Desensitization of 5-HT-1A Somatodentritic Receptors in Tryptophan Treated and Co-treated Rats Induced by Methylphenidate.

BACKGROUND: Psychostimulants can induce behavioral sensitization by their chronic use. The main target for the action of these drugs is dopamine, neither epinephrine nor serotonin transporters. Serotonin is synthesized by the precursor L-tryptophan. Tryptophan and methylphenidate being 5-HT agonists, both increase the level of serotonin thereby causing desensitization of 5-HT1a receptors. The present study investigated whether behavioral sensitization induced by Methylphenidate is decreased in tryptophan administrated animals. METHODS: The Experiment was divided into 2 phases (1). Behavioral effects of repeated administration of TRP 100 mg/kg and MPD for 14 days in three groups; (i) water (ii) MPD 1.0 mg/kg (iii) TRP. To explore the locomotor effects of treatment, the activity was monitored in a familiar and novel environment. (2) Behavioral consequences of repeatedly administrated MPD (1.0 mg/kg) on pretreated TRP (100 mg/kg) and MPD (1.0 mg/kg) animals following Co-MPD and TRP for 14 days, rats were divided in three groups (i) water, (ii) MPD and (iii) TRP as mentioned in Experiment no 1. After two weeks six subgroups were assigned i.e. (i) water-saline, (ii) water- MPD, (iii) TRP-saline (iv) TRP-MPD (v) MPD-saline and (vi) MPD-MPD+TRP and treated for further 14 days. Locomotor behavior was monitored in familiar environment on the next day and in novel environment on alternate days of each administration. RESULTS: The Results from phase 1 showed increased activity in both (TRP and MPD) treatments. However, the results of phase 2 showed significant decrease in methylphenidate-induced behavioral sensitization by both pretreatment and co-administration with TRP. CONCLUSION: The present study suggests the potential of tryptophan to decrease the risk of behavioral sensitization induced by methylphenidate.

Buspirone attenuates methylphenidate-induced growth inhibition.

Methylphenidate is effective in the treatment of attention deficit hyperactivity disorder (ADHD) in children and adults, but its long term use can cause potential adverse effect on growth rate and variable effects on appetite. Previous studies have shown that long term administration of psychostimulant drugs increases the effectiveness of somatodendritic 5-hydroxytryptamine (5-HT)-1A receptors. Repeated administration of buspirone attenuates the effectiveness of somatodendritic 5-HT1A receptors. The present study was designed to test the hypothesis that co-administration of buspirone may attenuate methylphenidate-induced effects on growth rate and food intake. Growth rate was calculated weekly in terms of change in body weight as percentage of preceding week's body weight and food intake was calculated weekly by subtracting the amount of food left in the hopper from the amount of food placed in the hopper as % in preceding week mg/gm of body weight after long-term administration of methylphenidate, buspirone and their co-administration. Long term oral administration of methylphenidate at a dose of 2.0 mg/kg/day decrease growth rate, but co-administration of buspirone at a dose of 10 mg/kg/day attenuates effect of methylphenidate on growth rate however food intake was significantly greater in all treated groups after 3 weeks of treatment. It is suggested that buspirone may oppose methylphenidate-induced growth inhibition by decreasing the sensitivity of somatodendritic 5-HT1A receptors. These findings may help to extend future therapeutics in ADHD.

The role of dopamine D1 and D2 receptors in adolescent methylphenidate conditioned place preference: sex differences and brain-derived neurotrophic factor.

This study analyzed the role of dopamine D1 and D2 receptors in methylphenidate (MPH) conditioned place preference (CPP) in adolescent male and female rats, in addition to the role of these receptors in the effects of MPH on brain-derived neurotrophic factor (BDNF) in the dorsal striatum and nucleus accumbens. Using a nonbiased CPP procedure, the animals were conditioned from postnatal day (PD) 33 to 37. On conditioning trials, animals were first administered saline or their respective antagonist (0.1 or 0.2 mg/kg SCH-23390; 0.01 or 0.03 mg/kg eticlopride HCl), followed by MPH (5 mg/kg). Approximately 10 min after MPH administration, the rats were placed into the paired context for a 10-min trial. One day after conditioning on PD38, a preference test was administered with dividers removed. One day following the preference test on PD39, brain tissue was removed, and the nucleus accumbens and striatum were analyzed for BDNF. Results revealed that MPH conditioning resulted in an increased preference that was blocked by either dose of SCH-23390, but generally not affected by either dose of eticlopride. Further, the higher dose of SCH-23390 resulted in a conditioned place aversion in males, presumably due to an increased number of dopamine D1 receptors in adolescent males. MPH produced a significant increase of striatal and accumbal BDNF alleviated by SCH-23390 or eticlopride. These results show that MPH results in CPP in adolescent male and female rats and these effects appear to be mediated by the dopamine D1 receptor, but the effects of MPH on BDNF appear to be mediated by both dopamine receptor families.

