Relative pre- and postsynaptic potencies of α-adrenoceptor agonists in the rabbit pulmonary artery

The rabbit pulmonary artery contains postsynaptic alpha-adrenoceptors which meidate smooth muscle contraction; its noradrenergic nerves contain presynaptic alpha-adrenoceptors which mediate inhibition of the release of the transmitter evoked by nerve impulses. Dose-response curves for the pre- and postsynaptic effects of eight alpha-receptor agonists were determined on superfused strips of the artery in the presence of cocaine, corticosterone and propranolo. 1. According to the concentrations which caused 20% of the maximal contraction (EC20 post), the postsynaptic rank order of potency was: adrenaline greater than noradrenaline greater than oxymetazoline greater than naphazoline greater than phenylephrine greater than tramazoline greater than alpha-methylnoradrenaline greater than methoxamine. The pA2 values of phentolamine againstoxymethazoline, phenylephrine, alpha-methylnoradrenaline and methoxamine were 7.43, 7.48, 7.59 and 7.69, respectively. 2. For the investigation of presynaptic effects, the arteries were preincubated with 3H-noradrenaline. All agonists inhibited the overflow of tritium evoked by transmural sympathetic nerve stimulation. According to the concentrations which reduced the stimulation-induced overflow by 20% (EC20 pre), the rank order of potency was: adrenaline greater than oxymetazoline greater than tramazoline greater than alpha-methylnoradrenaline greater than noradrenaline greater than naphazoline greater than phenylephrine greater than methoxamine. 10(-5) M phentolamine shifted the presynaptic dose-response curves for moradrenaline and oxymethazoline to the right. 3. The ratio EC20 pre/EC20 post was calculated for each agonist as an index of its relative post- and presynaptic potency. According to the ratios, the agonists were arbitrarily classified into three groups. Group 1 (ratio about 30: preferentially postsynaptic agonists) comprised methoxamine and phenylephrine; group 2 (ratio near 1; similar pre- and postsynaptic potencies) comprised noradrenaline, adrenaline and naphazoline; group 3 (ratio below 0.2; preferentially presynaptic agonists) comprised oxymetazoline, alpha-methylnoradrenaline and tramazoline (as well as clonidine). 4. Preferentially presynaptic and preferentially postsynaptic agonists had opposite effects on the basoconstrictor response to nerve stimulation. Methoxamine and phenylephrine either did not change or enhanced, but never reduced, the response. In contrast, oxymetazoline, alpha-methylnoradrenaline and tramazoline at low concentrations selectively inhibited the response to stimulation at low frequency (0.25-2Hz). 5. It is concluded that alpha-adrenoceptor agonists vary widely in their relative pre- and postsynaptic potencies, possibly because of structural differences between pre- and postsynaptic alpha-receptors. Pre- and postsynaptic components contribute to their overll postsynaptic effec in actively transmitting synapses. The preferential activation of presynaptic alpha-receptors results in alpha-adrenergic inhibition of synaptic transmission.     

Different alpha-adrenoceptors modulate the release of 5-hydroxytryptamine and noradrenaline in rat cortex.

1 The potassium-evoked release of [3H]-noradrenaline from slices of rat occipital cortex and the potassium-evoked release of [3H]-5-hydroxytryptamine from slices of rat frontal cortex were measured using a superfusion system. 2 The rank order of potency for a number of alpha-adrenoceptor agonists was different for the two neuronal systems, clonidine and azepexole being the most potent inhibitors of noradrenaline release and methoxamine and phenylephrine being the most potent against 5-hydroxytryptamine release. 3 The rank order of potency for a series of alpha-adrenoceptor antagonists in reversing the inhibition of noradrenaline release produced by clonidine was: phentolamine greater than rauwolscine = yohimbine = corynanthine much greater than WB4101, whereas against methoxamine-inhibition of 5-hydroxytryptamine release the rank order of potency was: WB4101 greater than phentolamine greater than corynanthine greater than yohimbine greater than rauwolscine. 4 The results suggest that the alpha-adrenoceptors which modulate potassium-evoked 5-hydroxytryptamine release are not identical with the alpha 2-adrenoceptors which modulate potassium-evoked 5-hydroxytryptamine release are not identical with the alpha 2-adrenoceptors located on noradrenergic nerve terminals and may more closely resemble alpha 1-than alpha 2-adrenoceptors.     

