The acute effect of 6-hydroxy-dopamine on the blood pressure responses to eserine and tyramine in the rat

Orphan neurones and amine excess: the functional neuropathology of Parkinsonism and neuropsychiatric disease

The aetiology and treatment of Parkinsonism is currently conceptualised within a dopamine (DA) deficiency-repletion framework. Loss of striatal DA is thought to cause motor impairment of which tremor, bradykinaesia and rigidity are prominent features. Repletion of deficient DA should at least minimise parkinsonian signs and symptoms. In Section 2, based on extensive pre-clinical and clinical findings, the instability of this approach to Parkinsonism is scrutinised as the existing negative findings challenging the DA deficiency hypothesis are reviewed and reinterpreted. In Section 3 it is suggested that Parkinsonism is due to a DA excess far from the striatum in the area of the posterior lateral hypothalamus (PLH) and the substantia nigra (SN). This unique area, around the diencephalon/mesencephalon border (DCMCB), is packed with many ascending and descending fibres which undergo functional transformation during degeneration, collectively labelled 'orphan neurones'. These malformed cells remain functional resulting in pathological release of transmitter and perpetual neurotoxicity. Orphan neurone formation is commonly observed in the PLH of animals and in man exhibiting Parkinsonism. The mechanism by which orphan neurones impair motor function is analogous to that seen in the diseased human heart. From this perspective, to conceptualise orphan neurones at the DCMCB as 'Time bombs in the brain' is neither fanciful nor unrealistic [E.M. Stricker, M.J. Zigmond, Comments on effects of nigro-striatal dopamine lesions, Appetite 5 (1984) 266-267] as the DA excess phenomenon demands a different therapeutic approach for the management of Parkinsonism. In Section 4 the focus is on this novel concept of treatment strategies by concentrating on non-invasive, pharmacological and surgical modification of functional orphan neurones as they affect adjacent systems. The Orphan neurone/DA excess hypothesis permits a more comprehensive and defendable interpretation of the interrelationship between Parkinsonism and schizophrenia and other related disorders.

Transmitter interactions in the central cholinergic control of blood pressure regulation

There are at least five mechanisms by which the central nervous system regulates neural and humoral systems that control the blood pressure (BP). Particular attention has been paid to central cholinergic-adrenergic interactions in the regulation of BP. Physostigmine and other anticholinesterases which penetrate the blood-brain barrier, both carbamates and organophosphates, produce an increase of BP. This effect can be abolished by atropine, but not by methylatropine. The available evidence indicates that physostigmine and other AChE inhibitors initially produce an activation of central muscarinic receptors, which subsequently leads to an increase of the peripheral adrenergic activity. The hypertensive response to physostigmine is possible only if a functionally competent ChE is present in the brain. This effect of physostigmine is regularly associated with a dose-related increase in the neural activity in the preganglionic fibers of the cervical sympathetic nerve. BP rise after physostigmine is significantly less in immunosympathectomized animals and almost completely abolished after chemical sympathectomy. Physostigmine significantly increased the plasma concentration of catecholamines. After electrocoagulation of the locus coeruleus, not only did a significant decrease occur in the basic level of noradrenaline in plasma, but there was also a strong depression of the noradrenaline plasma response to physostigmine and immobilization. Physostigmine increased lipolysis and glycogenolysis, whereas neostigmine did not produce any change. Several directly acting cholinergic agonists alter the functions of the cardiovascular system when injected directly into the cerebral ventricular system, or directly into various brain regions. The most probable sites of action of AChE inhibitors and directly acting cholinergic agonists are the locus coeruleus, the nucleus tractus solitarii and the rostral ventrolateral medulla (RVLM). The primary activation of the cholinergic synapse is believed to take place in RVLM. Met-enkephalin, Leu-enkephalin and beta-endorphin, when applied exogenously, depress or even abolish the hypertensive effect of physostigmine. The same type of response was obtained after application of substances which inhibit the enkephalin-degrading enzymes (bestatin, phosphoramidon). Thus, the exogenous or endogenous enkephalins activate the opioid receptors in the brain and at the same time produce a depression of the cholinergic-adrenergic interaction in the central nervous system, which is a prerequisite for the hypertensive response to physostigmine. The functional role of the central cholinergic mechanisms in BP control under physiological conditions has not been established with certainty. These mechanisms might have a more significant role under pathological or homeostatic disturbances. For example, physostigmine showed a life-saving effect in acute hypovolemic shock in rabbits.

