Estrogen-dependent and estrogen-independent effects of progesterone on the electrophysiological excitability of dorsal midbrain neurons in golden hamsters.

During the induction of lordosis responses in golden hamsters, the joint actions of estradiol (E) and progesterone (P) have been found to produce major changes in the activity levels, sensory responsiveness and movement-related firing of neurons in the dorsal midbrain, a region vital for lordosis in this species. The present study investigated the possibility that these effects of E and P on dorsal midbrain neurons might arise through changes in transsynaptic excitability or spike-generating processes. Single dorsal midbrain neurons were orthodromically or antidromically activated by ventromedial midbrain tegmental stimulation in ovariectomized, urethane-anesthetized hamsters before and after subcutaneous injection of P in propylene glycol vehicle. P injection produced two neurophysiological effects: (1) a strong depression of transsynaptic activation, and (2) in antidromically invaded neurons, a change in the amplitude (either an increase or decrease) of the soma-dendritic spike component. Both of these effects appeared rapidly (i.e. within 10-20 min) after P injection. The P effect on orthodromic neuronal excitability was contingent upon E priming of the hamsters, but, in contrast, the P effect on antidromic spike amplitude was not. Several features of these neurophysiological effects of P imply that the hormone may have been acting through nongenomic mechanisms, possibly involving receptors in neuronal membranes.