Maternal behaviour in lactating rats stimulates c-fos in glutamate decarboxylase-synthesizing neurons of the medial preoptic area, ventral bed nucleus of the stria terminalis, and ventrocaudal periaqueductal gray

Increased activity of the immediate-early gene c-fos can be observed in man y areas of the lactating rat brain after dams physically interact with pups and display maternal behaviour. These sites include the medial preoptic ar ea, ventral bed nucleus of the stria terminalis, and the ventrolateral caud al periaqueductal gray, each of which is critical for the normal performanc e of particular maternal behaviours. The phenotype of cells in these areas that show increased c-fos activity after maternal behaviour, however, is un known. Via double-label immunocytochemistry, we determined if the populatio n of cells in these sites that express c-fos after maternal behaviour in la ctating rats overlaps with the population that expresses the 67,000 mel. wt isoform of glutamate decarboxlyase, the synthesizing enzyme for the inhibi tory neurotransmitter GABA. Lactating rats were separated from pups beginni ng on day 5 postpartum, and 38 h later half were allowed to interact with a litter of pups for 60 min whereas the other half were not. Dams re-exposed to pups were highly maternal, retrieving and licking them as well as displ aying prolonged nursing behaviour that included milk letdown. Both groups o f darns had a similar number of 67,000 mel. wt glutamate decarboxylase-immu noreactive cells in each site, although the number of 67,000 mel. wt glutam ate decarboxylase-immunoreactive cells per microscopic field was significan tly greater in the caudal ventrolateral periaqueductal gray than in the ven tral bed nucleus of the stria terminalis, which in turn was greater than th e medial preoptic area. In pup-stimulated dams, two to fourfold more Fos-im munoreactive cells were found in these three sires compared with non-stimul ated controls. Labeling for Fos immunoreactivity and 67,000 mel. wt glutama te decarboxylase immunoreactivity was heterogeneous within each site. In th e medial preoptic area, more Fos-immunoreactive and 67,000 mel, wt glutamat e decarboxylase-immunoreactive cells (either single or dual-labeled) were f ound dorsally than ventrally. In the ventral bed nucleus of the stria termi nalis, more Fos-immunoreactive and 67,000 mel. wt glutamate decarboxylase-i mmunoreactive cells were found medially than laterally. Within the caudal v entrolateral periaqueductal gray, 67,000 mel. wt glutamate decarboxylase-im munoreactive labeling was greatest ventromedially, while high numbers of Fo s-immunoreactive nuclei were Found both ventromedially and ventrolaterally. In pup-stimulated darns, more than half (53% in the medial preoptic area, 59% in the ventral bed nucleus of the stria terminalis, and 61% in the caud al ventrolateral periaqueductal gray) of the total population of Fos-immuno reactive cells also expressed 67,000 mel. wt glutamate decarboxylase. These results suggest that many of the neurons in these sites that show elevated c-fos activity after maternal behaviour are either local inhibitory intern eurons or provide inhibitory input to other neural sites. These inhibitory mechanisms may be critical for the display of postpartum nurturance, possib ly facilitating maternal behaviour by removing tonic inhibition on sites ne cessary for maternal responding or by restricting activity in neural sites that inhibit it. (C) 2000 IBRO. Published by Elsevier Science Ltd. All righ ts reserved.