A study of the N-methyl-D-aspartate antagonistic properties of anticholinergic drugs

TRPV4 modulation participates in paraoxon-induced brain injury via NMDA and NLRP3 regulation

BACKGROUND

Organophosphorus pesticide poisoning can lead to severe brain damage, but the specific mechanisms involved are not fully understood. Our research aims to elucidate the function of the TRPV4 ion channel in the development of brain injury induced by paraoxon (POX).

METHODS

In vivo, we examined the survival rate, behavioral seizures, histopathological alterations, NMDA receptor phosphorylation, as well as the expression of the NLRP3-ASC-caspase-1 complex and downstream inflammatory factors in the POX poisoning model following intervention with the TRPV4 antagonist GSK2193874. In vitro, we investigated the effects of GSK2193874 on NMDA-induced inward current, cell viability, cell death rate, and Ca2+ accumulation in primary hippocampal neurons.

RESULTS

The treatment with the TRPV4 antagonist increased the survival rate, suppressed the status epilepticus, improved pathological damage, and reduced the phosphorylation level of NMDA receptors after POX exposure. Additionally, it inhibited the upregulation of NLRP3 inflammasome and inflammatory cytokines expression after POX exposure. Moreover, the TRPV4 antagonist corrected the NMDA-induced increase in inward current and cell death rate, decrease in cell viability, and Ca2+ accumulation.

CONCLUSION

TRPV4 participates in the mechanisms of brain injury induced by POX exposure through NMDA-mediated excitotoxicity and NLRP3-mediated inflammatory response.

Efficacy of a combined anti-seizure treatment against cholinergic established status epilepticus following a sarin nerve agent insult in rats

The development of refractory status epilepticus (SE) following sarin intoxication presents a therapeutic challenge. Here, we evaluated the efficacy of delayed combined double or triple treatment in reducing abnormal epileptiform seizure activity (ESA) and the ensuing long-term neuronal insult. SE was induced in rats by exposure to 1.2 LD50 sarin followed by treatment with atropine and TMB4 (TA) 1 min later. Double treatment with ketamine and midazolam or triple treatment with ketamine, midazolam and levetiracetam was administered 30 min post-exposure, and the results were compared to those of single treatment with midazolam alone or triple treatment with ketamine, midazolam, and valproate, which was previously shown to ameliorate this neurological insult. Toxicity and electrocorticogram activity were monitored during the first week, and behavioral evaluations were performed 2 weeks post-exposure, followed by biochemical and immunohistopathological analyses. Both double and triple treatment reduced mortality and enhanced weight recovery compared to TA-only treatment. Triple treatment and, to a lesser extent, double treatment significantly ameliorated the ESA duration. Compared to the TA-only or the TA+ midazolam treatment, both double and triple treatment reduced the sarin-induced increase in the neuroinflammatory marker PGE2 and the brain damage marker TSPO and decreased gliosis, astrocytosis and neuronal damage. Finally, both double and triple treatment prevented a change in behavior, as measured in the open field test. No significant difference was observed between the efficacies of the two triple treatments, and both triple combinations completely prevented brain injury (no differences from the naïve rats). Delayed double and, to a greater extent, triple treatment may serve as an efficacious delayed therapy, preventing brain insult propagation following sarin-induced refractory SE.

Central cholinergic transmission affects the compulsive-like behavior of mice in marble-burying test

The presence of the cholinergic system in the brain areas implicated in the precipitation of obsessive-compulsive behavior (OCB) has been reported but the exact role of the central cholinergic system therein is still unexplored. Therefore, the current study assessed the effect of cholinergic analogs on central administration on the marble-burying behavior (MBB) of mice, a behavior correlated with OCB. The result reveals that the enhancement of central cholinergic transmission in mice achieved by intracerebroventricular (i.c.v.) injection of acetylcholine (0.01 µg) (Subeffective: 0.1 and 0.5 µg), cholinesterase inhibitor, neostigmine (0.1, 0.3, 0.5 µg/mouse) and neuronal nicotinic acetylcholine receptor agonist, nicotine (0.1, 2 µg/mouse) significantly attenuated the number of marbles buried by mice in MBB test without affecting basal locomotor activity. Similarly, central injection of mAChR antagonist, atropine (0.1, 0.5, 5 µg/mouse), nAChR antagonist, mecamylamine (0.1, 0.5, 3 µg/mouse) per se also reduced the MBB in mice, indicative of anti-OCB like effect of all the tested cholinergic mAChR or nAChR agonist and antagonist. Surprisingly, i.c.v. injection of acetylcholine (0.01 µg), and neostigmine (0.1 µg) failed to elicit an anti-OCB-like effect in mice pre-treated (i.c.v.) with atropine (0.1 µg), or mecamylamine (0.1 µg). Thus, the findings of the present investigationdelineate the role of central cholinergic transmission in the compulsive-like behavior of mice probably via mAChR or nAChR stimulation.

Combination of acetylcholinesterase inhibitors and NMDA receptor antagonists increases survival rate in soman-poisoned mice

Organophosphorus nerve agents pose a global threat to both military personnel and civilian population, because of their high acute toxicity and insufficient medical countermeasures. Commonly used drugs could ameliorate the intoxication and overall medical outcomes. In this study, we tested the drugs able to alleviate the symptoms of Alzheimer's disease (donepezil, huperzine A, memantine) or Parkinson's disease (procyclidine). They were administered to mice before soman intoxication in terms of their: i) protection potential against soman toxicity and ii) influence on post-exposure therapy consisting of atropine and asoxime (also known as oxime HI-6). Their pretreatment effect was not significant, when administered alone, but in combination (acetylcholinesterase inhibitor such as denepezil or huperzine A with NMDA antagonist such as memantine or procyclidine) they lowered the soman toxicity more than twice. These combinations also positively influenced the efficacy of post-exposure treatment in a similar fashion; the combinations increased the therapeutic effectiveness of antidotal treatment. In conclusion, the most effective combination - huperzine A and procyclidine - lowered the toxicity three times and improved the post-exposure therapy efficacy more than six times. These results are unprecedented in the published literature.

Synthesis and preliminary biological evaluation of gabactyzine, a benactyzine-GABA mutual prodrug, as an organophosphate antidote

Organophosphates (OPs) are inhibitors of acetylcholinesterase and have deleterious effects on the central nervous system. Clinical manifestations of OP poisoning include convulsions, which represent an underlying toxic neuro-pathological process, leading to permanent neuronal damage. This neurotoxicity is mediated through the cholinergic, GABAergic and glutamatergic (NMDA) systems. Pharmacological interventions in OP poisoning are designed to mitigate these specific neuro-pathological pathways, using anticholinergic drugs and GABAergic agents. Benactyzine is a combined anticholinergic, anti-NMDA compound. Based on previous development of novel GABA derivatives (such as prodrugs based on perphenazine for the treatment of schizophrenia and nortriptyline against neuropathic pain), we describe the synthesis and preliminary testing of a mutual prodrug ester of benactyzine and GABA. It is assumed that once the ester crosses the blood-brain-barrier it will undergo hydrolysis, releasing benactyzine and GABA, which are expected to act synergistically. The combined release of both compounds in the brain offers several advantages over the current OP poisoning treatment protocol: improved efficacy and safety profile (where the inhibitory properties of GABA are expected to counteract the anticholinergic cognitive adverse effects of benactyzine) and enhanced chemical stability compared to benactyzine alone. We present here preliminary results of animal studies, showing promising results with early gabactyzine administration.

Neuroprotection by delayed triple therapy following sarin nerve agent insult in the rat

The development of refractory status epilepticus (SE) induced by sarin intoxication presents a therapeutic challenge. In our current research we evaluate the efficacy of a delayed combined triple treatment in ending the abnormal epileptiform seizure activity (ESA) and the ensuing of long-term neuronal insult. SE was induced in male Sprague-Dawley rats by exposure to 1.2LD50 sarin insufficiently treated by atropine and TMB4 (TA) 1 min later. Triple treatment of ketamine, midazolam and valproic acid was administered 30 min or 1 h post exposure and was compared to a delayed single treatment with midazolam alone. Toxicity and electrocorticogram activity were monitored during the first week and behavioral evaluation performed 3 weeks post exposure followed by brain biochemical and immunohistopathological analyses. The addition of both single and triple treatments reduced mortality and enhanced weight recovery compared to the TA-only treated group. The triple treatment also significantly minimized the duration of the ESA, reduced the sarin-induced increase in the neuroinflammatory marker PGE2, the brain damage marker TSPO, decreased the gliosis, astrocytosis and neuronal damage compared to the TA+ midazolam or only TA treated groups. Finally, the triple treatment eliminated the sarin exposed increased open field activity, as well as impairing recognition memory as seen in the other experimental groups. The delayed triple treatment may serve as an efficient therapy, which prevents brain insult propagation following sarin-induced refractory SE, even if treatment is postponed for up to 1 h.