Inhibition of neuronal uptake reduces the presynaptic effects of clonidine but not of alpha-methylnoradrenaline on the stimulation-evoked release of 3H-noradrenaline from rat occipital cortex slices

The iminoimidazolidine clonidine reduced concentration-dependently the release of 3H-noradrenaline evoked by electrical stimulation from the rate cerebral cortex. Exposure to the neuronal uptake inhibitors cocaine (10 micro M), desipramine (0.1 to 1 micro M) and amphetamine (1 micro M) significantly increased the stimulation-evoked overflow of tritium. These uptake inhibitors antagonized the effects of clonidine on stimulation evoked 3H-noradrenaline release but failed to modify the inhibition induced by the catecholamine alpha-methylnoradrenaline. Inhibition of monoamine oxidase by preincubation of cerebral cortex slices with 0.5 mM pargyline significantly increased the stimulation-evoked overflow of tritium, but clonidine was as effective as in the controls in inhibiting 3H-noradrenaline overflow. The antagonism by desipramine of the clonidine-induced inhibition of neurotransmission could not be attributed to a blockade of presynaptic alpha-adrenoceptors because: (1) the facilitating effect of phentolamine on 3H-noradrenaline overflow was not modified in the presence of desipramine; (2) the magnitude of the inhibition of the stimulation-evoked 3H-noradrenaline release elicited by alpha-methylnoradrenaline was the same in the presence of cocaine or desipramine; (3) exposure to desipramine in the presence of cocaine did not further increase the stimulation-evoked release of 3H-transmitter. Since the catecholamine alpha-methylnoradrenaline inhibited neurotransmission in the presence of desipramine or cocaine, we can conclude that inhibition of neuronal uptake of noradrenaline antagonized selectively the presynaptic inhibitory effects of imidazolines on alpha 2-adrenoceptors. The influence of the inhibition of neuronal uptake on the presynaptic effects of imidazolines and catecholamines should be taken into account when the relative order of potencies of various alpha 2-adrenoceptors agonists is determined.     

Prostaglandins of the E series inhibit release of noradrenaline in rat hypothalamus by a mechanism unrelated to classical alpha 2 adrenergic presynaptic inhibition

The effects of eight different prostanoid derivatives (PGs) on the in vitro release of noradrenaline (NA) from rat hypothalamic slices are reported. Prostaglandin E2 (10(-8)-10(-5) M), which does not interfere with the [3H]NA uptake mechanism, inhibited [3H]NA release induced by K+-evoked depolarization. The rank order of inhibition of release of NA for the PGs was: PGE2 greater than PGE1 greater than PGA2 greater than 16, 16-dimethyl-PGE2 greater than 11-epi-PGE2 greater than or equal to 8-iso-PGE2 greater than PGF2 alpha greater than PGD2. It has recently been shown that PGs of the E series specifically bind with a high affinity to membrane preparations of rat hypothalamus. A similar rank order was found for the activity of these PGs in displacing [3H]PGE2 from its binding sites, suggesting that the effect of PGEs on release of NA is mediated by an interaction with PGE2 receptors. Under the same experimental conditions, 10(-6) M clonidine (an alpha 2 adrenoceptor agonist) diminished, and 10(-6) M yohimbine (an alpha adrenoceptor antagonist) increased [3H]NA release, supporting the existence of alpha 2 auto-inhibition. Exposure to 10(-6) M of the alpha 1, alpha 2 adrenergic receptor antagonist phentolamine, a concentration which by itself had no effect on overflow of [3H]NA, blocked the inhibitory effect of clonidine, but failed to antagonize the inhibitory action of PGE2. Moreover, the action of clonidine and yohimbine remained unaffected when PG synthesis was blocked with indomethacin.(ABSTRACT TRUNCATED AT 250 WORDS)     

The electrically stimulated release of [3H]noradrenaline from nucleus tractus solitarii slices in vitro is modulated via mu-opioid receptors.

