Abstract
Neural signals necessary for gaze holding are produced in the excitatory networks of oculomotor neural integrators including the prepositus hypoglossi nucleus (PHN) and the interstitial nucleus of Cajal (INC). Our previous studies have shown that the activation of the networks can be evaluated by sustained excitatory postsynaptic current (EPSC) responses in vitro, in which a higher EPSC frequency after burst stimulation (100 Hz, 20 trains) than the frequency before the stimulation lasts for >1 s. Both the PHN and the INC receive serotonergic inputs mainly from the dorsal raphe nucleus, and serotonin (5-HT) induces depolarizing responses via 5-HT(2) or 5-HT(3) receptors and hyperpolarizing responses via 5-HT(1A) receptors in PHN and INC neurons. However, how 5-HT affects sustained EPSC responses remains unknown. In this study, we investigated the effects of 5-HT on sustained EPSC responses using whole-cell recordings in brainstem slices obtained from rats of either sex. Compared with the control treatment, bath application of 10 μM 5-HT significantly reduced the duration and frequency of the EPSC responses in the PHN and the INC. The application of 8-OH-DPAT, an agonist of the 5-HT(1A) receptor, suppressed sustained EPSC responses, but agonists of the 5-HT(2) and 5-HT(3) receptors had no effect on the responses, indicating that 5-HT has a suppressive effect on sustained EPSC responses via 5-HT(1A) receptors. These results suggest that neurons that express 5-HT(1A) receptors participate in excitatory networks and that the suppressive effect of 5-HT can facilitate exploratory behavior through eye movements rather than gaze holding.