Electrical activity at the sympathetic neuroeffector junction in the guinea-pig vas deferens

ATP as a sympathetic co-transmitter in rat vasomotor nerves--further evidence that individual release sites respond to nerve impulses by intermittent release of single quanta

A combination of intra- and extracellular recording was used to study neurotransmitter release in sympathetic vasomotor fibres in rat femoral and mesenteric arteries. The results show that neurotransmission in these preparations is similar to that in the 'short' sympathetic nerves in guinea-pig vas deferens and the 'long' sympathetic nerves in rat tail artery in the following respects: (1) The intracellularly recorded excitatory junction potentials (EJPs) and the extracellularly recorded junction currents (EJCs) presumably are caused by ATP secreted as a sympathetic co-transmitter. (2) The stimulus-evoked and spontaneous EJCs share the same brief time-course, which is similar to that of spontaneous EJPs, but much briefer than that of stimulus-evoked EJPs. (3) 'Successful' nerve impulses appear to release single transmitter quanta. (4) The probability of activation of individual release sites is low (0.002-0.02). (5) The low release probability cannot be accounted for by failure of the nerve impulse to invade the terminals. Moreover, it is also shown that application of tetrodotoxin to the medium within the recording electrode effectively abolishes transmitter secretion in the area enclosed by the tip of the electrode, indicating that the effective length constant for a passively propagating nerve action potential is probably very small and that activation of the release mechanisms in 'long' sympathetic nerve fibres seems to require that the varicosities are actively invaded.

Effects of reserpine pretreatment on neuroeffector transmission in the vas deferens

1. The effects of reserpine pretreatment on neurotransmission in the guinea-pig vas deferens have been re-examined with a view to study the role of noradrenaline (NA) in mediating postjunctional electrical responses and, in particular, excitatory junction potentials (EJP). 2. Reserpine (5 mg/kg, i.p.) caused almost total depletion (below detection levels) of the NA content of the vas deferens. However, spontaneous and evoked excitatory junction potentials (SEJP, EJP) and currents (SEJC, EJC) could still be recorded in NA-depleted tissues. The amplitude distribution of SEJC in control and reserpinized tissues was similar. 3. Facilitation of EJP and EJC was markedly slowed in reserpinized tissues, EJP taking 50-60 pulses to facilitate fully. However the amplitudes of fully facilitated EJP were comparable to EJP recorded in control tissues. 4. EJPs in reserpinized vasa deferentia were unaffected by the NA synthesis inhibitor, alpha-methyl tyrosine, but were abolished in the presence of the stable ATP analogue alpha,beta-methylene ATP which desensitizes postjunctional P2-purinoceptors. 5. Local application of ATP, but not NA, mimicked the EJP. These results indicate that EJP are mediated by a non-noradrenergic neurotransmitter possibly ATP.

Inhibition of transmitter release from sympathetic nerve endings by omega-conotoxin

1. The effects of the calcium channel blocker, omega-conotoxin, on sympathetic neuroeffector function in the guinea-pig vas deferens have been investigated using a combination of mechanical and electrophysiological recording techniques. 2. Biphasic contractions evoked by electrical field stimulation were irreversibly abolished by omega-conotoxin (10-100 nmol/L). 3. Electrically evoked excitatory junction potentials and currents were irreversibly blocked by omega-conotoxin (10-100 nmol/L). Spontaneous excitatory junction potentials and currents were unaffected by this treatment. 4. omega-Conotoxin did not block impulse propagation in the nerve terminals. However, in three of four experiments omega-conotoxin caused a decrease in the size of the nerve terminal impulse. 5. These findings support the suggestion the omega-conotoxin acts prejunctionally to inhibit sympathetic neuroeffector by interfering with depolarization-secretion coupling.

Some pharmacological applications of an extracellular recording method to study secretion of a sympathetic co-transmitter, presumably ATP

Extracellular recording in guinea-pig or mouse vas deferens or rat tail artery was used to study the effects of some pharmacological agents on the nerve terminal spike (NTS) and the secretion of a sympathetic co-transmitter (presumably ATP), as reflected in the excitatory junction current (EJC). A negative-going EJCi (i for inside) was assumed to reflect release from sites inside, and a positive-going EJCo (o for outside) release from sites outside the recording electrode. Passage into or out of the electrode seemed to be slow. Tetrodotoxin (TTX) in the outer medium blocked the NTS and ECJo as well as EJCi; TTX in the pipette blocked stimulus-evoked but not spontaneous EJCi. The dihydropyridine Ca2+ channel blocking agent, nifedipine, was without effect, but Cd2+ in the external medium blocked EJCo and also, by an effect apparently 'upstream' of varicosities, inhibited EJCi (i.e. release within the patch) but not the NTS. When present in the outer medium the alpha 2-adrenoceptor agonists, clonidine and xylazine, blocked both EJCo and EJCi, but not the NTS. The effects of clonidine were blocked by yohimbine, which in itself increased the EJCo by about 50%. Neuropeptide Y and met-enkephalin in the outer medium blocked EJCo; the effect of met-enkephalin was blocked by naloxone. The K+ channel blocking agents, tetraethylammonium and 4-aminopyridine, inside or outside the electrode, increased dramatically the size of EJCi or EJCo, respectively.

