SB-334867, a selective orexin-1 receptor antagonist, enhances behavioural satiety and blocks the hyperphagic effect of orexin-A in rats

Intracerebroventricular (i.c.v.) administration of the novel hypothalamic n europeptide orexin-A stimulates food intake in rats, and delays the onset o f behavioural satiety (i.e. the natural transition from feeding to resting) . Furthermore, preliminary findings with the selective orexin-1 receptor an tagonist, SB-334867, suggest that orexin-A regulation of food intake is med iated via the orexin-1 receptor. At present, however, little is known about either the intrinsic effects of SB-334867 on the normal structure of feedi ng behaviour, or its effects upon orexin-A-induced behavioural change. In t he present study, we have employed a continuous monitoring technique to cha racterize the effects of SB-334867 (3-30 mg/kg, i.p.) on the microstructure of rat behaviour during a 1-h test with palatable wet mash. Administered a lone, SB-334867 (30 mg/kg, but not lower doses) significantly reduced food intake and most active behaviours (eating, grooming, sniffing, locomotion a nd rearing), while increasing resting. Although suggestive of a behavioural ly nonselective (i.e. sedative) action, the structure of feeding behaviour was well-preserved at this dose level, with the reduction in behavioural ou tput clearly attributable to an earlier onset of behavioural satiety. As pr eviously reported, orexin-A (10 mug per rat i.c.v.) stimulated food intake, increased grooming and delayed the onset of behavioural satiety. Pretreatm ent with SB-334867 dose-dependently blocked these effects of orexin-A, with significant antagonism evident at dose levels (3-10 mg/kg) below those req uired to produce intrinsic behavioural effects under present test condition s. Together, these findings strongly support the view that orexin-A is invo lved in the regulation of feeding patterns and that this influence is media ted through the orexin-1 receptor.