Opposing central autonomic actions of alpha 1- and alpha 2-adrenoceptor antagonists on oculomotor tone

Control of organelle transport in melanophores: regulation of Ca2+ and cAMP levels

Melanophores of the cichlid Tilapia mossambica can be induced to aggregate pigment by addition of epinephrine to the medium, suggesting adrenergic control of this transport. The melanophore response to adrenergic stimulation was examined using agonists and antagonists that are highly specific for each alpha-adrenoceptor subclass. The signal transduction mechanism of each subclass is unique: stimulation of alpha 1 receptors results in a rise in intracellular free Ca2+, while alpha 2 stimulation results in decreased cAMP levels [Exton, 1985: Am. J. Physiol. 248:E633-E647]. Each alpha 1 or alpha 2 specific agonist tested showed a dose dependent ability to induce aggregation and each was able to effect complete aggregation of pigment, suggesting that aggregation can be mediated either by elevating Ca2+ or by lowering cAMP. However, in the presence of either an alpha 1 or an alpha 2 receptor antagonist, none of the agonists were able to induce significant aggregation, suggesting that changes in levels of both messengers are required for pigment aggregation in the melanophores. Moreover, experiments in which intracellular levels of Ca2+ or cAMP were perturbed, using BAPTA and forskolin, respectively, indicated that elevating Ca2+ in the presence of high cAMP is not sufficient to induce aggregation and, conversely, that lowering cAMP levels in the presence of reduced Ca2+ is not sufficient to induce pigment aggregation. These data indicate that the concentrations of both cAMP and Ca2+ are important in regulating pigment aggregation in teleost melanophores, and suggest that maximal aggregation of pigment requires altering the levels of both messengers.

Studies on the mechanism of prazosin induced sympatho-inhibition

Intravenous administration of the alpha 1-adrenoceptor antagonist, prazosin (3-300 micrograms/kg), produced a depression of sympathetic-cholinergic electrodermal responses evoked by electrical stimulation of the posterior hypothalamus in pentobarbital anesthetized cats. Pretreatment with the alpha 2-adrenoceptor antagonists yohimbine (0.5 mg/kg) or idazoxan (0.1 mg/kg) significantly blocked the depressant effects of prazosin but had no effect on hypothalamic evoked electrodermal responses when given alone. Electrodermal responses were readily elicited in animals depleted of CNS monoamines. Monoamine depletion, however, totally abolished prazosin's depression of centrally evoked electrodermal responses. Prazosin also depressed the amplitude of electrodermal responses evoked by electrical stimulation of the cervical spinal cord in spinalized cats. In contrast to hypothalamic stimulation, yohimbine when given alone potentiated spinally evoked electrodermal responses which suggests that both excitatory and inhibitory mechanisms were being activated. Taken together these results suggest that prazosin produces its CNS sympatholytic effect by enhancing inhibition mediated by alpha 2-adrenoceptor mechanisms and not directly by blockade of excitatory alpha 1-adrenergic receptors in the central nervous system. A spinal cord site of action for prazosin is also implicated.

Alpha 1- and alpha 2-adrenoreceptor antagonists produce opposing mydriatic effects by a central action

1. In anaesthetized rats, intravenous administration of alpha 2-adrenoreceptor antagonists yohimbine (0.3-3.0 mg kg-1), idazoxan (0.03-0.3 mg kg-1) and raulwolscine (0.3-3.0 mg kg-1) produced a dose-related inhibition of sciatic nerve-(ScN) mediated reflex pupillary dilation (parasympatho-inhibition). The rank order of potency was idazoxan greater than yohimbine greater than rauwolscine. 2. Under similar experimental conditions, intravenous administration of alpha 1-adrenoreceptor antagonists prazosin (0.03-1.0 mg kg-1), phenoxybenzamine (0.3-3.0 mg kg-1) and corynanthine (0.03-1.0 mg kg-1) produced a dose-dependent potentiation of the reflex mydriasis with the potency order being prazosin greater than corynanthine greater than phenoxybenzamine. Intravenous yohimbine (1.5 mg kg-1) reversed the potentiation caused by the alpha 1-adrenoreceptor antagonists and blocked the reflex mydriasis. 3. Parasympatho-inhibition and mydriasis elicited by hypothalamic stimulation was not affected by the alpha 2-adrenoreceptor antagonists yohimbine (0.3-3.0 mg kg-1), idazoxan (0.03-0.3 mg kg-1) or the alpha 1-adrenoreceptor antagonist prazosin (0.3-1.0 mg kg-1). 4. Microinjection of prazosin (3-30 ng) into the oculomotor nuclear complex (IIIn), produced a dose-related potentiation, whereas microinjection of yohimbine (0.3-3.0 micrograms) produced a dose-related blockade of reflex mydriasis. 5. The above findings support the hypothesis that ascending mechanisms (e.g. afferent ScN) produce inhibition of parasympathetic oculomotor tone to the iris by activation of central postsynaptic alpha 2-adrenoreceptors. Furthermore, these studies demonstrate that alpha 2-adrenoreceptor antagonists block and alpha 1-adrenoreceptor antagonists potentiate the reflex mydriasis. These actions appear to be localized within the pupilloconstrictor regions of the brain (oculomotor nuclear complex).