Characterization of tyrosine hydroxylase‐ and non‐tyrosine hydroxylase‐immunoreactive sexually dimorphic oxytocin receptor‐expressing neurons in the anteroventral periventricular nucleus of female mice

Oxytocin is involved in the regulation of maternal behavior by binding to the oxytocin receptor (OXTR) in various parts of the brain. Our previous studies demonstrated that OXTRs are specifically expressed in the anteroventral periventricular nucleus (AVPV) of female mice, but not in male mice. Furthermore, the activity of the OXTR neurons is essential for proper expression of maternal behavior. The present study aimed to characterize two different populations of OXTR neurons found in the AVPV in the previous study: tyrosine hydroxylase immunoreactive (TH⁺) and non-TH immunoreactive (TH^-) neurons. Whole-cell patch clamp recordings were used to observe the intrinsic electrophysiological properties of the OXTR neurons. TH⁺ neurons displayed a pacemaker-like intrinsic rhythmic short bursting activity, whereas TH^- neurons displayed either no firing at all, irregular firing, or phasic firing. Some TH^- OXTR neurons could switch back and forth among these firing patterns. The differences in the firing patterns between these two populations were likely derived from the difference in their expression of afterpotentials. TH⁺ OXTR neurons showed more depolarizing afterpotential (DAP) than after-hyperpolarization (AHP), while TH^- OXTR neurons exhibited more AHP than DAP. Activation of OXTR by a specific agonist caused a steady state depolarization and increase in Ca²⁺ transient resulting in changes in the firing activity in both TH⁺ and TH^- neurons. Lastly, biocytin was injected into the OXTR neurons during the whole-cell recordings to visualize the recorded neurons for immuno-identification of neuron type and morphological analysis. TH^- neurons displayed significantly more dendritic arborization than TH⁺ neurons. Therefore, TH⁺ and TH^- neurons are electrophysiologically and morphologically distinct. Moreover, because activation of OXTR caused a change in the firing activity of these neurons, oxytocin likely modulates the firing activity of both TH⁺ and TH^- OXTR neurons to influence maternal behavior.