Existence of a powerful inhibitory mechanism in the medial region of caudal medulla--with special reference to the paramedian reticular nucleus

Activation of neurons in cardiovascular areas of cat brain stem affects spinal reflexes

In 65 cats anesthetized with chloralose (40 mg/kg) and urethane (400 mg/kg), the effects of electrical stimulation and microinjection of sodium glutamate (0.25 M, 100-200 nl) in the pressor areas in the rostral brain stem on the evoked L5 ventral root response (EVRR) due to intermittent stimulation of sciatic afferents were compared to stimulating the dorsomedial (DM) and ventrolateral (VLM) medulla. In general, stimulating these rostral brain stem pressor areas including the diencephalon (DIC) and rostral pons (RP) produced increases in systemic arterial pressure (SAP). In most of the cases (85%) there were associated changes in the EVRR, predominantly a decrease in EVRR (72%). Stimulation of the midbrain (MB, principally in the periaqueductal grey) produced decreases in SAP and EVRR. Decreases in EVRR was observed in 91% of the DM and VLM stimulations in which an increase in SAP was produced. This EVRR inhibition was essentially unaltered after acute midcollicular decerebration. Increases in EVRR were also observed and occurred more often in the rostral brain stem than in the medulla. Since changes of both EVRR and SAP could be reproduced by microinjection of Glu into the cardiovascular-reactive areas of the brain stem, this suggests that neuronal perikarya in these areas are responsible for both actions. On some occasions, Glu induced changes in EVRR but not in SAP. This effect occurred more frequently in the rostral brain stem than in the medulla. The present data suggest that separate neuron population exist in the brain stem for the integration of SAP and spinal reflexes.(ABSTRACT TRUNCATED AT 250 WORDS)

Coronary vascular response to the cerebral ischemia reflex

Most centrally mediated sympathoexcitatory reflexes produce increases in arterial pressure, heart rate, and peripheral vascular resistance, including coronary vasoconstriction. Cerebral ischemia also causes large increases in arterial pressure and peripheral vasoconstriction but with modest or variable changes in heart rate. To examine the effect of cerebral ischemia on coronary vascular resistance, we produced cerebral ischemia in 14 cats by occluding the right brachiocephalic and left subclavian arteries for 30 seconds. After vagotomy and beta-blockade, a marked increase in arterial pressure (89 +/- 14%) and coronary vascular resistance (52 +/- 7%) was seen. After inhibition of the carotid baroreceptor reflex by surgical denervation and application of topical lidocaine, the increase in arterial pressure to cerebral ischemia was not affected, but the increase in coronary vascular resistance was attenuated (33 +/- 6%; p < 0.05 versus before denervation) to a level expected with autoregulation. To evaluate the possible contribution of the chemoreflex on coronary blood flow during cerebral ischemia, we conducted separate experiments in which nicotine was injected into both carotid arteries. Coronary constriction was not observed. Adrenalectomy and upper extremity ischemia likewise did not alter coronary vascular resistance. We conclude that cerebral ischemia elicits neurally mediated coronary vasoconstriction as a result of baroreceptor hypotension rather than directly. The relative absence of neurogenic coronary constriction and changes in heart rate suggest that sympathoexcitation during cerebral ischemia is directed more toward the peripheral vasculature than the heart.

Reduction of blood PO2 decrease and PCO2 increase during asphyxia by paramedian reticular nucleus in cats

Effects of activation of paramedian reticular nucleus (PRN) on the systemic arterial blood pressure (SAP), heart rate, renal nerve activity (RNA), and changes of the partial pressure of the arterial blood oxygen (PO2) and carbon dioxide (PCO2) during asphyxia were studied in cats anesthetized with chloralose (40 mg/kg) and urethane (400 mg/kg). During a 35-s period of asphyxial anoxia, SAP and RNA increased while heart rate decreased significantly. The arterial blood PO2 decreased by 64.6 +/- 4.7% while the PCO2 increased by 54.6 +/- 6.3%. Electrical stimulation of PRN produced a mild to moderate decrease of the SAP, heart rate, and RNA, but arterial PO2 and PCO2 did not change significantly. When PRN was stimulated simultaneously with asphyxia, increases of SAP and RNA and changes of blood gases subsequent to asphyxia reduced significantly. Arterial PO2 decreased only 54.0 +/- 4.9% while the PCO2 increased 39.4 +/- 10.5% (p < 0.01). Similar effects were observed in the venous blood from inferior vena cava. In addition, when the arteriovenous difference of PO2 and PCO2 was compared, simultaneous PRN stimulation during asphyxia produced a higher PO2 reserve (66.3%) and less PCO2 production (-7%) than without PRN stimulation; PO2 54.2%, PCO2 (-2.9%). The results suggest that PRN is a structure that can exert inhibition over a wide spectrum of body functions; not only autonomic system but probably also metabolism.

