Idazoxan increases rough-and-tumble play, activity and exploration in juvenile rats

The neurobiology of social play behaviour: Past, present and future

Social play behaviour is a highly energetic and rewarding activity that is of great importance for the development of brain and behaviour. Social play is abundant during the juvenile and early adolescent phases of life, and it occurs in most mammalian species, as well as in certain birds and reptiles. To date, the majority of research into the neural mechanisms of social play behaviour has been performed in male rats. In the present review we summarize studies on the neurobiology of social play behaviour in rats, including work on pharmacological and genetic models for autism spectrum disorders, early life manipulations and environmental factors that influence play in rats. We describe several recent developments that expand the field, and highlight outstanding questions that may guide future studies.

Noradrenergic modulation of play in Sprague-Dawley and F344 rats

RATIONALE

For many mammals, engaging in social play behavior as a juvenile is important for cognitive, social, and emotional health as an adult. A playful phenotype reflects a dynamic interplay between genetic framework and experiences that operate on hard-wired brain systems so the relative lack of play in an otherwise playful species may be useful for identifying neural substrates that modulate play behavior. The inbred F344 rat has been identified as a strain that is consistently less playful than other strains commonly used in behavioral research. Norepinephrine (NE) acting on alpha-2 receptors has an inhibitory effect on play and F344 rats differ from a number of other strains in NE functioning. As such, the F344 rat may be particularly useful for gaining insight into NE involvement in play.

OBJECTIVE

The objective of this study was to determine whether the F344 rat is differentially sensitive to compounds that affect NE functioning and that are known to affect play behavior.

METHODS

Using pouncing and pinning to quantify play, the effects of the NE reuptake inhibitor atomoxetine, the NE alpha-2 receptor agonist guanfacine, and the NE alpha-2 receptor antagonist RX821002 on play behavior were assessed in juvenile Sprague-Dawley (SD) and F344 rats.

RESULTS

Atomoxetine and guanfacine reduced play in both SD and F344 rats. RX821002 increased pinning to a comparable extent in both strains but F344 rats were more sensitive to the play-enhancing effects of RX821002 on pounces.

CONCLUSIONS

Strain differences in NE alpha-2 receptor dynamics may contribute to the lower levels of play in F344 rats.

How strain differences could help decipher the neurobiology of mammalian playfulness: What the less playful Fischer 344 rat can tell us about play

Play is common among the young of many mammalian species. How that play is exhibited results from a dynamic interplay between genetic framework and experiential influences that, in turn, operate on hard-wired brain systems. One approach towards understanding how genes and environment interact with brain substrates to yield a particular playful phenotype is to take advantage of inbred strains of rats that come with a known genetic identity and assess the effects of varying early social experiences and targeted neurobiological interventions on rats of these strains. This paper primarily summarizes research utilizing the F344 inbred strain, a rat that consistently plays less than most other strains.

Serotonin, motivation, and playfulness in the juvenile rat

The effects of the selective 5HT(1A) agonist 8-OH-DPAT were assessed on the play behavior of juvenile rats. When both rats of the test pair were comparably motivated to play, the only significant effect of 8-OH-DPAT was for play to be reduced at higher doses. When there was a baseline asymmetry in playful solicitation due to a differential motivation to play and only one rat of the pair was treated, low doses of 8-OH-DPAT resulted in a collapse of asymmetry in playful solicitations. It did not matter whether the rat that was treated initially accounted for more nape contacts or fewer nape contacts, the net effect of 8-OH-DPAT in this model was for low doses of 8-OH-DPAT to decrease a pre-established asymmetry in play solicitation. It is concluded that selective stimulation of 5HT(1A) receptors changes the dynamic of a playful interaction between two participants that are differentially motivated to play. These results are discussed within a broader framework of serotonergic involvement in mammalian playfulness.

