Central cardiovascular actions of D-Ala2-Met5-enkephalinamide in the rat: effects of naloxone and nucleus reticularis gigantocellularis lesion

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

Effects of bestatin on the central cardiovascular regulatory mechanisms in the rat

We evaluated the effects of bestatin, the specific aminopeptidase-B and leucine aminopeptidase inhibitor, on the central cardiovascular regulatory mechanisms in Sprague-Dawley rats anesthetized with pentobarbital sodium (40 mg/kg, i.p.). Intracerebroventricular injection of bestatin (100 or 200 nmol/5 microliters) consistently elevated the basal systemic arterial pressure and heart rate. At the same time, this degradative enzyme blocker increased the sensitivity of the baroreceptor reflex responses as well as the efficacy of the modulatory actions of the medullary nucleus reticularis gigantocellularis on these reflexes. We speculate that enhancing the tonic activities of the endogenous neuropeptides in the brain by protecting them from their catabolic enzymes may affect the central cardiovascular regulatory machinery by modifying the operations of the baroreceptor feedback controls and their modulatory mechanisms.

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.

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.