The present study was carried out to determine whether dorsal raphe serotonergic neurons are involved in the regulation of suckling-induced PRL release. Neurotoxin lesions were placed stereotaxically in lactating rats on day 1 of lactation by 5,7-dihydroxytryptamine microinjection into the dorsal raphe (DR), median raphe (MR), or superior colliculus (SC), an area devoid of serotonergic perikarya. Litters were adjusted to eight pups each and weighed daily to determine litter growth rates. On day 7 of lactation, litters were separated from mothers for 8 h, after which six healthy foster pups were provided for a 30-min suckling stimulus. Animals were killed by decapitation immediately after suckling, plasma was collected for RIA of PRL, and brains were frozen and dissected for determination of hypothalamic, caudate, and hippocampal serotonin (5-HT) using the enzymatic-isotopic assay procedure. Litter growth rates from days 1-7 of lactation were significantly different among lesion groups (P less than 0.005), with litters from SC-lesioned animals (SCL) growing similarly to the sham group (sham, 0.924; SCL, 0.941 g/pup . day). In contrast, growth rates of litters from both DR-lesioned (DRL) and MR-lesioned (MRL) animals were significantly depressed (DRL, 0.596; MRL, 0.449 g/pup . day; P less than 0.05 and P less than 0.01, respectively). 5-HT levels in hypothalamus, caudate nuclei, and hippocampus were similar in the sham and SCL groups, whereas hypothalamic 5-HT was depleted by 63% and 55%, respectively, in the DRL and MRL groups. Despite impairments in growth rate and litter survival in both the DRL and MRL groups, only DRL animals showed significant decrements in suckling-induced PRL release (DRL, 288 +/- 107; sham, 837 +/- 134 ng NIAMDD rat PRL RP-1/ml; P less than 0.05) after 5-HT-depleting lesions. The results suggest a specificity of function within the raphe system during lactation; DR 5-HT neurons which project to the hypothalamus provide stimulatory inputs to suckling-induced PRL release, whereas MR 5-HT neurons influence litter growth and survival via their role in maternal behavior.

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.