Spontaneous orofacial movements induced in rodents by very long-term neuroleptic drug administration: phenomenology, pathophysiology and putative relationship to tardive dyskinesia

Dose-related effects of a neonatal 6-OHDA lesion on SKF 38393- and m-chlorophenylpiperazine-induced oral activity responses of rats

Neonatal 6-hydroxydopamine (6-OHDA) treatment of rats is associated with concurrent supersensitization of dopamine (DA) D1 and serotonin 5-HT1C receptors, for agonist-induced oral activity. The present study was conducted to determine if graded reduction of striatal DA content and/or graded elevation of striatal 5-HT content by 6-OHDA would alter sensitivity of either receptor type, and thereby influence oral activity responses to DA and 5-HT agonists. At 3 days after birth, groups of rats were pretreated with desipramine (20 mg/kg i.p.), 1 h before administration of a range of doses of 6-OHDA HBr (15, 30, 60, 100, 150 and 200 micrograms, i.c.v., salt form; half in each lateral ventricle) or the vehicle, saline (0.85%)-ascorbic acid (0.1%). Between 2 and 4 months, a series of challenge doses of SKF 38393 HCl (0.30 to 3.0 mg/kg i.p.) and m-chlorophenylpiperazine 2HCl (0.30 to 6.0 mg/kg i.p.; m-CPP 2HCl) were administered to each group of rats and oral activity was observed. Oral activity was determined for 1 min every 10 min during a 60-min period, starting 10 min after injection of agonist or vehicle. SKF 38393 dose-response curves demonstrated enhanced oral activity responses in rats lesioned neonatally with 150 or 200 micrograms of 6-OHDA. m-CPP dose-response curves demonstrated enhanced oral activity responses in these 2 groups of rats, as well as those lesioned neonatally with 100 micrograms of 6-OHDA. Striatal DA content was reduced by > 97% in these 3 groups of rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Vacuous jaw movements in rats induced by acute reserpine administration: interactions with different doses of apomorphine

Two experiments were conducted to study the vacuous jaw movements induced in rats by acute administration of the monoamine-depleting agent reserpine. In the first experiment, different doses of reserpine (1.25, 2.5, and 5.0 mg/kg) were assessed for their ability to induce vacuous jaw movements. Acute administration of reserpine induced a dose-related increase in vacuous jaw movements, with the two highest doses being significantly different from the vehicle control. In the second experiment, interactions between 5.0 mg/kg reserpine and the dopamine agonist apomorphine were investigated. Coadministration of reserpine with the lowest dose of apomorphine (0.1 mg/kg) significantly increased vacuous jaw movements relative to reserpine alone. The two higher doses of apomorphine (0.5 and 1.0 mg/kg) significantly decreased vacuous jaw movements in reserpine-treated rats. These results demonstrate that vacuous jaw movements are induced by acute reserpine treatment in a dose-related manner. In addition, the interactions with apomorphine suggest that vacuous jaw movements are stimulated by decreases in dopamine release produced by low doses of apomorphine that are thought to have mainly presynaptic actions, but that these movements are decreased by higher doses of apomorphine that are known to act postsynaptically.

Investigation of the metabolic fate of the neuroleptic drug haloperidol by capillary electrophoresis-electrospray ionization mass spectrometry

Combined capillary electrophoresis-mass spectrometry (CE-MS) is a powerful analytical tool for the characterization of components of complex mixtures. Furthermore, when interfaced with an electrospray ionization (ESI) source, skimmer induced fragmentation can yield structural information that aids compound identification. We show that these techniques can be used to probe the metabolic fate of drugs by investigation of the phase I metabolism of the widely used neuroleptic drug haloperidol. In addition, we demonstrate differences of in vitro metabolism of haloperidol by mouse and guinea pig hepatic microsomes.