The relative playfulness of juvenile Lewis and Fischer-344 rats

The relative playfulness of inbred Lewis and Fischer-344 rats was characterized. Fischer rats were consistently less playful than Lewis rats, with rats of this strain less likely to initiate playful interactions with either responsive or unresponsive partners and also less likely to respond playfully when playful solicitations were directed to them. While less playful, Fischer rats were more socially inquisitive than Lewis rats when tested with an unresponsive partner, suggesting that Fischer rats are less likely to escalate a social encounter into a playful one. Strain differences in playful responsiveness were present with or without prior social isolation, suggesting that this aspect of play represents a relatively stable trait difference. Unlike play responsiveness, strain differences in play solicitation were only apparent after a period of social isolation. Low levels of play were still present in Fischer rats that had been reared by Lewis dams, suggesting a genetic source for the altered play in rats of this strain. Further studies of play behavior in Lewis and Fischer rats could illuminate relevant neural involvement in rough-and-tumble play and also help understand the genetic bases for this complex social behavior.

A further characterization of alpha-2 adrenoceptor involvement in the rough-and-tumble play of juvenile rats

In order to further understand the extent to which alpha-2 adrenoceptors are involved in play behavior, a more detailed behavioral assessment was made of the effects of the alpha-2 agonist clonidine and the alpha-2 antagonist RX821002 on the rough-and-tumble play of juvenile rats. Clonidine reduced overall levels of playfulness, as indicated by fewer attacks directed to the nape, a lower probability of rotating completely to supine in response to nape attacks and fewer pins. In contrast, RX821002 increased playfulness, as indicated by more attacks directed to the nape, a higher probability of rotating completely to supine in response to nape attacks and more pins. Testing rats in an unfamiliar environment did not affect the ability of RX821002 to increase play. Abruptly turning on a bright light during an ongoing play session also did not affect the ability of RX821002 to increase play or the ability of clonidine to reduce play. These data suggest that alpha-2 noradrenergic compounds do not affect play by altering levels of distractibility. However, the extent to which RX821002 and clonidine influenced overall activity during a play bout was compromised by turning on a bright light. While this pattern of results is consistent with a presynaptic site of action for alpha-2 involvement in modulating overall activity during a play bout, the same cannot be concluded about any putative explanatory mechanism for alpha-2 involvement in play behavior

Acute cardiovascular effects of the alpha2-adrenoceptor antagonist, idazoxan, in rats: influence of the basal sympathetic tone

Intravenous administration of the alpha2-adrenoceptor antagonist, idazoxan, elicits variable cardiovascular effects, depending on experimental conditions. In this study, the effects of idazoxan were investigated in rats with high, low, or no basal sympathetic tone. In a group of conscious Sprague-Dawley rats (n = 9), mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nervous activity (RSNA) were recorded. Idazoxan (250 microg/kg, i.v.) induced a transient decrease in MAP (-12+/-3 mm Hg) that was accompanied by increases in HR (49+/-14 beats/min) and RSNA (53+/-14%). In six of nine rats, a light pentobarbitone anesthesia was given. Basal RSNA was decreased (6.0+/-1.3 microV from 12.8+/-4.1 microV; p<0.05), and the depressor effect of idazoxan was reversed to a pressor effect (21+/-6 mm Hg) associated with bradycardia (-16+/-8 beats/min) and sympathoinhibition (-56+/-15%). In eight conscious intact rats, idazoxan (250 microg/kg, i.v.) attenuated by approximately 40% the pressor response to the selective alpha1-adrenoceptor agonist, cirazoline (0.5 microg/kg, i.v.). In three groups of six to seven ganglion-blocked (chlorisondamine, 2.5 mg/kg, i.v.) conscious rats, idazoxan dose-dependently increased mean arterial pressure (MAP: 39+/-2, 55+/-3, and 69+/-4 mm Hg at 125, 250, and 500 microg/kg, i.v., respectively) with minimal changes in HR. In contrast, the noradrenaline-releasing agent, tyramine (62.5, 125, and 250 microg/kg, i.v.), dose-dependently increased both MAP and HR. The alpha1-adrenoceptor antagonist, prazosin (1 mg/kg, i.v.; n = 8) blunted by approximately 70% (p<0.01) the pressor effect of 250 microg/kg idazoxan. It is concluded that in rats with high sympathetic tone, idazoxan has depressor effects, most likely related to its peripheral alpha-adrenoceptor antagonist properties. In rats with low or no sympathetic tone, idazoxan induced pressor responses mainly secondary to its partial agonist activity at vascular postjunctional alpha1-adrenoceptors.

