Neural mechanism of play fighting – neural circuitry, vasopressin, and CRH – in juvenile golden hamsters

Abstract

Play fighting is common in juvenile mammals as a peri-pubertal form of agonistic behavior preceding adult aggressive behavior. In golden hamsters, play fighting peaks in early puberty around postnatal day 35 (P-35), and gradually matures into adult aggression in late puberty. Though extensively studied, the neural mechanisms underlying play fighting remains poorly understood. My dissertation focuses on identifying the neural circuitry and neural transmitter systems that mediate this behavior in juvenile golden hamsters. Based on behavioral similarities between the offensive components of play fighting and adult aggression, I predicted that the neural circuitries mediating both behaviors shared common components. This possibility was tested by quantifying the immunolabeling of c-Fos expression in juvenile hamsters after the consummation of play fighting. In support of my hypothesis, I found that areas previously associated with offensive aggression in adult hamsters, including the ventrolateral hypothalamus (VLH), the posterior dorsal part of the medial amygdala (MePD), and the bed nucleus of the stria terminalis (BST), also showed enhanced c-Fos expression after play fighting, which supported my hypothesis. Vasopressin (AVP) facilitates aggression in adult hamsters. Therefore, I hypothesized that AVP also activates play fighting. To test my hypothesis, juvenile male golden hamsters were tested for play fighting after they received central microinjections of an AVP V1A-receptor antagonist into the anterior hypothalamus (AH). Also, immunocytochemistry was performed to identify possible AVP neurons associated with this behavior. I found that the AVP antagonist selectively inhibited the attack components of play fighting in experimental animals. In addition, AVP cells in the nucleus circularis (NC) and the medial division of the supraoptic nucleus (mSON), which were associated with offensive aggression, also showed increased c-Fos activity after play fighting. Together, these results show that AVP facilitates offensive behaviors throughout hamster development, from play fighting in juveniles to aggression in adults. A recent study shows that oral administration of a CRH receptor antagonist inhibits aggression in adult hamsters. Therefore, I predicted that CRH plays a similar role in play fighting. To test my prediction, juvenile hamsters were tested for play fighting after central microinjections of a CRH receptor antagonist. I found that microinjections of the CRH receptor antagonist within the lateral septum (LS) resulted in an inhibition of several aspects of play fighting. The possible source of CRH affecting the behavior was tested through combined immunocytochemistry to CRH and c-Fos. I found CRH neurons in the diagonal band of Broca (DBB), an area with extensive connections with the LS, were particularly activated in association with play fighting. In conclusion, I find that shared neural elements participating in the “vertebrate social behavior neural network” are associated with both aggression and play fighting in hamsters. This circuitry is activated before the onset of puberty and is affected by rising levels of steroid hormones during the developmental period leading to adult behaviors. Within the circuitry, vasopressin release in the AH appears to control the activation of play fighting attacks. In contrast, CRH release in the LS affects a broader range of aspects of play fighting, including not just consummatory aspects of the behavior, but apparently also appetitive components in the form of contact duration.