Marc A. Czapla

Brainstem activation of platelet-derived growth factor-β receptor modulates the late phase of the hypoxic ventilatory response

Abstract: The early phase of the biphasic ventilatory response to hypoxia in mammals is critically dependent on NMDA glutamate receptor activation within the nucleus of the solitary tract. However, the mechanisms underlying the subsequent development of the typical ventilatory roll‐off are unclear and could underlie important roles in the functional and molecular adaptation to oxygen deprivation. Because the growth factor platelet‐derived growth factor (PDGF)‐BB can modulate the open channel probability of NMDA receptors by activating PDGF‐β receptors, its contribution to hypoxic ventilatory roll‐off was examined. Administration of PDGF‐BB, but not PDGF‐AA, in the nucleus of the solitary tract was associated with significant attenuations of the early hypoxic ventilatory response in conscious rats. Furthermore, marked reductions in the magnitude of hypoxic ventilatory roll‐off occurred in mice heterozygous for a mutation in the PDGF‐β receptor. Administration of a PDGF‐β receptor antagonist to wild‐type littermates elicited similar declines in hypoxic ventilatory roll‐off. The relative abundance of PDGF‐β receptors was confirmed in the nucleus of the solitary tract and other nuclei implicated in the hypoxic ventilatory response. In nucleus of the solitary tract lysates, PDGF‐β receptor tyrosine phosphorylation was temporally correlated with hypoxic ventilatory roll‐off formation. Increased PDGF‐B chain mRNA expression was induced by hypoxia in the nucleus of the solitary tract, and PDGF‐B chain immunoreactivity colocalized with ≅40% of nucleus of the solitary tract neurons, demonstrating hypoxia‐induced c‐Fos enhancements. Thus, PDGF‐BB release and PDGF‐β receptor activation in the nucleus of the solitary tract are critical components of hypoxic ventilatory roll‐off and may have important functional implications in processes underlying survival and acclimatization to hypoxic environments.

Nociceptin, an endogenous ligand for the ORL1 receptor, decreases cardiac output and total peripheral resistance in the rat.

The heptadecapeptide nociceptin, also known as Orphanin FQ, is a newly discovered endogenous ligand for the opioid-like G-protein coupled receptor, ORL1. In the present study, responses to intravenous injections of nociceptin were investigated in the systemic vascular bed of the rat. Nociceptin induced dose-related decreases in systemic arterial pressure and total peripheral resistance when injected in doses of 1-30 nmol/kg i.v.. Nociceptin decreased heart rate and in doses of 10 and 30 nmol/kg i.v., significantly decreased cardiac output. In terms of relative vasodilator activity, nociceptin was approximately 10-fold less potent than the beta-adrenergic receptor agonist isoproterenol. These data show that nociceptin has novel vasodilator activity in the systemic vascular bed of the rat.

Endomorphin 1 and 2, endogenous μ-opioid agonists, decrease systemic arterial pressure in the rat

The endogenous opioid peptides, endomorphin 1 and 2, are newly isolated, potent, and selective mu-opioid receptor agonists. In the present study, responses to endomorphin 1 and 2 were investigated in the systemic vascular bed of the rat. Endomorphin 1 and 2 induced dose-related decreases in systemic arterial pressure when injected in doses of 1-30 nmol/kg i.v. In terms of relative vasodepressor activity, endomorphin 1 and 2 were approximately equipotent with each other and with the ORL1 ligand, nociceptin (orphanin FQ), and were about 10-fold more potent than met-enkephalin in decreasing systemic arterial pressure. Vasodepressor responses to endomorphin 1 and 2 and met-enkephalin, but not to nociceptin, were inhibited by the opioid receptor antagonist, naloxone. These results demonstrate that endomorphin 1 and 2 produce significant naloxone-sensitive decreases in systemic arterial pressure.

Responses to angiotensin peptides are mediated by AT1 receptors in the rat.

The effects of the angiotensin AT1 and AT2 receptor antagonists candesartan and PD-123,319 on hemodynamic responses to angiotensin peptides were investigated in the anesthetized rat. Injections of angiotensin II and III caused dose-related increases in systemic arterial and in hindquarters perfusion pressure that were reduced in an insurmountable manner by candesartan. Pressor responses to angiotensin IV were also attenuated, and a vasodepressor or vasodilator response to the angiotensin peptides was not unmasked by the AT1 receptor antagonists candesartan or losartan. The AT2receptor antagonist PD-123,319 had no significant effect on increases in systemic arterial and hindquarters perfusion pressure in response to the angiotensin peptides. Pressor responses to angiotensin peptides were not altered by adrenergic nerve terminal and α-receptor blocking agents or by the cyclooxygenase inhibitor sodium meclofenamate but were increased by an inhibitor of nitric oxide synthase. The present results suggest that pressor responses to the angiotensin peptides are mediated by the activation of AT1 receptors and that AT2 receptors, the adrenergic system, or cyclooxygenase products do not appear to modulate hemodynamic responses to the angiotensin peptides in the anesthetized rat.The effects of the angiotensin AT1 and AT2 receptor antagonists candesartan and PD-123,319 on hemodynamic responses to angiotensin peptides were investigated in the anesthetized rat. Injections of angiotensin II and III caused dose-related increases in systemic arterial and in hindquarters perfusion pressure that were reduced in an insurmountable manner by candesartan. Pressor responses to angiotensin IV were also attenuated, and a vasodepressor or vasodilator response to the angiotensin peptides was not unmasked by the AT1 receptor antagonists candesartan or losartan. The AT2 receptor antagonist PD-123,319 had no significant effect on increases in systemic arterial and hindquarters perfusion pressure in response to the angiotensin peptides. Pressor responses to angiotensin peptides were not altered by adrenergic nerve terminal and alpha-receptor blocking agents or by the cyclooxygenase inhibitor sodium meclofenamate but were increased by an inhibitor of nitric oxide synthase. The present results suggest that pressor responses to the angiotensin peptides are mediated by the activation of AT1 receptors and that AT2 receptors, the adrenergic system, or cyclooxygenase products do not appear to modulate hemodynamic responses to the angiotensin peptides in the anesthetized rat.

