The role of the glycine sensitive area of the ventral medulla in cardiovascular responses to carotid chemoreceptor and peripheral nerve stimulation

Chemoreceptors and cardiovascular control in acute and chronic systemic hypoxia

This review describes the ways in which the primary bradycardia and peripheral vasoconstriction evoked by selective stimulation of peripheral chemoreceptors can be modified by the secondary effects of a chemoreceptor-induced increase in ventilation. The evidence that strong stimulation of peripheral chemoreceptors can evoke the behavioural and cardiovascular components of the alerting or defence response which is characteristically evoked by novel or noxious stimuli is considered. The functional significance of all these influences in systemic hypoxia is then discussed with emphasis on the fact that these reflex changes can be overcome by the local effects of hypoxia: central neural hypoxia depresses ventilation, hypoxia acting on the heart causes bradycardia and local hypoxia of skeletal muscle and brain induces vasodilatation. Further, it is proposed that these local influences can become interdependent, so generating a positive feedback loop that may explain sudden infant death syndrome (SIDS). It is also argued that a major contributor to these local influences is adenosine. The role of adenosine in determining the distribution of O2 in skeletal muscle microcirculation in hypoxia is discussed, together with its possible cellular mechanisms of action. Finally, evidence is presented that in chronic systemic hypoxia, the reflex vasoconstrictor influences of the sympathetic nervous system are reduced and/or the local dilator influences of hypoxia are enhanced. In vitro and in vivo findings suggest this is partly explained by upregulation of nitric oxide (NO) synthesis by the vascular endothelium which facilitates vasodilatation induced by adenosine and other NO-dependent dilators and attenuates noradrenaline-evoked vasoconstriction.

Excitatory amino acid receptors in the paraventricular hypothalamic nucleus mediate pressor response induced by carotid body chemoreceptor stimulation in rats

In urethane-anesthetized rats with spinal transection, antagonists of excitatory amino acid receptors, P2 purinoceptors and adrenoceptors were microinjected into the paraventricular hypothalamic nucleus (PVN) and their effects on the pressor response evoked by carotid body chemoreceptor stimulation were examined. Microinjections of the non-selective excitatory amino acid antagonist kynurenate, the non-NMDA receptor antagonist CNQX and the NMDA antagonist 2-amino-5-phosphonovalerate (AP5) into the PVN inhibited the chemoreceptor reflex-induced pressor response. The excitatory amino acid agonist L-glutamate injected into the PVN produced an increase in blood pressure. The P2 purinoceptor antagonist suramin did not affect the pressor response and ATP did not affect basal blood pressure. The alpha adrenoceptor antagonist phentolamine, prazosin and yohimbine also inhibited the chemoreceptor-induced pressor response, while the beta antagonist propranolol did not affect it. These findings indicate that excitatory amino acid receptors and alpha adrenoceptors in the PVN are involved in mediating the pressor response induced by carotid body chemoreceptor stimulation in rats.

Evidence for the involvement of endogenous aspartate in the mediation of carotid chemoreceptor reflexes in the rostral ventrolateral medulla of the rat

2-Amino-5-phosphonovalerate (AP5; 153 pmol) injected into the rostral ventrolateral medulla (RVLM) inhibited pressor responses induced by carotid chemoreceptor stimulation. AP5 also inhibited pressor responses to aspartate (0.75 nmol) but not to glutamate (0.53 nmol) similarly injected. High K+ (50 mM) released endogenous aspartate and glutamate in a Ca2+-dependent manner from the RVLM. Chemoreceptor stimulation caused a release of aspartate but not of glutamate in the RVLM, and sinus nerve denervation abolished the release of aspartate. Increases in blood pressure induced by intravenous phenylephrine did not release aspartate. These results support the hypothesis that endogenous aspartate in the rat RVLM is involved in the mediation of chemoreceptor reflexes.

