A team of German scientists seeks to better understand the neural basis of play behavior in mammals through experiments in which researchers tickle rats and observe how their behavior correlates to their brain activity. In a 2023 paper in Neuron detailing these experiments, the authors argue that while the scientific community has an incomplete grasp of how neural pathways correspond to mammalian behavior patterns for any form of activity, the biological underpinnings of play behavior are some of the least understood.
The researchers hoped to identify a mechanism for play outside of the cerebral cortex as previous research suggests that play behaviors persist even if cortical structures are inactivated. Based on findings that play is associated with increased cellular activity in the region, the team focused their experiments on a structure in the midbrain (the uppermost part of the brain stem) called the periaqueductal gray area (PAG) which is typically associated with fear and escape behaviors.
The periaqueductal gray area and playful behavior
The team measured rat vocalizations at 50 Hz, an established proxy for playful behavior, as they tickled and played with rats. They compared the amount of these ultrasonic vocalizations (USV) between a control group and a group that had PAG activity blocked with with muscimol, a compound that acts on receptors for a neurotransmitter known as GABA to mimic GABA’s inhibitory effect. The muscimol blockade of PAG activity reduced the rats USVs in response to tickling and play.
Importantly, this muscimol inactivation did not decrease USV production altogether, only within the play context. This indicates that inactivating the PAG interferes with play behavior and not the rat’s physical ability to produce USVs. The researchers repeated the experiment, this time replacing muscimol with an anesthetic called lidocaine, and found that while this other method of inactivating the PAG also reduced USV production, the rats’ motor behavior changed as well.
Next, researchers recorded activity from individual rat neurons in the PAG during tickling and play. Tickling induced excitatory activity in the region, and the authors suggested a link between the strength of these neuronal responses and the USVs that rats produced during the experiment. They followed this experiment with a chasing-hand paradigm in which rats chase after the hand of an experimenter. This fun activity (indicated by the rats’ readiness to perform the chase and the USVs they produced during the experiment) also led to excitatory activity recorded in individual PAG neurons.
Diving deeper into periaqueductal gray area structure
In the chasing hand experiment, the researchers noticed that the neuronal response evoked by play varied based on the neuron’s location within the PAG. They performed cluster analysis on the neuron recordings taken during tickling and play and found that clusters of neurons with different response profiles were concentrated in different regions of the PAG known as columns.
Cells belonging to the cluster with the strongest excitatory response to both tickling and play were concentrated in the lateral PAG (LPAG) column. Cells with any excitatory response to both tickling and play were more common in both the LPAG column and the adjacent ventrolateral PAG (VLPAG) column. The dorsolateral PAG (DLPAG) and dorsomedial PAG (DMPAG) had the highest concentration of cells that responded neutrally to play but were inhibited by tickling; the DLPAG also had a significant concentration of cells with a neutral response to play and tickling. Neurons inhibited by both play and tickling were rare, but made up a small portion of the cells in the DMPAG, LPAG, and VPAG.
The role of the neuron cluster activated by tickling and play in modulating tickling and play activity was validated in further experiments. The researchers demonstrated that anxiety-inducing conditions, which are known to temper playful behavior, led to a decrease in activity for these neurons; no clear effect was observed for the other clusters. Recordings from all cell clusters showed that neurons in this tickling/play activated cluster fired during and before USVs. In a final experiment, researchers confirmed the relevance of the lateral PAG to play and tickling behavior in experiments where this column was selectively inactivated, which suppressed play behavior.
The authors conclude that the periaqueductal gray area has an important role in play behavior that can be dampened in anxiety-inducing conditions. While they acknowledge the need for more research into how each structural region in the PAG contributes to play behavior, they suggest that the lateral PAG may have an especially significant role.
While the biological pathways that govern behavior are complex and varied, this work paves the way for further experimentation into how mammals play that may one day translate to a better understanding of human behavior.