Three experiments investigated a possible effect of nitrous oxide (N2O
) on food intake in nondeprived male hooded rats in independent groups
designs. Experiment 1 demonstrated a concentration-related increase i
n intake with increasing level of nitrous oxide (10-40% N2O), reaching
statistical significance at 20% N2O when compared to room air control
s (p < 0.05). In experiment 2, pretreatment with 10 and 20 mg/kg of th
e benzodiazepine antagonist, flumazenil, failed to significantly atten
uate 30% N2O-induced hyperphagia. In Experiment 3, pretreatment with t
he opioid antagonist, naltrexone, effectively antagonized 30% N2O-indu
ced hyperphagia. Pronounced attenuation (to 59% of 30% N2O-induced int
ake level over a 1 h period) at the lowest dose of naltrexone (0.1 mg/
kg, p < 0.01) compared to vehicle level resulted in a shallow dose-res
ponse curve across the dose range tested (0.1-10.0 mg/kg). These resul
ts suggest that an endogenous opioid mechanism is prominently involved
in the N2O-induced ingestive response.