Glycine produced pressor responses when microinjected in the pressor areas of pons and medulla in cats

In 24 cats under chloralose/urethane anesthesia changes of systemic arterial pressure (SAP) and sympathetic vertebral nerve activities (VNA) were induced by microinjection of glycine (Gly, 1.0 M, 50 nl) into the pressor areas of the rostral pons, i.e., locus coeruleus-parabrachial nucleus (LC-PBN), nucleus of gigantocellular tegmental field-lateral tegmental field (FTG-FTL), and dorsomedial (DM) and ventrolateral (VLM) medulla. The effects were compared with those induced by microinjection of sodium glutamate (Glu, 0.25 M, 50 nl) into the same sites. In about 60% of the injections Gly produced increases in SAP and VNA similar to that of Glu. The increase in SAP was greater in VLM, while the increase in VNA was more marked in DM. In the rest of microinjections Gly and Glu produced changes of SAP and VNA in different combinations. The latency of Gly-induced increases in SAP and VNA was 1 to 3 s longer than that induced by Glu. Our findings show that although Gly is classified as an inhibitory transmitter, it often produced excitation of the pressor neurons in the pons and medulla similar to that of Glu. Whether Gly acts through the same cardiovascular neurons that respond to Glu or through activation of different kinds of neurons remains to be elucidated.

Excitatory amino acid receptors in the rostral ventrolateral medulla mediate hypertension induced by carotid body chemoreceptor stimulation

The rostral ventrolateral medulla (RVLM) is involved in the mediation of cardiovascular responses to peripheral chemoreceptor stimulation. To investigate whether excitatory amino acid inputs in the RVLM are related to the responses to chemoreceptor stimulation, we microinjected kynurenate, an amino acid antagonist, unilaterally into the RVLM and examined its effects on the pressor response to stimulation of carotid body chemoreceptors. Male Wistar rats were anesthetized with urethane, paralyzed and artificially ventilated. The carotid chemoreceptors were stimulated with isotonic solutions of inorganic phosphate solution. Stimulation of carotid body chemoreceptors produced increases in blood pressure. Kynurenate injected ipsilaterally but not contralaterally into the RVLM markedly inhibited the pressor response to chemoreceptor stimulation. In rats with spinal transection, stimulation of carotid body chemoreceptors also produced increases in blood pressure. The pressor response in rats with spinal transection was inhibited by intravenous injection of a vasopressin antagonist or by kynurenate injected ipsilaterally into the RVLM. Kynurenate injected into the RVLM inhibited the pressor response to NMDA, AMPA and kainate but not to acetylcholine in intact rats. These findings indicate that excitatory amino acid receptors are involved in mediating the pressor response to carotid body chemoreceptor stimulation in the rat RVLM. It appears that the chemoreceptor stimulation produces an increase in vasopressin release and the enhancement of vasopressin release is also mediated by an increase in excitatory amino acid inputs in the RVLM.

N-methyl-D-aspartate receptors but not non-N-methyl-D-aspartate receptors mediate hypertension induced by carotid body chemoreceptor stimulation in the rostral ventrolateral medulla of the rat

In urethane-anesthetized rats, excitatory amino acid antagonists were microinjected into the rostral ventrolateral medulla (RVLM) and their effects on the pressor response and tachycardia evoked by carotid chemoreceptor stimulation were examined. Microinjections of the N-methyl-D-aspartate (NMDA) receptor antagonists 2-amino-5-phosphonovalerate (AP5) and MK-801 into the RVLM inhibited these chemoreceptor reflex responses whereas these responses were not affected by injection of the non-NMDA receptor antagonist CNQX. AP5 and MK-801 but not CNQX abolished the pressor response evoked by NMDA whereas CNQX but not AP5 and MK-801 abolished that evoked by AMPA or kainate. These results provide evidence that NMDA receptors in the RVLM of the rat are involved in the carotid chemoreceptor reflex.

