Specific neurotoxin lesions of median raphe serotonergic neurons disrupt maternal behavior in the lactating rat.

Impairments in lactation after electrolytic lesions of the median raphe (MR) nucleus have been corrected by treatment with PRL. Specific serotonin neurotoxin lesions were used in the present study to determine whether decrements in litter growth after electrolytic lesions could be attributed to serotonergic neuron damage at the MR locus, and whether MR lesions (MRL) disrupted suckling-induced PRL release. Intracerebral microinjection of 5,7-dihydroxytryptamine (5,7-DHT) into the MR nucleus produced dose-related decrements in litter growth after either 4 micrograms (sham, 1.35 +/- 0.05; MRL, 1.04 +/- 0.05 g/pup X day; P less than 0.001) or 8 micrograms 5,7-DHT (sham, 1.35 +/- 0.06; MRL, 0.87 +/- 0.11 g/pup X day; P less than 0.001). Despite hypothalamic serotonin depletions of 15% and 55%, respectively, for the two doses of 5,7-DHT, there was no difference between sham and MRL animals in either basal or suckling-induced PRL release. When lesions were placed on day 1 of lactation (L) so that killing on day 7-L corresponded to the early maximal neurotoxin effect, MRL mothers still showed litter growth decrements (0.37 +/- 0.07; sham, 0.98 +/- 0.08 g/pup X day; P less than 0.001) and normal PRL values. When maternal behavior was examined, MRL animals exhibited a higher incidence of abnormal behaviors (failure to retrieve pups, cannibalism, and failure to initiate suckling during a 1-h test period; Fisher's exact P, Sham vs. MRL, less than 0.01, less than 0.05, and 0.15, respectively) than sham animals or animals with 5,7-DHT lesions in the dorsal raphe nucleus or superior colliculus. In addition, suckling behavior scores, determined from daily suckling behavior observations, were lowest in the MRL group and correlated with litter growth only in this group (r = 0.789; P less than 0.01). These data suggest that serotonergic elements in the MR nucleus play an obligatory role in maintaining normal maternal behavior during lactation, but they are not involved in suckling induced PRL release.