Persistent behavioural effect in apomorphine in 6-hydroxydopamine-lesioned rats

Quantifying muscle alterations in a Parkinson's disease animal model using electromyographic biomarkers

Parkinson's disease (PD) is a neurodegenerative disease currently diagnosed based on characteristic motor dysfunctions. The most common Parkinson's disease animal model induces massive nigrostriatal degeneration by intracerebral infusion of 6-hydroxydopamine (6-OHDA). Motor deficits in rat models of Parkinson's disease were previously addressed in other works. However, an accurate quantification of muscle function in freely moving PD-lesioned rats over time has not been described until now. In this work, we address the muscular activity characterization of a 6-OHDA-lesion model of PD along 6 weeks post-lesion based on spectral and morphological analysis of the signals. Using chronic implanted EMG electrodes in a hindlimb muscle of freely moving rats, we have evaluated the effect of the PD neurotoxic model in the muscular activity during locomotion. EMG signals obtained from animals with different time post-injury were analyzed. Power spectral densities were characterized by the mean and median frequency, and the EMG burst stationarity was previously verified for all animals. Our results show that as the time post-lesion increases both frequency parameters decrease. Probability distribution function analysis was also performed. The results suggest that contractile dynamics of the biceps femoris muscle change with time post-lesion. We have also demonstrated here the usefulness of frequency parameters as biomarkers for monitoring the muscular function changes that could be used for early detection of motor dysfunction.

Aplindore (DAB-452), a high affinity selective dopamine D2 receptor partial agonist

The pharmacology of aplindore (DAB-452) was characterized in CHO-K1 cells stably transfected with the human dopamine D(2) receptor short isoform (CHO-D(2s)) and in a behavioral model for post-synaptic agonism in rats. In [(3)H]-spiperone competition binding studies, aplindore showed high affinity for dopamine D(2) and D(3) receptors and low affinity for the dopamine D(4), serotonin (5-HT)(1A), 5-HT(2) receptors and the alpha1-adrenoceptor. The high potency partial agonist activity of aplindore was demonstrated in [(35)S]guanosine 5'-O-(3-thiotriphosphate) ([(35)S]GTPgammaS) binding, extracellular signal-regulated kinase (ERK)-phosphorylation and intracellular calcium flux assay using fluorometric plate reader ([Ca(2+)](i)-FLIPR) format. The [Ca(2+)](i)-FLIPR assay was conducted with CHO-D(2S) receptor cells also stably expressing chimeric G(alphaq/o)-proteins. In all assay modalities, the potencies and intrinsic activities of aplindore were lower than dopamine and higher than aripiprazole. In contrast to the [(35)S]GTPgammaS binding and ERK-phosphorylation assays, the [Ca(2+)](i)-FLIPR assay was able to detect the low partial agonist activity of SDZ 208-912. In unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, aplindore induced contralateral turning, which was blocked by the dopamine D(2) receptor antagonist raclopride. The dopamine D(2) receptor selective partial agonist profile of aplindore suggests that it should be effective for the treatment of dopaminergic-based disorders, such as schizophrenia and Parkinson's disease.

Rats with unilateral median forebrain bundle, but not striatal or nigral, lesions by the neurotoxins MPP+ or rotenone display differential sensitivity to amphetamine and apomorphine

Rotenone and 1-methyl-4-phenyl pyridinium (MPP+) are two mitochondrial neurotoxins known to produce Parkinson's disease (PD) in experimental animals. In the present study, we compared drug-induced rotational asymmetry in rats lesioned using these neurotoxins at three distinct basal ganglia sites, the striatum, substantia nigra pars compacta (SNpc) and median forebrain bundle (MFB). The levels of dopamine (DA) in the ipsilateral striata of these hemiparkinsonian animals were assayed employing an HPLC-electrochemical procedure 2 days after the final rotational study. Rats infused with rotenone or MPP+ into the SNpc, but not into the striatum or MFB, exhibited contralateral rotations immediately after recovery from anesthesia. Irrespective of the lesion site or the toxin used, all the animals exhibited ipsilateral rotations when challenged with D-amphetamine. Apomorphine administration caused contralateral circling behavior in MFB-lesioned animals, but ipsilateral rotations in rats that received rotenone or MPP+ in the striatum or SNpc. Stereotaxic administration of rotenone into the MFB, SNpc or striatum caused a significant loss of DA in the ipsilateral striatum to varying degrees (96%, 62% and 30%, respectively, as compared to the contralateral side). However, unilateral MPP+ administration into the MFB, SNpc or striatum caused respectively about 98%, 74% and 59% loss of striatal DA. Behavioural observations and the neurochemical results indicate that, among the three anatomically distinct loci-lesioned, MFB-lesioned animals mimicked behavioral aberrations similar to nigral lesions caused by 6-hydroxydopamine, a classical parkinsonian neurotoxin. Moreover, the results point out that while both d-amphetamine and apomorphine-induced rotations could be considered as valuable behavioral indices to test novel drugs against PD, yet apomorphine-induced contralateral bias proves to be a more reliable indicator of specific destruction in the nigrostriatal pathway and development of post-synaptic DA receptor supersensitivity.

Caffeine and the dopaminergic system

Caffeine is the most widely consumed psychostimulant substance, being self-administered throughout a wide range of conditions and present in numerous dietary products. Due to its widespread use and low abuse potential, caffeine is considered an atypical drug of abuse. The main mechanism of action of caffeine occurs via the blockade of adenosine A1 and A2A receptors. Adenosine is a modulator of CNS neurotransmission and its modulation of dopamine transmission through A2A receptors has been implicated in the effects of caffeine. This review provides an updated summary of the results reported in the literature concerning the behavioural pharmacology of caffeine and the neurochemical mechanisms underlying the psychostimulant effects elicited by caffeine. The review focuses on the effects of caffeine mediated by adenosine A2A receptors and on the influence that pre-exposure to caffeine may exert on the effects of classical drugs of abuse.

