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

Susceptibility to Pentylenetetrazole-Induced Seizures in Mice with Distinct Activity of the Endogenous Opioid System

Currently, pharmacotherapy provides successful seizure control in around 70% of patients with epilepsy; however, around 30% of cases are still resistant to available treatment. Therefore, effective anti-epileptic therapy still remains a challenge. In our study, we utilized two mouse lines selected for low (LA) and high (HA) endogenous opioid system activity to investigate the relationship between down- or upregulation of the opioid system and susceptibility to seizures. Pentylenetetrazole (PTZ) is a compound commonly used for kindling of generalized tonic-clonic convulsions in animal models. Our experiments revealed that in the LA mice, PTZ produced seizures of greater intensity and shorter latency than in HA mice. This observation suggests that proper opioid system tone is crucial for preventing the onset of generalized tonic-clonic seizures. Moreover, a combination of an opioid receptor antagonist-naloxone-and a GABA receptor agonist-diazepam (DZP)-facilitates a significant DZP-sparing effect. This is particularly important for the pharmacotherapy of neurological patients, since benzodiazepines display high addiction risk. In conclusion, our study shows a meaningful, protective role of the endogenous opioid system in the prevention of epileptic seizures and that disturbances in that balance may facilitate seizure occurrence.

KOR Control over Addiction Processing: An Exploration of the Mesolimbic Dopamine Pathway

Drug addiction is a complex, persistent, and chronically relapsing neurological disorder exacerbated by acute and chronic stress. It is well known that the dynorphin/kappa opioid receptor (KOR) system regulates stress perception and responsivity, while the mesolimbic dopamine system plays a role in reward and reinforcement associated with alcohol and substance use disorders. Interestingly, the dopamine and dynorphin/KOR systems are highly integrated in mesolimbic areas, with KOR activation leading to inhibition of dopamine release, further altering the perception of reinforcing and aversive stimuli. Chronic or repeated exposure to stress or drugs potentiates KOR function ultimately contributing to a hypodopaminergic state. This hypodopaminergic state is one of the hallmarks of hyperkatifeia, defined as the hypersensitivity to emotional distress that is exacerbated during drug withdrawal and abstinence. The relationship between stress and drug addiction is bidirectional; repeated/chronic stress promotes pro-addictive behaviors, and repeated cycles of drug exposure and withdrawal, across various drug classes, produces stress. Neuroadaptations driven by this bidirectional relationship ultimately influence the perception of the reinforcing value of rewarding stimuli. In this chapter, we address the involvement of the dopamine and dynorphin/KOR systems and their interactions in shaping reinforcement value processing after drug and stress exposure, as well as a combinatorial impact of both drugs and stress.

Social deprivation substantially changes multi-structural neurotransmitter signature of social interaction: Glutamate concentration in amygdala and VTA as a key factor in social encounter-induced 50-kHz ultrasonic vocalization

Ultrasonic vocalizations are important for coordinating social behavior in rats. Examination of the neurochemical mechanisms that govern social behavior and ultrasonic vocalization emission is crucial for understanding the social impairments that occur in many neuropsychiatric disorders. To elucidate neurochemical changes in the brain structures related to social behavior and their mutual relationships, we conducted three-phase experiment. Neurochemicals were measured in the following behavioral situations: without social encounter, with short social encounter, with long social encounter in isolated and non-isolated rats. The aims of this study were to: (1) extract the most important neurotransmitters and their metabolites that are involved in social encounter-induced emission of 50 kHz calls; (2) to elucidate mutual relationships among the neurochemical changes in the selected, six brain structures, and analyze compound relationships by step analysis; (3) create a model of all-to-all neurotransmitter correlations; (4) find the neurochemical basis of 50-kHz USVs emission during social encounter. Our behavioral and neurochemical analysis indicated that social encounter was a triggering factor of the glutamatergic neurotransmission in the ventral tegmental area (VTA), hippocampus, and amygdala; serotonergic neurotransmission in the NAcc, CPu, and amygdala; the dopaminergic neurotransmission in the caudate putamen (CPu) and hippocampus; GABAergic neurotransmission in the hippocampus and VTA. Social encounter-induced 50-kHz USVs were bound up with changes in glutamate in amygdala and VTA, glycine in the amygdala, VTA, hippocampus, nucleus accumbens and CPu, and dopamine metabolites in VTA and CPu.

