Central α-adrenergic systems as targets for hypotensive drugs

The neuropil surrounding sympathetic preganglionic neurons (SPNs) receives an abundant catecholaminergic innervation originating from the brain stem. The effect of catecholamines (CA) released at this spinal level on the activity of SPNs is still controversial as is the extent to which this particular CA transmission is affected by central antihypertensive drugs, such as clonidine and alpha-methyldopa. The present study was initiated, therefore, to determine the effects of iontophoretic applications of CAs and clonidine on the discharges of identified SPNs and to determine the type of receptor mediating the action of these compounds. Extracellular recordings were made with five- or six-barrel electrodes in 20 pigeons anesthetized with urethane, artificially ventilated, and immobilized. Data were obtained on 83 SPNs localized in the three first thoracic segments and identified on the basis of constancy of antidromic activation latency and collision. All of the cells sampled were inhibited by the application of low iontophoretic currents of clonidine and by a series of CAs, including alpha-methylnorepinephrine, epinephrine, and phenylephrine. For each compound, the amount of charge necessary to decrease the level of cell firing to 50% of control was calculated. Using this value as an index of drug potency, the following rank order could be determined: clonidine greater than alpha-methylnorepinephrine greater than epinephrine greater than norepinephrine greater than phenylephrine. The inhibitory effects of both clonidine and norepinephrine were antagonized by iontophoretic applications of the alpha antagonists, yohimbine, piperoxan, and phentolamine. In contrast, the beta antagonist, sotalol, and the alpha 1 antagonist, prazosin, were found ineffective when similarly applied. It is concluded that CAs and clonidine are inhibitory to the maintained activity of SPNs and that an alpha2-adrenergic receptor may be involved in the action of these compounds.

https://www.jneurosci.org/content/jneuro/1/8/908.full.pdf

Inhibition of sympathetic preganglionic neurons by catecholamines and clonidine: mediation by an alpha-adrenergic receptor

This report reviews a number of significant developments in the fields of noradrenergic transmission and adrenergic receptors which suggest that, in addition to the classical postsynaptic adrenoceptors, there are also presynaptic adrenoceptors that help modulate the release of norepinephrine (NE) from peripheral as well as central noradrenergic nerve endings during nerve stimulation. In particular, stimulation of presynaptic alpha-adrenoceptors reduces this release of transmitter and the reverse is observed after blockade of these receptors. Clearcut pharmacological differences exist between the postsynaptic alpha 1-adrenoceptors that mediate the responses of certain organs and the presynaptic alpha 2-adrenoceptors that modulate the NE release during nerve stimulation. Therefore, subclassification of alpha-adrenoceptors into alpha 1 and alpha 2 subtypes is warranted but must be considered to be independent of the anatomical location of these receptors. Some noradrenergic nerve endings have also been shown to possess beta-adrenergic receptors, the stimulation of which increases the quantity of transmitter released by nerve impulses. Physiologically, these receptors could be activated by circulating epinephrine (E) and be involved in essential hypertension. A third type of catecholamine receptor found at the noradrenergic nerve ending is the inhibitory dopamine (DA) receptor, which might be of significance in the development of new antihypertensive agents. Application of these new concepts of noradrenergic neurotransmission and the subclassification of alpha-adrenoceptors to the treatment of hypertension is presented. Clonidine, for example, appears to be a potent alpha 2-adrenoceptor agonist; the central receptor involved in its antihypertensive action is pharmacologically an alpha 2-type but located postsynaptically. Clonidine also induces activation of peripheral presynaptic alpha 2-adrenoceptors, which might contribute to its cardiovascular action. The antihypertensive effects of alpha-methyldopa are related to the formation of alpha-methylnorepinephrine, a preferential alpha 2-adrenoceptor agonist, which can stimulate peripheral presynaptic alpha 2-adrenoceptors leading to a decrease of NE release and a reduction in sympathetic tone. Prazosin is a new antihypertensive agent the mechanism of action of which involves a selective blockade of postsynaptic alpha 1-adrenoceptors. This drug does not antagonize several effects of clonidine that are mediated via alpha 2-adrenoceptors. The mechanisms presently considered to account for the antihypertensive activity of beta-adrenoceptor blocking agents are numerous. It is proposed that blockade of peripheral presynaptic facilitatory beta-adrenoceptors could be of significance in the antihypertensive action of these drugs.

Recent developments in noradrenergic neurotransmission and its relevance to the mechanism of action of certain antihypertensive agents.

Pupillary dilation as an index of central nervous system α2-adrenoceptor activation

1 In anaesthetized rats, bilateral injections of alpha-methylnoradrenaline, noradrenaline or adrenaline into the area of the nucleus tractus solitarii (NTS) of the brain stem caused dose-dependent decreases of systemic arterial blood pressure and heart rate. The effects of alpha-methylnoradrenaline were most pronounced and lasted longest. 2 The cardiovascular effects of alpha-methylnoradrenaline appeared to be restricted to the medio-caudal part of the NTS. 3 Prior administration of the alpha-adrenoceptor blocking agent, phentolamine, reversed the fall in blood pressure and heart rate induced by alpha-methylnoradrenaline into an increase. 4 Systemic administration of atropine combined with vagotomy potentiated the inhibitory effects of alpha-methylnoradrenaline on the cardiovascular system.

Centrally induced hypotension and bradycardia after administration of alpha-methylnoradrenaline into the area of the nucleus tractus solitarii of the rat.

Mean arterial blood pressure was recorded in conscious normotensive rats through indwelling arterial catheters. The effect of l-α-methyldopa (α-MD) (400 mg/kg, i.p.) was studied in animals pretreated with α-methyl-m-tyrosine (400 mg/kg i.p.) 27 and 15 h before α-MD, α-methyl-p-tyrosine methylester (H 44/68) (250 mg/kg, i.p.) 1 h before α-MD, and dl-α-hydrazino-α-methyl-β-(3,4-dihydroxyphenyl) propionic acid (MK 485, 100 mg/kg, i.p.) 30 min before α-MD. This pretreatment, which resulted in a severe depletion of endogenous catecholamines, did not alter the hypotensive effect of α-MD. The effect of α-MD (200 mg/kg, i.p.) was studied 30 min after pretreatment with the dopamine β-hydroxylase inhibitor, bis (4-methyl-l-homopiperazinyl-thiocarbonyl) disulphide (FLA-63) (25 mg/kg, i.p.). The hypotensive response to α-MD was completely abolished in these experiments. The formation of α-methylnoradrenaline from α-MD was prevented after FLA-63 but there was a significant increase in the amounts of α-methyldopamine formed.

M. Henning

Papers

Evidence that the hypotensive action of methyldopa is mediated by central actions of methylnoradrenaline.