U-50,488H inhibits dynorphin and glutamate release from guinea pig hippocampal mossy fiber terminals

κ-Opioid Signaling in the Lateral Hypothalamic Area Modulates Nicotine-Induced Negative Energy Balance

Several studies have reported that nicotine, the main bioactive component of tobacco, exerts a marked negative energy balance. Apart from its anorectic action, nicotine also modulates energy expenditure, by regulating brown adipose tissue (BAT) thermogenesis and white adipose tissue (WAT) browning. These effects are mainly controlled at the central level by modulation of hypothalamic neuropeptide systems and energy sensors, such as AMP-activated protein kinase (AMPK). In this study, we aimed to investigate the kappa opioid receptor (κOR)/dynorphin signaling in the modulation of nicotine’s effects on energy balance. We found that body weight loss after nicotine treatment is associated with a down-regulation of the κOR endogenous ligand dynorphin precursor and with a marked reduction in κOR signaling and the p70 S6 kinase/ribosomal protein S6 (S6K/rpS6) pathway in the lateral hypothalamic area (LHA). The inhibition of these pathways by nicotine was completely blunted in κOR deficient mice, after central pharmacological blockade of κOR, and in rodents where κOR was genetically knocked down specifically in the LHA. Moreover, κOR-mediated nicotine effects on body weight do not depend on orexin. These data unravel a new central regulatory pathway modulating nicotine’s effects on energy balance.

Pre-clinical to Clinical Translational Failures and Current Status of Clinical Trials in Stroke Therapy: A Brief Review

In stroke (cerebral ischemia), despite continuous efforts both at the experimental and clinical level, the only approved pharmacological treatment has been restricted to tissue plasminogen activator (tPA). Stroke is the leading cause of functional disability and mortality throughout worldwide. Its pathophysiology starts with energy pump failure, followed by complex signaling cascade that ultimately ends in neuronal cell death. Ischemic cascade involves excessive glutamate release followed by raised intracellular sodium and calcium influx along with free radicals' generation, activation of inflammatory cytokines, NO synthases, lipases, endonucleases and other apoptotic pathways leading to cell edema and death. At the pre-clinical stage, several agents have been tried and proven as an effective neuroprotectant in animal models of ischemia. However, these agents failed to show convincing results in terms of efficacy and safety when the trials were conducted in humans following stroke. This article highlights the various agents which have been tried in the past but failed to translate into stroke therapy along with key points that are responsible for the lagging of experimental success to translational failure in stroke treatment.

Opioid system and Alzheimer's disease

The opioid system may be involved in the pathogenesis of AD, including cognitive impairment, hyperphosphorylated tau, Aβ production, and neuroinflammation. Opioid receptors influence the regulation of neurotransmitters such as acetylcholine, norepinephrine, GABA, glutamate, and serotonin which have been implicated in the pathogenesis of AD. Opioid system has a close relation with Aβ generation since dysfunction of opioid receptors retards the endocytosis and degradation of BACE1 and γ-secretase and upregulates BACE1 and γ-secretase, and subsequently, the production of Aβ. Conversely, activation of opioid receptors increases the endocytosis of BACE1 and γ-secretase and downregulates BACE1 and γ-secretase, limiting the production of Aβ. The dysfunction of opioid system (opioid receptors and opioid peptides) may contribute to hyperphosphorylation of tau and neuroinflammation, and accounts for the degeneration of cholinergic neurons and cognitive impairment. Thus, the opioid system is potentially related to AD pathology and may be a very attractive drug target for novel pharmacotherapies of AD.

Presynaptic neuropeptide receptors

Presynaptic receptors for four families of neuropeptides will be discussed: opioids, neuropeptide Y, adrenocorticotropic hormone (ACTH), and orexins. Presynaptic receptors for the opioids (micro, delta, kappa, and ORL(1)) and neuropeptide Y (Y(2)) inhibit transmitter release from a variety of neurones, both in the peripheral and central nervous systems. These receptors, which were also identified in human tissue, are coupled to G(i/o) proteins and block voltage-dependent Ca(2+) channels, activate voltage-dependent K(+) channels, and/or interfere with the vesicle release machinery. Presynaptic receptors for ACTH (MC(2) receptors) have so far been identified almost exclusively in cardiovascular tissues from rabbits, where they facilitate noradrenaline release; they are coupled to G(s) protein and act via stimulation of adenylyl cyclase. Presynaptic receptors for orexins (most probably OX(2) receptors) have so far almost exclusively been identified in the rat and mouse brain, where they facilitate the release of glutamate and gamma-aminobutyric acid (GABA); they are most probably linked to G(q) and directly activate the vesicle release machinery or act via a transduction mechanism upstream of the release process. Agonists and antagonists at opioid receptors owe at least part of their therapeutic effects to actions on presynaptic receptors. Therapeutic drugs targeting neuropeptide Y and orexin receptors and presynaptic ACTH receptors so far are not available.

The antinociceptive effect of salvinorin A in mice

Salvia divinorum is a hallucinogenic plant used by the Mazatec Indians of Mexico for traditional spiritual ceremonies. The active constituent, salvinorin A, induces profound hallucinations, however the biological mechanism for this action is not known. Affinity-binding studies suggest that the biologic activity of salvinorin A involves the kappa-opioid receptor. The purpose of this study was to evaluate the antinociceptive effect of salvinorin A in mice. Salvinorin A and opioid receptor antagonists were administered intrathecally and the tail-flick latencies were used as a measure of antinociception. Salvinorin A increased tail-flick latencies in a dose-dependent manner (13.9-23.1 nmol) compared to control trials. Pretreatment with the kappa-opioid receptor antagonist nor-binaltorphimine attenuated the salvinorin A induced increase in tail-flick latency. In contrast, neither the mu-opioid receptor antagonist beta-funaltrexamine nor delta-opioid receptor antagonist naltrindole significantly affected the antinociceptive response of salvinorin A administration. These data support previous reports that salvinorin A represents a unique non-alkaloidal agonist for the kappa-opioid receptor.

Epilepsy, CNS viral injury and dynorphin

Epilepsy is a significant health problem. Despite the widespread use of both classic and newer pharmacological agents that target ion channels, amino acid transmission or receptors, there are numerous examples of mono- or polytherapy being ineffective. Seizures that are secondary to CNS infections are among the most refractory medically, and thus insult-specific agents are desirable. Recently, the study of the neuropharmacological actions of dynorphin in CNS viral injury has yielded new insights into epileptogenesis and epilepsy treatment. The opioid neuropeptide dynorphin modulates neuronal excitability in vitro in hippocampal slices and potentiates endogenous anti-ictal (i.e. protective) processes in animal models and humans. This work has renewed interest in the role of dysregulation of dynorphin in the pathogenesis of refractory seizures, including encephalitic seizures. The important role of dynorphin in epilepsy is also supported by new models of symptomatic epilepsies based on viral-induced seizures.

Mu opioid modulation of oxytocin secretion in late pregnant and parturient rats. Involvement of noradrenergic neurotransmission

We have investigated effects of micro- and kappa-opioid agonists and antagonists on plasma oxytocin levels and noradrenaline content in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of 20-day pregnant rats. beta-Endorphin, oxytocin, estrogen and progesterone profiles in late pregnant and parturient rats were also sought. Stage of estrous cycle was monitored by vaginal smear, and pro-estrous animals were left overnight with male. In the first set of experiments, pregnant rats were monitored and decapitated on days 20 and 21 and after the delivery of second pup. In the second set, 20-day pregnant rats were intracerebroventricularly infused with morphine (50 microg/10 microl), U50,488H (kappa-agonist; 50 microg/10 microl), clocinnamox (micro-antagonist; 50 microg/10 microl) and norbinaltorphimine (kappa-antagonist; 50 microg/10 microl). Controls received saline alone. Serum estrogen and progesterone levels were measured by enzyme immunoassay, and plasma oxytocin and beta-endorphin by radioimmunoassay. Noradrenaline and its metabolite (3,4-dihydroxyphenylglycol) were determined in micropunched hypothalamic nuclei by HPLC-ECD. In parturient rats, oxytocin levels were increased (p < 0.05) and beta-endorphin decreased (p < 0.01) compared to 20-day pregnant animals. Serum progesterone concentrations progressively declined towards parturition (p < 0.001). Clocinnamox raised oxytocin levels (p < 0.01) while U50,488H caused decreases (p < 0.05). Noradrenaline content was elevated by clocinnamox in the SON (p < 0.01) and PVN (p < 0.05) compared to control values. Other agonists and antagonists had no significant effect on the noradrenergic neurotransmission or oxytocin secretion. We suggest that noradrenaline may mediate the inhibitory effects of micro-opioids on oxytocin release. Our findings have also shown that kappa-opioid receptors are not involved in modulation of oxytocin neurons in late pregnant rats.

