Evidence for purinergic innervation of the anococcygeus muscle

Effect of mianserin on noradrenergic transmission in the rat anococcygeus muscle

1 The effects of mianserin on the accumulation of (-)-[(3)H]-noradrenaline and on contractile responses to field stimulation, exogenously applied (-)-noradrenaline, and to tyramine were studied in the rat anococcygeus muscle.2 Mianserin (10(-9) to 10(-5) M) and nortriptyline (10(-9) to 10(-5) M) inhibited the accumulation of (-)-[(3)H]-noradrenaline. In this aspect mianserin had a similar potency to nortriptyline, the most potent tricyclic antidepressant in inhibiting noradrenaline accumulation in this tissue.3 Mianserin 10(-9) or 10(-8) M alone had no effect on contractile responses to field stimulation and to (-)-noradrenaline but inhibited the responses to tyramine. The responses to field stimulation at low frequencies and to (-)-noradrenaline were potentiated by 10(-7) and 10(-6) M mianserin. It is suggested that the inhibitory effect mianserin has on neuronal accumulation is primarily responsible for these effects. Mianserin 10(-5) M inhibited responses to field stimulation and to (-)-noradrenaline.4 In the presence of nortriptyline (10(-6) M), the contractile responses to field stimulations were potentiated by mianserin (10(-8), 10(-7) and 10(-6) M), 10(-8) M being the most potent in this aspect. Mianserin 10(-8), 10(-7), 10(-6) and 10(-5) M had a similar inhibitory effect on responses to (-)-noradrenaline. In the absence of neuronal uptake, the potentiating effect of mianserin on responses to field stimulation may be due to antagonism at presynaptic alpha-adrenoceptors. In the presence of 10(-6) M nortriptyline, 10(-5) M mianserin abolished responses to field stimulation.5 Following incubation of the tissue in the presence of 6-hydroxydopamine (10(-3) M for 3 h), mianserin (10(-7), 10(-6) and 10(-5) M) nortriptyline (10(-7) and 10(-6) M) and phentolamine (5 x 10(-8) and 5 x 10(-7) M) inhibited contractile responses to (-)-noradrenaline. This illustrates the ability of these agents to inhibit the responses to noradrenaline at a postsynaptic site. The inhibitory effect was dose-related with nortriptyline and phentolamine; this illustrates the ability of these agents to antagonize postsynaptic alpha-adrenoceptors. The inhibitory effect observed with mianserin was not dose-related. This suggests that in addition to its reported ability to antagonize postsynaptic alpha-adrenoceptors, mianserin may have another post-synaptic action at the level of, or distal to, the alpha-adrenoceptor.6 These results illustrate that, in the rat anococcygeus muscle, mianserin is a potent inhibitor of noradrenaline accumulation and may be an antagonist at presynaptic alpha-adrenoceptors. Mianserin also inhibits the responses to exogenously applied noradrenaline in this tissue by an action or actions at the level of, or distal to, the postsynaptic alpha-adrenoceptor.

Pharmacological study of the anococcygeus muscle of the dog

1 The response of the dog anococcygeus muscle to field stimulation and to some drugs has been studied. The results are compared with those reported previously in the rat, cat and rabbit. 2 Field stimulation produced frequency-dependent contractions which were inhibited by guanethidine and phentolamine. When the tonus of the muscle was increased with guanethidine, field stimulation always produced frequency-dependent relaxation. The relaxation was not prevented by propranolol. 3 The muscle was contracted by noradrenaline, tyramine, acetylcholine, histamine (H1), 5-hydroxy-tryptamine, prostaglandin E2 and vasopressin. Phentolamine, atropine, promethazine (but not cimetidine) and methysergide inhibited the effect of the respective agonists. 4 After increasing the tonus of the muscle, it was relaxed by low concentrations of isoprenaline. The relaxation was antagonized by propranolol. 5 The response to adenosine triphosphate (ATP) was variable. In some preparations, it relaxed the muscle, in others it contracted the muscle prior to relaxation, in others still it only contracted the muscle. Indomethacin did not prevent ATP-induced contraction. 6 It is concluded that the anococcygeus of the dog, like that of rat, cat and rabbit, has an adrenergic motor innervation and an inhibitory innervation, the transmitter of which is not identified.

The inhibitory material in extracts from the bovine retractor penis muscle is not an adenine nucleotide

Methods are described for the removal of adenosine or purine nucleotides from extracts of bovine retractor penis muscle (BRP). These methods did not interfere with the biological test preparations. Removal of adenosine and purine nucleotides by these methods did not modify the inhibitory action of the extract on the BRP. The effect of the extract on the BRP resembles that of inhibitory nerve stimulation. If the inhibitory substance present in the extract is the inhibitory transmitter, then the results indicate that, in this tissue, the transmitter is neither adenosine nor a purine nucleotide.

Effects of pyroxamidine and guanethidine on contractile responses to field stimulation and to noradrenaline in the anococcygeus muscle and vas deferens of the rat

The effects of pyroxamidine (EMD 21192) and guanethidine on contractile responses were studied in the anococcygeus muscle and vas deferens of the rat. Pyroxamidine (10−6 and 10−5 M) and guanethidine (6 times 10−6 and 10−5 M) potentiated the responses to low concentrations of acetylcholine in the rat anococcygeus muscle. Following incubation of the muscle with 6-hydroxydopamine (10−3M for 3 h), pyroxamidine (10−5 M) and guanethidine (10−5 M) had no effect on responses to acetylcholine. This suggests that the potentiating effect of pyroxamidine and guanethidine on responses to acetylcholine is due to the release of subthreshold concentrations of noradrenaline.