Age-dependence of a 6-hydroxydopamine lesion on SKF 38393- and m-chlorophenylpiperazine-induced oral activity responses of rats

Neonatal 6-hydroxydopamine (6-OHDA) treatment is associated with destruction of dopamine (DA) fibers and subsequent sprouting of serotonin (5-HT) fibers in the striatum of rats. Enhanced oral activity responses to SKF 38393 and m-chlorophenylpiperazine (m-CPP), respective agonists for the DA D1 receptor complex and 5-HT2C receptor complex, ensue. To study the ontogenetic nature of this effect, rats were treated at birth, 3 days, 7 days, 10 days or 14 days with 6-OHDA-HBr (200 micrograms i.c.v.; salt form), following desipramine-HCl pretreatment (20 mg/kg i.p., 1 h; base form). Another group of rats was treated at 35 days and again at 42 days with 6-OHDA-HBr (300 micrograms i.c.v.), following desipramine-HCl (20 mg/kg i.p., 1 h) and pargyline-HCl (50 mg/kg i.p., 30 min). In rats treated from birth to 10 days, 6-OHDA reduced striatal DA content at 5 months by > or = 94%. Striatal 5-HT content was elevated by 28% to 51%, but only in rats treated with 6-OHDA at 7 days from birth or earlier. An enhanced oral activity response to SKF 38393-HCl (0.03 to 1.0 mg/kg i.p.) was absent in rats treated 7 days or later, and the change in SKF 38393 effect was correlated with a change in striatal DA content. An enhanced response to m-CPP.2HCl (0.3 to 6.0 mg/kg i.p.) was absent after treatment at 14 or 35 days, when striatal DA content was reduced only 44% to 63% and 5-HT content was not changed.(ABSTRACT TRUNCATED AT 250 WORDS)

MIF-1 fails to modify agonist-induced oral activity in neonatal 6-OHDA-treated rats

L-Prolyl-L-leucyl-glycinamide (MIF-1) is known to attenuate apomorphine-induced stereotypes in adult rats that are lesioned as neonates with 6-hydroxydopamine (6-OHDA). To test whether MIF-1 would affect dopamine (DA) agonist-induced and serotonin (5-HT) agonist-induced oral activity, both intact and neonatal 6-OHDA-treated rats were studied. Rats at 3 days from birth were injected with desipramine (20 mg/kg, IP), 1 h before 6-OHDA HBr (100 micrograms, salt form, in each lateral ventricle) or its vehicle, saline-ascorbic acid (0.1%). At approximately 6 months rats were treated with MIF-1 (0.1, 1.0, or 10.0 mg/kg, IP), 10 min before SKF 38393 HCl (1.0 mg/kg, IP) or m-chlorophenylpiperazine 2HCl (m-CPP 2HCl; 0.5 mg/kg, IP), DA D1 and 5-HT1C,2 receptor agonists, respectively. Although both agonists increased oral activity in control and neonatal 6-OHDA-treated rats, MIF-1 did not modify the response. In rats that received either of the three doses of MIF-1 for 21 consecutive days, there was still no observed effect of MIF-1 on the oral response of control and 6-OHDA-lesioned rats to SKF 38393 and m-CPP. These findings indicate that MIF-1 does not modify the oral activity response of supersensitized D1 and 5-HT1C receptors in adult rats that are lesioned neonatally with 6-OHDA.

Investigation of the metabolism of the neuroleptic drug haloperidol by capillary electrophoresis

Free solution capillary electrophoresis (FSCE) conditions were previously reported to be of limited use for the separation of pharmaceuticals, since many of these compounds are neutral. We show that by consideration of compound hydrophobicity and ionisable functional groups, FSCE conditions can be developed to effect the separation of a drug and its phase I metabolites. This is brought about by adding a suitable organic modifier to aid solubility, and modifying pH to effect a change in the mass to charge ratio of the metabolites present. Furthermore, we show that in this drug metabolism study, FSCE presents an advantage over both reversed-phase HPLC and micellar electrokinetic chromatography. We also demonstrate the use of FSCE for investigation of the phase I metabolites produced by the in vitro incubation of haloperidol (a neuroleptic agent) with both mouse and guinea pig hepatic microsomes and show that such an approach can be used to detect both qualitative and quantitative differences in species metabolism.

