FOREBRAIN EXPRESSION OF C-FOS DUE TO ACTIVE MATERNAL-BEHAVIOR IN LACTATING RATS

To reveal brain sites simultaneously active during the expression of m aternal behaviour in lactating rats, we used immunocytochemical visual ization of the nuclear protein product Fos of the immediate-early gene c-Sos as a marker of neuronal activity. After a 48 h separation from their litter, day 7 postpartum dams received a 1 h period of physical interaction with pups either capable or incapable of suckling, inacces sible pups in a wire-mesh box, an empty box, or no stimulation. Physic al interaction with pups elicited high levels of pronurturant maternal behaviour (retrieval, licking, mouthing), and suckling elicited nursi ng behaviour as well. Exposure to the box, with or without pups, elici ted high levels of investigatory sniffing, self-grooming, and general activity. Distal stimulation from pups did not differentially activate Fos in any of 20 sites, including olfactory-processing structures suc h as the piriform cortex and medial amygdala. Physical interaction wit h pups, with or without suckling, elicited higher levels of Fos-immuno reactive nuclei than that of other conditions in numerous sites, inclu ding many previously implicated in maternal behaviour (medial preoptic nucleus, nucleus accumbens, lateral septum, lateral habenula, and the bed nucleus of the stria terminalis). Similar group patterns of Fos e xpression also occurred in sites not previously implicated in maternal behaviour (somatosensory cortex and paraventricular thalamic nucleus) . Interaction with nonsuckling pups elicited the highest levels of Fos in the cortical amygdala, whereas suckling did not activate higher Fo s than nonsuckling interaction in any site included in this report, in cluding hypothalamic nuclei involved in lactation (paraventricular, su praoptic, and arcuate). There was little or no Fos in cingulate cortex , olfactory tubercle, medial septum, medial habenula, or ventromedial hypothalamus. These data suggest that trigeminal stimuli received by l actating rats during the performance of pronurturant maternal behaviou r promote cellular activity resulting in neuronal expression of c-fos in many forebrain sites including the medial preoptic nucleus, several sites connected with it that are part of the mesotelencephalic dopami ne system, and in the somatosensory cortex. In contrast, in these fore brain sites suckling does not elicit greater levels of Fos than that s een in nonsuckled rats and distal stimuli from pups are ineffective in increasing Fos levels compared with non-stimulated controls. (C) 1997 IBRO. Published by Elsevier Science Ltd.