Evidence that orexin-A-evoked grooming in the rat is mediated by orexin-1 (OX1) receptors, with downstream 5-HT2C receptor involvement

Rationale: Orexins A and B have recently been discovered and shown to be de rived from preproorexin, primarily expressed in the rat hypothalamus. Orexi n-A has been ascribed a number of in vivo functions in the rat after intrac erebroventricular (ICV) administration, including hyperphagia, neuroendocri ne modulation and, most recently, evidence for a behavioural response chara cterised by an increase in grooming. Objectives: Here, we have investigated the orexin-receptor subtypes involved in the grooming response to orexin-A (3 mug, ICV) in the rat. Methods: Male rats, habituated to clear Perspex b ehavioural observation boxes, were pretreated with antagonists with mixed s electivity for OX1, OX2, 5-HT2B, and 5-HT2C receptor subtypes prior to the administration of orexin-A and the intense grooming response elicited by th is peptide assessed. Results: Pretreatment of rats with a mixed OX1/5-HT2B/ 2C receptor antagonist 1-(4-methylsulfanylphenyl)-3-quinolin-4-ylurea (SB-2 84422), revealed a significant, but incomplete, blockade of orexin-A-induce d grooming. Despite the low potency of orexin-A at 5-HT2B and 5-HT2C recept ors in vitro (pKi<5), studies were undertaken to determine whether downstre am 5-HT2B or 5-HT2C receptors mediate in the grooming-elicited by orexin-A. Whilst the selective 5-HT2B receptor antagonist, SB- 215505 (3 mg/kg, PO, 5-HT2B, pKi=8.58; OX1, pK(B)<5.15) failed to effect orexin-A-induced groomi ng, the selective 5-HT2C receptor antagonist, SB-242084 (1 mg/kg, IP, 5-HT2 C, pKi=8.95; OX1, pK(B)<5.1) potently antagonised the grooming response to this peptide. This suggested that the partial blockade of orexin-A-induced grooming obtained with SB-284422 might be attributable to its 5-HT2C and/or OX1 receptor blocking activity. However, complete blockade of orexin-A-ind uced grooming by the subsequently identified selective OX, receptor antagon ist 1-(2-methylbenzoxazol-6-yl)-3-[1,5]naphthylidin-4-yl urea hydrochloride , SB-334867-A (OX1, pK(B)=7.4; OX2, pK(B),=5.7), devoid of appreciable affi nity for either 5-HT2B (pKi<5.3) or 5-HT2C (pKi<5.4) receptors, provides th e first definitive evidence that a central behavioural effect of orexin-A ( grooming) is mediated by OX1 receptors. Conclusions: This data suggests tha t orexin-A indirectly activates 5-HT2C receptors downstream from OX1 recept ors to elicit grooming in the rat. The use of SB-334867-A in vivo will enab le the role of OX1 receptors within the rat central nervous system to be fu rther characterised.