Mutually potentiating effects of mecamylamine and haloperidol in producing catalepsy in rats

Interactions between mecamylamine and alcohol in Long-Evans rats: flash-evoked potentials, body temperature, behavior, and blood alcohol concentration

Mecamylamine, a noncompetitive antagonist of nicotinic acetylcholine receptors, has many potential clinical applications, including treating alcohol dependency. However, little is known about the combined effects of mecamylamine and alcohol on visual system electrophysiology. We examined the separate and combined effects of mecamylamine (4.0mg/kg, ip) and alcohol (2.0 g/kg, ip) on flash-evoked potentials (FEPs) recorded from the visual cortex (VC) and superior colliculus (SC) of chronically implanted adult male Long-Evans rats. On separate days, either saline or mecamylamine was given 10 min prior to either saline or ethanol. FEPs were recorded 15 and 30 min after the second injection. In the VC, alcohol significantly decreased the amplitudes of components P23, N29, N39, P89, N143, and P237, but increased P46. N63 amplitude was not significantly altered. In contrast, mecamylamine increased the amplitude of P23, P46, and N63, but reduced the amplitude of N29 and P237. The combination of mecamylamine and alcohol resulted in amplitudes very similar to alcohol alone for components P23, N29, N63, P89, N143, and P237. However, mecamylamine pretreatment reduced the effects of alcohol on components N39 and P46. In the SC, FEP component amplitudes were generally decreased by alcohol but not significantly altered by mecamylamine. Mecamylamine pretreatment did not significantly alter the effects of alcohol on SC amplitudes. Latencies of nearly all components in both structures were significantly increased by all drug treatments, with the greatest increase produced by the combination treatment. Hypothermia was also produced by all drug treatments, with the greatest hypothermia (2.25 °C) produced by the combination treatment, most likely accounting for much of the drug-induced increase in latencies. All drug treatments reduced movement during FEP testing, but later in an open field alcohol increased ambulation while mecamylamine reduced movement. Separate groups of experimentally naïve adult male Holtzman albino and Long-Evans hooded rats were given (ip) either alcohol or mecamylamine plus alcohol. Tail vein samples were taken 30 min later. For both rat strains, blood alcohol concentration in the mecamylamine pretreatment group was significantly less at this time interval by about 50-60 mg/dL, suggesting a mechanism whereby mecamylamine can mitigate some of the acute effects of alcohol (e.g., on VC components N39 and P46).

Individual phenotype predicts nicotine-haloperidol interaction in catalepsy: possible implication for the therapeutic efficacy of nicotine in Tourette's syndrome

In individuals with Tourette's syndrome, the therapeutic efficacy of haloperidol can be augmented by nicotine. In laboratory rats, the dopamine antagonist haloperidol produces catalepsy and nicotine can potentiate it, although this effect is variable and not always observed. Our aim was to understand this variability. In rats, the locomotor response to a novel environment predicts the magnitude of the locomotor response to nicotine. Since the psychostimulant effect of nicotine might counter catalepsy, we hypothesized that rats with a high locomotor response to novelty would show reduced vulnerability to nicotine potentiation of haloperidol catalepsy. First, we administered haloperidol (0, 0.1 or 0.3mg/kg, ip) and found stronger catalepsy in rats with low reactivity to novelty. Second, we administered haloperidol (0.3mg/kg) or haloperidol plus nicotine (0.1mg/kg, ip) and found that nicotine indeed potentiated haloperidol catalepsy but only in rats with low reactivity to novelty. Nicotine did not induce catalepsy on its own. Thus, previously reported inconsistencies in the catalepsy potentiating effect of nicotine may have been due to differential vulnerability to its stimulant actions. As previously observed, the potentiation of haloperidol catalepsy was greatest 4h after injection. Given the short half-life of nicotine, the mechanism(s) underlying the delayed expression of its pro-cataleptic capacity remains obscure.

