Opiate receptor subtype binding in gerbil hippocampus is altered by forebrain ischemia

Effects of intracerebroventricular application of the delta opioid receptor agonist [D-Ala2, D-Leu5] enkephalin on neurological recovery following asphyxial cardiac arrest in rats

The delta opioid receptor (DOR) agonist [D-Ala2, D-Leu5] enkephalin (DADLE) has been implicated as a novel neuroprotective agent in the CNS. The current study was designed to evaluate the effects of intracerebroventricular (ICV) application of DADLE on neurological outcomes following asphyxial cardiac arrest (CA) in rats. Male Sprague-Dawley rats were randomly assigned to four groups: Sham group, CA group, DADLE group (DADLE+CA), and Naltrindole group (Naltrindole and DADLE+CA). All drugs were administered into the left cerebroventricle 30 min before CA. CA was induced by 8-min asphyxiation and the animals were resuscitated with a standardized method. DOR protein expression in the hippocampus was significantly increased in the CA group at 1 h after restoration of spontaneous circulation (ROSC) compared with the Sham group. As time progressed, expression of DOR proteins decreased gradually in the CA group. Treatment with DADLE alone or co-administration with Naltrindole reversed the down-regulation of DOR proteins in the hippocampus induced by CA at 24 h after ROSC. Compared with the CA group, the DADLE group had persistently better neurological functional recovery, as assessed by neurological deficit score (NDS) and Morris water maze trials. The number of surviving hippocampal CA1 neurons in the DADLE group was significantly higher than those in the CA group. However, administration of Naltrindole abolished most of the neuroprotective effects of DADLE. We conclude that ICV administration of DADLE 30 min before asphyxial CA has significant protective effects in attenuating hippocampal CA1 neuronal damage and neurological impairments, and that DADLE executes its effects mainly through DOR.

Opioids modulate post-ischemic progression in a rat model of stroke

Alterations in the opioidergic system have been found in cerebral ischemia. Neuroprotection studies have demonstrated the involvement of the opioidergic system in cerebral ischemia/reperfusion (I/R). However, the neuroprotective mechanisms remain largely unclear. This study was conducted to investigate whether intracerebroventricular administration of opioidergic agonists has a neuroprotective effect against cerebral ischemia in rats and, if this proved to be the case, to determine the potential neuroprotective mechanisms. Using a focal cerebral I/R rat model, we demonstrated that the opioidergic agents, BW373U86 (delta agonist) and Dynorphin A 1-13 (kappa agonist), but not TAPP (mu agonist), attenuated cerebral ischemic injury as manifested in the reduction of cerebral infarction and preservation of neurons. The antagonism assay showed that the neuroprotective effect of Dynorphin A was attenuated by nor-Binaltorphimine (kappa antagonist). Surprisingly, BW373U86-induced neuroprotection was not changed by Naltrindole (delta antagonist). These findings indicate that BW373U86 and Dynorphin A exerted distinct neuroprotection against ischemia via opioid-independent and -dependent mechanisms, respectively. The post-ischemic protection in beneficial treatments was accompanied by alleviations in brain edema, inflammatory cell infiltration, and pro-inflammatory cytokine interleukin 6 (IL-6) expression. In vitro cell study further demonstrated that the opioidergic agonists, delta and kappa, but not mu, attenuated IL-6 production from stimulated glial cells. Our findings indicate that opioidergic agents have a role in post-ischemic progression through both opioid-dependent and -independent mechanisms. In spite of the distinct-involved action mechanism, the potential neuroprotective effect of opioidergic compounds was associated with immune suppression. Taken together, these findings suggest a potential role for opioidergic agents in the therapeutic consideration of neuroinflammatory diseases. However, a better understanding of the mechanisms involved is necessary before this therapeutic potential can be realized.

