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Influence of Dopaminergic and Noradrenergic Systems on the Release of Opioid Peptides in Guinea‐pig Ileum

Neuroleptic drugs increase the biosynthesis and release of opioid peptides from the myenteric plexus of guinea‐pig ileum. In the present work, the involvement of dopamine receptors or α‐adrenoceptors in the release of opioids from the myenteric plexus of guinea‐pig was investigated. Acute or chronic treatment with prazosin, an α1‐blocking drug, produced no changes in the release of these peptides. Release was also unchanged after acute or chronic treatment with the α2‐blocking drug yohimbine. However, treatment with domperidone, a selective dopamine receptor antagonist, resulted in an increase in the release of opioids, as did treatment with (‐)−3−(3−hydroxyphenyl)‐N‐n‐propylpiperidine ((‐)−3−PPP), a dopamine autoreceptor stimulant. It is concluded that the effect of neuroleptics on the release of opioids from myenteric plexus is due to the blockade of dopamine receptors, and that interruption of dopaminergic transmission produces an increase in opioid release at this level.