Opiate receptors and the endorphin-mediated cardiovascular effects of clonidine in rats: evidence for hypertension-induced mu-subtype to delta-subtype changes

Co-localization of hypocretin-1 and leucine-enkephalin in hypothalamic neurons projecting to the nucleus of the solitary tract and their effect on arterial pressure

Experiments were done to investigate whether hypothalamic hypocretin-1 (hcrt-1; orexin-A) neurons that sent axonal projections to cardiovascular responsive sites in the nucleus of the solitary tract (NTS) co-expressed leucine-enkephalin (L-Enk), and to determine the effects of co-administration of hcrt-1 and D-Ala2,D-Leu5-Enkephalin (DADL) into NTS on mean arterial pressure (MAP) and heart rate. In the first series, in the Wistar rat the retrograde tract-tracer fluorogold (FG) was microinjected (50nl) into caudal NTS sites at which L-glutamate (0.25 M; 10 nl) elicited decreases in MAP and where fibers hcrt-1 immunoreactive fibers were observed that also contained L-Enk immunoreactivity. Of the number of hypothalamic hcrt-1 immunoreactive neurons identified ipsilateral to the NTS injection site (1207 ± 78), 32.3 ± 2.3% co-expressed L-Enk immunoreactivity and of these, 2.6 ± 1.1% were retrogradely labeled with FG. Hcrt-1/L-Enk neurons projecting to NTS were found mainly within the perifornical region. In the second series, the region of caudal NTS found to contain axons that co-expressed hcrt-1 and L-Enk immunoreactivity was microinjected with a combination of hcrt-1 and DADL in α-chloralose anesthetized Wistar rats. Microinjection of DADL into NTS elicited depressor and bradycardia responses similar to those elicited by microinjection of hcrt-1. An hcrt-1 injection immediately after the DADL injection elicited an almost twofold increase in the magnitude of the depressor and bradycardia responses compared to those elicited by hcrt-1 alone. Prior injections of the non-specific opioid receptor antagonist naloxone or the specific opioid δ-receptor antagonist ICI 154,129 significantly attenuated the cardiovascular responses to the combined hcrt-1-DADL injections. Taken together, these data suggest that activation of hypothalamic-opioidergic neuronal systems contribute to the NTS hcrt-1 induced cardiovascular responses, and that this descending hypothalamo-medullary pathway may represent the anatomical substrate by which hcrt-1/L-Enk neurons function in the coordination of autonomic-cardiovascular responses during different behavioral states.

Opioids and clonidine modulate cytokine production and opioid receptor expression in neonatal immune cells

OBJECTIVE

Opioids and clonidine, used in for sedation, analgesia and control of opioid withdrawal in neonates, directly or indirectly activate opioid receptors (OPRs) expressed in immune cells. Therefore, our objective is to study how clinically relevant concentrations of different opioids and clonidine change cytokine levels in cultured whole blood from preterm and full-term infants.

STUDY DESIGN

Using blood from preterm (≤ 30 weeks gestational age (GA), n=7) and full-term ( ≥ 37 weeks GA, n=19) infants, we investigated the changes in cytokine profile (IL-1β, IL-6, IL-8, IL-10, IL-12p70 and TNF-α), cyclic adenosine monophosphate (cAMP) levels and μ-, δ- and κ- opioid receptor (OPR) gene and protein expression, following in-vitro exposure to morphine, methadone, fentanyl or clonidine at increasing concentrations ranging from 0 to 1 mM.

RESULT

Following lipopolysaccharide activation, IL-10 levels were 146-fold greater in cultured blood from full-term than from preterm infants. Morphine and methadone, but not fentanyl, at >10(-5) M decreased all tested cytokines except IL-8. In contrast, clonidine at <10(-9) M increased IL-6, while at >10(-5) M increased IL-1β and decreased TNF-α levels. All cytokine changes followed the same patterns in preterm and full-term infant cultured blood and matched increases in cAMP levels. All three μ-, δ- and κ-OPR genes were expressed in mononuclear cells (MNC) from preterm and full-term infants. Morphine, methadone and clonidine, but not fentanyl, at >10(-5)M decreased the expression of μ-OPR, but not δ- or κ-OPRs.

CONCLUSION

Generalized cytokine suppression along with downregulation of μ-OPR expression observed in neonatal MNC exposed to morphine and methadone at clinically relevant concentrations contrast with the modest effects observed with fentanyl and clonidine. Therefore, we speculate that fentanyl and clonidine may be safer therapeutic choices for sedation and control of opioid withdrawal and pain in neonates.

