Opioids in cerebrospinal fluid in hypotensive newborn pigs

Therapeutic potential of nociceptin/orphanin FQ peptide (NOP) receptor modulators for treatment of traumatic brain injury, traumatic stress, and their co-morbidities

The nociceptin/orphanin FQ (N/OFQ) peptide (NOP) receptor is a member of the opioid receptor superfamily with N/OFQ as its endogenous agonist. Wide expression of the NOP receptor and N/OFQ, both centrally and peripherally, and their ability to modulate several biological functions has led to development of NOP receptor modulators by pharmaceutical companies as therapeutics, based upon their efficacy in preclinical models of pain, anxiety, depression, Parkinson's disease, and substance abuse. Both posttraumatic stress disorder (PTSD) and traumatic brain injury (TBI) are debilitating conditions that significantly affect the quality of life of millions of people around the world. PTSD is often a consequence of TBI, and, especially for those deployed to, working and/or living in a war zone or are first responders, they are comorbid. PTSD and TBI share common symptoms, and negatively influence outcomes as comorbidities of the other. Unfortunately, a lack of effective therapies or therapeutic agents limits the long term quality of life for either TBI or PTSD patients. Ours, and other groups, demonstrated that PTSD and TBI preclinical models elicit changes in the N/OFQ-NOP receptor system, and that administration of NOP receptor ligands alleviated some of the neurobiological and behavioral changes induced by brain injury and/or traumatic stress exposure. Here we review the past and most recent progress on understanding the role of the N/OFQ-NOP receptor system in PTSD and TBI neurological and behavioral sequelae. There is still more to understand about this neuropeptide system in both PTSD and TBI, but current findings warrant further examination of the potential utility of NOP modulators as therapeutics for these disorders and their co-morbidities. We advocate the development of standards for common data elements (CDE) reporting for preclinical PTSD studies, similar to current preclinical TBI CDEs. That would provide for more standardized data collection and reporting to improve reproducibility, interpretation and data sharing across studies.

Fetal Cerebrovascular Maturation: Effects of Hypoxia

The human cerebral vasculature originates in the fourth week of gestation and continues to expand and diversify well into the first few years of postnatal life. A key feature of this growth is smooth muscle differentiation, whereby smooth muscle cells within cerebral arteries transform from migratory to proliferative to synthetic and finally to contractile phenotypes. These phenotypic transformations can be reversed by pathophysiological perturbations such as hypoxia, which causes loss of contractile capacity in immature cerebral arteries. In turn, loss of contractility affects all whole-brain cerebrovascular responses, including those involved in flow-metabolism coupling, vasodilatory responses to acute hypoxia and hypercapnia, cerebral autoregulation, and reactivity to activation of perivascular nerves. Future strategies to minimize cerebral injury following hypoxia-ischemic insults in the immature brain might benefit by targeting treatments to preserve and promote contractile differentiation in the fetal cerebrovasculature. This could potentially be achieved through inhibition of receptor tyrosine kinase-mediated growth factors, such as vascular endothelial growth factor and platelet-derived growth factor, which are mobilized by hypoxic and ischemic injury and which facilitate contractile dedifferentiation. Interruption of the effects of other vascular mitogens, such as endothelin and angiotensin-II, and even some miRNA species, also could be beneficial. Future experimental work that addresses these possibilities offers promise to improve current clinical management of neonates who have suffered and survived hypoxic, ischemic, asphyxic, or inflammatory cerebrovascular insults.

Perivascular expression and potent vasoconstrictor effect of dynorphin A in cerebral arteries

Background

Numerous literary data indicate that dynorphin A (DYN-A) has a significant impact on cerebral circulation, especially under pathophysiological conditions, but its potential direct influence on the tone of cerebral vessels is obscure. The aim of the present study was threefold: 1) to clarify if DYN-A is present in cerebral vessels, 2) to determine if it exerts any direct effect on cerebrovascular tone, and if so, 3) to analyze the role of κ-opiate receptors in mediating the effect.

Methodology/Principal Findings

Immunohistochemical analysis revealed the expression of DYN-A in perivascular nerves of rat pial arteries as well as in both rat and human intraparenchymal vessels of the cerebral cortex. In isolated rat basilar and middle cerebral arteries (BAs and MCAs) DYN-A (1–13) and DYN-A (1–17) but not DYN-A (1–8) or dynorphin B (DYN-B) induced strong vasoconstriction in micromolar concentrations. The maximal effects, compared to a reference contraction induced by 124 mM K⁺, were 115±6% and 104±10% in BAs and 113±3% and 125±9% in MCAs for 10 µM of DYN-A (1–13) and DYN-A (1–17), respectively. The vasoconstrictor effects of DYN-A (1–13) could be inhibited but not abolished by both the κ-opiate receptor antagonist nor-Binaltorphimine dihydrochloride (NORBI) and blockade of G_i/o-protein mediated signaling by pertussis toxin. Finally, des-Tyr¹ DYN-A (2–13), which reportedly fails to activate κ-opiate receptors, induced vasoconstriction of 45±11% in BAs and 50±5% in MCAs at 10 µM, which effects were resistant to NORBI.

Conclusion/Significance

DYN-A is present in rat and human cerebral perivascular nerves and induces sustained contraction of rat cerebral arteries. This vasoconstrictor effect is only partly mediated by κ-opiate receptors and heterotrimeric G_i/o-proteins. To our knowledge our present findings are the first to indicate that DYN-A has a direct cerebral vasoconstrictor effect and that a dynorphin-induced vascular action may be, at least in part, independent of κ-opiate receptors.