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.

Effects of dynorphin on rat entopeduncular nucleus neurons in vitro

The entopeduncular nucleus (EP) receives dense neostriatal afferent axons that contain dynorphin (DYN, an endogenous kappa-receptor agonist), in addition to GABA and substance P. To examine the role of DYN in the EP, whole-cell recordings were performed in rat brain slice preparations. Based on the physiological and morphological characteristics, all the neurons recorded were similar to the Type-I EP neuron described in a previous study. The kappa-receptor agonist dynorphin A (1-13) (DYN13) hyperpolarized and decreased the input resistance of approximately one-quarter of the EP neurons examined. The hyperpolarization was due to an increase in potassium conductance since current-voltage relationship curves obtained before and after DYN13 application crossed at the potassium equilibrium potential. In the presence of the glutamate blocker 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide and 3-(2-carboxypiperzin-4-yl)-propyl-1-phosphonic acid in artificial cerebrospinal fluid, stimulation of the globus pallidus evoked bicuculline-sensitive multi-component GABAergic responses in EP neurons. Application of DYN13 equally reduced the amplitudes of the short-latency response, conceivably evoked by pallido-EP axons, and the medium-latency response, conceivably evoked by striato-EP axons. These effects were reversed by bath application of a non-selective opioid antagonist naloxone or by a kappa-opioid receptor-selective antagonist nor-binaltorphimine dihydrochloride (nor-BNI), but not by the partial differential -antagonist naltrindole or the mu-antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2). DYN13 also reduced the frequency of tetrodotoxin-insensitive miniature-inhibitory postsynaptic potential (mIPSPs) without changing their amplitude distributions. The decrease of the frequency of mIPSPs was reversible upon washing and was also completely blocked by nor-BNI. The results of the present study on the EP indicated that DYN released from striatal axons might exert at least three different effects on these target nuclei. Firstly, DYN might provide negative feedback regulation of striatal GABAergic outputs at their termination sites. Secondly, DYN released from the striatal terminals might diffuse to the pallidal terminals, regulating their GABA release. Thirdly, DYN might exert a direct inhibition of EP neurons. Thus, DYN released from striatal axons might control the activity of EP neurons by reducing the GABAergic transmission and also by hyperpolarizing postsynaptic membrane.

Kappa-Opioid and NMDA glutamate receptors are differentially targeted within rat medial prefrontal cortex

Activation of kappa-opioid receptors (KOR) in the medial prefrontal cortex (mPFC) modulates excitatory transmission, which may involve interactions with N-methyl-D-aspartate (NMDA) glutamate receptors. We investigated possible anatomical correlates of this modulation by using dual labeling electron microscopy to examine the cellular distributions of antibodies raised against KOR and the R1 subunit of the NMDA receptor (NR1). KOR immunoreactivity primarily was localized to plasma and vesicular membranes of axons and axon terminals that were morphologically heterogeneous. A small proportion of KOR immunoreactivity was associated with cytosolic compartments of dendrites and membranes of glial processes. NR1 labeling was mainly postsynaptic, associated most often with membranes of cytoplasmic organelles in cell bodies and large dendrites and plasmalemmal surfaces of distal dendrites. The remaining NR1-labeled profiles were axonal profiles and glial processes. Of all cellular associations between labeled profiles, the majority were KOR-labeled axons that contacted NR1-immunoreactive dendrites or cell bodies. Occasionally the two antigens were colocalized in axon terminals that formed either asymmetric synapses or displayed varicose morphology. KOR and NR1 also were colocalized within dendrites, and rarely were observed in the same cell bodies. Occasionally glial processes coursing adjacent to axo-spinous appositions expressed both KOR and NR1 immunoreactivity. These results indicate that ligand activation of KOR or NMDA receptors differentially modulates excitatory transmission in the mPFC through pre- and postsynaptic mechanisms, respectively. The data also suggest more minor roles for colocalized KOR and NMDA receptors in shared regulation of presynaptic transmitter release, postsynaptic responsivity, and glial function.

Subjects with major depression or bipolar disorder show reduction of prodynorphin mRNA expression in discrete nuclei of the amygdaloid complex

The dynorphin system has been associated with the regulation of mood. The expression of the prodynorphin mRNA was currently studied in the amygdaloid complex, a brain region critical for emotional processing, in subjects (14-15 per group) diagnosed with major depression, bipolar disorder, or schizophrenia and compared to normal controls. In situ hybridization histochemistry was used to characterize the anatomical distribution and expression levels of the prodynorphin mRNA within the amygdaloid complex. High prodynorphin mRNA levels were expressed in the parvicellular division of the accessory basal, posterior cortical, periamygdaloid cortex, and amygdalohippocampal area in normal subjects. Individuals with major depression had significantly reduced (41-68%) expression of the prodynorphin mRNA in the accessory basal (both parvicellular and magnocellular divisions; P < 0.01) and amygdalohippocampal area (P < 0.001) as compared to controls. The bipolar disorder group also showed a significant reduction (37-38%, P < 0.01) of the mRNA expression levels in the amygdalohippocampal area and in the parvicellular division of the accessory basal. No other amygdala nuclei studied showed any significant differences for the prodynorphin mRNA levels measured in the major depression and bipolar disorder subjects. Additionally, the prodynorphin mRNA expression levels did not differ significantly between the schizophrenic and normal control subjects in any of the amygdala areas examined. These findings indicate specific prodynorphin amygdala impairment in association with mood disorder.

Major coexpression of kappa-opioid receptors and the dopamine transporter in nucleus accumbens axonal profiles

The behavioral effects of psychostimulants, which are produced at least in part through inhibition of the dopamine transporter (DAT), are modulated by kappa-opioid receptors (KOR) in the nucleus accumbens (Acb). Using electron microscopic immunocytochemistry, we reveal that in the Acb KOR labeling is mainly, and DAT immunoreactivity is exclusively, presynaptic. From 400 KOR-labeled presynaptic structures, including axon terminals, intervaricosities, and small axons, 51% expressed DAT and 29% contacted another population of terminals exclusively labeled for DAT. Within axonal profiles that contained both antigens, DAT and KOR were prominently localized to plasma membrane segments that showed overlapping distributions of the respective immunogold-silver and immunoperoxidase markers. KOR labeling was also localized to membranes of small synaptic vesicles in terminals with or without DAT immunoreactivity. In addition, from 24 KOR-immunoreactive dendritic spines 42% received convergent input from DAT-containing varicosities and unlabeled terminals forming asymmetric, excitatory-type synapses. Our results provide the first ultrastructural evidence that in the Acb, KOR is localized to strategic sites for involvement in the direct presynaptic release and/or reuptake of dopamine. These data also suggest a role for KOR in the presynaptic modulation of other neurotransmitters and in the postsynaptic excitatory responses of single spiny neurons in the Acb. Dual actions on dopamine terminals and their targets in the Acb may account for KOR-mediated attenuation of drug reinforcement and sensitization.

