SCH 23390 affords protection against soman-evoked seizures in the freely moving guinea-pig: a concomitant neurochemical, electrophysiological and behavioural study

Allosteric Modulators of Sigma-1 Receptor: A Review

Allosteric modulators of sigma-1 receptor (Sig1R) are described as compounds that can increase the activity of some Sig1R ligands that compete with (+)-pentazocine, one of the classic prototypical ligands that binds to the orthosteric Sig1R binding site. Sig1R is an endoplasmic reticulum membrane protein that, in addition to its promiscuous high-affinity ligand binding, has been shown to have chaperone activity. Different experimental approaches have been used to describe and validate the activity of allosteric modulators of Sig1R. Sig1R-modulatory activity was first found for phenytoin, an anticonvulsant drug that primarily acts by blocking the voltage-gated sodium channels. Accumulating evidence suggests that allosteric Sig1R modulators affect processes involved in the pathophysiology of depression, memory and cognition disorders as well as convulsions. This review will focus on the description of selective and non-selective allosteric modulators of Sig1R, including molecular structure properties and pharmacological activity both in vitro and in vivo, with the aim of providing the latest overview from compound discovery approaches to eventual clinical applications. In this review, the possible mechanisms of action will be discussed, and future challenges in the development of novel compounds will be addressed.

Polypharmacology of dopamine receptor ligands

Most neurological diseases have a multifactorial nature and the number of molecular mechanisms discovered as underpinning these diseases is continuously evolving. The old concept of developing selective agents for a single target does not fit with the medical need of most neurological diseases. The development of designed multiple ligands holds great promises and appears as the next step in drug development for the treatment of these multifactorial diseases. Dopamine and its five receptor subtypes are intimately involved in numerous neurological disorders. Dopamine receptor ligands display a high degree of cross interactions with many other targets including G-protein coupled receptors, transporters, enzymes and ion channels. For brain disorders like Parkinsońs disease, schizophrenia and depression the dopaminergic system, being intertwined with many other signaling systems, plays a key role in pathogenesis and therapy. The concept of designed multiple ligands and polypharmacology, which perfectly meets the therapeutic needs for these brain disorders, is herein discussed as a general ligand-based concept while focusing on dopaminergic agents and receptor subtypes in particular.

The cannabinoid receptor antagonist AM251 increases paraoxon and chlorpyrifos oxon toxicity in rats

Organophosphorus anticholinesterases (OPs) elicit acute toxicity by inhibiting acetylcholinesterase (AChE), leading to acetylcholine accumulation and overstimulation of cholinergic receptors. Endocannabinoids (eCBs, e.g., arachidonoyl ethanolamide [AEA] and 2-arachidonoyl glycerol [2-AG]) are neuromodulators that regulate neurotransmission by reducing neurotransmitter release. The eCBs are degraded by the enzymes fatty acid amide hydrolase (FAAH, primarily involved in hydrolysis of AEA) and monoacylglycerol lipase (MAGL, primarily responsible for metabolism of 2-AG). We previously reported that the cannabinoid receptor agonist WIN 55,212-2 reduced cholinergic toxicity after paraoxon exposure. This study compared the effects of the cannabinoid receptor antagonist AM251 on acute toxicity following either paraoxon (PO) or chlorpyrifos oxon (CPO). CPO was more potent in vitro than PO at inhibiting AChE (≈ 2 fold), FAAH (≈ 8 fold), and MAGL (≈ 19 fold). Rats were treated with vehicle, PO (0.3 and 0.6 mg/kg, sc) or CPO (6 and 12 mg/kg, sc) and subsets treated with AM251 (3mg/kg, ip; 30 min after OP). Signs of toxicity were recorded for 4h and rats were then sacrificed. OP-treated rats showed dose-related involuntary movements, with AM251 increasing signs of toxicity with the lower dosages. PO and CPO elicited excessive secretions, but AM251 had no apparent effect with either OP. Lethality was increased by AM251 with the higher dosage of PO, but no lethality was noted with either dosage of CPO, with or without AM251. Both OPs caused extensive inhibition of hippocampal AChE and FAAH (>80-90%), but only CPO inhibited MAGL (37-50%). These results provide further evidence that eCB signaling can influence acute OP toxicity. The selective in vivo inhibition of MAGL by CPO may be important in the differential lethality noted between PO and CPO with AM251 co-administration.

