Effects of antidepressant drugs on noradrenaline accumulation and contractile responses in the rat anococcygeus muscle.

1 The effect of a series of antidepressant drugs on noradrenaline accumulation was studied in the isolated anococcygeus muscle of the rat. 2 The most potent inhibitors of noradrenaline accumulation were nortriptyline, desipramine and protriptyline. Opipramol, trimipramine and iprindole were active only in high concentrations. 3 Contractions of the anococcygeus muscle produced by noradrenaline were strongly potentiated by nortriptyline, desipramine and protriptyline. Other uptake inhibitors were less active in potentiating the noradrenaline response. 4 Nortriptyline, in concentrations that potentiated the action of noradrenaline, reduced or abolished the response to tyramine.     

Mianserin potentiates responses to acetylcholine in the rat anococcygeus muscle.

In the rat anococcygeus muscle, mianserin (1 and 10 muM) increased the magnitude of the maximal contractile responses to acetylcholine and carbamylcholine without affecting the maximal responses to (--)-noradrenaline. The potentiating effect of mianserin on responses to acetylcholine was maintained in the presence of 5muM phentolamine, following 6-hydroxy dopamine incubation of the tissues (1 mM for 3h) and in the presence of 1 muM nortriptyline. It is suggested that mianserin increases the sensitivity to acetylcholine by an action at the level of, or distal to, the cholinergic receptor.     

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.     

Neostigmine augments responses of the rat anococcygeus muscle to field stimulation.

1 The effects of neostigmine on noradrenergic transmission have been studied in the field stimulated, isolated anococcygeus muscle of the rat. 2 In those muscles where the excitatory response to field stimulation was not completely inhibited by guanethidine (5 X 10(-6) to 10(-5) M) or phentolamine (10(-6) M), atropine (5 X 10(-8) M) gave no further inhibition of the response. 3 The shape of the response to field stimulation was altered in a dose-dependent manner by neostigmine (5 X 10(-7) to 5 X 10(-6) M), such that a 'shoulder' appeared during the relaxation phase. The 'shoulder', present at all stimulation frequencies tested between 3 and 40 Hz, was abolished by atropine (5 X 10(-8) M) and unaffected by tubocurarine (10(-6) M). 4 Neostigmine (2.5 X 10(-6) M), whether alone or in the presence of atropine (5 X 10(-8) M), had no effect on the uptake or stimulation-induced release of [3H]-noradrenaline. 5 Using electron microscopy, small Schwann/axon bundles close to smooth muscle cells rarely showed cholinesterase staining, whereas larger bundles at the outer serosal aspects of the muscle exhibited dense staining. 6 It is concluded that the observed effects of neostigmine are not due to a presynaptic effect on noradrenergic transmission.     

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.     

Effect of neuropeptide Y on adrenergic and non-adrenergic, non-cholinergic responses in the rat anococcygeus muscle.

1. The effects of neuropeptide Y (NPY) were examined on adrenergic and non-adrenergic, non-cholinergic (NANC) neurotransmission in the rat anococcygeus muscle. 2. NPY (0.1-0.3 microM) greatly potentiated the contractile responses induced by field stimulation. Prazosin (0.1 microM) completely abolished the stimulation-induced responses either in the absence or presence of NPY. 3. NPY (0.1-0.3 microM) enhanced only the contractile responses to low doses of noradrenaline (NA, 0.003-0.01 microM). Responses to tyramine were unaffected by the same concentrations of NPY. 4. In superfused anococcygeus, previously loaded with [3H]-NA, NPY (0.1-0.3 microM) failed to modify the basal, as well as the stimulation-evoked, release of tritium at 2 and 4 Hz. 5. NANC relaxations induced by electrical stimulation were significantly reduced, in a concentration-related manner, by 0.1-0.3 microM NPY. 6. L-NG-nitro-arginine (L-NOARG, 30 microM) enhanced the stimulation (0.25-1 Hz)-induced motor responses. In the presence of L-NOARG (30 microM), NPY (0.1 microM) did not modify the motor responses induced by field stimulation (0.25-0.5 Hz). L-Arginine did not reverse the NPY-induced potentiation of stimulation-induced motor responses. 7. The relaxations of anococcygeus muscle induced by sodium nitroprusside (SNP, 0.01-0.3 microM) were diminished by NPY (0.1-0.3 microM). 8. Our study suggests that NPY, at concentrations devoid of contractile effect, potentiates the motor responses of rat anococcygeus muscle as a consequence, at least in part, of the inhibition of NANC relaxing responses by a different mechanism from L-NOARG.     

