Dorsal Raphe-Hypothalamic Projections Provide the Stimulatory Serotonergic Input to Suckling-Induced Prolactin Release^*

Abstract

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.

Role of the Midbrain Periaqueductal Gray in Maternal Nurturance and Aggression:c-fosand Electrolytic Lesion Studies in Lactating Rats

“…8C,D); these gains were 16 and 20% less overall in litters suckling prepartum lesioned (F (1,19) ϭ 1.26; p Ͻ 0.2755) and postpartum lesioned (F (1,19) ϭ 6.49; p Ͻ 0.0197) dams. Although large lesions of serotonergic perikarya in DR reduce hypothalamic serotonin necessary for sucklinginduced prolactin release (Borofsky et al, 1983), daily RLWGs did not differ between prepartum or postpartum PAG-x dams, with some (n ϭ 6, 6) or no (n ϭ 4, 5) DR damage ( p Ͼ 0.9, 0.3).…”

Differential Effects of Dorsal Raphe Nucleus Lesions on Pseudopregnancy Induced by Reserpine or Vaginal Stimulation in Female Rats.

“…The DRL inhibited the induction of PSP by VS in the previous study [30]. The prolactin release induced by nipple suckling has also been reported to be interrupted by the DRL in rats [40]. In addition, VS increases metabolic activity in the DRN [31].…”

Effects of Chronic Lesions in Mesencephalic Raphe Nuclei on Induction of Pseudopregnancy.

“…The serotonergic system is therefore thought to play a facilitatory role in the maintenance of PRL surges. The dorsal raphe nucleus lesions have been reported to suppress the suckling-induced PRL increase and estrogen-induced afternoon PRL surge in female rats [19,20]. In addition, lesions of the dorsal raphe nucleus inhibit the PRL-releasing effect of serotonin in male rats [21,22].…”