Opioid-induced depression in the lamprey respiratory network

The role of opioid receptors in modulating respiratory activity was investigated in in vitro brainstem preparations of adult lampreys by bath application of agonists and antagonists. The vagal motor output was used to monitor respiratory activity. Neuronal recordings were also performed to characterize the rostrolateral trigeminal region that has been suggested to be critical for respiratory rhythmogenesis. Microinjections of the μ-opioid receptor agonist [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) were also made into this region and at different locations within the brainstem. Bath application of DAMGO (0.5–2 μM) caused marked decreases in respiratory frequency up to complete apnea. Bath application of the δ-opioid receptor agonist [d-Pen2,5]-enkephalin (DPDPE) at 10–40 μM induced less pronounced depressant respiratory effects, while no changes in respiratory activity were induced by the κ-opioid receptor agonist trans-(1S,2S)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetamide (U50488) at 10–40 μM. Bath application of the opioid receptor antagonists naloxone and naltrindole did not affect baseline respiratory activity, but prevented agonist-induced effects. DAMGO microinjections (1 mM; 0.5–1 nl) at sites rostrolateral to the trigeminal motor nucleus, where respiration-related neuronal activity was recorded, abolished the respiratory rhythm. The results show that opioids may have an important role in the lamprey respiratory network and that μ-opioid receptor activation is the most effective in causing respiratory depression. They also indicate that endogenous opioids are not required for the generation of baseline respiratory activity. Apneic responses induced by DAMGO microinjections support the hypothesis that a specific opioid-sensitive region rostrolateral to the trigeminal motor nucleus, that we have termed the paratrigeminal respiratory group (pTRG), likely has a pivotal role in respiratory rhythmogenesis. Since the lamprey diverged from the main vertebrate line around 450 million years ago, our results also imply that the inhibitory role of opioids on respiration is present at an early stage of vertebrate evolution.