Scopolamine prevents tolerance to the effects of caffeine on rotational behavior in 6-hydroxydopamine-denervated rats

Continuous administration of caffeine has been shown to induce tolerance to its psychostimulant effects. In this study, using unilateral 6-hydroxydopamine nigrostriatal denervated rats, we tested the hypothesis that the muscarinic receptor antagonist, scopolamine, would prevent the tolerance to caffeine-induced contralateral rotational behavior. For that purpose we administered either caffeine (40 mg/kg) plus saline or scopolamine (5, 10 and 20 mg/kg) plus saline, as well as caffeine in combination with the various doses of scopolamine for 7 consecutive days, and measured ipsilateral and contralateral rotational behavior. The results showed that acute injections of scopolamine plus saline produced similar levels of both ipsilateral and contralateral turning, while caffeine produced more contralateral than ipsilateral turning. Tolerance to caffeine-induced contralateral turning was observed as of the second administration, while scopolamine plus saline injections did not produce significant changes in rotational behavior with repeated treatment. Scopolamine co-administered with caffeine significantly attenuated the increased contralateral turning produced by acute injections of caffeine plus saline, but significantly prevented the tolerance effects with repeated administration. These findings strongly suggest that muscarinic cholinergic processes may be involved in tolerance to caffeine-induced contralateral turning. The results are interpreted in terms of the possible interactions between dopamine, adenosine and acetylcholine neurotransmitter systems within the basal ganglia circuitry involved in motor behavior.

Motor stimulant effects of caffeine in 6-hydroxydopamine-lesioned rats are dependent on previous stimulation of dopamine receptors: a different role of D1 and D2 receptors

Caffeine has been reported to induce contralateral rotational behaviour in rats bearing a unilateral 6-hydroxydopamine lesion of the dopaminergic nigrostriatal pathway. In order to define the role of dopamine receptors in the mediation of this behaviour, we have evaluated the influence of previous exposure to a dopamine receptor agonist and the importance of the time elapsed from the 6-hydroxydopamine lesion on the rotational behaviour induced by caffeine. Separate groups of rats lesioned with 6-hydroxydopamine 2 weeks previously were exposed to four administrations of the D1/D2 receptor agonist apomorphine (0.3 mg/kg s.c.) (primed) or vehicle (drug-naive). Three days later, all rats received caffeine (30 mg/kg s.c.). Drug-naive 6-hydroxydopamine-lesioned rats did not rotate in response to caffeine, while rats primed with apomorphine rotate contralaterally in response to caffeine. When apomorphine priming was paired to the same environment (hemispherical bowls) where rats received caffeine, rotational behaviour was significantly higher than that obtained in rats primed in an unpaired environment (cylinders). Repeated priming with the D2/D3 receptor agonist quinpirole (0.2 mg/kg s.c.) induced a totally context-dependent contralateral rotation in response to caffeine, while caffeine contralateral rotation was not dependent from the context after repeated priming with the D1 agonist SKF 38393 [1-phenyl-2,3,4,5-tetrahydro-(1 H)-3-benzazepine-7,8-diol hydrochloride, 3 mg/kg s.c.]. Caffeine-mediated contralateral rotation was also evaluated in rats lesioned with 6-hydroxydopamine 12 weeks previously and exposed to four administrations of apomorphine or vehicle. As for rats repeatedly exposed to vehicle or apomorphine 2 weeks after 6-hydroxydopamine lesioning, caffeine failed to induce contralateral rotation in drug-naive rats, while it did induce a partially context-dependent contralateral rotation in apomorphine-primed rats. Different from rats receiving apomorphine priming 2 weeks after 6-hydroxydopamine lesioning, in 12 week-lesioned rats, caffeine also induced contralateral rotation after one priming with apomorphine (0.3 mg/kg s.c.), a condition which fails to induce context-dependent rotation. Administration of selective antagonists of A1 (8-cyclopentyl-1,3-dipropylxanthine), (DPCPX) or A2A (5-amino-2-(2-furyl)-7-(3-phenylpropyl)-pyrazolo[4,3-e]-1 ,2,4-triazolo[5c]pirimidine), (SCH 58261) adenosine receptors failed to induce contralateral rotation either alone or in combination in 12 week-6-hydroxydopamine-lesioned rats repeatedly primed with apomorphine. All together, the results indicate that: (i) caffeine does not induce any contralateral rotation in drug-naive 6-hydroxydopamine-lesioned rats; (ii) priming with a dopamine agonist enables caffeine to induce contralateral rotation, this rotation is, however, context independent only after priming with a selective D1 agonist; (iii) contralateral rotation in response to caffeine is dependent on the time from the 6-hydroxydopamine lesion; (iv) blockade of A1 and A2A adenosine receptors with selective antagonists does not induce contralateral rotational behaviour in 6-hydroxydopamine-lesioned rats.

Supersensitization of neurochemical responses by L-DOPA and dopamine receptor agonists in the striatum of experimental Parkinson's disease model rats

We studied the mechanisms of dopamine receptor agonist- and L-DOPA-mediated supersensitization in experimental Parkinson's disease model rats, by measuring in vivo acetylcholine (ACh) release, GTPase activities, and mRNA expression in the striatum of 6-hydroxydopamine-treated rats. D1 agonist (SKF38393) and D2/D3 agonists (bromocriptine and quinpirole) showed more potent stimulation or inhibitions on ACh release in the model rat than in the control. However, quinpirole-evoked stimulation of GTPase activity was enhance in the model rats, compared to the control, while there was no significant enhancement of the bromocriptine-evoked stimulation. On the other hand, L-DOPA at 0.3-10 pM showed a biphasic action including significant inhibition on the GTPase activity in the lesioned striatal membranes, but not in the control. In the RNAase protection assay, neither D1, D2, Gi1 alpha, GoA alpha nor Gs alpha mRNA expression in the model was significantly different from the control. These findings suggest that there is supersensitization of D1 and D2/D3 receptors in the experimental Parkinson's disease model, while the upregulation of their receptors or GTP-binding proteins (G-proteins) to be coupled to their receptors is unlikely involved in major parts of such mechanisms. In addition, the present report provides the first evidence that L-DOPA mediates neurochemical responses in the plasma membranes, possibly through its receptor.