Enhanced analgesic cholinergic tone in the spinal cord in a mouse model of neuropathic pain

Endogenous acetylcholine (ACh) is an important modulator of nociceptive sensory processing in the spinal cord. An increased level of spinal ACh induces analgesia both in humans and rodents while interfering with cholinergic signaling is allodynic, demonstrating that a basal tone of spinal ACh modulates nociceptive responses in naïve animals. The plasticity undergone by this cholinergic system in chronic pain situation is unknown, and the mere presence of this tone in neuropathic animals is controversial. We have addressed these issues in mice through behavioral experiments, histology, electrophysiology and molecular biology, in the cuff model of peripheral neuropathy. Our behavior experiments demonstrate the persistence, and even increased impact of the analgesic cholinergic tone acting through nicotinic receptors in cuff animals. The neuropathy does not affect the number or membrane properties of dorsal horn cholinergic neurons, nor specifically the frequency of their synaptic inputs. The alterations thus appear to be in the neurons receiving the cholinergic signaling, which is confirmed by the fact that subthreshold doses of acetylcholinesterase (AChE) inhibitors in sham animals become anti-allodynic in cuff mice and by the altered expression of the β2 nicotinic receptor subunit. Our results demonstrate that endogenous cholinergic signaling can be manipulated to relieve mechanical allodynia in animal models of peripheral neuropathy. Until now, AChE inhibitors have mainly been used in the clinics in situations of acute pain (parturition, post-operative). The fact that lower doses (thus with fewer side effects) could be efficient in chronic pain conditions opens new avenues for the treatment of neuropathic pain. SIGNIFICANCE STATEMENT: Chronic pain continues to be the most common cause of disability that impairs the quality of life, accruing enormous and escalating socio-economic costs. A better understanding of the plasticity of spinal neuronal networks, crucially involved in nociceptive processing, could help designing new therapeutic avenues. We here demonstrate that chronic pain modifies the spinal nociceptive network in such a way that it becomes more sensitive to cholinergic modulations. The spinal cholinergic system is responsible for an analgesic tone that can be exacerbated by acetylcholinesterase inhibitors, a property used in the clinic to relief acute pain (child birth, post-op). Our results suggest that lower doses of acetylcholinesterases, with even fewer side effects, could be efficient to relieve chronic pain.

TRPV4 Regulates Soman-Induced Status Epilepticus and Secondary Brain Injury via NMDA Receptor and NLRP3 Inflammasome

Nerve agents are used in civil wars and terrorist attacks, posing a threat to public safety. Acute exposure to nerve agents such as soman (GD) causes serious brain damage, leading to death due to intense seizures induced by acetylcholinesterase inhibition and neuronal injury resulting from increased excitatory amino-acid levels and neuroinflammation. However, data on the anticonvulsant and neuroprotective efficacies of currently-used countermeasures are limited. Here, we evaluated the potential effects of transient receptor vanilloid 4 (TRPV4) in the treatment of soman-induced status epilepticus (SE) and secondary brain injury. We demonstrated that TRPV4 expression was markedly up-regulated in rat hippocampus after soman-induced seizures. Administration of the TRPV4 antagonist GSK2193874 prior to soman exposure significantly decreased the mortality rate in rats and reduced SE intensity. TRPV4-knockout mice also showed lower incidence of seizures and higher survival rates than wild-type mice following soman exposure. Further in vivo and in vitro experiments demonstrated that blocking TRPV4 prevented NMDA receptor-mediated glutamate excitotoxicity. The protein levels of the NLRP3 inflammasome complex and its downstream cytokines IL-1β and IL-18 increased in soman-exposed rat hippocampus. However, TRPV4 inhibition or deletion markedly reversed the activation of the NLRP3 inflammasome pathway. In conclusion, our study suggests that the blockade of TRPV4 protects against soman exposure and reduces brain injury following SE by decreasing NMDA receptor-mediated excitotoxicity and NLRP3-mediated neuroinflammation. To our knowledge, this is the first study regarding the "dual-switch" function of TRPV4 in the treatment of soman intoxication.

Genetic Differences in Dorsal Hippocampus Acetylcholinesterase Activity Predict Contextual Fear Learning Across Inbred Mouse Strains

Learning is a critical behavioral process that is influenced by many neurobiological systems. We and others have reported that acetylcholinergic signaling plays a vital role in learning capabilities, and it is especially important for contextual fear learning. Since cholinergic signaling is affected by genetic background, we examined the genetic relationship between activity levels of acetylcholinesterase (AChE), the primary enzyme involved in the acetylcholine metabolism, and learning using a panel of 20 inbred mouse strains. We measured conditioned fear behavior and AChE activity in the dorsal hippocampus, ventral hippocampus, and cerebellum. Acetylcholinesterase activity varied among inbred mouse strains in all three brain regions, and there were significant inter-strain differences in contextual and cued fear conditioning. There was an inverse correlation between fear conditioning outcomes and AChE levels in the dorsal hippocampus. In contrast, the ventral hippocampus and cerebellum AChE levels were not correlated with fear conditioning outcomes. These findings strengthen the link between acetylcholine activity in the dorsal hippocampus and learning, and they also support the premise that the dorsal hippocampus and ventral hippocampus are functionally discrete.

Comparative physiology and efficacy of atropine and scopolamine in sarin nerve agent poisoning

Anticholinergic treatment is key for effective medical treatment of nerve agent exposure. Atropine is included at a 2 mg intramuscular dose in so-called autoinjectors designed for self- and buddy-aid. As patient cohorts are not available, predicting and evaluating the efficacy of medical countermeasures relies on animal models. The use of atropine as a muscarinic antagonist is based on efficacy achieved in studies in a variety of species. The dose of atropine administered varies considerably across these studies. This is a complicating factor in the prediction of efficacy in the human situation, largely because atropine dosing also influences therapeutic efficacy of oximes and anticonvulsants generally part of the treatment administered. To improve translation of efficacy of dosing regimens, including pharmacokinetics and physiology provide a promising approach. In the current study, pharmacokinetics and physiological parameters obtained using EEG and ECG were assessed in naïve rats and in sarin-exposed rats for two anticholinergic drugs, atropine and scopolamine. The aim was to find a predictive parameter for therapeutic efficacy. Scopolamine and atropine showed a similar bioavailability, but brain levels reached were much higher for scopolamine. Scopolamine exhibited a dose-dependent loss of beta power in naïve animals, whereas atropine did not show any such central effect. This effect was correlated with an enhanced anticonvulsant effect of scopolamine compared to atropine. These findings show that an approach including pharmacokinetics and physiology could contribute to improved dose scaling across species and assessing the therapeutic potential of similar anticholinergic and anticonvulsant drugs against nerve agent poisoning.

Efficacy of retigabine in ameliorating the brain insult following sarin exposure in the rat

BACKGROUND

Sarin is an irreversible organophosphate cholinesterase inhibitor. Following toxic signs, an extensive long-term brain damage is often reported. Thus, we evaluated the efficacy of a novel anticonvulsant drug retigabine, a modulator of neuronal voltage gated K⁺ channels, as a neuroprotective agent following sarin exposure.

METHODS

Rats were exposed to 1 LD₅₀ or 1.2 LD₅₀ sarin and treated at onset of convulsions with retigabine (5 mg/kg, i.p.) alone or in combination with 5 mg/kg atropine and 7.5 mg/kg TMB-4 (TA) respectively. Brain biochemical and immunohistopathological analyses were processed 24 h and 1 week following 1 LD₅₀ sarin exposure and at 4 weeks following exposure to 1.2 LD₅₀ sarin. EEG activity in freely moving rats was also monitored by telemetry during the first week following exposure to 1.2 LD₅₀ and behavior in the Open Field was evaluated 3 weeks post exposure.

