Properties of antidromically identified neurons in the enkephalinergic magnocellular dorsal nucleus of the guinea pig hypothalamus

Abstract

In the guinea pig, immunocytochemical and neuroanatomical studies have demonstrated that enkephalin-containing neurons in the hypothalamic magnocellular dorsal nucleus (MDN) terminate in the lateral septum (LS). In the present investigation, 114 MDN neurons, studied with extracellular recording techniques, were identified by antidromic activation from the LS. Latencies of responses from ipsilateral and contralateral LS were 13.5 and 18.78 ms, respectively, corresponding to an axonal conduction velocity of 0.1 m/s. By using the reciprocal collision test, evidence is presented for bilateral projection of individual MDN neurons to the LS. Fifty-one (44.73%) MDN-LS neurons discharged in a slow irregular pattern. Interspike time histograms were very similar and had a mode of about 280 ms. Peristimulus time histograms were compiled from 15 active MDN-LS neurons. Stimulation which elicited antidromic spikes resulted in a brief silent period in the spontaneous activity which was related to the normal interspike interval pattern of the firing. Prolonged silent periods as well as silent period occurring after subthreshold stimulus and increasing with the stimulus intensity were attributed to inhibitory synaptic effects. On the other hand, some MDN-LS neurons displayed orthodromic excitatory responses following LS stimulation. These observations provide electrophysiological evidence of a direct MDN-LS pathway, in all likelihood of enkephalinergic nature, and indicate that some MDN-LS neurons receive inhibitory and excitatory afferents from the LS.