Intra-orbitofrontal cortex injection of haloperidol removes the beneficial effect of methylphenidate on reversal learning of spontaneously hypertensive rats in an attentional set-shifting task.

Numerous studies suggest that attention-deficit/hyperactivity disorder (ADHD) is caused by deficits in catecholaminergic systems. Furthermore, dysfunctions of prefrontal cortex can impair inhibitory controls of ADHD patients, resulting in their impulsive behaviors. Researchers also find that rats with lesions in the orbitofrontal cortex show deficits in the reversal learning of attentional set-shifting task (ASST), a behavioral test frequently used in human studies to asses the inhibition system. However, the role of orbitofrontal dopamine system in the mechanism responsible for the dysfunctions of inhibitory controls in ADHD patients and animal models remains unknown. In the present study, we manipulated orbitofrontal dopamine activities of spontaneously hypertensive rats (SHR), a widely used ADHD animal model, through intra-peritoneal injection of methylphenidate (MPH) and central infusion of haloperidol, and observed their performances in ASST. The results show that juvenile SHRs learned slower than Wistar controls in the first and second reversal learnings of ASST. The deficits could be removed by intra-peritoneal injections of MPH. Furthermore, central infusions of haloperidol in the orbitofrontal cortex blocked the effects of MPH. In conclusions, dopamine activity in orbitofrontal cortex might play a crucial role in the neural mechanism of reversal learning deficits in this animal model of ADHD.

Differential sensitivity to psychostimulants across prefrontal cognitive tasks: differential involvement of noradrenergic α1 - and α2-receptors.

BACKGROUND: Psychostimulants improve a variety of cognitive and behavioral processes in patients with attention-deficit/hyperactivity disorder (ADHD). Limited observations suggest a potentially different dose-sensitivity of prefrontal cortex (PFC)-dependent function (narrow inverted-U-shaped dose-response curves) versus classroom/overt behavior (broad inverted U) in children with ADHD. Recent work in rodents demonstrates that methylphenidate (MPH; Ritalin) elicits a narrow inverted-U-shaped improvement in performance in PFC-dependent tests of working memory. The current studies first tested the hypothesis that PFC-dependent tasks, in general, display narrow dose sensitivity to the beneficial actions of MPH. METHODS: The effects of varying doses of MPH were examined on performance of rats in two tests of PFC-dependent cognition, sustained attention and attentional set shifting. Additionally, the effect of pretreatment with the α1-antagonist prazosin (.5 mg/kg) on MPH-induced improvement in sustained attention was examined. RESULTS: MPH produced a broad inverted-U-shaped facilitation of sustained attention and attentional set shifting. Prior research indicates α1-receptors impair, whereas α2-receptors improve, working memory. In contrast, attentional set shifting is improved with α1-receptor activation, whereas α2-receptors exert minimal effects in this task. Given the similar dose sensitivity of sustained attention and attentional set-shifting tasks, additional studies examined whether α1-receptors promote sustained attention, similar to attentional set shifting. In these studies, MPH-induced improvement in sustained attention was abolished by α1-receptor blockade. CONCLUSIONS: PFC-dependent processes display differential sensitivity to the cognition-enhancing actions of psychostimulants that are linked to the differential involvement of α1- versus α2-receptors in these processes. These observations have significant preclinical and clinical implications.

Methylphenidate-induced impulsivity: pharmacological antagonism by beta-adrenoreceptor blockade.