TsNTxP, a non-toxic protein from Tityus serrulatus scorpion venom, induces antinociceptive effects by suppressing glutamate release in mice

Neuropathic pain is a common type of chronic pain caused by trauma or chemotherapy. However, this type of pain is undertreated. TsNTxP is a non-toxic protein isolated from the venom of the scorpion Tityus serrulatus, and it is structurally similar to neurotoxins that interact with voltage-gated sodium channels. However, the antinociceptive properties of this protein have not been characterized. The purpose of this study was to investigate the antinociceptive effects of TsNTxP in acute and neuropathic pain models. Male and female Swiss mice (25-30 g) were exposed to different models of acute pain (tail-flick test and nociception caused by capsaicin intraplantar injection) or neuropathic pain (chronic pain syndrome induced by paclitaxel or chronic constriction injury of the sciatic nerve). Hypersensitivity to mechanical or cold stimuli were evaluated in the models of neuropathic pain. The ability of TsNTxP to alter the release of glutamate in mouse spinal cord synaptosomes was also evaluated. The results showed that TsNTxP exerted antinociceptive effects in the tail-flick test to a thermal stimulus and in the intraplantar capsaicin administration model. Furthermore, TsNTxP was non-toxic and exerted antiallodynic effects in neuropathic pain models induced by chronic constriction injury of the sciatic nerve and administration of paclitaxel. TsNTxP reduced glutamate release from mouse spinal cord synaptosomes following stimulation with potassium chloride (KCl) or capsaicin. Thus, this T. serrulatus protein may be a promising non-toxic drug for the treatment of neuropathic pain.

Kappa Opioid Receptors Mediate Heterosynaptic Suppression of Hippocampal Inputs in the Rat Ventral Striatum

Kappa opioid receptors (KORs) are highly enriched within the ventral striatum (VS) and are thought to modulate striatal neurotransmission. This includes presynaptic inhibition of local glutamatergic release from excitatory inputs to the VS. However, it is not known which inputs drive this modulation and what impact they have on the local circuit dynamics within the VS. Individual medium spiny neurons (MSNs) within the VS serve as a site of convergence for glutamatergic inputs arising from the PFC and limbic regions, such as the hippocampus (HP). Recent data suggest that competition can arise between these inputs with robust cortical activation leading to a reduction in ongoing HP-evoked MSN responses. Here, we investigated the contribution of KOR signaling in PFC-driven heterosynaptic suppression of HP inputs onto MSNs using whole-cell patch-clamp recordings in slices from adult rats. Optogenetically evoked HP EPSPs were greatly attenuated after a short latency (50 ms) following burst-like PFC electrical stimulation, and the magnitude of this suppression was partially reversed following blockade of GABA_ARs (GABA Type A receptors), but not GABA_BRs (GABA Type B receptors). A similar reduction in suppression was observed in the presence of the KOR antagonist, norBNI. Combined blockade of local GABA_ARs and KORs resulted in complete blockade of PFC-induced heterosynaptic suppression of less salient HP inputs. These findings highlight a mechanism by which strong, transient PFC activity can take precedence over other excitatory inputs to the VS.SIGNIFICANCE STATEMENT Emerging evidence suggests that kappa opioid receptor (KOR) activation can selectively modulate striatal glutamatergic inputs onto medium spiny neurons (MSNs). In this study, we found that robust cortical stimulation leads to a reduction in ongoing hippocampal-evoked MSNs responses through the combined recruitment of local inhibitory mechanisms and activation of presynaptic KORs in the ventral striatum (VS). These processes are likely to facilitate the efficient transfer of cortical information through the VS during critical decision making by dampening competing information from less salient excitatory inputs. These data provide a novel mechanism through which VS information processing could influence decision making, a function thought to occur primarily in the PFC.

Effects of N-, P/Q- and L-type Calcium Channel Blockers on Nociceptive Neurones of the Trigeminal Nucleus with Input from the Dura

In anaesthetized rats, extracellular recordings were made from neurones of the spinal trigeminal nucleus, involved in the processing of nociceptive input from the dura. Blockers of voltage-gated calcium channels (VGCCs) were administered topically to the exposed brainstem. Blockade of N-type (CaV2.2) channels reduced spontaneous activity and responses of the neurones to cold and chemical stimuli applied to the dura, suggesting that N-type channels regulate excitatory synaptic activation. Blockade of L-type (CaV1) channels enhanced spontaneous discharges of the neurones. Blockade of P/Q-type (CaV2.1) channels slightly decreased responses to chemical and cold stimuli but markedly increased spontaneous activity, an effect which was absent during concomitant application of GABA to the brainstem. The data suggest that P/Q-type VGCCs regulate a tonic synaptic inhibitory control of the brainstem neurones. The risk of migraine by genetic modifications of P/Q-type channels may thus be sought in disturbed inhibition in the network that processes nociceptive dura input.