Long-term alterations in mu, delta and kappa opioidergic receptors following middle cerebral artery occlusion in mice

Alterations in the opioidergic system may play a role in the molecular mechanisms underlying neurochemical responses to cerebral ischaemia. The present study aimed to determine the delayed expression of mu, delta and kappa opioid receptors, following 1, 2, 7, and 30 days of middle cerebral artery occlusion (MCAO) in mice. Using quantitative autoradiography, we highlighted significant decreases in mu, delta and kappa opioid receptor expression in ipsilateral cortices from day 1 post-MCAO. Moreover, in contralateral nucleus lateralis thalami pars posterior, ipsi- and contralateral nucleus medialis dorsalis thalami, and ipsilateral substantia nigra, pars reticulata (SNr), kappa receptors were increased; mu receptor densities were decreased in nucleus ventralis thalami, pars posterior (VThP), and SNr. delta-Binding sites were increased in the striatum on day 30 post-MCAO. The alterations in opioid receptors in cortical infarcts were correlated with strong histological damage. Further reductions in opioid receptor densities in cortical infarcts were observed at later time points. In subcortical brain regions, opioid receptor densities were also altered but no histological damage was seen, except in the VThP, in which cell density was increased on day 30. Delayed reductions in opioid receptor densities in the infarct appeared as the continuation of the early processes previously demonstrated. However, changes in subcortical opioid receptor expression may correlate with neuronal alterations in remote brain regions. Changes in opioidergic receptor expression in these regions may be involved in the long-term consequences of stroke and could be used as biomarker of neuronal alteration through the use of imaging techniques in the clinic.

Differential time-course decreases in nonselective, mu-, delta-, and kappa-opioid receptors after focal cerebral ischemia in mice

BACKGROUND AND PURPOSE

Neuroprotection studies have demonstrated the involvement of opioids in ischemia, and we have previously demonstrated alterations in Bmax of opioidergic receptors after 2 post-MCAO time points in mice.

METHODS

In the present study, we have investigated in a detailed manner the postischemic time course of variations in [3H]diprenorphine (nonselective), [3H]DAMGO (mu), [3H]DADLE (delta), and [3H]U69593 (kappa) relative binding densities after focal cerebral ischemia (0 to 48 hours) in mice.

RESULTS

In frontoparietal cortices, our results demonstrate decreases in (1) delta receptor densities at 1 to 3 hours after MCAO, (2) mu and nonselective binding sites at 6 to 12 hours after MCAO, and (3) kappa receptor densities between 6 and 24 hours after MCAO. In the rostral part of the infarct border zone, a decrease in delta-receptors was found concomitant with the extension of the infarct core; conversely, the decrease in delta-receptors appeared before (6 to 12 hours) macroscopic histological damage, which occurred between 12 hours and 24 hours after MCAO in the caudal part of this area. In this frontier, mu- and especially kappa-binding sites were decreased later (12 to 48 hours after MCAO).

CONCLUSIONS

These differential alterations in opioidergic receptors could be due to the selective sublocalization of receptors, postsynaptically on cortical interneurons for mu- and delta-receptors versus presynaptically on cortical afferent pathways for the kappa subtype. Further, our results suggest that delta- and mu-opioidergic receptors could be markers of infarct extension and neuronal death; the study of [3H]diprenorphine and selective binding sites argues in favor of the use of receptor-specific ligands. Finally, the relative preservation of kappa-receptors might be correlated with the neuroprotective role of kappa-agonists, as previously reported.

Maximal densities of mu, delta, and kappa receptors are differentially altered by focal cerebral ischaemia in the mouse

Though opioids are known to have neuroprotective properties, little information is available on the functional state of opioidergic receptors following focal cerebral ischaemia. The present study investigated the evolution of the Bmax and Kd for [3H]DAMGO, [3H]DADLE, and [3H]U69,593, respectively, for the mu, delta, and kappa opioidergic receptors after permanent focal cerebral ischaemia in mice. While the various Kd were unchanged, mu and delta Bmax values were precociously decreased in frontoparietal cortices, earlier than kappa receptors, reflecting infarct extension with time. The Bmax values for mu and delta receptors were also altered in non-infarcted tissues, such as tissues at risk (e.g., temporal auditory cortex) and exofocal (e.g., contralateral and non-infarcted) cortices. These results suggest that, in non-infarcted areas, the observed changes reflect functional modifications to focal ischaemia.

