Effect of opioid compounds on feeding and activity of the cockroach, Periplaneta americana

Opioid peptides have been implicated in regulation of feeding in invertebrates. Studies have suggested that receptors for opioids are present in cockroaches and that these receptors play roles in affecting both behaviour and feeding. We examined the effect of μ, δ, and κ opioid receptor agonists and antagonists on feeding, mass changes and activity in the cockroach, Periplaneta americana. The κ antagonist, nor-binaltorphimine, significantly increased food intake, while naltrexone (general antagonist) and naloxonazine (μ antagonist) both reduced feeding. A large mass loss was observed in cockroaches treated with nor-binaltorphimine, despite the increased food intake. Males did not lose as much mass during the 3 h as females, although drug treatment did have some effect on the loss. Time of activity (%) was not influenced by any drug. Water loss experiments suggested that nor-binaltorphimine increased water loss, accounting for the mass loss despite the increased feeding. We suggest that two populations of opioid receptors are present as previously reported, with one affecting feeding and the other involved with evaporative water loss.