Effects of clonidine and chlorpromazine on a sympathetic-cholinergic reflex

Autonomic activity in relation to cerebrospinal fluid neurochemistry in obsessive and disruptive children and adolescents

Electrodermal activity and heart rate were recorded during rest, simple tones, and a reaction time task in 43 male and female adolescents and children with obsessive compulsive disorder and 30 male adolescents and children with disruptive behavior disorders who had lumbar cerebrospinal fluid drawn during the same week. Partial correlations controlling for age and sex showed that in the obsessive group metabolites of serotonin and dopamine, but not of norepinephrine, were positively correlated with electrodermal responsivity, most consistently in the reaction time task. This result was not replicated in disruptive boys. Adrenocorticotropic hormone was positively related to electrodermal activity and heart rate throughout the session. The results for the obsessive adolescents suggest that nigrostriatal dopamine turnover and central serotonin turnover affect electrodermal activity, generally confirming and extending conclusions from pharmacological studies. Diagnosis may affect these relationships.

Sympathetic skin responses (SSRs) in monofocal brain lesions: topographical aspects of central sympathetic pathways

Lesions of the central sympathetic pathways are likely to be of clinical relevance. In patients with acute stroke, in particular, they may be responsible for the partially deleterious cardiac arrhythmia. There is little knowledge of the central organization of sympathetic pathways above the brainstem level for both cardio-efferent and sudomotor fibers. We studied the sympathetic skin response (SSR) in 29 patients with brainstem stroke or infarction in the territory of the middle cerebral artery (MCA) in order to evaluate the pathways mediating emotional sweating. In 24 patients (82.8%) the SSR was pathological. These abnormalities were bilateral with no clear asymmetry in 10 patients (34.5%), bilateral with marked contralateral pathology in 8 patients (27.6%), purely contralateral in 5 patients (17.2%) and purely ipsilateral in one patient (3.5%). Bilateral abnormalities were more frequent and more marked in brainstem than in MCA stroke. This is probably indicative of a more generalized sympathetic dysfunction. In contrast, contralateral abnormalities were more pronounced in MCA than in brainstem infarcts. Our results show that sympathetic skin responses are suppressed in the majority of stroke patients. Asymmetric responses may indicate that sudomotor fibers mediating emotional sweating have their origin or receive input from the contralateral MCA territory.

Functional influence of the central noradrenergic system on the skin conductance activity in rats

Pharmacological studies on neuroleptics and amphetamine strongly suggest that some dysfunction of the central catecholamine system may play a key role in the pathogenesis of schizophrenia. Our previous studies have demonstrated that intraventricular administration of 6-hydroxydopamine, a selective neurotoxin of the catecholamine neuron, can reproduce schizophrenia-like abnormalities in the skin conductance activity. In the present experiments, effects of pharmacological modulation of the central noradrenergic activity were studied in rats. Stimulation of the central noradrenergic activity by yohimbine (0.6 mg/kg, i.m.) slowed down the habituation of the skin conductance response (SCR) and increase the spontaneous fluctuation of the skin conductance (SF), while inhibition of the activity by clonidine (0.06 mg/kg, i.m.) accelerated or obliterated the SCR and decreased the SF frequency. If the functional significance of the central noradrenergic system lies in vigilance control, the present results are consistent with classical theory in psychophysiology: the habituation rate of SCR and the frequency of SF are correlated well with each other and both indices reflect arousal level. The disorder of the system should produce not only these psychophysiological abnormalities but also psychological disturbances; i.e., overarousal and underarousal syndromes. Therefore, the dysfunction of the noradrenergic system might constitute an essential aspect of schizophrenic disorder.

The effects of immobilization stress on electrodermal activity and brain catecholamine levels in rats

The effects of immobilization stress on electrodermal activity (EDA); skin conductance response magnitude and rate, skin conductance level and habituation number, and brain catecholamine levels; norepinephrine (NE) and dopamine (DA) were investigated in rats. Electrodermal activity was recorded using constant current method. Brain catecholamine levels were determined by a spectrophotophlorometric method. Electrodermal activity parameters (except skin conductance level) increased during immobilization. It was observed that, during immobilization stress, the alteration of norepinephrine and dopamine levels in rat brain was related to cerebral region and the duration of immobilization stress. It was concluded that these electrodermal activity alterations can be attributed to the changes in central norepinephrine metabolism induced by immobilization.