Effects of opioid receptor activation on cardiovascular responses and extracellular monoamines within the rostral ventrolateral medulla during static contraction of skeletal muscle

During static muscle contraction, activation of opioid receptors alters the extracellular glutamate concentrations within the rostral ventrolateral medulla (RVLM). In addition, microdialysis of glutamate in the ventrolateral medulla (VLM) increases the release of norepinephrine (NE), dopamine (DA), and serotonin (5-HT). Therefore, we hypothesized that extracellular concentrations of these monoamines as well as cardiovascular responses during static skeletal muscle contraction would be modulated following administration of [D-Ala(2)]methionine enkephalinamide (DAME), an opioid receptor agonist, into the RVLM. Microdialysis of 100 microM DAME into the RVLM of 10 rats significantly (P<0.01) decreased extracellular levels (in pg/10 microl) of NE (from 3.3+/-0.3 to 1.9+/-0.3), DA (from 5.5+/-0.2 to 3.7+/-0.3), and 5-HT (from 6.1+/-0.8 to 3.6+/-0.2) during static exercise. After microdialysis of DAME, the exercise pressor reflex also significantly (P<0.01) decreased mean arterial pressure (MAP) by 13+/-3 mmHg and heart rate (HR) by 16+/-6 bpm, compared with control (MAP=22+/-4 mmHg and HR=31+/-7 bpm). Subsequently, after 30 min microdialysis of naloxone, an opioid receptor antagonist, muscle contraction increased the extracellular monoamine levels (in pg/10 microl, 3.8+/-0.3 NE; 5.2+/-0.3 DA; and 5.5+/-0.4 5-HT) similar to the control groups and evoked a reversal of cardiovascular responses. Similarly, 30 min of microdialyzing naloxone, added to the perfusing medium containing DAME, reversed the attenuating effects of DAME on monoamines, MAP, and HR during a muscle contraction. Furthermore, microdialysis of 100 microM naloxone alone for 30 min potentiated cardiovascular responses and monoamine levels during a muscle contraction. In summary, the present data demonstrates that microdialysis of DAME into RVLM attenuates the exercise pressor reflex mediated increases in MAP, HR and extracellular levels of biogenic monoamines. A subsequent microdialysis of naloxone reversed the effects suggesting that an opioidergic mechanism within RVLM modulates the exercise pressor reflex. Overall, the present study provides further insights into the opioidergic modulation of the exercise pressor reflex.

Cellular and synaptic adaptations mediating opioid dependence

Although opioids are highly effective for the treatment of pain, they are also known to be intensely addictive. There has been a massive research investment in the development of opioid analgesics, resulting in a plethora of compounds with varying affinity and efficacy at all the known opioid receptor subtypes. Although compounds of extremely high potency have been produced, the problem of tolerance to and dependence on these agonists persists. This review centers on the adaptive changes in cellular and synaptic function induced by chronic morphine treatment. The initial steps of opioid action are mediated through the activation of G protein-linked receptors. As is true for all G protein-linked receptors, opioid receptors activate and regulate multiple second messenger pathways associated with effector coupling, receptor trafficking, and nuclear signaling. These events are critical for understanding the early events leading to nonassociative tolerance and dependence. Equally important are associative and network changes that affect neurons that do not have opioid receptors but that are indirectly altered by opioid-sensitive cells. Finally, opioids and other drugs of abuse have some common cellular and anatomical pathways. The characterization of common pathways affected by different drugs, particularly after repeated treatment, is important in the understanding of drug abuse.

Modulation of NMDA receptor subunit mRNA in butorphanol-tolerant and -withdrawing rats

The NMDA receptor has been implicated in opioid tolerance and withdrawal. The effects of continuous infusion of butorphanol on the modulation of NMDA receptor subunit NR1, NR2A, NR2B, and NR2C gene expression were investigated by using in situ hybridization technique. Continuous intracerebroventricular (i.c.v.) infusion with butorphanol (26 nmol/microl/h) resulted in significant modulations in the NRI, NR2A, and NR2B mRNA levels. The level of NR1 mRNA was significantly decreased in the cerebral cortex, thalamus, and CA1 area of hippocampus in butorphanol tolerant and withdrawal (7 h after stopping the infusion) rats. The NR2A mRNA was significantly decreased in the CA1 and CA3 of hippocampus in tolerant rats and increased in the cerebral cortex and dentate gyrus in butorphanol withdrawal rats. NR2B subunit mRNA was decreased in the cerebral cortex, caudate putamen, thalamus, CA3 of hippocampus in butorphanol withdrawal rats. No changes of NR1, NR2A, NR2C subunit mRNA in the cerebellar granule cell layer were observed in either butorphanol tolerant or withdrawal rats. Using quantitative ligand autoradiography, the binding of NMDA receptor ligand [3H]MK-801 was increased significantly in all brain regions except in the thalamus and hippocampus, at the 7 hr after stopping the butorphanol infusion. These results suggest that region-specific changes of NMDA receptor subunit mRNA (NR1 and NR2) as well as NMDA receptor binding ([3H]MK-801) are involved in the development of tolerance to and withdrawal from butorphanol.

Dynorphin exerts both postsynaptic and presynaptic effects in the Globus pallidus of the rat

The opioids contained in striato-pallidal axons are thought to play a significant role in motor control. We examined post- and presynaptic effects of the kappa (kappa)-receptor agonist dynorphin A (1-13) (DYN13) on the globus pallidus (GP) neurons in rat brain slice preparations using the whole cell recording method. DYN13 hyperpolarized and decreased the input resistance of approximately one-quarter of neurons examined. All of these DYN13-sensitive neurons had medium-sized somata, large aspiny dendrites and generated repetitive firing without strong accommodation. The hyperpolarization was blocked by barium and was independent of TTX and intracellular chloride levels. The hyperpolarization was also selectively blocked by the kappa-antagonist nor-binaltorphimine dihydrochloride but not by the mu- or delta-antagonists. These data suggested that DYN13 activates barium-sensitive potassium currents in some GP neurons. Low- and high-intensity stimulation of the neostriatum (Str) evoked long- and short-latency GABAergic responses, respectively. Previous data suggested that the long- and the short-latency responses were due to activation of the striato-pallidal axons and the local collaterals of pallido-striatal axons, respectively. DYN13 diminished the amplitude of both the short- and long-latency GABAergic responses in all the neurons tested. The effects of DYN13 on GABAergic postsynaptic responses were also selectively blocked by a kappa-antagonist. To investigate whether the effects were pre- or postsynaptic, the effects of DYN13 on spontaneous inhibitory postsynaptic potentials (IPSPs) and TTX-independent miniature-inhibitory postsynaptic currents (IPSCs) were examined. DYN13 decreased the frequency, but not the amplitude, of spontaneous IPSCs and calcium-dependent miniature-IPSCs. However, DYN13 did not alter the cadmium-insensitive miniature-IPSCs. These results suggested that DYN13 suppressed GABA release from presynaptic terminals. This possibility was tested using a paired-stimulation test. DYN13 reduced the probability of evoking IPSCs to the first stimulation and greatly increased the success probability to the second stimulus. The amplitude of successfully evoked IPSCs was not changed with DYN13. DYN13 did not affect the excitatory postsynaptic potentials (EPSPs) or the response to iontophoretically applied GABA and glutamate. Together, these results suggest that DYN released from striato-pallidal axons controls the activity of GP neurons 1) by directly hyperpolarizing a population of neurons and 2) by presynaptically inhibiting GABA release from striato-pallidal and intrapallidal terminals.

