Extracellular single-unit studies of suprachiasmatic neurons in brain slices. Effects of serotonin, dopamine, carbachol and LHRH.

Suprachiasmatic nucleus (SCN) is the major rhythm generating center in the central nervous system. Extracellular single-unit activities of the SCN neurons were recorded in brain tissue slices either from ovariectomized (OVX) rats or from OVX rats treated with estrogen (polyestradiol phosphate, PEP, 0.1 mg/rat, sc). Neuronal responses to luteinizing hormone-releasing hormone (LHRH) and various neurotransmitters, i.e., serotonin (5-HT), dopamine (DA) and carbachol (Carb, an acetylcholine agonist) were tested. A total of 137 units were recorded from OVX and 81 from OVX + PEP rats. The steady state firing rates of SCN neurons were between 0-9 spikes/sec in OVX and 0-23 in OVX + PEP group. Among them, most fired irregularly (86.9% in OVX and 71.6% in OVX + PEP rats), and a few fired regularly (10.2% and 25.9%, respectively). Few silent units were detected (2.9% and 2.5%, respectively). It seems that estrogen treatment somewhat increased the percentage of regular firing units. As for the agents tested in this study, LHRH by itself had no significant effect on the SCN neuronal activity; 5-HT affected more than 40% of the units, with about equal number of excitation (16.8%) and inhibition (24.4%), while Carb inhibited nearly half of the units tested (49.5%), excited only a few (10.3%). All the actions of these three agents were not significantly affected by in vivo estrogen pretreatment. Dopamine, however, excited more units (50%) in OVX than in OVX + PEP group (27.1%). Although LHRH had no direct effect by itself, it exerted a significant modulatory role on the actions of the other three agents. It either potentiated (42.9%) or inhibited (53.6%) the action of 5-HT, and it significantly potentiated the action of Carb (57.1%). For the action of DA, LHRH had a more significant potentiation effect on OVX + PEP than in OVX rats. (40.9% vs. 12.5%), while the inhibitory effect was the same. In conclusion, 5-HT, DA and Carb all exert significant effects on SCN neuronal activities. LHRH can have a modulatory role upon the action of 5-HT, DA and Carb, and estrogen pretreatment specifically affects that of DA in the SCN.