The involvement of gigantocellular reticular nucleus in clonidine-promoted hypotension and bradycardia in experimentally-induced hypertensive cats

Further Characterization of Nociception-Related and Arterial Pressure-Related Neuronal Responses in the Nucleus Reticularis Gigantocellularis of the Rat

The present study was undertaken to further characterize the nucleus reticularis gigantocellularis (NRGC) of the medulla oblongata in the central processing of nociceptive and cardiovascular signals, and its modulation by met-enkephalin. In Sprague-Dawley rats anesthetized with pentobarbital sodium, we found that all 125 spontaneously active NRGC neurons that responded to noxious stimuli (tail clamp) also exhibited arterial pressure-relatedness. Forty neurons additionally manifested cardiac periodicity that persisted even during nociceptive responses. While maintaining their cardiovascular responsive characteristics, the nociception-related NRGC neuronal activity was blocked, naloxone-reversibly (0.5 mg/kg, i.v.), by morphine (5 mg/kg, i.v.). Microiontophoretically applied met-enkephalin suppressed the responsiveness of NRGC neurons to individually delivered tail clamp or transient hypertension induced by phenylephrine (5 &mgr;g/kg, i.v.). Interestingly, in NRGC neurons that manifested both nociception and arterial pressure relatedness, the preferential reduction in the response to noxious stimuli upon simultaneous elevation in systemic arterial pressure was reversed to one that favored nociception in the presence of met-enkephalin. All actions of met-enkephalin were discernibly blocked by the opioid receptor antagonist, naloxone. Our results suggest that individual NRGC neurons may participate in the processing of both nociceptive and cardiovascular information, or in the coordination of the necessary circulatory supports during nociception. In addition, neuropeptides such as met-enkephalin may exert differential modulation on neuronal responsiveness according to the prevailing physiologic status of the animal. They also showed that NRGC may be a central integrator for pain and cardiovascular-related functions. Copyright 1996 S. Karger AG, Basel

Passive electrical properties of spontaneously active neurons in the nucleus reticularis gigantocellularis of the cat

We evaluated in chloralose-urethane anesthetized cats the passive electrical properties of 25 spontaneously active neurons in the nucleus reticularis gigantocellularis (NRGC) of the medulla oblongata. Compared to other mammalian brain regions, these reticular cells in general possessed higher input resistance, shorter membrane time constant and first equalizing time constant, and longer somatodendritic electrotonic length factor. It is discussed that, by providing a synaptic machinery for quick and sensitive response to afferent inputs and an efficacious interplay between temporal and spatial summation, these electrical membrane properties may account for the spontaneous and irregular discharge pattern characteristic of the NRGC neurons.

Characterization of alpha-adrenoceptors in the nucleus reticularis gigantocellularis involved in the cardiovascular depressant effects of guanabenz in the rat

The participation of alpha-adrenoceptors in the nucleus reticularis gigantocellularis in the hypotensive, negative inotropic and chronotropic effects induced by guanabenz, was examined in rats anesthetized with pentobarbital sodium (40 mg/kg, i.p.). Pretreatment with alpha-adrenoceptor antagonists yohimbine (10 micrograms), phentolamine (2.5 micrograms) and phenoxybenzamine (20 micrograms), which were injected bilaterally into the nucleus reticularis gigantocellularis, significantly antagonized the cardiovascular suppressant effects normally produced by systemic administration of guanabenz (10 micrograms/kg, i.v.). Pretreatment with prazosin (0.25 microgram) did not affect the vasodepressive, but significantly attenuated the bradycardic actions of guanabenz. The general trend of "antagonization potency" shown by the alpha-adrenergic blockers, against the cardiovascular effects of guanabenz, was in the order: yohimbine greater than phentolamine greater than phenoxybenzamine greater than prazosin. It is concluded that while the alpha 2-adrenoceptors in the nucleus reticularis gigantocellularis are more critically involved in the antihypertensive actions of guanabenz, the possibility exists that alpha 1-adrenoceptors may also participate, in part.

