Ultrastructural relationships of serotonin and GABA terminals in the rat suprachiasmatic nucleus. Evidence for a close interconnection between the two afferent systems

Abstract

Serotonin (5-HT) and γ-aminobutyric acid (GABA) nerve endings were identified in the rat suprachiasmatic nucleus (SCN) by combined [³H]5-HT uptake radioautography and glutamate decarboxylase (GAD) immunocytochemistry at the electron microscope level. In areas of good overlap between radioautographic and immunocytochemical labellings, there were no axonal varicosities exhibiting both labellings, indicating that 5-HT and GABA are not co-localized in the SCN. The systematic survey in these areas of all profiles that had accumulated [³H]5-HT and of all GAD-immunoreactive varicosities allowed the analysis of 247 of the former and 896, i.e. an almost four-fold greater number, of the latter. This seems concordant with the view that GABA endings would be the most numerous of all classes of nerve terminals so far identified in the SCN. More than 22% of the [³H]5-HT labelled profiles showed the membrane specialization typically associated with synap'tic junctions. Thereby, it was possible to evaluate that about 45% of the 5-HT terminals actually form a synapse in the SCN. Some 37% of the GAD-positive varicose profiles which could be formally interpreted also showed well differentiated synaptic contacts, suggesting that the GABAergic innervation of the SCN could be entirely junctional. Whereas 5-HT terminals usually innervated only one dendritic or somatic element, a convergence of several GABAergic terminals onto the same postsynaptic target also receiving a 5-HT input was frequently observed. Of all the [³H]5-HT labelled varicose profiles scanned, as much as 41% were directly apposed to at least one GAD-immunoreactive profile, indicating that these 5-HT/GABA axonal interfaces could well represent privileged sites of interactions between the two transmitters. Taken together, these data could be of potential value in determining the neurochemical mechanisms subserving cellular integration of rhythmic signals in the SCN.