An immunotoxin, anti-VIP antibody-ricin A chain conjugate eliminates neurons in the hypothalamic suprachiasmatic nucleus selectively and abolishes the circadian rhythm of water intake

In mammals, a master circadian oscillator is known to be located in the suprachiasmatic nucleus (SCN) of the hypothalamus. We examined the function of SCN neurons involved in the mechanism of circadian rhythm of water intake by lesioning them with an immunotoxin, anti-vasoactive intestinal polypeptide (VIP) antibody-ricin A conjugate. We found that the immunotoxin had a specific lethal effect on cultured PC12h cells when VIP was added to the medium. When the conjugate was infused into the third cerebral ventricle of rats above the SCN, two specific types of selective lesions of neurons were observed in the SCN: selective lesions of neurons containing arginine vasopressin (AVP) (AVP-neurons), and selective lesions of neurons containing VIP (VIP neurons). The former lesions caused disappearance of the circadian rhythm of drinking behavior, whereas the latter lesions did not affect the rhythm of water intake under constant dim lighting. Lesions that did not selectively affect one of these neurochemically identified SCN cell populations were also observed after the infusion of the conjugate or normal rabbit serum immunoglobulin G-ricin A chain conjugate. If these nonspecific lesions included entire region of the SCN, the circadian rhythm of water intake was abolished. These findings suggest that SCN neurons bearing VIP receptors such as AVP neurons, but not VIP neurons, may be involved in the mechanism of the circadian rhythm of water intake.