Skin conductance response after 6-hydroxydopamine lesion of central noradrenaline system in cats

Absence of skin conductance response (SCR) is a psychophysiological sign frequently observed among schizophrenic patients. This alteration of electrodermal activity can be reproduced in cats and rats by intraventricular administration of 6-hydroxydopamine (6-OHDA), a neurotoxin selectively destroying catecholamine neurons. The finding appears to be quite consistent with the catecholamine hypothesis of schizophrenia. To determine which catecholamine system--the dopamine (DA) or the noradrenaline (NA)--is responsible for the induction of this abnormality, the same dose of 6-OHDA was microinjected to either the ventral tegmental area (VTA, n = 3), through which most DA fibers ascend, or to the ascending noradrenergic bundle (ANB, n = 3), through which most NA fibers ascend. Four cats remained intact as a control for later brain catecholamine estimation. The skin conductance of all pretreatment intact cats (n = 10) showed not only spontaneous fluctuations but also SCRs to auditory stimuli (5 kHz, 100 dB, 1 sec) which habituated with repetition. In contrast, ANB-lesioned cats showed a complete abolition of auditory SCRs, few spontaneous fluctuations, and a low basal skin conductance level. These abnormalities were statistically significant. On the other hand, no change was found in the skin conductance activity of the VTA-lesioned cats. Catecholamine estimation after the experiment confirmed selective destruction of the appropriate system corresponding to each type of lesion. These findings are discussed in the context of the DA or NA theory of schizophrenia, and involvement of the NA system in the pathogenesis of this illness is suggested.

Clonidine inhibits electrodermal responses by an action on the spinal cord

Experiments were designed to determine the neural site of action for clonidine inhibition of sympathetic-cholinergic electrodermal responses (EDR) in anesthetized cats. Administration of clonidine (0.3-3.0 micrograms i.a.) directly to the stellate ganglion did not significantly decrease the amplitude of responses evoked by submaximal hypothalamic stimulation but did inhibit hypothalamic-evoked EDR when administered intrathecally at the C6 to T2 spinal levels. Administration of clonidine to the ganglion, however, did depress EDR evoked by the ganglionic stimulant, 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP, 10 micrograms i.a.). Intravenous clonidine (1-30 micrograms) also reduced EDR amplitude evoked by single pulse stimulation of both the pre- and postganglionic sympathetic nerves with responses elicited from both sites depressed to an equal extent. Yohimbine (0.5 mg/kg i.v.) uniformly antagonized clonidine's depression of EDR regardless of the site or mode of activation. These results indicate that clonidine depresses centrally evoked sudomotor responses by activation of alpha 2-adrenoceptors in the spinal cord and to a limited extent by direct action at the neuroeffector junction. Although a possible DMPP-clonidine interaction appears to take place at the level of the sympathetic ganglion, it is unlikely that ganglionic blockade contributes significantly to clonidine inhibition of EDR evoked by electrical activation of the nervous system.

Enhancement of central transmission to sympathetic preganglionic neurons by phosphodiesterase inhibitors and its prevention by clonidine

The effects of 3 phosphodiesterase inhibitors, aminophylline, isobutylmethylxanthine (IBMX), and RO 20-1724, were tested on descending intraspinal and spinal reflex transmission to sympathetic preganglionic neurons in unanesthetized spinal cats. Sympathetic discharges, recorded from upper thoracic preganglionic white rami, were evoked by stimulation (0.1 Hz) of descending excitatory pathways in the cervical dorsolateral funiculus (intraspinal) or of adjacent intercostal nerves (spinal reflex). Each phosphodiesterase rapidly and markedly enhanced transmission through intraspinal pathways but only slowly and modestly enhanced transmission through spinal reflex pathways. Pretreatment with a methyltyrosine-reserpine combination, chlorpromazine, or prazosin markedly restricted the enhancement of intraspinal transmission by IBMX to levels typically produced on spinal reflex pathways. Clonidine markedly depressed transmission through both pathways and prevented enhancement by the phosphodiesterase inhibitors. Yohimbine or tolazoline antagonized the depressant effects of clonidine and restored the ability of the phosphodiesterase inhibitors to enhance transmission. Somatic spinal reflexes were not affected by the phosphodiesterase inhibitors. The results suggest that descending norepinephrine pathways to sympathetic preganglionic neurons activate adenylate cyclase to generate cyclic AMP which increases neuronal excitability, possibly by phosphorylating membrane proteins. Clonidine appears to depress neuronal excitability by inhibiting adenylate cyclase through activation of alpha 2-adrenergic receptors.