Rostral ventrolateral medulla opioid receptor activation modulates glutamate release and attenuates the exercise pressor reflex

We previously reported that the administration of [D-Ala(2)]methionine enkephalinamide (DAME), an opioid receptor agonist, into the rostral (RVLM) but not into the caudal ventrolateral medulla (CVLM), attenuated increases in mean arterial pressure (MAP) and heart rate (HR) during static muscle contraction that had been blocked by prior microdialysis of the opioid receptor antagonist, naloxone [Am. J. Physiol. 274 (1998) H139-H146]. In this study, we determine whether this RVLM-mediated opioidergic-modulation of cardiovascular responses is associated with localized changes in extracellular concentrations of glutamate, an excitatory amino acid, using microdialysis techniques in anesthetized rats. Muscle contraction increased MAP and HR by 37+/-5 mmHg and 23+/-3 bpm, respectively. Extracellular glutamate concentrations, determined using HPLC-ECD, increased from 0.8+/-0.2 to 6.6+/-1.2 ng/5 microliter in the bilateral RVLM areas. Microdialysis of DAME (100 microM) for 30 min attenuated the contraction-evoked increases in MAP, HR, and glutamate levels (20+/-4 mmHg, 10+/-2 bpm, and 1.8+/-0.2 ng/5 microliter, respectively). After microdialysis of naloxone (100 microM) for 30 min into the RVLM, muscle contraction blocked the attenuations (35+/-5 mmHg, 26+/-4 bpm, and 5.8+/-1.0 ng/5 microliter, respectively). Developed muscle tensions were similar throughout the protocol (676+/-38, 678+/-37 and 687+/-37 g, respectively). These results suggest that an opioidergic receptor-mediated mechanism within the RVLM attenuates cardiovascular responses during static exercise via modulating extracellular concentrations of glutamate in the RVLM.

Kappa opioid receptor immunoreactivity in the nucleus accumbens and caudate-putamen is primarily associated with synaptic vesicles in axons

A rabbit polyclonal antiserum, raised against a C-terminal oligopeptide of the mouse kappa opioid receptor, was used to localize the cellular distribution of kappa receptors in the dorsal and ventral striatum of rats with light and electron microscopic immunocytochemistry. Prominent, diffuse kappa receptor immunoreactivity was present in the nucleus accumbens, particularly in the shell, ventral caudate-putamen and olfactory tubercle. The density of receptor immunoreactivity decreased in more dorsal areas of the caudate-putamen. In contrast, neuronal cell bodies stained clearly in the dorsal endopiriform nucleus, claustrum and layer VI of the adjacent cerebral cortex. Observations at the electron microscopic level in the dorsomedial shell of the nucleus accumbens and caudate-putamen revealed that the kappa receptor immunoreactivity was predominantly located in axons, often associated with synaptic vesicles, remote from the terminal or preterminal area. The few terminals which were labeled made slightly more asymmetrical than symmetrical contacts and the percentage of asymmetrical contacts observed was greater in the caudate than in the accumbens. A small number of postsynaptic spines was labeled; most of them were contacted by asymmetrical terminals. No labeling was observed in dendritic shafts.Thus, the predominant localization of kappa receptor immunoreactivity in axons is consistent with its role as a major inhibitor of glutamate and dopamine release in the dorsal and ventral striatum.

Arginine vasopressin release inhibitor RU51599 attenuates brain oedema following transient forebrain ischaemia in rats

RU51599 is an arginine vasopressin (AVP) release inhibitor and a selective kappa opioid agonist which has a pure water diuresis effect without associated electrolyte excretion. The effect of RU51599 on brain oedema following transient forebrain ischaemia in rats was examined. Under microscopy, the visible vertebral arteries at the second vertebra could be easily electrocauterized and completely cut by microscissors to yield complete cessation of circulation of both vertebral arteries. Transient forebrain ischaemia was induced by this improved highly reproducible technique of four-vessel occlusion model. Forty-three male Wistar rats were separated into six groups; saline-treated (1 ml/kg) normal rats (n = 10), RU51599-treated (1 mg/kg) normal rats (n = 4), saline-treated (1 ml/kg) rats with complete occlusion of both vertebral arteries (n = 5), RU51599-treated (1 mg/kg) rats with complete occlusion of both vertebral arteries (n = 5), saline-treated (1 ml/kg) rats with both complete occlusion of both vertebral arteries and carotid occlusion bilaterally during 45 minutes followed by 60 minutes of reperfusion (n = 11), RU51599-treated (1 mg/kg) rats with both complete occlusion of both vertebral arteries and carotid occlusion bilaterally during 45 minutes followed by 60 minutes of reperfusion (n = 8). The brain water content was determined by the dry-wet weight method. Cerebral blood flow was monitored during ischaemia and reperfusion was performed by laser Doppler flowmetry to make sure to obtain reversible forebrain ischaemia. Effects of RU51599 on concentration of glutamate released from the hippocampal CA1 of rats subjected to 5 minutes four-vessel occlusion and 60 minutes of reperfusion were also investigated by the microdialysis method. This modified four-vessel occlusion method produced reversible forebrain ischaemia with a high level of success. Bilateral carotid occlusion followed by 60 minutes reperfusion caused a significant increase in brain water content (P < 0.01), which was significantly attenuated by RU51599 (P < 0.01). These findings indicate that the AVP-release inhibitor RU51599 reduced brain oedema following transient forebrain ischaemia in rats.

Dynorphin A (1-13) neurotoxicity in vitro: opioid and non-opioid mechanisms in mouse spinal cord neurons

Dynorphin A is an endogenous opioid peptide that preferentially activates kappa-opioid receptors and is antinociceptive at physiological concentrations. Levels of dynorphin A and a major metabolite, dynorphin A (1-13), increase significantly following spinal cord trauma and reportedly contribute to neurodegeneration associated with secondary injury. Interestingly, both kappa-opioid and N-methyl-D-aspartate (NMDA) receptor antagonists can modulate dynorphin toxicity, suggesting that dynorphin is acting (directly or indirectly) through kappa-opioid and/or NMDA receptor types. Despite these findings, few studies have systematically explored dynorphin toxicity at the cellular level in defined populations of neurons coexpressing kappa-opioid and NMDA receptors. To address this question, we isolated populations of neurons enriched in both kappa-opioid and NMDA receptors from embryonic mouse spinal cord and examined the effects of dynorphin A (1-13) on intracellular calcium concentration ([Ca2+]i) and neuronal survival in vitro. Time-lapse photography was used to repeatedly follow the same neurons before and during experimental treatments. At micromolar concentrations, dynorphin A (1-13) elevated [Ca2+]i and caused a significant loss of neurons. The excitotoxic effects were prevented by MK-801 (Dizocilpine) (10 microM), 2-amino-5-phosphopentanoic acid (100 microM), or 7-chlorokynurenic acid (100 microM)--suggesting that dynorphin A (1-13) was acting (directly or indirectly) through NMDA receptors. In contrast, cotreatment with (-)-naloxone (3 microM), or the more selective kappa-opioid receptor antagonist nor-binaltorphimine (3 microM), exacerbated dynorphin A (1-13)-induced neuronal loss; however, cell losses were not enhanced by the inactive stereoisomer (+)-naloxone (3 microM). Neuronal losses were not seen with exposure to the opioid antagonists alone (10 microM). Thus, opioid receptor blockade significantly increased toxicity, but only in the presence of excitotoxic levels of dynorphin. This provided indirect evidence that dynorphin also stimulates kappa-opioid receptors and suggests that kappa receptor activation may be moderately neuroprotective in the presence of an excitotoxic insult. Our findings suggest that dynorphin A (1-13) can have paradoxical effects on neuronal viability through both opioid and non-opioid (glutamatergic) receptor-mediated actions. Therefore, dynorphin A potentially modulates secondary neurodegeneration in the spinal cord through complex interactions involving multiple receptors and signaling pathways.

Opioid peptides as possible neuromodulators of the afferent synaptic transmission in the frog semicircular canal

Vestibular receptors of the frog, Rana temporaria, were examined for the effect of bath-applied opioid peptide leu-enkephalin, its synthetic analogue dalargin and the specific opiate antagonist naloxone. Multiunit afferent activity of the whole vestibular nerve was recorded in an in vitro preparation. Leu-enkephalin (0.005-100 nM) and dalargin (0.1-100 nM) depress the resting discharge frequency. Naloxone (10 nM-1 microM) antagonizes responses induced by leu-enkephalin and dalargin that suggests a specific action of opioid peptides. Leu-enkephalin and delargin inhibit the excitatory action of L-glutamate. The effects of opioid peptides on L-glutamate-induced responses are unaffected by Co2+ block of transmitter release from hair cells that could speak in favour of the postsynaptic nature of these responses. At the same time, the other possible site of action of opioid peptides, such as efferent system, can not be excluded. The results indicate that opiate receptors are present in hair cells and that the neurotransmitter L-glutamate is involved in opiate action at the peripheral vestibular system of the frog. We suggest that opioid peptides may act as a neuromodulator in this system.

