Opioid modulation of social play reward in juvenile rats.

Social play behaviour is a vigorous form of social interaction abundant during the juvenile and adolescent phases of life in many mammalian species, including rats and humans. Social play is thought to be important for social, emotional and cognitive development. Being a rewarding activity, the expression of social play depends on its pleasurable and motivational properties. Since opioids have been widely implicated in reward processes, in the present study we investigated the role of opioids in the pleasurable and motivational properties of social play behaviour in rats. To assess social play motivation, an operant conditioning setup was used in which rats responded for social play under a progressive ratio schedule of reinforcement. Treatment with the opioid receptor agonist morphine reduced responding for social play at the highest dose tested, likely due to its rate-limiting effects. Morphine treatment increased the expression of social play behaviour during reinforced periods. The acquisition of social play-induced conditioned place preference (CPP) in a subeffective conditioning protocol was enhanced by treatment with morphine. Morphine treatment alone also induced CPP. In contrast, antagonizing opioid receptors with naloxone reduced responding for social play, the expression of social play and blocked the development of social play-induced CPP. These data implicate opioid neurotransmission in both the pleasurable and the motivational aspects of social play behaviour in rats. This article is part of the Special Issue entitled 'The neuropharmacology of social behavior: from bench to bedside'.

Maternal separation induces anhedonia in female heterozygous serotonin transporter knockout rats.

The serotonin transporter (SERT) gene has been linked to depression, especially the short allele of the serotonin transporter linked polymorphic region (5-HTTLPR). When short allele carriers are exposed to stressful life events, their risk for developing depression is increased. The neurochemical properties of the short allele of the 5-HTTLPR in humans can be mimicked in heterozygous serotonin transporter knockout (SERT+/-) rats. These animals have a similar reduction in SERT expression as humans with a 5-HTTLPR short allele. Several stress protocols have been used in SERT+/- animals but behavioural outcomes were mixed. Many studies used males to examine the behavioural effects of stress in SERT+/- rats, ignoring possible effects in females. However, women are depressed twice as often compared to men, therefore it is of great importance to study the effects of stress in females as well. Because early postnatal adversity can contribute to the psychopathology of depression, especially in vulnerable individuals, our aim was to investigate the effects of early-life stress in female SERT+/- rats and determine whether female SERT+/- rats could model the human short allele 5HTTLPR carriers. To this end, SERT+/- rats were maternally separated for six hours a day from postnatal day 2-15. Control rats were handled for 15 min from PND2-15 to control for litter disturbances. In adulthood, female rats were assessed for affective, social and coping behaviour. In addition, nerve growth factor (NGF) gene expression in the basolateral amygdala (BLA) and paraventricular nucleus of the hypothalamus (PVN) and basal plasma corticosterone levels were measured. Results show that maternal separation lowered sucrose preference in female SERT+/- rats compared to control SERT+/- rats, reflecting anhedonic behaviour. In addition, compared to control SERT+/- rats, maternal separation significantly lowered NGF gene expression in SERT+/- rats in both BLA and PVN, but did not affect plasma corticosterone levels. Together, these results show that early-life stress in female SERT+/- rats leads to depression-like behaviour and related plasticity impairments in the BLA and PVN.