Modulation of oral movements by intranigral 5-hydroxytryptamine receptor agonists in the rat

Bilateral infusion of 5-hydroxytryptamine (5-HT) agonists into the substantia nigra pars reticulata (SNr) of awake rats was shown to influence oral behavior. The 5-HT1A agonist (R)-8-hydroxy-2-(di-propylamino)- tetralin (8-OH-DPAT) (1.3-13 nmol on each side) produced a dose-dependent depression of vacuous chewing movements (VCMs) that lasted about 20 min. The (R)-8-OH-DPAT-induced depression of VCMs was blocked by the simultaneous intranigral infusion of a specific 5-HT1A antagonist [(-)-(S)-5-fluoro-8-hydroxy-2-(dipropylamino)tetralin HCl (UH-301)], which had no effect when given alone. Another 5-HT1A agonist [(5-methoxy-N,N-dimethyltryptamine hydrogen oxalate (5-MeO-DMT)] also reduced VCM frequencies. Intranigral infusion of the nonspecific 5-HT-agonists 1-(3-triflouro-methylphenyl) piperazine (TFMPP) and 1(m-chlorophenyl)-piperazine (mCPP) and a 5-HT3 agonist [2-methyl-5-hydroxytryptamine (2-Me-5-HT)] increased VCM after 5- to 10-nmol doses. Another 5-HT3 agonist (1-phenylbiguanide) and a 5-HT2 agonist [1-(4-bromophenyl-2,5-dimethoxy)-2-aminopropane (DOB)] had no significant effect. As most 5-HT receptors in the SNr are of the 5-HT1B subtype, these results suggest that the increased VCM frequency was mediated via nigral 5-HT1B receptors. The importance of 5-HTergic mechanisms in the development of drug-induced dyskinesias is discussed.

Effects of acute haloperidol and reserpine administration on vacuous jaw movements in three different age groups of rats

In these experiments three different age groups of rats were tested for vacuous jaw movements. The age groups included rats aged 3 months, rats aged 6-9 months, and rats aged 12-15 months. In the first experiment, rats were given a single IP injection of either 0.3% tartaric acid vehicle or 0.4 mg/kg haloperidol. In the second experiment, rats were given injections of vehicle or 5.0 mg/kg reserpine. Haloperidol and reserpine significantly increased vacuous jaw movements. There were significant effects of age on vacuous jaw movements, with rats aged 6-9 months and rats aged 12-15 months showing more jaw movements than 3-month-old rats. In both experiments, the drug x age interaction was not significant, indicating that the combined effects of age and neuroleptic treatment were additive and not synergistic. Haloperidol and reserpine also reduced rearing behavior in all age groups. It is concluded that age is an important variable in studies of vacuous jaw movements.

An evaluation of tandem mass spectrometry in drug metabolism studies

The use of precursor ion and constant neutral loss scanning as a means of rapidly detecting drug metabolites is evaluated. Four clinically useful drugs, namely (i) cyclophosphamide, (ii) mifentidine, (iii) cimetropium bromide and (iv) haloperidol, were subjected to microsomal incubations to afford phase I metabolites. Aside from a minor clean-up procedure involving zinc sulfate precipitation of microsomal proteins and solid-phase extraction of metabolites using a Sep-pak C-18 cartridge, the mixtures were analysed directly by fast atom bombardment tandem mass spectrometry. It is demonstrated that such screening strategies are important in detecting novel metabolites. However, there are some problems associated with only using such methods, including (i) the possibility of not detecting metabolites that undergo unusual collision-induced dissociation fragmentation pathways, (ii) the non-detection of metabolites that have undergone metabolic change at unusual sites of reactivity, and (iii) production of artifacts derived from the parent drug by the primary ionization process. Examples are discussed that highlight both the strengths and weaknesses of such an approach.