Control of glutamate release by calcium channels and kappa-opioid receptors in rodent and primate striatum

The modulation of depolarization (4-aminopyridine, 2 mM)-evoked endogenous glutamate release by kappa-opioid receptor activation and blockade of voltage-dependent Ca2+ -channels has been investigated in synaptosomes prepared from rat and marmoset striatum. 4-Aminopyridine (4-AP)-stimulated, Ca2+ -dependent glutamate release was inhibited by enadoline, a selective kappa-opioid receptor agonist, in a concentration-dependent and norbinaltorphimine (nor-BNI, selective kappa-opioid receptor antagonist)-sensitive manner in rat (IC50 = 4.4+/-0.4 microM) and marmoset (IC50 = 2.9+/-0.7 microM) striatal synaptosomes. However, in the marmoset, there was a significant (approximately 23%) nor-BNI-insensitive component. In rat striatal synaptosomes, the Ca2+ -channel antagonists omega-agatoxin-IVA (P/Q-type blocker), omega-conotoxin-MVIIC (N/P/Q-type blocker) and omega-conotoxin-GVIA (N-type blocker) reduced 4-AP-stimulated, Ca2+ -dependent glutamate release in a concentration-dependent manner with IC50 values of 6.5+/-0.9 nM, 75.5+5.9 nM and 106.5+/-8.7 nM, respectively. In marmoset striatal synaptosomes, 4-AP-stimulated, Ca2+ -dependent glutamate release was significantly inhibited by omega-agatoxin-IVA (30 nM, 57.6+/-2.3%, inhibition), omega-conotoxin-MVIIC (300 nM, 57.8+/-3.1%) and omega-conotoxin-GVIA (1 microM, 56.7+/-2%). Studies utilizing combinations of Ca2+ -channel antagonists suggests that in the rat striatum, two relatively distinct pools of glutamate, released by activation of either P or Q-type Ca2+ -channels, exist. In contrast, in the primate there is much overlap between the glutamate released by P and Q-type Ca2+ -channel activation. Studies using combinations of enadoline and the Ca2+ -channel antagonists suggest that enadoline-induced inhibition of glutamate release occurs primarily via reduction of Ca2+ -influx through P-type Ca2+ -channels in the rat but via N-type Ca2+ -channels in the marmoset. In conclusion, the results presented suggest that there are species differences in the control of glutamate release by kappa-opioid receptors and Ca2+ -channels.

Cellular sites for dynorphin activation of kappa-opioid receptors in the rat nucleus accumbens shell

The nucleus accumbens (Acb) is prominently involved in the aversive behavioral aspects of kappa-opioid receptor (KOR) agonists, including its endogenous ligand dynorphin (Dyn). We examined the ultrastructural immunoperoxidase localization of KOR and immunogold labeling of Dyn to determine the major cellular sites for KOR activation in this region. Of 851 KOR-labeled structures sampled from a total area of 10,457 microm2, 63% were small axons and morphologically heterogenous axon terminals, 31% of which apposed Dyn-labeled terminals or also contained Dyn. Sixty-eight percent of the KOR-containing axon terminals formed punctate-symmetric or appositional contacts with unlabeled dendrites and spines, many of which received convergent input from terminals that formed asymmetric synapses. Excitatory-type terminals that formed asymmetric synapses with dendritic spines comprised 21% of the KOR-immunoreactive profiles. Dendritic spines within the neuropil were the major nonaxonal structures that contained KOR immunoreactivity. These spines also received excitatory-type synapses from unlabeled terminals and were apposed by Dyn-containing terminals. These results provide ultrastructural evidence that in the Acb shell (AcbSh), KOR agonists play a primary role in regulating the presynaptic release of Dyn and other neuromodulators that influence the output of spiny neurons via changes in the presynaptic release of or the postsynaptic responses to excitatory amino acids. The cellular distribution of KOR complements those described previously for the reward-associated mu- and delta-opioid receptors in the Acb shell.

Hippocampal dynorphin B injections impair spatial learning in rats: a kappa-opioid receptor-mediated effect

The hippocampus plays a central role in the acquisition and storage of information. Long-term potentiation in the mossy fibre pathway to the CA3 region in the hippocampus, an animal model of memory acquisition, is modulated by dynorphin peptides. This study investigated the possible role of hippocampal dynorphin in spatial learning. Male rats were trained in the Morris Water Task after microinjection with different doses of dynorphin B (1, 3.3 or 10 nmol/rat) or artificial cerebrospinal fluid (as control) into the CA3 region of the dorsal hippocampus. Dynorphin B was found to impair spatial learning at all tested doses. The synthetic kappa1-selective opiate receptor antagonist nor-binaltorphimine (2 nmol) also given into the hippocampus fully blocked the acquisition impairment caused by dynorphin B (10 nmol), while nor-binaltorphimine alone did not affect learning performance. These findings suggest that dynorphin peptides could play a modulatory role in hippocampal plasticity by acting on hippocampal kappa-receptors and thereby impair spatial learning.

The role of glutamate in physical dependence on opioids

The present review will evaluate the interactions between kappa-opioid receptors and glutamate within the locus coeruleus (LC) during the development of opioid dependence and on expression of withdrawal from dependence on opioids. Hyperactivity of noradrenergic neurons in the LC has been proposed to play a critical role in the physiological and behavioral responses that comprise opioid withdrawal. Several studies indicate that the excitatory amino acid system, in particular, glutamate and its receptors, participate in both the withdrawal-associated increase in LC neuronal activity and the expression of opioid withdrawal behaviors. Most studies on opioid dependence have focused on the prototypical opioid morphine, which produces its physical dependence through agonist actions at the mu-opioid receptor. Butorphanol (Stadol), which exhibits a markedly different profile of opioid receptor activity than does morphine, produces its physical dependence primarily through actions at the kappa-opioid receptor. Studies from our laboratories using a rodent model in which butorphanol administration induces dependence indicate further that the kappa-opioid receptor is an important regulator of glutamate release within the LC. Glutamate exerts actions within the LC that mediate expression of behavioral symptoms of butorphanol withdrawal.

Brain myosin V is a synaptic vesicle-associated motor protein: evidence for a Ca2+-dependent interaction with the synaptobrevin-synaptophysin complex

Brain myosin V is a member of a widely distributed class of unconventional myosins that may be of central importance to organelle trafficking in all eukaryotic cells. Molecular constituents that target this molecular motor to organelles have not been previously identified. Using a combination of immunopurification, extraction, cross-linking, and coprecipitation assays, we demonstrate that the tail domain of brain myosin V forms a stable complex with the synaptic vesicle membrane proteins, synaptobrevin II and synaptophysin. While myosin V was principally bound to synaptic vesicles during rest, this putative transport complex was promptly disassembled upon the depolarization-induced entry of Ca2+ into intact nerve endings. Coimmunoprecipitation assays further indicate that Ca2+ disrupts the in vitro binding of synaptobrevin II to synaptophysin in the presence but not in the absence of Mg2+. We conclude that hydrophilic forces reversibly couple the myosin V tail to a biochemically defined class of organelles in brain nerve terminals.

Localization of a G-protein-coupled inwardly rectifying K+ channel, CIR, in the rat brain

The cellular localization of a G-protein-coupled K+ channel, CIR, in the rat brain has been demonstrated using a CIR-specific antibody, in combination with in situ hybridization. The CIR protein and messenger RNA were found in the cerebellar cortex, hippocampal formation, olfactory system, cerebral cortex, basal ganglia, several nuclei of the lower brain stem and the choroid plexus. In contrast to the messenger RNA, which was concentrated in the cell soma, the CIR protein was found in a subset of nerve fibers and, in other cases, in axon terminals. In the cerebellar cortex and hippocampus, the CIR protein was concentrated in the axon terminals of basket cells which are known to be GABAergic interneurons. This discrepancy between the distribution of protein and messenger RNA was observed in the substantia nigra, the interpeduncular, trigeminal, hypoglossal, oculomotor and red nuclei of the lower brain stem, and the tufted and mitral cells of the olfactory bulb. These observations suggested the translocation of the CIR protein into the nerve fibers following synthesis in the cell soma. Furthermore, its specific neuronal localization, especially in GABAergic interneurons, suggested the importance of CIR in synaptic transmission in neuronal systems.

