Summary
An attempt was made to determinepA2 values of antagonists at the presynaptic, release-inhibiting α2-autoreceptorsof rabbit and rat brain cortex under conditions when there was very little released noradrenaline in the autoreceptor biophase and, hence,pA2 values were not distorted by endogenous autoinhibition. Cortex slices were preincubated with3H-noradrenaline and then superfused and stimulated by trains of 4 pulses delivered at 100 Hz or, in a few cases, by trains of 36 pulses at 3 Hz. The α-adrenoceptor agonists clonidine, noradrenaline, and α-methylnoradren-aline concentration-dependently decreased the stimulation-evoked overflow of tritium. The a-adrenoceptor antagonists yohimbine, rauwolscine and idazoxan did not increase the overflow of tritium elicited by 4 pulses/100 Hz in rabbit brain slices and increased it only slightly in rat brain slices. In contrast, the antagonists increased markedly the overflow at 36 pulses/3 Hz. All antagonists caused parallel shifts to the right of the concentration-response curves of clonidine, noradrenaline, and α-methylnoradrenaline.pA2 values were calculated either from linear regression of log [agonist concentration ratio − 1] on log [antagonist concentration] or from sigmoid curve fitting. The slopes of the linear regression lines were close to unity, and thepA2 values calculated by the two methods agreed well. There was no consistent preferential antagonism of any antagonist to any agonist.pA2 values determined with stimulation by 4 pulses/100 Hz were by 0.53–0.80 log units higher than those determined with stimulation by 36 pulses/3 Hz. ThepA2 values (4 pulses/100 Hz) of yohimbine and rauwolscine in rabbit brain slices (approximately 7.9 and 8.2, respectively), were slightly higher than in rat brain slices (approximately 7.6 and 7.7, respectively), whereas thepA2 value of idazoxan in the rabbit. (about 7.1) was lower than itspA2 value in the rat (about 8.0). The experiments confirm thatpA2 values determined under conditions of autoinhibition are too low. Stimulation with short (30 ms) bursts of pulses permits the estimation ofpA2 values at presynaptic a2-autoreceptors without (rabbit) or almost without (rat) the complication of autoinhibition. The values suggest that α2-adrenoceptors in rabbit brain cortex differ slightly from those in rat brain cortex.
Similar content being viewed by others
References
Agrawal DK, Triggle CR, Daniel EE (1984) Pharmacological characterization of the postsynapticalpha adrenoceptors in vascular smooth muscle from canine and rat mesenteric vascular beds. J Pharmacol Exp Ther 229:831–838
Arunlakshana O, Schild HO (1959) Some quantitative uses of drug antagonists. Br J Pharmacol 14:48–58
Bonanno G, Maura G, Raiteri M (1986) Pharmacological characterization of release-regulating serotonin autoreceptors in rat cerebellum. Eur J Pharmacol 126:317–321
Feuerstein TJ, Bammert J, Meyer DK (1987) Endogenous agonists may change the concentration-response curves of exogenous agonists: source of quantitative information about the endogenous tone. Biol Cybern 56:419–429
Frankhuijzen AL, Wardeh G, Hogenboom F, Mulder AH (1988) α2-Adrenoceptor mediated inhibition of the release of radiolabelled 5-hydroxytryptamine and noradrenaline from slices of the dorsal region of the rat brain. Naunyn-Schmiedeberg's Arch Pharmacol 337:255–260
Frankhuyzen AL, Mulder AH (1982) Pharmacological characterization of presynaptic a-adrenoceptors modulating [3H]noradrenaline and [3H]5-hydroxytryptamine release from slices of the hippocampus of the rat. Eur J Pharmacol 81:97–106
Fuder H, Muscholl E, Spemann R (1983) The determination of presynapticpA2 values of yohimbine and phentolamine on the perfused rat heart under conditions of negligible autoinhibition. Br J Pharmacol 79:109–119