On the metabolism of haloperidol

1. p-Fluorobenzoyl-propionic acid, 4-(4-chlorophenyl)-4-hydroxy-piperidine, and reduced haloperidol were confirmed as metabolites of haloperidol. Their formation was catalysed by hepatic microsomes and was NADPH dependent. 2. The pyridinium metabolite of haloperidol (HP+) was identified. It is proposed that haloperidol first undergoes dehydration to form its 1,2,3,6-tetrahydropyridine analogue (HTP). HTP is then further metabolized to HP+, HTP N-oxide and its N-dealkylated product, 4-chlorophenyl-1,2,3,6-tetrahydropyridine (CPTP). HTPN-oxide was metabolized to CPTP and HTP. All these metabolites were confirmed by comparison with synthesized compounds using h.p.l.c. and h.p.l.c.-mass spectrometry. 3. Three unknown metabolites were present in microsomal metabolic incubations of haloperidol. One of them was tentatively characterized by h.p.l.c.-mass spectrometry as an oxygenated product of haloperidol, another appears to be the 2-pyridine analogue of haloperidol. The third metabolite was shown to be a neutral compound of unknown structure, which was not haloperidol N-oxide nor 4-hydroxy-4'-fluorobutyrophenone. In addition, HTP was metabolized to a further unknown product with a similar u.v. spectrum to that of HTP. 4. The identification of these metabolites led to the hypothesis that the metabolism of haloperidol is similar to that of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and may therefore produce motor neurone toxicity by a similar mechanism.

Involvement of the nucleus accumbens in oral behaviour in the freely moving rat

The role of the nucleus accumbens in oral behaviour was examined by intra-accumbens injections of a single dose of a selective dopamine D1 receptor agonist (SKF 38393: 5 micrograms/side), a selective dopamine D2 receptor agonist (quinpirole: 10 micrograms/side), and their combination in freely moving rats. Principal factor analysis revealed four factors to be involved in the scored behaviours, two of which concerned oral behaviour: a chew factor, comprising the behaviours chew, tongue protrusion, yawn and lick, and a groom factor, with high factor loadings of tremor and groom. The two remaining factors were the circle factor comprising circle, walk and rear, and the sniff factor comprising sniff, yawn and rear. Two-way ANOVA (independent variable D1 with H2O and SKF 38393 level; independent variable D2 with H2O and quinpirole level) of the factor scores revealed that SKF 38393 and quinpirole had similar or opposite effects which were additive or antagonistic, depending on which behaviour was studied. This study demonstrates that (a) the nucleus accumbens plays a major role in the oral behaviour of freely moving rats, and (b) an integrated study of all oral behavioural elements is necessary to describe the effects of drugs on oral behaviour.

Increases in rat striatal extracellular dopamine and vacuous chewing produced by two sigma receptor ligands

The present studies were carried out to analyze the neurochemical and behavioral effects of peripheral sigma ligand administration in the rat. Based upon previous studies which showed an increase in turning behavior following unilateral intranigral administration of sigma ligands, we determined the effects of two sigma ligands, 1,3-di-o-tolylguanidine (DTG) and (+)-pentazocine, on extracellular dopamine levels in the rat striatum. Dopamine levels were monitored via microdialysis in awake freely moving animals following i.p. injection of the ligands. Both DTG (1 and 3 mg/kg) and (+)-pentazocine (10 mg/kg) produced a significant (30-50%) increase in extracellular dopamine. Given the relatively high concentration of sigma receptors in brain nuclei involved in facial and mouth movements, we have also determined the effects of the two sigma ligands on facial movements. Both ligands produced a significant increase in vacuous chewing movements, suggesting that studies on the consequences of sigma receptor activation may have relevance to animal models of human dystonia and/or dyskinesia.

Persistent vacuous chewing in rats following neuroleptic treatment: relationship to dopaminergic and cholinergic function

In order to relate the effects of pharmacological intervention to neuroleptic induced increases in oral activity rats were treated continuously (7 mg/kg per week) or discontinuously (7 mg/kg per week or 2 mg/kg per week) with haloperidol for 6 months. Only the two intermittently treated groups developed persisting increases in vacuous chewing movements (VCM) following drug withdrawal. Opposed to control animals and continuously treated rats, the discontinuously treated groups demonstrated significant elevation in mouth movements following stimulation with the dopamine (DA) D1 receptor agonist SK&F 38393 (23 mg/kg), whereas they did not response to an acute challenge with the selective DA D1 receptor antagonist NNC-756 (0.1 mg/kg). The DA D2 receptor antagonist raclopride (1 mg/kg) provoked a general fall in VCM; however, this was only significant in rats treated intermittently with haloperidol 7 mg/kg per week and in control rats. Intermittent neuroleptic treatment also increased apomorphine-induced stereotypy. The effect of challenge with the anticholinergic drug scopolamine (0.25 mg/kg) was not related to oral activity; furthermore, the finding of severe agitation in rats tested with the latter drug points to caution in the interpretation of rating of rats treated with anticholinergics. These results support that intermittent ingestion of neuroleptic drugs lead to long-lasting increases in VCM. They also suggest a relation of persisting elevated oral activity to supersensitivity to DA receptor agonists, as opposed to subsensitivity to D1 receptor antagonists.