RESULTS

Treatment with retigabine following 1 LD₅₀ sarin exposure or in combination with TA following 1.2 LD₅₀ exposure significantly reduced mortality rate compared to the non-treated groups. In both experiments, the retigabine treatment significantly reduced gliosis, astrocytosis and brain damage as measured by translocator protein (TSPO). Following sarin exposure the combined treatment (retigabine+ TA) significantly minimized epileptiform seizure activity. Finally, in the Open Field behavioral test the non-treated sarin group showed an increased mobility which was reversed by the combined treatment.

CONCLUSIONS

The M current modulator retigabine has been shown to be an effective adjunct therapy following OP induced convulsion, minimizing epileptiform seizure activity and attenuating the ensuing brain damage.

Antidepressant-relevant concentrations of the ketamine metabolite (2R,6R)-hydroxynorketamine do not block NMDA receptor function

Preclinical studies indicate that (2R,6R)-hydroxynorketamine (HNK) is a putative fast-acting antidepressant candidate. Although inhibition of NMDA-type glutamate receptors (NMDARs) is one mechanism proposed to underlie ketamine's antidepressant and adverse effects, the potency of (2R,6R)-HNK to inhibit NMDARs has not been established. We used a multidisciplinary approach to determine the effects of (2R,6R)-HNK on NMDAR function. Antidepressant-relevant behavioral responses and (2R,6R)-HNK levels in the extracellular compartment of the hippocampus were measured following systemic (2R,6R)-HNK administration in mice. The effects of ketamine, (2R,6R)-HNK, and, in some cases, the (2S,6S)-HNK stereoisomer were evaluated on the following: (i) NMDA-induced lethality in mice, (ii) NMDAR-mediated field excitatory postsynaptic potentials (fEPSPs) in the CA1 field of mouse hippocampal slices, (iii) NMDAR-mediated miniature excitatory postsynaptic currents (mEPSCs) and NMDA-evoked currents in CA1 pyramidal neurons of rat hippocampal slices, and (iv) recombinant NMDARs expressed in Xenopus oocytes. While a single i.p. injection of 10 mg/kg (2R,6R)-HNK exerted antidepressant-related behavioral and cellular responses in mice, the ED₅₀ of (2R,6R)-HNK to prevent NMDA-induced lethality was found to be 228 mg/kg, compared with 6.4 mg/kg for ketamine. The 10 mg/kg (2R,6R)-HNK dose generated maximal hippocampal extracellular concentrations of ∼8 µM, which were well below concentrations required to inhibit synaptic and extrasynaptic NMDARs in vitro. (2S,6S)-HNK was more potent than (2R,6R)-HNK, but less potent than ketamine at inhibiting NMDARs. These data demonstrate the stereoselectivity of NMDAR inhibition by (2R,6R;2S,6S)-HNK and support the conclusion that direct NMDAR inhibition does not contribute to antidepressant-relevant effects of (2R,6R)-HNK.

Comparative effects of scopolamine and phencynonate on organophosphorus nerve agent-induced seizure activity, neuropathology and lethality

The efficacy of anticonvulsant therapies to stop seizure activities following organophosphorus nerve agents (NAs) has been documented as being time-dependent. We utilized the guinea pig NA-seizure model to compare the effectiveness of phencynonate (PCH) and scopolamine (SCP) when given at the early (at time of seizure onset) or late (40 min after seizure onset) phase of seizure progression. PCH possesses both anticholinergic and anti-NMDA activities, while SCP is a purely anti-muscarinic compound. Animals with cortical electrodes were pretreated with pyridostigmine bromide 30 min prior to exposure to a 2.0 x LD₅₀ subcutaneous dose of a NA (GA, GB, GD, GF, VR, or VX), followed one min later with atropine sulfate and 2-PAM. At either early or late phase, animals were treated with either PCH or SCP and the 24-h anticonvulsant ED₅₀ doses were determined. When administered at seizure onset, PCH, and SCP were both effective at terminating seizure activity against all NAs, with ED₅₀ values for SCP generally being lower. At the 40 min time, ED₅₀ values were obtained following GA, GD, GF, and VR challenges for SCP, but ED₅₀ value was obtained only following GD for PCH, indicating a superior efficacy of SCP. When seizure activity was controlled, a significant improvement in weight loss, neuropathology, and survival was observed, regardless of treatment or NA. Overall, these results demonstrate the differing efficacies of these two similarly structured anticholinergic compounds with delayed administration and warrant further investigation into the timing and mechanisms of the seizure maintenance phase in different animal models.

Sarin (GB, O-isopropyl methylphosphonofluoridate) neurotoxicity: critical review

Sarin (GB, O-isopropyl methylphosphonofluoridate) is a potent organophosphorus (OP) nerve agent that inhibits acetylcholinesterase (AChE) irreversibly. The subsequent build-up of acetylcholine (ACh) in the central nervous system (CNS) provokes seizures and, at sufficient doses, centrally-mediated respiratory arrest. Accumulation of ACh at peripheral autonomic synapses leads to peripheral signs of intoxication and overstimulation of the muscarinic and nicotinic receptors, which is described as "cholinergic crisis" (i.e. diarrhea, sweating, salivation, miosis, bronchoconstriction). Exposure to high doses of sarin can result in tremors, seizures, and hypothermia. More seriously, build-up of ACh at neuromuscular junctions also can cause paralysis and ultimately peripherally-mediated respiratory arrest which can lead to death via respiratory failure. In addition to its primary action on the cholinergic system, sarin possesses other indirect effects. These involve the activation of several neurotransmitters including gamma-amino-butyric acid (GABA) and the alteration of other signaling systems such as ion channels, cell adhesion molecules, and inflammatory regulators. Sarin exposure is associated with symptoms of organophosphate-induced delayed neurotoxicity (OPIDN) and organophosphate-induced chronic neurotoxicity (OPICN). Moreover, sarin has been involved in toxic and immunotoxic effects as well as organophosphate-induced endocrine disruption (OPIED). The standard treatment for sarin-like nerve agent exposure is post-exposure injection of atropine, a muscarinic receptor antagonist, accompanied by an oxime, an AChE reactivator, and diazepam.

Pretreatment and prophylaxis against nerve agent poisoning: Are undesirable behavioral side effects unavoidable?

The threat of chemical warfare agents like nerve agents requires life saving measures of medical pretreatment combined with treatment after exposure. Pretreatment (pyridostigmine) may cause some side effects in a small number of individuals. A comprehensive research on animals has been performed to clarify effects on behavior. The results from these studies are far from unambiguous, since pyridostigmine may produce adverse effects on behavior in animals in relatively high doses, but not in a consistent way. Other animal studies have examined the potential of drugs like physostigmine, galantamine, benactyzine, trihexyphenidyl, and procyclidine, but they all produce marked behavioral impairment at doses sufficient to contribute to protection against a convulsant dose of soman. Attempts have also been made to develop a combination of drugs capable of assuring full protection (prophylaxis) against nerve agents. However, common to all combinations is that they at anticonvulsant doses cause behavioral deficits. Therefore, the use of limited pretreatment doses may be performed without marked side effects followed by post-exposure therapy with a combination of drugs.

Supralethal poisoning by any of the classical nerve agents is effectively counteracted by procyclidine regimens in rats

A treatment regimen consisting of HI-6, levetiracetam, and procyclidine (termed the triple regimen) has previously been shown to work as a universal therapy against soman poisoning in rats, since it has capacities to function as both prophylactic and therapeutic measure. The purpose of the present study was to examine whether the triple regimen may have antidotal efficacy against intoxication by other classical nerve agents than soman. The treatment was given 1 and 5 min after exposure to a supralethal dose of nerve agents, and the results showed that the triple regimen successfully prevented or terminated seizures and preserved the lives of rats exposed to 5×LD50 of soman, sarin, cyclosarin, or VX, but solely 3×LD50 of tabun was managed by this regimen. To meet the particular antidotal requirements of tabun, the triple regimen was reinforced with obidoxime and was made to a quadruple regimen that effectively treated rats intoxicated by 5×LD50 of tabun. The rats recovered very well and the majority gained pre-exposure body weight within 7 days. Neuropathology was seen in all groups regardless of whether the rats seized or not. The most extensive damage was produced by sarin and cyclosarin. Differentiation between the nerve agents' potency to cause lesions was probably seen because the efficacious treatments ensured survival of supralethal poisoning. A combination of 2 oximes and 2 anticonvulsants may be a prerequisite to counteract effectively high levels of poisoning by any classical nerve agent.