Kappa opioid receptor-like immunoreactivity is present in substance P-containing subcortical afferents in guinea pig dentate gyrus

We have previously shown that kappa opioid receptor-like immunoreactivity (KT-LI) is present in axons and terminals in the granule cell layer and inner molecular layer of the guinea pig dentate gyrus. The distribution and ultrastructural appearance of processes with KT-LI were similar to those of the substance P (SP)-containing afferents which arise from the supramammillary region of the hypothalamus (SUM) and enter the hippocampal formation through the fimbria-fornix. The objective of the present study was to determine whether the terminals with KT-LI are likely to be SUM afferents. To accomplish this we 1) compared the intensity of KT- and SP-immunolabeling in the dentate gyrus ipsilateral and contralateral to a unilateral fornix transection and 2) used dual-labeling electron microscopy to determine whether terminals with KT-LI colocalize SP-LI in the dentate gyrus. Light microscopic examination of the dentate gyrus demonstrated that KT-LI and SP-LI were in thin processes with overlapping distributions in strata granulosum and moleculare. Following fornix transection, both KT-LI and SP-LI were dramatically reduced in these regions of the dentate gyrus ipsilateral to the transection, consistent with an SUM origin. By electron microscopy, most (71%) terminals with KT-LI also contained detectable SP-LI in single-section analysis. Many dual-labeled terminals formed thick asymmetric synaptic contacts with large dendritic shafts (2-5 microns) or granule cell perikarya, and a smaller proportion contacted dendritic spines; these characteristics resembled those of identified SUM afferents in other species. The demonstrations that 1) KT-LI colocalizes with SP-LI in a morphologically distinctive population of axon terminals and 2) most of the processes with KT-LI enter through the fimbria-fornix suggest that kappa opioid receptors are present in the SUM projection to the dentate gyrus.

Kappa opioid receptor-like immunoreactivity in guinea pig brain: ultrastructural localization in presynaptic terminals in hippocampal formation

Physiological and pharmacological studies have suggested that kappa opioid receptors (KORs) may be located presynaptically in the guinea pig hippocampal formation. In the present study, KOR-like immunoreactivity (-LI) was examined by using a rabbit antibody raised against a synthetic peptide from the carboxyl terminus of a cloned rat kappa receptor (KT). The specificity of affinity-purified KT antibody was confirmed by Western blotting, enzyme-linked immunosorbent assay, immunolabeling of KORs expressed in Xenopus oocytes, and immunocytochemical preadsorption controls. Specificity also was demonstrated by the light microscopic distribution of KT-LI in sections through the forebrain and the pons, which was largely consistent with the distribution of KORs previously reported, and resembled that of immunoreactivity for dynorphin B, an endogenous ligand for KORs. Detailed analysis of the hippocampal formation revealed that KT-LI was located predominantly in thin processes in the granule cell and inner molecular layers of the dentate gyrus. A few KT-labeled processes were also present in stratum lacunosum-moleculare of the CA1 region and all layers of the CA3 region of the hippocampus. By electron microscopy, KT-LI was restricted to unmyelinated axons and axon terminals, and was associated with plasma membranes, large dense-core vesicles, and cytoplasmic surfaces of small vesicles. In the dentate gyrus, immunolabeled terminals formed asymmetric synapses with granule cell perikarya and large unlabeled dendrites. In the CA3 region of hippocampus, KT-LI was present in small unmyelinated axons. The results of this study 1) demonstrate the specificity of the KT antibody, 2) show that the distribution of KT labeling corresponds well with previous KOR and dynorphin localization in many regions, and 3) provide ultrastructural evidence that KORs are located presynaptically in the guinea pig hippocampal formation.

Endogenous opioid regulation of hippocampal function

Endogenous opioid peptides modulate neural transmission in the hippocampus. Procnkephalin-derived peptides have been demonstrated to act at mu and delta opioid receptors to inhibit GABA release from inhibitory interneurons, resulting in increased excitability of hippocampal pyramidal cells and dentate gyrus granule cells. Prodynorphin-derived peptides primarily act at presynaptic kappa opioid receptors to inhibit excitatory amino acid release from perforant path and mossy fiber terminals. Opioid receptors reduce membrane excitability by modulating ion conductances, and in this way they may decrease voltage-dependent calcium influx and transmitter release. Synaptic plasticity in the hippocampus also is modulated by endogenous opioids. Enkephalins facilitate long-term potentiation, whereas dynorphins inhibit the induction of this type of neuroplasticity. Further, opioids may play important roles in hippocampal epilepsy. Recurrent seizures induce changes in the expression of opioid peptides and receptors. Also, enkephalins have proconvulsant effects in the epileptic hippocampus, whereas dynorphins may function as endogenous anticonvulsants.

Prior morphine exposure enhances ibogaine antagonism of morphine-induced dopamine release in rats

The present study examines the effect of prior morphine exposure on ibogaine antagonism of morphine-induced dopamine release. Female Sprague-Dawley rats were pretreated once a day for 2 days with morphine (20 mg/kg, i.p.) or saline and given a low dose of ibogaine (10 mg/kg, i.p.) or saline 5 hr after the last morphine or saline injection. Nineteen hours later, rats (awake and freely moving) were challenged with morphine (5 mg/kg, i.p.), and dopamine and its metabolites were monitored in the striatum and nucleus accumbens using in vivo microdialysis. Neither saline pretreatment, morphine pretreatment, nor ibogaine alone altered morphine-induced increases in extracellular dopamine and dopamine metabolites in either structure. However, when morphine pretreatment was combined with ibogaine, the morphine-induced elevation of dopamine, but not of metabolites, was completely blocked. These data suggest that prior morphine exposure enhances an opioid antagonist action of ibogaine on dopaminergic systems and that prior drug exposure may be a clinically significant determinant of ibogaine efficacy and/or potency in the treatment of opioid addiction.

kappa-opioid agonist U50488 inhibits P-type Ca2+ channels by two mechanisms

The effects of U50488, kappa-opioid agonist on P-type Ca2+ channels, were studied. U50488 inhibited depolarization-induced Ca2+ uptake into rat brain synaptosomes, which was sensitive to omega-Agatoxin IVA (omega-AgaIVA; P-type Ca2+ channel blocker) and inhibited P-type Ca2+ channel currents recorded from rat cerebellar Purkinje neurons by the whole-cell patch clamp method. Dynorphin A also inhibited P-type Ca2+ channel currents. The inhibition by U50488 was biphasic; high affinity component (21%, IC50 = 8.9 x 10(-8) M) and low affinity component (79%, IC50 = 1.1 x 10(-5) M). At low concentrations of U50488 (10(-6) M), P-type Ca2+ channel current inhibition was attenuated by norbinartorphimine (nor-BNI), kappa-opioid antagonist, and by dialysis of cells with a pipette solution containing guanosine 5'-O-(2-thiodiphosphate) (GDP-beta S). At high concentrations of U50488 (10(-5) M), P-type Ca2+ channel current inhibition was frequency-dependent. Thus U50488-induced current inhibition is mediated by two mechanisms. Its high affinity component is produced by activation of kappa-opioid receptors, whereas the low affinity component is due to its direct action on the P-type Ca2+ channel.