Electromyographical differentiation of the components of perioral movements induced by SKF 38393 and physostigmine in the rat

Facial electromyography (EMG) coupled with visual observation was used to investigate spontaneous and drug induced perioral movements in freely moving rats. Four separate perioral behaviours were identified; facial tremor, purposeless chewing, gaping and yawning. Facial tremor, yawning and gaping but not purposeless chewing produced characteristic EMG signals. Normal rats displayed a low level of purposeless chewing, occasional bursts of facial tremor but not gaping or yawning. Each burst of facial tremor was accompanied by a transient increase in purposeless chewing. Administration of the D1 agonist SKF 38393 induced a dose related increase in bursts of facial tremors and consequently an increase in the total number of purposeless chews. Gaping and yawning were not induced by SKF 38393 administration. Administration of the cholinesterase inhibitor physostigmine (0.1-0.4 mg/kg) induced a dose related increase in the total number of purposeless chews, but primarily these were not associated with facial tremor. Administration of physostigmine also increased gaping and yawning. Administration of the D1 antagonist SCH 23390 almost abolished facial tremor in normal treated rats but only partially reduced that induced by SKF 38393 and physostigmine. SCH 23390 reduced purposeless chewing in SKF 38393 treated rats but not in normal or physostigmine treated animals. Administration of the cholinergic antagonist atropine almost abolished facial tremor in normal and physostigmine treated rats, but only reduced by 46% that induced by SKF 38393. Atropine reduced purposeless chewing in normal, physostigmine and SKF 38393 treated animals. Physostigmine induced gaping and yawning were abolished by atropine administration.(ABSTRACT TRUNCATED AT 250 WORDS)

Vacuous jaw movements induced by sub-chronic administration of haloperidol: interactions with scopolamine

The present series of experiments was conducted to investigate the vacuous jaw movements induced by sub-chronic administration of haloperidol (HP). In the first experiment, daily injection of 0.4 mg/kg HP for 10 days increased vacuous jaw movements and decreased rearing behavior. The second and third experiments investigated the interaction between the effects of HP and the anticholinergic drug scopolamine. Co-administration of 0.5 mg/kg scopolamine with 0.4 mg/kg HP for 9 days reduced vacuous jaw movements and increased rearing responses relative to rats that received HP alone. Co-administration of HP with 0.25 mg/kg scopolamine for 9 days increased rearing relative to rats that received HP alone, but there was no effect of the lower dose of scopolamine on vacuous jaw movements. Administration of 0.5 mg/kg scopolamine plus 0.4 mg/kg HP on days 11-14 to rats that had received HP alone for 10 days reversed the effect of HP on rearing, but not on vacuous jaw movements. Rats that had received HP plus scopolamine for 10 days showed dramatic increases in vacuous jaw movements when scopolamine was withdrawn. Because vacuous jaw movements are produced within the first few days of administration, reduced by administration of scopolamine, and exacerbated by withdrawal of scopolamine, the pharmacological characteristics of these movements do not appear to bear a close relation to those of tardive dyskinesia in humans. The present results are consistent with the hypothesis that vacuous jaw movements in rats share some characteristics with Parkinsonian symptoms.

