bmax and Kd) and distribution pattern of the binding sites for 3 tritiated ligands [3H]muscimol ([3H]MUS), [3H]flunitrazepam ([3H]FLU) and [3H]baclofen ([3H]BAC) were analyzed and compared using measurements from discrete and anatomically well-defined thalamic regions. There was little correlation in the regional distribution of the 3 binding sites. The concentration of [3H]BAC binding sites in thalamic nuclei of interest was very low, practically at the limit of resolution of the quantitative autoradiographic technique; whereas appreciable quantities of [3H]MUS and [3H]FLU binding sites were present in the motor and adjacent limbic nuclei of the thalamus. There was more difference between the nuclei in regard to the number of high affinity GABA receptors than benzodiazepine receptors. Moreover, the ratio of Bmax[3H]MUS/Bmax[3H]FLU varied from 2.2 to 4.4 in different thalamic regions suggesting the presence of a diverse population of GABAA and benzodiazepine receptors. The distribution pattern of the 3 binding sites was compared to the topography of GABAergic afferents of the basal ganglia origin and the frequency of GABAergic synapses formed by thalamic local circuit neurons (LCN) in the motor thalamus that were established earlier. It was concluded that in the cat motor thalamus: (1) none of the ligands studied appear to reveal the receptors associated with nigro- or pallidothalamic synapses; (2) [3H]MUS binding sites may be associated with the dendrodendritic contacts formed by LCN; and (3) the [3H]FLU binding sites are physically unrelated to [3H]MUS binding sites. The concentration of [3H]FLU and [3H]MUS binding sites in the midline nuclei and of [3H]MUS binding sites in the limbic nuclei was remarkably high. It was concluded that in addition to previously suggested limbic structures, the midline nuclei with their very high content of benzodiazepine receptors may be considered as a neuroanatomical substrate of certain forms of anxiety.