The Ability of the Intracerebral Exposure to Progesterone on Consecutive Days to Facilitate Lordosis Behavior: An Interaction Between Progesterone and Estrogen

Abstract

To elucidate the action of progesterone (P) in the regulation of lordosis behavior, the effect of the repeated exposure of the mesencephalic reticular formation (MRF) or caudate-putamen (CPU) to P was determined in ovariectomized rats implanted 3 days earlier with estradiol benzoate (EB) in the preoptic-diagonal band area (POA). Crystalline P restricted to the lumens of 27 gauge cannulae markedly facilitated lordosis behavior when implanted into the MRF or CPU, in confirmation of previous studies. Similar cholesterol (C) implants were without effect. Both P and C were removed from the brain following behavioral testing; 24 h later, both groups received P implants (in the MRF or CPU as before). Lordosis behavior was facilitated only in those rats which had been exposed to C on the previous day. To determine if the local application of EB to the MRF or CPU could restore the ability of the 2nd application of P to facilitate lordosis, additional animals were subjected to the following test: lordosis behavior was facilitated by P exposure of either the MRF or CPU in spayed rats given an EB implant (POA) 3 days earlier. Following the behavioral test, however, the P cannulae were replaced by cannulae containing EB or C. Twenty-four hours later, these cannulae were also removed and P applied for a 2nd time. EB exposure of the MRF but not the CPU was highly effective in restoring behavioral responsiveness to intracerebral progesterone application. These results demonstrate that ovariectomized rats with EB implants in the POA do not respond equally to 2nd application of P to either the MRF or CPU when given within 24 h of a successful implantation of the same hormone. However, the application of EB directly to the MRF, but not to the CPU, restores the ability of local P to facilitate behavior.