Noradrenaline receptor-mediated potentiation of caffeine-induced Ca( 2+)-activated K(+) currents in bovine ciliary muscle cells

In vivo studies on the effects of α1-adrenoceptor antagonists on pupil diameter and urethral tone in rabbits

Alpha1-adrenoceptors mediate contraction of iris dilator smooth muscle and hence pupil dilatation. We compared the ability of i.v. bolus injections of alfuzosin, doxazosin, naftopidil, prazosin, tamsulosin and terazosin to antagonise phenylephrine-induced mydriasis relative to their potency for inhibiting phenylephrine-induced elevations of intraurethral pressure (IUP) in rabbits. Moreover, we compared the ability of these drugs to induce miosis in conscious rabbits in the absence of phenylephrine. All antagonists inhibited the effects of phenylephrine on pupil size and IUP, and the ratio of the respective ED50 values was close to unity in all cases. The doses required to induce statistically significant miosis in the absence of phenylephrine were 30- to 100-fold higher than those inhibiting phenylephrine-induced mydriasis for all antagonists, except for naftopidil. Moreover, the miotic effects of all alpha1-adrenoceptor antagonists were fully reversible within 8 h. We conclude that alfuzosin, doxazosin, naftopidil, prazosin, tamsulosin and terazosin inhibit phenylephrine-induced mydriasis in the same dose range as they inhibit elevations in IUP. Higher doses of all antagonists are required to induce miosis in the absence of an exogenous agonist, and such miosis is always reversible within hours.

Noradrenergic blockade prevents attacks in a model of episodic dysfunction caused by a channelopathy

Episodic neurological dysfunction often results from ion channel gene mutations. Despite knowledge of the mutations, the factors that precipitate attacks in channelopathies are not clear. In humans, mutations of the calcium channel gene CACNA1A are associated with attacks of neurological dysfunction in familial hemiplegic migraine and episodic ataxia type-2. In tottering mice, a mutation in the same gene causes attacks resembling paroxysmal dyskinesia. Stress, a trigger associated with human episodic disorders, reliably elicits attacks in tottering mice. Because noradrenergic neurotransmission is critical to the stress response and because noradrenergic hyperinnervation is observed in tottering mice, the role of norepinephrine in stress-induced attacks was investigated. Drugs that act at alpha-adrenergic receptors to block noradrenergic transmission prevented attacks. However, agents that facilitate noradrenergic neurotransmission failed to induce attacks. These results suggest that, while noradrenergic neurotransmission may be necessary for attacks, an increase in norepinephrine is not sufficient to induce attacks.