Expression of mu-opioid receptor mRNA in the medial preoptic area of juvenile rats

Parental brain through time: The origin and development of the neural circuit of mammalian parenting

This review consolidates current knowledge on mammalian parental care, focusing on its neural mechanisms, evolutionary origins, and derivatives. Neurobiological studies have identified specific neurons in the medial preoptic area as crucial for parental care. Unexpectedly, these neurons are characterized by the expression of molecules signaling satiety, such as calcitonin receptor and BRS3, and overlap with neurons involved in the reproductive behaviors of males but not females. A synthesis of comparative ecology and paleontology suggests an evolutionary scenario for mammalian parental care, possibly stemming from male-biased guarding of offspring in basal vertebrates. The terrestrial transition of tetrapods led to prolonged egg retention in females and the emergence of amniotes, skewing care toward females. The nocturnal adaptation of Mesozoic mammalian ancestors reinforced maternal care for lactation and thermal regulation via endothermy, potentially introducing metabolic gate control in parenting neurons. The established maternal care may have served as the precursor for paternal and cooperative care in mammals and also fostered the development of group living, which may have further contributed to the emergence of empathy and altruism. These evolution-informed working hypotheses require empirical validation, yet they offer promising avenues to investigate the neural underpinnings of mammalian social behaviors.

Mu opioid receptors in the medial preoptic area govern social play behavior in adolescent male rats

Neural systems underlying important behaviors are usually highly conserved across species. The medial preoptic area (MPOA) has been demonstrated to play a crucial role in reward associated with affiliative, nonsexual, social communication in songbirds. However, the role of MPOA in affiliative, rewarding social behaviors (eg, social play behavior) in mammals remains largely unknown. Here we applied our insights from songbirds to rats to determine whether opioids in the MPOA govern social play behavior in rats. Using an immediate early gene (ie, Egr1, early growth response 1) expression approach, we identified increased numbers of Egr1-labeled cells in the MPOA after social play in adolescent male rats. We also demonstrated that cells expressing mu opioid receptors (MORs, gene name Oprm1) in the MPOA displayed increased Egr1 expression when adolescent rats were engaged in social play using double immunofluorescence labeling of MOR and Egr1. Furthermore, using short hairpin RNA-mediated gene silencing we revealed that knockdown of Oprm1 in the MPOA reduced the number of total play bouts and the frequency of pouncing. Last, RNA sequencing differential gene expression analysis identified genes involved in neuronal signaling with altered expression after Oprm1 knockdown, and identified Egr1 as potentially a key modulator for Oprm1 in the regulation of social play behavior. Altogether, these results show that the MPOA is involved in social play behavior in adolescent male rats and support the hypothesis that the MPOA is part of a conserved neural circuit across vertebrates in which opioids act to govern affiliative, intrinsically rewarded social behaviors.

Mapping of Morphine-Induced OPRM1 Gene Expression Pattern in the Adult Zebrafish Brain

Morphine is a potent analgesic opiate commonly used in treating pain, and it is also a substance of abuse and highly addictive. Hence, it is vital to discover the action sites of morphine in the brain to increase its efficacy of treatment. In the present study, we aimed at identifying comprehensive neuroanatomical locations that are sensitive to morphine in the adult zebrafish (Danio rerio). We performed in situ hybridization to localize the mu opioid receptor (oprm1) gene and to map the morphine sensitive brain areas using neuronal PAS domain-containing protein 4a (npas4a), an early gene marker. Real-time PCR was used to detect changes in mRNA levels of oprm1 and npas4a in control and acute morphine treated fish (2 mg/L; 20 min). Intense positive oprm1 signals were seen in the telencephalon, preoptic area, habenula, hypothalamic area and periventricular gray zone of the optic tectum. Acute morphine exposure significantly increased oprm1 and npas4a mRNA levels in the medial zone of dorsal telencephalon (Dm), ventral region of the ventral telencephalon (Vv), preoptic area, and in the hypothalamus but a decrease in oprm1 and npas4a signals in the dorsal habenula. This study provides a detailed map of oprm1 localization in the brain, which includes previously unreported oprm1 in the habenula of teleost. Presence of oprm1 in multiple brain sites implies multiple action targets of morphine and potential brain functions which could include reward, cognitive and negative emotions.

Modulation of nociception by social factors in rodents: contribution of the opioid system

RATIONALE

The opioid system is involved in the regulation of several behavioral and physiological responses, controlling pain, reward, and addictive behaviors. Opioid administration, depending on drugs and doses, usually affects sociability reducing interactions between conspecifics, whereas some affiliative behaviors such as sexual activity, social grooming, and play behavior increase the endogenous opioid activity.

OBJECTIVES

The possible interaction between endogenous opioids released during socio/sexual behavior and their analgesic effect on pain response is reviewed in the rodent literature.

RESULTS

Direct evidence for socially mediated opioid changes resulting in increase in nociceptive threshold derives from studies exploring the effects of defeat experiences, social isolation, maternal, sexual behavior, and social reunion among kin or familiar animals in laboratory rodents. Indirect evidence for endogenous activation of the opioid system, possibly affecting pain sensitivity, derives from studies investigating the relevance of natural social reward using the conditioned place preference protocols or analyzing ultrasonic vocalizations associated to positive affective contexts. Finally, genetic and epigenetic factors that affect the opioid system during development are reported to be involved in modulating the response to social stimuli as well as nociception.

CONCLUSIONS

All studies highlight the relevance of affiliative contact behavior between conspecifics that is responsible for the activation of the endogenous mu-opioid system, inducing nociceptive threshold increase.

Biosocial Influences on the Family: A Decade Review

The past decade brought a remarkable increase in the number and quality of biosocial studies of family processes. The current review summarizes recent advances in biosocial family research by providing key exemplars of emerging research paradigms. Research in the past decade has substantiated the claim in the previous Decade Review (Booth, Carver, & Granger, 2000) that bidirectional influences between all levels of analysis are paramount. There is an emerging consensus that integrating factors at multiple biological and social levels is highly informative. Because ignoring biological factors often will underestimate mediating or moderating mechanisms, the review provides recommendations for biosocial family research. We also highlight the need for researchers who understand complex family environments to lend their expertise to biosocial studies.

Opioidergic projections to sleep-active neurons in the ventrolateral preoptic nucleus

Although opioids are known to influence sleep-wake regulation, the neuroanatomic substrate(s) mediating these effects remain unresolved. We hypothesized that the influence of opiates on sleep may be mediated, at least in part, by the ventrolateral preoptic nucleus (VLPO), a key cell group for producing behavioral sleep. By combining in situ hybridization for kappa and mu receptor mRNA with immunostaining of Fos expressed by VLPO cells during sleep we show that >85% of sleep-active VLPO neurons contain mRNA for either or both opioid receptors. Microinfusions of a kappa receptor agonist into the VLPO region increased NREM sleep by 51% during the subjective night, whereas a mu receptor agonist increased wakefulness by 60% during the subjective day. The sleep- and wake-promoting effects of the kappa and mu agonists were blocked by prior administration of their respective antagonist. Combining retrograde tracing from the VLPO with immunohistochemistry for dynorphin (Dyn, the endogenous kappa receptor agonist) or endomorphin 1 (EM1, the endogenous mu receptor agonist) we show that the central lateral parabrachial subnucleus (PBcl) provides Dyn inputs to the VLPO, whereas hypothalamic histaminergic neurons provide EM1 inputs to the VLPO. In summary, results from the present study suggest that central opioid inputs to the VLPO may play a role in sleep-wake regulation and that the VLPO likely mediates the hypnotic response to high levels of opioid analgesics.