Journal of Chemical Neuroanatomy 21 (2001) 63 – 73 Localization of 5-HT 7 receptors in rat brain by immunocytochemistry, in situ hybridization, and agonist stimulated cFos expression J.F. Neumaier a , T.J. Sexton a , J. Yracheta b , A.M. Diaz b , M. Brownfield b, * a Department of Psychiatry and Behaioral Sciences and Harboriew Medical Center Uniersity of Washington, Seattle WA 98195, USA b Department of Comparatie Biosciences and Neurosciences Training Program Uniersity of Wisconsin Madison WI 53706, USA Received 15 June 2000; received in revised form 31 August 2000; accepted 11 September 2000 Abstract 5-HT 7 receptors are recently identified members of the serotonin receptor family that have moderate to high affinity for several important psychotropic drugs. However, the lack of selective ligands has impeded the study of the brain distribution of these receptors. In this report, we describe the localization of 5-HT 7 receptor in rat forebrain by immunocytochemistry, in situ hybridization of 5-HT 7 mRNA, and functional stimulation of cFOS expression by 5-HT 7 receptor activation. The anatomical localization of 5-HT 7 mRNA in situ hybridization signal. Prominent immunostaining was apparent in numerous sites within the cerebral cortex, hippocampal formation, tenia tecta, thalamus and hypothalamus. 5-HT 7 receptors were detected in suprachias- matic nucleus by both immunocytochemistry and in situ hybridization. At a microscopic level, both cell bodies and proximal fibers were strongly stained in these regions, suggesting a somatodendritic subcellular distribution. 5-HT 7 receptor-like immunoreactivity was further compared with 5-HT 7 mediated biological function by administering 8-OH-DPAT intracerebroventricular injection (icv)with WAY 100135 (to block 5-HT 1A receptors) followed by double immunostaining localization of cFos activation and 5-HT 7 receptors. In all regions examined, cFos stimulation and 5-HT 7 -like immunoreactivity colocalized to the same neurons. Furthermore, cFos activation by 8-OH-DPAT was blocked by pimozide — a 5-HT 7 antagonist. Therefore, by using multiple strategies, we were able to localize 5-HT 7 receptors in rat brain unequivocally. The distribution of these receptors is consistent with their involvement in the control of circadian activity and the action of anti-depressants and atypical neuroleptics. © 2001 Elsevier Science B.V. All rights reserved. Keywords: 5-HT 7 receptor; cFos; In situ hybridization; Immunocytochemistry; Serotonin receptor www.elsevier.com/locate/jchemneu 1. Introduction The field of serotonin receptors has expanded from a few identified receptors to 14 cloned receptors during the last decade (Barnes and Sharp, 1999). These recep- tors are now being found to have diverse cellular distributions, modes of signal transduction, and biolog- ical functions. The recently cloned rat 5-HT 7 receptor (Bard et al., 1993; Lovenberg et al., 1993; Ruat et al., 1993; Shen et al., 1993; Tsou et al., 1994), has several alternative mRNA splice variants (Heidmann et al., 1997; Stam et al., 1997), all of which can stimulate cAMP synthesis by adenylate cyclase, when activated by serotonin. Understanding the biological function of 5-HT 7 receptors has been hampered, however, by the lack of highly selective agonists and antagonists to study its in vivo distribution and pharmacology of the receptor (Vanhoenacker et al., 2000). The 5-HT 7 recep- tor is unique in that it shares some pharmacological overlap with the 5-HT 1A receptor (high affinity for 8-hydroxy-2-(di-n -propylamino) tetralin (8-OH-DPAT) and 5-carboxamidotryptamine) and with the 5-HT 2A receptor (high affinity for clozapine, risperidone, mesu- lergine, and pimozide). Indeed, the earlier described effects of some of these drugs may actually be mediated by 5-HT 7 instead of 5-HT 1A or 5-HT 2A receptors. A case in point is the recent evidence that 5-HT 7 and not 5-HT 1A receptors account for 8-OH-DPAT induced * Corresponding author. Tel.: +1-608-2635863; fax: +1-608- 2633926. E-mail address: brownm@svm.vetmed.wisc.edu (M. Brownfield). 0891-0618/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0891-0618(00)00092-2