Identification of the neuroeffector transmitter in jejunal branches of the rabbit mesenteric artery.

Vasoconstriction or excitatory junction potentials (e.j.ps) evoked by nerve stimulation (15 field pulses at 2 Hz every 3 min) were recorded in rabbit isolated jejunal arteries. The resting diameter of the arteries and its decrease in response to stimulation was measured by a photoelectric method. Vasoconstriction was insensitive to prazosin 0.1 or 1 mumol/l. Yohimbine 1 mumol/l considerably enhanced, whereas alpha,beta-methylene ATP (alpha,beta-meATP) 1 mumol/l abolished the contractile response. In order to test the effect of exogenously applied transmitter candidates, noradrenaline (0.1-1 mumol/l) and ATP (10-30 mumol/l) were added in concentrations which evoked a vasoconstriction comparable to that induced by electrical stimulation. The action of noradrenaline was prevented by prazosin 0.1 mumol/l, but was unaffected by both yohimbine 1 mumol/l and alpha,beta-meATP 1 mumol/l. Alpha,beta-meATP 1 mumol/l depressed the effect of ATP. The e.j.ps evoked by a train of 15 pulses showed facilitation up to the third response and thereafter depression; a partial summation was also observed. Prazosin 0.1 mumol/l did not change the e.j.p. amplitudes. By contrast, when yohimbine 0.1 or 1 mumol/l was added to the prazosin-containing medium, both the late e.j.ps in the train and the summation were enhanced in a concentration-dependent manner. Alpha,beta-meATP 1 mumol/l almost abolished the e.j.ps. In conclusion, in rabbit jejunal arteries, stimulation of postganglionic sympathetic nerves may release noradrenaline together with ATP which is probably the sole neuroeffector transmitter under our conditions. Transmitter release seems to be modulated by the activation of presynaptic alpha 2-adrenoceptors. Under the stimulation conditions of the present experiments the released transmitter does not activate postsynaptic alpha 1-adrenoceptors.

Photoelectric measurement of neurogenic vasoconstriction in jejunal branches of the rabbit mesenteric artery reveals the presence of presynaptic opioid delta-receptors.

The perivascular nerves of rabbit mesenteric arteries were stimulated with 15 pulses at 2 Hz, and decreases in external diameter were measured by means of a photoelectric device. Both extra- and intraluminally added [Met5]enkephalin 1 mumol/l depressed vasoconstriction, although with the second mode of application a larger inhibition occurred. Therefore, in the subsequent experiments all opioids were added into the lumen. [Met5]enkephalin 0.1 mumol/l had no effect. [D-Pen2, L-Pen5]enkephalin 3 mumol/l was less potent than [Met5]enkephalin 1 mumol/l. ICI 174864 1 mumol/l was also without effect when given alone, but antagonized the action of [Met5]enkephalin 1 mumol/l. Ethylketocyclazocine, dynorphin A(1-13), normorphine and DAGO, all 1 mumol/l, were ineffective. [Met5]enkephalin 1 mumol/l did not change the vasoconstriction evoked by the application of noradrenaline (0.1-3 mumol/l). It is concluded that in the mesenteric artery action potential-induced transmitter release, and in consequence vasoconstriction can be inhibited by the activation of presynaptic opioid delta-receptors.

Presynaptic opioid delta-receptors in the rabbit mesenteric artery.

Excitatory junction potentials (e.j.p.s) evoked by nerve stimulation were recorded from muscle cells of the rabbit isolated mesenteric artery. At 0.03 Hz the e.j.p. amplitudes were stable. When a train of fifteen pulses was applied at 0.25 Hz or at higher frequencies (0.5, 1 and 2 Hz), e.j.p.s showed an initial facilitation followed by depression. [Met5]enkephalin 0.1 and 1 mumol/l, [D-Ala2,D-Leu5]enkephalin 0.1 and 1, but not 0.01 mumol/l, and [D-Pen2, L-Pen5]enkephalin 3 mumol/l all depressed the e.j.p.s evoked by trains of fifteen pulses at 1 Hz. When more than one concentration was used ([Met5]enkephalin, [D-Ala2,D-Leu5]enkephalin), the inhibition was concentration dependent. It was always greater for the first few e.j.p.s than for the later ones in a train. [Met5]enkephalin 1 mumol/l reduced the first e.j.p. at 1 Hz and the e.j.p.s evoked by 0.03 Hz to a similar extent. The inhibitory effect of [Met5]enkephalin 1 mumol/l on e.j.p.s persisted in the presence of yohimbine 0.3 mumol/l. Naloxone 1 mumol/l did not interfere with the effect of [Met5]enkephalin 1 mumol/l. Naloxone 10 mumol/l depressed some e.j.p.s and prevented the inhibition by [Met5]enkephalin 1 mumol/l. Neither ICI 154129 10 mumol/l nor ICI 174864 0.3 mumol/l had any effect of their own and both compounds antagonized the action of [Met5]enkephalin 1 mumol/l. Normorphine 10 mumol/l, fentanyl 1 mumol/l, ethylketocyclazocine 0.1 mumol/l, and dynorphin A(1-13) 1 mumol/l were all ineffective. Ethylketocyclazocine 1 mumol/l did not change the e.j.p.s either, but antagonized [Met5]enkephalin 1 mumol/l. [Met5]enkephalin 1 mumol/l failed to influence both the resting membrane potential of the muscle cells and the depolarizing effect of noradrenaline 3 and 30 mumol/l. We suggest that the axon terminals of post-ganglionic sympathetic neurones in the rabbit mesenteric artery possess opioid delta-, but not mu- or kappa-receptors. The activation of presynaptic delta-receptors inhibits the release of the neuroeffector transmitter. There is no evidence for any effect of co-released endogenous opioid peptides under our experimental conditions.

