Mechanism

Most of the research of neural mechanisms of play behavior has focused on the social play fighting of rodents, but some investigation has focused on brain regions associated with social play in primates.  In rodents, the brain areas involved in play fighting tend to differ than those involved in adult fighting, however overlap has been observed in certain more aggressive types of play in some species.  Phylogenetic primate research has found several regions associated with frequency of play, including the amygdala and hypothalamus.  It is theorized that the hypothalamus is involved in the motivational and rewarding aspects of play while the amygdala contributes to social and emotional aspects of play. Research into the reward and motivational aspects of play in rats suggest that the endogenous opioid system mediates the hedonic aspects of social play while dopamine modulates the motivational properties of social play.

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Figure 1. is pulled from Trezza et al. (2010) which can be found in REFERENCES.

Brain Areas Involved in Play

 

Rodents:

Studies of rodent play fighting have found that the brain areas involved in play are different than those involved in adult aggression.  Gordon et al. (2002) demonstrate this effect in rats by demonstrating differential c-Fos expression in juvenile rats after play fighting compared to adults in real fights.  This trend, however, is not demonstrated in golden hamsters, where the line between juvenile play and adult fighting is blurred.  These hamsters exhibit two different types of play fighting where one style of playing is less playful than the other and sets up hierarchical social structures.  Cheng and Delville (2008) demonstrate that this more aggressive type of playing activates many brain regions involved in adult fights. 

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Figure 2. Vervet monkeys play fighting. Photo by Manoj Shah. SOURCE.

Primates:

Less experimental data has been collected focusing on the neural mechanisms involved in primate social play, but some correlational evidence has pointed to the neocortex, cerebellum, amygdala, and hypothalamus as primary areas involved in social play.  The larger relative sizes of these regions in primates is correlated with increased play frequency.  The amygdala is hypothesized to be involved in the social and emotional aspects of play due to its relevance in emotional recognition outside of play.  Additionally, phylogenetic comparative analysis has suggested that the amygdala’s role in social play reflects the same regulation of the expression of aggression and submission in adult aggression. The hypothalamus’s role in play is theorized to reflect motivational aspects in play due to its role in the regulation of motivation of several behaviors.  (see Lewis & Barton, 2004 in REFERENCES)

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Figure 3. Sleeping rat pups. Photo by Andrea Gore. SOURCE.

Reward and Motivational Aspects of Play

The most successful and prevalent research on the motivational and hedonic mechanisms of social play have focused on neurotransmitters involved in rat play fighting. The endogenous opioid system mediates the hedonic aspects of social play while dopamine modulates the motivational properties of social play.  Evidence that opioids are involved in play comes from the fact that treatment of opioid agonists enhance social play while antagonists reduce social play.  This system is said to involve hedonic properties rather than motivational ones as opioid agonist treatment in a paradigm where rats had to work for social play increased frequency of play without improving performance.  The effect of dopamine is more complicated.  Treatment of dopamine antagonists reduce social play, while dopamine receptor agonists have been demonstrated to produce both increases and decreases in playing behavior, however, the reported increases have been very small.  The lack of effect of dopamine agonists likely reflects optimal dopamine signaling during play, where further stimulation of this signal no longer enhances social play.  (for review, see Trezza et al., 2010 in REFERENCES)


Modulation of Social Play

The modulation of play fighting has also been primarily studied in rats.  Rats are a useful species to study because they modulate their defensive tactics during play fighting according to age and partner; young male rats tend to use a complete rotation tactic while older males switch to a partial rotation tactic, and subordinate males engage in the complete rotational tactics while dominant males use the complete rotational tactic (females generally always use the complete rotational tactic).  The brain regions found to be important to the modulation of play fighting include the prefrontal cortex, the motor cortex, and the striatum.

Kamitakahara et al. (2007) demonstrate that the prefrontal cortex is necessary for partner related modulation, while the motor cortex is necessary for age related modulation of defensive techniques in a series of ablation studies.  These areas appear to be involved solely in the modulation of play fighting, as these ablations did not affect the rats ability to perform defensive tactics, nor did it affect frequency of play fighting.  The striatum, part of the forebrain involved in reward, may also play a role in the modulation of play fighting.  Lesions here prevented rats from making normal transitions between play behaviors without affecting the ability to produce these behaviors.