The electrically stimulated release of [3H]noradrenaline ([3H]NA) from slices of the nucleus tractus solitarii (NTS) from the rat in vitro was inhibited by the alpha 2-adrenoceptor agonist, clonidine, in a concentration-dependent manner and enhanced by the alpha 2-adrenoceptor antagonist, yohimbine. Phenylephrine, isoprenaline, carbachol, quinpirole and SKF 38393, all at 10(-6) M, did not affect the stimulus-evoked release of [3H]NA. The opioid peptides, alpha- and gamma-endorphin, did not have a significant effect on the stimulus-evoked release of [3H]NA; however, beta-endorphin reduced it in a concentration-dependent manner. [Leu5]Enkephalin also reduced [3H]NA release, but higher concentrations were necessary. The selective delta opioid receptor agonists, [D-Pen2,D-Pen5]enkephalin (DPDPE) and [D-Ser2(O-tert-butyl),Leu5]enkephalyl-Thr6 (DSTBULET), as well as the selective kappa opioid receptor agonist, U-69593, were not effective. The selective mu opioid receptor agonist, [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAGO), concentration dependently reduced the stimulus-evoked release of [3H]NA to the same extent as beta-endorphin did. Naloxone, while having no effect on stimulus-evoked [3H]NA release, antagonized the effect of DAGO. These results corroborate that the release of NA from noradrenergic terminals in the NTS region of the medulla oblongata of the rat is modulated via alpha 2-adrenoceptors and suggest that the release of NA in the NTS in rats is also modulated via mu opioid receptors.     

Does lithium in vitro and ex vivo alter the release of [3H]noradrenaline from brain tissue and the sensitivity of presynaptic autoreceptors?

The effect of lithium on the release of noradrenaline (NA) was investigated in slices of the cerebral cortex and hippocampus from the rat in vitro and ex vivo. In vitro, small concentrations of lithium chloride (1 and 2 mM) failed to alter the electrically stimulated tritiated overflow from slices preincubated with [3H]NA. Larger concentrations of lithium chloride (5 and 10 mM) significantly increased the electrically evoked overflow of [3H]NA by 18-40% as well as the basal 3H efflux. The alpha 2-adrenoceptor agonist clonidine inhibited, whereas the alpha 2-adrenoceptor antagonist rauwolscine facilitated the stimulated overflow of [3H]NA. These effects were attenuated by 10 mM lithium chloride but not by 2 mM. In slices of brain obtained from rats treated for 5 weeks with lithium chloride, the electrically evoked release of [3H]NA, as well as the inhibition of release of [3H]NA, induced by the alpha 2-adrenoceptor agonist clonidine were unaltered. It is concluded that therapeutically relevant concentrations of lithium do not influence the release of NA and that the function of presynaptic alpha 2-autoreceptors is not affected by chronic treatment with lithium. The increase in release of [3H]NA by larger concentrations of lithium may be relevant to its toxic effects.     

Noradrenaline release in the human pulmonary artery is modulated by presynaptic alpha 2-adrenoceptors

We used strips of human pulmonary arteries from patients undergoing surgery for lung tumor to investigate whether or not this human tissue (like that of the rabbit) is endowed with inhibitory presynaptic alpha 2-adrenoceptors. The strips were incubated with [3H]noradrenaline and subsequently superfused with physiological salt solution containing cocaine, corticosterone, and propranolol. The electrically (2 Hz) evoked overflow of tritium consisted of 89% unmetabolized [3H]noradrenaline and was abolished by tetrodotoxin or omission of Ca2+ from the superfusion fluid. Unlabeled noradrenaline, adrenaline, and the preferential alpha 2-adrenoceptor agonists B-HT 920, alpha-methylnoradrenaline, and clonidine concentration dependently inhibited the evoked overflow (maximum effect of clonidine lower than that of the other compounds): the alpha 1-selective agonist methoxamine was ineffective. The alpha 2-adrenoceptor antagonists BDF 6143 and rauwolscine facilitated the evoked overflow; the alpha 1-selective antagonist prazosin was ineffective. The concentration-response curve of B-HT 920 for its inhibitory effect on evoked overflow was shifted to the right by rauwolscine. It is concluded that the sympathetic nerve fibres of the human pulmonary artery possess presynaptic alpha 2-adrenoceptors. Stimulation-evoked release of noradrenaline is inhibited by activation and facilitated by blockade of these receptors.     

Chronic clonidine induces functional down-regulation of presynaptic alpha 2-adrenoceptors regulating [3H]noradrenaline and [3H]5-hydroxytryptamine release in the rat brain

Clonidine inhibited, through the activation of alpha 2 presynaptic receptors, the release of [3H]noradrenaline (pIC30 = 7.47) and [3H]5-hydroxytryptamine (pIC30 = 6.47) evoked by 15 mM KCl from superfused rat cerebral cortex synaptosomes. The natural agonist noradrenaline inhibited the K+-evoked release of the two [3H]amines less effectively after long-term (12 days) treatment with clonidine than after acute treatment. It can be concluded that chronic clonidine administration can induce down-regulation of both the alpha 2 presynaptic autoreceptors located on noradrenaline terminals and the alpha 2 presynaptic heteroreceptors located on serotonin terminals.     

