Beta-adrenoceptor binding correlates with behaviour of rats in the open field

Rats were injected IP once daily for 14 consecutive days with propranolol (5 mg/kg), yohimbine (2.5 mg/kg) or saline vehicle. A fourth group was unhandled during this time. Each rat was then placed in an open field for 4 min and its activity and defaecation recorded. Immediately after this, the animals were killed and cerebral cortices removed for radioligand binding to alpha 2- and beta-adrenoceptors and measurement of noradrenaline content. We report two sets of findings. First, beta-adrenoceptor density correlated positively, and affinity negatively, with the number of movements towards the centre of the field in the final 3 min of the trial. alpha 2-Adrenoceptor Kd, in contrast, correlated both with movements around the field and those directed towards the centre. Secondly, whereas the only specific drug effect was an increase in defaecation after treatment with propranolol, beta-adrenoceptor density was increased and affinity decreased in all injected groups, suggesting a non-specific effect of the stress of injection. Movements to and from the centre of the field were also increased in injected groups during the first minute of the trial. In both sets of findings the association of beta-adrenoceptor density with greater resistance to stress is hard to reconcile with existing theories of the role of beta-adrenoceptors in behavioural responses to stress.