Pupillary dilation as an index of central nervous system alpha 2-adrenoceptor activation

In recent years there has been increasing evidence that some antihypertensive drugs like clonidine and alpha-methyldopa (after conversion in the brain to alpha-methylnorepinephrine) may decrease sympathetic tone by stimulating central nervous system (CNS) alpha 2-adrenoceptors. These same drugs also produce pupillary dilation in cats and rats. In this review, evidence is presented supporting the hypothesis that clonidinelike drugs act either directly or indirectly on CNS postsynaptic alpha 2-adrenoceptors to cause pupillary dilation by reduction of parasympathetic neural tone to the iris. It is further suggested that the underlying physiologic mechanism for this mydriatic action is activation of an ascending pathway that provides tonic inhibitory input by releasing norepinephrine on neurons in the Edinger-Westphal complex. Yohimbine-sensitive pupillary dilation in these species may provide a simple and effective model for quantitatively accessing CNS alpha 2-adrenoceptor activity.

Permissive role of spinal alpha 1-adrenoceptors in sudomotor efferents

The electrodermal potential (EDP) recorded in the forepaws of anaesthetized cats in response to stimulation of the cholinergic-sympathetic nervous system at different levels was taken as a measure for sudomotor activity. Electrical stimulation of the hypothalamus with square wave pulses (1 ms duration, 0.5-64 Hz, 2 s train length) at intervals of 1 min induced rate-dependent EDPs which were inhibited at all rates of stimulation by intravenous (i.v.) injection of doses less than 40 micrograms/kg of the alpha 1-adrenoceptor blocking drug prazosin. However, prazosin (50-500 micrograms/kg i.v.) did not impair EDPs induced by preganglionic stimulation (0.5-64 Hz) or by injection of the nicotinic ganglion stimulant DMPP (50 micrograms/kg i.v.). Prazosin (50 micrograms/kg i.v.) inhibited EDPs induced by unilateral electrical stimulation (square wave pulses, 1 ms duration, 16 Hz, 2 s train length, 1 min intervals) of the spinal cord at C 1 in cats with an axotomy at the level of the medulla oblongata, thus indicating a spinal site of prazosin action and suggesting a permissive role of spinal catecholamines by activation of alpha 1-adrenoceptors. In spinal preparations pretreated with 250 micrograms/kg yohimbine i.v. to block inhibition by alpha 2-adrenoceptors of catecholamine reuptake, cocaine 2.5 mg/kg i.v. potentiated EDPs induced by spinal stimulation with 8 Hz. This effect could be antagonized by 50 micrograms/kg prazosin or 1000 micrograms/kg corynanthine i.v. In spinal preparations pretreatment with 5 mg/kg reserpine i.p. for depletion of catecholamines and 250 micrograms/kg yohimbine i.v., EDPs (16 Hz) were smaller than in undepleted preparations. Under these conditions injection of 100 micrograms/kg clonidine i.v. caused amplification of EDPs. This effect was antagonized by 50 micrograms/kg prazosin i.v. After i.v. pretreatment with 250 micrograms/kg yohimbine the i.v. injection of 2.5 mg/kg cocaine also potentiated EDPs which were induced by hypothalamic stimulation in intact cats. The results indicate that catecholaminergic neurons influence sudomotor activity by interaction with efferents of the cholinergic-sympathetic nervous system at the level of the spinal cord. Catecholamines seem to facilitate impulse transmission in non-catecholaminergic synapses by activation of alpha 1-adrenoceptors.

Alpha-2-adrenergic receptors in avian spinal cord: increases in apparent density associated with the sympathetic preganglionic cell column