Modulation of noradrenaline release in human saphenous vein via presynaptic alpha 2-adrenoceptors

Strips of human saphenous veins were incubated with [3H]noradrenaline and subsequently superfused with physiological salt solution containing cocaine, corticosterone and propranolol. The electrically (6 Hz) evoked overflow of tritium (78% of which was accounted for by unmetabolized [3H]noradrenaline) was abolished by tetrodotoxin or omission of Ca2+ from the superfusion fluid. Unlabelled noradrenaline, alpha-methylnoradrenaline, B-HT 920 and clonidine inhibited the evoked overflow (maximum effect of clonidine lower than that of the other compounds) whereas methoxamine was ineffective. The alpha 2-adrenoceptor antagonists, BDF 6143 and rauwolscine, facilitated the evoked overflow but no effect was obtained with prazosin. Rauwolscine produced a shift to the right of the concentration-response curve of B-HT 920 for its inhibitory effect on evoked outflow and BDF 6143 caused a shift to the right of the corresponding curve of clonidine. It is concluded that the stimulation-evoked release of noradrenaline from the sympathetic nerve fibres of the human saphenous vein is modulated via presynaptic alpha 2-adrenoceptors.     

Pharmacological characterization of presynaptic α-adrenoceptors modulating [3H]noradrenaline and [3H]5-hydroxytryptamine release from slices of the hippocampus of the rat

The experiments served to characterize the receptors mediating the inhibitory effect of α-adrenergic drugs on K⁺ (20 mM)-induced [³H]noradrenaline (NA) and [³H]5-hydroxytryptamine ([³H]5-HT) release from slices of the dorsal part of rat hippocampus. Dose-response curves were constructed using the cumulative dose-response technique (Frankhuyzen and Mulder, 1982). All of the adrenergic agonist drugs examined inhibited the K⁺-induced [³H]NA release. NA appeared to have the highest intrinsic activity followed by adrenaline. Clonidine and adrenaline had similar intrinsic activities, while that of oxymetazoline was lowest. The highest pD₂ values were observed for oxymetazoline and clonidine, being slightly higher than that of adrenaline followed by NA. By far the lowest pD₂ values was observed for phenylephrine. With the exception of phenylephrine, all of the agonists also inhibited the K⁺-induced [³H]5-HT release. NA, adrenaline and oxymetazoline appeared to have similar intrinsic activities, while that of clonidine was considerably lower. The pD₂ values of NA and adrenaline were not significantly different but were somewhat lower than those of oxymetazoline and clonidine. Similar antagonistic effects of phentolamine and yohimbine were observed with respect to the adrenergic inhibition of K⁺-induced [³H]NA and [³H]5-HT release. Prazosin, however, appeared to be ineffective in both instances. It is concluded from these results that the presynaptic adrenergic inhibition of [³H]NA as well as [³H]5-HT release is mediated by α₂-adrenoceptors located on noradrenergic and serotonergic varicosities, respectively. Furthermore, our data suggest that these α₂-adrenoceptors are not pharmacologically identical.     

Regulation of noradrenaline release in human cerebral arteries via presynaptic alpha 2-adrenoceptors

1. Electrical stimulation induced tritium release from branches of human middle cerebral arteries preincubated with [3H]noradrenaline (NA), which was reduced by the alpha 2-adrenoceptor agonists, clonidine and B-HT 920, and not affected by the alpha 1-agonist, methoxamine. 2. The stimulated tritium release was inhibited by yohimbine (alpha 2-antagonist), and increased by phentolamine (alpha-antagonist) and prazosin (alpha 1-antagonist). 3. The inhibitory effect of clonidine was antagonized by yohimbine. 4. NA uptake was markedly reduced when the interval between the death and the autopsy was greater than 5 hr. 5. These data indicates the existence of presynaptic inhibitory alpha 2-adrenoceptors, but not alpha 1, in human cerebral arteries, and that the adrenergic nerve endings start to degenerate from 5 hr after death.     