Endocannabinoid signaling in neurotoxicity and neuroprotection

The cannabis plant and products produced from it, such as marijuana and hashish, have been used for centuries for their psychoactive properties. The mechanism for how Delta(9)-tetrahydrocannabinol (THC), the active constituent of cannabis, elicits these neurological effects remained elusive until relatively recently, when specific G-protein coupled receptors were discovered that appeared to mediate cellular actions of THC. Shortly after discovery of these specific receptors, endogenous ligands (endocannabinoids) were identified. Since that time, an extensive number of papers have been published on the endocannabinoid signaling system, a widespread neuromodulatory mechanism that influences neurotransmission throughout the nervous system. This paper summarizes presentations given at the 12th International Neurotoxicology Association meeting that described the potential role of endocannabinoids in the expression of neurotoxicity. Dr. Raphael Mechoulam first gave an overview of the discovery of exogenous and endogenous cannabinoids and their potential for neuroprotection in a variety of conditions. Dr. Larry Parsons then described studies suggesting that endocannabinoid signaling may play a selective role in drug reinforcement. Dr. Carey Pope presented information on the role that endocannabinoid signaling may have in the expression of cholinergic toxicity following anticholinesterase exposures. Together, these presentations highlighted the diverse types of neurological insults that may be modulated by endocannabinoids and drugs/toxicants which might influence endocannabinoid signaling pathways.

Dopamine receptor modulation of hypoxic-ischemic neuronal injury in striatum of newborn piglets

Dopamine receptors regulate glutamatergic neurotransmission and Na(+),K(+)-ATPase via protein kinase A (PKA) and dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32)-dependent signaling. Consequently, dopamine receptor activation may modulate neonatal hypoxic-ischemic (H-I) neuronal damage in the selectively vulnerable putamen enriched with dopaminergic receptors. Piglets subjected to two durations of hypoxia followed by asphyxic cardiac arrest were treated with a D1-like (SCH23390) or D2-like (sulpiride) receptor antagonist. At 4 days of recovery from less severe H-I, the remaining viable neurons in putamen were 60% of control, but nearly completely salvaged by pretreatment with SCH23390 or sulpiride. After more severe H-I in which only 18% of neurons were viable, partial neuroprotection was seen with SCH23390 pretreatment (50%) and posttreatment (39%) and with sulpiride pretreatment (35%), but not with sulpiride posttreatment (24%). Dopamine was significantly elevated in microdialysis samples from putamen during asphyxia and the first 15 mins of reoxygenation. Pretreatment with SCH23390 or sulpiride largely attenuated the increased nitrotyrosine and the decreased Na(+),K(+)-ATPase activity that occurred at 3 h after severe H-I. Pretreatment with SCH23390, but not sulpiride, also attenuated H-I-induced increases in PKA-dependent phosphorylation of Thr34 on DARPP-32, Ser943 on the alpha subunit of Na(+),K(+)-ATPase, and Ser897 of the N-methyl-D-aspartate (NMDA) receptor NR1 subunit. These findings indicate that D1 and D2 dopamine receptor activation contribute to neuronal death in newborn putamen after H-I in association with increased protein nitration and decreased Na(+),K(+)-ATPase activity. Furthermore, mechanisms of D1 receptor toxicity may involve DARPP-32-dependent phosphorylation of NMDA receptor NR1 and Na(+),K(+)-ATPase.

Effects of anticonvulsants on soman-induced epileptiform activity in the guinea-pig in vitro hippocampus

Seizures arising from acetylcholinesterase inhibition are a feature of organophosphate anticholinesterase intoxication. Although benzodiazepines are effective against these seizures, alternative anticonvulsant drugs may possess greater efficacy and fewer side-effects. We have investigated in the guinea-pig hippocampal slice preparation the ability of a series of anticonvulsants to suppress epileptiform bursting induced by the irreversible organophosphate anticholinesterase, soman (100 nM). Carbamazepine (300 microM), phenytoin (100 microM), topiramate (100-300 microM) and retigabine (1-30 microM) reduced the frequency of bursting but only carbamazepine and phenytoin induced a concurrent reduction in burst duration. Felbamate (100-500 microM) and clomethiazole (100-300 microM) had no effect on burst frequency but decreased burst duration. Clozapine (3-30 microM) reduced the frequency but did not influence burst duration. Levetiracetam (100-300 microM) and gabapentin (100-300 microM) were without effect. These data suggest that several compounds, in particular clomethiazole, clozapine, felbamate, topiramate and retigabine, merit further evaluation as possible treatments for organophosphate poisoning.

