Effect of the kappa-receptor agonist, U-50,488H, on cerebral ischemia-induced impairment of working memory assessed in rats by a three-panel runway task

GJ-4 ameliorates memory impairment in focal cerebral ischemia/reperfusion of rats via inhibiting JAK2/STAT1-mediated neuroinflammation

ETHNOPHARMACOLOGICAL RELEVANCE

Gardenia jasminoides J. Ellis (Fructus Gardenia) is a traditional Chinese medicine with diverse pharmacological functions, such as anti-inflammation, anti-depression, as well as improvement of cognition and ischemia brain injury. GJ-4 is a natural extract from Gardenia jasminoides J. Ellis (Fructus Gardenia) and has been proved to improve memory impairment in Alzheimer's disease (AD) mouse model in our previous studies.

AIM OF THE STUDY

This study aimed to evaluate the therapeutic effects of GJ-4 on vascular dementia (VD) and explore the potential mechanisms.

MATERIAL AND METHODS

In our experiment, a focal cerebral ischemia and reperfusion rat model was successfully developed by the middle cerebral artery occlusion and reperfusion (MCAO/R). GJ-4 (10 mg/kg, 25 mg/kg, 50 mg/kg) and nimodipine (10 mg/kg) were orally administered to rats once a day for consecutive 12 days. Learning and memory behavioral performance was assayed by step-down test and Morris water maze test. The neurological scoring test was performed to evaluate the neurological function of rats. 2,3,5-Triphenyltetrazolium chloride (TTC) staining and Nissl staining were respectively employed to determine the infarct condition and neuronal injury of the brain. Iba1 immunohistochemistry was used to show the activation of microglia. Moreover, the synaptic damage and inflammatory level were detected by Western blot.

RESULTS

GJ-4 could significantly improve memory impairment, cerebral infraction, as well as neurological deficits of VD rats induced by MCAO/R. Further research indicated VD-induced neuronal injury was alleviated by GJ-4. In addition, GJ-4 could protect synapse of VD rats by upregulating synaptophysin (SYP) expression, post synaptic density 95 protein (PSD95) expression, and downregulating N-Methyl-D-Aspartate receptor 1 (NMDAR1) expression. Subsequent investigation of the underlying mechanisms identified that GJ-4 could suppress neuroinflammatory responses, supported by inhibited activation of microglia and reduced expression of inflammatory proteins, which ultimately exerted neuroprotective effects on VD. Further mechanistic study indicated that janus kinase 2 (JAK2)/signal transducer and activator of transcription 1 (STAT1) pathway was inhibited by GJ-4 treatment.

CONCLUSION

These results suggested that GJ-4 might serve as a potential drug to improve VD. In addition, our study indicated that inhibition of neuroinflammation might be a promising target to treat VD.

Kappa-Opioid Receptor Selectivity for Ischemic Neuroprotection with BRL 52537 in Rats

Kappa-opioid receptors (KOR) have been implicated in neuroprotection from ischemic neuronal injury, but less work has been performed with transient focal cerebral ischemia to determine the role of KOR during reperfusion. We tested the effects of a selective and specific KOR agonist, BRL 52537 hydrochloride [(+/-)-1-(3,4-dichlorophenyl)acetyl-2-(1-pyrrolidinyl)methylpiperidine], and the standard KOR antagonist, nor-binaltorphimine dihydrochloride [nor-BNI; 17,17'-(dicyclopropylmethyl)-6,6',7,7'-6,6'-imino-7,7'-binorphinan-3,4',14,14'-tetrol], on functional and histological outcome after transient focal ischemia in the rat. By use of the intraluminal filament technique, halothane-anesthetized adult male Wistar rats were subjected to 2 h of middle cerebral artery occlusion confirmed by laser Doppler flowmetry. In a blinded, randomized fashion, rats were treated with 1). saline (vehicle) 15 min before reperfusion followed by saline at reperfusion for 22 h, 2). saline 15 min before reperfusion followed by BRL 52537 (1 mg x kg(-1) x h(-1)) at reperfusion for 22 h, 3). saline 15 min before reperfusion followed by nor-BNI (1 mg x kg(-1) x h(-1)) at reperfusion for 22 h, or 4) nor-BNI (1 mg/kg) 15 min before reperfusion followed by BRL 52537 (1 mgx kg(-1)x h(-1)) and nor-BNI (1 mg x kg(-1) x h(-1)) at reperfusion for 22 h. Infarct volume (percentage of ipsilateral structure) analyzed at 4 days of reperfusion was significantly attenuated in saline/BRL 52537 rats (n = 8; cortex, 10.2% +/- 4.3%; caudoputamen [CP], 23.8% +/- 6.7%) (mean +/- SEM) compared with saline/saline treatment (n = 8; cortex, 28.6% +/- 4.9%; CP, 53.3% +/- 5.8%). Addition of the specific KOR antagonist nor-BNI to BRL 52537 completely prevented the neuroprotection (n = 7; cortex, 28.6% +/- 5.3%; CP, 40.9% +/- 6.2%) conferred by BRL 52537. BRL 52537 did not produce postischemic hypothermia. These data demonstrate that KORs may provide a therapeutic target during early reperfusion after ischemic stroke.