β-Endorphin involvement in the regulatory response to body sodium overload

The present study was performed to examine the role of the endogenous beta-endorphinergic system on blood pressure regulation, sympathetic and brain activity during body sodium overload. Beta-endorphin knockout (beta end-/-), heterozygous (beta end+/-) and wild-type (beta end+/+) mice were submitted for two weeks to either a normal- or a high-sodium diet (NSD and HSD, respectively), and systolic blood pressure (SBP), urinary catecholamines (as an index of sympathetic nervous system activity), and the brain pattern of Fos-like immunoreactivity (as a marker of neuronal activation) were evaluated in each group. HSD caused a significant increase in SBP in beta end-/- mutant mice compared with beta end+/+ mice kept in the same experimental conditions (P < 0.01), but no statistical differences were observed between beta end+/- and beta end+/+ on a HSD. Moreover, when animals from the three genetic lines were fed with a NSD no changes in SBP were evidenced. With regard to brain activity, beta end-/- mice maintained on a HSD showed a significant increase in Fos-like immunoreactive neurons in the median preoptic nucleus (P < 0.01) compared with beta end+/- and beta end+/+ animals. Additionally, beta end-/- mice had higher levels of urinary epinephrine excretion (P < 0.05) on a HSD in comparison to beta end+/+ and beta end+/- animals in the same experimental conditions. No differences, however, were registered in norepinephrine and dopamine urinary excretion in animals from the three genetic lines after two weeks on either a HSD or a NSD. In summary, our results indicate that the beta-endorphinergic system may play a part in the compensatory response to sodium overload, since the absence of beta-endorphin causes an increase in systolic blood pressure, and increases median preoptic nucleus neural activity and urinary epinephrine excretion.

Hemorrhage activates proopiomelanocortin neurons in the rat hypothalamus

Severe blood loss lowers arterial pressure through a central mechanism that is thought to include opioid neurons. In this study, we investigated whether hemorrhage activates proopiomelanocortin (POMC) neurons by measuring Fos immunoreactivity and POMC mRNA levels in the medial basal hypothalamus. Hemorrhage (2.2 ml/100 g body weight over 20 min) increased the number of Fos immunoreactive neurons throughout the rostral-caudal extent of the arcuate nucleus, the retrochiasmatic area and the peri-arcuate region lateral to the arcuate nucleus where POMC neurons are located. Double label immunohistochemistry revealed that hemorrhage increased Fos expression by beta-endorphin immunoreactive neurons significantly. The proportion of beta-endorphin immunoreactive neurons that expressed Fos immunoreactivity increased approximately four-fold, from 11.7+/-1.4% in sham-operated control animals to 42.0+/-5.2% in hemorrhaged animals. Hemorrhage also increased POMC mRNA levels in the medial basal hypothalamus significantly, consistent with the hypothesis that blood loss activates POMC neurons. To test whether activation of arcuate neurons contributes to the fall in arterial pressure evoked by hemorrhage, we inhibited neuronal activity in the caudal arcuate nucleus by microinjecting the local anesthetic lidocaine (2%; 0.1 or 0.3 microl) bilaterally 2 min before hemorrhage was initiated. Lidocaine injection inhibited hemorrhagic hypotension and bradycardia significantly although it did not influence arterial pressure or heart rate in non-hemorrhaged rats. These results demonstrate that hemorrhage activates POMC neurons and provide evidence that activation of neurons in the arcuate nucleus plays an important role in the hemodynamic response to hemorrhage.

Differential regulation of hippocampal progenitor proliferation by opioid receptor antagonists in running and non-running spontaneously hypertensive rats

Voluntary running in mice and forced treadmill running in rats have been shown to increase the amount of proliferating cells in the hippocampus. Little is known as yet about the mechanisms involved in these processes. It is well known that the endogenous opioid system is affected during running and other forms of physical exercise. In this study, we evaluated the involvement of the endogenous opioids in the regulation of hippocampal proliferation in non-running and voluntary running rats. Nine days of wheel running was compared with non-running in spontaneously hypertensive rats (SHR), a rat strain known to run voluntarily. On the last 2 days of the experimental period all rats received two daily injections of the opioid receptor antagonists naltrexone or naltrindole together with injections of bromodeoxyuridine to label dividing cells. Brain sections from the running rats showed approximately a five-fold increase in newly generated cells in the hippocampus, and this increase was partly reduced by naltrexone but not by naltrindole. By contrast, both naltrexone and naltrindole increased hippocampal proliferation in non-running rats. In non-running rats the administration of naltrexone decreased corticosterone levels and adrenal gland weights, whereas no significant effects on these parameters could be detected for naltrindole. However, adrenal gland weights were increased in naltrexone- but not in naltrindole-administered running rats. In addition, in voluntary running rats there was a three-fold increase in the hippocampal levels of Met-enkephalin-Arg-Phe compared with non-runners, indicating an increase in opioid activity in the hippocampus during running. These data suggest an involvement of endogenous opioids in the regulation of hippocampal proliferation in non-running rats, probably through hypothalamic-pituitary-adrenal axis modulation. During voluntary running in SHR naltrexone altered hippocampal proliferation via as yet unknown mechanisms.