Actions of carotid chemoreceptors on subretrofacial bulbospinal neurons in the cat

Thirty-one barosensitive bulbospinal neurons were recorded from the subretrofacial (SRF) nucleus in eight chloralose-anaesthetised, paralysed cats. Close arterial injections of CO2-saturated saline were used to stimulate carotid body chemoreceptors. Seven neurons were abruptly excited and six neurons abruptly inhibited by chemoreceptor stimuli: these were primary responses, independent of changes in blood pressure or central respiratory drive (monitored from the phrenic nerve). A further eight neurons responded only indirectly to chemoreceptor stimuli, showing enhanced modulation of their activity coupled to the enhanced central respiratory drive. The distinction between primary and secondary responses was shown more clearly when central respiratory drive was inhibited by superior laryngeal nerve stimulation. The remaining ten neurons showed no clear response to chemoreceptor stimuli. SRF bulbospinal neurons thus show the same range of responses to chemoreceptor stimuli as the sympathetic neurons they are believed to drive.

Spinal cholinergic modulation of cardiovascular tone and a somatosympathetic reflex response

The purpose of this study was to examine the relationship between spinal cholinergic pressor neurons and a somatosympathetic reflex response in rats. Intrathecal (IT) injection of the cholinesterase inhibitor, neostigmine (NEO), produced marked pressor and tachycardic responses without any changes in respiratory parameters. On the other hand, stimulation of the sciatic nerve produced increases in both cardiovascular and respiratory (tidal volume, minute volume, respiratory rate) responses. These cardiorespiratory responses to nerve stimulation were inhibited by IT NEO. A pressor response could also be induced by topical application of NEO to the surface of lower spinal cord, which was not altered by prior dorsal rhizotomy. These results indicate that two independent cholinergic systems exist in the spinal cord, one of which participates in the inhibitory modulation of the somatosympathetic reflex, and the other which mediates a sympathoexcitatory response. It is unlikely that the pressor response to spinal administration of NEO is mediated through this somatosympathetic response.

Responses of rostroventrolateral medulla spinal vasomotor neurones to chemoreceptor stimulation in rats

The activity of spinal projecting 'vasomotor' neurones of the rostroventrolateral medulla (RVL) has been recorded in anaesthetised and paralysed rats. They responded to carotid body chemoreceptor stimulation (0.208 M NaH2PO4) with an increase in discharge that preceded an evoked increase in arterial blood pressure. This initial excitatory response was followed by a fall in discharge correlated to the rise in blood pressure. RVL neurones, which had equivalent sensitivity to baroreceptor inputs but no spinal projecting axons, failed to exhibit the initial excitatory response on chemoreceptor stimulation. These observations suggest that the pressor response to chemoreceptor stimulation is mediated, at least partially, by excitation of RVL-spinal 'vasomotor' neurones.

Effects of anesthesia on efferent phrenic activity in neonatal swine

The effects of two levels of Saffan anesthesia (standard recording level: 2-4 mg/kg/min, and 10X recording level) and a single level of pentobarbital (5 mg/kg) on the power spectral density of efferent phrenic discharge were investigated in piglets aged from less than 1 day to 50 days. The phrenic high frequency oscillation (HFO) was present in decerebrate, unanesthetized piglets and in piglets anesthetized with Saffan, albeit reduced at 10 times recording level, but was absent under pentobarbital. The results indicate that Saffan does not have a significantly depressant effect on the phrenic HFO in developing swine.

Analysis of factors that contribute to cardiovascular changes induced in the cat by graded levels of systemic hypoxia

