Dibutyryl cyclic GMP, morphine and Met-enkephalin: lack of common actions on neuronal firing.

The effects of microiontophoretically administered dibutyryl cyclic GMP (dbcGMP), morphine and Met-enkephalin on neuronal firing in the mesencephalic reticular formation of the rat were examined. DbcGMP did not mimic the ability of either opioid to block neuronal firing evoked by a nociceptive stimulus. Furthermore, dbcGMP did not potentiate the ability of the opioid to block evoked neuronal firing. The only effect of dbcGMP was an occasional enhancement of both evoked and spontaneous neuronal firing. These data may indicate that the effects of morphine and Met-enkephalin in the mesencephalic reticular formation of the rat are not linked to cyclic GMP.