Evidence against VIP-involvement in neurogenic relaxations of the mouse anococcygeus muscle

Biology of the anococcygeus muscle

The anococcygeus is a smooth muscle tissue of the urogenital tract which, in the male, runs on to form the retractor penis. The motor innervation is classically sympathetic with noradrenaline as transmitter, but the relaxant parasympathetic transmitter has only recently been identified as nitric oxide. Indeed, the anococcygeus has provided an extremely useful model with which to probe the mechanisms underlying this novel nitrergic system, including the importance of physiological antioxidants in maintaining the potency of nitric oxide as a neurotransmitter. The cellular mechanisms of contraction and relaxation are slowly being clarified, with particular interest in the contribution of capacitative calcium entry and the guanylyl cyclase/cyclic GMP system. Many questions remain unanswered, however, including the precise physiological role of the muscle, the identity of substances released from subcellular vesicles of nitrergic nerves, the unusual sensitivity of the tissue to certain peptides (oxytocin and urotensin II), and the nature of store-operated channels through which calcium enters the cell to maintain contraction.

A comparison of the effects of capsaicin with inhibitory nerve stimulation in the rat anococcygeus muscle in vitro

Capsaicin was used to test whether centrifugal activation of sensory fibres in the rat anococcygeus muscle can contribute to non-adrenergic non-cholinergic (NANC) relaxation of the muscle. In a solution containing 0.5 mM Ca2+ and in the presence of carbachol (10 microM) capsaicin evoked a fast concentration-dependent relaxation of the muscle that was usually followed by a smaller, slower, relaxant response. The fast relaxant response was reduced when extracellular Ca2+ was raised to 2.5 mM, desensitized after a single application of capsaicin and was blocked by tetrodotoxin (1 microM) or ruthenium red (10 microM). The fast response was greatly reduced by haemoglobin, by cold storage of the muscles or by N-monomethyl-L-arginine (100 microM) in the absence but not in the presence of L-arginine (100 microM). It is concluded that centrifugal activation of sensory fibres evokes a nitric oxide-mediated relaxation of the anococcygeus muscles that probably contributes to electrically evoked NANC relaxation.

NADPH diaphorase innervation of the rat anococcygeus and retractor penis muscles

NADPH diaphorase histochemistry was used to determine whether the rat anoocccygeus (AC) and retractor penis (RP) muscles are innervated by nerves capable of synthesizing nitric oxide. In both tissues, muscle fascicles were enclosed by a varicose plexus of NADPH diaphorase positive (ND+) fibers. Perikarya of neurons on the surface of the AC muscle were also intensely stained for NADPH diaphorase. Many AC-RP ganglion cells in the pelvic plexus, located by the retrograde tracer Fluorogold, also stained for the enzyme. However, a significant population of AC-RP neurons in this location remained unstained. These results provide further evidence that nitric oxide may be an important neurotransmitter in these tissues.

NADPH-diaphorase reactivity in nerves supplying the rat anococcygeus muscle

NADPH-diaphorase histochemistry was used to establish the nature of neurons and nerve fibers in the rat anococcygeus muscle. NADPH-diaphorase-reactive nerve cell bodies were found in ganglia in the caudal dorsal fascia of the muscle and within the muscle itself. The reactive nerve cells had a relatively smooth surface and a single axon. They formed bundles of positive nerve fibers which entered the muscle and branched further to form a network extending towards the cranial end. In addition, bundles of NADPH-diaphorase-reactive fibers of extrinsic origin joined the loose plexus of local bundles and microganglia. The NADPH-diaphorase-reactive neurons supplying the rat anococcygeus muscle provide a neuroanatomical basis for the non-adrenergic non-cholinergic relaxation of the muscle mediated by nitric oxide.

Innervation of the anococcygeus muscle of the rat

The innervation of the anococcygeus muscle of the rat was investigated with regard to the histochemical features of nerve fibers within the muscle and to the location of the postganglionic autonomic neurons which are the source of these fibers. Acetylcholinesterase-positive fibers and catecholaminergic fibers are abundant in the anococcygeus as well as the related retractor penis muscle. Neuronal somata, either between muscle bundles of the anococcygeus or in the connective tissue sheath, are also acetylcholinesterase-positive. Nerve fibers and a minority of the ganglion cells in the anococcygeus and retractor penis muscles are immunoreactive for vasoactive intestinal polypeptide. Injection of the retrogradely transported dye Fluorogold into the anococcygeus muscle filled neurons in the abdomino-pelvic sympathetic chain, pelvic plexus and a small number of neurons in the inferior mesenteric ganglion. In the pelvic plexus, some neurons were located in the major pelvic ganglion but most were found along the main penile nerve and its branches to the anococcygeus muscle. Immunocytochemistry of these identified neurons indicates that about one half of them are positive for vasoactive intestinal polypeptide. These results raise the possibility that both acetylcholine and vasoactive intestinal polypeptide are important neurotransmitters in autonomic nerves to the anococcygeus muscle.

N-methylhydroxylamine inhibits and M&B 22948 potentiates relaxations of the mouse anococcygeus to non-adrenergic, non-cholinergic field stimulation and to nitrovasodilator drugs

1. The effects of N-methylhydroxylamine (NMH) and of M&B 22948 on relaxations of the mouse anococcygeus to non-adrenergic, non-cholinergic (NANC) field stimulation and to a number of smooth muscle relaxant drugs were investigated. 2. Relaxations to NANC field stimulation (10 Hz; 60 s train) were reversibly blocked by NMH (1-5 mM), which also caused weak, transient reductions of carbachol (50 microM)-induced tone. N,N-dimethylhydroxylamine (2 mM) and hydroxylamine (5 microM) reduced tone to the same extent as NMH, but neither produced any inhibition of NANC relaxations. 3. M&B 22948 10 microM, which by itself reduced tone by 12%, potentiated submaximal but not maximal relaxations to NANC field stimulation; overall the log frequency-response curve was displaced to the left by a factor of 2. 4. Sodium nitroprusside (0.01-1 microM), hydroxylamine (0.5-100 microM), and nitric oxide (2-200 microM) all relaxed carbachol-induced tone; relaxations to submaximal concentrations of these nitrovasodilators were reduced in the presence of 2 mM NMH, and potentiated in the presence of 10 microM M&B 22948. 5. Neither NMH (2 mM) nor M&B 22948 (10 microM) affected relaxations induced by submaximal concentrations of vasoactive intestinal peptide (VIP; 1 microM), papaverine (10 microM), 3-isobutyl-1-methyl-xanthine (10 microM), or 8-bromo-cyclic guanosine monophosphate (100 microM); relaxations to adenosine 5'-triphosphate (ATP, 2 mM) were unaffected by M&B 22948, but were potentiated by NMH. 6. The selective inhibition by NMH, and potentiation by M&B 22948, of NANC and nitrovasodilator-induced relaxations of the mouse anococcygeus suggests that the NANC transmitter is neither VIP nor ATP, but resembles the nitrovasodilator drugs in its mode of action. The NANC transmission system is therefore similar to that recently described in the bovine retractor penis.