Striatal synaptosomes from Hdh140Q/140Q knock-in mice have altered protein levels, novel sites of methionine oxidation, and excess glutamate release after stimulation

BACKGROUND

Synaptic connections are disrupted in patients with Huntington's disease (HD). Synaptosomes from postmortem brain are ideal for synaptic function studies because they are enriched in pre- and post-synaptic proteins important in vesicle fusion, vesicle release, and neurotransmitter receptor activation.

OBJECTIVE

To examine striatal synaptosomes from 3, 6 and 12 month old WT and Hdh140Q/140Q knock-in mice for levels of synaptic proteins, methionine oxidation, and glutamate release.

METHODS

We used Western blot analysis, glutamate release assays, and liquid chromatography tandem mass spectrometry (LC-MS/MS).

RESULTS

Striatal synaptosomes of 6 month old Hdh140Q/140Q mice had less DARPP32, syntaxin 1 and calmodulin compared to WT. Striatal synaptosomes of 12 month old Hdh140Q/140Q mice had lower levels of DARPP32, alpha actinin, HAP40, Na+/K+-ATPase, PSD95, SNAP-25, TrkA and VAMP1, VGlut1 and VGlut2, increased levels of VAMP2, and modifications in actin and calmodulin compared to WT. More glutamate released from vesicles of depolarized striatal synaptosomes of 6 month old Hdh140Q/140Q than from age matched WT mice but there was no difference in glutamate release in synaptosomes of 3 and 12 month old WT and Hdh140Q/140Q mice. LC-MS/MS of 6 month old Hdh140Q/140Q mice striatal synaptosomes revealed that about 4% of total proteins detected (>600 detected) had novel sites of methionine oxidation including proteins involved with vesicle fusion, trafficking, and neurotransmitter function (synaptophysin, synapsin 2, syntaxin 1, calmodulin, cytoplasmic actin 2, neurofilament, and tubulin). Altered protein levels and novel methionine oxidations were also seen in cortical synaptosomes of 12 month old Hdh140Q/140Q mice.

CONCLUSIONS

Findings provide support for early synaptic dysfunction in Hdh140Q/140Q knock-in mice arising from altered protein levels, oxidative damage, and impaired glutamate neurotransmission and suggest that study of synaptosomes could be of value for evaluating HD therapies.

The dynorphin/κ-opioid receptor system and its role in psychiatric disorders

The dynorphin/κ-opioid receptor system has been implicated in the pathogenesis and pathophysiology of several psychiatric disorders. In the present review, we present evidence indicating a key role for this system in modulating neurotransmission in brain circuits that subserve mood, motivation, and cognitive function. We overview the pharmacology, signaling, post-translational, post-transcriptional, transcriptional, epigenetic and cis regulation of the dynorphin/κ-opioid receptor system, and critically review functional neuroanatomical, neurochemical, and pharmacological evidence, suggesting that alterations in this system may contribute to affective disorders, drug addiction, and schizophrenia. We also overview the dynorphin/κ-opioid receptor system in the genetics of psychiatric disorders and discuss implications of the reviewed material for therapeutics development.

Group II metabotropic glutamate receptors in the striatum of non-human primates: dysregulation following chronic cocaine self-administration

A growing body of evidence has demonstrated a role for group II metabotropic glutamate receptors (mGluRs) in the reinforcing effects of cocaine. These receptors are important given their location in limbic-related areas, and their ability to control the release of glutamate and other neurotransmitters. They are also potential targets for novel pharmacotherapies for cocaine addiction. The present study investigated the impact of chronic cocaine self-administration (9.0mg/kg/session for 100 sessions, 900 mg/kg total intake) on the densities of group II mGluRs, as assessed with in vitro receptor autoradiography, in the striatum of adult male rhesus monkeys. Binding of [(3)H]LY341495 to group II mGluRs in control animals was heterogeneous, with a medial to lateral gradient in binding density. Significant elevations in the density of group II mGluRs following chronic cocaine self-administration were measured in the dorsal, central and ventral portions of the caudate nucleus (P<0.05), compared to controls. No differences in receptor density were observed between the groups in either the putamen or nucleus accumbens. These data demonstrate that group II mGluRs in the dorsal striatum are more sensitive to the effects of chronic cocaine exposure than those in the ventral striatum. Cocaine-induced dysregulation of the glutamate system, and its consequent impact on plasticity and synaptic remodeling, will likely be an important consideration in the development of novel pharmacotherapies for cocaine addiction.

