Evidence that orexin-A-evoked grooming in the rat is mediated by orexin-1 (OX1) receptors, with downstream 5-HT2C receptor involvement.
BACKGROUND
Orexins A and B have recently been discovered and shown to be derived from preproorexin, primarily expressed in the rat hypothalamus. Orexin-A has been ascribed a number of in vivo functions in the rat after intracerebroventricular (ICV) administration, including hyperphagia, neuroendocrine modulation and, most recently, evidence for a behavioural response characterised by an increase in grooming.


OBJECTIVE
Here, we have investigated the orexin-receptor subtypes involved in the grooming response to orexin-A (3 microg, ICV) in the rat.


METHODS
Male rats, habituated to clear Perspex behavioural observation boxes, were pretreated with antagonists with mixed selectivity for OX1, OX2, 5-HT2B and 5-HT2C receptor subtypes prior to the administration of orexin-A and the intense grooming response elicited by this peptide assessed.


RESULTS
Pretreatment of rats with a mixed OX1/5-HT2B/2C receptor antagonist 1-(4-methylsulfanylphenyl)-3-quinolin-4-ylurea (SB-284422), revealed a significant, but incomplete, blockade of orexin-A-induced grooming. Despite the low potency of orexin-A at 5-HT2B and 5-HT2C receptors in vitro (pKi<5), studies were undertaken to determine whether downstream 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, pKB < 5.15) failed to effect orexin-A-induced grooming, the selective 5-HT2C receptor antagonist, SB-242084 (1 mg/kg, IP, 5-HT2C, pKi = 8.95; OX1, pKB < 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-induced grooming by the subsequently identified selective OX1 receptor antagonist 1-(2-methylbenzoxazol-6-yl)-3-[1,5]naphthyridin-4-yl urea hydrochloride, SB-334867-A (OX1, pKB = 7.4; OX2, pKB = 5.7), devoid of appreciable affinity for either 5-HT2B (pKi < 5.3) or 5-HT2C (pKi < 5.4) receptors, provides the first definitive evidence that a central behavioural effect of orexin-A (grooming) is mediated by OX1 receptors.


CONCLUSIONS
This data suggests that orexin-A indirectly activates 5-HT2C receptors downstream from OX1 receptors to elicit grooming in the rat. The use of SB-334867-A in vivo will enable the role of OX,1 receptors within the rat central nervous system to be further characterised.
Duxon
M S
MS
Neuroscience Research, SmithKline Beecham Pharmaceuticals, Harlow, Essex, UK. Mark_S_Duxon@sbphrd.com
Stretton
J
J
Starr
K
K
Jones
D N
DN
Holland
V
V
Riley
G
G
Jerman
J
J
Brough
S
S
Smart
D
D
Johns
A
A
Chan
W
W
Porter
R A
RA
Upton
N
N
D000818:Animals; D002352:Carrier Proteins / Q000494:pharmacology*; D003001:Cloning, Molecular; D006120:Grooming / Q000187:drug effects*; D047908:Intracellular Signaling Peptides and Proteins; D008297:Male; D009043:Motor Activity / Q000187:drug effects; D009479:Neuropeptides / Q000494:pharmacology*; D018377:Neurotransmitter Agents / Q000494:pharmacology*; D064446:Orexin Receptors; D000068797:Orexins; D010879:Piperazines / Q000494:pharmacology; D011804:Quinolines / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D044405:Receptor, Serotonin, 5-HT2C; D043562:Receptors, G-Protein-Coupled; D018013:Receptors, Neuropeptide / Q000378:metabolism*; D011985:Receptors, Serotonin / Q000187:drug effects*; D017366:Serotonin Receptor Agonists / Q000494:pharmacology
D002352:Carrier Proteins; D047908:Intracellular Signaling Peptides and Proteins; D009479:Neuropeptides; D018377:Neurotransmitter Agents; D064446:Orexin Receptors; D000068797:Orexins; D010879:Piperazines; D011804:Quinolines; D044405:Receptor, Serotonin, 5-HT2C; D043562:Receptors, G-Protein-Coupled; D018013:Receptors, Neuropeptide; D011985:Receptors, Serotonin; C425003:SB284422; D017366:Serotonin Receptor Agonists; C015068:1-(3-chlorophenyl)piperazine