The neurobiology of social play behavior in rats

Social play behavior is one of the earliest forms of non-mother-directed social behavior appearing in ontogeny in mammalian species. During the last century, there has been a lot of debate on the significance of social play behavior, but behavioral studies have indicated that social play behavior is a separate and relevant category of behavior. The present review provides a comprehensive survey of studies on the neurobiology of social play behavior. Evidence is presented that opioid and dopamine systems play a role in the reward aspect of social play behavior. The role of cholinergic, noradrenergic and opioid systems in attentional processes underlying the generation of social play behavior and the involvement of androgens in the sexual differentiation of social play behavior in rats is summarized. It is concluded that there is not only behavioral, but also neurobiological evidence to suggest that social play behavior represents a separate category of behavior, instead of a precursor for adult social, sexual or aggressive behavior.

Rough-and-tumble play behavior in Fischer-344 and buffalo rats: effects of social isolation

Play behavior was assessed in two inbred strains of rats. Rats of the Fischer-344 strain (F344) were found to be less playful than Buffalo rats after social isolation, as evident from fewer pins and fewer playful attacks to the nape. When tested in same-strain pairings, overall defense of the nape did not differ between strains, although there were strain differences in the specific pattern of defense. When tested in cross-strain pairings, F344 rats were less likely to direct nape attacks toward a Buffalo play partner, and were also less likely to defend their nape when attacked by Buffalo rats. Although different levels of pinning and nape attacks in the two strains were dependent on the amount of isolation prior to the play period, differences in nape defense were not dependent on prior isolation. This pattern of results suggests that the neural mechanisms for playful attack differ from those underlying playful defense. These data also suggest that the F344 strain could be useful in better understanding the neural and genetic bases of mammalian playfulness.

Effects of MK-801 on rough-and-tumble play in juvenile rats

Effects of the noncompetitive N-methyl-D-aspartate (NMDA) channel blocker, MK-801, on play behavior and activity were assessed in juvenile rats. A low dose (0.025 mg/kg) of MK-801 significantly increased both pinning and dorsal contacts, two indices of play behavior in the rat. Higher doses (> or = 0.1 mg/kg) significantly reduced both measures of play, with play being virtually abolished at 0.2 mg/kg. Simultaneous measurements of activity indicated that the reductions of play at the higher doses were accompanied by significant increases in horizontal activity. There was also a dose-related reduction in rearing, which was significant at all doses. When juvenile rats were tested individually for activity in an open field, MK-801 increased horizontal activity at 0.2 mg/kg and decreased rearing at all doses. So while the effects of MK-801 on play are biphasic, the effects of this compound on locomotor activity, per se, are linear. These data suggest that the effects of NMDA channel blockade on overall activity and play are mediated through independent neural systems.

Noradrenergic alpha-1 and alpha-2 antagonists block L-dopa-induced air-stepping in neonatal rats

Five-day-old rat pups suspended in air and administered L-3,4-dihydroxyphenylalanine (L-DOPA) engage in a highly stereotyped and coordinated locomotor behavior termed air-stepping. L-DOPA is a precursor for dopamine and noradrenaline and one or both of these neurotransmitters could play a role in L-DOPA-induced air-stepping. The role of noradrenaline was investigated by assessing the abilities of the alpha-1 noradrenergic receptor antagonist prazosin and the alpha-2 noradrenergic receptor antagonist idazoxan to block L-DOPA-induced air-stepping in 5-day-old rats. Both antagonists decreased the duration of air-stepping. In addition, prazosin altered the topography of air-stepping by interfering with coordination of the hindlimbs. The results suggest that alpha-1 and alpha-2 noradrenergic receptor subtypes are involved in L-DOPA-induced air-stepping.