Glycyl-L-glutamine [beta-endorphin-(30-31)] attenuates hemorrhagic hypotension in conscious rats

The profound hypotension caused by acute hemorrhage is thought to involve opioid peptide neurons. In this study, we tested whether glycyl-L-glutamine [Gly-Gln; beta-endorphin-(30-31)], a nonopioid peptide derived from beta-endorphin processing, prevents the cardiovascular depression induced by hemorrhage in conscious and anesthetized rats. Previously, we found that Gly-Gln inhibits the hypotension and respiratory depression produced by beta-endorphin and morphine but does not affect opioid antinociception. Hemorrhage (2.5 ml/100 g body wt over 20 min) lowered arterial pressure in conscious rats (from 120.1 +/- 2.9 to 56.2 +/- 4.7 mmHg) but did not change heart rate significantly. Intracerebroventricular Gly-Gln (3, 10, or 30 nmol) pretreatment inhibited the fall in arterial pressure and increased heart rate significantly. The response was dose related and was sustained during the 35-min posthemorrhage interval. Pentobarbital sodium anesthesia potentiated the hemodynamic response to hemorrhage and attenuated the effect of Gly-Gln. Gly-Gln (10 or 100 nmol icv) did not influence arterial pressure or heart rate in normotensive rats. These data indicate that Gly-Gln is an effective antagonist of hemorrhagic hypotension.

Cardiovascular effects of anandamide in anesthetized and conscious normotensive and hypertensive rats

We previously showed that in anesthetized rats anandamide elicits bradycardia and a triphasic blood pressure response: transient hypotension secondary to a vagally mediated bradycardia, followed by a brief pressor and prolonged depressor response, the latter two effects being similar to those of delta 9-tetrahydrocannabinol (THC). The prolonged depressor but not the pressor response was reduced after alpha-adrenergic receptor blockade or cervical spinal cord transection and was inhibited by the cannabinoid type 1 (CB1) receptor antagonist SR141716A, suggesting CB1 receptor-mediated sympathoinhibition as the underlying mechanism. Here we examined the relationship between sympathetic tone and the cardiovascular effects of anandamide by testing these effects in both conscious and anesthetized, normotensive and spontaneously hypertensive rats. In urethane-anesthetized normotensive rats, SR141716A inhibited the prolonged depressor and bradycardic effects of anandamide and THC with similar potency, whereas it did not affect the pressor response to either agent. Anadamide caused similar hypotension in spontaneously breathing and in paralyzed, mechanically ventilated rats, suggesting that the hypotension is not secondary to respiratory effects. In conscious normotensive rats, anandamide elicited transient vagal activation and a brief pressor response, but the prolonged hypotensive component was absent. SR141716A potentiated and prolonged the brief pressor response to anandamide, suggesting that the depressor response may have been masked by an increased pressor response. All three phases of the anadamide response were present in both anesthetized and conscious spontaneously hypertensive rats, and the hypotensive component, inhibited by SR141716A in both, was more prolonged in the absence (> 50 minutes) than the presence (10 to 15 minutes) of anesthesia. We conclude that anandamide causes a non-CB1 receptor-mediated pressor and a CB1 receptor-mediated prolonged depressor response. The depressor response can be elicited in both conscious and anesthetized animals, but its magnitude depends on preexisting sympathetic tone.

Melanocortin antagonists define two distinct pathways of cardiovascular control by alpha- and gamma-melanocyte-stimulating hormones