Coexistence of autonomic and somatic mechanisms in the pressor areas of medulla in cats

The effects of electrical stimulation and microinjection of sodium glutamate (0.5 M) in the sympathetic pressor areas of the dorsal medulla (DM), ventrolateral medulla (VLM), and parvocellular nucleus (PVC) on the knee jerk, crossed extension, and evoked potential of the L5 ventral root produced by intermittent electrical stimulation were studied in 98 adult cats anesthetized with chloralose and urethane. During electrical and glutamate stimulation of these pressor areas, in addition to the rise of systemic arterial blood pressure marked inhibition of the spinal reflex was produced, indicating presence of neuronal perikarya responsible for these actions. Mild to moderate augmentation of spinal reflexes was also observed during brain stimulation but only in a few cases. The magnitude of the somatic effects among the pressor areas of the VLM, DM, and PVC subsequent to glutamate activation was about the same. Induced spinal reflex inhibition, independent from the baroreceptor and vagal influence, remained essentially unaltered after acute midcollicular decerebration. The inhibition was also observed in cats decerebellated 8-10 days in advance. The inhibition was not affected after bilateral electrolytic- or kainic-acid-induced lesions in the paramedian reticular nucleus (PRN). On the contrary, PRN-induced spinal reflex inhibition was attenuated after bilateral lesions in the DM or VLM. Data suggest that there coexists neuronal subpopulations in the VLM, DM, and PVC that can affect both the sympathetic pressor systems and spinal reflexes.

Differential actions of the medial region of caudal medulla on autonomic nerve activities

1. The inhibitory effects produced by activation of the medial region of caudal medulla on activities of the left and right cardiac sympathetic, vagus and greater splanchnic nerves were studied in chloralose-urethane anaesthetized cats. 2. Electrical stimulation of the medial region produced an 80-92% inhibition of the sympathetic nerve activities, and a 45% and 58% inhibition of the left and right cardiac vagal nerve activities, respectively. There were no significant differences between effects elicited in the left and right autonomic nerves. Similar but smaller inhibitory effects were produced by micro-injection of sodium glutamate (0.5 mol/L) or DL-homocysteic acid (50 mmol/L) to the same medullary sites. 3. These data suggest that neurons residing in the medial medullary region exert strong inhibitory effects on autonomic nerve activities. Since the vasculature is principally innervated by sympathetic nerves, inhibition of sympathetic nerve activities might be the principal factor responsible for the depressor effects caused by activation of the medial region of caudal medulla. The heart is innervated both by sympathetic and parasympathetic nerves. Thus, their simultaneous inhibition during activation of the medial region elicits only a weak and variable inhibition of the heart.

Sympathoadrenal excitation and inhibition by lower brainstem stimulation in cats

Effects of stimulation of brainstem sites on hemodynamics and plasma catecholamine levels were assessed in cats under chloralose-urethane anesthesia. Pressor areas of the dorsal medulla (DM) and ventrolateral medulla (VLM) and the depressor area of the paramedian reticular nucleus (PRN) were stimulated electrically using a monopolar electrode, or chemically using sodium glutamate microinjection. Plasma levels of norepinephrine (NE) and epinephrine (EPI) were measured in caval blood above the adrenal veins. Electrical stimulation of the DM and VLM produced increases in blood pressure and in plasma NE and EPI levels that were enhanced after acute vagotomies. The NE and EPI responses were attenuated after acute, bilateral adrenalectomies, confirming augmented adrenomedullary secretion, whereas the pressor responses were intact. Injection of sodium glutamate into the same pressor regions of the DM or VLM also produced pressor responses and elevated plasma catecholamine levels, indicating that the responses resulted from activation of neuronal perikarya. Stimulation of the PRN attenuated pressor and catecholamine responses during stimulation of the DM and VLM. The results indicate that pressor responses during stimulation of the DM and VLM are due at least partly to activation of perikarya in these regions, are associated with but not dependent on adrenomedullary activation, and are enhanced after vagotomy; and that neurons of the PRN exert inhibitory modulation of the pressor and adrenomedullary responses during stimulation of VLM and DM.

Paramedian reticular nucleus: a thermolytic area in the rat medulla oblongata

The effects of stimulation or ablation of the paramedian reticular nucleus (PRN) of the rat medulla oblongata on the thermal responses induced by ambient temperature changes, a pyrogen, or a hypothermic substance were assessed. Electrical stimulation of the PRN elicited thermolytic reactions (including decreased metabolism, cutaneous vasodilation and hypothermia) which could be mimicked by micro-injection of kainic acid (an excitotoxic amino acid) into the same region. Bilateral electrolytic lesions in the PRN prevented the animals from responding to heat stress (35 degrees C for 30 min) to some extent, but did not prevent responses to cold stress (4 degrees C for 60 min). In addition, the thermogenic reactions induced by intrahypothalamic injection of polyriboinosinic acid: polyribocytidylic acid (a pyrogenic substance), or the thermolytic reactions induced by intraperitoneal administration of chlorpromazine (a tranquilizer), were antagonized respectively by activation or ablation of the PRN. This suggests that the PRN of the caudal medulla may function as a thermolytic area.