Vertebrate spinal cord receives a dense and diversified catecholaminergic innervation from brainstem and diencephalon. Within the spinal gray, the densest terminations appear to be within the neuropil surrounding sympathetic preganglionic neurons (SPNs) in thoracic spinal cord. Results of recent iontophoresis investigations showed that several catecholamines and clonidine, an alpha-2 agonist, uniformly inhibited the maintained discharge activity of SPNs [19]. These experiments raised the possibility that the inhibitory effects might be mediated by activation of an alpha-2-adrenergic receptor. The present series of ligand binding studies provide biochemical evidence suggesting the presence of alpha-2-adrenergic receptors in the SPN cell column. Total specific binding (Bmax) of the radiolabeled agonists clonidine (CLO) and para-amino-clonidine (PAC) (at concentrations above and below apparent KDS) was significantly greater in thoracic spinal cord in comparison with cervical spinal cord (P less than 0.001). The elevated levels in thoracic spinal cord were entirely accounted for by increases in apparent receptor density in dorsal horn and the SPN cell column (inclusive of the adjoining intermediate spinal laminae) (P less than 0.005). Adrenergic receptor subtype specificity of [3H]PAC was tested in competitive inhibition experiments. The results confirmed that [3H]PAC is a preferential alpha-2 agonist in thoracic and cervical spinal cord, and indicated the following rank order of potency for its displacement: norepinephrine = yohimbine much greater than prazosin greater than propranolol.

Clonidine inhibits electrodermal potentials induced by preganglionic stimulation

The electrodermal potential (EDP) recorded with surface electrodes between the palm and the shaven back of the right forepaw of anaesthetized and vagotomized cats was taken as a measure of the activity of cholinergic-sympathetic sudomotor nerves. EDPs were induced by preganglionic electrical stimulation of the stellate ganglion for 2 s with trains of DC pulses (2 ms duration, 0.5-128 Hz) at regular intervals of 60 s. The EDPs amounted to 12 mV and increased little with stimulation rate (14 mV). The i.v. injection of 30 micrograms/kg of the alpha 2-adrenergic agonist clonidine did not change the EDPs significantly. A consistent result was obtained in cats pretreated i.p. with 5 mg/kg reserpine 18 h beforehand for depletion of catecholamines. Three hours after the i.v. injection of 3 mg/kg guanethidine, clonidine (30 micrograms/kg i.v.) induced significant reduction of EDPs in the lower range of the stimulation rate but did not affect those at 16 and 32 Hz. Partial blockade of ganglionic nicotinic receptors by i.v. infusion of 0.08-0.3 mg/kg per min hexamethonium diminished EDPs (1 Hz) by 30-50%. Under these conditions the i.v. injection (30 micrograms/kg) or topical application of 1 microgram clonidine to the right stellate ganglion inhibited EDPs at all rates of stimulation. The inhibitory effects of clonidine could be antagonized by 200 micrograms/kg yohimbine i.v. Partial ganglionic blockade by i.v. infusion of the depolarizing blocker suxamethonium (0.2-0.4 mg/kg per min) decreased EDPs. However, the topical application of 1 microgram clonidine to the stellate ganglion during infusion of suxamethonium caused no further decrease.(ABSTRACT TRUNCATED AT 250 WORDS)

Clonidine inhibits nicotinic effects in ganglia of the cholinergic-sympathetic system

The electrodermal potential (EDP) recorded with surface electrodes between the palm and the shaven back of the forepaws of anaesthetized cats was taken as a measure of sudomotor activity in response to submaximal activation of cholinergic-sympathetic efferents with the nicotinic ganglionic stimulant DMPP. The intravenous injection of 30 micrograms/kg clonidine inhibited EDPs in despinalized cats and in vagotomized cats with chronic denervation of the stellate ganglion. In vagotomized cats clonidine (30 micrograms/kg i.v.) shifted the dose-response curve of DMPP (4-256 micrograms/kg i.v.) to the right. An inhibitory effect was also observed in despinalized cats upon topical application of 0.1 and 1 microgram clonidine to the stellate ganglion. In all preparations the inhibitory effect of clonidine could be antagonized by intravenous injection of 200 micrograms/kg yohimbine. As all substances used were also active when injected during arrest of blood flow to the foreleg by means of a tourniquet, actions at the level of the sweat gland could be excluded. The results lead to the conclusion that the activation of postsynaptic somadendritic alpha 2-adrenoceptors in sudomotor ganglia of the cholinergic-sympathetic nervous system inhibits the effects of nicotinic ganglionic stimulation. It is assumed that this action is due to the known ganglionic hyperpolarization induced by alpha 2-adrenoceptor stimulants.