Effect of chronic trifluoperazine administration and subsequent withdrawal on the production and persistence of perioral behaviours in two rat strains

The effect of chronic administration of trifluoperazine on the perioral movement profile of Wistar and Sprague-Dawley rats was examined. Perioral movements were characterised by visual observations, coupled with electromyographic recording from the masseter muscle. In drug-naive animals from both strains the spectrum of perioral behaviours was essentially identical, primarily consisting of purposeless chewing, accompanied by occasional bursts of facial tremor and teeth chattering, with occasional yawning. Each burst of facial tremor was accompanied by a transient increase in the rate of purposeless chewing. Wistar rats exhibited a higher level of spontaneous purposeless chewing compared to Sprague-Dawley rats. In both strains, chronic administration of trifluoperazine (5 mg/kg per day, PO) for 5 months induced an increase in perioral behaviour, which primarily consisted of enhanced purposeless chewing. In Wistar rats the drug-induced increase in purposeless chewing was accompanied by an increase in the incidence of yawning, with no change in the incidence of either facial tremor or teeth chattering. In contrast, Sprague-Dawley rats displayed a drug-induced increase in purposeless chewing, accompanied by an increase in the incidence of facial tremor and teeth chattering, but not yawning. In Wistar rats withdrawal of trifluoperazine diminished but did not reverse the drug-induced increase in purposeless chewing. Drug withdrawal also precipitated a transient increase in the incidence of facial tremor and teeth chattering, but had no effect on yawning. In Wistar rats, the level of purposeless chewing and the incidence of yawning remained elevated above control levels for at least 13 weeks after drug withdrawal.(ABSTRACT TRUNCATED AT 250 WORDS)

Jaw muscle activity, the nucleus accumbens and dopaminergic agonists: A new approach

A method was developed to analyze electromyographic (EMG) signals in terms of power, viz., a measure for overall muscle activity, and number of seconds marked by distinct frequency ranges. With the help of this method, the effects of intraaccumbens administration of distilled water, the D1 receptor agonist SK&F 38393 (SKF; 5 micrograms), the D2 receptor agonist LY 171555 (LY; 10 micrograms), and their combination upon the EMG signals of the masseter and the digastric muscle were analysed in freely moving rats. Only the combined treatment affected the power: The noted increase was limited to the digastric muscle. The time/frequency analysis was limited to frequency ranges 3-4 Hz (class A), 4-5 and 5-6 Hz (class B), and 6-7, 7-8, ..., 12-13, and 13-14 Hz (class C). Apart from a small effect of SKF alone and of SKF in combination with LY on class B of the masseter muscle, neither SKF nor LY affected class A or B. SKF and LY increased and decreased, respectively, class C in both muscles. The data suggest that SKF and LY elicited both opposite and synergistic effects. The method is a new tool to analyze EMG signals in freely moving rats.

Cholinergic/dopaminergic interaction in the rat striatum assessed from drug-induced repetitive oral movements

The role of striatal dopaminergic/cholinergic interactions in the regulation of oral behaviour in rats was studied using methods which resolve distinct patterns of jaw movements, allowing a more accurate quantitative and qualitative analysis. Both dopamine and acetylcholine receptor agonists given either systemically or into the ventral striatum induced repetitive oral movements. However, the cholinergic movements differed from dopaminergic movements as to pattern of activity. Oral movements induced by apomorphine (0.2 mg/kg i.v.) were potentiated by carbachol (0.1 microgram/0.2 microliters) injected into the dorsal striatum, while inhibition was observed when carbachol was injected into the ventral striatum. Pilocarpine (4 mg/kg)-induced oral movements were reduced by injecting flupentixol (10 micrograms/0.2 microliters), but not a combination of SKF 38393 (3 micrograms)+quinpirole (10 micrograms/0.2 microliter), into either the dorsal or the ventral striatum. Oral movements induced by the injection of carbachol (1 microgram/0.2 microliter) into the ventral striatum were enhanced by previous injection of this combination of dopamine receptor agonists into the same site and were inhibited by flupentixol. These results suggest that cholinergic and dopaminergic oral movements are separate behaviors and that the striatal dopamine/acetylcholine interaction in their regulation is neither simply antagonistic or synergistic, nor reciprocal.

