INFLUENCE OF MORPHINE ON THE ACTIVITY OF LOW-THRESHOLD VISCERAL MECHANORECEPTORS IN CATS WITH ACUTE PERICARDITIS

The purpose of this investigation was to test whether morphine (morphi num hydrochloricum) applied to the receptive field of the thoracic vis ceral afferent fibres modifies their activity. Experiments were perfor med on chloralose-anaesthetised cats, paralysed and artificially venti lated, in a state of pericarditis that was induced by intrapericardial injection of h-carrageenan and kaolin. Resulting acute inflammation w as proven histopathologically and documented electrocardiographically. Single afferent fibres with receptive fields in thoracic viscera were dissected from thoracic sympathetic chain (19 fibres), as well as the vagus nerve (9 fibres). All tested fibres transmitted sensory informa tion from the low-threshold mechanoreceptors. As a final result, it wa s found that morphine (0.001-1.0 mg/ml) when applied locally activates , depending on the dose, afferent fibres as follows: 12 sympathetic af ferents (out of 12 tested), and 7 vagal afferents (out of 9 tested). I n examining the specificity of morphine action, the preliminary local application of naloxone (1.0 mg/ml) just before morphine, blocked all excitatory responses. The excitatory response was present whether the receptive field was located in the inflammatory area, or outside it, i n group III or IV fibres.