Noradrenergic, purinergic, and cholinergic transmission in the mouse vas deferens: influence of field-stimulation parameters, reserpinization, 6-hydroxydopamine and 4-aminopyridine

Dynamic monitoring of NET activity in mature murine sympathetic terminals using a fluorescent substrate

BACKGROUND AND PURPOSE

To validate a fluorescence approach for monitoring norepinephrine transporter (NET) transport rate in mature sympathetic terminals, and to determine how prejunctional muscarinic receptors affect NET rate.

EXPERIMENTAL APPROACH

Confocal imaging of a fluorescent NET substrate [neurotransmitter transporter uptake assay (NTUA)] as it accumulates in the mature sympathetic nerve terminals of the mouse isolated vas deferens. Fluorescence recovery after photobleaching (FRAP), enhanced green fluorescence protein (EGFP)-transgenic mice and contraction studies were also used.

KEY RESULTS

NTUA fluorescence accumulated linearly in nerve terminals, an effect that was prevented with NET inhibition with desipramine (1 microM). Such accumulation was reversed by amphetamine (10 microM), which is known to reverse the direction of transport of NET substrates. NTUA labelling was not present in cholinergic terminals (identified using EGFP fluorescence expressed in transgenic mice under a choline acetyltransferase promoter). FRAP experiments, altered nerve terminal distribution with reserpine pretreatment and co-imaging in terminals filled with a cytoplasmic marker (Alexa 594 dextran) indicated that the NTUA labelling was largely confined to vesicles within varicosities; vesicular exchange between varicosities was rare. The rate of NTUA accumulation was slower in the presence of the muscarinic agonist carbachol (10 microM) demonstrating muscarinic inhibition of NET rate.

CONCLUSIONS AND IMPLICATIONS

A straightforward protocol now exists to monitor NET transport rate at the level of the single nerve terminal varicosity, providing a useful tool to understand the physiology of NET regulation, the action of NET inhibitors on mature sympathetic terminals, dynamic vesicular tracking and to identify sympathetic terminals from mixed terminal populations in living organs.

The effect of epibatidine on spontaneous and evoked neurotransmitter release in the mouse and guinea pig isolated vas deferens

BACKGROUND AND PURPOSE

Nicotinic agonists increase sympathetic field-stimulus-evoked contraction of the rodent vas deferens, presumably by increasing evoked neurotransmitter release. This presumption was tested in two species.

EXPERIMENTAL APPROACH

The effect of the nicotinic acetylcholine receptor (nAChR) agonist epibatidine on neurotransmitter release in mouse and guinea pig isolated vas deferens was investigated using contraction studies and conventional intracellular recording techniques.

KEY RESULTS

In 12 of 14 mouse vasa deferentia, slow bath application of epibatidine (100 nM) had no significant effect on excitatory junction potential (EJP) amplitude and spontaneous EJP (SEJP) frequency. However, rapid application of epibatidine to the mouse vas deferens caused an increase in SEJP frequency (by 530%), with no effect on EJP amplitude. Despite the absence of an effect on EJPs, electrically-evoked contractions of the mouse vas deferens were significantly increased in the presence of epibatidine (by 50%). A transient contraction was reliably induced by a higher epibatidine concentration (1 microM). This contraction was significantly reduced in the presence of prazosin, tetrodotoxin, or alpha,beta-methyleneATP. Epibatidine did not induce a contraction in the presence of a combination of prazosin, alpha,beta-methyleneATP and cyclopentolate. In guinea pig vasa deferentia, bath-applied epibatidine potentiated EJP amplitude in a biphasic pattern, lasting for at least 30 minutes.

CONCLUSION AND IMPLICATIONS

The nAChR-mediated augmentation of neurogenic contraction is indeed prejunctional, but in the mouse arises from an increase in spontaneous neurotransmitter release that primes smooth muscle for subsequent contraction, while in the guinea pig there is a direct augmentation of evoked neurotransmitter (ATP) release.