Arachidonic acid- and prostaglandin E2-induced cerebral vasodilation is mediated by carbon monoxide, independent of reactive oxygen species in piglets

Arachidonic acid (AA) and prostaglandin (PG) E(2) stimulate carbon monoxide (CO) production, and AA metabolism is known to be associated with the generation of reactive oxygen species (ROS). This study was conducted to address the hypothesis that CO and/or ROS mediate cerebrovascular dilation in newborn pigs. Experiments were performed on anesthetized newborn pigs with closed cranial windows. Different concentrations of AA (10(-8)-10(-6) M), PGE(2) (10(-8)-10(-6) M), iloprost (10(-8)-10(-6) M), and their vehicle (artificial cerebrospinal fluid) were given. Piglets with PGE(2) and iloprost received indomethacin (5 mg/kg iv) to inhibit cyclooxygenase. AA, PGE(2), and iloprost caused concentration-dependent increases in pial arteriolar diameter. The effects of both AA and PGE(2) in producing cerebral vascular dilation and associated CO production were blocked by the heme oxygenase inhibitor chromium mesoporphyrin (2 × 10(-5) M), but not by the prostacyclin analog, iloprost. ROS inhibitor tempol (SOD mimetic) (1 × 10(-5) M) and the H(2)O(2) scavenger catalase (1,000 U/ml) also do not block these vasodilator effects of AA and PGE(2). Heme-L-lysinate-induced cerebrovascular dilation and CO production was blocked by chromium mesoporphyrin. Hypoxanthine plus xanthine oxidase, a combination that is known to generate ROS, caused pial arteriolar dilation and CO production that was inhibited by tempol and catalase. These data suggest that AA- and PGE(2)-induced cerebral vascular dilation is mediated by CO, independent of ROS.

Significance of endogenous opioids in the maintenance of cerebral and spinal vascular CO2-sensitivity in deep hemorrhagic hypotension

High CO(2)-sensitivity, one of the major characteristics of the cerebrovascular bed, has been shown to be influenced by a variety of factors. There are no reports, however, on the involvement of the endogenous opioid peptides in the modulation of the CO(2)-sensitivity of the cerebral and spinal cord vessels, either in normotensive or, in hypotensive conditions. The effect of general opiate receptor blockade (1.0mg/kg naloxone, i.v.) on regional cerebrovascular CO(2)-sensitivity was studied with radiolabeled microspheres in 10 distinct brain and spinal cord regions of the anesthetized cat. The CO(2)-induced flow changes were investigated in normotensive, in moderately hypotensive (MAP=80 mmHg) and in deep hypotensive cats (MAP=40 mmHg). The systemic arterial pressure was lowered by hemorrhage. In the normotensive cats, opiate receptor blockade caused no changes in the vascular CO(2)-sensitivity in the investigated cerebral and spinal cord regions. In moderate hypotension, cerebral and spinal CO(2)-sensitivity was significantly reduced by the hemorrhage itself, but remained unaffected by the naloxone administration. In deep hemorrhagic hypotension, however, general opiate receptor blockade resulted not only in a further reduction of the already impaired CO(2)-sensitivity, but even in a reversal of the effect of CO(2) from flow increase to flow decrease. These results indicate that endogenous opioid peptides, which do not seem to influence cerebrovascular reactions in steady-state, normotensive conditions, may contribute significantly to the maintenance of the normal vasodilatory response of the cerebral and spinal cord vessels to CO(2) during hemorrhage-induced deep arterial hypotension.

Role of Nociceptin/Orphanin FQ in the physiologic and pathologic control of the cerebral circulation

Nociceptin/orphanin FQ is a newly described member of the opioid family. Previous minireviews in this series have described the contribution of important factors, including opioids, in the regulation of the cerebral circulation during physiologic and pathologic conditions. The present review extends these initial comments to an opioid whose vascular actions have only very recently been appreciated. In particular, this review discusses the contribution of nociceptin/orphanin FQ to impaired cerebral hemodynamics after cerebral hypoxia/ischemia and traumatic brain injury.

Role of nociceptin/orphanin FQ in age-dependent cerebral hemodynamic effects of brain injury

This study was designed to compare the role of the newly described endogenous opioid nociceptin/orphanin FQ (NOC/oFQ) in the reductions of cerebral blood flow (CBF) and pial artery diameter observed following fluid percussion brain injury (FPI) in chloralose anesthetized newborn and juvenile pigs as a function of time postinsult. FPI elevated CSF NOC/oFQ concentration from 70 +/- 3 to 444 +/- 51 within 1 h and to 1,931 +/- 112 pg/mL (n = 7) within 8 h, whereas concentrations returned to control value within 168 h in the newborn. In contrast, FPI elevated CSF NOC/oFQ from 77 +/- 4 to 202 +/- 16 pg/mL (n = 7) within 1 h, while values returned to control value within 8 h in the juvenile. Topical NOC/oFQ (10(-8), 10(-6) M) induced vasodilation was reversed to vasoconstriction by FPI in the newborn while such responses were only attenuated in the juvenile at 1 h post insult (control, 9 +/- 1 and 16 +/- 1%; FPI newborn, -8 +/- 1 and -14 +/- 1%; FPI juvenile, 2 +/- 1 and 5 +/- 1%, n = 7). Such altered dilation returned to control value within 168 h in newborns and 8 h in juveniles. Blood flow in the cerebrum was reduced from 57 +/- 4 to 23 +/- 3 mL x min(-1) x 100 g(-1) (n = 7) within 1 h and returned to control value with 168 h post FPI in newborns. In animals pretreated with [F/G] NOC/oFQ (1-13) NH2 (1 mg/kg, i.v.), a NOC/oFQ antagonist, however, CBF only fell to 39 +/- 4 mL x min(-1) x 100 g(-1) (n = 7) at 1 h post insult in newborns. In contrast, CBF was only reduced from 57 +/- 6 to 32 +/- 2 in untreated and to 39 +/- 3 mL/min(-1) x 100 g(-1) (n = 7) in treated juveniles within 1 h post FPI. Similar observations for reductions in pial artery diameter were made in untreated and treated newborns and juveniles. These data suggest that an elevated CSF NOC/oFQ concentration and altered vascular responsiveness to this opioid contribute to reductions in CBF and pial artery diameter observed following FPI. Because such NOC/oFQ changes were greater in newborns versus juveniles, these data further suggest that NOC/oFQ contributes to age-related cerebral hemodynamic differences in the effects of FPI.

