Infusions of lidocaine into the amygdala, but not the preoptic area, block pseudopregnancy in the rat.

Abstract

In the rat, vaginocervical stimulation (VCS) received during mating is required for the subsequent expression of 10-12 days of twice-daily prolactin surges that are necessary for pregnancy or pseudopregnancy (PSP). This temporal separation of sensory stimulus and neuroendocrine response suggests that a mnemonic of the vaginocervical stimulation is created in the brain that triggers and sustains the daily prolactin surges. We investigated the possible involvement of the medial preoptic area (mPOA) and the medial amygdala (mAMYG) as potential neural sites involved in the processing of this neuroendocrine arc. Cycling female rats were bilaterally implanted with intracerebral cannulae in either the mPOA or mAMYG. On proestrus, females were manually palpated to confirm sexual receptivity and then received bilateral infusions of either the local anesthetic lidocaine, the Ca(++) channel blocker, verapamil, or phosphate-buffered saline (PBS) into either brain site before or both before and after receipt of 15 intromissions from an experienced male. Unmated control females received comparable infusions of lidocaine or verapamil, and were placed in the empty test arena for 10 min. Infusions consisted of either a single bilateral infusion 15 min before mating (Expt. 1), bilateral infusions both 15 min before and after mating (Expt. 2) or eight bilateral infusions separated by 30 min intervals spanning a period beginning 45 min before and ending 2 h 45 min after mating (Expt. 3). None of the lidocaine infusions into the mPOA prevented the establishment of PSP, and neither verapamil infusions into the mAMYG nor the shorter-term neural block (i.e. single or double lidocaine infusions) of the mAMYG prevented mating-induced PSP. However, the longer-term neural block (i.e. multiple lidocaine infusions) of the mAMYG significantly reduced the incidence of PSP. These data support previous findings that the mAMYG receives sensory input from VCS, and suggest that the mAMYG is a site at which a mnemonic of VCS is established.