A search for presynaptic imidazoline receptors at rabbit and rat noradrenergic neurones in the absence of α2-autoinhibition
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Presynaptic, release-inhibiting imidazoline receptors have hitherto been detected mainly under conditions of ongoing α2-autoinhibition. We tried to find them under α2-autoinhibition-free conditions, in the majority of experiments in the rabbit pulmonary artery but in a few experiments also in rabbit atria, rabbit brain cortex and rat brain cortex. Tissue segments were preincubated with [3H]noradrenaline and then superfused and stimulated electrically. Ten compounds, some thought to inhibit noradrenaline release through activation of presynaptic imidazoline receptors, some thought to act through α2-adrenoceptors, were tested. Rauwolscine and 6-chloro-N-methyl-2,3,4,5-tetrahydro-1-H-3-benzazepine (SKF 86466) were used as antagonists. In rabbit pulmonary artery segments stimulated by trains of 20 pulses/50 Hz (α2-autoinhibition-free conditions), all ten “agonists” [medetomidine, aganodine, 4-chloro-2-guanidyl-isoindoline (BDF 7579), 4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline (BDF 6143), 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304), α-methylnoradrenaline, clonidine, moxonidine, cirazoline and idazoxan] reduced the stimulation-evoked overflow of tritium, with potency decreasing in that order and with maximal inhibition by 59% (idazoxan) to 95% (moxonidine). Rauwolscine competitively antagonized the effects of all ten “agonists” with similar potency, the pK d-values lying between 7.6 and 8.2. Similarly, SKF 86466 competitively antagonized the effects of clonidine and moxonidine with the same potency (pK d 7.6). Cirazoline was also studied in the other three tissues. In rabbit atrial segments stimulated by 20 pulses/50 Hz and rabbit brain cortex slices stimulated by 4 pulses/100 Hz (both autoinhibition-free), cirazoline reduced the evoked overflow of tritium with concentration-inhibition curves similar to the curve in the pulmonary artery. In the brain cortex, the pK d-value of rauwolscine against cirazoline was 7.7 (pulmonary artery: 7.6). In rat brain cortex slices stimulated by 3 pulses/100 Hz (autoinhibition-free), cirazoline failed to change the evoked overflow of tritium but antagonized the inhibitory effect of UK 14304, pK d of cirazoline against UK 14304 6.9. In rat brain cortex slices stimulated by trains of 36 pulses/3 Hz, finally (marked α2-autoinhibition), cirazoline increased the evoked overflow of tritium. In the rabbit pulmonary artery, rauwolscine and SKF 86466 acted with the same potency against typical presynaptic imidazoline receptor agonists (such as clonidine) and typical α2-adrenoceptor agonists (such as moxonidine). Hence, presynaptic imidazoline receptors were not detectable, in a tissue that is prototypical for presynaptic imidazoline receptors, in the absence of α2-autoinhibition, i.e. under conditions usually thought to be optimal for presynaptic receptor characterization. The pK d-values of rauwolscine and SKF 86466 indicate that all ten agonists activated the α2A-autoreceptors of the pulmonary artery. Cirazoline behaved as an α2(A)-autoreceptor agonist in rabbit tissues but as an α2(D)-autoreceptor antagonist in rat tissues. Perhaps cirazoline generally possesses greater intrinsic activity at (human, rabbit) α2A-adrenoceptors than the orthologous (rodent) α2D-adrenoceptors.
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