IMPLICATIONS

The neuroprotective effect of selective kappa-opioid agonists in transient focal ischemia is via a selective action at the kappa-opioid receptors.

Neuroprotective kappa-opioid receptor agonist BRL 52537 attenuates ischemia-evoked nitric oxide production in vivo in rats

BACKGROUND AND PURPOSE

Kappa-opioid receptors (KOR) have been implicated in neuroprotection from ischemic neuronal injury. We tested the effects of a selective and specific KOR agonist, BRL 52537 hydrochloride [(+/-)-1-(3,4-dichlorophenyl)acetyl-2-(1-pyrrolidinyl) methylpiperidine], on infarct volume and nitric oxide production after transient focal ischemia in the rat.

METHODS

With the use of the intraluminal filament technique, halothane-anesthetized male Wistar rats (weight, 250 to 300 g) were subjected to 2 hours of focal cerebral ischemia confirmed by Doppler flowmetry. In a blinded randomized fashion, rats were treated with intravenous saline or 1 mg/kg per hour BRL 52537 infusion, initiated 15 minutes before occlusion and maintained until 2 hours of reperfusion. In a second experiment, rats were treated during reperfusion with saline or 1 mg/kg per hour BRL 52537, initiated at onset of reperfusion and continued for 22 hours. In a final experiment, in vivo striatal nitric oxide production was estimated via microdialysis by quantification of citrulline recovery after labeled arginine infusion in striatum of intravenous BRL 52537- or saline-treated rats.

RESULTS

In rats treated with BRL 52537 during ischemia and early reperfusion, infarct volume was significantly attenuated in cortex (16+/-6% versus 40+/-7% of ipsilateral cortex in saline group) and in caudoputamen (30+/-8% versus 66+/-6% of ipsilateral caudoputamen in saline group). Infarct volume was also reduced by treatment administered only during reperfusion in cortex (19+/-8% in BRL 52537 group [n=10] versus 38+/-6% in saline group) and in caudoputamen (35+/-9% versus 66+/-4% in saline group). BRL 52537 treatment markedly attenuated NO production in ischemic striatum compared with saline-treated controls.

CONCLUSIONS

These data demonstrate that (1) the selective KOR agonist BRL 52537 provides significant neuroprotection from focal cerebral ischemia when given as a pretreatment or as a posttreatment and (2) attenuation of ischemia-evoked nitric oxide production in vivo may represent one mechanism of ischemic neuroprotection.

Expression of the prodynorphin gene after experimental brain injury and its role in behavioral dysfunction

Traumatic brain injury (TBI) causes excess release of neurotransmitters, such as glutamate, and increases intracellular calcium levels. Elevated levels of calcium, and perhaps other intracellular second messengers, as a result of TBI can alter the expression of many genes. The protein products of some of these genes may be signals for TBI-associated memory dysfunction. Therefore, identification of genes whose expression is altered after TBI in the hippocampus, a structure in the medial temporal lobe that plays a critical role in memory formation and storage, and elucidation of the role(s) of their protein products may shed light on the molecular mechanisms underlying TBI-elicited memory dysfunction. The prodynorphin gene is expressed in hippocampal granule cells, and its expression has been reported to be enhanced as a result of elevated intracellular calcium. The prodynorphin protein is proteolytically cleaved to generate multiple dynorphin peptides, which can modulate neurotransmitter release through the activation of presynaptic kappa opioid receptors. In this study, we report that 1) TBI transiently increases prodynorphin mRNA in the hippocampus, 2) dynorphin peptide immunoreactivity is enhanced for up to 24 hr after TBI and 3) intracerebroventricular infusion of the kappa receptor antagonist nor-binaltorphimine (nor-BNI) impairs subsequent performance in a spatial memory task. These results suggest that dynorphin action may serve a beneficial role after TBI.

Mu-, delta- and kappa-opioid receptor populations are differentially altered in distinct areas of postmortem brains of Alzheimer's disease patients

The putative role of the opioid system in cognitive and memory functions prompted us to search for possible changes in the cohort of the major opioid receptors, mu, delta and kappa, in Alzheimer's disease. The present study examines alterations in opioid receptor levels by quantitative autoradiography. These experiments were carried out on coronal sections of postmortem brains from Alzheimer's disease patients and from aged-matched, dementia-free individuals. Brain sections were labeled with the tritiated forms of mu-, delta- and kappa-opioid ligands; DAMGO ([D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin), DPDPE ([D-Pen2,5]-enkephalin) and bremazocine (in the presence of mu- and delta-ligands), respectively. Nonspecific binding was determined in the presence of naloxone (10 microM). Brain areas analyzed were caudate, putamen, amygdaloid complex, hippocampal formation and various cerebral and cerebellar cortices. Image analyses of autoradiographs show, that in comparison to the same areas in control brain, statistically significant reductions in mu-opioid receptor binding occur in the subiculum and hippocampus of Alzheimer's disease brains. Binding of delta-opioid receptors is also decreased in the amygdaloid complex and ventral putamen of Alzheimer's disease brains. In contrast, large increases of kappa-opioid receptor binding are found in the dorsal and ventral putamen as well as in the cerebellar cortex of Alzheimer's disease brains. Levels of mu- delta- and kappa-opioid receptor binding are unaltered in the caudate, parahippocampal gyrus and occipito-temporal gyrus. These results may suggest an involvement of the endogenous opioid system in some of the multitude of effects that accompany this dementia.

