Branched output neurons of the rat subthalamic nucleus: electrophysiological study of the synaptic effects on identified cells in the two main target nuclei, the entopeduncular nucleus and the substantia nigra.

The synaptic responses of entopeduncular and nigral cells to subthalamic stimulation were studied with extracellular recording techniques in rats with and without chronic lesions. Entopeduncular output cells were identified by antidromic activation from the lateral habenula, ventral anterior thalamic nucleus and tegmenti pedunculopontine nucleus. Nigral cells projecting to superior colliculus were identified by antidromic discharge. Stimulation of the subthalamic nucleus produced a short latency suppression of spontaneous activity (10-60 ms duration) of 89% of the entopeduncular cells tested in chronically lesioned rats. Of these cells, 50% were identified as projecting to lateral habenula. On the other hand, subthalamic nucleus stimulation produced a short latency excitation of 73% of the nigral cells tested (4.16 +/- 0.07 ms). Forty-eight percent of these cells projected to superior colliculus. The subthalamic fibres which terminate in entopeduncular nucleus and substantia nigra, come from the same neuronal population since the majority, if not all, rat subthalamic neurones send branched projections to both these nuclei. Therefore, the two different types of responses recorded in these nuclei are elicited by the activation of a single neuronal population. This dual effect could be easily explained if one of the responses is mediated by local interneurones. If not, the same transmitter induces the two responses. The entopeduncular nucleus and substantia nigra which are the main target nuclei of the subthalamic nucleus, are also the only known outputs of the striatum. The subthalamic efferent cells could thus modulate the activity of the entire striatal descending output. It is noteworthy that this subthalamic control is different in entopeduncular nucleus than in substantia nigra.