Dynorphin and the pathophysiology of drug addiction

Drug addiction is a chronic relapsing disease in which drug administration becomes the primary stimulus that drives behavior regardless of the adverse consequence that may ensue. As drug use becomes more compulsive, motivation for natural rewards that normally drive behavior decreases. The discontinuation of drug use is associated with somatic signs of withdrawal, dysphoria, anxiety, and anhedonia. These consequences of drug use are thought to contribute to the maintenance of drug use and to the reinstatement of compulsive drug use that occurs during the early phase of abstinence. Even, however, after prolonged periods of abstinence, 80-90% of human addicts relapse to addiction, suggesting that repeated drug use produces enduring changes in brain circuits that subserve incentive motivation and stimulus-response (habit) learning. A major goal of addiction research is the identification of the neural mechanisms by which drugs of abuse produce these effects. This article will review data showing that the dynorphin/kappa-opioid receptor (KOPr) system serves an essential function in opposing alterations in behavior and brain neurochemistry that occur as a consequence of repeated drug use and that aberrant activity of this system may not only contribute to the dysregulation of behavior that characterizes addiction but to individual differences in vulnerability to the pharmacological actions of cocaine and alcohol. We will provide evidence that the repeated administration of cocaine and alcohol up-regulates the dynorphin/KOPr system and that pharmacological treatments that target this system may prove effective in the treatment of drug addiction.

Nalfurafine, the kappa opioid agonist, inhibits icilin-induced wet-dog shakes in rats and antagonizes glutamate release in the dorsal striatum

Icilin, a cooling compound, produces vigorous wet-dog shakes in rats. We have reported previously that icilin-induced wet-dog shakes are blocked by the kappa opioid receptor agonists, nalfurafine and U50,488H, and that icilin evokes a dose- and time-dependent increase in glutamate within the dorsal striatum. Since activation of kappa opioid receptors inhibits glutamate release intrastriatally, we targeted glutamate release within the dorsal striatum using nalfurafine and examined the role of the dorsal striatum in icilin-induced wet-dog shakes, more specifically, the effect that icilin-evoked intrastriatal glutamate release has on the overt stimulant behavior. We report that nalfurafine (0.04mg/kg) inhibits icilin (0.50mg/kg)-induced wet-dog shakes and that this inhibition is reversed by intrastriatal perfusion of the kappa opioid receptor antagonist, norbinaltorphimine (100nM). Furthermore,we antagonized icilin-evoked glutamate release with nalfurafine (0.04mg/kg), and reversed inhibition of glutamate release with intrastriatal norbinaltorphimine (100nM). These findings support a central component in the behavioral response to icilin and suggest that activation of kappa opioid receptors antagonizes icilin-induced wet-dog shakes in rats by inhibiting glutamate release within the dorsal striatum.

Relevance of the MPTP primate model in the study of dyskinesia priming mechanisms

For nearly 20 years, the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) primate model has allowed great strides to be made in our understanding of the maladaptive changes underlying the levodopa-related motor response complications occurring in most parkinsonian patients. Studies indicate that sustained dopamine D2 receptor occupancy can prevent and reverse existing dyskinesias. Recent experiments in levodopa-treated MPTP animals, co-administered either a threshold dose of cabergoline or a glutamate NMDA NR2B-selective antagonist (CI-1041), have afforded protection against dyskinesia, perhaps through presynaptic inhibition of glutamate release and blockade of supersensitive postsynaptic NMDA receptors in the striatum, respectively. Some of the biochemical events that have correlated with dyskinesias, namely upregulated GABA(A) receptors in the internal pallidum, rise in pre-proenkephalin-A gene expression in the striatum, and upregulated striatal glutamate ionotropic receptors and adenosine A(2a) receptors, may be counteracted by these preventive strategies.