Effects of mu- and delta-opioid-receptor antagonists on the stimulus properties of cholecystokinin

Melton and Riley recently reported that the relatively selective mu-opioid-antagonist naloxone potentiated the stimulus properties of the gut peptide cholecystokinin (CCK). To assess whether such opioid potentiation is limited to activity at the mu-receptor subtype, in the present experiment the effects of the highly selective delta-antagonist naltrindole on CCK's stimulus properties were examined. Because in the initial report of naloxone's potentiation of CCK a relatively high, nonphysiologic dose of CCK (i.e., 13 micrograms/kg) was used as the training drug, in the current analysis subjects were trained to discriminate 5.6 micrograms/kg CCK from its vehicle and the assessments and comparisons of the effects of naloxone and naltrindole were based on this dose. Specifically, rats were administered 5.6 micrograms/kg CCK before saccharin-LiCl pairings and the CCK vehicle before saccharin alone. With such training, they rapidly acquired the drug discrimination, avoiding saccharin consumption when it was preceded by CCK and consuming the same saccharin solution when it was preceded by its vehicle. In subsequent generalization tests, doses of CCK that were ineffective in suppressing saccharin consumption (i.e., did not substitute for the training dose of CCK) did result in the suppression of saccharin consumption when combined with doses of the mu antagonist naloxone that alone had no effect on saccharin intake. On the other hand, the highly selective delta-opioid-receptor antagonist naltrindole was ineffective in potentiating the effects of CCK. Specifically, when naltrindole was combined with ineffective doses of CCK, subjects drank at control levels. The ability of naloxone to potentiate CCK's stimulus effects is consistent with a range of other demonstrations of the role of the mu-opioid-receptor subtype in CCK-opioid interactions, although the specific basis for the interaction remains unknown. Given recent findings on the effects of delta agonists and antagonists on CCK-induced activity, the failure of naltrindole to potentiate CCK's stimulus effects may be due to the absence of delta activity within this preparation, rather than the absence of delta mediation of CCK-opioid interactions in general.

ZTTA, a postproline cleaving enzyme inhibitor, improves cerebral ischemia-induced deficits in a three-panel runway task in rats

We investigated the effect of N-benzyloxycarbonyl-thioprolyl-thioprolinal-dimethylaceta l (ZTTA), a novel postproline cleaving enzyme (prolyl endopeptidase, PPCE) inhibitor, on the in vitro activity of rat brain PPCE and memory impairment induced by cerebral ischemia. ZTTA noncompetitively inhibited rat brain PPCE (ki = 2.9 microM). Cerebral ischemia for 5 min increased the number of errors in a working memory task with a three-panel runway paradigm. ZTTA at 6 mg/kg, administered immediately after blood flow reperfusion, significantly reduced the increase in working memory errors expected to occur 24 h after 5 min of ischemia. The antiamnesic action of ZTTA may be ascribable to a neuroprotective effect on the central nervous system due to some neuropeptides that are substrates of PPCE in the brain.

The effects of chronic morphine on the generalization of buprenorphine stimulus control: an assessment of kappa antagonist activity

Rats trained to discriminate the mixed mu agonist/kappa antagonist buprenorphine from its vehicle generalize buprenorphine control to morphine. Buprenorphine, however, does not generalize to MR2266. The generalization to morphine suggests that buprenorphine's mu agonist properties mediated in part its discriminative control. The failure to generalize to MR2266, a compound reported to block kappa-mediated effects, however, suggests that its kappa antagonist activity was not involved in its discriminative effects. The ability of buprenorphine's mu (but not kappa) activity to establish stimulus control may be a function of the overshadowing of the kappa properties of buprenorphine by its concurrent mu activity. To test this possibility, in the present experiment rats were chronically exposed to morphine prior to buprenorphine discrimination training. This procedure has been reported to result in tolerance to buprenorphine's mu agonist effects and a more pronounced display of its kappa antagonist properties. The rats were then tested for the generalization of buprenorphine control to morphine, MR2266, and pentobarbital. As expected, buprenorphine failed to generalize to the nonopioid pentobarbital. Although subjects were tolerant to morphine (as evidenced by reductions in morphine-induced behavioral effects and a rightward shift in the doses of morphine substituting for buprenorphine), buprenorphine still failed to generalize to MR2266. The failure of buprenorphine to generalize to MR2266 under conditions that should have allowed for the development of stimulus control by buprenorphine's kappa antagonist activity was discussed in terms of the general inability of kappa antagonist activity to support discrimination learning.