Noradrenaline-dopamine interactions mediate increases in locomotor activity, development of sensitisation and subjective effects of psychostimulant drugs. However, the modulatory effects of noradrenaline on psychostimulant-induced impulsivity are less clear. This article examined the relative roles of noradrenaline and dopamine in the modulation of methylphenidate-induced impulsive responding in rats performing the 5-choice serial reaction time task. Experiment 1 examined the systemic antagonism of methylphenidate-induced impulsivity with either propranolol, a beta-adrenoreceptor blocker, or prazosin, an alpha1-adrenoreceptor antagonist, which antagonises the locomotor activating effects of amphetamine. Propranolol completely abolished methylphenidate-induced impulsivity. This effect was centrally rather than peripherally mediated, as nadolol, a peripheral beta-blocker failed to affect methylphenidate-induced premature responding. Prazosin partially attenuated the methylphenidate-mediated increase in premature responding. A second experiment examined the effects of selective anti-D beta H saporin-induced cortical noradrenaline depletion on methylphenidate-induced impulsivity. Contrary to the effects of beta-adrenoreceptor blockade, cortical noradrenergic depletion did not alter methylphenidate-induced impulsivity. Other experiments examined the comparative effects of selective dopamine and serotonin receptor blockade. D4 dopamine receptor blockade with systemically administered L-745,870 also attenuated methylphenidate-induced impulsivity. The other antagonists had no effect on methylphenidate-induced impulsivity. Taken together, these studies provide evidence for a modulatory role of beta-adrenoreceptors on methylphenidate-induced impulsive responding.

Inhibition of methylphenidate-induced gene expression in the striatum by local blockade of D1 dopamine receptors: interhemispheric effects.

Psychostimulants change the function of cortico-basal ganglia circuits. Some of these effects are mediated by altered gene regulation in projection neurons of the striatum which participate in these circuits. Psychostimulant-induced changes in gene expression in these neurons are a consequence of excessive stimulation of G-protein-coupled receptors, particularly the D1 dopamine receptor subtype. Recent findings show that the psychostimulant methylphenidate, which causes dopamine overflow in the striatum, produces changes in striatal gene regulation similar, but not identical, to those induced by psychostimulants such as cocaine and amphetamine. We investigated, in rats, the role of striatal D1 receptors in methylphenidate-induced gene expression, by intrastriatal administration of the D1 receptor antagonist SCH-23390. Effects on the expression of two plasticity-related molecules, the transcription factor zif 268 and the synaptic plasticity factor Homer 1a, in the striatum and cortex were assessed. Intrastriatal infusion of SCH-23390 (2-10 microg) attenuated zif 268 and Homer 1a mRNA expression induced by methylphenidate (10 mg/kg, i.p.) in a dose-dependent manner. Moreover, this unilateral SCH-23390 infusion not only inhibited gene induction at the infusion site in the central striatum, but also in distant striatal regions including the nucleus accumbens, as well as throughout the entire contralateral striatum. These results indicate that striatal D1 receptors are critical for gene induction by methylphenidate. Moreover, the ipsilateral and contralateral effects of local SCH-23390 administration suggest that D1 receptor-stimulated striatal output exerts robust control over widespread striatal activities/gene expression via regulation of input to the striatum.

Methylphenidate-induced motor activity in rats: modulation by melatonin and vasopressin.

Methylphenidate (MPH), a dopamine (DA) reuptake inhibitor, is well known to enhance motor activity, in part depending on the time of its application during the light-dark cycle. Moreover, after MPH administration, the hypothalamo-neurohypophysial axis including the neuropeptide vasopressin (AVP) was found influenced. Both the latter and behavioural effects of central AVP can also be modulated by the pineal gland with its light-dark-dependent activity. The present study was performed to investigate whether the pineal gland, its hormone melatonin (Mel), and AVP are involved in the MPH-evoked stimulation of activity. After application of 10 mg/kg MPH, the motor activity in pinealectomised (PE) rats was significantly higher than in sham-operated (SO) animals. After application of 250 microg Mel before MPH treatment, the stimulation of motor activity was diminished in PE rats and augmented in SO animals; however, when SO and PE rats were compared after Mel pretreatment, the reaction to MPH was nearly identical. Blocking the endogenous AVP by 25 or 1 microg of the V1a receptor antagonist d(CH(2))(5)[Tyr(Me)(2)]AVP (AAVP) before MPH treatment significantly augmented the motor activity in SO rats only and abolished the differences seen between SO and PE animals after MPH application. The present results indicate that the behavioural stimulation of MPH was modulated by both the pineal gland with its hormone Mel as well as the neuropeptide AVP.

Methylphenidate sensitization is modulated by valproate.

