Stress and water balance: the roles of ANP, AVP and isatin.

Stress is often associated with water retention and its resolution with diuresis. The biological systems for the control of stress and water balance are very closely related. Corticotrophin releasing hormone (CRH) and arginine vasopressin (AVP) are co-localised in the hypothalamus and often act synergistically. Atrial natriuretic peptide (ANP) can exert a feedback control on the hypothalamic/pituitary/adrenal axis. ANP has been shown to be anxiolytic, whereas AVP may be anxiogenic. AVP and ANP levels have been found to be abnormal in a range of stress disorders and psychiatric illnesses. Isatin is an endogenous anxiogenic factor which is also a potent inhibitor of the ANP receptor. It may provide a link between the function of monoamines during stress, and the control of water balance by ANP.

Effects of some anxiogenic agents on rat brain monoamine oxidase (MAO) A and B inhibitory (tribulin) activity.

Anxiogenic agents, yohimbine, pentylenetetrazole (PTZ), quinine, bufotenine, flumazenil and isatin were administered (ip) to rats at doses known to induce anxiety in this species. All the drugs exhibited anxiogenic response on the elevated plus-maze and induced a parallel increase in endogenous brain monoamine oxidase (MAO) inhibitory (tribulin) activity. The intensity of the drug-induced anxiety was fairly well correlated with the magnitude of increase in the MAO A inhibitory component of tribulin but not so with its MAO B inhibitory component. Thus, in the doses used, the degree of anxiogenic activity was PTZ > yohimbine > bufotenine > quinine > isatin > flumazenil, in terms of % entries on the open arms of the maze, whereas the magnitude of endogenous MAO A inhibition was PTZ > yohimbine > bufotenine > quinine > flumazenil > isatin. The results indicate that the MAO A inhibitory component of tribulin, rather than its MAO B inhibitory component, may be responsible for the postulated function of tribulin as an endogenous marker of anxiety.