1. In cats anaesthetized with Saffan, which does not block afferent activation of the brain stem defence areas, we have analysed the cardiovascular changes induced by 3 min periods of graded systemic hypoxia (fraction of O2 in inspirate, Fi,O2, 0.15, 0.12, 0.08, 0.06). 2. At light levels of Saffan anaesthesia, hypoxia (particularly Fi, O2 0.08 and 0.06) or selective stimulation of carotid chemoreceptors evoked the pattern of tachycardia, decrease in renal and mesenteric vascular conductance (RVC, MVC), but increase in femoral vascular conductance (FVC) which is characteristic of the alerting-defence response. This supports our view that activation of the defence areas is an integral part of the response to systemic hypoxia. 3. Hypoxia also induced an increase in frequency of augmented breaths which was graded with the level of hypoxia: 0.6 min-1 at Fi, O2 0.21 to 1.1 min-1 at Fi, O2 0.06; in some cats each of these was accompanied by a transient fall in arterial pressure (ABP) and increase in FVC. It is proposed that these responses were all part of a reflex elicited by lung irritant receptors and facilitated by peripheral chemoreceptors. However, their low rate of occurrence and the liability of the vasodilatation suggests they do not make major contributions to the overall response. 4. The above short-lasting responses were superimposed upon gradual changes whose magnitudes were graded with the level of hypoxia: hyperventilation, slight tachycardia, but bradycardia at Fi, O2 0.6, small increases in ABP, FVC and MVC allowing femoral and mesenteric blood flow to increase, but decreases in RVC which maintained renal blood flow constant. 5. Vagotomy had no significant effect on these changes. Further, hyperinflation of the lungs with pressures of 10 mmHg evoked the Breuer-Hering reflex but had no noticeable cardiovascular effect. It is proposed that, in the cat, reflex tachycardia and vasodilatation elicited by lung stretch receptors play no significant part in the response to hypoxia. 6. By contrast, after pneumothorax, with ventilation and thereby arterial PCO2 (Pa, CO2) maintained constant, graded hypoxia produced graded bradycardia, decrease in MVC and RVC and no change in FVC. Taken together, these results suggest that in the spontaneously breathing cat, the response to hypoxia is dominated by the effects of hypocapnia secondary to hyperventilation, which by inhibiting peripheral and central chemoreceptor activity effectively counteracts the primary bradycardia and peripheral vasoconstriction elicited by hypoxic stimulation of peripheral chemoreceptors. 7. These proposals are compared with those drawn for other species.

Analysis of the cardiovascular changes induced in the rat by graded levels of systemic hypoxia

1. In rats anaesthetized with Saffan, we have further analysed the respiratory, cardiac and regional vascular responses induced by 3 min periods of graded hypoxia (breathing 15, 12, 8 or 6% O2 in N2). 2. Frequently, hypoxia evoked an episode, lasting 1-1.5 min, of tachycardia, renal and mesenteric vasoconstriction and skeletal muscle vasodilatation. The tachycardia and muscle vasodilatation persisted after vagotomy indicating they were not initiated by pulmonary stretch receptors secondary to hyperventilation. We propose that such episodes represented the cardiovascular components of the alerting-defence response initiated by activation of the brain stem defence areas by peripheral chemoreceptors. 3. Each of these episodes was superimposed upon gradual hyperventilation, tachycardia, fall in arterial pressure and vasodilatation in renal, mesenteric and muscle circulation the magnitudes of which at 2 min were generally graded with the level of hypoxia. In the 3rd minute, respiration and heart rate tended to wane below control levels. 4. Vagotomy had little effect on the heart rate changes and only slightly reduced the peripheral vasodilatation allowing the conclusion that the gradual tachycardia and peripheral vasodilatation was not a reflex initiated by pulmonary stretch receptors. 5. Guanethidine given after vagotomy abolished the tachycardia indicating it was sympathetically mediated; possible initiating factors are discussed. But the secondary bradycardia persisted indicating it reflected the direct effect of hypoxia on cardiac pacemaker tissue. 6. The peripheral vasodilatation persisted after guanethidine or phentolamine indicating it was mainly attributable to the local vasodilator effects of tissue hypoxia. 7. It is proposed that the components of the alerting response are an integral part of the response to systemic hypoxia. Further, that in the rat this response is superimposed upon, but may be overcome by the direct effects of hypoxia on peripheral vasculature, heart and central nervous system.