Effects of alpha-2 adrenoceptor antagonists on rough-and-tumble play in juvenile rats: evidence for a site of action independent of non-adrenoceptor imidazoline binding sites

The pharmacological specificity of alpha-2 adrenoceptor involvement in the modulation of rough-and-tumble play behavior was assessed in juvenile rats. The alpha-2 adrenoceptor antagonists idazoxan and RX821002 both increased the frequency of pinning in individually housed rats that were given a brief opportunity to play. Dorsal contacts, a measure of play solicitation, were not consistently affected by these compounds. Since RX821002 shows little affinity for non-adrenoceptor imidazoline binding sites, it is likely that the facilitation of play following administration of these two compounds is due to blockade of alpha-2 receptors. The effect of RX821002 and idazoxan is unlikely to be an artifact associated with using rats that are reared in isolation, as RX821002 also increased pinning, as well as dorsal contacts, in group-housed rats that were isolated for a short period (4h) before the play session. The alpha-1 adrenoceptor antagonist prazosin, which also binds to alpha-2B receptors, reduced the frequency of both pinning and dorsal contacts. There was a strong trend for St 587, a centrally active alpha-1 agonist, to attenuate the effect of prazosin on play. While this leaves open the possibility that prazosin may be reducing play through alpha-1 blockade, antagonist activity at alpha-2B receptors cannot be ruled out. From these data, we conclude that the facilitation of play following idazoxan and RX821002 is likely due to blockade of alpha-2A adrenoceptors. These findings add further support for a specific role of alpha-adrenoceptors in the modulation of playfulness in the juvenile rat.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin on behavior of monkeys in peer groups

Adult female rhesus monkeys were fed diets containing 0, 5, or 25 ppt 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) for approximately 4 years. They were bred to unexposed males during TCDD exposure (Experiment 1) and again after TCDD exposure ended (Experiment 2). Offspring from both experiments were weaned at 4 months and socialized for 1.5 h/day in groups of four monkeys each beginning at approximately 8 months of age. Each social group contained both control and TCDD-exposed monkeys. In Experiment 2, the offspring were later placed in new social groups containing only monkeys from the same TCDD exposure condition. The TCDD-exposed offspring born concurrent with maternal TCDD exposure (Experiment 1) initiated more rough-tumble play, retreated less during play bouts, and were less often displaced from preferred positions in the playroom. They also engaged in more self-directed behaviors. The behavior of offspring born after maternal TCDD exposure ended (Experiment 2) was not altered when they were socialized with control monkeys. However, some behavioral changes did emerge when they were placed in social groups containing only TCDD-exposed monkeys.

Place conditioning reveals the rewarding aspect of social interaction in juvenile rats

Rewards, as diverse as food, sweetened solutions, copulation, electrical brain stimulation, and drugs abused by humans, have been shown to condition place preferences in rats. Juvenile rats will readily learn to traverse a T-maze for the opportunity to interact with another similar-aged rat. This suggests that play behavior is rewarding. Experiment 1 examined whether play (as quantified by rough-and-tumble pinning) would act as a sufficient reward to condition a place preference (CPP). Experiment 2 examined whether pairings with a nonplaying partner would decrease the time spent in the preferred side and thus suggest a conditioned place aversion (CPA). In Experiment 1, dominant juvenile rats were given free access to a CPP apparatus and a side preference for one of the two physically distinct sides was determined. Dominant rats were then conditioned twice daily over four days in the CPP apparatus. They spent their first session confined in their preferred side with a scopolamine-treated partner (that rendered the partner unable to respond to play solicitations) and during the second session, dominant rats were confined to their less preferred side with a submissive play partner. The number of dorsal contacts, as well as frequency and duration of pinning, were recorded. Following conditioning, side preference was redetermined. A similar procedure was used in Experiment 2 except that the subjects underwent conditioning on their less-preferred side without a play partner. Results of Experiment 1 demonstrated that the dominant rats significantly increased (198.6%) the time spent on the originally less-preferred side after play conditioning.(ABSTRACT TRUNCATED AT 250 WORDS)