The discriminative stimulus effects of mecamylamine in nicotine-treated and untreated rhesus monkeys

The extent to which chronic nicotine treatment can alter the effects of the nicotinic acetylcholine receptor antagonist mecamylamine, and whether those effects can be attenuated by nicotine have not been clearly established in the literature. Here, the discriminative stimulus effects of mecamylamine were compared between one group of rhesus monkeys receiving a continuous infusion of nicotine base (5.6 mg/kg/day subcutaneously) and another group of monkeys not receiving nicotine treatment. Both groups responded under a fixed ratio 5 schedule of stimulus-shock termination. Stimulus control was obtained at doses of 1.78 mg/kg mecamylamine in monkeys receiving continuous nicotine and 5.6 mg/kg mecamylamine in monkeys not receiving continuous nicotine treatment. Nicotine did not attenuate the discriminative stimulus effects of mecamylamine in either group. Discontinuation of continuous nicotine produced responding on the mecamylamine lever within 24 h in some but not all monkeys. This may indicate a qualitative difference in the discriminative stimulus effects of mecamylamine between groups, perhaps reflecting antagonism of nicotine and nicotine withdrawal in monkeys receiving continuous nicotine. The failure of nicotine to reverse the effects of mecamylamine is consistent with a noncompetitive interaction at nicotinic acetylcholine receptors and indicates that mecamylamine-induced withdrawal cannot be readily modified by nicotine.

Potential pharmacological strategies for the improved treatment of organophosphate-induced neurotoxicity

Organophosphates (OP) are highly toxic compounds that cause cholinergic neuronal excitotoxicity and dysfunction by irreversible inhibition of acetylcholinesterase, resulting in delayed brain damage. This delayed secondary neuronal destruction, which arises primarily in the cholinergic areas of the brain that contain dense accumulations of cholinergic neurons and the majority of cholinergic projection, could be largely responsible for persistent profound neuropsychiatric and neurological impairments such as memory, cognitive, mental, emotional, motor, and sensory deficits in the victims of OP poisoning. The therapeutic strategies for reducing neuronal brain damage must adopt a multifunctional approach to the various steps of brain deterioration: (i) standard treatment with atropine and related anticholinergic compounds; (ii) anti-excitotoxic therapies to prevent cerebral edema, blockage of calcium influx, inhibition of apoptosis, and allow for the control of seizure; (iii) neuroprotection by aid of antioxidants and N-methyl-d-aspartate (NMDA) antagonists (multifunctional drug therapy), to inhibit/limit the secondary neuronal damage; and (iv) therapies targeting chronic neuropsychiatric and neurological symptoms. These neuroprotective strategies may prevent secondary neuronal damage in both early and late stages of OP poisoning, and thus may be a beneficial approach to treating the neuropsychological and neuronal impairments resulting from OP toxicity.

Caramiphen edisylate: an optimal antidote against organophosphate poisoning

Potent cholinesterase inhibitors such as sarin, induce an array of harmful effects including hypersecretion, convulsions and ultimately death. Surviving subjects demonstrate damage in specific brain regions that lead to cognitive and neurological dysfunctions. An early accumulation of acetylcholine in the synaptic clefts was suggested as the trigger of a sequence of neurochemical events such as an excessive outpour of glutamate and activation of its receptors. Indeed, alterations in NMDA and AMPA central receptors' densities were detected in brains of poisoned animals. Attempts to improve the current cholinergic-based treatment by adding potent anticonvulsants or antiglutamatergic drugs produced unsatisfactory results. In light of recent events in Syria and the probability of various scenarios of military or terrorist attacks involving organophosphate (OP) nerve agent, research should focus on finding markedly improved countermeasures. Caramiphen, an antimuscarinic drug with antiglutamatergic and GABAergic facilitating properties, was evaluated in a wide range of animals and experimental protocols against OP poisoning. Its remarkable efficacy against OP exposure was established both in prophylactic and post-exposure therapies in both small and large animals. The present review will highlight the outstanding neuroprotective effect of caramiphen as the optimal candidate for the treatment of OP-exposed subjects.

Treatment of status epilepticus with ketamine, are we there yet?

Status epilepticus (SE), a neurological emergency both in adults and in children, could lead to brain damage and even death if untreated. Generalized convulsive SE (GCSE) is the most common and severe form, an example of which is that induced by organophosphorus nerve agents. First- and second-line pharmacotherapies are relatively consensual, but if seizures are still not controlled, there is currently no definitive data to guide the optimal choice of therapy. The medical community seems largely reluctant to use ketamine, a noncompetitive antagonist of the N-methyl-d-aspartate glutamate receptor. However, a review of the literature clearly shows that ketamine possesses, in preclinical studies, antiepileptic properties and provides neuroprotection. Clinical evidences are scarcer and more difficult to analyze, owing to a use in situations of polytherapy. In absence of existing or planned randomized clinical trials, the medical community should make up its mind from well-conducted preclinical studies performed on appropriate models. Although potentially active, ketamine has no real place for the treatment of isolated seizures, better accepted drugs being used. Its best usage should be during GCSE, but not waiting for SE to become totally refractory. Concerns about possible developmental neurotoxicity might limit its pediatric use for refractory SE.

N-methyl D-aspartate (NMDA) receptor antagonists and memantine treatment for Alzheimer's disease, vascular dementia and Parkinson's disease

Memantine, a partial antagonist of N-methyl-D-aspartate receptor (NMDAR), approved for moderate to severe Alzheimer's disease (AD) treatment within the U.S. and Europe under brand name Namenda (Forest), Axura and Akatinol (Merz), and Ebixa and Abixa (Lundbeck), may have potential in alleviating additional neurological conditions, such as vascular dementia (VD) and Parkinson's disease (PD). In various animal models, memantine has been reported to be a neuroprotective agent that positively impacts both neurodegenerative and vascular processes. While excessive levels of glutamate result in neurotoxicity, in part through the over-activation of NMDARs, memantine-as a partial NMDAR antagonist, blocks the NMDA glutamate receptors to normalize the glutamatergic system and ameliorate cognitive and memory deficits. The key to memantine's therapeutic action lies in its uncompetitive binding to the NMDAR through which low affinity and rapid off-rate kinetics of memantine at the level of the NMDAR-channel preserves the physiological function of the receptor, underpinning memantine's tolerability and low adverse event profile. As the biochemical pathways evoked by NMDAR antagonism also play a role in PD and since no other drug is sufficiently effective to substitute for the first-line treatment of L-dopa despite its side effects, memantine may be useful in PD treatment with possibly fewer side effects. In spite of the relative modest nature of its adverse effects, memantine has been shown to provide only a moderate decrease in clinical deterioration in AD and VD, and hence efforts are being undertaken in the design of new and more potent memantine-based drugs to hopefully provide greater efficacy.

Acquisition, expression, and reinstatement of ethanol-induced conditioned place preference in mice: effects of exposure to stress and modulation by mecamylamine

Nicotinic acetylcholine receptors mediate some of the rewarding and motivational effects of ethanol, including relapses. Relapses are common in drug addicts during abstinence when exposure to any stressor ensues. However, the role of nicotinic acetylcholine receptors in the ethanol- and stress-induced reinstatement of ethanol-induced conditioned place preference has not yet been explored. Therefore, the present study investigated the influence of mecamylamine, a nicotinic acetylcholine receptors antagonist on acquisition, expression, and reinstatement of ethanol-induced conditioned place preference in adult male Swiss mice. The results revealed that mecamylamine (0.1-10 µg/mouse, intracerebroventricularly) dose dependently prevented the development, expression, and reinstatement of ethanol-induced conditioned place preference. Further, acute treatment with mecamylamine blocked the restraint stress and forced swim stress-induced reinstatement of ethanol-induced conditioned place preference. All of these treatments had no influence on the locomotor activity. Therefore, it is concluded that mecamylamine blocks the acquisition, expression and reinstatement of conditioned reinforcing effects of ethanol without per se reinforcing or aversive influence. This ability of mecamylamine might be a potential advantage in the treatment of alcoholism.