Melanocortin peptides and at least two subtypes of melanocortin receptors (MC3-R and MC4-R) are present in brain regions involved in cardiovascular regulation. In urethane-anesthetized rats, unilateral microinjection of alpha-melanocyte-stimulating hormone (MSH) into the medullary dorsal-vagal complex (DVC) causes dose-dependent (125-250 pmol) hypotension and bradycardia, whereas gamma-MSH is less effective. The effects of alpha-MSH are inhibited by microinjection to the same site of the novel MG4-R/MC3-R antagonist SHU9119 (2-100 pmol) but not naloxone (270 pmol), whereas the similar effects of intra-DVC injection of beta-endorphin (1 pmol) are inhibited by naloxone and not by SHU9119. Hypotensive and bradycardic responses to electrical stimulation of the arcuate nucleus also are inhibited by ipsilateral intra-DVC microinjection of SHU9119. gamma-MSH and ACTH(4-10), but not alpha-MSH, elicit dose-dependent (0.1-12.5 nmol) pressor and tachycardic effects, which are much more pronounced after intracarotid than after intravenous administration. The effects of gamma-MSH (1.25 nmol) are not inhibited by the intracarotid injection of SHU9119 (1.25-12.5 nmol) or the novel MC3-R antagonist SHU9005 (1.25-12.5 nmol). We conclude that the hypotension and bradycardia elicited by the release of alpha-MSH from arcuate neurons is mediated by neural melanocortin receptors (MC4-R/MC3-R) located in the DVC, whereas the similar effects of beta-endorphin, a peptide derived from the same precursor, are mediated by opiate receptors at the same site. In contrast, neither MC3-R nor MC4-R is involved in the centrally mediated pressor and tachycardic actions of gamma-MSH, which, likely, are mediated by an as yet unidentified receptor.

Melanocortin antagonists define two distinct pathways of cardiovascular control by alpha- and gamma-melanocyte-stimulating hormones

Melanocortin peptides and at least two subtypes of melanocortin receptors (MC3-R and MC4-R) are present in brain regions involved in cardiovascular regulation. In urethane-anesthetized rats, unilateral microinjection of alpha-melanocyte-stimulating hormone (MSH) into the medullary dorsal-vagal complex (DVC) causes dose-dependent (125-250 pmol) hypotension and bradycardia, whereas gamma-MSH is less effective. The effects of alpha-MSH are inhibited by microinjection to the same site of the novel MG4-R/MC3-R antagonist SHU9119 (2-100 pmol) but not naloxone (270 pmol), whereas the similar effects of intra-DVC injection of beta-endorphin (1 pmol) are inhibited by naloxone and not by SHU9119. Hypotensive and bradycardic responses to electrical stimulation of the arcuate nucleus also are inhibited by ipsilateral intra-DVC microinjection of SHU9119. gamma-MSH and ACTH(4-10), but not alpha-MSH, elicit dose-dependent (0.1-12.5 nmol) pressor and tachycardic effects, which are much more pronounced after intracarotid than after intravenous administration. The effects of gamma-MSH (1.25 nmol) are not inhibited by the intracarotid injection of SHU9119 (1.25-12.5 nmol) or the novel MC3-R antagonist SHU9005 (1.25-12.5 nmol). We conclude that the hypotension and bradycardia elicited by the release of alpha-MSH from arcuate neurons is mediated by neural melanocortin receptors (MC4-R/MC3-R) located in the DVC, whereas the similar effects of beta-endorphin, a peptide derived from the same precursor, are mediated by opiate receptors at the same site. In contrast, neither MC3-R nor MC4-R is involved in the centrally mediated pressor and tachycardic actions of gamma-MSH, which, likely, are mediated by an as yet unidentified receptor.

Cardiovascular excitatory effects of adenosine in the nucleus of the solitary tract

Adenosine is an inhibitory neuromodulator in several brain regions. In the nucleus tractus solitarius (NTS), however, adenosine exerts excitatory cardiovascular effects. The purpose of the present study was to elucidate the involvement of other endogenous mechanisms that could contribute to the final hemodynamic response to adenosine in this nucleus. In normotensive Sprague-Dawley rats, intra-NTS microinjection of adenosine (2.3 nmol/60 nl) decreased blood pressure and heart rate. These effects were blocked by prior administration of the specific adenosine receptor antagonist 1,3-dipropyl-8-p-sulfophenylxanthine (0.92 nmol) and by the two glutamate receptor antagonists kynurenic acid and glutamic diethylester. The specificity of the adenosine-glutamate interaction in the NTS was demonstrated with adrenergic and angiotensin receptor antagonists that did not affect the adenosine response and by experiments with glutamate receptor antagonists that did not affect nicotine actions in the NTS. Furthermore, an increase in glutamate levels was demonstrated during perfusion of adenosine through a microdialysis probe in the NTS of anesthetized rabbits. These findings indicate that adenosine increases the release of glutamate in the NTS and, thus, are at variance with the concept of a "universal" inhibitory effect of adenosine in the central nervous system.