Repeated administration of the stimulant methylphenidate (MPD) produces sensitization to its own effects. Glutamate, dopamine, and GABA have been implicated in the underlying mechanism of sensitization to stimulants such as amphetamine and cocaine. We have investigated effects of the GABAergic agent sodium valproate (VAL) on the locomotor response to MPD. Activities of male Sprague-Dawley rats were continuously recorded by a computerized activity monitoring system for 15 days. We studied the dose effect of valproate 1) at 50, 100, and 200 mg/kg (i.p.) on motor activities, 2) on the acute response of motor activities to 2.5 mg/kg MPD, and 3) on behavioral sensitization to subsequent repeated injections of MPD. Valproate alone did not significantly affect motor activities. All three doses of valproate attenuated the acute locomotor effects of MPD, while only the 50 mg/kg dose blocked the development of sensitization to subsequent administration. Possible mechanisms involving substrates for the effect of GABA agonists on sensitization are discussed.

Valproate modulates the expression of methylphenidate (ritalin) sensitization.

Repeated administration of amphetamine, cocaine, and methylphenidate (MPD) has been reported to elicit behavioral sensitization to their locomotor and stereotypic effects in rodents. GABAergic drugs have been shown to inhibit behavioral effects of stimulants. The objective of the present study was to determine whether single or multiple administration of sodium valproate, a GABA agonist, would prevent the expression of sensitization to the locomotor effect of MPD once it has developed. Twenty-eight male Sprague-Dawley rats were randomized into three treatment groups: a control group received only 2.5 mg/kg MPD during the 14-day cycle, a group received a single 50 mg/kg valproate injection on Day 9, and a group received multiple 50 mg/kg valproate injection on Days 9-13. Rats were housed in test cages and behavioral activities were recorded for 14 consecutive days. All injections were given between 12:00 h and 14:00 h. Multiple injection of MPD elicited sensitization to its locomotor and stereotypic effects. Single administration of valproate did not block the expression of sensitization in the four locomotor indices measured. However, multiple administration of valproate prevented MPD sensitization to horizontal activity, total distance and number of stereotypic movements.

Role of 5-HT1A receptors in the ability of mixed 5-HT1A receptor agonist/dopamine D2 receptor antagonists to inhibit methylphenidate-induced behaviors in rats.

Behavioral effects produced by the indirect-acting dopamine receptor agonist, methylphenidate (40 mg/kg i.p.) were examined in rats after administration of the 5-HT1A receptor agonists (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and flesinoxan, the mixed 5-HT1A receptor agonist/dopamine D2 receptor antagonists buspirone and 1-[-4-fluorobenzoylamino)ethyl]-ethyl]-4-(7-methoxynaphthyl) piperazine (S 14506), the neuroleptics haloperidol and clozapine, and the sigma receptor ligand/partial 5-HT1A receptor agonist alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazine-butanol (BMY 14802). All of the compounds produced dose-related decreases in methylphenidate-induced stereotyped gnawing, and, as gnawing was inhibited, other methylphenidate-induced responses (i.e. sniffing, rearing and locomotion) appeared. Higher doses of haloperidol and buspirone, but none of the remaining compounds, inhibited these other responses, so that the behavior of the methyphenidate-treated animals became similar to that of normal controls. Pretreatment with the 5-HT1A receptor antagonist N-[2-4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridinyl)- cyclohexanecarboxamide (WAY-100635; 0.63 mg/kg s.c.) blocked the ability of 8-OH-DPAT, S 14506 and flesinoxan to inhibit methylphenidate-induced gnawing, demonstrating the involvement of 5-HT1A receptors in their ability to inhibit methylphenidate-induced behaviors. In contrast, pretreatment with WAY-100635 did not alter the ability of haloperidol, clozapine, buspirone, or BMY 14802 to inhibit methylphenidate-induced gnawing, or in the case of haloperidol and buspirone, to normalize behavior. The results indicate that mixed compounds with 5-HT1A receptor agonist and dopamine receptor antagonist properties can be differentiated on the basis of the ability of WAY-100635 to reverse their effects on methylphenidate-induced behaviors.

Does haloperidol block methylphenidate? Motivation or attention?