Minimum effective drug concentrations of a transdermal patch system containing procyclidine and physostigmine for prophylaxis against soman poisoning in rhesus monkeys

A transdermal patch system containing procyclidine, an N-methyl-d-aspartate receptor antagonist possessing anticholinergic action, and physostigmine, a reversible cholinesterase inhibitor, was developed, and its prophylactic efficacy against soman intoxication was investigated. Male rhesus monkeys were shaved on the dorsal area, attached with a matrix-type patch with various sizes (2×2 to 7×7 cm) for 24 or 72 h, and challenged with 2×LD₅₀ doses (13μg/kg) of soman. The smallest patch size for the protection against lethality induced by soman intoxication was 3×3cm, resulting in blood procyclidine concentration of 10.8 ng/ml, blood physostigmine concentration of 0.54 ng/ml, which are much lower concentrations than maximum sign-free doses, and blood cholinesterase inhibition of 42%. The drug concentrations and enzyme inhibition rate corresponding to a diverging point of survivability were presumably estimated to be around 7 ng/ml for procyclidine, 0.35 ng/ml for physostigmine, and 37% of enzyme inhibition. Separately, in combination with the patch treatment, the post treatment consisting of atropine (0.5 mg/kg) plus 1-[([4-(aminocarbonyl)pyridinio]methoxy)methyl]-2-[(hydroxyimino)methyl]pyridinium (HI-6, 50 mg/kg) exerted protection against 5×LD₅₀ challenge of soman, which means the posttreatment remarkably augmented the efficacy of the patch. Additionally, it was found that brain injuries induced by soman toxicity were effectively prevented by the patch treatment according to histopathological examinations. These results suggest that the patch system could be an effective alternative for diazepam, an anticonvulsant, and the current pyridostigmine pretreatment, and especially in combination with atropine plus HI-6, could be a choice for quality survival from nerve-agent poisoning.

Inhibitory influence of mecamylamine on ethanol withdrawal-induced symptoms in C57BL/6J mice

Several reports show the involvement of neuronal nicotinic acetylcholine receptors (nAChRs) in the behavioral effects of ethanol, including ethanol drinking and relapse. Therefore, this study evaluated the effects of mecamylamine, a nAChR antagonist, on ethanol withdrawal signs. Ethanol dependence was induced in C57BL/6J mice by ethanol liquid diet administration. Animals were provided with nutritionally balanced control liquid diet (600 kcal/l) as their sole nutrient source on day 0; from days 1 to 4, 3% v/v of ethanol, followed by 6% v/v of ethanol (from days 5 to 7), and 10% v/v of ethanol (from days 8 to 10) were incorporated into the liquid diet. On day 11, ethanol liquid diet was replaced with nutritionally balanced control liquid diet, and ethanol withdrawal-induced physical signs were recorded. Results showed that acute administration of mecamylamine (1-4 mg/kg, intraperitoneally) dose-dependently attenuated ethanol withdrawal-induced signs, and these effects were comparable with those of diazepam (1-2 mg/kg, intraperitoneally). In addition, chronic administration of mecamylamine into ethanol diet-fed mice markedly attenuated the ethanol withdrawal sign scores, thus supporting the contention that nAChR is involved in ethanol dependence. In conclusion, our results suggest that mecamylamine exhibited inhibitory effects on ethanol withdrawal signs which could be mediated through nAChR.

Inhibitory influence of mecamylamine on the development and the expression of ethanol-induced locomotor sensitization in mice

Several evidences have indicated the involvement of neuronal nicotinic acetylcholine receptors (nAChR) in behavioral effects of drugs of abuse, including ethanol. nAChRs are implicated in ethanol-induced behaviors as well as neurochemical responses to ethanol. Recently, it is demonstrated that mecamylamine, a nAChR antagonist blocks cocaine-, d-amphetamine-, ephedrine-, nicotine-, and methylphenidate-induced psychomotor sensitization. However, no reports are available on its role in ethanol-induced psychomotor sensitization. Therefore, an attempt was made to evaluate its effect on ethanol-induced locomotor sensitization using a model previously described by us. The results revealed that acute administration of mecamylamine (1 and 2mg/kg, i.p.) blocked the acute stimulant effect of ethanol (2.0g/kg, i.p.). In addition, treatment with mecamylamine (0.5-2.0mg/kg, i.p.), 30min prior to the challenge dose of ethanol (2.0g/kg, i.p.) dose dependently attenuated expression of sensitization to locomotor stimulant effect of ethanol. Moreover, administration of mecamylamine (1 and 2mg/kg, i.p.) during development (prior to each ethanol injection on days 1, 4, 7, and 10) blocked acquisition as well as expression (day 15) of sensitization to locomotor stimulant effect of ethanol. Mecamylamine per se did not affect locomotor activity. Further, it also did not influence blood ethanol levels and rotarod performance in mice. These results support the hypothesis that neuroadaptive changes in nAChRs may participate in the development and the expression of ethanol-induced locomotor sensitization.

TOLERANCE TO PENTYLENTETRAZOL-INDUCED CONVULSIONS AND PROTECTION OF CEREBROVASCULAR INTEGRITY BY CHRONIC NICOTINE

The authors' previous studies have shown that in nicotine-induced seizures sensitivity was decreased and blood-brain barrier (BBB) disruption was prevented as a consequence of nicotine pretreatment. This study aimed to investigate the possible protective actions of nicotine on cerebrovascular permeability and seizures induced by pentylentetrazol (PTZ) injection. Cerebrovascular effects of nicotine were evaluated by measuring the permeability changes of BBB using Evans-Blue (EB) dye and specific gravity (SG), which indicates brain water and protein content. The experiments were carried out on Wistar rats. Animals were randomly divided into two groups. Convulsions were induced by injection of PTZ (80 mg/kg i.v.) in rats either pretreated with nicotine daily with a low dose of 0.8 mg/kg day for 21 days or injected with a single dose of 6 mg/kg mecamylamine. The same procedures were followed in control rats with the exception that they were injected only with saline. PTZ injection caused tonic-clonic convulsions and increased the EB dye leakage and specific gravity values in saline-injected control rat brains. Daily injection of nicotine lessened the intensity of seizures. These were accompanied by marked decreases in both the leakage of EB and brain water content. Acute administration of a nAChR antagonist mecamylamine significantly increased seizure latency and decreased the duration of seizures. Thereby, mecamylamine reduced the EB leakage and water content in most brain regions. These results indicate that development of tolerance to PTZ convulsions can be produced by chronic nicotine administration in rats. The mechanism for this effect currently needs clarification. Moreover, the data also suggest that cholinergic activity may account for occurrence of PTZ-induced convulsions.

Antiparkinson drugs used as prophylactics for nerve agents: studies of cognitive side effects in rats

Antiparkinson agents possess excellent anticonvulsant properties against nerve agent-induced seizures by exerting both cholinergic and glutamatergic antagonisms. It is important, however, that drugs used as prophylactics not by themselves cause impairment of cognitive capability. The purpose of the present study was to make a comparative assessment of potential cognitive effects of benactyzine (0.3 mg/kg), biperiden (0.11 mg/kg), caramiphen (10 mg/kg), procyclidine (3 mg/kg), and trihexyphenidyl (0.12 mg/kg) separately and each in combination with physostigmine (0.1 mg/kg). The results showed that benactyzine, caramiphen, and trihexyphenidyl reduced rats' innate preference for novelty, whereas biperiden and procyclidine did not. When benactyzine, caramiphen, and trihexyphenidyl were combined with physostigmine the cognitive impairment disappeared. This counteracting effect, however, caused changes in locomotor and rearing activities not seen by each drug alone. Acetylcholinesterase inhibitors and anticholinergics used as prophylactics can offset each other, but exceptions are observed in a previous study when a very potent anticholinergic (scopolamine) or a high dose of procyclidine still results in cognitive deficits in spite of coadministration with physostigmine. Among the present drugs tested, procyclidine appears to be a robust anticonvulsant with few cognitive side effects.

Measurements with fluorescent probes in primary neural cultures; improved multiwell techniques

INTRODUCTION

Fluorescence imaging techniques are valuable tools for the pharmacological characterization of CNS drugs. Dissected cerebellar granule neurons (CGN) are an important model system in the study of mechanisms of excitotoxicity, glutamate receptors and transporters. Widely applied techniques use fluorescent probes loaded in neural cells cultured on glass supports. CGN, however, require at least 7 days for differentiation and over time cells tend to cluster and loose adherence to the glass substrate. This problem is accentuated in small wells (e.g. 96-well plates).

METHODS

CGN were grown on large coverslips (60 x 24 mm) and measurements made with a designed mountable multiwell in 48 regions on 4 coverslips at a time. The UV ratiometric probe fura-2 was used to measure glutamatergic calcium ([Ca(2+)](i)) responses induced by NMDA. The IC(50) of NMDA receptor antagonists was determined from inhibition curves with 6 doses and 8 parallels per experiment.

