The posterodorsal medial amygdala (MePD) is an area of the brain that integrates chemosensory inputs from the environment, hormonal signals and sociobehavioural cues to mediate various sexual behaviours in males and females. To better understand MePD neuronal function, Maria Elisa Calcagnotto, Alberto Rasia-Filho and Francine Dalpian (Dalpian et al., 2019) now characterise its electrophysiological and functional properties in rats. Their analysis identifies two subpopulations of MePD neurons, which they term Class I and Class II. Class I neurons are largely bi-tufted cells that show a reduced firing rate and irregular spikes when depolarised, with currents between 20 and 100 pA. In contrast, Class II neurons are mostly stellate cells that fire a maximum of two action potentials upon depolarisation with 20–100 pA currents. Interestingly, Class I neurons show increased firing frequency in the left hemispheric MePD cells of male rats, an observation likely explained by the finding that left hemispheric MePD neurons in males receive more excitatory input than females. Moreover, excitatory input in females is increased during dioestrus, the period of sexual unreceptiveness during the female reproductive cycle. Male MePD neurons in the right hemisphere additionally obtain less inhibitory synaptic input than female MePD neurons, and inhibitory synaptic input in females is elevated during pro-oestrus, the time of ovarian follicle maturation. Together, these data demonstrate the presence of two neuronal subpopulations in the MePD, and indicate important differences in the properties and stimulation of MePD neurons, which vary depending on sex, hemispheric localisation and stage of the oestrous cycle.