Characterization of the contractile activity of dopamine on the rat isolated seminal vesicle

The mechanism of the contractile effect of dopamine (DA) on the rat isolated seminal vesicle was studied. Cocaine (10 microM/1 in the organ bath, 30 min before DA) and 6-OHDA (50 mg/kg i.v. 24 hr before removal of the seminal vesicle) almost completely prevented the contractile effect of DA. Drugs known to have an affinity for DA receptors or for alpha-adrenoceptors antagonized the contractile effect of DA, the rank order of potency being: prazosin greater than phentolamine greater than yohimbine greater than clonidine greater than sulpiride greater than apomorphine greater than haloperidol. The antagonism was in each case greater against DA than against noradrenaline (NA), used for comparison; selectivity for DA being highest in the case of prazosin and sulpiride. Taken together, these findings indicate that DA makes the rat seminal vesicle contract mostly by means of an indirect mechanism, binding presynaptic DA-receptors and, in part, presynaptic alpha-adrenoceptors as well; or, alternatively, binding presynaptic DA-receptors which have some links with alpha 2-adrenoceptors; the consequence being in either case the release of NA from sympathetic nerve endings.     

Presynaptic serotonin receptors and alpha-adrenoceptors on central serotoninergic and noradrenergic neurons of normotensive and spontaneously hypertensive rats

Rat brain tissues preincubated with [3H]serotonin ([3H]5-HT) or [3H]norepinephrine ([3H]NE) were superfused in the presence of an inhibitor of serotonin or NE uptake, respectively. The electrically (3 Hz) evoked [3H]5-HT release from slices of the medulla oblongata [containing the nucleus tractus solitarii (NTS)] from Wistar rats was inhibited by 5-HT and NE, and these effects were counteracted by metitepine and phentolamine, respectively. The evoked [3H]5-HT release in slices from the cortex, medulla oblongata, and hypothalamus of 5-7-, 9-11-, and 19-22-week-old spontaneously hypertensive rats (SHR) did not differ from that in slices from age-matched Wistar-Kyoto rats (WKY). Nor was there any difference between strains for: the inhibitory effects of 5-HT and NE and the facilitatory effect of metitepine on the evoked [3H]5-HT release; the shift to the right of the concentration-response curves of 5-HT and NE by metitepine and phentolamine, respectively; the potassium (12 mM)-evoked [3H]5-HT release from cortex synaptosomes and its inhibition by 5-HT; the electrically evoked [3H]NE release in cortex slices, its inhibition by NE, and the shift to the right of the concentration-response curve of NE by phentolamine. The results provide evidence that 5-HT release in the rat brain NTS can be inhibited by 5-HT receptors and alpha-adrenoceptors. 5-HT release and its modulation by presynaptic 5-HT1 receptors and alpha 2-adrenoceptors as well as NE release and its modulation by presynaptic alpha 2-adrenoceptors do not differ between SHR and WKY rats. It may be of therapeutic relevance that according to these results the effects of 5-HT1 receptor agonists activating presynaptic 5-HT autoreceptors are not attenuated in this type of hypertension. It has been suggested that such agonists can be developed as a new class of antihypertensive drugs.     

Clonidine differentially modulates the release of endogenous GABA in various rat brain areas.

The effects of clonidine on the release of endogenous gamma-aminobutyric acid (GABA) have been studied in superfused synaptosomes prepared from rat hypothalamus, nucleus tractus solitarii (NTS) and cerebellum. Clonidine enhanced in a concentration-dependent manner the basal release of GABA from hypothalamus and NTS synaptosomes. In contrast, the imidazoline did not affect the release of the amino acid from the cerebellar preparation. The alpha 2-adrenoceptor antagonist yohimbine prevented the releasing effect of clonidine, while the alpha 1-adrenoceptor antagonist prazosin was ineffective. The results show that the release of GABA from hypothalamus or NTS nerve endings can be enhanced by clonidine through the activation of adrenoceptors of the alpha 2 subtype. The GABAergic nerve terminals of cerebellum do not seem to possess presynaptic adrenoceptors by which clonidine can regulate the basal outflow of the amino acid. The results suggest that some of the clonidine effects may occur through activation of the GABA system.     

