Vasopressin and oxytocin. Their presence in the central nervous system and their functional significance in brain processes related to behaviour and memory.

Vasopressin and oxytocin exert pronounced effects on behaviour by a direct action on the brain. A single injection of vasopressin results in a long-term inhibition of extinction of a conditioned avoidance response suggesting that vasopressin triggers a long-term effect on the maintenance of a learned response, probably by facilitation of memory processes. In addition vasopressin improves passive avoidance behaviour, delays extinction of appetitive discrimination tasks, affects approach behaviour to an imprinting stimulus in ducklings, improves copulation rewarded behaviour of male rats in a T-maze, prevents or reverses amnesia induced by electroconvulsive shock, CO2 inhalation, pentylenetetrazol or puromycin. The majority of these effects of vasopressin in the various and sometimes relatively complex tasks may be explained by stimulatory influences of this neuropeptide on memory processes. Generally oxytocin exerts effects which are opposite to those of vasopressin and it has been suggested that oxytocin may be an amnesic neuropeptide. Various limbic system structures seem to act as the anatomical substrate for the behavioural effects of vasopressin. In particular the amygdala, the dentate gyrus of the hippocampal complex, the ventral hippocampus and the dorsal septum seem to be involved. Evidence has been obtained from experiments with homozygous diabetes insipidus rats and from experiments in which antisera were applied that endogenous vasopressin and oxytocin play a physiological role in brain processes related to memory. It appears that highly active fragments can be generated from vasopressin and experiments in which a fragment of vasopressin ([pGlu4, Cyt6]AVP-(4-8)) as well as an AVP-antagonist were used, reveal that the vasopressin receptors mediating the behavioural effects are situated in the brain and differ in specificity from the peripheral (blood pressure) vasopressin receptors. Generally the clinical data obtained so far with vasopressin treatment are in agreement with the results from animal experiments and they support the notion on the involvement of vasopressin in memory function. The sometimes reported conflicting results on vasopressin effects in certain patients (Korsakoff or Alzheimer) may have to do with the wide-spread pathology in these diseases.