CNS adrenergic inhibition of parasympathetic oculomotor tone

Inhibition of parasympathetic neural tone to the iris was produced by electrical stimulation of the afferent sciatic nerve, medullary reticular formation, and posterior hypothalamus in anesthetized cats in which only the parasympathetic nerves to the eye were intact. Stimulation of all 3 sites of activation produced a graded pupillary dilation and reduction of tonic nerve activity in the short ciliary nerves. Intravenous administration of the alpha-2-adrenoceptor antagonist, yohimbine hydrochloride, (0.03-1.0 mg/kg) produced a dose-dependent antagonism of the mydriasis elicited by activation of the ascending (sciatic nerve and medullary) mechanisms but did not block the pupillary dilation evoked by stimulation of the system descending from the hypothalamus. This differential action of yohimbine was confirmed directly by means of nerve recordings taken from the parasympathetic nerve to the eye. Depletion of CNS monoamines with reserpine and alpha-m-p-tyrosine reduced the norepinephrine concentration of the medulla and midbrain by 95% and 97%, respectively. In these depleted preparations, stimulation of the hypothalamus still produced the characteristic mydriasis and inhibition of parasympathetic tonic activity whereas activation of ascending mechanisms (sciatic or medullary) were no longer effective in producing these effects. Taken together, these results suggest that ascending parasympatho -inhibition is mediated by a monoamine (probably norepinephrine) and that inhibition descending from the hypothalamus is mediated by a non-monoaminergic mechanism.

Evidence for two distinct sympathoinhibitory bulbo-spinal systems

Yohimbine hydrochloride (0.5 mg/kg, i.v.) caused a long-lasting potentiation of electrodermal (sympathetic-cholinergic) reflexes in intact anaesthetized and decerebrate unanaesthetized cats. Transection of the cervical spinal cord also resulted in an increased amplitude of the sudomotor reflex in unanaesthetized decerebrate preparations. Depletion of monoamines in the CNS by pretreatment with reserpine (5 mg/kg, i.p.) and alpha-methyl-p-tyrosine (2 X 300 mg/kg, i.p.) reduced the concentrations of norepinephrine, dopamine and serotonin to less than 93% of control levels in the thoracic spinal cord. In monoamine-depleted preparations, yohimbine no longer facilitated the reflex amplitude whereas the effect of spinal transection was not altered. These results suggest that there are two distinct sympathoinhibitory systems in the lower brain stem that converge on spinal sympathetic neurons, one of which is monoaminergic and one of which is not. Evidence for the baroreceptor-independent nature of these descending inhibitory systems is discussed.

Analysis of CNS sympatho-inhibition produced by guanabenz

The effects of guanabenz on several autonomic systems were observed in vagotomized, anesthetized cats with the aim of determining, in a quantitative sense, the degree to which guanabenz produces a clonidine-like central nervous system action. Guanabenz given as a single dose (50 micrograms/kg i.v.) produced a transient hypertension associated with a more sustained bradycardia and depression of centrally (hypothalamic) evoked electrodermal responses (EDR). Increasing cumulative doses of guanabenz (3-1000 micrograms/kg i.v.) also resulted in a dose-related depression of EDR amplitude, transient hypertension followed by hypotension, sustained bradycardia, and mydriasis. All responses were antagonized by pretreatment with yohimbine (0.5 mg/kg i.v.). The ED50 for depression of the centrally evoked EDR was in the range of 50-100 micrograms/kg i.v. in the non-pretreated preparations. Guanabenz (100 micrograms/kg i.v.) was shown to be devoid of significant ganglionic blocking properties. These experiments suggest that guanabenz acts like clonidine in the CNS and that an alpha 2-adrenergic inhibitory mechanism is involved in its myriad of central autonomic effects.

Bronchopulmonary effects of clonidine on the bronchomotor responses of the guinea-pig

