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 (4 h) before the play session. The alpha-1 adrenoceptor antagonist prazosin, which also binds to alpha-2_B 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-2_B 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-2_A adrenoceptors. These findings add further support for a specific role of alpha-adrenoceptors in the modulation of playfulness in the juvenile rat.