The presynaptic modulation of corticostriatal afferents by mu-opioids is mediated by K+ conductances

Population spikes associated with the paired pulse ratio protocol were used to measure the presynaptic inhibition of corticostriatal transmission caused by mu-opioid receptor activation. A 1 microM of [D-Ala(2), N-MePhe(4), Gly-ol(5)]-enkephalin (DAMGO), a selective mu-opioid receptor agonist, enhanced paired pulse facilitation by 44+/-8%. This effect was completely blocked by 2 nM of the selective mu-receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-NH (CTOP). Antagonists of N- and P/Q-type Ca(2+) channels inhibited, whereas antagonists of potassium channels enhanced, synaptic transmission. A 1 microM of omega-conotoxin GVIA, a blocker of N-type Ca(2+) channels, had no effect on the action of DAMGO, but 400 nM omega-agatoxin TK, a blocker of P/Q-type Ca(2+)-channels, partially blocked the action of this opioid. However, 5 mM Cs(2+) and 400 microM Ba(2+), unselective antagonists of potassium conductances, completely prevented the action of DAMGO on corticostriatal transmission. These data suggest that presynaptic inhibition of corticostriatal afferents by mu-opioids is mediated by the modulation of K(+) conductances in corticostriatal afferents.

Effect of enucleation on the expression of c-Fos protein in the supraoptic nucleus of the Japanese monkey (Macaca fuscata)

The aim of this study was to examine the effects of one eye enucleation on the expression of c-Fos protein in the hypothalamus of the Japanese monkey (Macaca fuscata). Compared with an intact monkey, significantly increased numbers of c-Fos positive neurons were observed in the supraoptic nuclei on both sides at 1 h after eye enucleation. This maximal c-Fos expression then started to decrease at 3 h after eye enucleation. Furthermore, by a dual-labeled immunocytochemical study, the c-Fos immunoreactivity was found mainly in the vasopressinergic but not in the oxytocinergic neurons within the supraoptic nucleus. These results suggest that vasopressinergic but not oxytocinergic neurons within the supraoptic nucleus may have critical roles in the stimulation of this nucleus in response to eye enucleation.

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.

Presynaptic modulation of synaptic transmission by opioid receptor in rat subthalamic nucleus in vitro

Presynaptic modulation of synaptic transmission in rat subthalamic nucleus (STN) neurons was investigated using whole-cell patch-clamp recordings in brain slices. Evoked GABAergic inhibitory postsynaptic currents (IPSCs) were reversibly reduced by methionine enkephalin (ME) with an IC(50) value of 1.1 +/- 0.3 microM. The action of ME was mimicked by the mu-selective agonist [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO), and was partially blocked by the mu-selective antagonists naloxonazine and D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2) (CTAP). Evoked GABA(A) IPSCs were also inhibited by the delta-selective agonist [D-Pen(2,5)]-enkephalin (DPDPE), but not by the kappa-selective agonist (+)-(5 alpha,7 alpha,8 beta)-N-methyl-N-[7-(1-pyrrolidinyl)-1-oxaspiro[4.5]dec-8-yl]-benzeneacetamide (U-69593) and the orphan receptor agonist orphanin FQ/nociceptin (OFQ). DPDPE-induced inhibition was completely blocked by the delta-selective antagonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174,864). ME, DAMGO and DPDPE increased the paired-pulse ratio of IPSCs. Evoked excitatory postsynaptic currents (EPSCs) were reversibly reduced by ME with an IC(50) value of 0.35 +/- 0.14 microM. Inhibition by ME was associated with an increase in the paired-pulse ratio of EPSCs. The action of ME was mimicked by DAMGO, and blocked by naloxonazine. DPDPE had little effect on evoked EPSCs. Neither U-69593 nor OFQ had any effect. ME significantly decreased the frequency of spontaneous miniature EPSCs (mEPSCs) without change in their amplitude. The action of ME was mimicked by DAMGO. DPDPE had no effect. The presynaptic voltage-dependent potassium conductance blocker 4-aminopyridine (4-AP, 100 microM) abolished the inhibitory effects of ME on evoked IPSCs and EPSCs. In contrast, 4-AP only partially blocked the actions of baclofen. These results suggest that opioids inhibit inhibitory synaptic transmission in the STN through the activation of presynaptic mu- and delta- receptors. In contrast, inhibition of excitatory synaptic inputs to the STN occurs through the activation of only mu-receptors. Both inhibitions may be mediated by blockade of voltage-dependent potassium conductance.