Behavioral asymmetries and recovery in rats with different degrees of unilateral striatal dopamine depletion

A detailed behavioral analysis during the first postoperative week was performed in rats which had sustained various degrees of unilateral neostriatal dopamine (DA) lesions by administration of the neurotoxin 6-hydroxydopamine into the substantia nigra. These animals were assigned to different groups according to their residual DA levels in the damaged neostriatum (as percentage of the intact side). On the first day after toxin injection into the substantia nigra, turning asymmetries (tight turns) toward the side of the lesion were observed in animals with a mean residual DA level of 32% or less. Out of these, the strongest asymmetries were observed in animals with a mean residual DA of 3%. After one week, the asymmetry in tight turns had totally recovered except in those groups with mean residual DA levels of 17% or less. Partial recovery was found in animals with mean residual DA of 9 and 17%, whereas no indication for recovery was found in animals with the most severe lesions (mean residual DA 3%). Measurement of thigmotactic scanning also revealed an asymmetry for the side of the lesion on the first post-operative day. This asymmetry was observed over a wider range of DA lesion than that observed in turning, namely up to a mean residual DA level of 78%. Furthermore, recovery to symmetry was observed in all lesion-groups except in those with more severe lesions (mean residual DA 17% or less). In contrast to turning, the strongest asymmetries were not displayed by the animals with the most severe lesions. Furthermore, locomotor activity was affected by the lesion, since on the first postoperative day locomotion was reduced in animals with mean residual DA of 39% or less. On day 7, this lesion-dependent deficit had recovered to control levels. Finally, the analysis of net turns allowed the prediction of lesion size in animals with residual DA levels of less than 15%. These results are discussed with respect to mechanisms of recovery, the role of lesion size, and the value of different behavioral measures to predict the degree of DAergic lesion.

Conditioned apomorphine-induced turning in 6-OHDA-lesioned rats

Apomorphine-induced turning has been used to evaluate the extent of unilateral nigrostriatal denervation after 6-hydroxydopamine (6-OHDA) lesions and subsequent functional striatal reinnervation by catecholaminergic grafts. It has been noted that the pregraft rotational pattern is usually double peaked and that fetal ventral mesencephalic grafts or dopaminergic drugs will alter the second peak but leave the first relatively unchanged. We hypothesized that the first peak may be the result of factors extrinsic to the nigrostriatal dopamine system, specifically a conditioned turning response, and would, therefore, be unperturbed by the above treatments which increase dopaminergic (DA) inputs. This was investigated by injecting 6-OHDA, unilaterally, into the nigrostriatal pathway of several groups of young Fisher 344 rats. One experimental group was repeatedly tested with 0.05 mg/kg apomorphine and the rotations quantified. A second group received similar injections of apomorphine but were prevented from rotating. Vehicle control animals were also studied for both of the above experimental groups. Subsequent to the above treatment, all animals were tested unrestrained repeatedly on apomorphine. Our results support the conditioned response hypothesis in that the first peak is not present with the initial unrestrained apomorphine behavioral trial but is present upon the second and subsequent unrestrained trials. Moreover, the restrained but apomorphine-injected rats, as well as the control animals, manifest no first peak upon their first freely moving apomorphine test; the second and subsequent unrestrained apomorphine trials, in these groups, do manifest a first peak.(ABSTRACT TRUNCATED AT 250 WORDS)

Supersensitivity of quinpirole-evoked GTPase activation without changes in gene expression of D2 and Gi protein in the striatum of hemi-dopaminergic lesioned rats

Quinpirole with D2 agonist activity showed a two-fold increase in the GTPase activity in the 6-hydroxydopamine-lesioned side of the striatum, compared with that in the control side. However, neither qualitative nor quantitative changes were observed in the gene expression of two alternative splicing products derived from dopamine D2 receptor gene or of Gil alpha, in the RNAase protection assay. These results suggest that D2 receptor-mediated supersensitization does not come from the D2 receptor upregulation.

Conditioned increases in mesolimbic dopamine overflow by stimuli associated with cocaine

Stimuli associated with cocaine come to acquire incentive-motivational as well as secondary reinforcing properties which can energize and maintain behavior in laboratory animals as well as precipitate craving in addicts. Environmental stimuli paired with a large dose of cocaine for one training session elicited significant increases in locomotor activity and in extracellular dopamine in the nucleus accumbens of rats during a second test session with a low dose of cocaine. The increases in extracellular dopamine are not likely a secondary consequence of this increase in locomotor output of rats conditioned to cocaine, since doses of MK-801 which produced similar increases in locomotor behavior had no effect on mesolimbic dopamine. These findings provide a neurochemical mechanism for understanding the incentive motivational properties of stimuli associated with cocaine and may help to explain recidivism of cocaine addicts when they return to an environment in which the drug was used.

Long-term sensitization of apomorphine-induced rotation behavior in rats with dopamine deafferentation or excitotoxin lesions of the striatum

Following unilateral 6-hydroxydopamine (6-OHDA)-induced deafferentation or unilateral kainic acid (KA) lesions of the striatum, rats displayed rotation behavior in response to apomorphine (0.25 or 1 mg/kg, SC, for 6-OHDA- and KA-lesioned rats, respectively). Three days following the initial apomorphine injection, rats were challenged under identical conditions with the same dose of apomorphine received previously. A third trial with apomorphine was again repeated after 3 days. Two more sets of behavioral data, each consisting of three trials, were collected under the same conditions as the first. Each set was separated by a period of 5-6 weeks. Following the second trial of the first set, rats showed a significant increase in the maximal number of rotations, demonstrating behavioral sensitization. Following the two 5-week intervals, rats were still sensitized to apomorphine, showing behavioral responses similar to the sensitized. Following the two 5-week intervals, rats were still sensitized to apomorphine, showing behavioral responses similar to the sensitized responses observed after the initial trials. Thus, the postsynaptically mediated sensitization of apomorphine-induced rotation behavior in 6-OHDA- or KA-lesioned rats is a long-lasting phenomenon. That lesions producing postsynaptic dopaminergic hypersensitivity and hyposensitivity can both show long-lasting sensitization may indicate multiple mechanisms underlying the sensitization.

