Adenosine A2A Receptor Gene Disruption Provokes Marked Changes in Melanocortin Content and Pro-Opiomelanocortin Gene Expression

A2A receptor knockout (A2AR-/-) mice are more anxious and aggressive, and exhibit reduced exploratory activity than their wild-type littermates (A2AR+/+). Because alpha-melanocyte-stimulating hormone (alpha-MSH) influences anxiety, aggressiveness and motor activity, we investigated the effect of A2AR gene disruption on alpha-MSH content in discrete brain regions and pro-opiomelanocortin (POMC) expression in the hypothalamus and pituitary. No modification in alpha-MSH content was observed in the hypothalamus and medulla oblongata where POMC-expressing perikarya are located. In the arcuate nucleus of the hypothalamus, POMC mRNA levels were not affected by A2AR disruption. Conversely, in A2AR-/- mice, a significant increase in alpha-MSH content was observed in the amygdala and cerebral cortex, two regions that are innervated by POMC terminals. In the pars intermedia of the pituitary, A2AR disruption provoked a significant reduction of POMC mRNA expression associated with a decrease in alpha-MSH content. By contrast, in the anterior lobe of the pituitary, a substantial increase in POMC mRNA and adrenocorticotropin hormone concentrations was observed, and plasma corticosterone concentration was significantly higher in A2AR-/- mice, revealing hyperactivity of their pituitary-adrenocortical axis. Together, these results suggest that adenosine, acting through A2A receptors, may modulate the release of alpha-MSH in the cerebral cortex and amygdala. The data also indicate that A2A receptors are involved in the control of POMC gene expression and biosynthesis of POMC-derived peptides in pituitary melanotrophs and corticotrophs.