kappa-Opioid receptor agonist protects against ischemic reduction of 2-deoxyglucose uptake in morphine-tolerant rats

We examined the effects of mu-opioid receptor agonist and antagonists, and kappa-opioid receptor agonist on the hypoxia/hypoglycemia-induced reduction in 2-deoxyglucose uptake of rat hippocampal slices. Naloxone, a mu-opioid receptor antagonist and (5,7,8)-(+)-3,4-dichloro-N-methyl-N-(7,8,1-pyrrolidinyl)-1-oxaspirol+ ++ (4,5)dec-8-yl)-benzeneacetamide methanesulfonate, U-62,066E, a kappa-opioid receptor receptor agonist, showed neuroprotective actions against the hypoxia/hypoglycemia-induced deficit in glucose uptake. In contrast, morphine exhibited an exacerbating action. These results suggest that blockade of mu-opioid receptor- and stimulation of kappa-opioid receptor-mediated functions has a protective role against the hypoxia/hypoglycemia-induced decreases in glucose metabolism in hippocampal slices. Chronic administration of morphine (10 mg/kg) for 9 days affected neither the basal nor the hypoxia/hypoglycemia-induced reduction in 2-deoxyglucose uptake. Rats treated with morphine chronically exhibited not only tolerance to the analgesic effect but also tolerance to the exacerbating action. However, chronic morphine did not modify U-62,066E-induced neuroprotection. These findings indicate that the receptor mechanisms of neuroprotection produced by the activation of kappa-opioid receptors may not be involved in mu-opioid receptor function.

Effects of naloxone, morphine and kappa-opioid receptor agonists on hypoxia/hypoglycemia-induced reduction of 2-deoxyglucose uptake in hippocampal slices from U-50,488H-tolerant rats

The aim of the present study was to determine whether U-50,488H and U-62,066E, kappa-opioid receptor agonists cause a neuroprotective action against hypoxia/hypoglycemia-induced reduction in 2-deoxyglucose (2-DG) uptake of hippocampal slices from U-50,488H-tolerant rats. Both U-50,488H and U-62,066E exhibited an attenuating effect on hypoxia/hypoglycemia-induced reduction in 2-DG uptake of hippocampal slices. Hypoxia/hypoglycemia-induced deficit of 2-DG uptake was prevented by cotreatment with naloxone, an opioid receptor antagonist, but potentiated by cotreatment with morphine, a mu-opioid receptor agonist. Chronic administration of U-50,488H resulted in the development of tolerance to the analgesic effect as well as the neuroprotective effect whereas this treatment affected neither basal- nor hypoxia/hypoglycemia-induced decreases in 2-DG uptake. Chronic administration of U-50,488H did not modify naloxone-induced attenuation of 2-DG uptake deficit but slightly potentiated the morphine-induced exacerbation. These findings suggest that the tolerance to kappa-opioid receptors does not affect the mu-opioid receptor-mediated neuroprotective or neurotoxic action.

Cognitive deficits induced by global cerebral ischaemia: relationship to brain damage and reversal by transplants

The CA1 and hilar fields of the hippocampus are highly vulnerable to lack of oxygen after interruption of blood flow to the brain. Severe anterograde memory loss, seen in a significant proportion of heart attack survivors, has been attributed to selective bilateral ischaemic damage to the hippocampus. Animal models of global ischaemia, induced by extracranial occlusion of the major ascending arteries, enable assessment of the neuropathological and functional consequences of transient interruption of cerebral blood flow, and can inform strategies to reduce or alleviate ischaemic brain damage. This review focuses firstly on the nature of cognitive deficits induced by global ischaemia, how far they are consistent with lesion-based accounts of hippocampal function, and the extent to which these deficits can be correlated with CA1 cell loss. The second focus of the review is to examine the limited evidence for graft-induced recovery of cognitive function in animals subjected to global ischaemia. Recent findings that grafted foetal cells from discrete hippocampal fields follow appropriate laminar routes to form functional connections with host neurons, and that growth factors protect cells from ischaemic damage, have suggested that CA1 or trophic grafts placed in the region of ischaemic CA1 cell loss might restore or protect this vulnerable sector, and reduce cognitive deficits.

U50488 reduces the severity of tissue damage in a rabbit model of focal cerebral ischemia

Many pharmacotherapies for stroke that have been successful in the laboratory have proven to be ineffective in the clinical setting, often because patients do not arrive for treatment until hours after the onset of the ischemic insult. Kappa opioid treatment of cerebral ischemia has been successful in the cat and mouse with treatment delays of up to 6 h. The purpose of the present study was to develop a model of delayed kappa opioid treatment of cerebral ischemia in the rabbit. Fourteen rabbits underwent permanent, unilateral occlusion of the internal carotid, middle cerebral, and anterior cerebral arteries via a transorbital, microsurgical approach. At 6 h postocclusion, animals received a blinded bolus injection and continuous infusion of either saline or the kappa agonist, U50488. Survival was not improved after U50488 treatment. U50488 treatment did, however, reduce areas of severe tissue damage and increase areas of modest tissue damage. This suggests U50488 arrested the progression of damage from noninfarcted to fully infarcted tissue. The present results show beneficial effects of delayed treatment with kappa agonists in a species similar in vasculature to humans, and much less costly than primates or cats.