On-off effects of dopamine receptor agonists in the hemi-parkinsonian rat

Rats lesioned in one substantia nigra were treated daily with bromocriptine, SKF-77434 or cocaine and their rotational (circling) behavior was quantified. Within animal fluctuations in response to the direct-acting agonists were remarkable. In consecutive daily sessions individual animals completed from zero to hundreds or thousands of rotations. Every rat was totally unresponsive to drug treatment at least once in 14 days, with a response failure rate > or = 30% for both 1.0 mg/kg bromocriptine and 0.25 mg/kg SKF-77434. When the bromocriptine dose was increased from 1 to 2.5 to 5 mg/kg the response stabilized in some animals, and overall failure rate declined. In the SKF-77434-treated rats the interval between administrations was manipulated. The response failure rate did not decrease when drug was given every third day, but decreased significantly upon weekly administration. Rats treated repeatedly with the indirect agonist cocaine exhibited no response failures indicating that failures in response to the direct agonists were unlikely to be due to procedural artifact. The results suggest the possibility that large changes in responsivity to direct dopamine receptor agonists are a characteristic of their interaction with the denervated striatum. This approach may prove useful for study of the on-off effects that typically develop in parkinsonism and for identification of drugs with lesser propensity to induce such effects.

A video image analyzing system for open-field behavior in the rat focusing on behavioral asymmetries

A video image analyzing system is presented which measures turning behavior, thigmotactic scanning and locomotion in rats. The system works by analyzing digitized video images obtained by a black/white video camera. Turning behavior is expressed in different diameter classes and as partial or full turns. Thigmotactic scanning is expressed as distance or time locomoted with the left or right side of the body along one of the walls of the testing environment. Locomotion is measured as distance travelled and is expressed in meters. Examples for the application of these behavioral measures are given which include: the measurement of spontaneous or drug-dependent behavioral asymmetries after brain lesion (the unilateral 6-hydroxydopamine model), a unilateral peripheral manipulation (hemivibrissotomy), and the measurement of open-field behavior (spontaneous or drug-induced) in intact animals. Among others, these examples show that the analysis of thigmotactic scanning may provide an alternative behavioral measure, which may be especially useful in the study of functional asymmetries.

Sensitization, response fluctuation and long-term effect of SKF-82958 and bromocriptine in the hemi-parkinsonian rat

Rats with a unilateral 6-hydroxydopamine lesion of substantia nigra were treated with the dopamine agonists SKF-82958 (D1 receptor selective) or bromocriptine (D2 receptor-selective) and their circling response recorded. Both of the compounds induced an acute episode of rotation directed away from the lesioned side. Consecutive daily treatments with either compound usually resulted in a significantly increased average response (sensitization) over a 3- to 6-day treatment period. But nearly all animals treated with low doses of either SKF-82958 or bromocriptine exhibited one or more days when they were totally unresponsive to drug treatment. Response fluctuations thus were not exclusively associated with D1 or D2 receptor agonist treatment. When subsequently tested, undrugged, in the drug-associated environment, 2, 4 and 10 weeks after their last drug treatment, rats that had previously been treated with SKF-82958 exhibited rapid contralateral rotation while rats that had previously been treated with bromocriptine showed no such undrugged rotation. This result is consistent with previous findings that the D1 receptor agonist, SKF-38393, but not the D2 receptor agonist, quinpirole, had long-term behavioral effect in nigral rats, and suggests that persistent motor consequences of limited treatment with dopamine receptor agonists are D1 receptor-related.

Rotational behavior as a classically conditioned response to pentobarbital administration

Pentobarbital stimulus control of rotational behavior was investigated in rats with unilateral 6-hydroxydopamine lesions of substantia nigra. In conditioning trials, lesioned rats were injected simultaneously with 10 mg/kg pentobarbital and 0.05 mg/kg apomorphine and their rotational (circling) behavior observed and counted. Subsequent to three consecutive daily conditioning sessions, animals were re-introduced to the rotation environment and tested with saline or pentobarbital. Pentobarbital, but not saline, administration was followed by a brief epoch of rapid contralateral rotation. After additional conditioning trials in which pentobarbital and apomorphine administration were paired, test sessions with 1 g/kg ethanol and with 10 mg/kg chlordiazepoxide were conducted. Most animals did not rotate in response to ethanol administration and most did rotate in response to chlordiazepoxide. Finally, in order to determine the persistence of the conditioned effect, animals were tested with pentobarbital 15 weeks after their last conditioning session and were found to rotate actively in response.

Pavlovian conditioning of L-dopa induced movement

Using the unilateral 6-hydroxydopamine (6-OHDA) substantia nigra pars compacta (SNC) lesion rat model and a Pavlovian conditioning protocol, the present investigation determined that the contralateral rotation response induced by the antiparkinsonian dopaminergic drug L-dopa can become conditioned to exteroceptive test environment stimuli. Two non-drug conditioning tests indicated that contralateral rotation was elicited by the test environment without the presence of L-dopa. This conditioned response had a rotation diameter profile that was qualitatively the same as the L-dopa induced contralateral rotation response. Additionally, drug tests with the combined dopaminergic receptor antagonists, SCH 23390 (0.1 mg/kg) and haloperidol (0.5 mg/kg), at doses sufficient to block spontaneous behavior and L-dopa (20 mg/kg)-induced rotation, revealed that the conditioned contralateral rotation response, unlike L-dopa-induced contralateral rotation, is not affected by D1/D2 receptor blockade. Thus, the conditioned stimuli of the test environment can elicit the contralateral rotation response even in animals rendered akinetic by D1/D2 antagonists. This activation of a conditioned dopaminergic drug response by the situational stimuli, independent of dopaminergic mechanisms, may, therefore, contribute to the untoward overstimulation clinical effects of L-dopa through summation of conditioned and drug-induced effects. Furthermore, the use of conditioning procedures to elicit movement in akinetic animals may provide a new research methodology to investigate the phenomenon of paradoxical kinesia.