Role of NOC/oFQ in impaired opioid-induced pial artery dilation following brain injury

Previous studies in piglets show that opioid-induced pial artery dilation was impaired following fluid percussion brain injury (FPI). This study was designed to determine the role of the newly described opioid nociceptin orphanin FQ (NOC/oFQ) in such impaired dilation to other opioids after FPI. CSF NOC/oFQ concentration was elevated from 70+/-6 to 444+/-56 pg/ml ( approximately 10(-10) M) within 1 h of FPI. Coadministration of NOC/oFQ (10(-10) M) with methionine enkephalin (10(-10), 10(-8), 10(-6) M) attenuated pial dilation induced by this opioid (7+/-1, 13+/-2, and 19+/-2 vs. 2+/-1, 6+/-1, and 7+/-2%) under non-brain injury conditions. Similar inhibition by NOC/oFQ was observed for leucine enkephalin and dynorphin. Methionine enkephalin (10(-10), 10(-8), 10(-6) M)-induced pial artery dilation was also inhibited within 1 h of FPI, but such responses were partially restored in animals pretreated with the NOC/oFQ receptor antagonist [F/G] NOC/oFQ (1-13) NH(2) (10(-6) M) (8+/-1, 14+/-1, and 21+/-1 vs. 1+/-1, 3+/-1, and 4+/-1 vs. 7+/-1, 11+/-1, and 17+/-1% for sham control, FPI and FPI pretreated with the NOC/oFQ receptor antagonist). Leucine enkephalin and dynorphin-induced pial artery dilation were similarly altered by FPI and partially restored by [F/G] NOC/oFQ (1-13) NH(2). These data indicate that the NOC/oFQ released by FPI contributes to impaired dilation to other opioids observed following this insult.

Cerebral hemodynamics after traumatic brain injury of immature brain

These studies were designed to characterize the cerebral hemodynamic effects of fluid percussion brain injury (FPI) in the newborn pig equipped with a closed cranial window. Reductions in cerebral blood flow, pial artery diameter, and cerebral oxygenation following FPI were greater in newborn (1-3 days old) vs. juvenile (3-4 weeks old) pigs, suggesting that newborns were exquisitely sensitive to brain injury. Additionally, in piglets, there was decremented dilation to nitric oxide, cGMP, and cAMP dependent stimuli following FPI. The membrane potential of vascular muscle is an important contributor to vascular tone and the activity of K+ channels is an important regulator of membrane potential. Recent studies indicate that altered dilator responsiveness and cerebral hemodynamic control following FPI results from impaired K+ ATP sensitive (KATP) and calcium sensitive (Kca+2) channel function. Impaired KATP channel function results, at least in part, from protein kinase C activation by the peptide endothelin-1. These observations indicate that the effects of brain injury on cerebral hemodynamics in the newborn are multifaceted and multifactorial.

Vasopressin contributes to dynorphin modulation of hypoxic cerebrovasodilation

Because pial artery dilation during a 20- or 40-min hypoxic exposure was less than that observed during a 5- or 10-min exposure, stimulus duration determines the vascular response to hypoxia. Dynorphin (Dyn) modulates hypoxic pial dilation and contributes to decremented dilation during longer hypoxic exposures. This study was designed to determine whether vasopressin (VP) contributes to Dyn modulation of hypoxic pial dilation in newborn pigs equipped with a closed cranial window. Moderate (M) and severe (S) hypoxia (arterial PO2 approximately 35 and 25 mmHg, respectively) had no effect on cerebrospinal fluid VP during a 5-min exposure but increased its concentration during longer exposure periods. The VP antagonist [beta-mercapto-beta,beta-cyclopentamethylenepropionyl1,O-Me-Tyr2, Arg8]vasopressin (MEAVP) had no influence on pial dilation during the 5-min exposure but potentiated the 20- and 40-min M and S hypoxic exposure dilations: 21 +/- 2 vs. 29 +/- 3% and 23 +/- 2 vs. 33 +/- 2% for 20- and 40-min S hypoxic dilation before and after MEAVP. Topical VP during 5 min of hypoxia elicited dilation that was reversed to vasoconstriction during 20 min of S and 40 min of M and S hypoxia. Similarly, during 5 min of hypoxia, Dyn elicited dilation that was reversed to vasoconstriction during longer hypoxic periods. MEAVP blunted this Dyn-induced vasoconstriction. These data show that VP modulates hypoxic pial dilation in a stimulus duration-dependent manner and that VP contributes to the reversal of Dyn from a dilator to a constrictor during prolonged hypoxia. Finally, these data suggest that VP contributes to Dyn modulation of hypoxic cerebrovasodilation.

