Opioids play a big role in pain transmission, reward mechanisms and addiction in mammals, and were already established when vertebrates evolved jaws. So how old are the opioids, and why might we care? By tracing the history of opioids we might gain some insight into their original function as well as their complete functions higher vertebrates. Logically, if you are an insect physiologist in training working in the laboratory of Dr Paul Cooper, a good way to look for physiological activity of opioids in invertebrates, would be to test them on insects, e.g. cockroaches…
…so we took a bunch of different opioids and injected them into cockroaches to see what happened.
Here the abstract of the paper:
Opioid peptides have been implicated in regulation of feeding in invertebrates and studies in the ’70s and ’80s suggested that receptors for opioids were present in cockroaches and that these receptors play roles in affecting both behaviour and feeding. We examined the effect of three classes – μ, δ, and κ – of 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
Publications arising from this research:
Cooper, P.D., Dennis, S.R., Woodman, J. D., Cowlings, A., Donnelly, C. (2010) Effect of opioid compounds on feeding and activity of the cockroach, Periplaneta americana. Comparative Biochemistry and Physiology, Part C 151:298-302