Burst firing of oxytocin neurons in male rat hypothalamic slices

Burst firing and single spike activity play different roles in the modulation of local neuronal circuit activity and neurosecretion. In hypothalamic oxytocin (OT) neurons in vivo, burst firing is associated with pulsatile secretion of OT in the milk ejection reflex, and can be observed in slices from both immature and lactating rats in vitro. Whether OT neurons from male rats also possess burst firing capability is still an open question. To examine this possibility, whole-cell patch clamp recordings were made in supraoptic nucleus OT neurons in brain slices from male rats. In low Ca2+ medium, the α1-adrenoceptor agonist, phenylephrine evoked bursts that were highly similar to those from lactating rats in vivo and in vitro: explosive onset, short-duration, quickly reaching peak firing rate and displaying an exponential decay in returning to the pre-burst rate. During bursts, spike durations increased, and spike amplitudes decreased, while riding on an arc of depolarization around peak rate. In comparison to those from lactating rats in vitro, the rising phase of male bursts was more rapid, the decay phase was slower, and the rising phase of the spike after hyperpolarization was faster. No significant differences, however, were seen in burst characteristics that are most important in determining the amount of peptide release: burst amplitudes (the number of spikes in a burst), firing frequency within bursts or peak firing rate. Thus, we conclude that OT neurons in males are capable of burst firing highly similar to that seen in lactating rats.