Role of opioids in hypoxic pial artery dilation is stimulus duration dependent

Because methionine enkephalin contributes to and dynorphin opposes dilation during a 10-min hypoxic exposure, opioids modulate pial artery dilation to this stimulus. However, such modulation may be dependent on the duration of hypoxia. The present study was designed to characterize the modulation of hypoxic pial dilation by opioids as a function of stimulus duration in newborn pigs equipped with a closed cranial window. Hypoxic dilation was decremented in both moderate and severe groups (PO2 approximately 35 and 25 mmHg, respectively) during 20-min and 40-min exposure periods compared with the response during 5 or 10 min of stimulation (24 +/- 1, 25 +/- 1, 18 +/- 1, and 14 +/- 1% for 5, 10, 20, and 40 min of moderate hypoxia; means +/- SE). Moderate and severe hypoxia had no effect on cerebral spinal fluid (CSF) methionine enkephalin or dynorphin concentration during a 5-min exposure period. During a 10-min exposure, however, both opioids were increased in CSF. During 20- and 40-min exposure periods, CSF dynorphin continued to increase, whereas methionine enkephalin steadily decreased (962 +/- 18, 952 +/- 21, 2,821 +/- 15, 2,000 +/- 81, and 1,726 +/- 58 pg/ml methionine enkephalin for control, 5, 10, 20, and 40 min of moderate hypoxia, respectively). The mu-opioid (methionine enkephalin) antagonist beta-funaltrexamine had no influence on dilation during the 5-min exposure, decremented the 10- and 20-min exposures, but had no effect on 40-min exposure hypoxic dilation. Whereas the kappa-opioid (dynorphin) antagonist norbinaltorphimine similarly had no effect on a 5-min exposure dilation, it, in contrast, potentiated 10-, 20-, and 40-min exposure hypoxic dilations (23 +/- 1 vs. 23 +/- 1, 24 +/- 1 vs. 32 +/- 1, 16 +/- 1 vs. 24 +/- 2, and 13 +/- 1 vs. 23 +/- 3% for 5, 10, 20, and 40-min hypoxic dilation before and after norbinaltorphimine). These data show that opioids do not modulate hypoxic pial dilation during short but do so during longer exposure periods. Moreover, hypoxic pial dilation is diminished during longer exposure periods. Decremented hypoxic pial dilation during longer exposure periods results, at least in part, from decreased release of methionine enkephalin and accentuated release of dynorphin. These data suggest that the relative role of opioids in hypoxic pial dilation changes with the stimulus duration.

Role of endothelial nitric oxide synthase in hypoxia-induced pial artery dilation

Nitric oxide (NO) contributes to hypoxia-induced pial artery dilation, at least in part, through the formation of cGMP and the subsequent release of methionine enkephalin and leucine enkephalin in the newborn pig. In separate studies, these opioids also were observed to elicit NO-dependent pial artery dilation, whereas light/dye endothelial injury reduced hypoxic pial dilation. The current study was designed to investigate the role of the endothelial isoform of NO synthase in hypoxic pial dilation, associated opioid release, and opioid dilation in piglets equipped with a closed cranial window. N-iminoethyl-L-ornithine (L-NIO) (10(-6) mol/L), an antagonist that may have greater endothelial NO synthase inhibitory selectivity, had no effect on dilation elicited by hypoxia (PO2 approximately 35 mm Hg) (24 +/- 2 versus 24 +/- 2% in the absence and presence of L-NIO, respectively, n = 8). Hypoxic dilation was accompanied by increased CSF cGMP, which also was unchanged in the presence of L-NIO (394 +/- 19 and 776 +/- 63 versus 323 +/- 13 and 739 +/- 25 fmol/mL for control and hypoxia in the absence and presence of L-NIO, respectively, n = 6). Additionally, hypoxic pial dilation was associated with increased CSF methionine enkephalin, which also was unchanged in the presence of L-NIO (992 +/- 73 and 2469 +/- 197 versus 984 +/- 18 and 2275 +/- 185 pg/mL, respectively, n = 6). In contrast, methionine enkephalin-induced dilation was blocked by L-NIO (6 +/- 1, 10 +/- 1, and 16 +/- 1 versus 1 +/- 1, 1 +/- 1, and 2 +/- 1% for 10(-10), 10(-8), 10(-6) mol/L methionine enkephalin, respectively, before and after L-NIO, n = 8). Substance P-induced pial dilation was blunted by L-NIO, whereas responses to sodium nitroprusside and N-methyl-D-aspartate were unchanged. These data indicate that endothelial NO synthase contributes to opioid-induced pial artery dilation but not hypoxia-induced dilation. Additionally, these data suggest that neuronally derived NO contributes to hypoxic pial dilation.

Effects of nalmefene, CG3703, tirilazad, or dopamine on cerebral blood flow, oxygen delivery, and electroencephalographic activity after traumatic brain injury and hemorrhage

Hemorrhage after traumatic brain injury (TBI) in cats produces significant decreases in cerebral oxygen delivery (DcereO2) and electroencephalographic (EEG) activity. To determine whether effective treatments for the separate insults of TBI and hemorrhagic shock would also prove effective after the clinically relevant combination of the two, we measured the effects of a kappa-opiate antagonist (nalmefene), an inhibitor of lipid peroxidation (tirilazad), a thyrotropin-releasing hormone analog (CG3703), a clinically useful pressor agent (dopamine) or a saline placebo on cerebral blood flow (CBF), and EEG activity after TBI and mild hemorrhagic hypotension. Cats (n = 40, 8 per group) were anesthetized with 1.6% isoflurane in N2O:O2 (70:30) and prepared for fluid-percussion TBI and microsphere measurements of CBF. Cats were randomized to receive nalmefene (1 mg/kg), tirilazad (5 mg/kg), CG3703 (2 mg/kg), dopamine (20 microg x kg(-1) x min[-1]) or a saline placebo (2 ml, 0.9% NaCl). Animals were injured (2.2 atm), hemorrhaged to 70% of preinjury blood volume, treated as just described and resuscitated with a volume of 10% hydroxyethyl starch equal to shed blood. CBF was determined and EEG activity recorded before injury, after hemorrhage, and 0, 60, and 120 min after resuscitation (R0, R60, and R120). CBF increased significantly after resuscitation (R0) in the nalmefene- and CG3703-treated groups. CBF did not differ significantly from baseline in any group at R60 or R120. DcereO2 was significantly less than baseline in the saline-, dopamine-, and tirilazad-treated groups at R60 and in the dopamine-, tirilazad-, and CG3703-treated groups at R120. EEG activity remained unchanged in the nalmefene-treated group but deteriorated significantly at R60 or R120 compared to baseline in the other groups. Nalmefene and CG3703 preserved the hyperemic response to hemodilution (otherwise antagonized by TBI), and nalmefene prevented the deterioration in DcereO2 and EEG activity that occurs after TBI and hemorrhage.