The actions of leukotrienes C4 and D4 on guinea-pig isolated hearts

1 Noradrenaline and a series of imidazolines were used to characterized and differentiate the postsynaptic alpha-adrenoceptors of rat and rabbit aortae. 2 Dose-response curves in each tissue revealed marked differences in the profile of agonist activity among the compounds. Based on the ED50 values for each compound, a rank order of potency of oxymetazoline greater than noradrenaline greater than tramazoline greater than tetrahydrozoline greater than clonidine was obtained in rabbit aorta and an order of noradrenaline greater than clonidine greater than tramazoline greater than oxymetazoline was obtained in rat aorta. Tetrahydrozoline had no agonist activity in rat aorta. 3 Dissociation constants were determined for each agonist in rat and rabbit aortae. Again, differences between the tissues were observed to the extent that the rank order of affinities for the imidazolines were exactly opposite for the two tissues. In rabbit aorta the order was, oxymetazoline greater than tramazoline greater than tetrahydrozoline greater than clonidine, whereas in rat aorta it was, clonidine greater than tetrahydrozoline greater then tramazoline greater than oxymetazoline. The extremes in tissue selectivity were observed with clonidine, which had approximately 125 fold higher affinity in rat aorta, and oxymetazoline, which had approximately 4 times higher affinity in rabbit aorta. 4 The absolute values of relative efficacies of the imidazolines studied, and their rank order, also differed between the two tissues. The relative efficacies of oxymetazoline and tramazoline were more than 15 fold greater in rabbit aorta than in rat aorta. Furthermore, tetrahydrozoline had a greater relative efficacy than clonidine in rabbit aorta while the converse was true in rat aorta. 5 Differences in the rank order of potency, affinity and relative efficacy of noradrenaline and a series of imidazolines in rat and rabbit aortae indicate that the postsynaptic alpha-adrenoceptors in these tissues are different. While the postsynaptic alpha-adrenoceptor of rabbit aorta is clearly of the alpha 1-subtype, the exact nature of the postsynaptic alpha-receptor of rat aorta is not clear. The unique alpha-receptor of rat aorta has properties of both alpha 1- and alpha 2-adrenoceptors.     

Prejunctional alpha 1-adrenoceptors modify release of [3H]noradrenaline in the guinea-pig vas deferens

1. Several alpha 1- and alpha 2-adrenoceptor agonists and antagonists were examined for effects on spontaneous and stimulus-evoked release of [3H]noradrenaline from sympathetic nerves in guinea-pig vas deferens. 2. Prazosin (0.1 and 1 microM), phentolamine (30 microM) and yohimbine (10 microM) each enhanced the stimulus-evoked release of [3H]noradrenaline. 3. Prazosin and phentolamine increased the spontaneous outflow of [3H]noradrenaline, whereas yohimbine was without effect. 4. Methoxamine (10 microM) and clonidine (0.1 microM) inhibited the stimulus-evoked release of [3H]noradrenaline, whereas only methoxamine (1 microM) decreased the spontaneous outflow of [3H]noradrenaline. 5. The identity of prejunctional alpha-adrenoceptors in the guinea-pig vas deferens is discussed.     

Presynaptic noradrenergic alpha-receptors and modulation of 3H-noradrenaline release from rat brain synaptosomes.

The depolarization (15 mM K+)-induced release of 3H-NA from superfused rat brain synaptosomes and the effects of alpha-noradrenergic drugs thereon were studied. Noradrenaline (NA; in the presence of the uptake inhibitor desipramine) reduced synaptosomal 3H-NA release. Reduction of the concentration of calcium ions in the medium during K+ stimulation greatly enhanced the sensitivity of 3H-NA release to alpha-receptor-mediated inhibition. Under these conditions NA dose-dependently inhibited 3H-NA release from synaptosomes obtained from cortex or hypothalamus, but did not affect 3H-NA release from striatal (i.e dopaminergic) synaptosomes. Adrenaline, clonidine and oxymetazoline potently inhibited 3H-NA release from cortex synaptosomes at concentrations in the nanomolar range. Phentolamine by itself did not affect synaptosomal 3H-NA release, but antagonized the inhibitory effects of both noradrenaline and adrenaline. The data obtained further substantiate the hypothesis that the alpha-receptors mediating a local negative feedback control of NA release are localized on the varicosities of central noradrenergic neurons, Furthermore, noradrenergic nerve terminals in the hypothalamus appear to be less senstive to alpha-receptor-mediated presynaptic inhibition than those in the cortex.     

