Animal studies indicate that central cholinergic neurotransmission stimulates CRH secretion, but several human studies suggest that the hypothalamo-pituitaryadrenal cortical (HPA) axis may be activated only by doses of cholinergic agonists that produce noxious side effects and, by inference, a nonspecific stress response. Physostigmine (PHYSO), a reversible cholinesterase inhibitor, was administered to normal women and men at a dose that elevated plasma ACTH cortisol, and arginine vasopressin (AVP) concentrations but produced few or no side effects. Exogenous AVP also was administered alone and following PHYSO, to determine if it would augment the effect of PHYSO on the HPA axis. Fourteen normal women and 14 normal men matched to the women on age and race underwent four test sessions 5 to 7 days apart: PHYSO (8 g/kg IV), AVP (0.08 U/kg IM), PHYSO plus AVP, and saline control. Serial blood samples taken before and after pharmacologic challenge were analyzed for ACTH , cortisol, and AVP. PHYSO and AVP administration produced no side effects in about half the subjects and mild side effects in the other half, with no significant female-male differences overall. There also were no significant female-male differences in ACTH or cortisol responses to AVP. In contrast, the men had significantly greater ACTH There is ample evidence from animal studies that cholinergic neurotransmission stimulates both CRH and arginine vasopressin (AVP) secretion (Gregg 1985;Tuomisto and Männistö 1985;Assenmacher et al. 1987 Received March 20, 1998; revised July 1, 1998; accepted July 10, 1998. N EUROPSYCHOPHARMACOLOGY 1999 -VOL . 20 , NO . 5 Pituitary-Adrenal Responses to Physostigmine and AVP 435 and Grossman 1990;Michels et al. 1991;Okuda et al. 1993;Whitnall 1993;Coiro et al. 1995;Calogero 1995;Ohmori et al. 1995), both of which stimulate ACTH secretion (Rivier et al. 1990;Antoni 1993). Several studies in humans, however, suggest that the hypothalamo-pituitary-adrenal cortical (HPA) axis is activated only by doses of cholinergic agonists that produce noxious side effects, especially nausea (Carroll et al. 1980;Davis et al. 1982;Doerr and Berger 1983;Nurnberger et al. 1983;Lewis et al. 1984;Krieg et al. 1987;Freeman et al. 1990), and, by inference, a nonspecific stress response, nausea being a powerful stimulus to AVP release (Nussey et al. 1988;Koch et al. 1990;Kohl 1992). In the present study, a dose of physostigmine (PHYSO), a reversible cholinesterase inhibitor, was established that discernibly elevated plasma ACTH and cortisol concentrations in normal subjects but produced few or no side effects. The purpose was to develop a pharmacologic cholinergic challenge to the central nervous system as specific as possible, with which to test the cholinergic overactivity hypothesis of major depression (Dilsaver 1986;Janowsky and Overstreet 1995), as reflected by HPA axis hormone responses in patients compared to controls.Under the hypothesis that a low, minimal side-effect producing dose of PHYSO would enhance the secretion...