Expression of mu-opioid receptor mRNA in the medial preoptic area of juvenile rats.
Previous studies indicate that the latency to initiate parental behavior in both male and female rats increases with age; weanling (21 days old) rats display parental behavior 0-2 days after exposure to newborn pups, while older juveniles (30 days old) require 5-6 days of pup exposure before they express the behavior. Furthermore, activation of mu-opioid receptors inhibits parental behavior in juvenile and adult rats. We hypothesized that the age-related increase in behavioral latency could be modulated by the induction of mu-receptor expression in the medial preoptic area (MPOA), a region in which mu-receptors regulate parental behavior. In situ hybridization histochemistry was used to measure mu-receptor mRNA expression in the MPOA of male and female Sprague-Dawley rats that were 21, 24, 27, 30, or 33 days old. Using autoradiographic film analysis, we observed that neurons within part of the MPOA expressed very dense mu-receptor mRNA. Comparison of mRNA distribution with histological boundaries indicated that neurons within the medial preoptic nucleus (MPN), excluding the central part, exhibited the highest density of mu-receptor mRNA within the MPOA. High densities of mu-receptor mRNA extended dorsolaterally and caudally from the MPN toward the bed nucleus of the stria terminalis. MPN mu-receptor mRNA expression was not altered with age and no sex difference was observed. The dense presence of mu-receptor mRNA in the MPN suggests that ample substrate exists on which mu-receptor ligands could modulate the latency to begin parental behavior in juvenile rats, but such behavioral expression apparently is not mediated by a change in mu-receptor mRNA production.
Gulledge
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CC
Department of Pharmacology and Experimental Therapeutics, Tufts University, Boston, MA 02111, USA.
Mann
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PE
Bridges
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RS
Bialos
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M
Hammer
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RP
D000367:Age Factors; D000818:Animals; D015342:DNA Probes; D005260:Female; D018507:Gene Expression Regulation, Developmental; D017403:In Situ Hybridization; D008297:Male; D008425:Maternal Behavior / Q000502:physiology; D011247:Pregnancy; D011301:Preoptic Area / Q000737:chemistry* / Q000254:growth & development / Q000502:physiology*; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000235:genetics*; D012737:Sex Factors
D015342:DNA Probes; D012333:RNA, Messenger; D017450:Receptors, Opioid, mu