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

Evaluation of 18F-IAM6067 as a sigma-1 receptor PET tracer for neurodegeneration in vivo in rodents and in human tissue

The sigma 1 receptor (S1R) is widely expressed in the CNS and is mainly located on the endoplasmic reticulum. The S1R is involved in the regulation of many neurotransmission systems and, indirectly, in neurodegenerative diseases. The S1R may therefore represent an interesting neuronal biomarker in neurodegenerative diseases such as Parkinson's (PD) or Alzheimer's diseases (AD). Here we present the characterisation of the S1R-specific 18F-labelled tracer 18F-IAM6067 in two animal models and in human brain tissue. Methods: Wistar rats were used for PET-CT imaging (60 min dynamic acquisition) and metabolite analysis (1, 2, 5, 10, 20, 60 min post-injection). To verify in vivo selectivity, haloperidol, BD1047 (S1R ligand), CM398 (S2R ligand) and SB206553 (5HT2B/C antagonist) were administrated for pre-saturation studies. Excitotoxic lesions induced by intra-striatal injection of AMPA were also imaged by 18F-IAM6067 PET-CT to test the sensitivity of the methods in a well-established model of neuronal loss. Tracer brain uptake was also verified by autoradiography in rats and in a mouse model of PD (intrastriatal 6-hydroxydopamine (6-OHDA) unilateral lesion). Finally, human cortical binding was investigated by autoradiography in three groups of subjects (control subjects with Braak ≤2, and AD patients, Braak >2 & ≤4 and Braak >4 stages). Results: We demonstrate that despite rapid peripheral metabolism of 18F-IAM6067, radiolabelled metabolites were hardly detected in brain samples. Brain uptake of 18F-IAM6067 showed differences in S1R anatomical distribution, namely from high to low uptake: pons-raphe, thalamus medio-dorsal, substantia nigra, hypothalamus, cerebellum, cortical areas and striatum. Pre-saturation studies showed 79-90% blockade of the binding in all areas of the brain indicated above except with the 5HT2B/C antagonist SB206553 and S2R ligand CM398 which induced no significant blockade, indicating good specificity of 18F-IAM6067 for S1Rs. No difference between ipsi- and contralateral sides of the brain in the mouse model of PD was detected. AMPA lesion induced a significant 69% decrease in 18F-IAM6067 uptake in the globus pallidus matching the neuronal loss as measured by NeuN, but only a trend to decrease (-16%) in the caudate putamen despite a significant 91% decrease in neuronal count. Moreover, no difference in the human cortical binding was shown between AD groups and controls. Conclusion: This work shows that 18F-IAM6067 is a specific and selective S1R radiotracer. The absence or small changes in S1R detected here in animal models and human tissue warrants further investigations and suggests that S1R might not be the anticipated ideal biomarker for neuronal loss in neurodegenerative diseases such as AD and PD.

[Electroacupuncture protects the brain against acute ischemic injury via up-regulation of delta-opioid receptor in rats]

OBJECTIVE

To explore the effect of delta-opioid receptor (DOR) in electroacupuncture (EA) protecting the brain against acute ischemic injury.

METHODS

Fifty-one rats were randomly divided into sham ischemia group, ischemia group, sham EA group, EA group, and EA+DOR antagonist (naltrindole) group. Transient focal cerebral ischemia (1 hour) was induced in rat brain by middle cerebral artery occlusion (MCAO) method. EA was applied on Shuigou (GV 26) and Neiguan (PC 6) for 30 min, starting immediately after the onset of reperfusion. Neurological deficit scores and volume of cerebral infarction were detected after 24-hour reperfusion. Other 12 rats were randomly divided into sham ischemia group, ischemia group, EA group and EA + naltrindole group. DOR protein expressions were assessed by Western blotting after 24-hour reperfusion.

RESULTS

In comparison with the ischemia group and sham EA group, EA significantly reduced ischemic infarction and neurological deficits (P<0.05); EA significantly increased the expression of 60 kD DOR protein (P<0.05) and tended to increase that of 36 kD DOR protein (P>0.05). When naltrindole was combined with EA, the naltrindole completely abolished the EA-induced protection in ischemic infarction and neurological deficits, and also arrested the expression of DOR.

CONCLUSION

EA can up-regulate DOR expression and protect the brain from ischemia-reperfusion injury.

