Effects of pimozide and naloxone on latency for hypothalamically induced eating

Latency to feed in response to lateral hypothalamic electrical stimulation was assessed at a variety of stimulation frequencies using moderate stimulation intensity; shorter latencies accompanied higher stimulation frequencies. Pimozide (0.0625, 0.125, 0.25 and 0.5 mg/kg, i.p.) and naloxone (0.5, 1.0, 2.0 and 4.0 mg/kg, i.p.) increased latencies in a dose-dependent manner. At 0.5 mg/kg (4 X the lowest effective dose), pimozide blocked eating completely for most trials and in all animals. Naloxone attenuated eating at 1.0 mg/kg, but 4.0 mg/kg was not more effective than 2.0 mg/kg and did not block the behavior completely in any animal. With each drug, latencies were normal or near-normal at the beginning of each session and anorexic effects developed with repeated testing. Thus neither drug caused a simple overall impairment in performance capability. In both cases it is inferred that the proximal food cues lost the ability to maintain eating well before the distal food cues lost the ability to elicit eating. The similarity between the effects of pimozide and naloxone raises the possibility that opioid-containing and dopamine-containing neurons interact to influence feeding through common brain circuitry.