Nociceptin, An Endogenous Ligand for the ORL1 Receptor, has Vasodilator Activity in the Hindquarters Vascular Bed of the Rat

The heptadecapeptide nociceptin, also known as Orphanin FQ, is a newly discovered endogenous ligand for the G-protein coupled, opioid-like receptor ORL1. In the present study, responses to intra-arterial injections of nociceptin were investigated in the hindquarters vascular bed of the rat. Under constant-flow conditions nociceptin induced dose-dependent decreases in hindquarters perfusion pressure when injected in doses of 1-30 nmol into the hindquarters perfusion circuit. The decreases in hindquarters perfusion pressure were rapid in onset and pressure returned to control values over a 3-6 min period. In terms of relative vasodilator activity, nociceptin was approximately equipotent to the nitric oxide donor, DEA/NO, and 30-fold less potent than adrenomedullin. These data demonstrate that nociceptin has significant vasodilator activity in the hindquarters vascular bed of the rat.

DECREASES IN SYSTEMIC ARTERIAL AND HINDQUARTERS PERFUSION PRESSURE IN RESPONSE TO NOCICEPTIN ARE NOT INHIBITED BY NALOXONE IN THE RAT

Nociceptin, the endogenous ligand for the ORL1 receptor, has been shown to decrease systemic arterial and hindquarters perfusion pressures in the rat. The present study was undertaken to determine if decreases in systemic arterial and hindquarters perfusion pressures, in response to nociceptin, are mediated by a naloxone-sensitive mechanism. Injections of nociceptin decreased systemic arterial and hindquarters perfusion pressures in a dose-related manner. The decreases in systemic arterial and hindquarters perfusion pressure in response to nociceptin were not altered by the administration of naloxone in a dose of 2 mg/kg i.v. Met-enkephalin decreased systemic arterial and hindquarters perfusion pressures and responses to the opioid receptor agonist were significantly reduced by naloxone, whereas decreases in systemic arterial pressure in response to the nitric oxide donor, DEA/NO, were not altered. The results of the present study show that decreases in systemic arterial and hindquarters perfusion pressure in response to nociceptin are not mediated by a naloxone-sensitive mechanism in the rat.

Tone-Dependent Vasodilator Responses to Proadrenomedullin NH2-Terminal 20 Peptide in the Hindquarters Vascular Bed of the Rat

Responses to proadrenomedullin NH2-terminal 20 peptide (PAMP) were investigated in the systemic and hindquarters vascular bed of the rat. Intravenous injections of PAMP and adrenomedullin (ADM) produced dose-related decreases in systemic arterial and hindquarters perfusion pressure, which were not altered by alpha-receptor or adrenergic nerve terminal blocking agents. PAMP was 100-fold less potent than ADM, and hindquarters vasodilator responses to both peptides were similar in innervated and denervated preparations. When baseline tone was increased with phenylephrine and U46619 or decreased with sodium nitroprusside, vasodilator responses to PAMP and ADM were correlated with the basal level of tone, suggesting that responses to both peptides are dependent on the baseline level of vasoconstrictor tone in the hindquarters vascular bed of the rat.

Reduced suppression of CO2-induced ventilatory stimulation by endomorphins relative to morphine.

Opioids are among the most effective analgesics, but a major limitation for their therapeutic usefulness is their induction of respiratory depression. Endomorphin-1 (EM1), in contrast to several other mu opioids, exhibits a threshold for respiratory depression that is well above its threshold for analgesia. Its effect on sensitivity to CO(2), however, remains unknown. Minute ventilation (V(E)) in 2, 4, and 6% CO(2) was measured before and after systemic administration of EM1, endomorphin-2 (EM2), DAMGO, and morphine in the conscious rat. EM1 and EM2 attenuated the hypercapnic ventilatory response (HCVR) only in high doses, while DAMGO and morphine diminished the HCVR in much lower doses. The ventilatory effects of high doses of all 4 agonists were blocked by the mu-opioid antagonist naloxone (0.4 mg/kg i.v.), but not by the peripherally restricted mu-opioid antagonist, methyl-naloxone (0.4 mg/kg i.v.). It was concluded that the endomorphins attenuated the HCVR only in large doses, well beyond the analgesic threshold, and did so through a centrally mediated mu-opioid mechanism.