Ionic storm in hypoxic/ischemic stress: can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K(+) efflux and Na(+)-, Ca(2+)- and Cl(-)-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly delta-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na(+) influx through the membrane and reduces the increase in intracellular Ca(2+), thus decreasing the excessive leakage of intracellular K(+). Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na(+) channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.

In vivo administration of corticotropin-releasing hormone at remote intervals following ischemia enhances CA1 neuronal survival and recovery of spatial memory impairments: a role for opioid receptors

The contribution of corticotropin-releasing hormone (CRH) in the modulation of ischemia-induced cell death in vivo remains unclear. We characterized the impact of pre-ischemic administration of CRH (0, 0.1, 1, 5 microg, i.c.v., 15 min prior to vessel occlusion) on neuronal damage following global ischemia in rats. The injection of 5 microg CRH led to a 37% increase in CA1 neuronal survival compared to vehicle-treated ischemic animals, while pre-treatment with alpha-helical CRH (9-41) abolished this neuronal protection. A second objective aimed to determine whether CRH protection is maintained over weeks when the peptide is administered at remote time intervals following ischemia. Compared to vehicle-treated ischemic animals, administration of CRH 8h following global ischemia led to a 61% increase in CA1 neuronal survival observed 30 days post-ischemia. Neuronal protection translated into significant improvement of ischemia-induced spatial memory deficits in the radial maze. Finally, our findings demonstrated that selective blockade of kappa- and delta-opioid receptors (using nor-binaltorphimine and naltrindole, respectively) prior to CRH administration significantly reduced CA1 neuronal protection. These findings represent the first demonstration of enhanced neuronal survival following in vivo CRH administration in a global model of ischemia in rats. They also support the idea that CRH-induced neuroprotection involves opioid receptors activation.

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.

Potential use of early alterations in mu and delta opioid receptors as a predictive index for delayed brain ischemic damage

We previously reported differential alterations of the mu, delta, and kappa opioid receptors following permanent middle cerebral artery occlusion. The present work studied the evolution of opioid receptor types following transient focal cerebral ischemia (tMCAO), as well as the putative predictive potential of early neurochemical alterations on the delayed ischemic damage. delta receptors were significantly decreased as early as 6 h post tMCAO (-22% approximately -57% vs. sham group), followed by a decrease in the mu binding site density at 24 h post tMCAO (-18% approximately -65%), in infarcted and penumbral cortices. Finally, early decreases in cortical opioid mu and delta receptor densities were found to significantly correlate (P < 0.001, r(2) = 0.48 and 0.75, respectively) with the occurrence of delayed histological damage. The high correlation between decreases in mu and delta receptor densities at 6 h post tMCAO and the histological damage that occurred at 24 h post tMCAO suggests that these early neurochemical alterations could be used as predictive markers of delayed ischemic damage.

Exofocal alterations in opioidergic receptor densities following focal cerebral ischemia in the mouse

In previous studies of our group, we have reported differential alterations in opioidergic receptor subtypes densities in infarcted and periinfarcted brain tissue following middle cerebral artery occlusion (MCAO) in mice. Other studies have also described subcortical alterations consecutive to focal cortical ischemia. For a better understanding of ischemic processes in exofocal areas, we have investigated the evolution of opioidergic receptors following focal cortical ischemia through the quantification of relative binding densities, B(max) and K(d) values for the mu, delta, and kappa subtypes. Our results demonstrate that opioid receptor subtypes exhibit adaptations at distance from the ischemic core, mainly in the striatum, the thalamus, and the substantia nigra. Indeed, mu and delta B(max) values were increased in ventral thalamic nuclei, while kappa relative binding densities were transiently increased in nucleus medialis dorsalis and nucleus lateralis, pars posterior. Moreover, the B(max) of mu and delta receptors were transiently decreased at 6 h post-MCAO in ipsi- and contralateral patches and matrices of the striatum. Conversely, the mu B(max) values were increased in ipsi- and contralateral substantia nigra, pars compacta, and pars reticulata, 24 h following MCAO. In contralateral substantia nigra, pars compacta, kappa B(max) was found to be decreased at 24 h post-MCAO. These alterations could reflect neuronal dysfunction in exofocal brain structures, consecutively to the degeneration of defined neuroanatomical pathways. Our study indicates that opioidergic receptors could be used as markers of the neuronal reorganization that take place in subcortical areas following an ischemic insult of the brain cortex.

Endogenous opiates: 1998

This paper is the twenty-first installment of our annual review of research concerning the opiate system. It summarizes papers published during 1998 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

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.