SOME ASPECTS OF THE SYNAPTIC CIRCUITRY UNDERLYING INHIBITION IN THE VENTROBASAL THALAMUS

We describe here, and review, the ultrastructural features and synapti c relationships of flat-vesicle containing, presumptively inhibitory p resynaptic elements in the glomerular and extraglomerular neuropils of the thalamic ventrobasal (VB) nucleus in monkey, cat and rat. This ac count is based on EM study of normal material, LM and EM immunocytoche mistry for GABA, anterograde tracing with HRP and EM of physiologicall y characterized interneurons intracellularly injected with HRP. It eme rges clearly from this study that attempts to categorize flat-vesicle containing terminals in thalamic tissue as either F-boutons (axon term inals with flattened synaptic vesicles and Gray type II synaptic speci alizations) or P-boutons (dendritic appendages of interneurons with fl attened vesicles) by examining only single sections are likely to prod uce unreliable results. In many cases it is only by studying serial se ctions that such profiles can be unambiguously identified. Within glom eruli the P-boutons participate in triplet (triadic) synapses which ar e thought to mediate rapid feed forward inhibition of projection cells , and serial synaptic arrays involving other P-boutons. Since P-bouton s from more than one interneuron are present in individual VB glomerul i, P-bouton to P-bouton synapses may mediate disinhibition of interneu rons. We show that dendritic shafts of interneurons make and receive s ynaptic contacts and that in the monkey, at least, reciprocal synaptic contacts between shafts or between a shaft and a P-bouton are not unc ommon. Finally, we confirm that in the rat VB there are insignificant numbers of P-boutons or cells with the morphological and transmitter c haracteristics of interneurons and we suggest that comparative electro physiological studies of inhibitory events in rat VB versus those in c at or monkey VB during transmission of somatosensory information might help to clarify the roles of thalamic intrinsic neurons.