Co-administration of progabide inhibits haloperidol-induced oral dyskinesias in rats

Vacuous chewing movements in rats may be an animal analogue of the human motor disorder, tardive dyskinesia. The movements are phenomenologically and pharmacologically similar to tardive dyskinesia. The pathophysiology of these involuntary oral movements, and perhaps of tardive dyskinesia, are likely to include both dopamine receptor changes, and alterations in GABA (gamma-aminobutyric acid) system function. In an attempt to test the involvement of GABA system dysfunction in these movements, we treated rats chronically with water alone, haloperidol alone, the GABA agonist progabide alone, and haloperidol plus progabide. Sprague-Dawley rats received haloperidol (1.5 mg/kg per day) in their drinking water and progabide (100 mg/kg per day) in their food for 12 months. After 12 months of treatment, haloperidol had induced vacuous chewing movements when administered alone, but the prevalence of the movements was decreased by 40% with the coadministration of progabide. Moreover, the haloperidol-progabide-treated animals did not merely demonstrate movement suppression but actual inhibition of movement onset, as determined by an additional progabide-withdrawal experiment. These data would suggest that progabide and perhaps other GABAmimetic compounds can prevent the development of tardive dyskinesia in man.

Haloperidol-induced vacuous chewing in rats: suppression by α-methyl-tyrosine

Chronic treatment of rats with haloperidol (1 mg/kg twice daily for 4 weeks) induced repetitive vacuous chewing movements (VC), that persisted for over 72 h after haloperidol withdrawal. Haloperidol-induced VC were inhibited by the s.c. administration of the specific dopamine D1, receptor antagonist, SCH 23390 (0.025-0.100 mg/kg), in a dose-dependent manner, and were totally suppressed by an acute challenge with haloperidol (2 mg/kg i.p.) and by the dopamine synthesis inhibitor, alpha-methyl-tyrosine (AMT) (200 mg/kg i.p.). In AMT-treated rats, VC were reinstated by the administration of the selective D1 agonist, SKF 38393. The results support the hypothesis that chronic haloperidol-induced VC are mediated by dopamine acting selectively upon D1 receptors.

Changes in dopamine D1, D2 and D3 receptor mRNA levels in rat brain following antipsychotic treatment

The effects of administration of antipsychotic drugs (1-32 days, twice per day) on the rat brain mRNA levels of dopamine D1, D2 and D3 receptors has been assessed by a novel procedure utilising solution hybridisation with oligonucleotides. Saline and sulpiride (10 mg/kg/injection) had no effect on D1, D2 and D3 receptor mRNA levels. Haloperidol (1.5 mg/kg/injection) elicited increases in D1, D2 and D3 receptor mRNA levels of 100%, 100% and 300% respectively, after 32 days and loxapine (2 mg/kg/injection) elicited increases of 450%, 150% and 550%, respectively. These results indicate that the up-regulation of dopamine receptors may be associated with the occurrence of tardive dyskinesia but not the clinical mode of action of antipsychotics.

Functional interaction between dopamine D1 and D2 receptors in rat jaw movements

The functional interaction between dopamine D1 and D2 receptors in dopamine-mediated jaw movements was studied in ketamine-anaesthetized rats after C1 spinal transection. Jaw movements were recorded by means of a light-emitting diode attached to the mandible; the method permits a detailed qualitative and quantitative analysis of jaw movements. D1 stimulation with SKF38393 (10 mg/kg i.v.) produced frequent bursts of teeth chattering, which were abolished by pretreatment with SCH23390 (0.25 mg/kg i.v.). D2 stimulation by quinpirole (1-10 mg/kg i.v.) produced infrequent bursts of jaw movements, which were characterized by low frequency jaw opening and closure movements from the rest position of the jaw, and absence of tongue protrusions. An additional stimulation of D1 receptors by giving SKF38393 30 min later produced an almost continuous pattern of jaw openings but less closure movements from the rest position, and the openings were accompanied by frequent tongue protrusions. These results clearly demonstrate that the type of oral behaviour produced by stimulation of D1 and D2 receptors together is qualitatively different from that produced by stimulation of either D1 or D2 receptors alone.

Effects of pimozide on the hedonic properties of sucrose: analysis by the taste reactivity test

The ability of the neuroleptic agent, pimozide, to modify sucrose palatability was assessed using three 10-min taste reactivity test sessions. Pimozide was found to suppress the ingestive response of tongue protrusions, but enhance the mildly ingestive/neutral response of mouth movements elicited by an intraoral infusion of sucrose solution. Since the pattern of taste reactivity responding shifted from highly ingestive to mildly ingestive/neutral, our results suggest that pimozide pretreatment reduces the palatability of sucrose solution. The temporal pattern of the modification of these taste reactivity responses was predicted by the Anhedonia Hypothesis.