Is co-transmission involved in the excitatory responses of the rat anococcygeus muscle?

1 Electrical and mechanical responses to field (transmural) and extrinsic nerve stimulation were recorded simultaneously in the rat anococcygeus muscle. Membrane potential changes recorded intracellularly following either method of stimulation were indistinguishable. Single stimuli usually produced a slow depolarization; trains of pulses produced a fast excitatory junction potential (e.j.p.) initially, followed by a slow depolarization similar to that produced by single pulses. The fast e.j.ps, the slow depolarizations and the accompanying contractions were abolished by the alpha-adrenoceptor antagonists, phentolamine (1 X 10(-6)M) or prazosin (1 X 10(-7)M) and by tetrodotoxin (TTX, 1 X 10(-6)M) but unaffected by alpha, beta-methylene adenosine triphosphate (alpha, beta-MeATP, 1(-10) X 10(-6)M), an agent known to desensitize purinoceptors. 2 Application of noradrenaline (NA, 1 X 10(-8)-1 X 10(-6)M), by pressure ejection from a micropipette, depolarized the membrane and produced a localized contraction, both of which were abolished by phentolamine (1 X 10(-6)M) or prazosin (1 X 10(-7)M). 3 Application of adenosine-5'-triphosphate (ATP, 1 X 10(-4)-1 X 10(-3)M), by pressure ejection from a micropipette, produced a small membrane depolarization and localized contraction which were unaffected by phentolamine (1 X 10(-6)M) or prazosin (1 X 10(-7)M) but abolished by alpha, beta-MeATP (1 X 10(-6)M). 4 The results show that, in the rat annococcygeus muscle, (1) field or extrinsic nerve stimulation released only one excitatory transmitter, namely NA, although receptors for both NA and ATP were present on the muscle, (2) alpha, beta-MeATP was selective for purinoceptors and (3) there was no evidence for excitatory co-transmission in this tissue.     

Effect of alpha-adrenergic agonists and antagonists on neurotransmission in the rat anococcygeus muscle.

The isolated rat anococcygeus muscle was found to be a sensitive preparation for the study of the pre- and postsynaptic actions of α-adrenergic agonists and antagonists. The pre- and postsynaptic actions of α-agonists were characterized by comparing the order of potency (1) for directly contracting smooth muscle, (2) for inhibiting the field-stimulated release of tritium (³H) or ³H-norepinephrine, following labelling of adrenergic stores with ³H-NE and (3) for inhibiting twitch contractions elicited by field-stimulation. Clonidine, oxymetazoline tetrahydrozoline and naphazoline, listed in their order of potency, inhibited field-stimulated ³H-release and twitch contractions. With the exception of clonidine, a clear separation of pre- and postsynaptic α-mediated effects was not achieved with the other imidazolines tested. In contrast, methoxamine and phenylephrine did not inhibit twitch response or ³H-release, but did cause a direct contraction of the smooth muscle. The α-antagonists, phentolamine, yohimbine and phenoxybenzamine increased the stimulated release of ³H in a concentration dependent manner. At 10^?7 M, yohimbine preferentially blocked presynaptic α-receptors. Phentolamine was the most potent antagonist at pre- or postsynaptic α-receptors, but like phenoxybenzamine showed no selectivity for pre- or postsynaptic α-receptors. The percent inhibition of neurotransmitter released produced by the α-agonists was found to be inversely proportional to the train lenght, frequency of stimulation and extracellular calcium. Because of the differential effects of α-agonists at pre- and postsynaptic sites, α-receptors on nerve varicosities may have different structural requirements for blockade and activation than the α-receptors on smooth muscle.     

Temperature-dependent specific desensitization in the rat anococcygeus muscle to angiotensin II

The effect of angiotensin II (A II) on isolated rat anococcygeus muscle was investigated. A II, when used submaximally (4 X 10(-9) mol/l), produced a rapid contraction of the rat anococcygeus muscle. However, a profound tachyphylaxis developed with repeated administrations. This tachyphylaxis is specific in nature, since the tissue responded normally to noradrenaline even after the establishment of a complete tachyphylaxis to the peptide. Incubation with phentolamine or indometacin had no effect on the A II-induced tachyphylaxis. On the other hand, a marked prevention of tachyphylaxis was obtained by lowering the bath temperature to 31 or 25 degrees C. These results indicate that neither noradrenaline nor prostaglandin release participate in the development of tachyphylaxis to A II in rat anococcygeus muscle, and this tachyphylaxis can be prevented by lowering the temperature of the bath solution.