Role of impaired cAMP and calcium-sensitive K+ channel function in altered cerebral hemodynamics following brain injury

Previous studies have shown that pial arteries constricted and responses to dilator opioids were blunted after fluid percussion injury (FPI) in newborn pigs. Membrane potential of vascular muscle is a major determinant of vascular tone and activity of K+ channels is a major regulator of membrane potential. Recent data show that opioids elicit dilation via the sequential production of cAMP and subsequent activation of calcium-sensitive K+ (K(Ca2+)) channels by this second messenger. The present study was designed to investigate the effect of FPI on cAMP and K(Ca2+) channel function. Chloralose-anesthetized piglets equipped with a closed cranial window were connected to a percussion device consisting of a saline-filled cylindrical reservoir and a metal pendulum. Brain injury of moderate severity (1.9-2.1 atm) was produced by allowing the pendulum to strike a piston on the cylinder. FPI blunted dilation to the cAMP analogs 8-Bromo cAMP and Sp 8-Bromo cAMPs (10(-8), 10(-6) M), (9 +/- 1 and 16 +/- 1 vs. 2 +/- 1 and 3 +/- 1% dilations to 8-Bromo cAMP before and after FPI, respectively, n = 8). Similarly, FPI attenuated dilation to pituitary adenylate cyclase activating peptide (PACAP), an endogenous activator of adenylate cyclase, and NS 1619, a K(Ca2+) channel agonist (9 +/- 1 and 16 +/- 1 vs. 3 +/- 1 and 5 +/- 1% for NS 1619 10(-8), 10(-6) M before and after FPI, respectively, n = 8). Moreover, FPI attenuated PACAP, methionine enkephalin, leucine enkephalin, and dynorphin induced elevations in CSF cAMP concentration (940 +/- 2, 1457 +/- 50, and 2191 +/- 53 vs. 810 +/- 17, 1033 +/- 36, and 1218 +/- 49 fmol/ml for control, PACAP 10(-8), 10(-6) M before and after FPI, respectively, n = 8). These data show that cAMP and K(Ca2+) channel function is impaired after FPI. Further these data suggest that impaired cAMP and K(Ca2+) channel function contribute to altered cerebral hemodynamics following FPI.

Role of activation of calcium-sensitive K+ channels and cAMP in opioid-induced pial artery dilation

The present study was designed to investigate the role of activation of Kca+2 channels and cAMP in opioid-induced pial artery dilation in newborn pigs equipped with closed cranial windows. Methionine enkephalin, an endogenous mu agonist, elicited dilation that was modestly attenuated by the Kca+2 channel antagonist, iberiotoxin (10(-7) M) (7 +/- 1, 11 +/- 1 and 16 +/- 1 vs. 4 +/- 1, 7 +/- 1, and 11 +/- 1% for methionine enkephalin 10(-10), 10(-8), 10(-6) M in the absence and presence of iberiotoxin, respectively). Dilator responses to leucine enkephalin and dynorphin, endogenous delta and kappa agonists, as well as the synthetic analogues DAMGO, DPDPE, deltorphin and U50488H all were similarly attenuated by iberiotoxin. Dilation in response to methionine enkephalin was accompanied by increased CSF cAMP concentration (1170 +/- 21, 1358 +/- 22, 1473 +/- 26, and 1575 +/- 24 fmol/ml for control, 10(-10), 10(-8), 10(-6) M methionine enkephalin, respectively). Methionine enkephalin-induced dilation was attenuated by Rp 8-bromo cAMPs (10(-5) M), a cAMP antagonist (7 +/- 1, 11 +/- 1 and 17 +/- 1 vs. 2 +/- 1, 4 +/- 1, and 7 +/- 1% for methionine enkephalin 10(-10), 10(-8), and 10(-6) M in the absence and presence of Rp 8-bromo cAMPs, respectively). Dilation by the other endogenous and synthetic opioid analogues was also accompanied by elevated CSF cAMP and attenuated by Rp 8-bromo cAMPs. Additionally, dilation produced by the cAMP analogue, 8-bromo cAMP, was blunted by iberiotoxin. These data show that both cAMP and activation of Kca+2 channels contribute to opioid-induced pial artery dilation. Further, these data suggest that opioids elicit dilation, at least in part, via the sequential release of cAMP and subsequent activation of Kca+2 channels by this second messenger.