Histamine H3 receptor activation inhibits glutamate release from rat striatal synaptosomes

The release of glutamate from striatal synaptosomes induced by depolarisation with 4-aminopyridine (4-AP) was studied by a method based on the fluorescent properties of the NAPDH formed by the metabolism of the neurotransmitter by glutamate dehydrogenase.Ca(2+)-dependent, depolarisation-induced glutamate release was inhibited in a concentration-dependent manner by the selective histamine H(3) agonist immepip. Best-fit estimates were: maximum inhibition 60+/-10% and IC(50) 68+/-10 nM. The effect of 300 nM immepip on depolarisation-evoked glutamate release was reversed by the selective H(3) antagonist thioperamide in a concentration-dependent manner (EC(50) 23 nM, K(i) 4 nM). In fura-2-loaded synaptosomes, the increase in the intracellular concentration of Ca(2+) ([Ca(2+)](i)) evoked by 4-AP-induced depolarisation (resting level 167+/-14 nM; Delta[Ca(2+)](i) 88+/-15 nM) was modestly, but significantly reduced (29+/-5% inhibition) by 300 nM immepip. The action of the H(3) agonist on depolarisation-induced changes in [Ca(2+)](i) was reversed by 100 nM thioperamide. Taken together, our results indicate that histamine modulates the release of glutamate from corticostriatal nerve terminals. Inhibition of depolarisation-induced Ca(2+) entry through voltage-dependent Ca(2+) channels appears to account for the effect of H(3) receptor activation on neurotransmitter release. Modulation of glutamatergic transmission in rat striatum may have important consequences for the function of basal ganglia and therefore for the control of motor behaviour.

Biphasic dose-related effects of morphine on dopamine release

The effects of morphine on extracellular dopamine levels in brain have never been studied over a wide range of doses within a single study. This has made it difficult to make definitive interpretations of drug interactions with morphine. An inhibition of morphine-induced increases in dopamine could be interpreted as either antagonism or potentiation depending the shape of the morphine dose-response curve. Accordingly, the aim of the present study was to determine the effects of a wide range of morphine doses (0, 5, 10, 20 and 30 mg/kg, i.p.) on extracellular dopamine, DOPAC and HVA levels in the nucleus accumbens and striatum of awake and freely moving female Sprague-Dawley rats. The results show that, in both brain regions, the dose-response curve for morphine-induced increases in dopamine is non-monotonic while the dose-response curve for morphine-induced increases in DOPAC and HVA is monotonic in the nucleus accumbens. The results of this study are discussed in terms of their implications for interpreting drug interactions with morphine and with relationship to morphine's mode of action at mu and kappa opioid receptors.

High-affinity inhibition of glutamate release from corticostriatal synapses by omega-agatoxin TK

To know which Ca(2+) channel type is the most important for neurotransmitter release at corticostriatal synapses of the rat, we tested Ca(2+) channel antagonists on the paired pulse ratio. omega-Agatoxin TK was the most effective Ca(2+) channel antagonist (IC(50)=127 nM; maximal effect=211% (with >1 microM) and Hill coefficient=1.2), suggesting a single site of action and a Q-type channel profile. Corresponding parameters for Cd(2+) were 13 microM, 178% and 1.2. The block of L-type Ca(2+) channels had little impact on transmission, but we also tested facilitation of L-type Ca(2+) channels. The L-type Ca(2+) channel agonist, s-(-)-1,4 dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-3-pyridine carboxylic acid methyl ester (Bay K 8644 (5 microM)), produced a 45% reduction of the paired pulse ratio, suggesting that even if L-type channels do not participate in the release process, they may participate in its modulation.

Antiparkinsonian action of a delta opioid agonist in rodent and primate models of Parkinson's disease