Effects of mecamylamine on flash-evoked potentials, body temperature, and behavior in Long-Evans rats

This experiment examined the effects of mecamylamine, a nicotinic acetylcholine receptor antagonist, on flash-evoked potentials (FEPs) recorded from the visual cortex (VC) and superior colliculus (SC) of chronically implanted male Long-Evans rats, and on body temperature and open field behavior. FEPs were recorded at 20 and 35 min following intraperitoneal injections of saline, and of 0.3, 3.0, and 10.0 mg/kg mecamylamine on separate days. The 0.3 mg/kg dose did not produce significant effects. The amplitude of VC components N₃₀, P₄₈, and P₈₇ increased, N₁₅₀ and P₂₃₁ decreased, and P₂₃, N₄₀, N₅₈, and N₆₈ were unchanged following administration of the 10.0 mg/kg dose. In the SC, component P₂₈ was unaffected, P₃₉ was reduced, and N₄₉ was augmented by the 10.0 mg/kg dose. All component peak latencies were increased by the 3.0 and 10.0 mg/kg doses. Significant hypothermia was also produced by the 3.0 and 10.0 mg/kg doses, suggesting that this was the basis for the increased latencies. The 10.0 mg/kg dose produced a significant decrease in movement during the recording sessions. In subsequent open field observations, both line crossings and rearings were reduced by the 3.0 and 10.0 mg/kg doses. The results suggest that endogenous acetylcholine acting on nicotinic acetylcholine receptors plays at most a modest role in producing FEPs recorded from the VC and SC.

Emerging drugs in Tourette syndrome

Proper education of the patient is the first step in the treatment of Tourette syndrome (TS). Before deciding how to treat the patient, it is important to decide whether to treat the TS-related symptoms. Counselling and behavioural modification may be sufficient for those with mild symptoms. Medications, however, may be considered when symptoms begin to interfere with peer relationships, social interactions, academic or job performance, or with activities of daily living. Therapy must be individualised and the most troublesome symptoms should be targeted first. Antidopaminergic agents are clearly the most effective drugs in the treatment of tics. Although haloperidol and pimozide are the only drugs currently approved by the FDA for the treatment of TS, other dopamine receptor-blocking drugs and tetrabenazine, a dopamine depleting drug, as well as botulinum toxin injections, have been used to treat tics associated with TS. Carefully designed, comparative, longitudinal trials assessing the efficacy and adverse-effect profiles of these drugs, including tardive dyskinesia, are lacking. Selective serotonin reuptake inhibitors are recommended for the treatment of obsessive-compulsive behaviour: a common comorbidity. Psychostimulants, such as methylphenidate, are the treatment of choice for attention deficit hyperactivity disorder. Even though these drugs may transiently increase tics, this does not necessarily constitute a definite contraindication to the use of these drugs in patients with TS. Here, existing and emerging medical treatments in patients with tics and comorbid behavioural disorders associated with TS are reviewed.

The treatment of Tourette's syndrome: current opinions

Correct education of the patient is one of the most important aspects in the treatment of Tourette's syndrome. Pharmacotherapy is often unsatisfactory and therefore should be limited to those patients who are significantly impaired. Therapy must be individualised and the most troublesome symptom should be targeted first. In the treatment of tics, dopamine receptor blocking agents are currently the most effective drugs. It is currently unknown whether classic neuroleptics, such as pimozide, selective dopamine receptor antagonists, such as sulpiride, or newer atypical antipsychotics, such as risperidone, have the best adverse effect profile. Tiapride can be used as an alternative, particularly in children. Selective serotonin-re-uptake inhibitors are recommended for the treatment of obsessive-compulsive behaviour. In children suffering from attention deficit hyperactivity disorder, psychostimulants, such as methylphenidate, are the treatment of choice. Recent studies have provided increasing evidence that stimulants do not cause a significant increase in tics in the majority of patients.

Multicenter, double-blind, placebo-controlled study of mecamylamine monotherapy for Tourette's disorder

OBJECTIVE

The safety and efficacy of mecamylamine as a monotherapy in children and adolescents with Tourette's disorder (TD) was investigated in an 8-week multicenter, double-blind, placebo-controlled study.

METHOD

Eligible subjects included subjects with TD (DSM-IV), with a naturalistic mix of comorbid diagnoses, nonsmokers, aged 8 to 17 years, whose behavioral and emotional symptoms (according to parents) were more disturbing than tics. After a washout period of all psychotropic medication, subjects were randomly assigned to either mecamylamine (n = 29) or placebo (n = 32). Mecamylamine doses ranged from 2.5 to 7.5 mg/day. Primary efficacy measures included the Tourette's Disorder Scale-Clinician Rated (TODS-CR) and 21-point Clinical Global Improvement scale; secondary efficacy measures included the Yale Global Tic Severity Scale and a rage-attack scale (RAScal).

