Neuroimmunology NeuroReport 0959-4965 # Lippincott Williams & Wilkins Localization of the 5-HT 1A receptors in the brain of opossum Monodelphis domestica Rouzanna L. Djavadian, CA Ewa Wielkopolska, Katarzyna Bialoskorska and Kris Turlejski Department of Neurophysiology, Nencki Institute of Experimental Biology, 3 Pasteur St, 02-093 Warsaw, Poland CA Corresponding Author THIS paper describes the distribution of 5-HT 1A recep- tors in the brain of opossum Monodelphis domestica. They were visualized by immunohistological staining with an antibody against the amino acid sequence (170± 186) of this receptor that was previously successfully used in the rat and monkey. As in Eutherians, high levels of immunostaining were present in the septum, hippocampus, raphe nuclei and some other brain stem nuclei. Neocortex, several thalamic nuclei and hypotha- lamus showed moderate density of the labeled struc- tures. Moderate levels of 5-HT 1A receptors were also observed in the caudate nucleus and putamen, unlike in the rat, in which labeling in these nuclei was almost absent. Another difference with the rat was observed in the neocortex: in the opossum immunostaining was absent in the layer 4 of many cortical areas. In general, distribution and density of this important receptor in the opossum is very similar to that described in the rat and monkey and therefore it follows a general mamma- lian pattern. NeuroReport 10:3195±3200 # 1999 Lippin- cott Williams & Wilkins. Key words: Cerebral cortex; Hippocampus; 5-HT 1A re- ceptors; Immunocytochemistry; Opossum; Raphe nuclei Introduction Serotonin (5-HT) is an important neurotransmitter in the CNS of adult mammals, in¯uencing neurons in both synaptic and paracrine ways [1,2]. It is also an important developmental signal in the immature brain [3]. Serotonin exerts those various in¯uences through a diverse family of 5-HT receptors, consist- ing of seven types and at least 14 subtypes [4]. One of the most frequently expressed and functionally important members of this family is the 5-HT 1A receptor subtype. Its amino acid sequence in the rat was described by Alberts et al. [5]. Activation of the 5-HT 1A receptor hyperpolarizes neuronal membrane and lowers probability of spike generation [6,7]. Recently, investigators using either radioligand auto- radiography, mRNA in situ hybridization and im- munohistology showed that high levels of this receptor are expressed in the rat's hippocampus, septum, cerebral cortex and brain stem [8±13]. A similar distribution of this receptor was found in monkey and human [14,15]. The grey short-tailed opossum ( Monodelphis do- mestica), one of the South-American marsupial spe- cies, was recently established as a laboratory animal [16]. An advantage of this easily bred mammal is that its pups are born at a very early stage of development, in most structures of the central nervous system corresponding to that in the 14-day- old rat embryo or 6- to 8-week-old human embryo [17]. Therefore, various experimental procedures may be applied ex utero during the phase of cortical plate formation and before axons start to extend into the optic nerve. Descriptions of the brain structures of this spe- cies, though still fragmentary, include several im- portant studies, like that of development of serotoninergic innervation in the early postnatal period [18]. Distribution of the serotoninergic re- ceptors has not yet been investigated in the opos- sum, even though their role in the developmental processes may be important. As a ®rst step towards that goal, we attempted to describe the distribution of the 5-HT 1A receptors in the brain of adult Monodelphis, and compare it to the distribution of this receptor in the brains of the previously investi- gated Eutherian species. We found that the same polyclonal antibody as has been previously used for detection of the 5-HT 1A receptors in the brain and spinal cord of the rat and monkey [14,19], binds to the 5-HT 1A receptors in the brain of Monodelphis, and therefore can be used for their immunohisto- chemical labeling. This antibody recognizes amino acid sequence 170±186 of the rat 5-HT 1A receptor that is present in its second extracellular loop. This sequence was shown to be highly conserved in evolution, as it is identical in rats and human [5,20]. NeuroReport 10, 3195±3200 (1999) Vol 10 No 15 19 October 1999 3195