The neurohypophysial hormone vasopressin exerts antidiuretic, vasopressor and behavioural effects (for example, facilitation of memory processes). Vasopressin may alter animal behaviour via direct effect on brain processes. Recently, however, it has been suggested that vasopressin acts mainly at peripheral receptor systems and influences behavioural mechanisms by altering visceral afferent signals. We now present data showing that (1) central administration of [Arg8]vasopressin (AVP) and more potently [pGlu, Cyt]AVP(4-8), the desglycinamide derivative of a peptide generated from AVP by brain synaptic membranes, produce the behavioural effect (promotion of passive avoidance behaviour) without the pressor effect; (2) central administration of a vasopressor antagonist blocks the behavioural but not the pressor effect of systemically administered AVP; and (3) [pGlu, Cyt]AVP(4-8) induces the behavioural effect in the absence of the pressor effect. The results indicate that AVP and related peptides affect passive avoidance behaviour by a direct central action and that the structural requirement for activation of central vasopressin receptors differs from that of the peripheral cardiovascular receptors, although both can be blocked by the same vasopressor antagonist.

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The effects of ventral mesencephalic tegmentum (VMT) radio-frequency lesions on spontaneous and complex behaviors, such as food hoarding and alimentary patterns, were investigated. Activity measures indicated that VMT lesions increased horizontal activity in a circular corridor as well as in an open field whereas vertical activity (rearing) was decreased. After 12 hr of food deprivation, food hoarding and alimentary patterns were evaluated in a square open field to which rats had free access from their home cage. Control rats hoarded food and had organized feeding behavior, but experimental rats did not hoard. Moreover, increasing the open-field illumination did not reverse this deficit. Similarly, the VMT lesion induced disruption of the adaptative alimentary behavior in response to food deprivation. A strong correlation was observed between hoarding scores and the extent of locomotor hyperactivity. These results are discussed in terms of a possible implication of limbic-midbrain mechanisms in which the meso-cortico-limbic dopaminergic AlO neurons originating in the VMT might have a primary role.