Estimation ofpA2 values at presynaptic ?2-autoreceptors in rabbit and rat brain cortex in the absence of autoinhibition

1 Rats received an intraperitoneal injection of 1.6 mg kg-' N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) to achieve an irreversible inactivation of presynaptic release-modulating (z2-autoreceptors. Cerebral cortex slices were prepared at different times after the injection (24, 48, 96, 192, 336, 774 h), incubated with [3H]-noradrenaline ([H]-NA), superfused and stimulated electrically with 4 pulses at 100 Hz (= autoinhibition-free condition). Overflow of radioactivity was used to measure release. Furchgott analysis was used to estimate agonist dissociation constants (KA) and pool size of resynthesized receptors (q). 2 The KA values of the three X2-autoreceptor agonists, bromoxidine (UK-14304), clonidine and noradrenaline (NA) were 187 nM, 72 nM, and 1202 nM, respectively. 3 The release-inhibiting effects of the agonists returned considerably faster than the receptor pool. The calculated half-lives for the recovery of the maximal release-inhibiting effects of bromoxidine, clonidine and NA were 30.7, 63.6 and 20.8 h, respectively, whereas the half-life for the recovery of the receptor pool was 445 h. 4 The data indicate a large receptor reserve at presynaptic x2-autoreceptors for the agonists used and validate the use of EEDQ as a tool for the determination of agonist dissociation constants.

Section: Resultsmentioning

confidence: 93%

Section: Preparation Of Tissue Incubation and Superfusionmentioning

confidence: 99%

Central α₂‐autoreceptors: agonist dissociation constants and recovery after irreversible inactivation

Agneter

Drobny

Singer

1993

“…Studies on presynaptic a-autoreceptors also first indicated heterogeneity within the 2 class (Doxey & Everitt, 1977;Dubocovich, 1979). A species difference between the rabbit and the rat, for example, was a consistent finding (Reichenbacher et al, 1982;Ennis, 1985;Lattimer & Rhodes, 1985;Alabaster et al, 1986;Limberger et al, 1989).…”

Section: Introductionmentioning

confidence: 78%

“…The outflow of tritium was expressed as fractional rate (min-'). The stimulation-evoked overflow was calculated by subtraction of the basal outflow and was expressed as a percentage of the tritium content of the tissue (Limberger et al, 1989;. For further evaluation of basal tritium efflux, ratios were calculated of the fractional rate during the 5 min before S2, S3 and S4, and the fractional rate during the 5 min before SI (b2/b1, b3/b1, b4/b1).…”

Section: Methodsmentioning

confidence: 99%

Pharmacological characterization of presynaptic α₂‐autoreceptors in rat submaxillary gland and heart atrium

Limberger

Trendelenburg

Starke

1992

1 The pharmacological properties of presynaptic M2-autoreceptors were studied in rat isolated submaxillary glands and atria. Tissue pieces were preincubated with [3H]-noradrenaline, then superfused with medium containing desipramine, and stimulated electrically. In one series of experiments, pEC30 values of 12 x-adrenoceptor antagonists were determined, i.e., negative logarithms of concentrations that increased the electrically evoked overflow of tritium by 30%. In another series, pKD values of 9 aadrenoceptor antagonists against the release-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304), and of 3 antagonists against the release-inhibiting effect of methoxamine, were determined.2 In submaxillary glands, the pEC30 values of the antagonists correlated well with their pKD values against UK 14304 (r = 0.93). The same was true for atria (r = 0.92).3 In submaxillary glands, the pKD values of 3 antagonists against UK14304 were very similar to their pKD values against methoxamine, with a maximal difference of 0.4. The same was true for atria where the maximal difference was 0.3. 4 The pEC30 values obtained in submaxillary glands correlated significantly with those obtained in atria (r = 0.81). The same was true for the pKD values (r = 0.79). However, the pEC30 and pKD values also indicated consistent differences between the two tissues. 5 It is concluded that the sites of action of the imidazoline UK 14304 (x2-selective), the phenylethylamine noradrenaline, and the phenylethylamine methoxamine (a,-selective) are exclusively a2-adrenoceptors. There is no indication for presynaptic a,-adrenoceptors or for an effect of UK 14304 mediated by presynaptic imidazoline receptors. The 02-autoreceptor population in the submaxillary gland differs from that in the atrium. 6 Comparison with studies from the literature indicates that the submaxillary autoreceptors are closely similar to the a2D radioligand binding site found in the bovine pineal gland and probably the rat submaxillary gland. The atrial autoreceptors also conform best to this site, but the agreement is more limited; the atrial autoreceptors may represent a type related to, but distinct from, the a2D site, or a mixture of different types.

“…This was achieved by stimulating the tissue with short (91 ms) trains of electric pulses, too short to allow autoinhibition to develop (Singer, 1988). Autoinhibition-free conditions are required for valid determination of dissociation constants (Fuder et al, 1983;Limberger et al, 1989;see Starke et al, 1989).…”

Section: Resultsmentioning

confidence: 99%

α_2c‐Adrenoceptor‐modulated release of noradrenaline in human right atrium

Rump

Böhmann

Schaible

et al. 1995

Self Cite

1 The aim of the present study was to characterize the presynaptic a2-autoreceptors in human right atrium in terms of the a2AD system. Segments of atrial appendages were preincubated with [3H]-noradrenaline and then superfused in the presence of cocaine and stimulated electrically. pEC30% values of eight a-adrenoceptor antagonists with discriminatory power were determined. pEC30% is the negative logarithm of the antagonist concentration that increased the stimulation-induced overflow of tritium by 30%. For four antagonists, the dissociation constant KD was determined, in addition to pEC30%, against the overflow-inhibiting effect of 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14,304) under autoinhibition-free conditions. 2 pEC30% and KD values yielded identical rank orders of antagonist affinity (rauwolscine > WB 4101> phentolamine > prazosin) suggesting that both released noradrenaline and the exogenous agonist UK 14,304 activated the same receptor to inhibit release. In conclusion, the presynaptic a2-autoreceptors in human right atrium are X2C. In this they agree with the previously characterized a2-autoreceptors in human kidney. The a2C classification possibly separates, in general, human a2-autoreceptors from those in lagomorph (rabbit) and rodent (rat, mouse, guinea pig) species that have been proposed to be predominantly a2A or a2D.