Fuder H, Braun HJ, Schimkus R (1986) Presynapticalpha-2 adrenoceptor activation and coupling of the receptor-presynaptic effector system in the perfused rat heart: affinity and efficacy of phenethylamines and imidazoline derivatives. J Pharmacol Exp Ther 237:237–245
Furchgott RF (1972) The classification of adrenoceptors (adrenergic receptors). An evaluation from the standpoint of receptor theory. In: Blaschko H, Muscholl E (eds). Handbook of experimental pharmacology, vol 33, Springer Berlin Heidelberg New York, pp 283–335
Lattimer N, Rhodes KF (1985) A difference in the affinity of some selective α2-adrenoceptor antagonists when compared on isolated vasa deferentia of rat and rabbit. Naunyn-Schmiedeberg's Arch Pharmacol 329:278–281
Limberger N, Starke K (1983) Partial agonist effect of 2-[2-(1,4benzodioxanyl)]-2-imidazoline (RX 781094) at presynaptic α2-adrenoceptors in rabbit ear artery. Naunyn-Schmiedeberg's Arch Pharmacol 324:75–78
Limberger N, Singer EA, Starke K (1988) Only activated but not non-activated presynaptic α2-autoreceptors interfere with neighbouring presynaptic receptor mechanisms. Naunyn-Schmiedeberg's Arch Pharmacol 338:62–67
MacKay D (1978) How should values ofpA2 and affinity constants for pharmacological competitive antagonists be estimated? J Pharm Pharmacol 30:312–313
Mayer A, Limberger N, Starke K (1988) Transmitter release patterns of noradrenergic, dopaminergic and cholinergic axons in rabbit brain slices during short pulse trains, and the operation of presynaptic autoreceptors. Naunyn-Schmiedeberg's Arch Pharmacol 338:632–643
Nasseri A, Minneman KP (1987) Relationship between α2-adrenergic receptor binding sites and the functional receptors inhibiting norepinephrine release in rat cerebral cortex. Mol Pharmacol 32:655–662
Reichenbacher D, Reimann W, Starke K (1982) α-Adrenoceptor-mediated inhibition of noradrenaline release in rabbit brain cortex slices. Naunyn-Schmiedeberg's Arch Pharmacol 319: 71–77
Ruffolo RR, Turowski BS, Patil PN (1977) Lack of cross-desensitization between structurally dissimilar α-adrenoceptor agonists. J Pharm Pharmacol 29:378–380
Singer EA (1988) Transmitter release from brain slices elicited by single pulses: a powerful method to study presynaptic mechanisms. Trends Pharmacol Sci 9:274–276
Starke K (1987) Presynaptic α-autoreceptors. Rev Physiol Biochem Pharmacol 107:73–146
Starke K, Montel H, Gayk W, Merker R (1974) Comparison of the effects of clonidine on pre- and postsynaptic adrenoceptors in the rabbit pulmonary artery. Naunyn-Schmiedeberg's Arch Pharmacol 285:133–150
Valenta B, Drobny H, Singer EA (1988) Presynaptic autoinhibition of central noradrenaline releasein vitro: operational characteristics and effects of drugs acting atalpha-2 adrenoceptors in the presence of uptake inhibition. J Pharmacol Exp Ther 245:944–949
Wallenstein S, Zucker CL, Fleiss JL (1980) Some statistical methods useful in circulation research. Circulation Res 47:1–9
Wand DR (1975) Analysis of dose-response curves. In: Daniel EE, Paton DM (eds) Methods in pharmacology, vol 3, Plenum Press, New York London, pp 471–506
Zier G, Drobny H, Valenta B, Singer EA (1988a) Evidence against a functional link between noradrenaline uptake mechanisms and presynaptic alpha-2 adrenoceptors. Naunyn-Schmiedeberg's Arch Pharmacol 337:118–121
Zier G, Drobny H, Valenta B, Singer EA (1988b) Determination of truepA2 values for antagonists at presynaptic alpha-2 autoreceptors in the brain. Naunyn-Schmiedeberg's Arch Pharmacol 338:R68
Author information
Authors and Affiliations
Additional information
Send offprint requests to E. A. Singer at the above address
Rights and permissions
About this article
Cite this article
Limberger, N., Mayer, A., Zier, G. et al. Estimation ofpA2 values at presynaptic α2-autoreceptors in rabbit and rat brain cortex in the absence of autoinhibition. Naunyn-Schmiedeberg's Arch. Pharmacol. 340, 639–647 (1989). https://doi.org/10.1007/BF00717739
Received:
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/BF00717739