Correlated effects of guanabenz on single-neuron activity in the nucleus reticularis gigantocellularis, systemic arterial pressure and heart rate in the rat

The simultaneous effects of guanabenz on the activity of single-neurons in the nucleus reticularis gigantocellularis of the medulla oblongata, systemic arterial pressure and heart rate were assessed in Sprague-Dawley rats, anesthetized with pentobarbital sodium (40 mg/kg, i.p.). Of the 35 arterial pressure-related neurons (neurons that temporally altered their spike rhythm subsequent to fluctuations in arterial pressure) which were evaluated in this reticular nucleus, 32 changed their discharge frequencies that exhibited a degree and time-course parallel to the hypotension promoted by the amino-guanidine compound (5, 10 or 20 micrograms/kg, i.v.). More importantly, these alterations neuronal activity preceded the occurrence of the induced vasodepression, signifying a causative relationship between the two events. Four of the 5 non-arterial pressure-related neurons in the gigantocellular reticular nucleus, on the other hand, manifested no basic modification in their spike frequencies in relation to the hypotension induced by guanabenz. These observations provided further support for the idea that the nucleus reticularis gigantocellularis participates actively in the cardiovascular suppressive actions of the aminoguanidine compound.

Interactions between guanabenz and clonidine in their antinociceptive effects in the rat

Based on the hot-plate algesiometric assay in conscious Sprague-Dawley rats, we observed that both guanabenz and clonidine, at a subcutaneous dose of 5 and 1 mg/kg, respectively, exhibited significant antinociceptive potencies. Paradoxical actions on the hot-plate responses, however, were exerted by guanabenz at lower doses, varying from hyperalgesia (1 mg/kg) to no effect (2 mg/kg). A pretreatment of clonidine (1 mg/kg) substantially potentiated the antinociceptive efficacy of the two latter doses, but failed to further augment the pain-suppressive action of guanabenz at 5 mg/kg. These interactive effects of the two alpha-adrenoceptor agonists suggest that they may share a common mechanism(s) in their analgesic actions.

Anatomic and physiologic evaluation of a link between the nucleus reticularis gigantocellularis and nucleus ambiguous in the rat

Localized microinjection of wheat germ agglutinin-conjugated horseradish peroxidase into the nucleus ambiguous (NA) of Sprague-Dawley rats retrogradely labeled neurons that were bilaterally located within the confine of the nucleus reticularis gigantocellularis (NRGC). These cells conformed with the Golgi description of cytoarchitectonic configurations and size distributions for neurons in this reticular nucleus. Furthermore, field potentials antidromically evoked at the NA by stimulating the vagus nerve were augmented by concurrent activations of bilateral NRGC sites that were themselves capable of eliciting hypotension and bradycardia. We conclude that the gigantocellular reticular nucleus may at least employ the cardiac vagal preganglionic neurons in the NA to exert its actions on the heart.

The relationship between cardiovascular and pain regulatory systems

An increasing amount of anatomical, physiological, and pharmacological evidence suggest that pain inhibitory circuitry is linked with cardiovascular regulatory systems in man and laboratory animals. Induction of hypertension in rats by different methods (mineralocorticoid treatment, stenosis of renal artery, or social deprivation) is associated with reduced responsiveness to noxious thermal stimuli (hot-plate) or to noxious mechanical stimuli (paw pressure). Genetically hypertension-prone rats derived from the SABRA strain and spontaneously hypertensive rats derived from Wistar/Kyoto strain also display a similar hypoalgesia. Acute increases in blood pressure are associated with reduced sensitivity to painful stimuli. Additionally, the interaction between blood pressure and pain perception has also been supported by the demonstration that various experimental interventions that diminish the magnitude of hypertension also attenuate the hypoalgesia. Recent clinical findings are also in agreement with the laboratory animal findings since sensory and pain thresholds have been shown to be significantly higher in unmedicated essential hypertensive subjects compared to normotensive controls. Thus, the human data corroborate animal data and suggest that a relation between blood pressure and pain sensitivity is likely to be a general phenomenon. It is unlikely that damage to peripheral pain fibers caused by a change in blood pressure contributes to the observed hypoalgesia. Naloxone, which has no effect on blood pressure, returns the pain sensitivity to normal levels. Behavioral tests (open field and motor activity cage) of normotensive and of renal and genetically (SBH and SHR) hypertensive rats exclude the possibility of a general motor deficit in hypertensive rats. Endogenous opioid peptides in central and peripheral nervous systems as well as in endocrine organs are implicated, although non-opioid mechanisms are also evident. Activation of baroreceptor afferents by acute or chronic increases in arterial or venous blood pressure may play an important role in the somatosensory responses associated with the increase in blood pressure. Coordinated cardiovascular-pain regulatory responses may be part of an adaptive mechanism that helps the body to face stressful events.