Basic features of an extracellular recording method to study secretion of a sympathetic co-transmitter, presumably ATP

An extracellular recording method is described which permits in suitable model tissues (e.g. guinea-pig or mouse vas deferens) study of the nerve impulse in sympathetic terminals and the release of transmitter from sites inside or outside the recording electrode. Negative- or positive-going potentials were assumed to reflect the excitatory junction current (EJC) caused by transmitter released inside or outside the electrode, respectively, and hence termed 'EJCi' (i for inside) or 'EJCo' (o for outside). The EJCo were shown to be Ca2+-dependent, blocked by addition of tetrodotoxin or guanethidine, resistant to the alpha 1-adrenoceptor blocking agent prazosin but suppressed by desensitization of P2-purinoceptors by alpha,beta-methylene ATP, and hence, presumably, are caused by release of ATP as a sympathetic co-transmitter. The amplitude of the EJCo was voltage-dependent and increased with the length and frequency of stimulus trains within the range of 1-50 shocks at 0.1-2.5 Hz. In conclusion, combined use of EJCi and EJCo provides a useful tool for physiological and pharmacological analysis of pre- and post-junctional events associated with the secretion of a sympathetic co-transmitter, presumably ATP.

On the secretory activity of single varicosities in the sympathetic nerves innervating the rat tail artery

1. Nerve terminal impulses (NTIs) and spontaneous or stimulus-evoked excitatory junction currents (SEJCs or EJCs), reflecting secretion of transmitter quanta from release sites in the sympathetic nerves of rat tail artery, were recorded by extracellular electrodes. 2. The release of transmitter quanta from single varicosities was analysed on a pulse-by-pulse basis. 3. Since the SEJCs were tetrodotoxin-resistant, and hence probably caused by single quanta, they were employed to analyse the quantal content of EJCs. 4. In the majority of recordings, EJCs were large compared to SEJCs from the same attachment, and preceded by prominent NTIs. This type of activity appeared to reflect simultaneous activation of several nerve fibres and numerous varicosities. 5. By focal stimulation, it was usually possible to improve the resolution by examining spots in which a large proportion of the suprathreshold stimuli failed to cause EJCs. Here, averaged NTIs preceding large EJCs were indistinguishable from averaged NTIs not followed by EJCs. Thus, failure of invasion by the nerve impulse was not a cause of the frequent secretory failure. 6. In these attachments the amplitude distribution of nerve stimulus-evoked EJCs was similar to that of the SEJCs and many individual EJCs could be matched in amplitude and time course by SEJCs. Thus, transmitter secretion from these sympathetic nerve varicosities seems to be basically monoquantal. 7. Under conditions when all EJCs were smaller than or equal to the largest SEJCs some characteristic EJC profiles appeared only a few times in response to several hundred suprathreshold stimuli at low frequency (0.5-1 Hz). Using tentatively these EJCs as 'fingerprints' of single quanta from particular release sites, the probability for activation of individual release sites ranges from 0.002 to 0.02.

Simultaneous intracellular and focal extracellular recording of junction potentials and currents, and the time course of quantal transmitter action in rodent vas deferens

Simultaneous recordings were made of spontaneous excitatory junction potentials and the underlying spontaneous excitatory junction currents in guinea-pig and mouse vas deferens using adjacent intracellular and focal extracellular electrodes. Concurrent spontaneous excitatory junction potentials and spontaneous excitatory junction currents were observed in a small proportion of smooth muscle cells penetrated intracellularly within 50-200 microns of the extracellular electrode. These simultaneous events had identical variations in time course, indicating that they were caused by the same transmitter release event. Their amplitudes were not related. Concurrent spontaneous excitatory junction potentials and currents had identical durations, rise times and time constants of decay, showing that the spontaneous excitatory junction potential reflects the time course of quantal transmitter action. In contrast, spontaneous "discrete events" obtained by differentiating the rising phases of spontaneous excitatory junction potentials with respect to time were brief compared with the underlying currents. Excitatory junction potentials evoked by electrical stimulation of the hypogastric nerve were prolonged compared to the underlying excitatory junction currents. The peaks in the first time differential of individual excitatory junction potentials (evoked discrete events) were brief compared to corresponding excitatory junction currents. It is concluded that at the neuroeffector junction of the rodent vas deferens the membrane potential response to a quantum of spontaneously released transmitter is a good estimate of the duration of transmitter action, in accordance with some of the predictions for three-dimensional syncytial tissues. The first time differential of the membrane potential, the "discrete event", does not reflect the time course of spontaneous or evoked quantal transmitter action in these syncytial tissues.