Blockade of hippocampal M1 muscarinic receptors impairs working memory performance of rats

In order to clarify the roles of hippocampal M1 and M2 muscarinic receptors in working and reference memory performance of rats, the effects of intrahippocampal injections of selective antagonists at both receptors on this behavior were examined with a three-panel runway task. In the working memory task, the M1 muscarinic receptor antagonist pirenzepine, injected bilaterally at 0.32 and 1.0 microgram/side into the dorsal hippocampus, significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points). This effect of intrahippocampal pirenzepine (1.0 microgram/side) on working memory was attenuated by concurrent injection of 10 micrograms/side AF102B, the selective M1 muscarinic receptor agonist. Intrahippocampal injection of the M2 muscarinic receptor antagonist methoctramine at doses up to 1.0 microgram/side had no significant effect on the number of working memory errors. Intrahippocampal methoctramine injection at 3.2 micrograms/side produced a significant increase in working memory errors, an effect that was reversed by concurrent injection of 10 micrograms/side AF102B. Concurrent injection of 0.32 microgram/side methoctramine significantly reduced the increase in working memory errors induced by intrahippocampal pirenzepine (1.0 microgram/side). In the reference memory task, neither pirenzepine nor methoctramine affected the number of errors when injected into the hippocampus at doses up to 1.0 and 3.2 micrograms/side, respectively. These results suggest that processes mediated by M1 muscarinic receptors in the hippocampus are involved in working memory, but not in reference memory, and that blockade of hippocampal M2 muscarinic receptors ameliorates working memory deficits produced by M1 muscarinic blockade, possibly by increasing acetylcholine release.

Intrahippocampal administration of the NO synthase inhibitor L-NAME prevents working memory deficits in rats exposed to transient cerebral ischemia

A 5-min period of cerebral ischemia increased the number of errors in a working memory task with three-panel runway paradigm, while it had no effect on reference memory errors. The nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), infused into the bilateral dorsal hippocampus at 100 micrograms/side immediately after blood flow reperfusion, significantly reduced the increase in working memory errors expected to occur 24 h after 5 min of ischemia. Intrahippocampal administration of the inactive isomer D-NAME at 100 micrograms/side immediately after reperfusion had no effect on the increase in working memory errors in the ischemic rats. These findings suggest that the mechanism mediated by hippocampal NO synthesis during the early reperfusion phase contributes to the postischemic impairment of working memory.

Evaluation of neuroprotection and behavioral recovery by the kappa-opioid, PD117302 following transient forebrain ischemia

The effects of the selective kappa-opioid, PD117302 ((+/-)-trans-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]benzo[b]thiophene-4-acetamide), on transient (15 min) global forebrain ischemia, induced by four-vessel occlusion, was evaluated using a multiple fixed-ratio, fixed-interval schedule of food presentation in rats. The schedule produced distinctive patterns of responding in the fixed-ratio and fixed-interval components. Ischemia produced CA1 hippocampal necrosis and prolonged suppression of responding under both schedule components. When responding resumed, the pattern of responding rapidly returned. Response disruption and CA1 hippocampal necrosis were minimal or nonexistent in sham-occluded rats. Behavioral recovery time under both components of the schedule of reinforcement correlated with CA1 necrosis. On average, CA1 necrosis was less, and behavioral recovery time was shorter, in rats treated with PD117302 postocclusion as compared with vehicle-treated rats. The difference, however, did not reach statistical significance. These results demonstrate the utility of schedule-controlled responding for evaluating potentially therapeutic compounds for the treatment of ischemic injury. These results also further characterize the neuroprotective actions of kappa opioids.

Deficits in working memory following inhibition of hippocampal nitric oxide synthesis in the rat

In order to elucidate the roles of hippocampal nitric oxide (NO) synthesis in working and reference memory performance of rats, the effects of intrahippocampal injections of the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME), on this behavior were examined with a three-panel runway task. In the working memory task, L-NAME, injected bilaterally at 10 and 32 micrograms/side into the dorsal hippocampus, significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points). This effect of intrahippocampal L-NAME (32 micrograms/side) on working memory was attenuated by concurrent injection of 100 micrograms/side L-arginine, the precursor of NO. Intrahippocampal injection of the inactive isomer D-NAME at doses up to 32 micrograms/side had no effect on the number of working memory errors. In the reference memory task, neither L-NAME nor D-NAME affected the number of errors when injected into the hippocampus at doses up to 32 micrograms/side. These results suggest that processes mediated by NO synthesis in the hippocampus are involved in working memory, but not in reference memory.

