Increased sexual behavior in the male rat following lesions in the mammillary region

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Abstract
Copulatory output of the mature male rat was measured over an eight week period under conditions of each male having free access to a constant stimulus female.Of 71 males tested 31 of these (44%) had to be discarded as they did not reach our arbitrary minimum criterion of producing copulation plugs on 4 out of 14 nights during the initial (control) observation period. An extreme individual variability in sex drive was noted among the animals used for further testing.Employing equal sized groups of about ten animals and using a two week interval as the basic unit of time; the mean sexual activity for the group measured as number of copulation plugs found and mean percentage of nights on which copulation occurred indicated an extreme similarity of all groups. Maximum sexual response occurred during the initial 14 day test period and both measures of activity used (copulation plugs and days on which copulation occurred) declined by approximately 50% over the eight week test period.A similar 50% decline in response occurred following lesions in the middle hypothalamus dorsal to the arcuate nucleus or lesions dorsal to the hypothalamus involving the habenula and surrounding cortex. It was concluded these areas are not involved in regulation of copulatory behavior in the male rat.Lesions in the mammillary region (mostly medial) resulted in 50% of the animals showing a sustained constant copulatory output during the eight week test, while the remaining 50% (mostly with lateral lesions) showed marked increases which expressed as the mean of the group amounted to 90% copulation plugs and 50% in nights on which copulation occurred.Thus it was concluded that structures in the mammillary region at the junction of the diencephalon and mesencephalon normally exert a regulatory (inhibitory) influence on the sex drive in the male rat.

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