Studies on the mode of action of bretylium and guanethidine in post-ganglionic sympathetic nerve fibres

1. The effects of bretylium and guanethidine on the nerve terminal impulse and transmitter release from sympathetic postganglionic nerve terminals in the guinea-pig vas deferens have been studied in vitro using focal extracellular recording. Excitatory junction currents (EJCs) were used as a measure of transmitter release. 2. Both bretylium and guanethidine altered the configuration of the nerve terminal impulse in a manner consistent with their being local anaesthetics. 3. Bretylium (1-3 microM) only completely inhibited transmitter release when impulse propagation in the sympathetic nerve terminal was blocked. 4. In contrast, guanethidine (1-10 microM) could block transmitter release with little effect on the configuration of the nerve terminal impulse. 5. The inhibitory effects of these agents on both the nerve terminal impulse and on transmitter release were reversed by the indirectly acting sympathomimetic agent, d-amphetamine (1-10 microM). 6. Using this technique the mechanisms of action of drugs known to modify the transmitter release in sympathetic nerve terminals can be more precisely determined.

Electrophysiological analysis of the inactivation of sympathetic transmitter in the guinea-pig vas deferens

1. The properties of junction potentials evoked by nerve stimulation and by local application of drugs, and currents evoked by nerve stimulation, in the smooth muscle cells of the guinea-pig vas deferens have been investigated. The effects of temperature on these responses have been studied using intracellular and extracellular recording. 2. Local, brief (5-15 ms) application of 10(-4) M-adenosine-5'-triphosphate (ATP) from glass micropipettes onto the surface of the vas deferens, using pressure pulses (103-206 kPa), elicited a depolarization of the smooth muscle cell membranes which closely resembled the nerve stimulation-evoked excitatory junction potential (EJP). 3. Local application of 10(-4) M-noradrenaline (NA) failed to produce any detectable membrane potential response. Junction potentials elicited by a mixture of 10(-4) M-ATP and 10(-4) M-NA (ratio by volume 1:50) in the drug ejection micropipette were similar in shape to those evoked by ATP alone. 4. Cooling the tissue from 35 to 25 degrees C did not significantly alter resting membrane potentials but resulted in a significant prolongation of the rising and decaying phases of the EJPs. Fifty per cent decay times for EJPs at 35 and 25 degrees C were (mean +/- S.D.) 236 +/- 20 and 434 +/- 30 ms respectively (P less than 0.01). 5. Extracellularly recorded excitatory junction currents (EJCs) elicited by nerve stimulation, believed to reflect the transmembrane current underlying the EJPs, were prolonged in parallel at low temperatures (50% decay times of EJCs at 35 and 25 degrees C: 11.73 +/- 3.94 and 26.15 +/- 8.4 ms, respectively, P less than 0.01). 6. Junction potentials evoked by locally applied, exogenous ATP were also significantly prolonged by cooling (50% decay times: 663 +/- 88 ms at 35 degrees C and 1955 +/- 79 ms at 25 degrees C, P less than 0.01). 7. Bath application of 10(-6) M-alpha,beta-methylene ATP, the enzymatically stable, desensitizing analogue of ATP, reversibly abolished nerve-evoked EJPs. Local application of 10(-6) M-alpha,beta-methylene ATP led to a prolonged depolarization of the smooth muscle cells lasting between 20 and 60 s. 8. Junction potentials elicited by locally applied alpha,beta-methylene ATP were not prolonged or otherwise significantly altered on cooling. The durations of the depolarizations were 46.0 +/- 12.1 s at 35 degrees C and 43.4 +/- 10.6 s at 25 degrees C (P greater than 0.1).(ABSTRACT TRUNCATED AT 400 WORDS)

Time course of transmitter action at the sympathetic neuroeffector junction in rodent vascular and non-vascular smooth muscle

1. Transmitter release from sympathetic postganglionic nerve terminals innervating the guinea-pig and mouse vas deferens and the rat tail artery has been studied in vitro by focal extracellular recording with particular emphasis on the time course of transmitter action underlying the intracellular potential changes. 2. In the absence of stimulation, spontaneous excitatory junction currents (SEJCs) were recorded with amplitudes up to 500 microV and durations between 40 and 100 ms. SEJCs were unaffected by the competitive alpha-adrenoceptor antagonist prazosin but blocked by alpha, beta-methylene ATP which desensitizes P2-purinoceptors. 3. During trains of supramaximal stimuli at 0.1-4 Hz stimulus locked excitatory junction currents (EJCs) were evoked intermittently from the population of varicosities located under the suction electrode. 4. SEJCs were similar in amplitude and time course to EJCs evoked by low-frequency stimulation in the same attachment in all three tissues. 5. SEJCs recorded using either a conventional AC amplifier or a patch clamp amplifier had the same time course. 6. These studies show that the time course of the current underlying the excitatory junction potential is brief and essentially the same in three different tissues. The prolonged time course of the excitatory junction potential in different tissues can be accounted for by the passive membrane properties.