Nicotinic acetylcholine receptors at sites of neurotransmitter release to the guinea pig intestinal circular muscle

Experiments were designed to test the hypothesis that nicotinic acetylcholi ne receptors (nAChRs) are present at sites of neurotransmission to the guin ea pig ileum circular smooth muscle. Circular smooth muscle preparations, f rom which the myenteric plexus had been removed (circular muscle-axon prepa ration), were used for this purpose. Nicotine and dimethylphenyl piperazini um iodide (10-100 mu M) induced contraction of the circular smooth muscle. Agonist-induced contraction was inhibited by 1 mu M scopolamine and abolish ed in the combined presence of 1 mu M scopolamine and 0.3 mu M CP 96,345-01 , a neurokinin-1 receptor antagonist. Contractions induced by electric fiel d stimulation (30 pulses, 0.5 ms, 70 V, 10 Hz) were abolished by 0.3 mM tet rodotoxin (TTX); in contrast, agonist-induced contraction was attenuated bu t not abolished by 0.3 mM TTX. Mecamylamine (3 or 30 mu M), an nAChR antago nist, blocked agonist-induced contractions. Frequency-response curves for b oth "ON" and "OFF" electric field stimulation contractions were abolished b y the combined presence of 1 mM scopolamine and 0.3 mM CP 96,345-01 or by 0 .3 mM TTX. At stimulation frequencies greater than 2 Hz, the ON contraction was increased in the presence of 100 mu M nitro-L-arginine. Mecamylamine ( 3 mu M) was used to block the stimulatory prejunctional nAChRs located near sites of neurotransmitter release to the circular smooth muscle; however, ON and OFF contractions were not affected by mecamylamine. Although the pre junctional nAChRs are not targets for endogenously released acetylcholine u nder the conditions tested here, these receptors may be targets for the dev elopment of new prokinetic agents.