In conscious guinea pigs, clonidine (10 and 100 micrograms/kg i.v.) lowered diastolic (-7.9 +/- 3.5 and -12.4 +/- 5.2%) and systolic (-8.6 +/- 3.0 and -11.9 +/- 4.2%) arterial pressure and reduced heart rate (-14.5 +/- 3.9 and -27.7 +/- 3.8%), but did not significantly modify pulmonary airway resistance. Hypotension was suppressed by yohimbine and bradycardia was partially suppressed by atropine and yohimbine, which demonstrates in this animal an alpha 2-adrenergic effect for hypotension and a mixed cholinergic and alpha 2-adrenergic effect for bradycardia. Clonidine (10 and 100 micrograms/kg i.v.) enhanced the bronchoconstrictor effects of histamine 20 micrograms/kg (+80.0 +/- 22.5 and 89.1 +/- 26.5%), acetylcholine 25 micrograms/kg (+66.4 +/- 19.8 and +95.4 +/- 25.4%) and serotonin 15 micrograms/kg (+68.5 +/- 23.2 and +81.4 +/- 34.1%). The duration of this effect was comparable to that of the hypotensive and cardiac effects of clonidine. The effects of clonidine were suppressed after pretreatment with propranolol, reserpine or pentobarbitone, all drugs which enhance the bronchoconstrictor effect of ACh. Yohimbine (1 mg/kg), piperoxan (0.3 mg/kg) or prazosin in high dosage (0.3 mg/kg) inhibited the potentiation by clonidine of ACh-induced bronchoconstriction, whereas prazosin in lower doses (0.03 mg/kg) or AR-C 239 (0.05 mg/kg) had no action. A specific involvement of alpha 2-adrenoceptors stimulated by clonidine with subsequent reduction of the adrenergic activity associated with bronchospasm could therefore be demonstrated in the conscious guinea-pig during bronchomotor reactions.

Use of a sympathetic-cholinergic system in the analysis of sympatho-inhibition produced by clonidine and some congeneric derivatives of clonidine

The effects of clonidine and five analogs of clonidine were tested with regard to their ability to depress centrally and peripherally evoked electrodermal responses (EDR) in control cats, as well as in animals pretreated with yohimbine hydrochloride. With the exception of St-91, all of the clonidine-like substances selectively reduced the amplitude of centrally (hypothalamic) evoked responses in a dose-dependent fashion. Clonidine was found to have no significant inhibitory effect at the level of the sympathetic ganglion. The order of central nervous system sympatho-inhibitory potency of these compounds was clonidine (St-155) greater than St-375 greater than St-606 greater than St-600 greater than St-608 much much greater than St-91. Prior treatment with yohimbine hydrochloride (0.5 mg/kg i.v.) antagonized the depressant effect of all of these drugs. These results indicate that clonidine and the clonidine congeners tested (with the exception of St-91) all produce sympatho-inhibition by an action on a CNS alpha-adrenergic mechanism and demonstrate the usefulness of this electrodermal model system for the analysis of drugs affecting central sympathetic reactivity.

Spinal sympathetic neurons: possible sites of opiate-withdrawal suppression by clonidine

Morphine, methadone, meperidine, fentanyl, and clonidine rapidly depressed transmission through sympathetic preganglionic neurons in cats with the spinal cord transected. Naloxone promptly antagonized this effect of the opiates but not that of clonidine which was reversed by alpha 2-adrenergic receptor antagonists. The independent depression of preganglionic neurons by clonidine may contribute to the ability of this drug to depress the symptoms of opiate withdrawal that are characterized by sympathetic hyperactivity.

Differential sensitivity of four central sympathetic pathways to depression by clonidine

The dose-response effects of clonidine HCl (2.5-200 micrograms/kg i.v.) on transmission through somatospinal reflex, viscerospinal reflex, intraspinal, and spinal-bulbospinal reflex pathways were determined in spinal or chloralose-anesthetized cats to assess principle sites of drug action. Evoked sympathetic discharges were recorded from upper thoracic preganglionic rami. Clonidine rapidly produced parallel, dose-dependent depression of transmission through each pathway which was rapidly antagonized by tolazoline or yohimbine. The two spinal reflex pathways were least sensitive to depression which was identical and was limited to 60%. In contrast, both descending pathways could be depressed completely. Although the spinal-bulbospinal reflex pathway was more sensitive to depression than its efferent, descending intraspinal pathway alone, analysis of the relative depression of transmission at spinal and at brainstem levels indicates that the spinal site is more sensitive to clonidine that it is generally considered to be.

Functional roles of monoaminergic pathways to sympathetic preganglionic neurons

Despite considerable progress in mapping the central monoaminergic pathways to sympathetic preganglionic neurons in the spinal cord, the respective functional roles of these pathways have not been resolved. Evidence for both excitation and inhibition has been advanced for each of the three monoamines, serotonin, norepinephrine, and epinephrine. Our previous studies on spinal sympathetic pathways to sympathetic preganglionic neurons support the prevailing opinion that serotonin pathways are inhibitory but did not satisfactorily resolve the functional role of the norepinephrine pathways. However, more recent studies showing that intraspinally evoked sympathetic discharges were rapidly and markedly enhanced by phosphodiesterase inhibitors and that this effect was prevented by clonidine have led to formulation of a coherent hypothesis which accommodates much of the conflicting evidence regarding norepinephrine. In addition, evidence for the role of the epinephrine pathways has been obtained by using a selective inhibitor of epinephrine synthesis. The results of these recent studies complement our previous results and suggest that the excitability of sympathetic preganglionic neurons is regulated by excitatory norepinephrine pathways and inhibitory epinephrine pathways that activate or suppress adenylate cyclase to control intraneuronal levels of cyclic AMP.