Alpha-methyltyrosine blocks the expression of rotation classically conditioned with apomorphine

Rats with unilateral 6-hydroxydopamine lesions of substantia nigra rotate (circle) when placed, undrugged, in the environment in which they have previously been treated with apomorphine. This conditioned rotation, like the unconditioned rotation which acutely follows the administration of apomorphine, is directed away from the side with the lesion, i.e., the rotation is contralateral. Here, rats that had been administered apomorphine weeks earlier were tested, in a crossover design, for the expression of conditioned rotation following treatment with saline and with alpha-methyltyrosine. When administered four hours prior to testing, 100 mg/kg alpha-methyltyrosine significantly antagonized the expression of classically conditioned rotation. In a second group of animals, alpha-methyltyrosine had no effect on the unconditioned rotation induced by 0.05 mg/kg apomorphine.

Conditioning of apomorphine effects: simultaneous analysis of the alterations in cortical electroencephalogram and behaviour

The possible conditioning of pharmacological effects of apomorphine on the electroencephalogram was studied using telemetric recordings in rats. Previous studies have shown that apomorphine-induced stereotyped behaviour can be conditioned: after repeated pairings of defined stimuli with the drug effect, the presentation of the external stimuli alone elicited stereotype sniffing, licking, and gnawing. Since apomorphine, an agonist at dopamine receptors, also produces a characteristic EEG pattern with an increase of power in the alpha-1 band, the possibility that this effect could also be conditioned was studied. In fact, conditioning with a dose of 0.5 mg/kg apomorphine (s.c.) led to a significant increase in the number of short-lasting episodes with enhancement of the power in the alpha-1 range in the presence of the conditioned stimuli, according to a comparison of the results obtained in the conditioned group and those of the controls ("pseudoconditioned"). Moreover, behavioural studies were performed simultaneously in order to find possible correlations between conditioned effects on EEG and conditioned alterations in behaviour. In general, a fair correlation between the increase of power in the alpha-1 band and stereotyped behaviour was found. This was also the case during extinction, when the conditioned stimuli were repeatedly uncoupled from apomorphine administration: both behavioural parameters and EEG alterations showed similar time-courses and had almost disappeared during the fourth extinction session.

Pavlovian conditioning between co-administered drugs: elicitation of an apomorphine-induced antiparkinsonian response by scopolamine

Sprague-Dawley rats with unilateral 6-OHDA substantia nigra lesions were given combined scopolamine (0.5 mg/kg IP) and apomorphine (0.05 mg/kg SC) treatments. In this animal model, scopolamine, when administered separately, induces ipsilateral rotation and apomorphine, contralateral rotation. When these drugs are co-administered at 0.5 mg/kg and 0.05 mg/kg dose levels, respectively, animals rotate in the contralateral direction, creating the opportunity for the stimulus effect of scopolamine to become associated with the response effect of apomorphine. In tests with scopolamine (0.5 mg/kg), animals that previously had scopolamine and apomorphine co-administered rotated contralaterally in the test chamber, thereby behaving as if they had received apomorphine. Thus, scopolamine exhibited a functionally acquired conditioned stimulus (CS) property by eliciting the apomorphine response of contralateral rotation as a conditioned response. This acquired CS property was extinguished with separate scopolamine trials and reacquired following one scopolamine-apomorphine co-administration trial.

Sensitization of rotation behavior in rats with unilateral 6-hydroxydopamine or kainic acid-induced striatal lesions

Following unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra or unilateral kainic acid (KA) lesions of the striatum rats displayed rotation behavior in response to apomorphine (0.25 mg/kg SC or 1 mg/kg SC for the 6-OHDA- and KA-lesioned rats respectively). Three to five days following the initial apomorphine challenge rats were challenged under identical conditions with the same dose of apomorphine received previously. Both 6-OHDA- and KA-lesioned rats demonstrated a significant increase in the total number of rotations. Following a subsequent challenge with apomorphine, rats showed further increases in the total number of rotations. With the second and the subsequent apomorphine challenges there were significant increases in the maximal number of rotations, a significant decrease in the time of onset of rotation behavior and in some cases an increase in the duration of the rotation behavior. These increases in rotation behavior following repeated challenges with apomorphine indicate a supersensitivity to dopamine receptor agonists distinct from that elicited by lesions and chronic antagonist treatments. Furthermore, the utility of the rotation behavior model for testing the efficacy of dopaminergic agonists might be compromised if repeated challenges in individual animals are employed.

Classically conditioned ethanol stimulus control of a motor behavior

Rats with extensive unilateral 6-hydroxydopamine lesions of substantia nigra rotate (turn in circles) in response to administration of dopamine agonists. Here apomorphine administration, but not single or repeated administrations of ethanol, resulted in rotation in such lesioned animals. When apomorphine and ethanol were administered simultaneously to lesioned animals, rapid contralateral rotation resulted. After five of these conditioning trials in which ethanol and apomorphine were paired, ethanol alone elicited apomorphine-like rotation. Saline administration did not result in rotation in the conditioned animals, indicating that ethanol rather than the injection procedure exerted stimulus control over the behavior. Control animals which were treated five times with apomorphine and ethanol in an unpaired manner showed no conditioned rotation to ethanol.

Direct dopamine agonist-like activity conditioned to cocaine

Rats were lesioned unilaterally by infusions of 6-hydroxydopamine aimed at substantia nigra. In subsequent behavioral testing, apomorphine treatment resulted in rotation (circling) directed contralaterally with respect to the lesion and cocaine treatment induced ipsilaterally directed rotation. When 0.05 mg/kg apomorphine and 10 mg/kg cocaine were administered simultaneously, rotation appropriate for apomorphine resulted. After a number of paired administrations, treatment with cocaine alone resulted in apomorphine-like rotation.