RESULTS

The method was validated by comparing with published data for the dose response to NMDA and glycine and IC(50) values for ion-channel block by Mg(2+) and MK-801.

DISCUSSION

Resolution is enhanced with the new technique since it allows measurement of multiple doses on cells from the same batch. It has advantages to cuvette techniques because cells have intact dendritic tree and synaptic function and it is a convenient method to obtain reliable dose-response curves for NMDA channel modulators on differentiated neural cells.

Neuronal structures involved in the induction and propagation of seizures caused by nerve agents: Implications for medical treatment

In epilepsy research, studies have been made to identify brain areas critical for triggering and/or controlling propagated seizure activity. The purpose of the present study was to focus on a similar approach in nerve agent research by reviewing relevant literature to map potential trigger sites and propagation pathways for seizures. The piriform cortex and medial septal area emerge as prime target areas for soman-induced seizures. The cholinergic hyperactivation in the latter structures seems to induce increased glutamatergic activity in the piriform, entorhinal, and perirhinal cortices along with the hippocampal region. For prophylactic or early treatment, mapping of muscarinic subreceptors in the piriform cortex and medial septum would be guiding for designing anticholinergic drugs with optimal properties. Sustained seizures governed by glutamatergic over-activity may primarily be terminated by drugs with optimal glutamatergic antagonism primarily in the piriform, entorhinal, and perirhinal cortices. Studies of radiolabeled ligands to map subreceptors may provide specification of wanted drug properties to guide the choice among existing agents or to synthesize novel ones.

Anticonvulsant efficacy of drugs with cholinergic and/or glutamatergic antagonism microinfused into area tempestas of rats exposed to soman

A group of antiparkinson drugs (benactyzine, biperiden, caramiphen, procyclidine, and trihexyphenidyl) has been shown to possess both anticholinergic and antiglutamatergic properties, making these agents very well suited as anticonvulsants against nerve agents. The first purpose of this study was to make a comparative assessment of the anticonvulsant potencies of the antiparkinson agents when microinfused (1 microl) into the seizure controlling area tempestas (AT) of rats 20 min before subcutaneous injection of soman (100 microg/kg). The second purpose was to determine whether cholinergic and/or glutamatergic antagonism was the effective property. The results showed that only procyclidine (6 microg) and caramiphen (10 microg) antagonized soman-induced seizures. Cholinergic, and not glutamatergic, antagonism was likely the active property, since atropine (100 microg), and scopolamine (1 microg) caused anticonvulsant effects, whereas MK-801 (1 microg), and ketamine (50 microg) did not. Soman (11 nmol) injected into AT resulted more frequently in clonic convulsions than full tonic-clonic convulsions. AT may serve as both a trigger site for soman-evoked seizures and a site for screening anticonvulsant potencies of future countermeasures.

The combination of donepezil and procyclidine protects against soman-induced seizures in rats

Current treatment of nerve agent poisoning consists of prophylactic administration of pyridostigmine and therapy using atropine, an oxime and a benzodiazepine. Pyridostigmine does however not readily penetrate the blood-brain barrier giving ineffective protection of the brain against centrally mediated seizure activity. In this study, we have evaluated donepezil hydrochloride, a partial reversible inhibitor of acetylcholinesterase (AChE) clinically used for treating Alzheimer's disease, in combination with procyclidine, used in treatment of Parkinson's disease and schizophrenia, as prophylaxis against intoxication by the nerve agent soman. The results demonstrated significant protective efficacy of donepezil (2.5 mg/kg) combined with procyclidine (3 or 6 mg/kg) when given prophylactically against a lethal dose of soman (1.6 x LD(50)) in Wistar rats. No neuropathological changes were found in rats treated with this combination 48 h after soman intoxication. Six hours after soman exposure cerebral AChE activity and acetylcholine (ACh) concentration was 5% and 188% of control, respectively. The ACh concentration had returned to basal levels 24 h after soman intoxication, while AChE activity had recovered to 20% of control. Loss of functioning muscarinic ACh receptors (17%) but not nicotinic receptors was evident at this time point. The recovery in brain AChE activity seen in our study may be due to the reversible binding of donepezil to the enzyme. Donepezil is well tolerated in humans, and a combination of donepezil and procyclidine may prove useful as an alternative to the currently used prophylaxis against nerve agent intoxication.

Pharmacological therapies against soman-induced seizures in rats 30 min following onset and anticonvulsant impact

Systemic administration does not allow a clear differentiation between the anticonvulsant properties of GABAA (gamma-aminobutyric acid) modulators. For this reason, various GABAA modulators have previously been micro-infused into seizure controlling substrates (area tempestas, substantia nigra) in the rat brain as a screening method for potential systemic administration. The purpose of the present study was to examine the anticonvulsant impact of the GABAergic modulators muscimol, ethanol, and propofol (screened by micro-infusions) when each drug was combined with procyclidine and administered systemically. The results showed that all 3 combinations could effectively terminate soman-induced (100 microg/kg s.c.) seizures when administered 30-35 min after onset. Procyclidine and propofol were considered as the most relevant double regimen to replace a previous triple regimen (procyclidine, diazepam, pentobarbital) against soman-induced seizures. Additionally, it was shown that unilateral implantation of hippocampal electrodes resulted in increased resistance to aphagia/adipsia and neuropathology, but not to lethality following soman. Efficient pharmacological treatment of soman-induced seizures at an early stage (< 20 min) is crucial to avoid neuropathology and cognitive deficits.

Pre-junctional effects of oximes on [3H]-acetylcholine release in rat hippocampal slices during soman intoxication

In this study, the non-reactivating effects of oximes in the hippocampus of the rat are investigated. The potassium (51 mM) evoked release of [(3)H]-acetylcholine and the liberation of [(3)H]-choline were determined in hippocampal slices following in vitro exposure to soman and five oximes (toxogonin, HI-6, HLö-7, P2S and 2-PAM) in separate experiments by superfusion. In the absence of soman, toxogonin and HLö-7 in particular induced a concentration dependent significant increase in the evoked release of [(3)H]-acetylcholine. There was also a significant effect of HI-6, but the effect was much smaller. Two pralidoxime salts, P2S (methanesulfonate salt) and 2-PAM (methiodide salt), had similar but lower effects that were only observed at relatively high concentrations. Experiments performed following complete inhibition of the acetylcholinesterase activity by soman (1.0 microM) showed that HI-6 and HLö-7 induced a significant decrease in the potassium-evoked release of [(3)H]-acetylcholine, while the liberation of [(3)H]-choline increased. Toxogonin, P2S and 2-PAM did not reduce significantly the evoked release of [(3)H]-acetylcholine. Only limited reactivation of the acetylcholinesterase activity was observed in superfusion experiments with toxogonin, HI-6, P2S and 2-PAM following exposure of hippocampal slices to soman. However, HLö-7 was proved to be relatively more effective in reactivating the acetylcholinesterase activity at high concentrations (50 and 200 microM). The acetylcholinesterase activity was reactivated to approximately 12% and 40% of control, respectively. It is concluded that HI-6 and HLö-7 have important non-acetylcholinesterase reactivating properties following soman poisoning, as may be seen by the significant reduction in the evoked release of [(3)H]-acetylcholine effected by these oximes. HLö-7 is of particular interest in view of its ability to additionally improve reactivation of the acetylcholinesterase activity.

Protection by sustained release of physostigmine and procyclidine of soman poisoning in rats

The efficacy of a combinational prophylactic regimen on the lethality, convulsions, and loss of morphological and functional integrities of the brain induced by an organophosphate soman was investigated in rats. The rats were implanted subcutaneously with osmotic minipumps containing the combinational prophylactic regimen composed of physostigmine, a reversible cholinesterase inhibitor, and procyclidine, an N-methyl-D-aspartate antagonist possessing anticholinergic action, for 3 days, and intoxicated subcutaneously with soman (160 microg/kg, 1.3 LD50). The doses of combinational regimen in minipumps were optimized to achieve 30-35% inhibition of blood cholinesterase activity by physostigmine and 50-100 ng/ml of blood concentrations of procyclidine as clinically available doses, respectively. In comparison, 1-[([4-(aminocarbonyl)pyridinio]methoxy)methyl]-2-[(hydroxyimino)methyl]pyridinium (HI-6, 125 mg/kg) was administered intraperitoneally 30 min prior to the soman challenge in control groups to reduce mortality of rats without affecting convulsions. Soman induced profound limbic convulsions and 30% mortality, leading to increased blood-brain barrier permeability, neural injuries, learning and memory impairments, and physical incapacitation of survived rats pretreated with HI-6. The combinational regimen, at optimal doses without adverse effects on passive avoidance performances (72 microg/kg/h of physostigmine plus 432 microg/kg/h of procyclidine), exerted full protective effects against lethality, convulsions, blood-brain barrier opening, brain injuries, learning and memory impairments, and physical incapacitation induced by soman. Taken together, it is suggested that the combination of physostigmine and procyclidine, at adequate doses, could be a choice to provide the victims of organophosphate poisoning with chance of intensive care for survival and neuroprotection.