Analysis of cardiovascular responses evoked following changes in peripheral chemoreceptor activity in the rat

1. Comparisons have been made between rats anaesthetized with pentobarbitone and Saffan (Glaxovet), of respiratory and cardiovascular changes evoked by (1) brief stimulation of carotid body chemoreceptors (c.b.); (2) systemic hypoxia induced by N2 breathing for 5 s; (3) brief unloading of peripheral chemoreceptors with dopamine; and (4) O2 breathing for 10 s. The results are discussed in relation to responses reported in other species. 2. Under pentobarbitone, c.b. stimulation evoked hyperventilation, tachycardia, and vasoconstriction in hindlimb muscle and renal and mesenteric circulation. The effects of vagotomy and/or of holding ventilation constant indicated that the primary cardiac response to c.b. stimulation was bradycardia which could be overcome by tachycardia, due to a reflex mediated by pulmonary stretch receptors with vagal afferents and to other secondary effects of hyperventilation. However, reflex vasodilatation initiated by hyperventilation did not modulate the chemoreceptor-induced peripheral vasoconstriction. 3. Under light pentobarbitone, N2 evoked a similar pattern of response to c.b. stimulation, except that the tachycardia apparently also reflected the known effects of increased central inspiratory drive and central nervous hypoxia on cardiac vagal and sympathetic activity. However, under deep pentobarbitone or after guanethidine, N2 induced generalized vasodilatation. It is proposed that these responses reflected the local vasodilator actions of hypoxia. 4. Under light Saffan anaesthesia, both c.b. stimulation and N2 evoked the autonomic components of the alerting stage of the defence response which includes tachycardia and vasodilatation in hindlimb muscle, which are not secondary to hyperventilation, with renal and mesenteric vasoconstriction, pupillary dilatation and exophthalmus. However, under deep Saffan anaesthesia, c.b. stimulation and N2 produced the patterns of response they each evoked under deep pentobarbitone. It is proposed that light Saffan anaesthesia allows chemoreceptor stimulation to activate the defence areas and that under such conditions the primary response to c.b. stimulation and direct effects of hypoxia may be overridden. 5. Under pentobarbitone or Saffan, the hypoventilation induced by I.V. dopamine and by O2 indicated that almost 50% of eupnoeic ventilation was due to drive from peripheral chemoreceptors. This drive apparently played no significant role in setting the baseline level of heart rate, but could account for 10% of total peripheral resistance and of the baseline level of arterial pressure under Saffan, rather less under pentobarbitone.

Modulation of the centrally-evoked visceral alerting/defence response by changes in CSF pH at the ventral surface of the medulla oblongata and by systemic hypercapnia

In the present study on nine cats, repeated tests were made of the effects of superfusion of the ventral surface of the medulla oblongata with acid or alkaline CSF. Only two animals showed slight hyperventilation, tachycardia, mesenteric vasoconstriction and variable changes in hindlimb vascular conductance when the ventral surface was superfused with acid CSF; alkaline CSF produced opposite effects. These changes are qualitatively similar to, but much smaller than, published results which support the idea that the central chemoreceptor areas for CO2 are near the surface of the ventral medulla. But, in accord with those who have disputed this idea, the remaining 7 animals showed no response to superfusion with acid or alkaline CSF. Yet, all 9 animals showed marked hyperventilation in response to inhalation of 5% or 8% CO2. These findings accord with the view that chemosensitive structures on the ventral medulla represent part, but not all of the central chemosensitive mechanism for CO2. Inhalation of CO2 also induced bradycardia, mesenteric vasodilatation and either vasodilatation or vasoconstriction in hindlimb, attributable to a predominance of the direct myocardial depressant and local vasodilator effects of CO2, over the increase in sympathetic activity produced by central hypercapnia. But, despite the different effects of acid CSF and inhaled CO2 on baselines, they produced comparable effects on the visceral altering/defence response evoked by electrical stimulation in the ventral amygdalo-hypothalamic pathway viz, the magnitude of the characteristic hindlimb dilatation was reduced while that of the mesenteric constriction was increased.(ABSTRACT TRUNCATED AT 250 WORDS)