1 Isolated, desheathed preparations of the rabbit rectococcygeus muscle were relatively insensitive to spasmogens other than
muscarinic drugs. Transmural stimulation with 1-50 pulses (0.2-0.4 ms
at 10 Hz) elicited graded twitches which were abolished by
tetrodotoxin and were therefore neurogenic; longer pulses sometimes triggered
tetrodotoxin-resistant myogenic contractions.2 Twitches elicited by 0.2-0.4 ms pulses were due to post-ganglionic excitation because they were not reduced by
hexamethonium,
pentolinium or
dimethyltubocurarine, or by
ganglion-paralyzing concentrations of
nicotine.3 The acetyl- and butyryl-
cholinesterase activities of the rectococcygeus were determined manometrically and could be selectively inhibited by BW 284C51 (1:5-bis-(4-allyl-dimethylammonium-phenyl)-pentan-3-one dibromide) and
iso-OMPA (tetramonoisopropylpyrophosphortetramide), respectively. Single-pulse twitches were greatly potentiated in amplitude and duration only when both
cholinesterases were inhibited.4 The preparations could not be made to contract by
nicotine (2.1-21 muM) even after
cholinesterase inhibition, suggesting an absence of
ganglion-cells; with
nicotine (105-210 muM) small,
atropine-susceptible responses were elicited, which were non-ganglionic because they were not reduced by
tetrodotoxin.5 Rectococcygeus preparations that had been treated with
physostigmine released
acetylcholine into the bath fluid on electrical stimulation.6 The motor transmission was paralyzed by
botulinum toxin (Type A) and abolished by
atropine; the block of
muscarinic receptors by
atropine was effective against both endogenous and exogenous
acetylcholine.7 Inhibitory
adrenoceptors and scanty motor alpha-
adrenoceptors were detected in the smooth muscle.8 Strong inhibitions of motor transmission and of rhythmic activity were produced by
noradrenaline (but not by
tyramine), by
isoprenaline, and, after
phentolamine, also by
adrenaline and
phenylephrine. These inhibitions were only slightly reduced by
propranolol and rather more by
pindolol.9 Experiments on preparations retaining their extrinsic nerve supply suggest an absence of ganglionic relays in the last 1-2 cm of the motor nerve pathway to this muscle.10 Some contrasting properties of the adjacent caudo-anal muscle, including the poor motor responses to transmural stimulation, are described.