Investigation of postjunctional alpha1- and alpha2-adrenoceptor subtypes in vas deferens from wild-type and alpha(2A/D)-adrenoceptor knockout mice

1. The subtypes of alpha(1)- and alpha(2)-adrenoceptor mediating contractions of vas deferens have been examined in wild-type and alpha(2A/D)-adrenoceptor knockout mice. 2. Maximum contractions to noradrenaline but not phenylephrine were significantly greater in vas from wild-type. The alpha(1A)-adrenoceptor antagonist RS100329 (5-methyl-3-[3-[4-[2-(2,2,2,-trifluoroethoxy)phenyl]-1-piperazinyl]propyl]-2,4-(1H)-pyrimidinedione) (10 nM) significantly shifted the potency of noradrenaline. The alpha(2D)-adrenoceptor antagonist BRL 44408 (2-[(4,5-dihydro-1H-imidazol-2-yl)methyl]-2,3-dihydro-1-methyl-1H-isoindole) significantly reduced the maximum contraction to noradrenaline in wild-type but not in knockout. 3. Following prazosin (0.1 micro M), a component of the contraction to noradrenaline in wild-type but not in knockout was sensitive to the alpha(2)-adrenoceptor antagonist yohimbine. 4. Nifedipine (10 micro M) or suramin (100 micro M) reduced the contraction to 10 Hz stimulation for 4 s to an early peak and small maintained response. The peak was abolished by the alpha(1)-adrenoceptor antagonist prazosin. 5. RS100329 or prazosin inhibited 10 Hz stimulation evoked contractions with a U-shaped concentration-response curve: inhibiting responses up to 0.1 micro M, with a reversal of inhibition above this concentration. In the presence of suramin or nifedipine, the reversal of inhibition by high concentrations of prazosin was reduced. 6. The alpha(1D)-adrenoceptor selective antagonist BMY7378 (8-[2-(4-(2- methoxyphenyl)piperazin-1-yl)ethyl]-8-azaspiro[4,5]decane-7,9-dione) produced inhibition of 10 Hz evoked contractions only in high concentrations. 7. In conclusion, contractions to nerve stimulation in mouse vas deferens involve largely alpha(1A)-adrenoceptors and purinoceptors. alpha(1)-Adrenoceptor antagonists in high concentrations increase the purinergic response presumably by blocking prejunctional alpha(2)-adrenoceptor-mediated inhibition. In the presence of nifedipine, responses are predominantly alpha(1)-adrenoceptor mediated. Contractions to exogenous noradrenaline involved both alpha(1A)- and alpha(2A/D)-adrenoceptors in wild-type mice.

Investigation of neurotransmission in vas deferens from alpha(2A/D)-adrenoceptor knockout mice

1. We have investigated pre- and post-junctional responsiveness in vas deferens from wild-type and alpha(2A/D)-adrenoceptor knockout mice. The response to a single stimulus was not significantly different between wild-type and knock-out mice. The isometric contraction to 10 Hz stimulation for 4 s was significantly larger in vas deferens from knockout as compared with wild-type. 2. The maximum potentiation of 10 Hz stimulation-evoked contractions by yohimbine was to 206.2+/-38.0% of control in wild-type but to 135.8+/-13.6% of control in knockout. The alpha(2A/D)-adrenoceptor selective antagonist BRL 44408 significantly increased the 10 Hz stimulation-evoked contraction in wild-type but not knockout, and the reverse was true for the alpha(2C)-adrenoceptor selective antagonist spiroxatrine. The alpha(2B)-adrenoceptor antagonist imiloxan had no effect on the evoked contraction except at high concentrations, and only in wild-type. Following cocaine (3 microM) and BRL 44408 (1 microM), 10 Hz responses were similar in shape and maximum between wild-type and knock-out. 3. The alpha(2)-adrenoceptor agonist xylazine virtually abolished the early component of the contraction to 10 Hz stimulation in the presence of nifedipine (10 microM) in vas deferens from knockout mice in a way consistent with a change of receptor subtype but without clear evidence for a reduced receptor number. However, the late component of the contraction to 10 Hz stimulation was significantly potentiated by xylazine in tissues from knock-out mice. 4. It is concluded that, although non-alpha(2A/D)-adrenoceptors replace alpha(2D)-adrenoceptors in this knockout, the alpha(2)-adrenoceptor agonist and antagonist data are contradictory. The antagonist data suggest a major loss of prejunctional alpha(2)-adrenoceptors, but this is not necessarily supported by the agonist data.