Cognitive side effects in rats caused by pharmacological agents used to prevent soman-induced lethality

It is important that prophylactics used to protect military and emergency personnel against lethal doses of nerve agents do not by themselves produce impairment of cognitive capability. The purpose of the present study was to examine whether physostigmine, scopolamine, and various doses of procyclidine might reduce rats' innate preference for novelty. When these drugs were tested separately, the results showed that physostigmine (0.1 mg/kg) and procyclidine (3 mg/kg) did not affect preference for novelty, whereas scopolamine (0.15 mg/kg) and procyclidine in a higher dose (6 mg/kg) resulted in a preference deficit (Experiment 1). In Experiment 2, the combination of physostigmine and scopolamine or physostigmine and procyclidine (6 mg/kg) caused a marked deficit in preference for novelty. A much milder deficit was observed when physostigmine was combined with lower doses (1 or 3 mg/kg) of procyclidine. The latter combinations also had milder adverse impact on the animals' interest in the test environment and activity measures than the former combinations. By combining physostigmine with anticholinergics, a potentiation of adverse effects on behavior was seen. It is concluded that a slight cognitive impairment might be unavoidable with effective prophylactics.

Putative role of proteolysis and inflammatory response in the toxicity of nerve and blister chemical warfare agents: implications for multi-threat medical countermeasures

Despite the contrasts in chemistry and toxicity, for blister and nerve chemical warfare agents there may be some analogous proteolytic and inflammatory mediators and pathological pathways that can be pharmacological targets for a single-drug multi-threat medical countermeasure. The dermal-epidermal separation caused by proteases and bullous diseases compared with that observed following exposure to the blister agent sulfur mustard (2,2'-dichlorodiethyl sulfide) has fostered the hypothesis that sulfur mustard vesication involves proteolysis and inflammation. In conjunction with the paramount toxicological event of cholinergic crisis that causes acute toxicity and precipitates neuronal degeneration, both anaphylactoid reactions and pathological proteolytic activity have been reported in nerve-agent-intoxicated animals. Two classes of drugs already have demonstrated multi-threat activity for both nerve and blister agents. Serine protease inhibitors can prolong the survival of animals intoxicated with the nerve agent soman and can also protect against vesication caused by the blister agent sulfur mustard. Poly (ADP-ribose) polymerase (PARP) inhibitors can reduce both soman-induced neuronal degeneration and sulfur-mustard-induced epidermal necrosis. Protease and PARP inhibitors, like many of the other countermeasures for blister and nerve agents, have potent primary or secondary anti-inflammatory pharmacology. Accordingly, we hypothesize that drugs with anti-inflammatory actions against either nerve or blister agent might also display multi-threat efficacy for the inflammatory pathogenesis of both classes of chemical warfare agent.

Ethanol elevates accumbal dopamine levels via indirect activation of ventral tegmental nicotinic acetylcholine receptors

It was previously demonstrated that the central nicotinic acetylcholine receptor antagonist mecamylamine perfused in the ventral tegmental area (VTA) counteracts the elevation of extracellular dopamine levels in the nucleus accumbens after systemic ethanol, as measured by in vivo microdialysis. In the present study we investigated the effect of different concentrations of ethanol perfused locally in the VTA or in the nucleus accumbens on extracellular accumbal dopamine levels. Ethanol (10-1000 mM) perfused in the VTA did not influence dopamine output in the nucleus accumbens. However, ethanol (300 mM) perfused in the nucleus accumbens increased accumbal dopamine levels to approximately the same extent (30%) as observed after systemic ethanol, whereas ethanol (1000 mM) decreased the dopamine output by approximately 50%. Next, the hypothesis that endogenous acetylcholine is required for the increased accumbal dopamine levels after ethanol was challenged. It was shown that in animals pre-treated with vesamicol, a potent inhibitor of vesicular acetylcholine storage, ethanol (300 mM) in the nucleus accumbens failed to elevate extracellular accumbal dopamine levels. Similarly, in animals perfused with mecamylamine in the VTA, but not in the nucleus accumbens, ethanol in the nucleus accumbens (300 mM) failed to increase accumbal dopamine levels. However, whereas dihydro-beta-erythroidine (antagonist for the nicotinic receptor subtype alpha4beta2) perfused in the VTA prevented the increase in accumbal dopamine after systemic nicotine, the antagonist was unable to prevent the dopamine elevating effects of ethanol. Finally, to investigate whether mecamylamine exerts its antagonizing effect of ethanol induced accumbal dopamine levels through an interaction with the NMDA receptor MK-801, the effects of the prototypic NMDA receptor antagonist were examined and compared to those of mecamylamine. After perfusion in the VTA, MK-801 enhanced accumbal dopamine levels by itself but did not antagonize the enhancing effect of ethanol. The present set of experiments indicate that the mesolimbic dopamine activating effects of ethanol may be due to an indirect rather than direct activation of ventral tegmental nicotinic acetylcholine receptors of a subtype composition different from the alpha4beta2. Furthermore, it is argued that the primary site of action of ethanol in its accumbal dopamine elevating effect may be located to the nucleus accumbens or nearby regions.

Effects of inhaled toluene and 1,1,1-trichloroethane on seizures and death produced by N-methyl-D-aspartic acid in mice

Evidence exists that some abused solvents have N-methyl-D-aspartic acid (NMDA) antagonist activity, although which of their effects may be related to this mechanism is not well understood. The effects of toluene and 1,1,1-trichloroethane (TCE) on NMDA-induced seizures in mice were studied using three experimental protocols: (a) animals injected i.p. with 120 or 170 mg/kg NMDA and immediately afterwards exposed to solvent vapors or air for 30 min (co-exposure protocol); (b) mice exposed for 30 min to solvent or air, then injected with NMDA and placed in the chamber for a second 30-min exposure (pre-exposure+co-exposure protocol); and (c) mice that inhaled 4000 ppm toluene or air for 30 min twice a day, 6 h apart, for 7 days, and were injected with 120 mg/kg NMDA immediately before a 30-min toluene exposure (repeated exposure protocol). When given acutely, toluene, but not TCE, produced concentration-dependent protection against NMDA-induced seizures. Higher concentrations of toluene were also effective against the lethal effects produced by 170 mg/kg NMDA. Clearer effects were seen when the pre-exposure+co-exposure protocol was followed. Under these conditions the IC(50) for toluene was 739 ppm (653-825) against seizure occurrence and 2127 ppm (1966-2288) against lethality. Repeated exposure to toluene did not result in tolerance to its anticonvulsant effects. These results are consistent with the in vitro effects described for toluene as a noncompetitive NMDA antagonist and as a compound that enhances GABAergic transmission. The lack of protective effects of TCE is not consistent with its in vitro actions.

Effects of acute procyclidine administration on prepulse inhibition of the startle response in schizophrenia: a double-blind, placebo-controlled study

Prepulse inhibition (PPI) of the startle response refers to a reduction in response to a strong stimulus (pulse) if this is preceded shortly by a weak non-startling stimulus (prepulse). Consistent with theories of deficiencies in early stages of information processing, PPI is found to be reduced in patients with schizophrenia. Atypical antipsychotics are found to be more effective than typical antipsychotics in improving PPI in this population. Anticholinergic drugs are often used to control extrapyramidal symptoms induced by antipsychotic medication, especially by typical antipsychotics, in schizophrenic patients and are known to disrupt cognitive functions in both normal and schizophrenic populations. The effect of anticholinergics on PPI in schizophrenia has not yet been examined. This study determined the effects of procyclidine, an anticholinergic drug, on PPI in patients with schizophrenia given risperidone or quetiapine and not on any anticholinergic drugs, employing a placebo-controlled, cross-over design. Under double-blind conditions, subjects were administered oral 15 mg procyclidine and placebo on separate occasions, 2 weeks apart, and tested for acoustic PPI (prepulse 8 dB and 15 dB above the background and delivered with 30-ms, 60-ms and 120-ms prepulse-to-pulse intervals). Procyclidine significantly impaired PPI compared to placebo (assessed as percentage reduction) with 60-ms prepulse-to-pulse trials and increased the latencies to response peak across all trials. The use of anticholinergics needs to be carefully controlled/examined in investigations of information processing deficits using a PPI model and reduced to the minimum level in clinical care of schizophrenia.