Characteristics of transient cerebral ischemia-induced deficits on various learning and memory tasks in male Mongolian gerbils

We examined the characteristics of 5-min cerebral ischemia-induced behavioral deficits in spontaneous locomotor activity and their effects on the performance of habituation (HAB), passive avoidance (PA) and 8-arm radial maze (RM) tasks in Mongolian gerbils. Performances in HAB, PA and RM were impaired within 2 days after occlusion, and gerbils showed hyperlocomotion during this period. Ten days after ischemia, the hyperlocomotion disappeared and performance in the HAB and PA was the same as that in the sham-operated group. Retention in the RM was impaired at that period, but this impairment was overcome, and retention recovered easily to the sham-operated level with a few additional trials. When the acquisition trial in the RM began at 11 days after occlusion, severe learning impairment was found. Destruction of hippocampal CA1 neurons appears from 2-3 days after ischemic insult, with most CA1 neurons having disappeared by day 7. These findings suggest that the impairment of performance in the HAB and PA within 2 days after occlusion may be related to an early phase of CA1 neuronal death and to hyperlocomotion, although the impairment of spatial learning and memory was clearly associated with CA1 injury 10 days after ischemia.

Amygdaloid NMDA and muscarinic receptors involved in working memory performance of rats

In a three-panel runway task, the competitive N-methyl-D-aspartate (NMDA) receptor antagonist, 3-[(+-)-2-carboxypiperazin-4-yl]propyl-1-phosphonic acid (CPP) (56 ng/side), injected bilaterally into the basolateral subdivision of the amygdala, significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel gates at four choice points) in a test of working memory. Intra-amygdaloid injection of the muscarinic receptor antagonist scopolamine (5.6 micrograms/side) also significantly increased working memory errors. In a test of reference memory, neither CPP nor scopolamine affected the number of errors when injected into the basolateral amygdala at doses up to 56 ng/side or 5.6 micrograms/side, respectively. These results indicate that processes mediated by NMDA and muscarinic receptors in the basolateral amygdala are involved in the performance of working memory, but not in reference memory.

Effect of U-50488h, a selective opioid kappa receptor agonist, on vascular injury after spinal cord trauma

U-50488H, a selective opioid kappa receptor agonist has been shown to be a neuroprotective agent in animal models of spinal cord injury. The mechanism of action of U-50488H is not known. Methylprednisolone, the only neuroprotective drug proven in patients with acute spinal cord injury may prevent the secondary injury after an initial trauma. Secondary vascular injury develops after experimental spinal cord trauma. In this study we examined the effects of U-50488H on post-traumatic vascular injury based on the measurement of vascular permeability, edema and neutrophil infiltration in a rat spinal cord injury model. Vascular permeability was assessed by vascular extravasation of fluorescein isothiocyanate conjugated dextran (FITC-D), a macromolecular tracer. Tissue edema was determined by percentage water content and neutrophil infiltration by myeloperoxidase (MPO) activity, a marker enzyme for neutrophils. U-50488H at doses of 5, 10, 20 and 40 mg/kg i.p. administered twice (0.5 h before and 0.5 h after trauma) reduced vascular permeability in a dose-dependent manner. More frequent dosing (10 mg/kg, 0.5 h before and 0.5, 2, 8 and 22 h after injury) reduced vascular permeability 24 h after injury. U-50488H also reduced edema formation but did not affect neutrophil infiltration. Results from this study raise the possibility that the neuroprotective effect of U-50488H involves a secondary vascular event.

U-50,488H, a kappa-opioid receptor agonist, markedly prevents memory dysfunctions induced by transient cerebral ischemia in mice

Transient ischemia produced marked memory dysfunctions in mice on three different tasks, spontaneous alternation, elevated plus-maze and passive avoidance, as tested 1, 1-2, and 2-3 days after ischemic insult, respectively. U-50,488H, a kappa-opioid receptor agonist, administered 20 min before ischemic insult markedly prevented the impairment of spontaneous alternation, the prolongation of transfer latency in elevated-plus maze and the shortening of step-through latency in passive avoidance induced by transient ischemia. The protective effect of U-50,488H (30 mg/kg) on ischemia-induced memory dysfunctions observed in the three tasks was almost completely reversed by pretreatment with nor-binaltorphimine (4 micrograms, i.c.v.), a kappa-selective opioid antagonist. Although U-50,488H (30 mg/kg) did not affect body temperature in sham mice, it blocked hypothermia induced by ischemic insult. These results suggest that the protective effect of U-50,488H on memory dysfunctions in ischemic mice is associated with the activation of kappa-opioid receptors and is not based upon hypothermia.

Working memory deficits following muscarinic blockade combined with depletion of brain somatostatin in rats

In a working memory task with three-panel runway paradigm, cysteamine, a depletor of somatostatin, at 100 or 200 mg/kg i.p. given 24 h before testing, had no effect on the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points). Cysteamine at 100 mg/kg caused a significant reduction in somatostatin-like immunoreactivity in the rat brain, including the hippocampus and cerebral cortex. Working memory errors were significantly increased by scopolamine, a muscarinic receptor antagonist, at 0.32 mg/kg i.p. given 20 min before testing, whereas errors were not affected by the 0.1 mg/kg dose. Combined administration of 100 mg/kg cysteamine and 0.1 mg/kg scopolamine significantly increased the number of working memory errors. However, cysteamine at 100 mg/kg and scopolamine at 0.1 mg/kg had no effect on reference memory errors, whether they were administered alone or in combination. These results suggest that depletion of brain somatostatin aggravates working memory deficits induced by blockade of muscarinic receptors.

