Lactation-induced reduction in rats' acoustic startle is associated with changes in noradrenergic neurotransmission

The acoustic startle response (ASR) with or without fear conditioning was c ompared between cycling (CYC) and lactating (LACT) female rats. ASR sensiti vity to changes in endogenous noradrenergic (NA) release was examined using the alpha-2 NA receptor drugs yohimbine and clonidine. Groups of CYC and L ACT females were also tested in the open field. ASR was reduced in all LACT compared with that in CYC females. Both groups exhibited a robust response to fear conditioning and unpotentiated ASR subsequent to conditioning was increased in LACT females. The lowest dose of yohimbine significantly incre ased ASR in LACT females, but not in CYC females. Clonidine reduced ASR in both groups of females, with a greater potency in CYC females. In the open field, LACT females displayed a shorter latency to emerge, less freezing be havior, and more entries into the field than did CYC females. The authors c oncluded that (a) LACT females are less anxious in a novel environment and that decreased anxiety can be efficiently counteracted by fear conditioning , and (b) changes in NA neurotransmission contribute to lactation-induced m odifications in ASR.