Antiparkinsonian effects of caffeine depend upon pavlovian drug conditioning processes

Caffeine induces the antiparkinsonian response of contralateral rotation in rats with unilateral 6-hydroxydopamine (6-OHDA) lesions following exposure to the direct-acting dopamine agonist apomorphine. This effect of caffeine has been attributed to a 'priming' effect of apomorphine which enables the supersensitive dopamine receptors to respond to caffeine and other methylxanthines. These studies, however, did not differentiate between conditioning and pharmacological variables. To resolve this issue, a Pavlovian conditioning procedure was employed in which separate groups of 6-OHDA rats were given 0.05 mg/kg (s.c.) apomorphine treatments paired or unpaired with a test environment. The animals in the paired or conditioning treatment groups subsequently exhibited contralateral rotation when tested with caffeine (10 mg/kg, i.p.), whereas animals in the unpaired or 'priming' treatment groups displayed only enhanced ipsilateral rotation. The activation of the conditioned contralateral rotation by caffeine in the paired group occurred even after the response was suppressed by extinction. This preferential activation following extinction of a previously conditioned drug response by caffeine implicates caffeine mechanisms (e.g. adenosine antagonism) in drug conditioning processes.

Dopamine receptors mediate drug-induced but not Pavlovian conditioned contralateral rotation in the unilateral 6-OHDA animal model

Following Pavlovian conditioning treatment sessions with apomorphine, animals receiving the paired treatment showed substantial contralateral rotation when placed without drug into the test environment previously paired to the apomorphine (0.5 mg/kg) injection while animals in the unpaired control treatment showed only ipsilateral rotation. Subsequent tests with the D1 antagonist (SCH 23390), or the D2 antagonist (haloperidol) partially suppressed and the combined D1-D2 antagonists completely suppressed the apomorphine-induced response of contralateral rotation. The identical contralateral rotation response occurring as a Pavlovian conditioned response in the paired apomorphine treatment group was not attenuated by dopamine receptor blockade. In both paired and unpaired groups, the spontaneous ipsilateral rotation response was completely blocked. Thus, non-dopaminergic mechanisms mediate conditioned rotation whereas the drug-induced as well as the spontaneous rotation responses require stimulation of striatal dopamine receptors.

Reinstatement by caffeine of an extinguished conditioned dopaminergic drug response

An experimental study of extinction of conditioned drug-induced effects was carried out to determine: 1) duration of the extinction effect; and 2) stability of extinction as determined by a challenge with a stimulant drug. Twelve animals with unilateral 6-hydroxydopamine (6-OHDA) substantia nigra lesions were assigned to paired and unpaired treatment groups (n = 6) in a Pavlovian conditioning paradigm. The paired animals received apomorphine (0.05 mg/kg SC) immediately prior to placement into a test chamber and the unpaired animals received the apomorphine 30 min following test chamber placement. The two groups were matched for apomorphine-induced contralateral rotation prior to the conditioning treatment. Following Pavlovian conditioning, the paired group, but not the unpaired group, exhibited contralateral rotation in a nondrug test trial. This conditioned response underwent extinction after one nondrug extinction trial and the extinction effect persisted for 2 months. When tested with caffeine (10 mg/kg), the paired animals again exhibited substantial contralateral rotation. In contrast, the unpaired animals showed only an increase in ipsilateral rotation in response to the caffeine treatment. The drastically different response to caffeine in the paired and unpaired animals was not due to prior apomorphine exposure per se or due to 6-OHDA lesion-induced differences in striatal dopamine depletion. Rather, the effect of caffeine on rotation behavior was determined by the Pavlovian drug conditioning procedures carried out several months earlier prior to caffeine testing.

Cocaine and local anesthetics: stimulant activity in rats with nigral lesions

Cocaine and several other local anesthetics were tested for their ability to induce rotational behavior in rats with unilateral 6-hydroxydopamine lesions of substantia nigra. Acute administration of bupivacaine, chloroprocaine, etidocaine, lidocaine, mepivacaine, procaine or tetracaine failed to induce active rotation in this sensitive assay of dopamine agonist activity. On the other hand, cocaine or dimethocaine treatment induced active rotation directed ipsilaterally to the lesioned side, indicating indirect dopamine agonist activity. Repeated administration of cocaine or dimethocaine at 1-week intervals resulted in increased rotational response (i.e., sensitization) while there was no suggestion of sensitization or induction of rotational behavior after weekly repeated administration of procaine or tetracaine. Daily administration of mepivacaine, procaine or tetracaine for 5 days also failed to induce rotation. Dimethocaine thus was found similar to cocaine and different from the other local anesthetics tested both in terms of frank stimulant activity and development of sensitization upon repeated administration.

Kupalov's concept of shortened conditional reflexes: psychophysiological and psychopharmacological implications

In the regulation of cortical excitability tonus, Kupalov described a particular acquired, learned mechanism, the shortened conditional Reflex (SCR). SCR is an essential mechanism by which an adequate cortical tonus is established by environmental cues as an anticipatory set-up preparing the individual qualitatively and quantitatively for expected forthcoming events. An original physiological example is given, the conditional "transfer" of motor behavior at cortical electrical stimulation. Three psychopharmacological implications are presented, namely environmental-dependent, stress-related events, environmental-dependent drug conditioning, and drug-tolerance experiments. Future eventual psychophysiological and psychopharmacological lines of research related to the shortened conditional reflex concept are discussed.