Central pressor effects induced by muscarinic receptor agonists: evidence for a predominant role of the M2 receptor subtype

The cardiovascular effects induced in the rat by several muscarinic receptor agonists were studied. All the agonists produced a clear decrease in heart rate. This decrease appeared to be peripherally mediated, because it was antagonized by methylscopolamine. The effects on blood pressure varied depending on the presence of anaesthesia, previous treatments and the type of agonists tested. When peripheral muscarinic activity was blocked by administration of methylscopolamine, a dose-dependent hypertension was obtained following the injection of oxotremorine, arecoline and aceclidine, by both intraperitoneal and intracerebroventricular routes. The muscarinic receptor agonist RS 86 produced a slight increase in blood pressure but the increase was weaker than those observed with the agonists cited above. On the other hand, the muscarinic receptor agonists pilocarpine, AF-30 and McN-A-343, considered as partially M1-selective compounds, did not produce any effect on blood pressure. Moreover, the hypertension induced by oxotremorine was completely blocked by intracerebroventricular administration of the non-subtype-selective muscarinic receptor antagonist scopolamine but was unaffected by the M1-selective antagonist pirenzepine. We propose that the central hypertensive response induced by muscarinic receptor agonists in the unanaesthetized rat is, at least partially, mediated through the stimulation of the so-called M2 muscarinic receptor subtype.

The effect of atropine or atropine methylnitrate on salt-induced hypertension in the inbred S/JR and R/JR Dahl rat

The cholinergic antagonists atropine (ATR) and atropine methylnitrate (ATRMN) were chronically administered to inbred Dahl hypertension-sensitive (S/JR) and -resistant (R/JR) rats maintained on an 8.0% NaCl-containing diet. The effects on blood pressure (BP), heart rate (HR), and mortality were then examined during and after a four week period of treatment. Administration of ATR (7.2 mg/day) or ATRMN (2.4 mg/day) attenuated the development of salt-induced hypertension (HT) in the S/JR strain but had relatively little effect on BP in the R/JR strain. HR during the treatment period was significantly greater in S/JR and R/JR rats that received ATR or ATRMN than vehicle-treated controls. Each drug also reduced HT-related mortality in S/JR rats. In general, the effects of ATR on BP and mortality were greater than those of ATRMN. However, the results suggest that the central and peripheral cholinergic systems participate in the development of salt-induced HT in the S/JR rat.

Influence of a cholinergic agent, oxotremorine, on sympathetic reflexes

Administration of cholinergic substances by various routes has been reported to produce marked cardiovascular effects. The present study examined the influence of a muscarinic agonist, oxotremorine, on aspects of sympathetic function. The compound reduced blood pressure and spontaneous sympathetic outflow in anesthetized cats after blockade of peripheral muscarinic receptors with methylatropine. Effects on potentials evoked in the splanchnic nerve by stimulation of an intercostal nerve, i.e., the somatosympathetic reflex, were also evaluated. Oxotremorine did not modify the initial or "spinal" component of the reflex but markedly reduced the amplitude and increased the latency of the secondary or "supraspinal" phase. Atropine antagonized the inhibitory response. Effects of oxotremorine differed importantly from those produced by baroreceptor and coronary chemoreceptor activation and administration of clonidine and methyldopa which also attenuated the amplitude of the supraspinal component but did not alter its latency. Consequently, oxotremorine depresses sympathetic function at loci within the brainstem and cord by mechanisms not identical with those mediating responses to these reflexes and drugs.

Reduction of adverse acute cardiovascular actions of 6-hydroxydopamine in dogs by adrenergic receptor blockade

Injection of dogs with 50 mg/kg of 6-hydroxydopamine (6-OHDA) caused large increases in blood pressure and heart rate, followed after 1 or 2 hr by severe hypotension. Prior administration of the adrenergic receptor blocking agents phentolamine and propranolol attenuated both the hyper- and hypotensive phases of 6-OHDA. Phentolamine and propranolol did not prevent depletionof norepinephrine or development of the characteristic morphological changes induced in adrenergic fibers by 6-OHDA.

Acute and chronic cardiovascular effects of 6-hydroxydopamine in dogs

Acute and chronic cardiovascular effects of repeated injections of 6-hydroxydopamine were studied in unanesthetized dogs. The acute response to initial injections of small doses of 6-hydroxydopamine consisted of an intense sympathomimetic effect characterized by a marked increase in blood pressure and a reflex bradycardia. Three days after the administration of a total dose of 50 mg/kg of 6-hydroxydopamine, significant decreases in heart rate, blood pressure, and total peripheral resistance were observed, but cardiac output remained unchanged. The absence of a response to intravenously injected tyramine, to electrical stimulation of the right stellate ganglion, and to bilateral carotid occlusion was consistent with a state of complete peripheral sympathectomy. Along with the functional impairment of transmission at the sympathetic nerve endings, marked endogenous norepinephrine depletion was observed in the heart, carotid sinus, and spleen. In contrast, endogenous norepinephrine was increased in the stellate ganglion. The response to electrical stimulation of the distal end of the right vagus was not altered by treatment with 6-hydroxydopamine. In anesthetized dogs treated with 6-hydroxydopamine, clamping of the hilar vessels of the adrenal glands for 10 minutes caused hypotension, but the same procedure in normal dogs did not affect blood pressure, demonstrating the importance of the compensatory role of the adrenal medulla in sympathectomized animals. It is concluded that 6-hydroxydopamine can be used effectively to obtain a generalized chemical sympathectomy in unanesthetized dogs and provides a useful means for studying the role of the autonomic nervous system in the regulation of cardiovascular functions.