A quantitative assessment of CNS sympatho-inhibition produced by psychotropic drugs

As one index of sympathetic reactivity, electrodermal responses (EDR) were evoked from central (hypothalamic) and peripheral (ulnar nerve) sites in pentobarbital-anesthetized cats. When compared with intravenous chlorpromazine (ED50 approximately 1.0 mg/kg), only thioridazine, trifluoperazine, and pimozide were less potent than chlorpromazine in reducing the amplitude of these centrally-evoked sympathetic-cholinergic responses. Perphenazine and methotrimeprazine (a non-neuroleptic phenothiazine) were about twice as potent as chlorpromazine. Haloperidol and triflupromazine were about 5 times as potent and chlorprothixine was more than 10 times as potent. None of these agents reduced the peripherally-evoked electrodermal response, indicating a CNS mode of action. Diazepam was without effect at either site. In addition, pretreatment with yohimbine (0.5 mg/kg. i.v.) did not significantly alter the ED50 for any of the above drugs. These results demonstrate that all of the phenothiazines and non-phenothiazine neuroleptics tested produce a dose-dependent central sympatho-inhibition and that diazepam does not. The results also suggest that there is no significant correlation between central sympatho-inhibition and the antipsychotic potency of these compounds and that their depression of central sympathetic outflow is independent of alpha-adrenergic mechanisms in the CNS.

Clonidine attenuation of a cardiogenic hypertensive chemoreflex

Clonidine is an antihypertensive agent with a primary action mediated by alpha adrenergic stimulation in the central nervous system, thus inhibiting sympathetic efferent activity. Serotonin activates a cardiogenic hypertensive chemoreflex which induces discharges of sympathetic efferent neurons. The purpose of this study was to determine the effects of intravenous clonidine upon the thoracic sympathetic efferent discharges in chloralose-anesthetized dogs, their peripheral autonomic receptors being blocked with atropine, propranolol, and phentolamine. Efferent nerve traffic was quantified using a Schmitt trigger and Digital PDP8/e computer. Control spontaneous activity (tone) following autonomic blockade was normalized at 100%. Serotonin (100 microgram/ml, 2 ml, left atrium) caused an increase in the reflex efferent sympathetic activity to 192% +/- 16% of control (p less than 0.001). Following clonidine, the tone was decreased to 63% +/- 6% of control, and the reflex sympathetic discharge elicited by serotonin was significantly (p less than 0.001) reduced from 192% to 116% +/- 9% of control tone (before clonidine). The attenuation of the reflexly elicited discharge was significantly (p less than 0.05) greater than the attenuation of the tone. In four dogs that did not receive atropine, the vago-vagal reflex sinus bradycardia induced by serotonin was not affected by clonidine.

Contrasting effects of clonidine and 5-hydroxytryptophan on spinal sympathetic pathways

The effects of clonidine HCI were compared with those of 5-HTP on transmission through two spinal sympathetic pathways, segmental spinal reflex pathways and descending intraspinal excitatory pathways, in unanesthetized spinal cats. Evoked sympathetic discharges were recorded from upper thoracic preganglionic rami. Clonidine (5-50 microgram/kg) produced a parallel, dose-dependent depression of transmission through each pathway. The intraspinal pathway was five time more sensitive than the spinal reflex pathway (ED50's, 6 and 30 microgram/kg), and the spinal reflex pathway could not be depressed by more than 60% even by higher doses. In contrast, 5-HTP was more effective in depressing the spinal reflex than the intraspinal pathway (ED50's 32 and 44 mg/kg), and both pathways could be depressed completely. Small doses of tolazoline or yohimbine rapidly antagonized the effects of clonidine but not 5-HTP. Clonidine and 5-HTP appear to depress the excitability of sympathetic preganglionic neurons by activating alpha2- and 5-HT receptors, respectively. Each mechanism may contribute independently to regulation of the sympathetic outflow.