Persistent augmented dopamine release after acute cocaine requires dopamine receptor activation

Pretreatment of rats with a single injection of cocaine produces a persistent augmentation of amphetamine-induced [3H]dopamine [( 3H]DA) release measured using the in vitro striatal slice preparation. Cocaine has several actions in the nigrostriatal DA system: it blocks DA uptake and thereby indirectly stimulates DA receptors and it also acts as a local anesthetic. We investigated which of these actions is responsible for the augmented amphetamine-stimulated [3H]DA release by determining whether pretreatment with drugs sharing one or more of these actions also augmented release. Release was increased in striatal slices one week after a single injection of either mazindol, a DA uptake blocker and indirect DA receptor agonist, or apomorphine, a direct-acting receptor agonist, whereas the local anesthetic lidocaine had no effect. The prerequisite of DA receptor stimulation was confirmed by pretreatment prior to the cocaine injection with either a nonselective, a D-1 selective or a D-2 selective DA receptor antagonist. Each of these blocked the long-lasting augmentation of release. From these experiments, we conclude that cocaine indirectly activates both D-1 and D-2 DA receptors to produce the persistent augmentation of striatal amphetamine-stimulated [3H]DA release.

Conditioning of rotational behavior after the administration of a single dose of apomorphine in rats with unilateral denervation of the dopaminergic nigrostriatal pathway: relevance to drug addiction

Our aim is to study the relationship of drug activation of the dopamine neurotransmission system and the conditioning of environmental stimuli present at the time of drug administration. We injected a single dose of apomorphine (0.05 mg/kg SC) in rats with the nigrostriatal dopamine pathways unilaterally denervated with 6-hydroxydopamine, which generates rotational behavior contralateral to the lesioned hemisphere. We observed rotational behavior without apomorphine administration when animals were reexposed at different time intervals to the same environment in which they performed turning behavior. The present findings show that this rotational behavior can be conditioned to environmental stimuli in a strong and long-lasting way. In light of the relationship between opioids and the dopaminergic system, similar conditioning could take place in the learning processes implicated in drug addiction.

LSD: permanent behavioral effect in rats

Rats with a unilateral nigrostriatal lesion produced by infusion of 6-hydroxydopamine (6-OHDA) exhibited acute contralateral (apomorphine-like) rotation in response to systemic LSD administration. When re-introduced to the environment in which LSD had been administered weeks earlier, the animals exhibited a 2-3 min burst of contralateral rotation. This behavior developed over a period of weeks and could be produced by a single LSD treatment. This latent, undrugged rotation is similar to that which we previously reported to result from one or more apomorphine treatments.

Conditioning of the early behavioral response to apomorphine in the rotational model of Parkinson's disease

The rotational model of Parkinson's disease has been widely used to investigate the action of dopamine agonists. This animal model involves the production of a unilateral nigrostriatal lesion in rats. The lesion is produced with 6-hydroxydopamine which causes dopamine depletion and subsequent supersensitivity of striatal receptors. Thus, administration of drugs which are therapeutically effective in Parkinson's disease, elicits circling behavior contralateral to the lesioned side. The origin of the paradoxical response and the undrugged rotations observed after saline injections in apomorphine pretreated rats has not been explained. We report here that the undrugged response can be associated with the early rotational response elicited by the drug. Furthermore, we show the effect of inhibitory conditioning on this early response. Our results suggest that pharmacological conditioning plays a significant role in the modification of the therapeutic effectiveness of CNS-active drugs after extended treatments.

Apomorphine induces latent rotation in lesioned rats, amphetamine does not

Rats with unilateral 6-hydroxydopamine (6HDA) lesions of one nigrostriatal pathway were used to compare the efficacy of apomorphine (Apo) and (+)-amphetamine (Amph) at inducing latent rotational behavior. Rats treated with Apo rotated contralaterally acutely after drug treatment and subsequently, weeks after drug treatment, exhibited rapid contralateral circling when re-introduced to the rotation environment. Amph treated animals, on the other hand, rotated ipsilaterally acutely after drug treatment and exhibited no latent drug effect.

Heterogeneous rotational responsiveness in 6-hydroxydopamine-denervated rats: pharmacological and neurochemical characterization

Qualitative differences in pharmacological responsiveness to various types of dopamine agonists have been reported in rats that have undergone unilateral 6-hydroxydopamine (6-OHDA)-induced denervation of the nigro-striatal pathway. The present experiments further characterize these differences, pharmacologically and neurochemically. Rats were classified as having high rotational sensitivity (0.03 mg/kg SC apomorphine sufficient to induce more than 100 rotations/20 min) or low sensitivity (0.3 mg/kg SC apomorphine required to meet this criterion). High sensitivity rats showed marked contralateral rotational behavior (approximately 150 rotations/20 min) in response to apomorphine (ED50 = 0.08 mg/kg IP), CGS 15855A (ED50 = 0.07 mg/kg), CGS 15873A (ED50 = 0.43 mg/kg), (+)-3-PPP (ED50 = 2.3 mg/kg), (-)-3-PPP (ED50 = 0.87 mg/kg) and quinpirole (peak effective dose, 0.03 mg/kg). In low sensitivity rats, 3- to 10-fold higher doses of apomorphine induced a maximal rate of rotational behavior, but only partial effects were produced by quinpirole, CGS 15855A, CGS 15873A, (+)-3-PPP, and (-)-3-PPP (40-80 rotations/20 min). Because apomorphine is a nonselective D1 and D2 agonist, it is proposed that activation of either D1 or D2 receptors suffices to induce high rates of rotation in high sensitivity rats, whereas in low sensitivity rats, D1 or D2 agonism alone induces submaximal rotation rates. The ipsilateral rotational behavior induced by d-amphetamine was more pronounced and occurred at lower doses in the high-sensitivity rats. Striatal dopamine depletion on the lesioned side did not differ between the groups, but low sensitivity rats showed two-fold higher DOPAC/DA ratios on the lesioned side than did high-sensitivity rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Application of the unilateral 6-hydroxydopamine rat model of rotational behavior to the study of conditioned drug effects

In rats with unilateral 6-hydroxydopamine lesions of the substantia nigra apomorphine reliably induces a response of contralateral rotation. The present study shows that this response can readily be conditioned to a test environment which the rats are briefly exposed to during the onset of the apomorphine-induced rotation. This conditioned response can be repeatedly conditioned, extinguished, and differentially conditioned, thereby providing a useful behavioral model for the study of the conditioning of drug-induced movements.