RESULTS

Of the 61 subjects who were randomized, 50 (82%) completed at least 3 weeks on medication and 38 (62%) completed the full 8-week trial. Study withdrawals included 12/29 on mecamylamine and 11/32 on placebo. For the total sample, mecamylamine was no more effective than placebo on any of the outcome measures. However, an item analysis of the TODS-CR suggested that mecamylamine may have reduced sudden mood changes and depression in moderately to severely affected subjects. Except for a slight increase in heart rate during the 1st week in both the mecamylamine and the placebo groups, there where no significant mecamylamine-related changes in vital signs, electrocardiogram, complete blood cell count, or blood chemistry values.

CONCLUSIONS

Mecamylamine, in doses up to 7.5 mg/day, is well tolerated in children and adolescents, but as a monotherapy it does not appear to be an effective treatment for tics or for the total spectrum of symptoms associated with TD. However, further studies should be conducted to investigate its possible therapeutic effects in subjects with comorbid mood disorders and as an adjunct to neuroleptic medication.

Mecamylamine effects on haloperidol-induced catalepsy and defecation

Recent clinical experience with Tourette syndrome (TS) patients suggests that the nicotinic receptor antagonist, mecamylamine (Inversine), may be a useful adjunct to neuroleptic therapy for controlling tic symptom. This is consistent with previous preclinical findings demonstrating that mecamylamine can potentiate the cataleptic effects of neuroleptics in rats. However, these earlier preclinical studies employed high doses (1-2.5 mg/kg) of mecamylamine that may not be clinically relevant since human doses of mecamylamine used to treat TS have been much lower (0.03-0.1 mg/kg). In order to test the potential therapeutic properties of mecamylamine preclinically, we conducted catalepsy experiments in rats employing both a low and high dose of mecamylamine in combination with haloperidol. Sixty-four male Sprague Dawley rats were randomized into four treatment groups (n = 16/group). Each rat received an injection of either saline or mecamylamine (0.1 or 3.0 mg/kg s.c.) followed one hour later with a second injection of either saline or haloperidol (0.4 mg/kg s.c.). The bar test was used to measure duration of catalepsy at 3 hrs following the second injection. The results demonstrated that only the mecamylamine treated rats showed statistically significant haloperidol-induced catalepsy when measured at 3 hrs. In addition, haloperidol-induced defecation was not affected by the 0.1 mg/kg mecamylamine dose, but completely abolished by the 3.0 mg/kg dose. These findings suggest that a clinically relevant dose of mecamylamine (0.1 mg/kg) can affect the duration of haloperidol-induced catalepsy without having significant effects on gastrointestinal function.

Enhancement of haloperidol-induced catalepsy by nicotine: an investigation of possible mechanisms

Nicotine has been reported to potentiate the cataleptic effect of the dopamine receptor antagonist haloperidol in rats. This effect is paradoxical, since nicotine alone tends to increase nigrostriatal dopamine release. In the present experiments, a pro-cataleptic effect of nicotine was confirmed statistically but was small and variable. Three potential mechanisms underlying this effect were investigated. (i) Desensitization of brain nicotinic receptors appears to make little if any contribution to the pro-cataleptic effect of nicotine, insofar as the latter was not mimicked by two centrally active nicotinic antagonists (mecamylamine and chlorisondamine). (ii) Depolarization inactivation resulting from combined treatment with haloperidol and nicotine does not appear to be critical, since the pro-cataleptic effect of nicotine was not enhanced by chronic haloperidol administration, a treatment designed to enhance depolarization inactivation. (iii) The slow emergence and persistence of the acute pro-cataleptic effect of nicotine suggested possible mediation by a nicotine metabolite. However, neither cotinine nor nornicotine, the principal pharmacologically-active metabolites of nicotine, exerted a significant pro-cataleptic effect. In conclusion, the pro-cataleptic effect of nicotine was weak and variable in the present study, and its mechanism remains obscure.Key words: catalepsy, nicotine, haloperidol, Tourette's Syndrome, dopamine, nornicotine.