Stimulation frequency-noradrenaline release relationships examined in α2A-, α2B- and α2C-adrenoceptor-deficient mice

Abstract

The stimulation frequency-noradrenaline release relationship was studied in the vas deferens and the cerebral cortex of NMRI mice, mice in which the alpha2A-, the alpha2B-, the alpha2C- or both the alphaCA- and the alpha2C-adrenoceptor gene had been disrupted (alpha2AKO, alpha2BKO, alpha2CKO and alpha2ACKO), and the wildtype mice from which the knockout animals had been generated. Tissue pieces were preincubated with 3H-noradrenaline and then superfused and stimulated electrically with a constant number of pulses (30 in vas deferens and 50 in brain cortex) at frequencies between 0.03 and 100 Hz. The frequency-evoked tritium overflow curves ascended monophasically in the vas deferens of wildtype and NMRI mice. Disruption of the alpha2B-adrenoceptor gene caused no change. In the vas deferens of alpha2CKO mice, the overflow evoked by low frequencies (0.3 and 1 Hz) was slightly increased. In the vas deferens of alpha2AKO and alpha2ACKO mice, the evoked overflow was increased to a greater extent. Rauwolscine (1 microM) caused a marked increase of the evoked overflow of tritium from the vas deferens of NMRI, wildtype, alpha2BKO and alpha2CKO mice. Rauwolscine also increased the evoked overflow of tritium from the vas deferens of alpha2AKO and alphaC2ACKO mice, but to a smaller extent. The gene disruptions and rauwolscine slightly steepened the slope of the vas deferens frequency-overflow curve. In the brain cortex of wildtype and NMRI mice, the frequency-evoked tritium overflow curves were U-shaped. In the brain cortex of alpha2BKO and alpha2CKO mice, the evoked overflow was slightly reduced. In the brain cortex of alpha2AKO and alpha2AcKO mice, in contrast, the evoked overflow was increased. Rauwolscine (1 microM) caused a marked increase of the evoked overflow of tritium from the brain cortex of NMRI, wildtype, Q2BKO and alpha2CKO mice. Rauwolscine also increased the evoked overflow of tritium from the brain cortex of alpha2AKO and alpha2ACKO mice, but to a smaller extent. The gene disruptions and rauwolscine flattened the U shape of the brain cortex frequency-overflow curve. It is concluded that alpha2-autoinhibition is one factor that shapes the frequency-noradrenaline release relationships in the mouse vas deferens and cerebral cortex. The autoreceptors are mainly alpha2A and to a minor extent, and well detectable in the vas deferens only, alpha2C. When both the alpha2A- and the alpha2C-adrenoceptor have been deleted, alpha2B-adrenoceptors may be expressed as autoreceptors in noradrenergic neurons. It seems possible that alpha2C-autoreceptors depress mainly release at low (around 1 Hz) whereas alpha2A-autoreceptors depress mainly release at high (around 10 Hz) frequencies.

Evidence that the plant cannabinoid cannabigerol is a highly potent α2‐adrenoceptor agonist and moderately potent 5HT1A receptor antagonist

“…Additional experiments are now required first to establish whether the inhibitory effect of cannabigerol in the vas deferens and its stimulatory effect in the [ 35 S]GTPγS binding assay are mediated by α 2A ‐, α 2B ‐ and/or α 2C ‐adrenoceptors, and second to investigate why the potency that cannabigerol displays in these two bioassays is so different (Table 1 and Figures 2 and 3). In the meantime, it is noteworthy that there is evidence that electrically evoked neuronal release of noradrenaline from the vas deferens can be inhibited by the activation not only of α 2A ‐adrenoceptors but also of α 2C ‐adrenoceptors (Scheibner et al. , 2001), particularly when the frequency of the electrical stimulation is relatively low as it was in this investigation.…”

α2-Adrenergic blockade mimics the enhancing effect of chronic stress on breast cancer progression

“…Several reports have shown that α 2 -adrenergic receptors mediate an autoinhibition of sympathetic transmission (Hein et al, 1999; Scheibner et al, 2001; Trendelenburg et al, 2001) and that disruption of this negative feedback signal with α 2 -adrenergic antagonists or gene inactivation of α 2 -adrenergic receptors enhances norepinephrine release (Starke, 2001). Subsequently, phentolamine, with affinity for both α 1 - and α 2 -adrenergic receptors, has been shown to increase norepinephrine release from peripheral tissues ex vivo (Schelb et al, 2001).…”

“…Multiple reports have shown that pre-synaptic α 2 -adrenergic receptors can mediate an autoinhibition of sympathetic transmission (Hein et al, 1999; Scheibner et al, 2001; Trendelenburg et al, 2001) and that disruption of this negative feedback signal with α 2 -adrenergic antagonists or gene inactivation of α 2 -adrenergic receptors enhances norepinephrine release (Starke, 2001). Thus, it is conceivable that blocking inhibitory α 2 -autoreceptors might increase sympatho-neural discharge and enhance β-adrenergic signaling with its known downstream pro-tumor consequences for breast cancer progression (Thaker et al, 2006; Sloan et al, 2010; Madden et al, 2011).…”

Physiological significance of α2-adrenergic receptor subtype diversity: one receptor is not enough

“…1B) (9,26,62). When ␣ 2A -and ␣ 2C -receptors were stably expressed together with N-type Ca 2ϩ channels or with G protein-coupled inwardly rectifying K ϩ (GIRK) channels, no differences in the activation kinetics of these two receptor subtypes were detected at identical levels of receptor expression (10).…”