Functional reconfiguration of midbrain neurons by ovarian steroids in behaving hamsters

Previous studies have demonstrated the importance of the midbrain, especially the tectum, in the sensorimotor control of the sexually-receptive posture, lordosis, in female golden hamsters. In the present study, midbrain single unit activity was recorded during hormonal induction of lordosis, to identify neuronal activity mediating the sensorimotor control of the response and to observe functional changes in these neurons associated with the hormones' behavioral effect. Progesterone administration to estrogen-primed hamsters initiated pronounced changes in neuronal activity levels, somatosensory responsiveness and movement-related firing. These changes began in some neurons within 10 min of subcutaneous hormone injection and affected progressively more neurons until lordosis was elicitable 2-4 hr later. The pattern of these changes, including increased neuronal responsiveness to lumbosacral stimuli, appearance of lordosis-related firing (especially in the tectum) and reduced incidence of firing associated with lordosis-incompatible behaviors, constituted a transformation, or reconfiguration of midbrain sensorimotor function. It is proposed that this reconfiguration enables the elicitation and maintenance of lordosis by lumbosacral stimuli. Neural effects of comparable magnitude didn't result from control progesterone injections which failed to induce lordosis.