Neurochemical correlates of behavioural responses to frustrative nonreward in the rat: implications for the role of central noradrenergic neurones in behavioural adaptation to stress.

We tested whether the stress of nonreward has neurochemical effects on noradrenergic neurones which resemble those reported for other forms of stress. Rats trained to run in a straight runway for food reward were subjected to either 1 or 10 extinction trials. Half the rats in each group were injected before the start of acquisition with IP 6-hydroxydopamine to deplete peripheral noradrenaline stores. All animals were killed immediately after their final test in the runway, together with untrained controls. Noradenaline depletion had no behavioural or neurochemical effects. The rate of extinction in the 10-trial group, which was indexed by the slope of the linear regression of running time on trial, correlated negatively with both alpha 2- and beta-adrenoceptor number (Bmax). There were no differences between groups in cerebral cortical noradrenaline content, or alpha 2- or beta-adrenoceptor binding. These results substantially conflict with those predicted from Stone's hypothesis relating beta-adrenoceptor sensitivity to the behavioural response to stress. A further finding was that alpha 2-, but not beta-adrenoceptor number, negatively correlated with levels of noradrenaline in the tissue, suggesting that noradrenaline is less involved in the regulation of beta- than in that of alpha 2-adrenoceptors.