Some aspects of the synaptic circuitry underlying inhibition in the ventrobasal thalamus.

We describe here, and review, the ultrastructural features and synaptic relationships of flat-vesicle containing, presumptively inhibitory presynaptic elements in the glomerular and extraglomerular neuropils of the thalamic ventrobasal (VB) nucleus in monkey, cat and rat. This account is based on EM study of normal material, LM and EM immunocytochemistry for GABA, anterograde tracing with HRP and EM of physiologically characterized interneurons intracellularly injected with HRP. It emerges clearly from this study that attempts to categorize flat-vesicle containing terminals in thalamic tissue as either F-boutons (axon terminals with flattened synaptic vesicles and Gray type II synaptic specializations) or P-boutons (dendritic appendages of interneurons with flattened vesicles) by examining only single sections are likely to produce unreliable results. In many cases it is only by studying serial sections that such profiles can be unambiguously identified. Within glomeruli the P-boutons participate in triplet (triadic) synapses which are thought to mediate rapid feed forward inhibition of projection cells, and serial synaptic arrays involving other P-boutons. Since P-boutons from more than one interneuron are present in individual VB glomeruli, P-bouton to P-bouton synapses may mediate disinhibition of interneurons. We show that dendritic shafts of interneurons make and receive synaptic contacts and that in the monkey, at least, reciprocal synaptic contacts between shafts or between a shaft and a P-bouton are not uncommon. Finally, we confirm that in the rat VB there are insignificant numbers of P-boutons or cells with the morphological and transmitter characteristics of interneurons and we suggest that comparative electrophysiological studies of inhibitory events in rat VB versus those in cat or monkey VB during transmission of somatosensory information might help to clarify the roles of thalamic intrinsic neurons.