The opioid peptides localized in striatal projection neurons are of great relevance to Parkinson's disease, not only as a consequence of their distribution, but also due to the pronounced changes in expression seen in Parkinson's disease. It has long been suspected that increased expression of enkephalin may represent one of the many mechanisms that compensate for dopamine (DA) depletion in Parkinson's disease. Here we demonstrate that a systemically delivered, selective delta opioid agonist (SNC80) has potent antiparkinsonian actions in both rat and primate models of Parkinson's disease. In rats treated with either the D2-preferring DA antagonist haloperidol (1 mg/kg) or the selective D1 antagonist SCH23390 (1 mg/kg), but not a combination of D1 and D2 antagonists, SNC80 (10 mg/kg) completely reversed the catalepsy induced by DA antagonists. In rats rendered immobile by treatment with reserpine, SNC80 dose-dependently reversed akinesia (EC(50) 7.49 mg/kg). These effects were dose-dependently inhibited (IC(50) 1.05 mg/kg) by a selective delta opioid antagonist (naltrindole) and by SCH23390 (1 mg/kg), but not by haloperidol (1 mg/kg). SNC80 also reversed parkinsonian symptoms in the MPTP-treated marmoset. At 10 mg/kg (ip), scores measuring bradykinesia and posture were significantly reduced and motor activity increased to levels comparable with pre-MPTP-treatment scores. Any treatment that serves to increase delta opioid receptor activation may be a useful therapeutic strategy for the treatment of Parkinson's disease, either in the early stages or as an adjunct to dopamine replacement therapy. Furthermore, enhanced enkephalin expression observed in Parkinson's disease may serve to potentiate dopamine acting preferentially at D1 receptors.

Delayed cerebral edema and fatal coma after minor head trauma: role of the CACNA1A calcium channel subunit gene and relationship with familial hemiplegic migraine

Trivial head trauma may be complicated by severe, sometimes even fatal, cerebral edema and coma occurring after a lucid interval ("delayed cerebral edema"). Attacks of familial hemiplegic migraine (FHM) can be triggered by minor head trauma and are sometimes accompanied by coma. Mutations in the CACNA1A calcium channel subunit gene on chromosome 19 are associated with a wide spectrum of mutation-specific episodic and chronic neurological disorders, including FHM with or without coma. We investigated the role of the CACNA1A gene in three subjects with delayed cerebral edema. Two subjects originated from a family with extreme FHM, and one subject was the previously asymptomatic daughter of a sporadic patient with hemiplegic migraine attacks. In all three subjects with delayed severe edema, we found a C-to-T substitution resulting in the substitution of serine for lysine at codon 218 (S218L) in the CACNA1A gene. The mutation was absent in nonaffected family members and 152 control individuals. Haplotype analysis excluded a common founder for both families. Neuropathological examination in one subject showed Purkinje cell loss with relative preservation of granule cells and sparing of the dentate and inferior olivary nuclei. We conclude that the novel S218L mutation in the CACNA1A calcium channel subunit gene is involved in FHM and delayed fatal cerebral edema and coma after minor head trauma. This finding may have important implications for the understanding and treatment of this dramatic syndrome.

The adrenergic receptor agonist, clonidine, potentiates the anti-parkinsonian action of the selective kappa-opioid receptor agonist, enadoline, in the monoamine-depleted rat

1. The treatment of Parkinson's disease relies predominantly upon dopamine replacement therapy, usually with l-dihydroxyphenylalanine (L-DOPA). However, side-effects of long-term treatment, such as L-DOPA-induced dyskinesias can be more debilitating than the disease itself. Non-dopaminergic treatment strategies might therefore be advantageous. 2. The aim of this study was to investigate the potential anti-parkinsonian efficacy of the kappa-opioid receptor agonist, enadoline, and the alpha-adrenoreceptor agonist, clonidine, both alone or in combination, in the reserpine-treated rat model of Parkinson's disease. 3. Rats were treated with reserpine (3 mg kg-1), and experiments carried out 18 h later, at which time they exhibited profound akinesia (normal animals 1251+/-228 mobile counts h-1, reserpine-treated animals 9+/-2 mobile counts h-1). Both enadoline and clonidine increased locomotion in reserpine-treated rats in a dose-dependent manner. The maximum locomotor-stimulating effect of enadoline alone was seen at a dose of 0.2 mg kg-1 (208+/-63 mobile counts h-1). The maximum effect of clonidine was seen at a dose of 2 mg kg-1 (536+/-184 mobile counts h-1). 4. Co-administration of enadoline (0.1 mg kg-1) and clonidine (0.01 - 0.1 mg kg-1) at sub-threshold doses, synergistically increased locomotion. 5. The synergistic stimulation of locomotion in the reserpine-treated rat involved activation of kappa-opioid receptors and a combination of both alpha1 and alpha2-adrenoreceptors. 6. The results presented suggest a need for further studies on the potential of stimulating kappa-opioid and/or alpha-adrenoreceptors as a therapy for Parkinson's disease. Furthermore, the studies may offer potential mechanistic explanations of the ability of alpha2-adrenergic receptor antagonist to reduce L-DOPA-induced dyskinesia in Parkinson's disease.