Brain implantations of engineered GABA-releasing cells suppress tremor in an animal model of Parkinsonism.

Traditional approaches in the treatment of Parkinson's disease have typically been directed at restoring dopaminergic tone in the neostriatum of the basal ganglia. Nevertheless, the vast majority of neostriatal efferent projections use GABA as their neurotransmitter. Substantia nigra pars reticulata (SNr) is a major basal ganglia output area that is a target of these GABAergic projections, and research from animal models has indicated that stimulation of GABA receptors in SNr can produce motor effects consistent with an antiparkinsonian action. In the present study, implantation of engineered GABA-releasing cells into SNr reduced tremulous movements in an animal model of parkinsonian tremor. These results suggest that implantation of GABA cells into SNr, or possibly into other basal ganglia structures, could provide an alternative transplantation strategy for the treatment of Parkinsonism.

Nucleus accumbens dopamine depletions make animals highly sensitive to high fixed ratio requirements but do not impair primary food reinforcement.

It has been suggested that dopamine in nucleus accumbens is involved in the process of enabling organisms to overcome work-related response costs. One way of controlling work costs with operant schedules is to use fixed ratio schedules with different ratio requirements. In the present study, the effects of nucleus accumbens dopamine depletions were investigated using six schedules: fixed ratio 5, 20, 50, 100, 200, and 300. In the first three schedules the food reinforcement consisted of one 45 mg food pellet per ratio completed. In the remaining schedules the food reinforcement per ratio completed was increased to two pellets for fixed ratio 100, four pellets for fixed ratio 200, and six pellets for fixed ratio 300. All rats were trained extensively prior to surgery, and rats were able to maintain high levels of responding on all schedules up to the fixed ratio 300. After training, rats were injected with either ascorbate vehicle or 6-hydroxydopamine into the nucleus accumbens. Rats were tested post-surgically on each of the schedules, with 3 days of testing per schedule. Rats with nucleus accumbens dopamine depletions exhibited behavioral deficits that were highly dependent upon the ratio value. There were small and transient effects of dopamine depletion on fixed ratio 5 lever pressing, but as the ratio value got larger the impairment became greater. On the fixed ratio 20 and 50 schedules, response rates were partially reduced in dopamine-depleted rats. Responding on the fixed ratio 200 and 300 schedules was severely impaired, and on the last day of fixed ratio 300 testing no dopamine-depleted rats obtained a single reinforcer. These data are consistent with previous reports that accumbens dopamine depletions enhance 'ratio strain', making rats more sensitive to high ratio values. The induction of ratio strain by dopamine depletions does not appear to be related to a loss of appetite, and seems to be relatively independent of the baseline rate of responding and the overall density of food reinforcement across the session. We conclude that dopamine in nucleus accumbens may be important for enabling rats to overcome behavioral constraints such as work-related response costs, and may be critical for the behavioral organization and conditioning processes that enable animals to emit large numbers of responses in the absence of primary reinforcement.

D1 or D2 antagonism in nucleus accumbens core or dorsomedial shell suppresses lever pressing for food but leads to compensatory increases in chow consumption.

Although interference with dopamine (DA) systems can suppress lever pressing for food reinforcement, it is not clear whether this effect occurs because of a general disruption of food motivation. One way of assessing this has been a choice procedure in which a rat responds on an fixed ratio 5 (FR5) schedule for preferred Bioserve pellets while a less preferred lab chow is concurrently available in the operant chamber. Untreated rats consume little of the chow, preferring to respond for the Bioserve pellets. Previous studies have shown that depleting DA in the accumbens substantially decreased lever pressing while increasing chow consumption. In the present study, low doses (0.0625-1.0 microg) of the D1 antagonist SCH 23390 or the D2 antagonist raclopride were injected into the either the core or shell subregions of nucleus accumbens, and rats were tested on the concurrent lever pressing/feeding task. Analysis of the dose response curves showed that injections of SCH 23390 into the core were more potent than injections into the shell for suppressing lever pressing (i.e., the ED(50) was lower in the core). Nevertheless, injections of either drug into either site suppressed lever pressing and increased intake of the concurrently available chow. Across both drugs and at both sites, the amount of chow consumed was negatively correlated with the total number of responses. Neither drug significantly increased response duration, suggesting that accumbens DA antagonism did not produce the type of motor impairment that leads to severe alterations in the form of lever pressing. In summary, the blockade of D1 or D2 receptors in nucleus accumbens core or shell decreased lever pressing for food reinforcers, but rats remained directed toward the acquisition and consumption of food. These results indicate that accumbens D1 antagonism does not decrease lever pressing because of a general reduction in food motivation. Nevertheless, interference with accumbens DA does appear to set constraints upon which responses are selected for obtaining food, and may impair the ability of animals to overcome work-related response costs in order to obtain food.

Substantia nigra pars reticulata is a highly potent site of action for the behavioral effects of the D1 antagonist SCH 23390 in the rat.