Bretylium or 6-OHDA-resistant, action potential-evoked Ca2+ transients in varicosities of the mouse vas deferens

Action potential-evoked calcium transients in varicosities in mouse vas deferens were monitored using laser scanning confocal microscopy. Their significance was examined by comparison with excitatory junction potentials (EJPs) and neurogenic contractions, both indirect measurements of transmitter release. Bretylium abolished EJPs, as well as the ATP and NA-mediated phases of contraction. However, bretylium revealed a prominent late component of contraction that was atropine-sensitive. Bretylium abolished calcium transients in 21%, enhanced in 16% and had no effect in 63% of varicosities examined. Pre-treatment with 6-OHDA reduced NA levels to below detectable levels but many strings of varicosities still responded to nerve impulses with 'normal' calcium transients. Varicosities in which calcium transients were abolished by these agents were sympathetic. The identity of those varicosities in which calcium transients were resistant to bretylium (sympathetic but no uptake-1 sites, parasympathetic, sensory) remains to be established.

Release of endogenous ATP from the vasa deferentia of the rat and guinea-pig by the indirect sympathomimetic tyramine

1. Adenosine 5'triphosphate (ATP) as well as [3H]-noradrenaline ([3H]-NA) is released by perfusion of the vas deferens with the indirect sympathomimetic tyramine (100 microM); this result is consistent with the concept of sympathetic cotransmission. 2. While tyramine produced a strong contraction in the vas deferens of the rat, it had little mechanical action in the guinea-pig vas deferens. This appears to be largely because tyramine induces considerably lower levels of release of both ATP and NA from the guinea-pig vas deferens compared to that of the rat. Furthermore, NA released by tyramine appears to release ATP from a secondary pool in the rat vans deferens, but not that of the guinea-pig, since prazosin reduced the tyramine-induced release of ATP in the rat vas deferens. 3. alpha,beta-Methylene ATP (alpha,beta-meATP) increased both the spontaneous release of ATP and the tyramine-evoked efflux of ATP and [3H]-NA. The basal and tyramine-induced efflux of [3H]-NA was also enhanced by the alpha 1-adrenoceptor antagonist, prazosin, suggesting that prejunctional alpha 1-adrenoceptors may modulate neurotransmitter release.

Electrophysiological and electrochemical analysis of the secretion of ATP and noradrenaline from the sympathetic nerves in rat tail artery: effects of ?2-adrenoceptor agonists and antagonists and noradrenaline reuptake blockers

The aim of this study was to investigate whether or not nerve impulses release ATP and noradrenaline in parallel from the sympathetic nerve terminals of the rat tail artery. The extracellularly recorded excitatory junction current (EJC) was used to study, pulse by pulse, the release of ATP. An electrochemical method was used to study online the nerve stimulation-induced rise in the extracellular concentration of endogenous noradrenaline at the probe, a carbon fibre electrode (CF). This parameter, which does not directly represent noradrenaline release, but reflects release minus clearance, has been termed delta[NA]CF. The effects of a number of pharmacological agents on the EJCs were examined both at 0.1 and 2 Hz, and the effects on the EJC response to 100 pulses at 2 Hz compared with that on the delta[NA]CF response. Clonidine and xylazine were used as alpha 2-agonists, yohimbine and idazoxan as alpha 2-antagonists and desipramine and cocaine as blockers of noradrenaline reuptake. Most of these agents had unwanted side effects, especially at higher concentrations. However, clonidine and xylazine depressed at lower concentrations the EJC and delta[NA]CF responses to about the same extent; these effects were partially or completely reversed by yohimbine. Yohimbine or idazoxan did not affect the EJCs at 0.1 Hz but enhanced the EJC and delta[NA]CF responses to 100 pulses at 2 Hz to the same extent. All effects of desipramine (1 microM) seemed explainable as a result of block of noradrenaline reuptake, while cocaine (10 microM) in addition exerted an 'unspecific' depressant (probably local anesthetic) effect. Under control conditions, both agents depressed the EJC but dramatically enhanced the delta[NA]CF response to 100 pulses at 2 Hz. Addition of yohimbine prevented the depressant effect of desipramine on the EJCs completely and reduced that of cocaine, but increased their effects on the delta[NA]CF response. These results are compatible with the view that ATP and noradrenaline are released in parallel from the sympathetic nerve terminals of this tissue. The different, and under some conditions even opposite, effects of desipramine or cocaine on the EJC and delta[NA]CF responses are explainable in terms of the known post-secretory effects of these agents.