Influence of cAMP on cerebrospinal fluid opioid concentration: role in cAMP-induced pial artery dilation

Previously, it has been observed that cGMP analogs and agents that elevate cGMP levels markedly increase the concentration of the opioids [Met5]enkephalin and [Leu5]enkephalin in cortical periarachnoid cerebrospinal fluid (CSF) of the newborn pig. However, such agents had no effect on CSF dynorphin-(1-13) concentration. The present study was designed to: (1) investigate the influence of cAMP on the CSF concentration of the opioids [Met5]enkephalin, [Leu5]enkephalin and dynorphin-(1-13); and (2) determine the role of these opioids in cAMP-induced pial artery vasodilation. Piglets equipped with closed cranial windows were used to measure pial artery diameter and collect cortical periarachnoid CSF for assay of opioids. The cAMP analog, 8-Bromoadenosine-3',5'-cyclic monophosphate (8-Bromo cAMP) elicited pial dilation that was blunted by a cAMP antagonist, Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate (10(-5) M) (11 +/- 1 and 19 +/- 1 vs. 1 +/- 1 and 1 +/- 1 for 10(-8) M, 10(-6) M 8-Bromo cAMP before and after Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate, respectively). The dilation produced by 8-Bromo cAMP was accompanied by modest increases in CSF [Met5]enkephalin and co-administration of Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate with 8-Bromo cAMP blocked these increases in CSF opioid concentration (1179 +/- 48, 1593 +/- 92 and 2079 +/- 88 vs. 1054 +/- 32, 1038 +/- 15 and 1071 +/- 17 pg/ml for control, 10(-8) M and 10(-6) M 8-Bromo cAMP before and after Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate, respectively). The release of CSF [Leu5]enkephalin by 8-Bromo cAMP was also blocked by Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate. In contrast 8-Bromo cAMP produced marked increases in CSF dynorphin-(1-13) (38 +/- 3, 61 +/- 3 and 88 +/- 6 vs. 27 +/- 3, 28 +/- 3 and 30 +/- 4 pg/ml for control, 10(-8) M and 10(-6) M 8-Bromo cAMP before and after Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate, respectively). Similar blunted vascular and biochemical responses were observed with the co-administration of Sp 8-Bromoadenosine-3',5'-cyclic monophosphorothioate, another analog of cAMP, with Rp 8-Bromoadenosine-3',5'-cyclic monosphorothioate. The opioid receptor antagonist naloxone (1 mg/kg i.v.) attenuated 8-Bromo cAMP-induced dilation (9 +/- 1 and 17 +/- 1 vs. 5 +/- 1 and 8 +/- 1 for 10(-8) M, 10(-6) M 8-Bromo cAMP before and after naloxone). These data show that cAMP contributes to the release of the CSF opioids [Met5]enkephalin, [Leu5]enkephalin and dynorphin-(1-13), and suggest that, while cGMP is more important relative to cAMP in elevating CSF [Met5]enkephalin and [Leu5]enkephalin concentration, the converse is true for dynorphin-(1-13). Further, these data indicate that opioids contribute to cAMP-induced pial artery vasodilation.

Role of vasopressin in altered pial artery responses to dynorphin and beta-endorphin following brain injury

Pial artery constriction following fluid percussion brain injury (FPI) is associated with elevated CSF dynorphin and beta-endorphin concentration in newborn pigs. Additionally, dynorphin is a dilator under control conditions and a vasoconstrictor under decreased cerebrovascular tone conditions. Vasopressin contributes to beta-endorphin-induced pial constriction and the constrictor potential for dynorphin. Recently, it has been observed that FPI reverses vasopressin from a dilator to a constrictor. The present study was designed to characterize the effect of FPI on beta-endorphin-induced constriction and the role of vasopressin in that constriction as well as in the reversal of dynorphin's vascular response following FPI. Brain injury of moderate severity (1.9 - 2.3 atm) was produced in anesthetized newborn pigs equipped with a closed cranial window. Dynorphin in physiologic and pharmacologic concentrations (10(-10), 10(-8), 10(-6) M) was reversed from a dilator to a constrictor following FPI (7 +/- 1, 11 +/- 1, and 16 +/- 1 vs -4 +/- 1, -7 +/- 1, and -11 +/- 1% before and after FPI, respectively). Dynorphin-induced vascular changes were accompanied by increased cortical periarachnoid CSF vasopressin and these biochemical changes were potentiated following FPI (24 +/- 4 vs 134 +/- 7 and 53 +/- 7 vs 222 +/- 14 pg/mliter for control and dynorphin (10(-6) M) before and after FPI, respectively). In contrast, in animals pretreated with the vasopressin receptor antagonist [1-(beta-mercapto-beta beta-cyclopentamethylene propionic acid) 2-(O-methyl)-Tyr-AVP] (MEAVP, 5 micrograms/kg iv), dynorphin-induced constriction following FPI was attenuated (6 +/- 1, 12 +/- 1, and 16 +/- 1, vs -2 +/- 1, -4 +/- 1, and -7 +/- 1% before and after FPI, respectively). Additionally, beta-endorphin-induced pial constriction was potentiated following FPI (-7 +/- 1, -10 +/- 1, -15 +/- 1 vs -10 +/- 1 -15 +/- 2, and -21 +/- 2% for beta-endorphin (10(-10), 10(-8), 10(-6) M) before and after FPI, respectively). beta-endorphin-induced CSF vasopressin release was similarly potentiated following FPI. Further, MEAVP blunted the augmented constrictor responses to beta-endorphin observed following FPI (-5 +/- 1, -9 +/- 1, -14 +/- 1 vs -2 +/- 1, -5 +/- 1, and -8 +/- 1% before and after FPI, respectively). These data indicate that FPI potentiates beta-endorphin-induced pial construction and reverses dynorphin from a dilator to a constrictor. Additionally, these data show that vasopressin contributes to augmented beta-endorphin pial constriction and the reversal of dynorphin's vascular effects following FPI. Further, since CSF dynorphin and beta-endorphin concentrations are increased following FPI, these data suggest that these two opioids contribute to pial artery constriction observed following FPI, at least, in part, via the release of vasopressin.