Conditioned response to apomorphine in nigro-striatal system-lesioned rats: the origin of undrugged rotational response

Development and time-course characteristics of Early rotational response (ER) to apomorphine in 6-hydroxydopamine-lesioned rats is explored. We show here how this ER can be considered a conditioned response that arises when the drug is repeatedly administered, according to a classical conditioning paradigm. In this way, the ER to apomorphine can be considered a non-pharmacological, conditioned, placebo response, the drug action being the unconditioned stimulus (UCS). In our model, the undrugged rotational response elicited by saline injections two weeks after drug treatment can be considered as the conditioned response (CR) to the conditioned stimulus, the CS being the environment associated with the drug treatment. This CR had not previously been identified during the drug treatment. Thus, we studied the acquisition of the ER, nonexistent after the first injection of apomorphine. Furthermore, we distinguish between this ER and the later, strictly pharmacological rotational response (LR) to apomorphine. Finally, we related this ER to the undrugged, paradoxical response to saline. In conclusion, we demonstrate the paradigm of pharmacological conditioning using this animal model of Parkinson's disease, supported by our own results and those of Silverman and Ho (1981).

A review of the biochemical and neuropharmacological actions of lithium

The pharmacological actions central to the therapeutic effects of lithium have not yet been established, despite almost 40 years of clinical use and scientific investigation. We review the biochemical and neuropharmacological data relating to this problem and attempt to identify profitable areas for further research.

Undrugged rotational response in nigro-striatal system-lesioned rats is related to the previous early response to apomorphine when repeatedly administered

Development and time-course characteristics of undrugged rotational response weeks or months after repeated apomorphine administration in 6-hydroxydopamine-lesioned rats were explored. In one series of experiments, all groups received several drug doses two-three weeks post-lesion and remained undrugged at different intervals from the last drug injection. Weeks or months later, they were injected with saline in the same environment where they previously had received apomorphine. In this way we studied acquisition, time-course and extinction of the rotational response after saline. Furthermore, we related this undrugged response which does not fully develop until two weeks after treatment, and which previously had not been related to a specific parameter of the rotational response to the drug, to a critical point of the time-course response to the drug, i.e., the early rotational response in the first minute after injection. This early response is a learning phenomenon based on the environmental cues where drug has been repeatedly administered. Finally, we state the concept of pharmacological conditioning using this animal model of Parkinson's disease, in agreement with our own results and the previous results of Silverman and Ho (1981).

Ethanol induces rotational behavior in 6-hydroxydopamine lesioned mice

Mice with unilateral striatal lesions created by 6-hydroxydopamine (6HDA) injection were screened for rotational (circling) behavior in response to injection of amphetamine and apomorphine. Those that rotated ipsilaterally in response to amphetamine and contralaterally in response to apomorphine were subsequently challenged with 1 to 3 g/kg (i.p.) ethanol. Surprisingly, ethanol induced dose related contralateral (apomorphine-like) rotation which, despite gross intoxication, was quite marked in most animals. No significant correlation was found between the number of turns made following ethanol and made after apomorphine or amphetamine.

Evidence for heterogeneous rotational responsiveness to apomorphine, 3-PPP and SKF 38393 in 6-hydroxydopamine-denervated rats

Several novel dopamine (DA) agonists (SKF 38393, 3-PPP, TL-99) have been reported to induce rotational behavior (RB) in rats unilaterally denervated of the nigro-striatal pathway by 6-hydroxydopamine. Other reports have indicated no RB, however, and these drugs do not cause other behavioral manifestations of postsynaptic DA agonism. In the present experiments, two groups of 6-hydroxydopamine-denervated rats were distinguished by their relative responsiveness to apomorphine-induced RB. A highly sensitive group showed maximal RB in response to doses as low as 0.03 mg/kg, while a less sensitive group exhibited comparable RB only in response to 15- to 20-fold higher doses. The high sensitivity group exhibited RB in response to SKF 38393, 3-PPP and pergolide, but the low sensitivity group did not show appreciable RB after these drugs, even at doses 50 to 100-fold higher. Haloperidol markedly attenuated apomorphine-induced RB in the low sensitivity subgroup, but only reduced by approximately one-half the number of turns induced by apomorphine or SKF 38393 in the high sensitivity group. The atypical antipsychotics, clozapine and RMI 81582, and the muscle relaxant, methocarbamol, reduced RB in all groups, but only at doses that caused performance impairment in a rotorod test. These results appear to reflect qualitative differences in responsiveness to different DA agonists. Behavioral preselection of 6-hydroxydopamine-denervated animals is necessary to achieve consistent pharmacological results with the 6-hydroxydopamine RB model.

Apomorphine-induced biphasic circling behaviour in 6-hydroxydopamine-lesioned rats. A pharmacological kindling phenomenon

Factors governing the development of apomorphine-induced biphasic circling behaviour in rats having unilateral 6-hydroxydopamine lesions of the substantia nigra were investigated. It was found that a post-lesion time of at least 2--3 weeks and the repeated exposure to apomorphine were essential for its development. Optimal results were obtained when animals received weekly apomorphine, 0.05 mg/kg sc, in post-lesion weeks 6, 7 and 8. Pretreatment with haloperidol, 1.0 and 2.0 mg/kg ip 1 h beforehand in post-lesion week 9, converted the biphasic response into an enhanced, uniphasic one. The findings suggest that the development of the biphasic response to apomorphine is a multi-factorial process representing a pharmacological kindling phenomenon.

A quantitative analysis of stereotyped gnawing induced by apomorphine

An apparatus was designed and constructed to enable a quantitative analysis of the stereotyped gnawing produced by the dopamine agonist apomorphine. Using this apparatus it was discovered that increasing the subcutaneous dose of apomorphine increased (1) the number of animals that gnawed, and (2) the duration of gnawing in those animals that gnawed at all doses. Other aspects of apomorphine-induced gnawing, in particular the latency to respond and the frequency and duration of individual gnaws, were relatively unaffected. Likely properties of the system responsible for the organization of sterotyped gnawing are discussed.