Induction of c-Fos-like and FosB-like immunoreactivity reveals forebrain neuronal populations involved differentially in pup-mediated maternal behavior in juvenile and adult rats

Juvenile rats can exhibit maternal behavior after being exposed continuousl y to rat pups, a process called sensitization. Maternal behavior in juvenil es is robust and is similar to adult maternal behavior (Mayer and Rosenblat t [1979] Dev. Psychobiol. 12:407-424; Gray and Chesley [684] J. Comp. Psych ol. 98:91-99). In this study, immunocytochemical detection of the protein p roducts of two immediate-early genes, c-fos and fosB, was used as a tool to identify forebrain neuronal populations involved in the maternal behavior of 27-day-old juvenile rats compared with 60-day-old adults. To sensitize t hem, rats were exposed continuously to foster pups. Once they were maternal , they were isolated from pups overnight, reexposed to pups for 2 hours, an d then killed. Nonmaternal control animals also were isolated overnight and were either reexposed to pups for 2 hours or kept isolated from pups befor e killing. The lateral habenula (LH) was the only area in which both matern al juveniles and maternal adults had more c-Fos-immunoreactive (-Ir) neuron s compared with controls. In maternal adults, the number of neurons that ex pressed c-Fos and FosB immunoreactivity increased in the medial preoptic ar ea (MPO) and the ventral bed nucleus of the stria terminalis (BSTv, whereas the dorsal bed nucleus of the stria terminalis (BSTd) and the medial and c ortical nuclei of the amygdala (MEA and COA respectively) had increases onl y in the number of neurons that expressed c-Fos immunoreactivity. in contra st, juveniles, whether or not they were maternal, had the same number of c- Fos-IR and FosB-Ir neurons in all these areas. The adult-like increase in t he number of c-Fos-Lr neurons found in maternal juveniles suggests that the juvenile LH participates in the neural circuit that supports maternal beha vior in an adult-like manner. The lack of c-fos or fosB induction in the MP O, BSTv, BSTd, COA, or MEA of maternal juveniles compared with maternal adu lts may reflect the immaturity of these brain regions in juvenile rats. Exa ctly what this immaturity consists of and when the responses of these regio ns become adult-like remain to be determined. J. Comp. Neurol. 416:45-78, 2 000. (C) 2000 Wiley-Liss, Inc.