Inhibition of spinal reflexes by paramedian reticular nucleus

The inhibitory actions of the paramedian reticular nucleus (PRN), and its neighbouring structures, i.e., midline raphe nuclei (MRN) and dorsal medullary depressor area (DMD) on the knee jerk (KnJ) and crossed extension movement (CEM) induced by central sciatic stimulation and on the L5 ventral root response (EVRR) evoked by central tibial stimulation, were studied in cats under urethane (400 mg/kg) and alpha-chloralose (40 mg/kg) anesthesia alone, IP or further paralyzed with atracurium besylate (0.5 mg/kg/30 min), IV. Electrical stimulation of the above areas with rectangular pulses (80 Hz, 1.0 msec, 100-200 microA) decreased systemic arterial blood pressure (SAP) in an average value of: 36 +/- 3 mmHg for PRN; 19 +/- 2 mmHg for MRN; and 23 +/- 3 mmHg for DMD. The KnJ and CEM were almost completely suppressed by simultaneous PRN stimulation. The EVRR, including mono- and polysynaptic spinal reflexes with transmission velocity from 10 to 60 m/sec or above, were also suppressed. MRN stimulation only inhibited the KnJ, CEM and polysynaptic spinal reflexes with transmission velocities between 25 and 60 m/sec, but facilitated spinal reflexes with conduction velocities below 10 m/sec. On the other hand, DMD stimulation resulted in small suppression of KnJ, CEM and inhibition of polysynaptic spinal reflexes with conduction velocities between 25 and 60 m/sec. Even though MRN and DMD partially inhibited polysynaptic spinal reflexes, the magnitude of such inhibition was much smaller than that produced by PRN (-20% and -22% vs. -48%). The above-mentioned PRN effects on SAP and EVRR persisted in chronic animals decerebellated 9-12 days before.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular and muscle tone changes produced by microinjection of cholinergic and glutamatergic agonists in dorsolateral pons and medial medulla

Cardiovascular and muscle responses to L-glutamic acid (Glut) and cholinergic agonists injected into the dorsolateral pontine tegmentum and medial medullary reticular formation (MMRF) were examined in unanesthetized, decerebrated cats. Glut, or cholinergic agonists acetylcholine (ACh) or carbachol (Carb), were injected into pons and MMRF at sites from which electrical stimulation produced bilateral suppression of muscle tone. Glut injection in MMRF produced hypotension without change in heart rate at doses as low as 1 mM. At higher doses (0.1-0.4 M), Glut induced hypotension with bradycardia in 23 out of 40 injections in both pons and MMRF. High concentrations of microinjected Glut decreased muscle tone or produced complete atonia in pons and rostral MMRF. Both N-methyl-D-aspartic acid (NMDA) and non-NMDA receptor blockers attenuated or completely blocked the cardiovascular response, while only non-NMDA antagonists blocked muscle inhibition to Glut injection. Microinjection of cholinergic agonists produced consistent hypotension in all of the injections in pons and MMRF, however, the heart rate response was variable with increase (27/42), decrease (2/42), or no change (13/42) in rate seen. Cholinergic injection produced muscle atonia in pons and caudal MMRF but not in rostral MMRF. Both muscle and cardiovascular responses were blocked by atropine but not by hexamethonium. The time course of muscle atonia and cardiovascular change differed in most of the experiments. We conclude that muscle tone suppression and cardiovascular response to Glut or cholinergic agonists use different receptor mechanisms and possibly different neurons. However, the co-localization of these mechanisms suggests that neuronal networks in the medial medulla and dorsolateral pons coordinate motor and cardiovascular responses.

Paramedian reticular nucleus--sympathetic inhibition in spontaneously hypertensive rats

The cardiovascular reactivity of various areas in the medulla related to sympathetic or parasympathetic activation, or to sympathetic inhibition, was compared in spontaneously hypertensive rats (SHR) and in normotensive rats Wistar-Kyoto (WKY) or Sprague-Dawley (SD). In SHR, which has an elevated resting systemic arterial blood pressure (SAP), the sympathetic pressor responses elicited from electrical stimulation of the dorsomedial medulla (DMM), parvocellular lateral nucleus (PVC) or ventrolateral medulla (VLM) were more profound than those in WKY and SD. The depressor and bradycardia responses elicited from electrical stimulation of the paramedian reticular nucleus (PRN) (which exerts both sympathetic and parasympathetic inhibitions) or from the area of the solitary nucleus/dorsomotor nucleus of vagus (NTS/DMV) (where stimulation leads to both parasympathetic activation and sympathetic inhibition) were also more intensive in SHR than in WKY and SD. The elicited pressor and depressor responses, however, were not significantly different between WKY and SD. Our results are consistent with previous findings (15) that in SHR an increased sympathetic activity of the pressor areas of medulla contributes to the pathogenesis of hypertension. Sympathetic inhibition (PRN and NTS/DMV areas) and parasympathetic activation (NTS/DMV area) from these areas, however, may not be critically involved.