Hypotensive stress retards associative learning in rabbits

This study examined the effects of hypotensive stress on classical conditioning of the rabbit's nictitating membrane response. Hypotension, consisting of an approximately 45% decrease in blood pressure, was maintained for 30 min by the i.v. infusion of sodium nitroprusside. Twenty minutes later animals were exposed to a conditioning session consisting of 60 pairings of a 200 ms tone conditioned stimulus with a 100 ms airpuff unconditioned stimulus directed at the cornea. This procedure was repeated for four consecutive days. Animals exposed to the hypotensive stress demonstrated a significantly retarded acquisition of conditioned responses as measured by their frequency and onset latency as well as by an increase in the number of trials required to reach acquisition criteria of five and 10 consecutive conditioned responses as compared with controls. A separate group of animals received a nitroprusside infusion one day after the acquisition of conditioned responses to the tone conditioned stimulus. These animals demonstrated a normal retention of conditioned responses and a normal response to varying intensities of the conditioned stimulus. Hypotensive stress also had no effect on the frequency and topography of the unconditioned response. It was concluded that a decrease in blood pressure can serve as a physiological stressor. One of the reactions to this stress consists of a retardation in the formation of associations during a learning task, without any decrease in the ability to retrieve previously learned material.

Relationship between opioids and activation of phospholipase C and protein kinase C in brain injury induced pial artery vasoconstriction

Previously, it has been observed that newborn pig pial artery constriction after fluid percussion brain injury was associated with elevated CSF dynorphin and beta endorphin concentration. Additionally, brain injury reversed dynorphin-induced pial artery vasodilation to vasoconstriction. The present study was designed to characterize the relationship between opioids and activation of phospholipase C (PLC) and protein kinase C (PKC) in brain injury-induced pial vasoconstriction. Anesthetized newborn pigs equipped with a closed cranial window were connected to a percussion device consisting of a saline-filled cylindrical reservoir with a metal pendulum. Brain injury of moderate severity (1.9-2.3 atm) was produced by allowing the pendulum to strike a piston on the cylinder. Brain injury decreased pial arteriolar diameter within 10 min of injury and continued to fall progressively for 3 h (130 +/- 5, 108 +/- 4 and 102 +/- 5 microns for 0, 10 and 180 min postinjury). In contrast, the PLC inhibitor, neomycin (10(-4) M), blunted brain injury-induced pial vasoconstriction (133 +/- 4, 129 +/- 4 and 135 +/- 5 microns for 0, 10 and 180 min postinjury, respectively). Similarly, staurosporine (10(-7) M), a PKC inhibitor, also blunted brain injury-induced vasoconstriction. beta endorphin (10(-8), 10(-6) M)-induced pial artery vasoconstriction was blunted by neomycin (12 +/- 1, 19 +/- 1 vs. 2 +/- 1, 4 +/- 2% constriction before and after neomycin, respectively). Staurosporine similarly blunted beta endorphin pial constriction (10 +/- 1, 15 +/- 1 vs. 1 +/- 1, 1 +/- 1% constriction before and after staurosporine, respectively). The constrictor potential for dynorphin was also inhibited by neomycin and staurosporine.(ABSTRACT TRUNCATED AT 250 WORDS)

The contribution of delta 1- and delta 2-opioid receptors to hypoxia-induced pial artery dilation in the newborn pig

Previously, it has been observed that mu-opioid receptors contribute to while kappa-opioid receptors oppose pial artery dilation in response to hypoxia. The present study was designed to investigate the contribution of delta 1- and delta 2-opioid receptor activation to hypoxia-induced pial vasodilation. Newborn pigs equipped with a closed cranial window were used to measure pial artery diameter and collect cortical periarachnoid CSF for assay of opioids. Hypoxia increased CSF leucine enkephalin (a delta -agonist) from 36 +/- 6 to 113 +/- 17 pg/ml (n = 5). Hypoxia-induced pial artery vasodilation was attenuated during moderate hypoxia (PaO2 approximately 35 mm Hg), while this response was blunted during severe hypoxia (PaO2 approximately 25 mm Hg), by the delta 1-opioid receptor antagonist 7-benzylidenenaltrexone (BNTX; 10(-8) M) (23 +/- 2 vs. 13 +/- 2 and 34 +/- 6 vs. 10 +/- 3% for moderate and severe hypoxia in the absence and presence of BNTX, respectively; n = 5). In contrast, the delta 2-opioid receptor antagonist naltrindole (10(-9) M) blunted pial vasodilation during moderate hypoxia, but only attenuated the vasodilator response during severe hypoxia (22 +/- 2 vs. 8 +/- 2 and 33 +/- 4 vs. 23 +/- 4% for moderate and severe hypoxia in the absence and presence of naltrindole, respectively; n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

The role of nitric oxide in opioid-induced pial artery vasodilation

The present study was designed to investigate the role of nitric oxide (NO) and the production of cGMP in the vasodilator response to opioid agonists in newborn pigs equipped with a closed cranial window. Methionine-enkephalin (10(-8), 10(-6) M), an endogenous mu opioid agonist, produced pial artery dilation that was attenuated by L-nitroarginine (LNA, 10(-6) M), an NO synthase inhibitor (10 +/- 1 vs. 4 +/- 1 and 16 +/- 1 vs. 7 +/- 1% for 10(-8), 10(-6) M methionine-enkephalin, respectively). Methionine-enkephalin-induced vasodilation was associated with increased cortical periarachnoid CSF cGMP and these changes in CSF cGMP were attenuated by LNA (354 +/- 11 and 596 +/- 32 vs. 278 +/- 13 and 266 +/- 19 fmol/ml for control and methionine-enkephalin 10(-6) M before and after LNA, respectively). Leucine enkephalin, an endogenous delta agonist, elicited similar changes in pial diameter and CSF cGMP while dynorphin, an endogenous k agonist, produced dilation associated with large increases in CSF cGMP (374 +/- 18 vs. 1054 +/- 45 fmol/ml for control and dynorphin 10(-6) M, respectively). Vascular and biochemical changes for these two opioids were similarly attenuated by LNA. The synthetic selective opioid receptor agonists, DAMGO, DPDPE, deltorphin, and U50,488H (10(-8), 10(-6) M) mu, delta 1, delta 2, and kappa agonists, respectively, also elicited increases in pial artery diameter and CSF cGMP that were similarly attenuated by LNA.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of opioids in newborn pig fluid percussion brain injury