Blockade of hippocampal nicotinic receptors impairs working memory but not reference memory in rats

In a three-panel runway task, intrahippocampal injection of the nicotinic receptor antagonist, mecamylamine (10 and 18 micrograms/side), significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) in a test of working memory. This increase in errors also occurred after rats were given IP mecamylamine (10 mg/kg). Mecamylamine did not affect the number of errors in a test of reference memory whether it was given at doses up to 18 micrograms/side intrahippocampally or up to 10 mg/kg IP. These results suggest that mechanisms mediated by hippocampal nicotinic receptors play a role in working memory but not in reference memory.

Kappa-opioid receptors on astrocytes stimulate L-type Ca2+ channels

Cultured astrocytes from the cerebral cortex of the rat respond to opioid kappa-receptor stimulation with a substantial elevation of the cytoplasmic free calcium, visualized through the use of the fluorescent calcium indicator Fura-2. The stimulation of kappa-receptors with U-50488H increases the level of calcium through a dose-related stimulatory effect on the transmembrane calcium influx. The kappa-receptor stimulation was completely blocked by the selective kappa-receptor blocker nor-binaltorphimine. Furthermore, the transmembrane calcium influx was completely blocked by nifedipine, indicating the involvement of L-type channels. The presence of L-type channels was verified by stimulation of L-type channels with Bay K8644. The effects of Bay K8644 were completely blocked by nifedipine. L-type channel-coupled kappa-receptors on astrocytes might represent a novel mechanism contributing to the depressant action of opioids on synaptic transmission via decreasing the availability of extracellular calcium necessary for presynaptic transmitter release.

Working memory deficits induced by intrahippocampal administration of 8-OH-DPAT, a 5-HT1A receptor agonist, in the rat

In a test of working memory using a three-panel runway task, the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), at 1.0 mg/kg i.p. significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel gates at four choice points), but it had no effect on errors in a test of reference memory. Intrahippocampal injection of 8-OH-DPAT at 10 micrograms/side also significantly increased the number of working memory errors, without affecting that of reference memory errors. The effect of intrahippocampal 8-OH-DPAT (10 micrograms/side) on working memory was blocked by the 5-HT1A receptor antagonist, (-)-propranolol, at 5 mg/kg i.p. These results suggest that activation of hippocampal 5-HT1A receptors impairs working memory, but does not affect retention of reference memory.

Dynorphin A-(1-13) potently prevents memory dysfunctions induced by transient cerebral ischemia in mice

The effect of dynorphin A-(1-13), an endogenous kappa-opioid receptor agonist, on memory dysfunctions induced by transient cerebral ischemia in mice was investigated by using three different tasks, namely, spontaneous alternation, elevated plus-maze performance, and passive avoidance behavior. Transient ischemia produced a marked memory dysfunction in mice, as assessed in the three tasks, which were carried out consecutively 1 to 3 days after the ischemic insult. The i.c.v. injection of dynorphin A-(1-13) before the ischemic insult potently prevented the impairment of spontaneous alternations, the prolongation of transfer latency in the elevated plus-maze and the shortening of step-through latency in the passive avoidance task induced by transient ischemia. Dynorphin A-(1-13) (10 micrograms), however, did not affect the body temperature of the sham-operated or the ischemic mice. The protective effect of dynorphin A-(1-13) (10 micrograms) on ischemia-induced memory dysfunctions observed in the three tasks was almost completely reversed by pretreatment with nor-binaltorphimine (4 micrograms, i.c.v.), a kappa-selective opioid receptor antagonist. These results suggest that dynorphin A-(1-13) prevents memory dysfunctions in ischemic mice through the activation of kappa-opioid receptors.

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.

Evaluation of transient forebrain ischemia induced by four vessel occlusion using schedule-controlled behavior

Transient (5-min) global forebrain ischemia, induced by four- vessel occlusion, was assessed using a multiple fixed-ratio, fixed-interval schedule of food presentation in five rats. Under control conditions, the schedule produced distinctive response rates in each schedule component. Initially, ischemia disrupted responding under both schedule components, and to approximately the same degree. In general, total session responses returned to pre-occlusion levels during the course of 45 post-occlusion days, however, response rates under the fixed-interval component showed slightly less recovery than those under the fixed-ratio component. Histological assessment revealed considerable variability in hippocampal damage between rats. Severe damage in the CA1, CA2, and CA3 formations was observed in a single rat, and that rat also showed the greatest degree of response disruption. These results suggest that schedule-controlled responding may be a valuable method for assessing the effects of ischemic injury, and thus, putative neuroprotective compounds, on complex behavior.

