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Notes: Abstract A range of agonists and antagonists were used to characterize the receptors through which 5-hydroxytryptamine (5-HT) contracts and relaxes the longitudinal muscle of segments of guinea-pig distal colon, in vitro. 5-HT contracted the longitudinal muscle over the concentration range 10−9 to 10−4 mol/l. The 5-HT3 receptor agonist, 2-methyl-5-HT, produced concentration dependent contractions over the range 10−6 to 10−4 mol/l. 5-methoxytryptamine, an agonist at 5-HT4 receptors, caused contractions over a concentration range of 10−8 to 10−4 mol/l. The 5-HT4 antagonist, SDZ 205-557 (5 × 10−7 mol/l) substantially suppressed the responses to low concentrations of 5-HT and to 5-methoxytryptamine, but had no effect on the responses to higher concentrations of 5-HT. In contrast, the 5-HT3 antagonist, granisetron (10−6 mol/l), blocked the effect of 2-methyl-5-HT and substantially depressed responses to high concentrations of 5-HT, but had no effect on lower concentrations of 5-HT. Granisetron produced a small reduction in the response to 5-methoxytryptamine. Tetrodotoxin (TTX) (3 × 10−7 mol/l) almost abolished the response to 5-methoxytryptamine and markedly suppressed the response to 2-methyl-5-HT, but the responses to 5-HT were only partially reduced. The 5-HT, antagonist, methiothepin 10−6 mol/l. depressed the response to 5-HT 10−7 to 10−4 mol/l. and blocked its TTX insensitive component. The 5-HT2 antagonist, ketanserin, in concentrations up to 10−5 mol/l, had no effect on the contractions evoked by 5-HT. The response to 5-HT was substantially depressed by hyoscine (3 × 10−6 mol/l. The tachykinin antagonist, spantide 10−5 mol/l. depressed the response to 5-HT but to a lesser extent than hyoscine. Spantide and hyoscine combined completely blocked the contractile responses to 5-HT Responses to 2-methyl-5-HT were partially suppressed by hyoscine (3 x 10−6 mol/l. and spantide (10−5 mol/l) and completely blocked when both byoscine and spantide were present. Contractions evoked by 5-methoxytryptamine were partially blocked by hyoscine (3 × 10−6 mol/l) and were unaffected by spantide (10−5 mol/l), but a combination of hyoscine and spantide completely blocked such responses. When the excitatory transmission was blocked with hyoscine (3 × 10−6 mol/l) and spantide 10−5 mol/l) and the tone of the muscle raised, an inhibitory response to 5-HT was revealed that had a threshold concentration between 10−7 mol/l) and 3 × 10−7 mol/l, and a maximum effect at 10−4 mol/l. It was blocked by TTX (3 × 10−7 mol/l) and granisetron 10−6 mol/l. while N-nitro-l-arginine (NOLA) (10−4 mol/l) and SDZ 205-557 (5 × 10−7 mol/l) had no effect. Apamin A 10−6 mol/l. partially suppressed this response. It is concluded that 5-HT3, 5-HT4 and 5-HT1-like receptors mediate contraction of the longitudinal muscle of the distal colon. The 5-HT3 and 5-HT4 receptors are located on the excitatory motor neurons innervating the longitudinal muscle and the 5-HT1-like receptor is located on the muscle. 5-HT3 receptors are also found on inhibitory neurons to the muscle.