The roles of alpha 2-adrenoceptors in the nucleus reticularis gigantocellularis and vagal mechanism in the cardiovascular suppressive effects of guanabenz in the rat

In pentobarbital-anesthetized rats, pretreatment with yohimbine (10 micrograms), which was microinjected into the bilateral nucleus reticularis gigantocellularis (NRGC), significantly antagonized the reduction in arterial pressure, and the force and rate of heart contraction normally promoted by systemic administration of guanabenz (10 micrograms/kg, i.v.). At the same time, the vasodepressive as well as negative inotropic and chronotropic effects of direct application of guanabenz (500 ng) into the NRGC were attenuated by bilateral cervical vagotomy or atropine sulfate (1 mg/kg, i.v.). We conclude that guanabenz may promote antihypertension by activating the alpha 2-adrenoceptors in the NRGC, which in turn elicits cardiovascular suppression by at least facilitating the vagal outflows to the heart.

Participation of nucleus reticularis gigantocellularis in guanabenz-promoted hypotension, decrease in cardiac contractility and bradycardia in rats

The possible participation of the nucleus reticularis gigantocellularis in the hypotension, decrease in cardiac contractility and bradycardia induced by guanabenz was examined in rats that were anesthetized with pentobarbital sodium (40 mg/kg, i.p.). Guanabenz (10 micrograms/kg, i.v.) elicited an initial, transient hypertension, accompanied by an increase in cardiac contractility, followed by a significant and sustained hypotension, as well as decrease in the force and rate of cardiac contraction. In rats receiving bilateral focal electrolytic lesions of the nucleus reticularis gigantocellularis, the same injection produced only the initial transient responses, without the subsequent depressant effects. Microinjection of guanabenz directly into the ventro-medial portion of the nucleus reticularis gigantocellularis, at an ineffective systemic concentration (500 ng), produced significant and prolonged reduction in arterial pressure, cardiac contractility and heart rhythm. On the other hand, local application of the same concentration (500 ng) of guanabenz into the lateral portion of the same nucleus produced only minor hypotension and bradycardia, with no decrease in cardiac contractility. It is concluded that the nucleus reticularis gigantocellularis is at least one of the central sites through which guanabenz may produce its cardiovascular suppressant effects.

Arterial pressure- and cardiac rhythm-related single-neuron activities in the nucleus reticularis gigantocellularis of the rat

This communication presents an analysis of 531 extracellularly recorded spontaneously active nucleus reticularis gigantocellularis (NRGC) single-neurons in pentobarbital anesthetized rats, with particular reference to their cardiovascular-related activities. Fifty-nine percent of the gigantocellular neurons evaluated had temporal relationship with induced or spontaneous arterial pressure fluctuations. Twenty-eight percent of the NRGC neurons manifested definite discharge patterns and periodicity with reference to the cardiac cycle. It is discussed that these gigantocellular neurons may serve as a primary component of the cardiovascular regulatory mechanism. Alternatively, they may coordinate the necessary circulatory support for activities varying from respiration to somatomotor functions or sleep-wakefulness.