Influence of chronic opioid delta receptor antagonism on blood pressure development and tissue contents of catecholamines and endogenous opioids in spontaneously hypertensive rats

Opioid delta receptors seem to be involved in blood pressure regulation of spontaneously hypertensive rats (SHR), possibly by an interaction with the sympathetic nervous system. In the present study the effect of four weeks' chronic delta receptor antagonism with ICI 154 129 on development of blood pressure was evaluated in young SHR. Contents of adrenaline and noradrenaline and the opioid peptides beta-endorphin and leucine-enkephalin were measured in brain stem, mid brain, hypothalamus, and adrenal glands. After four weeks' treatment, systolic blood pressure was lower when compared with control SHR. During chronic delta antagonism, concentrations of adrenaline were higher in hypothalamus, mid brain and adrenal glands, contents of noradrenaline were higher in hypothalamus and adrenal glands than in control rats, contents of opioid peptides were not altered with the exception of an increase of beta-endorphin concentration in the hypothalamus. The changes in concentrations of catecholamines following chronic delta antagonism may reflect an alteration of sympathetic activity and could contribute to the retardation of blood pressure development.

Altered pituitary hormone response to naloxone in hypertension development

Endogenous opioid regulation of blood pressure is altered during stress in young adults at risk for hypertension. We studied the effects of the opioid antagonist naloxone on the secretion of corticotropin and beta-endorphin during psychological stress in young adults with mildly elevated casual arterial pressures. Naloxone-induced secretion of both corticotropin and beta-endorphin was significantly diminished in persons at enhanced risk for hypertension compared with the low blood pressure control group. Results suggest that opioidergic inhibition of anterior pituitary function is altered in hypertension development.

Biological detectors in liquid chromatography

The applicability of isolated organ preparations as chromatographic detectors was demonstrated. An appropriately designed detector cell (biological detector) applicable in both on-line and off-line modes was developed. Deproteinized sera from healthy volunteers and schizophrenic patients and human amniotic fluid were fractionated by gel-permeation, ion-exchange and reversed-phase liquid chromatography. Ultraviolet absorption and biological activity were compared. The results show that isolated organ preparations selected according to the needs of particular experiments meet the essential criteria of conventional chromatographic detectors. The use of isolated organs allows the detection of biologically active substances in a matrix of physicochemically closely related, but biologically distinct, fluid components. Biological detectors may also provide valuable additional information concerning the chemical structure of biologically active agents in an early stage of isolation.

Cardiovascular effects of neuropeptide Y in rat brainstem nuclei

Central catecholaminergic neurons are involved in cardiovascular regulation. Neuropeptide Y (NPY) coexists with adrenaline and noradrenaline in the rat brain, and interactions among these substances have been studied. The purpose of this study was to investigate the possible role of NPY in central cardiovascular control. Male Sprague-Dawley rats were anesthetized with urethane, and blood pressure was monitored intra-arterially. Intramedullary microinjection (60 nl) of NPY (0, 46.5 fmol, 465 fmol, 1.5 pmol, and 4.65 pmol) was made into the nucleus tractus solitarii (NTS), into the area postrema, and into the C1 area in the rostroventrolateral medulla. Injection site was identified by L-glutamate administration and confirmed histologically. Unilateral injection of NPY into the NTS produced a prominent dose-related decrease in heart rate and systolic and diastolic blood pressure (-106 +/- 8 beats/min, -56 +/- 2 mm Hg, and -33 +/- 2 mm Hg, respectively after 4.65 pmol NPY, n = 7, p less than 0.001). Maximal changes occurred at 30 seconds and recovered in 10 minutes for blood pressure and 20 minutes for heart rate. Injection into the area postrema produced an initial increase in heart rate and mean blood pressure (+23 +/- 2 beats/min and +18 +/- 2 mm Hg) followed by a prolonged decrease in heart rate and mean blood pressure (-14 +/- 4 beats/min and -15 +/- 2 mm Hg, respectively, n = 7, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous opioid involvement in the cardiovascular effects of clonidine in rats: role of the parasympathetic nervous system

The interaction between intravenously administered clonidine and naloxone on blood pressure and heart rate was studied in urethane-anesthetized, normotensive Sprague-Dawley rats. In rats pretreated with propranolol, 1 mg/kg i.v., to eliminate sympathetic tone in the heart clonidine, 5 micrograms/kg i.v. produced hypotension and bradycardia which were inhibited by naloxone, 2 mg/kg i.v. These effects were similar to effects observed in earlier studies in the absence of propranolol. In contrast, in rats pretreated with atropine, 5 mg/kg i.v., to eliminate the effects of changes in vagal tone, the hypotensive and bradycardic effects of clonidine were not influenced by naloxone. These findings are interpreted to indicate that an endogenous opioid is involved in the clonidine-induced increase in parasympathetic outflow to the myocardium.