A role for hippocampal opioids in long-term functional plasticity

In the granule cells of the hippocampus, glutamate coexists with opioid peptides derived from the proenkephalin and prodynorphin genes. The functional significance of this coexistence has been unclear but recent evidence suggests that the dynorphins and enkephalins play a crucial role in regulating the efficiency of neurotransmission at granule-cell synapses. Together with evidence that the level of opioid activity in this pathway can change dramatically according to the physiological or pathological state of the tissue, this information focuses attention on granule-cell opioids as primary mediators of hippocampal plasticity.

Ultrastructural heterogeneity of enkephalin-containing terminals in the rat hippocampal formation

Opioid peptides, including leu-enkephalin (LE), are important neuromodulators in the hippocampal formation where they may play a role in learning and memory as well as epileptogenesis. We examined the cellular substrates that underlie the function of LE in each lamina of the rat hippocampal formation by immunocytochemistry at the electron microscopic level in single section analysis. LE-like immunoreactivity (LE-LI) was primarily associated with large dense-core vesicles (80-100 nm), usually found in axons and axon terminals, but was also observed in perikarya and occasionally in dendrites. The morphology and synaptic associations of LE-LI-containing terminals were strikingly distinct in each region of the hippocampal formation. In the molecular layer of the dentate gyrus, terminals with LE-LI were typically small (0.6 microns) and formed primarily asymmetric (excitatory type) synapses on single dendritic spines, which is consistent with the presence of LE in the lateral perforant path. In the hilus of the dentate gyrus, two types of LE-containing terminals were present: (1) small round terminals that were heterogeneous in size (0.4-1 microns) and in type of contact formed and (2) larger (3-5 microns) terminals exhibiting the characteristic morphology of mossy fiber boutons that formed asymmetric synapses on spines. This variation in morphology and the type of contact suggests LE may have a heterogeneous influence on diverse hilar interneurons. In the CA3 region of the hippocampus, LE-LI was localized to large mossy fiber boutons (3-7 microns) that formed multiple asymmetric synapses on complex spiny dendritic processes and often formed puncta adherentia with the shafts of large CA3 pyramidal cell dendrites, indicating that this peptide may be directly released onto pyramidal cells. At the border of stratum radiatum and lacunosum moleculare in the CA1 region of the hippocampus, LE-labeled terminals averaged 0.8 microns in diameter and often formed symmetric (inhibitory type) synapses on dendritic shafts, which is consistent with a role in disinhibition. In conclusion, these heterogeneous cellular interactions indicate that LE has diverse functional roles and mechanisms of action within each lamina of the hippocampal formation and may directly and indirectly modulate hippocampal cell activity.

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.

Lipid hydroperoxide-induced apoptosis: lack of inhibition by Bcl-2 over-expression

Increased membrane lipid peroxidation has recently been implicated as being associated with apoptosis. In the present study the addition of 15-hydroperoxyeicosatetraenoic acid (15-HPETE) or 13-hydroperoxydodecadienoic acid (13-HPODE) to A3.01 T cells is shown to induce marked chromatin condensation coincident with DNA fragmentation, indicative of apoptosis. 15-HPETE also evoked an immediate and sustained rise in cytoplasmic calcium which was required for the induction of apoptosis. A3.01 cells transfected with the bcl-2 proto-oncogene were 6- to 8-fold more resistant to apoptotic killing by tumor necrosis factor-alpha, but only 0.4-fold more resistant to 15-HPETE. Thus, Bcl-2 is not capable of protecting cells from undergoing apoptosis following the direct addition of lipid hydroperoxides.

Effects of adrenal steroid manipulations and repeated restraint stress on dynorphin mRNA levels and excitatory amino acid receptor binding in hippocampus

Adrenal steroid and stress effects were determined in hippocampus on levels of dynorphin (DYN) mRNA, expressed in dentate gyrus, and excitatory amino acid receptors, measured in Ammon's horn and dentate gyrus. Adrenalectomy (ADX) decreased DYN mRNA levels in dentate gyrus and replacement with aldosterone (ALDO), a specific type I adrenal steroid receptor agonist, prevented the decrease. Ru28362, a specific type II receptor agonist, had no effect. Likewise, kainate receptor binding to the stratum lucidum and hilus region of dorsal hippocampus was decreased after ADX and this decrease was prevented by ALDO but not by Ru28362 treatment. Similar though smaller effects were found for CNQX binding to AMPA receptors but only in the dentate gyrus molecular or infra- and supragranular layers. Although corticosterone (CORT) treatment of intact rats (40 mg/kg for 3 weeks) elevated DYN mRNA levels in dentate gyrus, up to 14 days of daily restraint stress (1 or 6 h/day) had no significant effect. Neither CORT treatment nor repeated restraint stress altered NMDA and non-NMDA glutamate receptors in hippocampus. The results of this study showing ADX-induced decreases of DYN mRNA and CNQX binding in dentate gyrus and decreased kainate binding in mossy fiber terminal regions are consistent with morphological evidence showing that adrenal steroids maintain normal integrity and structure of dentate gyrus neurons and do so via type I adrenal steroid receptors. These same parameters are apparently not sensitive to chronic restraint stress although the effects of other stressors must be examined.

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.

Kappa opioid receptor agonists inhibit the pilocarpine-induced seizures and toxicity in the mouse

Involvement of the kappa opioid receptor in regulation of the pilocarpine-induced seizures and neurodegeneration was studied in mice. Administration of pilocarpine (400 mg/kg i.p.) resulted in a sequence of behavioral alterations including motor limbic seizures. Pretreatment of mice with the selective kappa opioid receptor agonist U69,593 (2 and 20 mg/kg i.p.) or PD117,302 (0.1 and 1 mg/kg i.p.) increased the latency of motor seizures and decreased the seizure severity and mortality. Those effects were abolished in animals pretreated with the specific kappa opioid receptor antagonist nor-binaltorphimine (Nor-BNI, 10 mg/kg i.p.). Examination of frontal forebrain sections by light microscopy revealed widespread damage, especially within the hippocampal formation, in pilocarpine-treated mice. Both U69,593 and PD117,503 protected the integrity of hippocampal neurons, especially in the CA1 region, that effect being reversed by Nor-BNI. The above data indicate that activation of the kappa opioid receptor exerts an inhibitory effect on the pilocarpine-induced limbic seizures and neurotoxicity.

Distribution of effects of the kappa-opioid agonist CI-977 on cerebral glucose utilization in rat brain

The effects of the kappa-opioid agonist CI-977 upon local cerebral glucose utilization have been examined in conscious, lightly restrained rats to gain insight into the potential adverse effects of this neuroprotective agent. Cerebral glucose utilization was assessed quantitatively in 45 anatomically discrete brain regions by means of [14C]2-deoxyglucose autoradiography. The i.v. administration of CI-977 (0.03-3 mg/kg) induced relatively homogeneous patterns of altered cerebral glucose utilization with moderate statistically significant reductions (approximately 25%) being observed in 29 brain regions, and a statistically significant increase (approximately 40%) in one brain region, the lateral habenular nucleus. Glucose use throughout the entire neocortex and inferior colliculus was particularly sensitive to reduction (approximately 35%) following CI-977 administration, although there was only a limited dose dependency to the response. Minimal alterations in glucose use were observed in 15 of the 45 brain regions, particularly in the lower brain stem (e.g. superior olives, cochlear nucleus and median raphe) and forebrain limbic regions (e.g. septal nucleus, nucleus accumbens and mediodorsal thalamus). These data demonstrate that CI-977 produces widespread, anatomically organized alterations in function-related glucose use which contrast those seen previously with the NMDA receptor antagonists, thereby suggesting that CI-977 may be intrinsically safer as an in vivo neuroprotective agent.

