Stimulation of vagal afferent neurones serving the gut inhibits food intake and activates autonomic reflexes regulating digestion. Some of these effects are mediated by the intestinal hormone cholecystokinin (CCK) which acts on CCK-A receptors expressed by vagal afferent neurones (Burdyga et al. 2001). The neuropeptides orexin A and B are produced in the hypothalamus and gut, and act at orexin-1 or -2 receptors (OX-R1, -R2) to stimulate food intake (Kirchgessner, 2002). We have examined OX-R1 expression by vagal afferent neurones.

The expression of OX-R1 in rat nodose ganglia was examined by RT-PCR, cloning and sequencing, and by immunocytochemistry, using methods similar to those described recently (Burdyga et al. 2001). The effect of orexin A on the discharge of afferent nerve fibres from the jejunum was studied in rats anaesthetised with pentobarbitone (60 mg kg^-1, I.P.) and then humanely killed (Lal et al. 2001).

Primers specific for rat OX-R1 revealed a band of the predicted size in rat nodose ganglia following RT-PCR. Cloning and sequencing of the product confirmed its identity. Primers specific for the orexin precursor and OX-R2 did not yield products in RT-PCR of the same starting material. OX-R1-, but not OX-2R-like immunoreactivity was identified in a subset of neurones where the localisation was predominantly vesicular. Approximately 80 % of these neurones also expressed the CCK-A receptor.

Administration of orexin A (up to 30 nmol, I.A.) did not change the discharge of intestinal afferent nerve fibres in anaesthetised rats. In contrast, CCK8 (100 pmol, I.V.) stimulated the discharge of these nerve fibres. Administration of orexin A (1 nmol, I.V.) immediately prior to CCK8 significantly inhibited responses (peak increase in discharge after vehicle and CCK, 114 ± 11 % compared with CCK alone; vs. 72 ± 5 % after orexin A and CCK; mean ± S.E.M., n = 5Ð8, P < 0.05, Mann-Whitney rank sum test).

We conclude that vagal afferent neurones expressing the CCK-A receptor may also express the OX-R1, and that stimulation of the latter inhibits responses to CCK. Stimulation of food intake by orexin might therefore be mediated, partly, by suppressing CCK-evoked satiety signals from the gut.

All procedures accord with current UK legislation.