Oxytocin linked antistress effects--the relaxation and growth response.

Stress or noxious stimuli of various kind may induce the fight-flight response. In this situation a number of physiological and behavioural adaptations leading to defense of the organism occur. At a central level increased activity in the noradrenergic locus coeruleus (LC) and an enhanced secretion of corticotrophin-releasing factor (CRF) and vasopressin produced in the paraventricular nucleus (PVN) integrate stress response. Here the existence of an opposite psycho-physiological pattern associated with relaxation and growth and which is activated by certain types of non-noxious stimuli and integrated by oxytocin is proposed. In support of this, administration of oxytocin to male and female rats gives rise to effects of antistress nature in particular after repeated administration. Thus a five day treatment period with oxytocin 1 mg/kg s.c. or 1 micro g/kg i.c.v gives rise to sedation, lowering of blood pressure, increased withdrawal latency in the tail flick test and also a decrease of corticosterone levels and a rise of certain vagally controlled hormones. Weight gain is also increased under certain conditions. These effects persist several weeks after administration of oxytocin and cannot be reversed by oxytocin antagonists when established, suggesting that secondary mechanisms have been activated. Naloxone temporarily reverses the increased withdrawal of the tail flick test suggesting that opioid mechanisms have been activated to cause this particular effect. In contrast the sedative and blood pressure lowering effect seems to be induced by an enhanced activity in central alpha 2 receptors. Oxytocin levels increase in blood and CSF after various kinds of non-noxious sensory stimulation such as touch, light pressure and warm temperature in both female and male rats. It is suggested that other types of non-noxious stimuli as well may increase oxytocin release. If so, a release of oxytocin could be responsible for not only the antistress effects occurring during lactation but also why relationships, social contact and networks may have health promoting effects in particular by preventing cardiovascular disease.