Interactions between cardiovascular and pain regulatory systems

A review of pharmacological, neuroanatomical, electrophysiological, and behavioral data indicates that systems controlling cardiovascular function are closely coupled to systems modulating the perception of pain. This view is directly supported by experiments from our laboratory showing that activation of either the cardiopulmonary baroreceptor reflex arc or the sinoaortic baroreceptor reflex arc induces antinociception. The outcomes of studies using pharmacological treatments, peripheral nerve stimulation, peripheral nerve resection, and CNS lesions are also presented as a preliminary means of characterizing cardiovascular input to pain regulatory systems. The network formed by these systems is proposed to participate in the elaboration of adaptive responses to physical and psychological stressors at various levels of the neuroaxis, and possibly to participate in "diseases of adaptation." In particular, the present analysis suggests that the inhibition of pain brought about by elevations in either arterial or venous blood pressure may provide a form of psychophysiological relief under situations of stress and contribute to the development of essential hypertension in humans.

Differential effects of clonidine on pain, arterial blood pressure, and heart rate in the cat: lack of interactions with naloxone

In cats anesthetized with alpha-chloralose and urethane, intravertebral administration of clonidine (4 and 10 micrograms/kg) dose-dependently suppressed the jaw-opening reflex, arterial blood pressure, and heart rate. For a given dose, there was a differential degree of inhibition in the order of analgesia much greater than hypotension greater than bradycardia. Naloxone injections (0.4 and 1.0 mg/kg, i.vert.) essentially failed to antagonize these effects, suggesting the lack of involvement of the opiate receptors or endogenous opioids in these processes. Furthermore, pain suppression by clonidine appeared to be independent of the vasodepression and cardioinhibition it promoted. It is possible that neural mechanisms responsible for clonidine-induced antinociception, hypotension, and bradycardia are likely to have differential sensitivities to the imidazoline compound, regardless of whether they exist in separate central sites or in subpopulations of neurons within common neural substrates.

Correlated effects of clonidine on single-neuron activities in the gigantocellular reticular nucleus, arterial pressure and heart rate in the cat

In chloralose-urethane anesthetized cats, intravenous administration of clonidine (10 micrograms/kg) promoted an increase in single-neuron discharges in the gigantocellular reticular nucleus that exhibited a degree and time-course parallel to the hypotension and bradycardia produced by the imidazoline compound. This observation was taken to be a positive indication that clonidine may pharmacologically activate (or disinhibit) neurons in this reticular nucleus, which in turn elicits vasodepression and cardioinhibition.

Qualitative analysis of intracellular characteristics of spontaneous neurons in the nucleus reticularis gigantocellularis of the cat

The intracellular characteristics of spontaneous neurons in the nucleus reticularis gigantocellularis (NRGC) of cats were evaluated. Apart from an irregular pattern of spontaneous discharge, we demonstrated that out of 895 successfully impaled NRGC neurons, 744 had a relatively low transmembrane potential of -12 to -40 mV. Many reticular neurons manifested spontaneous pre-potentials and post-spike depolarizations, from which action potentials were observed to arise. It is discussed that the spontaneous nature of NRGC neuronal discharge may be the result of continuous presynaptic bombardments because of the special anatomic and physiologic features of the reticular formation. Alternatively, the NRGC neurons may possess pacemaker-like membrane properties.

Alpha-Adrenergic and 5-hydroxytryptaminergic receptor stimulants as new antihypertensive drugs, with observations on involvement of opiate receptors

Cardiovascular actions of central alpha-adrenergic and 5-hydroxytryptaminergic stimulants are discussed in terms of overall effects, correlation of various activities, receptor activation and side-effects. The involvement of opiate receptors in their actions is also examined.

Morphine suppression of bradycardia induced by electrical stimulation of gigantocellular reticular nucleus in the cat

In chloralose-urethane-anesthetized cats, intravertebral injection of morphine (1,2 and 4 mg/kg) promoted a drastic suppression of the bradycardia elicited by stimulating the medullary gigantocelular reticular nucleus (GRN). The degree of blockadge of GRN-induced cardioinhibition was directly dependent upon the dose of morphine and inversely related to the reticular stimulus train intensity and pulse frequency. The possibility that the GRM may be a site of action for morphine in its production of hypotension and bradycardia was discussed.