Stimulation of alpha 1-adrenoceptors increases electrically evoked [3H]acetylcholine release from the rat phrenic nerve

The modulation by sympathomimetic amines of the electrically evoked [3H]acetylcholine release from the motor nerve was investigated. Phenylephrine (10 mumol/l), alpha-methylnoradrenaline (10 mumol/l) and adrenaline (1 mumol/l) enhanced the electrically evoked [3H]acetylcholine release from the rat phrenic nerve. The enhancing effect of phenylephrine was completely prevented by low concentrations of prazosin (0.1 or 0.01 mumol/l) whereas a high concentration of yohimbine (1 mumol/l) was necessary to abolish the effect of phenylephrine. The simultaneous blockade of alpha- and beta-adrenoceptors, i.e. propranolol (0.1 mumol/l) together with prazosin (0.01 mumol/l) or yohimbine (1 mumol/l), was required to abolish the enhancing effect of alpha-methylnoradrenaline. Likewise, the enhancing effect of adrenaline could be abolished only by a combination of two antagonists (propranolol together with yohimbine or prazosin). Neither clonidine nor oxymetazoline (1 or 10 mumol/l) modulated the evoked [3H]acetylcholine release. The experiments showed the existence of alpha-adrenoceptors which are present on the motor nerve and whose stimulation mediates an increase in evoked transmitter release. The different potencies found for prazosin and yohimbine indicate that an alpha 1-subtype of the receptors was involved. Increased sympathetic activity may facilitate neuromuscular transmission by stimulation of presynaptic alpha- and beta-adrenoceptors.     

Phenoxybenzamine partially inhibits alpha 2-adrenoceptors without affecting their presynaptic function

The turnover of alpha-adrenoceptors was assessed by administering phenoxybenzamine (PBZ) intraperitoneally to rats in order to block the receptors irreversibly. The reappearance of the binding of [3H]prazosin, [3H]clonidine and [3H]rauwolscine in membranes from cerebral cortices was then measured. Maximum inhibition of binding occurred 3 hr after administration of phenoxybenzamine. The binding of [3H]prazosin was inhibited by 95% after administration of phenoxybenzamine (2 X 4 mg/kg, i.p.), and the half life (t1/2) for the alpha 1-adrenoceptor was 1.87 days. The "turnover" of binding for the alpha 2-adrenoceptor ligands ([3H]clonidine and [3H]rauwolscine) was similar: with doses of phenoxybenzamine up to 15 mg/kg (i.p.), the binding of both ligands was inhibited to a maximum of 30%. Maximum recovery occurred 3 days after treatment with phenoxybenzamine and the alpha 2-adrenoceptor has an apparent half life for recovery of 12 hr. Since only partial blockade of alpha 2-adrenoceptors was possible with phenoxybenzamine the possibility that these blocked sites included functional presynaptic autoreceptors was investigated. Clonidine (1 microM) attenuated K+-induced release of preloaded [3H]noradrenaline from cortical synaptosomes prepared from control rats by some 35%. Clonidine inhibited this release of [3H]noradrenaline to the same extent in synaptosomes prepared from rats treated with phenoxybenzamine 3 hr prior to sacrifice. This indicates that the alpha 2-adrenoceptors which are blocked by phenoxybenzamine are not part of the functional receptor population.     

Modulation of noradrenaline release from cat cerebral arteries by presynaptic alpha 2-adrenoceptors. Effect of chronic treatment with desipramine and cocaine

1. Field electrical stimulation elicited an increase of the tritium efflux over the basal level from cat cerebral arteries previously incubated with (+/-) [3H]noradrenaline ([3H]NA). 2. This efflux was: (a) reduced by clonidine, NA or B-HT 920; (b) unaffected by methoxamine, prazosin and yohimbine (10(-6) M); (c) reduced by yohimbine (5 x 10(-6) M), and (d) increased by phentolamine. 3. The effect of clonidine was blocked by yohimbine. 4. The daily treatment with the neuronal uptake blockers desipramine (10 mg/kg, i.p.) or cocaine (10 mg/kg, i.p.) [during 12 days], antagonized the inhibitory action of clonidine totally or partially, respectively. 5. These results suggest: (1) the existence of presynaptic alpha 2-adrenoceptors in these arteries, which modulate the NA release, and (2) that chronic treatment with desipramine or cocaine induces a subsensitivity of these alpha 2-receptors, which facilitates the NA release.