Supersensitized D1 receptors mediate enhanced oral activity after neonatal 6-OHDA

Enhanced oral responses have been observed in rats that are treated shortly after birth with 6-hydroxydopamine (6-OHDA). A series of studies was conducted to characterize this effect. A dose-response curve demonstrated that the dopamine D1 receptor agonist, SKF 38393, produced a maximal response in 6-OHDA-treated rats at a dose of 0.10 mg/kg (IP). With the D2 receptor antagonist, spiperone, a bell-shaped dose-response curve was seen, with a maximal effect in the 6-OHDA group occurring at 80 micrograms/kg. There were only slight increases in oral activity with different SKF 38393 or spiperone doses in the saline group, indicating that there was an overt supersensitization of D1 receptors in the 6-OHDA-treated rats. The D1 antagonist SCH 23390 (0.30 mg/kg, IP) attenuated the response to both SKF 38393 and spiperone. The oral response to the D2 agonist, quinpirole (0.10 mg/kg, IP) was not preferentially increased in the 6-OHDA group of rats. These findings indicate that the enhanced oral response in neonatal 6-OHDA-treated rats is mediated by supersensitive dopamine D1 receptors. The persistence of the enhanced oral response in 6-OHDA-treated rats at 8 months demonstrates that this sensitization of D1 receptors is a long-lived phenomenon.

Cholinergic modulation of oral activity in drug-naive and chronic haloperidol-treated rats

The cholinergic agonists pilocarpine, physostigmine, oxotremorine, and arecoline were administered IP at various doses to rats. Oral activity was assessed in these animals with a computerized video analysis system that determined the number and form of jaw openings and closings (computer scored movelets "CSMs"). The different cholinergic drugs produced distinctive changes in the number of CSMs at various amplitudes and in the frequency distribution of CSMs as determined by fast fourier analysis. Rats treated for 28 weeks with haloperidol showed a previously described, late onset oral dyskinesia characterized by increases in small amplitude CSMs, decreases in CSM slope, increased energy at the 1-3 Hz range and decreased energy at the 5-7 Hz range. Administration of pilocarpine (1.0 mg/kg) reversed all of these effects, while the anticholinergic drug, scopolamine (0.05 mg/kg), had no effect. These results indicate that different cholinomimetics can uniquely alter oral activity in rats and that symptoms of late onset, neuroleptic-induced oral dyskinesia are modified by a cholinergic agonist.

Chronic ascorbate potentiates the effects of chronic haloperidol on behavioral supersensitivity but not D2 dopamine receptor binding

Ample behavioral evidence suggests that ascorbate parallels the action of haloperidol, a widely used neuroleptic. To determine the extent to which this parallel extends to chronic treatment, 21 days of exposure to ascorbate (100 or 500 mg/kg) alone or combined with haloperidol (0.5 mg/kg) were assessed on stereotyped behavior and neostriatal D2 dopamine receptor binding in rats. Our results indicate that when challenged with the dopamine agonist, apomorphine (0.5 mg/kg), animals chronically treated with haloperidol or high-dose ascorbate alone display a supersensitive sniffing response relative to controls, while animals chronically treated with the combination of haloperidol and high-dose ascorbate display a further potentiation of sniffing relative to the haloperidol groups. In addition, [3H]spiperone saturation studies showed, as expected, an up-regulation of striatal D2 dopamine receptors in rats treated with haloperidol as reflected by a change in receptor density (Bmax) but not affinity (KD). Ascorbate treatment, however, had no effect on D2 receptor density or the distribution of [3H]apomorphine in whole brain. Even though chronic treatment with the haloperidol-high-dose-ascorbate combination produced an up-regulation of striatal D2 dopamine receptors, this treatment did not cause a further up-regulation relative to haloperidol alone nor did it have any effect on [3H]apomorphine distribution. Taken together, these findings indicate that although chronic ascorbate produces behavioral supersensitivity to apomorphine through central mechanisms, they appear to differ from those induced by chronic haloperidol.