Cardiovascular effects of microinjection of dynorphin-A(1-8) into the hippocampus in conscious, spontaneously hypertensive and normotensive Wistar-Kyoto rats

The possibility that the dynorphinergic system in the hippocampal formation (HF) may be implicated in the central regulation of peripheral circulatory activity was examined in chronically catheterized and conscious spontaneously hypertensive rats (SHR) and their normotensive control Wistar-Kyoto (WKY) rats. The ipsilateral microinjection of dynorphin-A(1-8) (DA1-8) at a dose of 10 nmol into the dorsal region of HF, where injection of control saline failed to affect peripheral cardiovascular activities, lowered mean blood pressure (MBP) by 19.2 +/- 1.2 mmHg in WKY and 25.9 +/- 3.2 mmHg in SHR. However, no significant alteration of heart rate (HR) was found in either WKY or SHR following the drug administration. The depressor response to intra-HF DA1-8 was dose-related (0.05 to 50.0 nmol) in the two strains of rats. Intra-HF injection of the kappa opioid receptor antagonist norbinaltorphimine at a dose of 2 nmol, which by itself produced only minimal fluctuations of basal MBP and HR, markedly reversed subsequent reduction of blood pressure induced by intra-HF injection of DA1-8 in both strains of rats. The results indicate that DA1-8 administered into the HF acts on kappa opioid receptors to produce a profound decrease in blood pressure, but not heart rate, in conscious hypertensive and normotensive rats.

Comparative effects of intrahippocampal injection of dynorphin A(1-8), dynorphin A(1-13), dynorphin A(1-17), U-50,488H, and dynorphin B on blood pressure and heart rate in spontaneously hypertensive and normotensive Wistar-Kyoto rats

We previously demonstrated centrally mediated hypotensive and bradycardic effects of dynorphin A(1-8) (DA1-8) on microinjection into various areas of the hippocampal formation (HF) of both anesthetized and conscious male normotensive and spontaneously hypertensive rats (SHR). The purpose of the present study was to determine whether other dynorphin fragments also had this activity. We microinjected DA1-8, dynorphin A(1-13), dynorphin A(1-17), dynorphin B (DB), and the nonpeptide kappa-opioid agonist U-50,488H into HF areas previously found to react to DA1-8, at doses ranging from 0.05 to 50 nmol. The subjects were male SHR and normotensive Wistar-Kyoto (WKY) rats in which arterial pressure and heart rate were monitored. Dose-related centrally mediated hypotension and bradycardia were found in both strains with all agents used, except for DB, which had no effects. Similarly injected drug vehicle was also without effect. In general, the responses were greater in SHR than in WKY rats. Preinjection of the active HF areas with 2 nmol of nor-binaltorphimine (nor-BNI), a selective kappa-opioid receptor antagonist, which itself had no blood pressure or heart rate effects, abolished both the decrease in blood pressure and heart rate of all dynorphins and U-50,488H. The results demonstrated the equivalent abilities of all the dynorphin fragments studied, except DB, to cause HF-mediated hypotension and bradycardia. The results with U-50,488H and nor-BNI strongly implicate kappa-opiate receptor activation of the HF in these effects.

Kappa-selective agonists decrease postsynaptic potentials and calcium components of action potentials in the supraoptic nucleus of rat hypothalamus in vitro

To investigate the effects of the endogenous kappa-receptor agonists dynorphin and leumorphin on neurons of the supraoptic nucleus in the rat hypothalamus, intracellular recordings were made from 62 supraoptic neurons in slice preparations. Bath application of dynorphin and leumorphin at 10(-7) M to 3 x 10(-6) M decreased the spontaneous firing rate with slight hyperpolarization of the membrane potential (-3.8 +/- 0.5 mV, mean +/- S.E.M.) but did not detectably change input resistance. The inhibitory effects were blocked by the relatively selective kappa-antagonist MR-2266. The synthetic kappa-receptor agonist U-50,488H had similar inhibitory effects on supraoptic neurons. Postsynaptic potentials evoked by electrical stimulation dorsal or dorsolateral to the supraoptic nucleus were suppressed by dynorphin and leumorphin. Morphine and [D-Ala, D-Leu]enkephalin, which are relatively selective to mu- and delta-receptors, respectively, influenced the postsynaptic potentials less. Dynorphin and leumorphin also decreased the duration of action potentials that were prolonged by either bath application of tetraethylammonium chloride at 5-10 mM or intracellular injection of Cs ions from the recording electrodes which were filled with 3 M cesium citrate. The prolongation was blocked by 1 mM MnCl2 and 2 mM CoCl2, which suggested that the components were due to voltage-dependent Ca2+ influx. The results suggest that endogenous kappa-receptor agonists inhibit neurosecretory cells of the supraoptic nucleus to suppress synaptic events and Ca2+ components of action potentials.

Opioid receptor antagonist nalmefene stereospecifically inhibits glutamate release during global cerebral ischemia

The opioid receptor antagonist nalmefene improves cellular bioenergetics and attenuates the reduction in tissue glutamate levels after global cerebral ischemia/reperfusion. The latter finding suggests that nalmefene might inhibit glutamate release during ischemia. To test this hypothesis, we used microdialysis techniques to examine the effect of nalmefene pretreatment on extracellular excitatory amino acid levels during global cerebral ischemia in rats. Saline, (-)-nalmefene (20, 100 or 500 micrograms/kg) or the inactive nalmefene enantiomer (+)-nalmefene (100 micrograms/kg) were given 15 min prior to induction of ischemia using a multi-vessel occlusion model. Pretreatment with (-)-nalmefene decreased peak dialysate glutamate in a dose-dependent fashion as compared to saline-treated controls, whereas (+)-nalmefene had no effect. These results suggest that opioid receptors may modulate glutamate release during ischemia and that inhibition of excitatory amino acid release may contribute to the protective actions of opioid receptor antagonists in cerebral ischemia.

Effect of the kappa-1 opioid agonist CI-977 on ischemic brain damage and cerebral blood flow after middle cerebral artery occlusion in the rat

The effects of the kappa-1 opioid agonist CI-977 upon the volume of ischemic brain damage (defined using quantitative neuropathology) and local cerebral blood flow (CBF) (defined using quantitative [14C]iodoantipyrine autoradiography) have been examined at 4 h and 30 min, respectively, after permanent middle cerebral artery (MCA) occlusion in halothane-anesthetised rats. Treatment with CI-977 (0.3 mg/kg, s.c.) 30 min before and 30 min after occlusion of the MCA reduced the volume of infarction in the cerebral hemisphere (reduced by 27% when compared to vehicle; P < 0.05) and cerebral cortex (reduced by 32%; P < 0.05), despite a marked and sustained hypotension, with only minimal effect on damage in the caudate nucleus. In the hemisphere contralateral to the occluded MCA, treatment with CI-977 (0.3 mg/kg, s.c.) 30 min prior to the induction of ischemia failed to demonstrate any significant effect on either the level of local CBF in any of the 25 regions examined or on the volume of low CBF determined by frequency distribution analysis. In the hemisphere ipsilateral to MCA occlusion, CI-977 failed to produce statistically significant alterations in either the level of local CBF in 23 of the 25 regions or on the volume of low CBF, but areas of hyperemia were observed in both the medial caudate nucleus and lateral thalamus (local CBF increased by 65% and 86%, respectively, when compared to vehicle).(ABSTRACT TRUNCATED AT 250 WORDS)

Prostaglandins F2 alpha synthesis in the hippocampal mossy fiber synaptosomal preparation: II. Effects of receptor activation

Mossy fiber nerve endings were isolated from rat hippocampi and used to determine the effects of receptor activation on the production of prostaglandin F2 alpha (PGF2 alpha). Glutamate and its agonists had no effect on PGF2 alpha synthesis. Similarly, acetylcholine, gamma-aminobutyric acid, histamine and purinergic receptor agonists did not affect PGF2 alpha accumulation in this preparation. However, norepinephrine, serotonin and dopamine exerted receptor-mediated stimulations of PGF2 alpha production. The agonist-evoked increases in PGF2 alpha production were attenuated by phospholipase A2 inhibitors, L-type voltage-sensitive Ca2+ blockers and a K+ channel activator, but they were insensitive to tetrodotoxin. In addition, a kappa opioid agonist decreased PGF2 alpha synthesis in unstimulated and depolarized synaptosomes. It appeared, therefore, that certain receptor agonists were able to modulate PGF2 alpha synthesis in the hippocampal mossy fiber synaptosomal preparation.