Blockade of alpha 2-adrenoceptors permits the operation of otherwise silent opioid kappa-receptors at the sympathetic axons of rabbit jejunal arteries.

We sought for presynaptic, release-inhibiting opioid receptors at the postganglionic sympathetic axons innervating the jejunal arteries of rabbits. Evoked excitatory junction potentials (e.j.p.s; trains of 15 pulses at 1 Hz) as well as evoked overflow of tritium after preincubation with [3H]-noradrenaline (trains of 120 pulses at 1 Hz) were used to estimate transmitter release. In otherwise untreated tissues ethylketocyclazocine reduced neither the e.j.p. amplitudes nor the evoked overflow of tritium; [Met5]-enkephalin depressed the evoked overflow of tritium. Ethylketocyclazocine reduced e.j.p. amplitudes, however, in tissues exposed to either yohimbine, tolazoline or phentolamine, but not in tissues exposed to prazosin. Ethylketocyclazocine also depressed the evoked overflow of tritium when yohimbine was present. The inhibition produced by ethylketocyclazocine in the presence of yohimbine was antagonized by (-)-3-furylmethyl)-alpha-noretazocine (MR 2266) but not by N,N-diallyl-Tyr-alpha-aminoisobutyric acid-alpha-aminoisobutyric acid-Phe-Leu-OH (ICI 174864). It is concluded that the sympathetic neurones of rabbit jejunal arteries possess presynaptic kappa-receptors in addition to the previously identified delta-receptors. The kappa-receptors become operative only when presynaptic alpha 2-adrenoceptors have been blocked.

Differential effect of stimulation strength in mouse vas deferens on inhibition of neuroeffector transmission by receptor type selective opioids.

In the mouse isolated vas deferens the amplitude of excitatory junction potentials (e.j.p.s) recorded intracellularly from smooth muscle cells varied with the strength of stimulation. Receptor type selective opioids were tested in this preparation. The mu-agonist normorphine (2,000 nmol/l) reduced the amplitude of e.j.p.s and shifted the stimulus-response curve in a parallel way to the right. By contrast, the kappa-agonist U-50488 (1,000 nmol/l) and the delta-agonist [D-Ala2,D-Leu5]-enkephalin (2 nmol/l) caused a non-parallel shift of the curve. In addition, opioids having a lower selectivity for one type of receptor were also used. The preferential kappa-agonists ethylketocyclazocine (40 nmol/l) and dynorphin A1-13 (100 nmol/l) produced parallel and non-parallel shifts, respectively. Thus, normorphine and ethylketocyclazocine were more effective in depressing e.j.p.s evoked by low intensities of stimulation than those evoked by high intensities of stimulation. U-50488, dynorphin A1-13 and [D-Ala2,D-Leu5]-enkephalin caused an equal depression of e.j.p.s evoked by either intensity of stimulation. The preferential mu- and delta-antagonists naloxone (1,000 nmol/l) and ICI 154129 (10,000 nmol/l), reversed the action of the respective agonists normorphine and [D-Ala2, D-Leu5]-enkephalin. In addition, ICI 154129 (10,000 nmol/l) reversed the action of dynorphin A1-13, as well. The preferential kappa-antagonist MR-2266 (1,000 nmol/l) prevented the effect of both ethylketocyclazocine and U-50488. It is concluded that under the conditions of these experiments normorphine and ethylketocyclazocine acted at mu-, U-50488 at kappa-, and dynorphin A1-13 and [D-Ala2,D-Leu5]-enkephalin at delta-receptors.(ABSTRACT TRUNCATED AT 250 WORDS)