Capsaicin-sensitive innervation of the guinea-pig taenia caeci

1. Stimulation (2--50 Hz) of the mesenteric nerves of the guinea-pig taenia caeci gave rise to contraction of the muscle obtained from animals pretreated with the adrenergic neuron-blocking agent guanethidine. Contraction was the response to stimulation at low frequencies (2--5 Hz) in about half of the untreated preparations as well. 2. The response was abolished by hyocine (4.5 x 10(-7) M), but was unaffected by the ganglionic blocking agent mecamylamine (4.9 x 10(-5) M). Physostigmine (2.4 x 10(-8) M) enhanced the contractions. 3. Capsaicin (9.8 x10(-6) M) elicited a contraction of the taenia caeci followed by a long-lasting tachyphylaxis. Contraction in response to stimulation of the mesenteric nerves was absent after this pretreatment. 4. Neither the response to direct excitation of the cholinergic neural elements of the myenteric plexus, nor the relaxation caused by stimulation of adrenergic fibres were influenced by capsaicin. "Purinergic" relaxation produced by field stimulation (0.5--10 Hz) remained also unchanged. 5. No functional evidence has been found for the presence of parasympathetic preganglionic fibres among the perivascular nerves applying the taenia. 6. It is concluded that capsaicin-sensitive nerves excite cholinergic neurons of the myenteric plexus.