In the anococcygeus, pyroxamidine (10−6 and 10−5 M) and guanethidine (10−6 and 10−5 M) inhibited responses to field stimulation and potentiated responses to exogenously applied (−)-noradrenaline. The responses to field stimulation in the vas deferens were also inhibited by 10−5M pyroxamidine and by 10−5 M guanethidine. 10−6 M guanethidine, but not 10−5 M pyroxamidine, potentiated responses to (−)-noradrenaline in the vas deferens. In the presence of nortriptyline (10−6 M), a potent inhibitor of neuronal uptake, the inhibitory effects of pyroxamidine and guanethidine on responses to field stimulation were reduced or reversed and these drugs had no effect on responses to (−)-noradrenaline. This suggests that pyroxamidine is a noradrenergic neuron blocker and that its action is dependent on continued neuronal uptake. Following 6-hydroxydopamine incubation, 10−5 M pyroxamidine and 10−5 M guanethidine inhibited the responses to (−)-noradrenaline in the rat anococcygeus muscle. Thus it seems likely, at high concentrations, that these compounds have postsynaptic blocking activity.

Ultrastructural identification of non-adrenergic, non-cholinergic nerves in the rat anococcygeus muscle

The innervation of the rat anococcygeus muscle has been investigated ultrastructurally following fixation with a modified chromaffin reaction for the demonstration of biogenic amines (Tranzer and Richards, 1976). Three types of nerve profiles were revealed: (1) 60-70% of the profiles are adrenergic; (2) less than 5% of the profiles appear to be cholinergic; (3) up to 40% of the profiles are distinguished by the presence of a characteristically high proportion of electron-opaque, chromaffin-negative vesicles, 85-110 nm in diameter. This third type of profile was not affected by 6-OHDA, and is considered to represent the non-adrenergic, non-cholinergic inhibitory innervation of this tissue. Because of the morphological similarity of this nerve type, apart from the smaller vesicle size, to classical peptidergic nerve endings, they have been termed "small p-type" (sp-type). These results are discussed in relation to a previous report describing only two types of nerve profiles in this tissue (Gillespie and Lüllmann-Rauch, 1974).

Effects of neuronal polypeptides on intestinal smooth muscle; a comparison with non-adrenergic, non-cholinergic nerve stimulation and ATP

Substance P, somatostatin, enkephalin and vasoactive intestinal polypeptide (VIP) did not mimic the inhibitory responses to non-adrenergic, non-cholinergic nerve stimulation. Substance P (0.1-10 microgram/ml) always caused contraction, enkephalin (0.1-10 microgram/ml) and somatostatin (0.1 microgram/ml) were inactive, while VIP (0.1-1 microgram/ml) produced very slow relaxation, taking about 4 min to reach a maximum after a latency of about 60 sec. Low concentrations of neurotensin (1-10 ng/mg) caused contraction, but at higher concentrations (50-1000 ng/ml) it produced a biphasic response which consisted of an initial contraction followed by a slow relaxation. In high tone preparations, the slow relaxation did not mimic the nerve-mediated response, taking approximately 43 sec. to reach maximum, after a long latency of about 15 sec. In contrast, ATP (0.1-50 microgram/ml) mimicked closely the rapid responses to non-adrenergic, non-cholinergic nerve stimulation in all preparations, whether the tone was low, medium or high. The time for the inhibitory response to reach maximum was about 15 sec after a latency of approximately 1 sec. Indomethacin (3.4-34 microgram/ml) did not unmask any inhibitory responses to any of the peptides. It is concluded that ATP remains the most likely substance to be the inhibitory transmitter released from non-adrenergic, non-cholinergic nerves supplying the smooth muscle of the taenia coli.

Non-adrenergic, non-cholinergic (purinergic?) inhibitory innervation of the rabbit rectococcygeus muscle

A powerful non-adrenergic, non-cholinergic inhibitory innervation of the rabbit rectococcygeus was unmasked by atropine in preparations in which tone had been raised either by carbachol or prostaglandin E2-Flourescent histochemical localization of quinacrine (which binds ATP) revealed ganglia and associated nerve bundles which were neither adrenergic nor cholinergic both on the surface and within the rectococcygeus muscle. Treatment of the rabbit with 6-hydroxydopamine abolished catecholamine fluorescence without affecting quinacrine-stained neural elements. Release of ATP from the rectococcygeus increased 2-4 times above background levels during stimulation of intramural inhibitory nerves. In preparations in which the tone was raised, ATP caused an inhibitory response comparable to that produced by non-adrenergic, non-cholinergic nerve stimulation. Indomethacin reduced the increase in tone and spontaneous activity following washout of ATP in low tone preparations, and potentiated the relaxations produced by ATP in high tone preparations. These results suggest that the rabbit rectococcygeus, as well as receiving a cholinergic excitatory innervation and an adrenergic inhibitory innervation, is also innervated by purinergic inhibitory nerves.