Effect of pentobarbital on postischemic MK-801, muscimol, and naloxone bindings in the gerbil brain

We investigated the postischemic alterations in [3H]MK-801, [3H]muscimol, and [3H]naloxone binding in the gerbil brain, and examined the effect of pentobarbital against these alterations. [3H]MK-801, [3H]muscimol, and [3H]naloxone were used to label N-methyl-D-aspartate (NMDA), gamma-aminobutyric acidA (GABAA), and opiate receptors, respectively. Transient cerebral ischemia was induced for 10 min, and pentobarbital (40 mg/kg) was administered intraperitoneally 30 min before ischemia. Five hours after ischemia, no conspicuous alteration in [3H]MK-801, [3H]muscimol, and [3H]naloxone binding was found in the striatum and hippocampus. Seven days after ischemia, [3H]MK-801 and [3H]naloxone binding was significantly decreased in the striatum and hippocampal area where histological neuronal damage was noted. By contrast, no significant change in [3H]muscimol binding was seen in the above regions except for the hippocampal CA3 sector. The treatment of pentobarbital caused a significant alteration in the binding of [3H]naloxone and [3H]muscimol in various brain areas 5 h after ischemia. However, this drug showed no significant change in [3H]MK-801 binding in the brain. Seven days after ischemia, pentobarbital partly ameliorated a significant reduction in [3H]MK-801 and [3H]naloxone binding in the striatum and hippocampus. A histological study also showed that pentobarbital afforded neuronal protection against the damage to the brain except for the hippocampal CA1 sector 7 days after ischemia. These results suggest that NMDA and opiate receptors are damaged after ischemia, whereas GABAA receptors are unaffected. They also demonstrate that opiate receptors are severe affected by the treatment of pentobarbital, compared with NMDA and GABAA receptors. These findings are of interest in relation to the mechanism of ischemic neuronal damage.

The effects of naloxone on retinal ischemia in rats

The efficacy of naloxone (NL), a broad spectrum opioid antagonist, on retinal ischemia, was evaluated in a rat model of retinal ischemia with histopathologic and morphometric criteria. Two intraperitoneal injections of naloxone 3 mg/kg given immediately and 6 hr after reperfusion showed beneficial effects to the retina as evaluated at 2, 7, and 14 days after reperfusion. Morphologically, the naloxone-treated group showed better-preserved ganglion cells, nerve fiber layer, and inner nuclear layer. Morphometrically, in the treated groups, inner retinal thickness at all three time points and ganglion cell counts at 7 days showed higher values than vehicle controls. This beneficial effect of naloxone was dose-dependent with a minimal effective total dose of 6 mg/kg. A possible role of opiate receptors in retinal ischemia is suggested.

Naloxone receptor binding in gerbil striatum and hippocampus following transient cerebral ischemia

Receptor autoradiographic and histological techniques were used to investigate sequential alteration of naloxone receptors in the gerbil brain 1 h-7 days after transient cerebral ischemia. Transient ischemia was induced for 10 min. [3H]Naloxone binding showed a transient elevation in the striatum 1 h after ischemia, whereas the hippocampus revealed no significant alteration in the binding. Thereafter, no conspicuous alteration in [3H]naloxone binding was seen in the striatum and hippocampus up to 24 h after ischemia. However, a significant elevation in [3H]naloxone binding was found in the hippocampal region 48 h after ischemia. In contrast, the striatum showed no significant alteration in [3H]naloxone binding. Seven days after ischemia, a severe reduction in [3H]naloxone binding was seen not only in the dorsolateral striatum and hippocampal CA3 pyramidal cell layer, where irreversible neuronal damage was found, but also in the histopathological intact dentate gyrus. However, the hippocampal CA1 sector which was most vulnerable to ischemia, revealed no conspicuous alteration in [3H]naloxone binding. These results demonstrate that alteration of naloxone receptors precedes ischemic neuronal damage to the striatum and hippocampus. They also suggest that the damage between striatum and hippocampus may be produced with different processes.

Forebrain ischemia in the gerbil increases lambda opiate binding in hippocampal mossy fibers

Transient forebrain ischemia was produced in gerbils by short-term occlusion of the common carotid arteries under halothane anesthesia. Histological analysis of brains 7 days post-ischemia demonstrated characteristic destruction of CA1 pyramidal cells. lambda Opiate binding (measured with [3H]naloxone in the presence of 300 nM diprenorphine) at 7 days post-ischemia was significantly increased in the stratum lucidum of the hippocampus (the mossy fiber layer), but not in any other region measured, including other hippocampal regions, cortex, amygdala, caudate putamen, thalamus, and hypothalamus. The increase in mossy fiber lambda binding was slow to develop (no increase detected up to 48 h post-ischemia), and long-lasting (binding remained elevated at 32 days post-ischemia). While MK-801 significantly inhibited CA1 pyramidal cell destruction when administered 20 min prior to ischemia, the increase in mossy fiber lambda binding was still evident. None of seven different opioid agonists and antagonists examined had an effect on either the pyramidal cell damage or increased mossy fiber lambda binding seen 7 days after ischemia.