The present study was designed to characterize the relationship between cerebral opioid concentration, cerebral hemodynamics, and cerebral oxygenation following percussion brain injury in neonatal pigs. Previous research found that opioids represent a significant vasoactive component in the regulation of the neonatal piglet cerebral circulation. Anesthetized newborn (1-5 days old) pigs equipped with a closed cranial window were connected to a percussion device consisting of a saline-filled cylindrical reservoir with a metal pendulum. Brain injury of moderate severity (1.9-2.3 atm.) was produced by allowing the pendulum to strike a piston on the cylinder. Fluid percussion brain injury decreased pial arteriolar diameter (132 +/- 5 to 110 +/- 5 microns within 10 min). Cerebral blood flow also fell within 10 min of injury and continued to fall progressively for 3 h, resulting in a 46 +/- 4% decrease. Within 30 s of brain injury, there was a transient increase in cerebral hemoglobin-O2 saturation that was reversed to a progressive profound decrease in cerebral hemoglobin-O2 saturation for the next 3 h, as measured by near infrared spectroscopy. CSF opioid concentrations were increased 10 min after brain injury; dynorphin showed the largest proportional increase (5.8 +/- 0.9 fold). The CSF concentration for other opioids continued to increase over 180 min while the dynorphin concentration progressively decreased with time. In naloxone (1 mg/kg i.v.) pretreated piglets, the brain injury induced decrease in arteriolar diameter was attenuated (129 +/- 5 to 121 +/- 5 microns within 10 min). Similarly, the decrease in regional cerebral blood flow and cerebral hemoglobin-O2 saturation observed following brain injury were also blunted by naloxone.(ABSTRACT TRUNCATED AT 250 WORDS)

Met5-enkephalin is localized within axon terminals in the subfornical organ: vascular contacts and interactions with neurons containing gamma-aminobutyric acid

Met5-enkephalin inhibits sodium and water excretion and antagonizes the central actions of angiotensin II in subfornical organ of rat brain. We examined the ultrastructural basis for enkephalin modulation in this circumventricular region. Additionally, we examined the possibility that there might be cellular sites for functional interactions involving Met5-enkephalin and gamma-aminobutyric acid (GABA), a known inhibitory transmitter throughout the central nervous system. Met5-enkephalin and GABA were identified in single coronal sections through the subfornical organ using immunoperoxidase and silver-enhanced immunogold labeling methods, respectively. Enkephalin-like immunoreactivity was most prominently localized within axon terminals. These were distributed primarily in the central, highly vascular, regions of the subfornical organ. Enkephalin-labeled terminals were apposed to the basement membranes of fenestrated capillaries and also formed symmetric, inhibitory type synapses with neurons. In terminals associated with either blood vessels or neurons, the enkephalin immunoreactivity was enriched in large (80-150 nm) dense core vesicles. The immunoreactive vesicles were usually located within portions of the axon in close proximity to astrocytic processes. In contrast, smaller vesicles in the same terminals were more often aggregated near the basement membrane of the capillaries and the active zone of the synapse. The targets of enkephalin-immunoreactive terminals were either unlabeled or GABA-labeled dendrites of local neurons. Enkephalin was also co-localized with GABA in perikarya and in axon terminals. Terminals containing only GABA were far more abundant than those containing enkephalin or enkephalin and GABA. GABA-immunoreactive terminals formed symmetric synapses on unlabeled dendrites some of which also received convergent input from terminals containing enkephalin. Additionally, the enkephalin-immunoreactive terminals were closely apposed to GABA-labeled and unlabeled terminals. These results suggest sites for nonsynaptic release of Met5-enkephalin from dense core vesicles in contact with astrocytes near blood vessels and synaptic complexes in the rat subfornical organ. Moreover, the observed dual localization and pre- and postsynaptic associations between neurons containing Met5-enkephalin and GABA indicate that inhibitory effects of opioids in the subfornical organ may be mediated or potentiated by GABA.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

The influence of opioids on local cerebral glucose utilization in the newborn pig

Topical methionine enkephalin, leucine enkephalin, and dynorphin (10(-6)M) previously have been observed to produce prominent pial arteriolar dilation. Dilation to these opioids could be caused directly by opioids acting on vascular receptors, or indirectly, as a consequence of increased metabolism. Therefore, we examined this possibility by determining the influence of opioids on cerebral glucose utilization in piglets with closed cranial windows using the [14C]deoxyglucose method. Qualitatively, the autoradiographic images expressed as a change in relative optical density from vehicle were unchanged by these opioids. Quantitatively, the opioids similarly had no effect on cerebral glucose utilization (53 +/- 5, 70 +/- 8, 63 +/- 5, and 52 +/- 3, mumol.100 g-1.min-1 for vehicle, methionine enkephalin, leucine enkephalin, and dynorphin, respectively). In contrast, topical glutamate (10(-3) M) produced similar dilation but increased cerebral glucose utilization (41 +/- 3 vs 89 +/- 8 mumol.100 g-1.min-1 for vehicle and glutamate, respectively). Therefore, these opioids do not appear to produce vascular effects through a change in cerebral metabolic utilization of glucose.