Neuropathy-specific analgesic action of intrathecal nicotinic agonists and its spinal GABA-mediated mechanism

The effect of the nicotinic acetylcholine receptor (nAChR) agonists at the spinal level has not been well studied in neuropathic states. In the present report, we demonstrate the efficacy of intrathecal nicotinic agonists in partial sciatic nerve injury-induced neuropathy model mice. Intrathecal (i.t.) administration of (-)nicotine and (+)epibatidine, at doses without undesirable effects, had no antinociceptive action in sham-operated mice. However, they completely reversed the thermal and mechanical hyperalgesia in partial sciatic nerve-injured mice. This neuropathy-specific analgesic action of i.t. (-)nicotine and (+)epibatidine is attributed to different subtypes of nAChRs at the spinal level. After antagonism experiments with mecamylamine, dihydro-beta-erythroidine and methyllycaconitine, it was observed that (-)nicotine-induced analgesia was mediated through the alpha4beta2 subtype of nAChR while the (+)epibatidine-induced one was mediated through non-alpha4beta2 subtype of nAChR. Moreover, i.t. pretreatment with NMDA receptor antagonist did not block nicotinic analgesia. Similar to nicotinic agonists, gamma-aminobutyric acid receptor (GABA(A)) agonist muscimol (i.t.) produced neuropathy-specific analgesic action giving analgesia only in nerve-injured mice. The GABA(A) antagonists bicuculline and picrotoxin significantly blocked the analgesic effects of muscimol as well as that of (-)nicotine and (+)epibatidine. On the other hand, i.t. injection of nicotinic antagonist mecamylamine and GABA(A) antagonist picrotoxin in sham-operated mice induced a thermal hyperalgesia without any effects in nerve-injured animals suggesting the presence of a tonic inhibitory tone on nociceptive transmission through the spinal cholinergic-GABAergic system. These results also suggest that the neuropathy-specific analgesic action of intrathecal nicotinic agonists was due to stimulation of this cholinergic-GABAergic system whose inhibitory tone had been reduced due to injury.

Clinical pharmacokinetic and pharmacodynamic properties of drugs used in the treatment of Parkinson's disease

Current research in Parkinson's disease (PD) focuses on symptomatic therapy and neuroprotective interventions. Drugs that have been used for symptomatic therapy are levodopa, usually combined with a peripheral decarboxylase inhibitor, synthetic dopamine receptor agonists, centrally-acting antimuscarinic drugs, amantadine, monoamine oxidase-B (MAO-B) inhibitors and catechol-O-methyltransferase (COMT) inhibitors. Drugs for which there is at least some evidence for neuroprotective effect are certain dopamine agonists, amantadine and MAO-B inhibitors (selegiline). Levodopa remains the most effective drug for the treatment of PD. Several factors contribute to the complex clinical pharmacokinetics of levodopa: erratic absorption, short half-life, peripheral O-methylation and facilitated transport across the blood-brain barrier. In patients with response fluctuations to levodopa, the concentration-effect curve becomes steeper and shifts to the right compared with patients with stable response. Pharmacokinetic-pharmacodynamic modelling can affect decisions regarding therapeutic strategies. The dopamine agonists include ergot derivatives (bromocriptine, pergolide, lisuride and cabergoline), non-ergoline derivatives (pramipexole, ropinirole and piribedil) and apomorphine. Most dopamine agonists have their specific pharmacological profile. They are used in monotherapy and as an adjunct to levodopa in early and advanced PD. Few pharmacokinetic and pharmacodynamic data are available regarding centrally acting antimuscarinic drugs. They are characterised by rapid absorption after oral intake, large volume of distribution and low clearance relative to hepatic blood flow, with extensive metabolism. The mechanism of action of amantadine remains elusive. It is well absorbed and widely distributed. Since elimination is primarily by renal clearance, accumulation of the drug can occur in patients with renal dysfunction and dosage reduction must be envisaged. The COMT inhibitors entacapone and tolcapone dose-dependently inhibit the formation of the major metabolite of levodopa, 3-O-methyldopa, and improve the bioavailability and reduce the clearance of levodopa without significantly affecting its absorption. They are useful adjuncts to levodopa in patients with end-of-dose fluctuations. The MAO-B inhibitor selegiline may have a dual effect: reducing the catabolism of dopamine and limiting the formation of neurotoxic free radicals. The pharmacokinetics of selegiline are highly variable; it has low bioavailability and large volume of distribution. The oral clearance is many-fold higher than the hepatic blood flow and the drug is extensively metabolised into several metabolites, some of them being active. Despite the introduction of several new drugs to the antiparkinsonian armamentarium, no single best treatment exists for an individual patient with PD. Particularly in the advanced stage of the disease, treatment should be individually tailored.

Effects of 10 mg and 15 mg oral procyclidine on critical flicker fusion threshold and cardiac functioning in healthy human subjects

The critical flicker fusion threshold (CFFT) is thought to index alertness and cortical arousal. Sedative drugs reduce CFFT while psychostimulants increase it. Procyclidine is an anticholinergic that is used to control the extrapyramidal side-effects of antipsychotics in schizophrenia. This study examined the effects of clinically relevant doses of oral procyclidine administration on CFFT and heart rate in two separate experiments (Experiment 1, drug dose: 10 mg, n = 16; Experiment 2, drug dose: 15 mg, n = 12) involving healthy subjects using a double-blind, placebo-controlled, cross-over design. 10 mg procyclidine had no significant effect on CFFT, heart rate or self-ratings of mood, but the 15 mg dose significantly lowered CFFT at 1 h and 2 h after procyclidine administration, increased drowsiness ratings and produced a drop in heart rate. The effects observed in this study may have implications for treatment compliance of schizophrenic patients, choice of antipsychotics, prescribing to patients with heart disease and monitoring of cardiac function under treatment. Further investigations are required to quantify the effects of procyclidine on CFFT and cardiac function in patients with schizophrenia.

Effects of combinational prophylactics composed of physostigmine and procyclidine on soman-induced lethality, seizures and brain injuries

The antidotal, anticonvulsant and neuroprotective effects of physostigmine (PhS) and procyclidine (PC), the combinational prophylactics for organophosphate poisoning, were evaluated. For the investigation of dose-response relationship in rats and guinea pigs, various doses (0-6 mg/kg) of PC in combination with a fixed dose (0.1 mg/kg) of PhS were pretreated subcutaneously 30 min prior to subcutaneous poisoning with soman. Procyclidine in combination with PhS exhibited remarkable synergistic effects in a dose-dependent manner, leading to 1.92-5.07 folds of protection ratio in rats and 3.00-4.70 folds in guinea pigs. On the other hand, a low effect (1.65 fold) was achieved with the traditional antidotes atropine (17.4 mg/kg) plus 2-pralidoxime (30 mg/kg) treated immediately after soman poisoning, compared with a marked protection (5.50 fold) with atropine (17.4 mg/kg) plus HI-6 (125 mg/kg) in unpretreated rats. Noteworthy, the combinational prophylactics greatly potentiated the effect of atropine plus 2-pralidoxime to 6.13 or 12.27 folds and that of atropine plus HI-6 to 12.00 or 21.50 folds with 1.0 or 3.0 mg/kg of PC, respectively. A high dose (100 μg/kg, 1.3×LD(50)) of soman induced severe epileptiform seizures in rats pretreated with HI-6 (125 mg/kg), resulting in brain injuries in discrete brain regions under histopathological examination in 24 h. Interestingly, such seizures and excitotoxic brain injuries were fully prevented by pretreatment with PhS (0.1 mg/kg) and PC (1 mg/kg). Taken together, it is proposed that the prophylactics composed of PhS and PC could be a promising regimen for the prevention of lethality, seizures and brain injuries induced by soman poisoning.