Changes of neuropeptides and their receptors in experimental stroke gerbil brains

Eight kinds of neuropeptides and four kinds of neuropeptide receptors were examined in the right and left hemispheres of mongolian gerbils after unilateral carotid ligation-induced stroke and in normal controls. Five hours after ligation of the right common carotid artery, beta-endorphin concentration in the right hemisphere (ischemic side) of the stroke group was significantly increased compared with that in the contralateral hemisphere (non-ischemic side), but there were no differences between sides in other neuropeptides either with or without stroke. Furthermore, although there were no differences in [3H]naloxone binding, [3H]thyrotropin-releasing hormone binding or 125I-vasoactive intestinal polypeptide binding in the brain in this model of stroke, [3H]enkephalin binding was significantly lower on the ischemic side than on the non-ischemic side in the stroke group. These results suggest that increased activity in the beta-endorphinergic system in the brain might be partly caused by ischemic brain failure.

Effects of traumatic brain injury in rats on binding to forebrain opiate receptor subtypes

Sprague-Dawley rats were subjected to a moderate level (2.2 atm) of traumatic brain injury (TBI) using fluid percussion. Injured animals were allowed to survive posttrauma for periods of 5 min, 3 h, and 24 h. The effect of TBI on binding to forebrain opiate receptors was assessed using quantitative receptor autoradiography, and compared to a sham control group. Binding of [3H]DAGO to mu receptors in neocortex and the CA1 pyramidal layer of the hippocampus was significantly decreased in the 24-h group (p less than 0.05). [3H]Bremazocine binding to kappa receptors was unchanged at 5 min and 24 h, but showed large decreases 3 h after TBI in the CA1 pyramidal layer (65%, p less than 0.05) and dentate gyrus (43%, p less than 0.05). Levels of delta binding (measured with [3H]DSLET) and lambda binding (measured with [3H]naloxone) were unaffected by TBI. These data support previous suggestions of a role for endogenous opioids in TBI, and provide further evidence that mu and kappa opioid receptor subtypes in neocortex and hippocampus may have different functions in TBI.

Effect of opioids on delayed neuronal death in the gerbil hippocampus

The effect of opioids on delayed neuronal death was evaluated in the gerbil hippocampus. Male Mongolian gerbils were subjected to transient forebrain ischemia and neuronal density was evaluated in the hippocampus 7 days following ischemia. When hypothermia during and after ischemia was prevented, treatment with morphine, U-50488H, or naloxone provided no significant protection. In contrast, a spontaneous drop in rectal temperature to 32 degrees C at the end of ischemia produced near-complete protection of CA1 pyramidal neurons. No opioids modulate the protective effect of hypothermia.

Evaluation of U-50,488H analogs for neuroprotective activity in the gerbil

U-50,488H, a kappa (kappa) opioid ligand with moderate potency at sigma (sigma) receptors, protects against mechanical and ischemia-induced injury. The purpose of this study was to evaluate the possibility that sigma-receptors may be involved in mediating the neuroprotective actions of U-50,488H. This possibility was examined by testing the potential of a series of U-50,488H analogs, which are potent sigma-ligands with minimal activity at kappa-opioid receptors, to protect against ischemia-induced neuronal damage in the gerbil. Like U-50,488H, BD-449 (20 mg/kg), the cis-diastereomer of U-50,4888H, protected against ischemia-induced neuronal damage as did BD-737 (50 and 30 mg/kg) and BD-738 (50 mg/kg). All 3 compounds interacted selectively with sigma-receptors. In contrast, BD-601 (50 mg/kg), did not protect against ischemia-induced neuronal damage, although it also interacted potently with sigma-receptors. One difference between the compounds that were neuroprotective and BD-601 is that only BD-601 produced sigma-like behavioral effects in the rat. Thus, it is possible that BD-601 may interact differently or at a different sigma-subtype than BD-449, BD-737 and BD-738 with sigma-receptors. However, these results clearly indicate that an interaction with kappa-opioid receptors is not required for anti-ischemic activity, and that sigma-receptors may play a role in neuroprotection.

Endogenous opiates: 1989

This paper is the twelfth installment of our annual review of the research published during 1989 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 functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; locomotor activity; sex, development, pregnancy, and aging; immunological responses; and other behavior.