The effect of methylphenidate preceded by a moderate dose of haloperidol on reaction times over the duration of a continuous performance test (CPT), was investigated in ten male children, with a DSM-III diagnosis of attention deficit disorder with hyperactivity disorder (ADDH). Using a within-subject double-blind design, the effects of methylphenidate preceded by haloperidol on reaction time during the first and second blocks of CPT test were compared. Methylphenidate maintained a significantly improved reaction time in the second block of the CPT test. When methylphenidate, preceded by placebo, was preceded by haloperidol this effect was not observed, suggesting opposing effects on attentional systems by methylphenidate versus haloperidol. The study is the first to examine the "blocking" effect of haloperidol over the course of a CPT. The results suggest that dopamine systems are involved in the maintenance of the CPT response, and support an "incentive motivation" theory of sustained attention.

Chronic lithium attenuates dopamine D1-receptor mediated increases in acetylcholine release in rat frontal cortex.

The effects of chronic lithium treatment on methylphenidate-, D1 dopamine receptor agonist (A-77636)-, and tactile stimulation-induced increases in frontal cortical acetylcholine release were studied in the rat using in vivo brain microdialysis. Cortical acetylcholine release in control rats was maximally stimulated by methylphenidate (1.25 and 2.5 mg/kg) to 173% and 212% above baseline, respectively. The effect of methylphenidate (2.5 mg/kg) was blocked by pretreatment with the dopamine D1 receptor antagonist SCH 23390 (0.3 mg/kg). Chronic treatment with lithium chloride (3-4 weeks) produced plasma lithium concentrations of 0.45 +/- 0.02 meq/l. Chronic lithium significantly reduced increases in cortical acetylcholine release produced by methylphenidate. Stimulation of dopamine D1 receptors with the full D1 receptor agonist A-77636 (0.73 mg/kg) increased cortical acetylcholine release. Chronic lithium significantly reduced this effect of A-77636. In contrast, lithium failed to influence the increases of cortical acetylcholine release produced by tactile stimulation. These results suggest that while lithium does not influence normal, arousal-related increase in cortical acetylcholine release, this ion selectively attenuates dopamine mediated increases and/or abnormally large increases, which in the present circumstances were pharmacologically induced. The relevance of these findings to the antimanic actions of lithium is discussed.

Omega-conotoxin GVIA inhibits the methylphenidate-induced but not methamphetamine-induced behavior.

We investigated the effects of antagonists for omega-conotoxin GVIA (omega-CTX)-sensitive N-type voltage-sensitive calcium channels (N-channels) on methylphenidate- and methamphetamine-induced behavior. I.c.v. injection of omega-CTX or neomycin, both N-channel antagonists, caused a dose-dependent inhibition of methylphenidate-induced hypermotility in mice but failed to inhibit methamphetamine-induced hyperactivity. Further, omega-CTX inhibited the circling behavior induced by methylphenidate in rats that had kainic acid-induced unilateral striatal lesions. These results suggest that calcium influx through omega-CTX-sensitive N-channels plays an important role in methylphenidate-induced behavior.

Inhibition of methylphenidate-induced behaviors in rats: differences among neuroleptics.

Methylphenidate (MPD) increased in rats the incidence of sniffing, rearing and locomotion, and this along dose-response curves that had an inverted U-shape; at 40 mg/kg, MPD exclusively induced stereotyped gnawing, which was inhibited by neuroleptics. However, as gnawing induced by 40 mg/kg MPD was inhibited in a dose-dependent manner, other responses (i.e., sniffing, rearing and locomotion) appeared. Higher doses of neuroleptics also inhibited these latter responses, so that the behavior of the MPD-treated animals became similar to that of normal controls. Only some nonsedative neuroleptics appeared able to normalize the behavior of MPD-treated rats at doses that induced neither complete behavioral suppression nor adverse effects. The neuroleptics differed markedly, however, in terms of the relative doses at which they 1) inhibited gnawing, 2) inhibited the other effects of MPD and 3) induced complete behavioral suppression and/or adverse effects. This variation among neuroleptics, which appears to represent a novel aspect of their ability to antagonize behavioral effects of central nervous system stimulants, may be based on differences in the extent to which they exert agonist activity at dopamine receptors. Assuming that MPD-induced behaviors model the positive symptoms of schizophrenia, it may be hypothesized that the differences among the neuroleptics observed here provide an indication of their efficacy to reduce positive symptoms in schizophrenics.

Locomotor stereotypy is produced by methylphenidate and amfonelic acid and reduced by haloperidol but not clozapine or thioridazine.

