Neuro-Correlate of Limbic-Hypothalamo-Pituitary-Gonadal Axis in the Rat: Change in Limbic-Hypothalamic Unit Activity Induced by Vaginal and Electrical Stimulation
The present study deals with the effect of vaginal stimulation (VS) with a glass rod on unit activity in the brain and the correlation of the activity of each area under certain hormonal conditions. The frequency characteristics of the neuronal discharges in the brain stem, limbic structures, and hypothalamus were recorded, using stereotaxically oriented steel micro-electrodes, in adult female rats under light urethane anesthesia. The following results were obtained: (1) The level of ‘spontaneous’ activity in the anterior hypothalamic, preoptic, and septal areas of estrogen-primed spayed rats was higher than that observed in ovariectomized, non-estrogen-primed rats. (2) Significant and discriminated responses induced by VS were observed only in the estrous state, that is, after estrogen priming in the case of the ovariectomized animals. (3) These responses were almost independent of the EEG activity level, and could be discriminated from the response to other peripheral stimuli such as pinching. (4) The firing rate of hypothalamic neurones, depending on their locations, increased or decreased during and only immediately after VS (‘phasic response’), by means of which the EEG after-reaction was induced. Reciprocal changes were observed between the periventricular arcuate nucleus (ARC) (facilitated) and ventromedial hypothalamus (VMH) (inhibited), and between the basal and medial parts of the anterior hypothalamic area (AHA) (inhibited) and the medial preoptic area (MPO) (facilitated). (5) Neurone activity in the ventromedial part of the midbrain reticular formation (MRF), in the dorsal hippocampus (DHPC), in the medial complex of the amygdala (AMYG) and lateral septum, was enhanced during and for some minutes after VS (‘tonic response’), with the appearance of the EEG after-reaction. (6) The MRF may play a significant role in discriminating vaginal impulses from other peripheral sensory afferents. Interrelations of the unit activity in the above portions of the brain in estrogen-primed rats were examined by electrical stimulation and the results were as follows: (1) The VMH and ARC nuclei were reciprocally interrelated, and there exists an inhibitory pathway between them that may subserve changes in functional activity. (2) Electrical stimulation of the MPO and the AHA decreased unit activity of the VMH and increased that in the ARC. (3) The cornu ammonis of DHPC and the medial complex of the AMYG had a reciprocal relationship with the hypothalamus. In other words, the DHPC increased AHA activity and decreased MPO activity, while the AMYG influenced these nuclei conversely. Similarly, the DHPC increased and the AMYG decreased unit discharge in the ARC, while they exerted a reverse influence on the VMH activity. It is therefore suggested that the DHPC mainly activates the AHA-ARC circuit and the medial AMYG activates the MPO-ARC circuit, while both the hippocampus and amygdala may participate in different functional reorganizations activated by responses to VS.