Immobilization stress modifies locomotor response to catecholamine receptor agonists in rats.

The effects of dopaminergic (apomorphine, nomifensine, B-HT 920) and noradrenergic (methoxamine, clonidine, salbutamol) agonists on locomotor activity were investigated in rats submitted to acute (3 h) or repeated (3 h/4 days) immobilization stress. The stress-induced functional changes were monitored by the blood level of corticosterone and the number of lymphocytes as well as the brain utilization of NA and DA. The rats subjected to acute immobilization stress displayed 30 min later an enhanced locomotor activation after apomorphine, nomifensine, or methoxamine and reduced sedative effect of clonidine, salbutamol or B-HT 920. 24 h after the repeated stress only the locomotor responses to apomorphine, nomifensine, B-HT 920 and salbutamol were modified. Spontaneous locomotor activity was not significantly changed under the influence of stressful stimuli. Increased plasma corticosterone level, strong reduction of blood lymphocytes and enhanced NA and DA utilization in the brain of rats after acute stress, together with above mentioned results, suggest that short-lasting stress evokes (30 min later) significant functional changes not only in the blood but also in the brain: enhanced CA neurons activity as well as the increased alpha 1-adrenergic and DA-post-synaptic receptors responsiveness in parallel with reduced alpha 2 - and beta-adrenergic and DA-presynaptic receptors reactivity. On the other hand, 24 h after last session of repeated stress CA brain neurons activity was not changed, however DA and beta-adrenergic responsivity was farther modified. It is postulated that the stress conditions produce in NA and/or DA brain systems a state of readiness to locomotion activating stimuli.