Comparative evaluation of lisuride and terguride, ergot alkaloid derivatives, on the field-stimulated vas deferens of mouse

The ergot alkaloid derivatives, lisuride (LIS) and terguride (TDHL), known to interact with central dopamine receptors as agonist and partial agonist, respectively, were studied on the field-stimulated mouse vas deferens, where recently the existence of presynaptic dopamine receptors has been evidenced. LIS was a competitive antagonist at prejunctional alpha 2 and DA1 receptors situated on the sympathetic nerve terminals of the mouse, with a pA2 value of 9.2 and 9.1, respectively. LIS was also able to antagonize the effects of LY 171555, selective DA2 agonist, but the type of interaction cannot be conceptualized in terms of competitive antagonism. Likewise, the type of interaction of TDHL with dopaminergic and adrenergic agonist-activated sites is not suggestive of a competitive antagonism. Based on these results, it seems that central and peripheral pharmacologic profiles of LIS and TDHL cannot be overlapped, LIS being a potent DA1- and alpha 2-antagonist with a high degree of specifity for these receptors.

Pre- and postjunctional effects of N-ethylmaleimide in the isolated mouse vas deferens

The influence of N-ethylmaleimide (NEM) on contractions due to exogenously applied noradrenaline and bethanechol and on the inhibitory effects of clonidine, of the enkephalin derivative, FK 33-824, and 2-chloroadenosine (2-CLA) on field stimulation-response curves and [3H]noradrenaline [( 3H]NA) release was studied in the isolated mouse vas deferens. Exposure to NEM (60 microM: 10 min) caused a 30% reduction of the maximal contraction due to NA but nearly abolished the response to bethanechol. NEM partially reversed the depression of the pulse width-response curves by clonidine and FK 33-824 but was without effect with 2-CLA. The contractions evoked by stimulation frequencies above 20 Hz were depressed by NEM both in presence and absence of the agonists. NEM diminished the inhibition of the stimulation-evoked release of [3H]NA by the three agonists. The prejunctional effect of NEM was markedly influenced by the stimulation parameters. These findings support the suggestion that the inhibition mediated by alpha 2-adrenoceptors, mu- and P1-receptors in the mouse vas deferens is NEM-sensitive and possibly transmitted by a pertussis toxin-sensitive G-protein.

Activation of alpha 2-adrenoceptors attenuates the inotropic effect of field stimulation in atria

Guinea-pig left atria were driven to contract at a rate of 0.5 Hz by stimulation with punctate electrodes. For additional field stimulation, a train of one to eight field pulses (30 V; 0.05-0.4 ms duration) was applied during each refractory period. Cholinergic effects were blocked by atropine and thus the resulting increase of the contractile force was caused by noradrenaline released from sympathetic nerve endings. Trains of several short field pulses delivered in the refractory period after each contraction produced a significantly greater inotropic effect than one single pulse of the same total duration delivered in each refractory period. Phentolamine, rauwolscine and idazoxan, by blocking prejunctional alpha 2-adrenoceptors selectively increased the inotropic effect of single field pulses. Selective blockers of alpha 1-adrenoceptors (prazosin, corynanthine) were ineffective in this respect. The effect of alpha 2-adrenoceptor blockers persisted in the presence of noradrenaline uptake blockers (cocaine or desipramine plus corticosterone) or phenylephrine, but was overcome by the alpha 2-adrenoceptor agonists, clonidine and guanabenz. It is concluded that activation of presynaptic alpha 2-adrenoceptors limits the release of noradrenaline by long-lasting single field pulses. Autoinhibition of transmitter release was not seen with trains of short field pulses.

