SHORT COMMUNICATION Decreased glucocorticoid receptor mRNA and dysfunction of HPA axis in rats after removal of the cholinergic innervation to hippocampus. Jung-Soo Han, 1 Jennifer L. Bizon, 1 Hyun-Ja Chun, 2 Courtney E. Maus 1 and Michela Gallagher 1 1 Department of Psychological and Brain Science, Johns Hopkins University, 3400 North Charles Street, MD 21218, USA 2 Department of Newly Developed Drugs, Professional Graduate School of Oriental Medicine, Won Kwang University, South Korea Keywords: cholinergic lesion, glucocorticoid, GR, hippocampus, MR, stress Abstract Excess exposure to glucocorticoids can have deleterious effects on physiology and cognition. Glucocorticoids acting via receptors located in hippocampal neurons contribute to negative feedback after stress by terminating the further release of glucocorticoids. The current study investigated the effects of selective immunolesions of septo-hippocampal cholinergic neurons on hippocampal corticosterone receptor mRNA and on hypothalamic-pituitary-adrenal (HPA) activity. As evaluated by in situ hybridization, hippocampal glucocorticoid receptor (GR) mRNA, but not mineralocorticoid receptor (MR) mRNA, was signi®cantly decreased in lesioned rats compared to controls. In a companion study, the peak corticosterone response to one hour of restraint stress did not differ between lesion and control groups but the post-stress decline of corticosterone was more protracted in the lesioned rats. These ®ndings are discussed in terms of their possible relevance to ageing as age-related degeneration of the basal forebrain cholinergic system may contribute to the commonly observed dysfunction of the HPA axis in older animals. Introduction A pro®cient regulation of the stress response is important because protracted exposure to glucocorticoids can have damaging physio- logical and cognitive effects in both humans (e.g. Newcomer et al., 1994; Keenan et al., 1995) and rodents (e.g. Joels & Kloet, 1994; Pavlides et al., 1996). Principal neurons in hippocampus that bear glucocorticoid receptors have been identi®ed as a target for steroids, through which negative feedback on corticosterone release is mediated (Sapolsky et al., 1983), speci®cally, rats with damage to the hippocampal formation show de®cient feedback regulation of the stress response (Sapolsky et al., 1984; Herman et al., 1995). The status of particular neurotransmitter systems can regulate the hippocampal corticosterone receptors, which in turn, could alter an organism's ability to respond to stress. Selective lesions, which remove serotonergic or noradrenergic projections to forebrain, markedly decrease hippocampal MR and GR mRNA expression as well as glucocorticoid binding sites (Seckl & Fink, 1991; Maccari et al., 1992; Casolini et al., 1993). The effects of such lesions can occur independently of changes in circulating glucocorticoids, as alterations in receptor expression are seen in 6-hydroxydopamine lesioned rats maintained on controlled levels of corticosterone (Maccari et al., 1992). Several lines of evidence also implicate the septo-hippocapmpal cholinergic projections in the regulation of the HPA system. The loss of cholinergic afferent input to the hippocampus is an important feature of ageing (McGeer et al., 1984; Smith & Booze, 1995), and alterations of hippocampal corticosterone receptors are also seen in aged rats (Issa et al., 1990; Bizon et al., 2001). Moreover, lesions of the septal region alter MR and GR expression and HPA function (Yau et al., 1992). In contrast to the use of selective toxins to study monoamines, manipulations of the cholinergic system, thus far, have used less selective methods, that destroy the innervation of hippocampus by other systems and/or damage noncholinergic neurons at the site of injection (Yau et al., 1992; Alema et al., 1995). Microinjection of 192 IgG-saporin into the basal forebrain produces near complete removal of septo-hippocampal cholinergic neurons while sparing both ®bers of passage and neighbouring noncholinergic neurons (Baxter et al., 1995). The current study investigated the effects of such lesions on hippocampal MR and GR mRNA using in situ hybridization, and on their effect on HPA function in its basal state as well as in response to an acute episode of restraint stress. The results showed that although no changes occur in basal corticosterone, young rats with selective cholinergic lesions have reduced hippocampal GR mRNA and dysregulation of HPA function in response to acute stress. Methods Experimental subjects Naive male Long-Evans rats were used in these experiments (300± 325g; Charles River Laboratories, Wilmington, MA). Separate groups were used to assess the effects of cholinergic lesions on corticoster- one receptor mRNA and on the corticosterone response to stress. All Correspondence: Dr Jung-Soo Han, as above. E-mail: jsh@jhu.edu Received 25 June 2002, revised 8 July 2002, accepted 25 July 2002 doi:10.1046/j.1460-9568.2002.02191.x European Journal of Neuroscience, Vol. 16, pp. 1399±1404, 2002 ã Federation of European Neuroscience Societies