Occurrence, pharmacology and function of presynaptic α₂-autoreceptors in α_{2 A/D}-adrenoceptor-deficient mice

Abstract.

The occurrence, pharmacological properties and function of α₂-autoreceptors were studied in hippocampal slices, occipito-parietal cortex slices, segments of heart atria and segments of the vas deferens of wildtype (WT) mice and mice in which the α_2 A/D-adrenoceptor gene had been disrupted (α_2 A/DKO). Tissues were preincubated with [³H]-noradrenaline and then superfused and stimulated electrically. Stimulation periods for brain slices consisted either of 1 pulse or of 2–64 pulses delivered at 1-s intervals; stimulation periods for peripheral tissues consisted either of 1 POP (pseudo-one-pulse; brief burst of 20 pulses/50 Hz) or of 2–4 POPs delivered at 1-s intervals. Single pulses or POPs were used to study the effect of medetomidine and its interaction with antagonists. One or more pulses or POPs per stimulation period were used to study α₂-autoinhibition.

Medetomidine decreased the evoked overflow of tritium in WT tissues. In α_2 A/DKO tissues, the inhibition was slightly (peripheral tissues) or greatly (brain slices) attenuated but not abolished. Phentolamine, rauwolscine, spiroxatrine, 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane (WB 4101), tolazoline and prazosin antagonized the effect of medetomidine in all tissues. Their pK_d values against medetomidine were compared with pK_d values at prototypical α₂ binding sites by means of a correlation analysis. For WT brain and atrial autoreceptors, the correlations indicated an α_2D pharmacology, whereas for WT vas deferens autoreceptors they favoured an α_2B pharmacology. In the KO tissues, any correlation with α_2D was lost, and the non-α_2 A/D-autoreceptors displayed α_2B or α_2C pharmacology. When 2 or more pulses or POPs were applied to WT tissues per stimulation period, the pulse number-overflow curve (POP number-overflow curve) was flat, indicating that overflow elicited by p pulses (POPs) was much smaller than p times the overflow elicited by a single pulse (POP); moreover, rauwolscine caused a pulse (POP) number-dependent and, at high pulse (POP) numbers, large increase in evoked tritium overflow. In α_2 A/DKO tissues, the pulse (POP) number-overflow curve was much steeper, indicating that overflow elicited by p pulses (POPs) was closer to p times the overflow elicited by a single pulse (POP); moreover, rauwolscine caused no (atria) or only a small increase in overflow, and did so in brain slices only at high pulse numbers (16 and 64).

In conclusion, the predominant α_2D pharmacology of the autoreceptors in WT tissues supports the idea that the main mammalian presynaptic α₂-autoreceptors belong to the α_2 A/D subtype. However, α_2 A/D-deficient animals also possess autoreceptors. As expected, these non-α_2 A/D-autoreceptors display α_2B or α_2C pharmacology. In WT animals, α_2B- or α_2C-autoreceptors or both may coexist with α_2 A/D-autoreceptors, at least in peripheral tissues. Little autoinhibition by released noradrenaline in trains of pulses remains when the α_2 A/D-adrenoceptor is lacking, again in accord with a predominance of α_2 A/D-autoreceptors.