Pulse duration and alpha 2-adrenoceptors modify noradrenaline release from field-stimulated atria

In guinea-pig atria preloaded with 10 muCi [3H]noradrenaline, field stimulation during the refractory period increased the release of radioactivity and the force of contraction. Both effects were dependent on the extracellular Ca2+ concentration and were abolished by 3 x 10(-8) mol/l tetrodotoxin. When applied during each refractory period, two short (0.05 ms) pulses released significantly more radioactivity than one pulse of 0.1 ms duration applied during each refractory period. Similarly, a train of four pulses (0.05 ms each) was more effective than one pulse lasting 0.2 ms. The radioactivity released by single, long-lasting pulses in each refractory period was increased by phentolamine, idazoxan and N-ethylmaleimide. A small increase was also obtained with prazosin. The effect of phentolamine was antagonized by clonidine but not by phenylephrine. Clonidine did not prevent the effect of N-ethylmaleimide (all drugs 3 x 10(-5) mol/l; atropine 10(-7) mol/l and cocaine 3 x 10(-6) mol/l present in all experiments). It is concluded that activation of prejunctional alpha 2-adrenoceptors reduces the amount of noradrenaline release by one long field pulse in each refractory period. In most experiments, the inotropic effect of the two stimulation protocols was not significantly different. However, when atria were stimulated with four pulses immediately before stimulation with one pulse, the autoinhibition thus caused was strong enough to also reduce the inotropic effect of this stimulation protocol.

Evidence for the existence of prejunctional receptor sites for dopamine in the mouse vas deferens

The present work is focused on the effects of newly developed dopaminergic agonists and antagonists on the field-stimulated vas deferens. Both LY 171555 and SK&F 38393, relatively selective DA2 and DA1 receptor agonists, respectively, produced concentration-dependent inhibition of the field stimulation-evoked contractions in the mouse vas deferens; both compounds did not modify the baseline tone nor the contractile responses to exogenous noradrenaline. Control LY 171555 and SK&F 38393 concentration-response curves, were shifted rightward in a parallel manner in the presence of sulpiride (relatively specific DA2 antagonist) and SCH 23390 (relatively specific DA1 antagonist), respectively. Control concentration-response curves for dopaminergic agonists were not modified in the presence of specific blockers for H1 and H2 histamine receptors, serotonin receptors and alpha 2-adrenoceptors. These preliminary findings are suggestive of the existence of two dopaminergic receptor types both presumably located prejunctionally.

The field-stimulated vas deferens of the streptozotocin-diabetic mouse: effects of prazosin, alpha, beta-methylene ATP, and variation of stimulation parameters

The question of diabetic neuropathy was studied in the field-stimulated isolated vas deferens of the mouse. The animals were treated with either buffer or streptozotocin (170 mg/kg i.v.) 2 or 4 weeks, respectively, beforehand. Stimulus-response relationships were tested by variation of frequency (VF) at constant pulse width and by variation of pulse width (VP) at constant frequency. The adrenergic twitch component was eliminated by prazosin (1 microM) and the purinergic component by alpha, beta-methylene ATP (MeATP, 10 microM). The diabetes did not alter the muscular contractility (tested with KCl) and left the twitch-inhibiting effects of prazosin and MeATP unchanged, thereby revealing no difference in susceptibility between noradrenergic and purinergic mechanisms. However, in diabetic vasa, the maximal effectiveness of stimulation was decreased with VF but not VP, whereas the sensitivity of intramural neurons (50% effective frequency or pulse width, respectively) was unchanged with VF and reduced with VP. This may suggest that the diabetic neuron release less transmitter (VF), which can be compensated for by the activation of less sensitive neurons (VP). Actually, the uptake of 3H-noradrenaline into the (4 weeks-) diabetic vas was normal but the stimulation-induced fractional release of tritium was decreased by 26%. It is concluded that a sympathetic neuropathy occurred in the vas deferens of the streptozotocin-diabetic mouse.