Diurnal variations in the feeding response to 8-OH-DPAT injected into the dorsal or median raphe

To determine the impact on food intake of the 5-HT1A agonist, 8-OH-DPAT, administered at various times throughout the nocturnal cycle, rats were implanted with cannulae in either the dorsal or median raphe nuclei and injected with saline or 8-OH-DPAT (0.4, 0.8 or 1.6 nmol) immediately following dark onset, or during the mid or late dark periods. 8-OH-DPAT potentiated feeding when administered in the mid or late dark phase, whereas treatment during the early dark actually suppressed food intake. These findings indicate that the feeding responses to dorsal and median raphe 8-OH-DPAT vary greatly depending on the time of administration in the diurnal cycle and are consistent with recent reports of a biphasic action of this compound on ingestive behaviour.