In addition to its well-known behavioral effects in rats, amphetamine also produces patterned locomotion (referred to below as locomotor stereotypy) in an open field. Locomotor stereotypy may be mediated by different mechanisms than those mediating the better-known behavioral effects of amphetamine. To determine whether the ability to produce locomotor stereotypy is an exclusive property of amphetamine or is a property of many amphetamine-like stimulants, several doses of methylphenidate and amfonelic acid were tested. The ability of both atypical and typical neuroleptics to block amphetamine-induced locomotor stereotypy was also tested. Both amfonelic acid and methylphenidate produced some degree of locomotor stereotypy. In addition, amphetamine-induced locomotor stereotypy was reduced by haloperidol but not by clozapine or thioridazine. These data suggest that locomotor stereotypy is more closely related to focused stereotypy than to hyperlocomotion.

[Characteristics of antagonism between ceruletide and various central-acting drugs: investigation by means of ambulatory activity in mice].

Behavioral characteristics of ceruletide, a cholecystokinin-like decapeptide, were investigated by means of ambulatory activity in mice. Ceruletide at 100 and 300 micrograms/kg, i.p. slightly but significantly decreased the mouse's activity for 20 min. Therefore, 100 micrograms/kg of ceruletide was used in the experiment of combined administration with the central-acting drugs. Ceruletide reduced the increased activity which was produced by methamphetamine (2 mg/kg, s.c.), ephedrine (80 mg/kg, i.p.), methylphenidate (4 mg/kg, s.c.), cocaine (20 mg/kg, s.c.), mazindol (2.5 mg/kg, s.c.), apomorphine (0.5 mg/kg, s.c.), bromocriptine (8 mg/kg, i.p.), scopolamine (0.5 mg/kg, s.c.), caffeine (10 mg/kg, s.c.) and morphine (20 mg/kg, s.c.) with different potencies and durations. The mice that had experienced ceruletide at 3 micrograms/kg for 5 times at intervals of 3-4 days demonstrated a significant increase in the sensitivity to methamphetamine, although the same treatment with 10-300 micrograms/kg of ceruletide was without effect. On the other hand, when 3-300 micrograms/kg of ceruletide was combined with 2 mg/kg of methamphetamine, the development of reverse tolerance to the ambulation-increasing effect of methamphetamine was inhibited dependently on the doses of ceruletide. However, the reverse tolerance to methamphetamine once established was scarcely modified by ceruletide when it was administered afterwards.

Analgesic action of methylphenidate on parkinsonian sensory symptoms. Mechanisms and pathophysiological implications.

Intravenous administration of methylphenidate hydrochloride, a central stimulant, was unexpectedly found to exert a potent analgesic effect on primary sensory symptoms in a group of patients with Parkinson's disease. This effect, which has now been studied in a short-term, double-blind, placebo-controlled experiment, subsequently disappeared if patients were pretreated with a beta-blocker or with a serotonin antagonist. Cerebrospinal fluid monoamine metabolites were determined in some of these patients, and the 5-hydroxyindoleacetic acid level was found to be significantly lower than in parkinsonian patients without pain and in normal volunteers. Given the mechanism of action of methylphenidate on the central nervous system, the adrenergic and serotoninergic mediation of its analgesic effect, and the demonstration of impaired central serotonin metabolism in the patient group, it is concluded that not only central dopaminergic deficiency but also altered noradrenergic and serotoninergic transmission in the spinal cord are quite likely to play a role in the pathophysiology of pain in Parkinson's disease.

Comparative behavioral effects of CNS cholinesterase inhibitors.

Phenylmethanesulfonyl fluoride, methanesulfonyl fluoride, and physostigmine were compared on the efficacy with which each could suppress methylphenidate-induced stereotyped gnawing, an extrapyramidal motor behavior. Whereas physostigmine produced powerful suppression of the stereotypy, the sulfonyl fluorides did not produce any clear behavioral effect. Biochemical experiments conducted with the behavioral tests demonstrated that the sulfonyl fluorides produced inhibition of whole brain, caudate, cortex, cerebellum, hippocampus and brain stem cholinesterate equal to that produced by physostigmine. The reason for the marked discrepancy between the behavioral effect of physostigmine and the sulfonyl fluorides is unknown. It is, however, clear that the effect of the various drugs on extrapyramidal motor behaviors is not a simple function of the degree to which each inhibited CNS cholinesterase.