60 Brain Researctt. 512 (1990) 60-t~9 Elsevier BRES 15289 Corticotropin-releasing factor receptors in the rabbit brain visualized by in vitro autoradiography S.Y. Chai 1, E. Tarjan 2, M.J. McKinley 2, G. Paxinos 3 and F.A.O. Mendelsohn J 1 University of Melbourne, Department of Medicine, Austin Hospital, Heidelberg, Vict. (Australia), 2Howard Florey Institute for Experimental Physiology and Medicine, Parkville, Vict. (Australia) and 3School of Psychology, University of New South Wales, Kensington, N.S. W (Australia) (Accepted 15 August 1989) Key words: Corticotropin-releasing factor; Receptor; Rabbit; Pineal gland; Autoradiography Corticotropin-releasing factor (CRF) binding sites were visualized in the rabbit brain by in vitro autoradiography using the radioligand lzsI-[Tyr°]ovineCRF. The radioligand binding to sections of rabbit cingulate cortex were competed for by ovine and rat CRF with inhibitory constants (Ki) of 26 and 37 nM, respectively, whereas sauvagine and alpha-helical CRFg_41were approximately 10-fold less potent. In the rabbit brain, the highest densities of binding sites for CRF are found in the pineal gland and the choroid plexus. The cerebral cortex is labelled throughout, with the highest concentration of binding sites in the piriform and primary olfactory divisions. In the cerebellar cortex, the granular layer is more intensely labelled than the molecular layer. The distribution of CRF binding sites in the hippocampus follows a laminar pattern; the molecular layer of the dentate gyrus is intensely labelled, the oriens, radiatum and lacunosum moleculare layers of Ammon's horn contain moderate densities of binding and no binding is observed in the granular layer of the dentate gyrus and the pyramidal cell layer. The ventral subnucleus of the lateral septum, the zonal and superficial layers of the superior colliculus contain high densities of receptors. A moderate concentration of binding sites is observed in the caudate nucleus, putamen, bed nucleus of the stria terminalis, paraventricular, anterodorsal and anteroventral thalamic nuclei and the medial nucleus of the mammillary body. The fundus striati, the dorsal subnucleus of the lateral septum, the lateral dorsal thalamic, lateral and medial geniculate nuclei, the lateral and anterior cortical amygdaloid nuclei, central and posterior interpeduncular nuclei all contain low but significant levels of CRF binding sites. No detectable binding is observed in the pons, medulla oblongata or fibre tracts. The high densities of CRF binding sites found in the cortex, hippocampus and structures associated with the limbic system supports a role for CRF in the modulation of behavioural and physiological responses associated with stress. However, the extremely high density of CRF binding sites found in the pineal gland and choroid plexus suggest a more extensive role for CRF in the rabbit central nervous system. INTRODUCTION Corticotropin-releasing factor (CRF) was first isolated from ovine hypothalami as the secretagogue of ACTH and fl-endorphinz°. The role of this peptide in the central nervous system, however, was found to be more exten- sive. In the rat, intracerebro-ventricular administration of CRF increased blood pressure and heart rate 16, increased grooming is and locomotor activity 18, potentiated the effects of exposure to a novel environment 7 and de- creased food intake 1°. The central administration of CRF induced behavioural effects which resemble responses to acute stress. Similar effects were also observed in the rabbit after intracerebroventricular administration of CRF, namely, increased respiratory rate 1, increased exploratory activity and reduced food intake 19. In addi- tion, infusion of CRF into the cerebral ventricles of the rabbit induced a large sodium appetite 19. CRF-like immunoreactivity is distributed extensively within the rat central nervous system where high con- centrations of CRF-immunoreactive cell bodies were found in regions including the paraventricular and su- praoptic hypothalamic nuclei, the median preoptic area, locus coeruleus and parabrachial nucleus3"15. The major site of CRF synthesis in the rat brain is the paraventric- ular hypothalamic nucleus21, although CRF mRNA has also been detected in the inferior olivary nucleus and nucleus of the solitary tract 12. Receptor binding sites for CRF have been demonstrated in the rat brain where they are concentrated in the cortex and structures associated with the limbic system 4'22. This study investigates the distribution of CRF binding sites in the rabbit central nervous system using quanti- tative in vitro autoradiography in order to elucidate possible central sites where CRF may exert its action. MATERIALS AND METHODS Source of peptide~ The radioligand used was 125I-[Tyr°]ovine CRF, (spec. act. 2200 Ci/mmol) which had been purified by reverse-phase high pressure Correspondence: S.Y. Chai, University of Melbourne, Department of Medicine, Austin Hospital, Heidelberg, Vict. 3084, Australia. 0006-8993/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)