Involvement of cholinergic mechanisms in impairment of working memory in rats following basolateral amygdaloid lesions

In order to clarify the role of the amygdala in the working and reference memory of rats in the three-panel runway task, the effects of lesions of subnuclei of the amygdaloid complex on this behavior were studied. Rats that had been trained preoperatively, until they achieved the criterion of learning, were subjected to lesions of the amygdala. In the working memory task, lesions of the basolateral subdivision of the amygdala caused a significant increase in the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points), while lesions of the corticomedial amygdala had no effect on working memory errors. The increase in working errors, observed in basolateral amygdaloid-lesioned rats, declined gradually as retraining sessions were given once each day, reverting to control levels on and after the sixth session. In the reference memory task, the number of errors was not affected by lesions of the basolateral or corticomedial amygdala. The increase in working memory errors, induced by lesions of the basolateral amygdala was significantly reduced by intraperitoneal administration of the inhibitors of cholinesterase, tetrahydroaminoacridine (0.32-1.0 mg/kg) and physostigmine (0.032-0.1 mg/kg), and the muscarinic receptor agonist, oxotremorine (0.1 mg/kg), before the runway test. These findings suggest that the basolateral amygdala is selectively involved in working memory but not in reference memory and that the lowering of central cholinergic function may account for the impairment of working memory, induced by lesions of the basolateral amygdala.

Intrahippocampal injections of benzodiazepine and muscimol impair working memory but not reference memory of rats in the three-panel runway task

In a three-panel runway task, the benzodiazepine chlordiazepoxide at 3.2 and 10 mg/kg i.p. significantly increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) in a test of working memory, but it had no effect on errors in a test of reference memory. This effect of 10 mg/kg chlordiazepoxide on working memory was blocked by the benzodiazepine receptor antagonist flumazenil at 10 mg/kg. Intrahippocampal injection of chlordiazepoxide at 10 and 32 micrograms/side significantly increased the number of working memory errors. This effect of intrahippocampal chlordiazepoxide (32 micrograms/side) was attenuated not only by flumazenil at 10 mg/kg but also by the gamma-aminobutyric acid (GABA)A receptor antagonist bicuculline at 3.2 mg/kg. Intrahippocampal injection of the GABAA receptor agonist muscimol at 100 and 320 ng/side also significantly increased working memory errors. Neither chlordiazepoxide nor muscimol affected the number of reference memory errors when injected into the hippocampus at doses up to 32 micrograms/side or 320 ng/side, respectively. These results suggest that activation of the GABAA/benzodiazepine receptor complex in the hippocampus impairs working memory, but does not affect reference memory.

Protection by N-methyl-D-aspartate receptor antagonists against impairment of working memory in rats following transient cerebral ischemia

A 5-min period of cerebral ischemia increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) assessed by the working memory procedure applied in a three-panel runway task. The selective and competitive N-methyl-D-aspartate (NMDA) receptor antagonist CGS 19755 (3.2 and 10 mg/kg), administered i.p. immediately after blood flow reperfusion, significantly reduced the increase in errors expected to occur 24 h after 5 min of ischemia. CGS 19755 10 mg/kg had no effect on the increase in errors when injected 6 h after ischemia. The i.p. administration of the non-competitive NMDA antagonists dextrorphan 10 and 32 mg/kg and MK-801 1.0 mg/kg immediately after reperfusion decreased the increase of errors in the ischemic rats. The protective effects of NMDA antagonists suggest that the mechanism mediated by NMDA receptors during the early reperfusion phase plays a pivotal role in the postischemic impairment of working memory.

Effect of staurosporine, a protein kinase C inhibitor, on impairment of working memory in rats exposed to cerebral ischemia

A 5-min period of cerebral ischemia induced in rats by the four-vessel occlusion method increased the number of errors (attempts to pass through two incorrect panels of the three panel-gates at four choice points) assessed by a working memory procedure applied in a three-panel runway task. The protein kinase C (PKC) inhibitor, staurosporine 0.03 and 0.1 mg/kg, administered immediately after blood flow reperfusion, significantly reduced the increase in errors expected to occur 24 h after 5 min of ischemia. However, administration of staurosporine 0.1 mg/kg 6 h after ischemia had no effect on the increase in errors. The protective effect of staurosporine suggests that the enhanced PKC activity during the early reperfusion phase plays a crucial role in the post-ischemic impairment of working memory.

Blockade of 5-HT2 receptors protects against impairment of working memory following transient forebrain ischemia in the rat

The effects of the 5-HT2 receptor antagonists pirenperone, cinanserin and ritanserin on impairment of working memory in an animal model of cerebral ischemia were investigated, using a three-panel runway task. A 5-min period of ischemia caused a significant increase in the number of errors (attempts to pass through two incorrect panels of the 3 panel gates at 4 choice points). Pirenperone at 0.32 and 1.0 mg/kg, cinanserin 10 mg/kg and ritanserin 3.2 mg/kg administered i.p. immediately after blood flow reperfusion significantly reduced the increase in errors expected to occur 24 h after the 5 min of ischemia. These results suggest that the blockade of 5-HT2 receptors prevents the impairment of working memory following transient forebrain ischemia.