Intracerebral microdialysis: 30 years as a tool for the neuroscientist

1. Microdialysis is an established technique for studying physiological, pharmacological and pathological changes of a wide range of low molecular weight substances in the brain extracellular fluid. Many studies have proven its sensitivity in sampling the extracellular space in discrete brain locations, such as the striatum, and monitoring the action of exogenous substances. 2. The two main areas of application of microdialysis are the recovery of endogenous substances, primarily the neurotransmitters, and the infusion of drugs through the microdialysis cannula (retrodialysis). 3. Microdialysis in awake animals is the tool of choice for studying the relationship between changes in behaviour and neurotransmitters in certain brain areas. In addition, the concomitant recording of the electroencephalogram at the site of microdialysis has been shown to be extremely useful in determining the role of certain neurotransmitters in paroxysmal activity. 4. Clinical applications of microdialysis have included monitoring of ischaemic injury, subarachnoid haemorrhage, trauma and epilepsy. With the recent availability of standardized equipment, the use of microdialysis in the neurological clinic is likely to become more common.

A review of membrane sampling from biological tissues with applications in pharmacokinetics, metabolism and pharmacodynamics

This review provides an overview of membrane sampling techniques, microdialysis and ultrafiltration, and cites illustrations of their applications in pharmacokinetics, metabolism and/or pharmacodynamics. The review organizes applications by target tissue and general type of information gleaned. It focuses on recently published microdialysis studies (1999 to this writing) and offers the first review of ultrafiltration sampling studies. The advantages and limitations of using microdialysis and ultrafiltration sampling as tools for obtaining pharmacokinetic and metabolism data are discussed. Numerous examples are described including studies in which several types of data are collected simultaneously. Reports that study local metabolism of drug delivered through the probe are also presented.

Changes in striatal electroencephalography and neurochemistry induced by kainic acid seizures are modified by dopamine receptor antagonists

We investigated the involvement of striatal dopamine release in electrographic and motor seizure activity evoked by kainic acid in the guinea pig. The involvement of the dopamine receptor subtypes was studied by systemic administration of the dopamine D(1) receptor antagonist, R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride (SCH 23390; 0.5 mg kg(-1)), or the dopamine D(2) antagonist, (5-aminosulphonyl)-N-[(1-ethyl-2-pyrrolidinyl)-methyl]-2-methoxybenzamide (sulpiride, 30 mg kg(-1)). Microdialysis and high performance liquid chromatography were used to monitor changes in extracellular levels of striatal dopamine and its metabolites, glutamate, aspartate and gamma-amino-butyric acid (GABA). These data were correlated with changes in the striatal and cortical electroencephalographs and clinical signs. We found that, although neither dopamine receptor antagonist inhibited behavioural seizure activity, blockade of the dopamine D(1)-like receptor with SCH 23390 significantly reduced both the 'power' of the electrical seizure activity and the associated change in extracellular striatal concentration of glutamate, whilst increasing the extracellular striatal concentration of GABA. In contrast, blockade of the dopamine D(2)-like receptor with sulpiride significantly increased the extracellular, striatal content of glutamate and the dopamine metabolites. These results confirm previous evidence in other models of chemically-evoked seizures that antagonism of the dopamine D(1) receptor tends to reduce motor and electrographic seizure activity as well as excitatory amino-acid transmitter activity, while antagonism of the dopamine D(2) receptor has relatively less apparent effect.

SCH 23390: the first selective dopamine D1-like receptor antagonist

SCH 23390, the halobenzazepine (R)-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5- tetrahydro-1H-3-benzazepine, is a highly potent and selective dopamine D1-like receptor antagonist with a K(i) of 0.2 and 0.3 nM for the D1 and D5 dopamine receptor subtypes, respectively. In vitro, it also binds with high affinity to the 5-HT2 and 5-HT1C serotonin receptor subtypes. However, the doses required to induce a similar response in vivo are greater than 10-fold higher than those required to induce a D1-mediated response. Previous in vivo pharmacological studies with SCH 23390 have shown it to abolish generalized seizures evoked by the chemoconvulsants: pilocarpine and soman. These studies provide evidence of the potential importance of D1-like dopaminergic receptor mechanisms in facilitating the initiation and spread of seizures. The inference from a majority of studies is that the activation of dopamine D1 receptors facilitates seizure activity, whereas activation of D2 receptors may inhibit the development of seizures. SCH 23390 has also been used in studies of other neurological disorders in which the dopamine system has been implicated, such as psychosis and Parkinson's disease. Apart from the study of neurological disorders, SCH 23390 has been extensively used as a tool in the topographical determination of brain D1 receptors in rodents, nonhuman primates, and humans. In summary, SCH 23390 has been a major tool in gaining a better understanding of the role of the dopamine system, more specifically the D1 receptor, in neurological function and dysfunction.