RATIONALE: Considerable evidence indicates that dopaminergic drugs, including drugs that act on D1 receptors, exert their effects by actions on forebrain dopamine terminal regions. Nevertheless, anatomical studies also have demonstrated that there is a high concentration of D1 receptors in the substantia nigra pars reticulata (SNr). The D1 receptors in SNr are located largely on the terminals of gamma-aminobutyric acid (GABA)-ergic striatonigral neurons. The present studies were undertaken to determine whether the D1 antagonist SCH 23390 was effective if locally injected into SNr and to compare the results of SNr injections with those obtained from other brain sites. Fixed ratio 5 (FR5) lever pressing and open-field locomotion were used as the behavioral tests because these tasks are sensitive to systemic SCH 23390. METHODS: Rats received bilateral implantations of guide cannulae into either nucleus accumbens, neostriatum, SNr, or control sites in the cortex or brainstem. Rats in the FR5 study were trained prior to surgery. All rats received one of the following local injections (0.5 microl per side): vehicle, 0.25, 0.5, 1.0, or 2.0 microg SCH 23390. RESULTS: In the FR5 study, the SNr was by far the most potent site for suppression of lever pressing, with an ED50 (dose that produces half maximal response) of 0.33 microg per side. Nucleus accumbens and neostriatum injections were less potent than those in SNr, but more potent than injections into the control regions. With open-field locomotion, the SNr, nucleus accumbens, and neostriatum were approximately equipotent sites, and all three were more potent than the control sites. CONCLUSIONS: SNr was a very potent site for suppression of lever pressing and open-field locomotion. These data suggest that D1 antagonists have multiple sites of action, including not only the forebrain dopamine terminal regions but also the SNr. It is possible that blockade of SNr D1 receptors modulates GABA release from striatonigral neurons.

Neostriatal muscarinic receptor subtypes involved in the generation of tremulous jaw movements in rodents implications for cholinergic involvement in parkinsonism.

Several studies have shown that a number of pharmacological and neurochemical conditions in rats can induce jaw movements that are described as "vacuous" or "tremulous". For several years, there has been some debate about the clinical significance of various drug-induced oral motor syndromes. Nevertheless, considerable evidence now indicates that the non-directed, chewing-like movements induced by cholinomimetics have many of the characteristics of parkinsonian tremor. These movements are characterized largely by vertical deflections of the jaw, which occur in the same 3-7 Hz peak frequency that is typical of parkinsonian tremor. Cholinomimetic-induced tremulous jaw movements are suppressed by a number of different antiparkinsonian drugs, including scopolamine, benztropine, L-DOPA, apomorphine, bromocriptine, ropinirole, pergolide, amantadine, diphenhydramine and clozapine. A combination of anatomical and pharmacological research in rats has implicated M4 receptors in the ventrolateral neostriatum in the generation of tremulous jaw movements. Mice also show cholinomimetic-induced jaw movements, and M4 receptor knockout mice demonstrate subtantially reduced levels of jaw movement activity, as well as increased locomotion. Taken together, these data are consistent with the hypothesis that a centrally-acting M4 antagonist may be useful as a treatment for parkinsonian symptoms, including tremor.

Effects of H1 antagonists on cholinomimetic-induced tremulous jaw movements: studies of diphenhydramine, doxepin, and mepyramine.

In several previous studies, tremulous jaw movements in rats have been used to assess the effects of antiparkinsonian drugs and atypical antipsychotics. Because antihistamines such as diphenhydramine are used as antiparkinsonian agents, and atypical antipsychotic drugs such as clozapine and olanzapine have high affinity for histamine H1 receptors, the present study investigated the effects of H1 antagonists on cholinomimetic-induced jaw movements. Diphenhydramine, doxepin, and mepyramine (all injected IP 2.5-20.0 mg/kg) were assessed for their ability to block the jaw movements induced by 5.0 mg/kg of the anticholinesterase tacrine. Within this dose range, only diphenhydramine produced a robust and significant reduction in jaw movement activity. Thus, diphenhydramine was subjected to further testing, which employed procedures previously used to assess the effects of other antitremorogenic drugs, such as clozapine. Diphenhydramine did not induce jaw movement activity. In addition to suppressing jaw movement activity after acute injections, diphenhydramine also suppressed tacrine-induced jaw movements after repeated (14-day) administration. In summary, the present results show that diphenhydramine suppresses cholinomimetic-induced jaw movements, an effect that is similar to other antiparkinsonian or antitremor drugs such as anticholinergics, L-DOPA, DA antagonists, and clozapine. Nevertheless, doxepin produced only mild effects, and mepyramine, which has a higher affinity and selectivity than diphenhydramine for H1 receptors, failed to suppress cholinomimetic-induced jaw movements. These results suggest that diphenhydramine suppresses tremulous movements through a mechanism that does not depend upon antagonism of histamine H1 receptors.

Behavioral assessment of atypical antipsychotics in rats: studies of the effects of olanzapine (Zyprexa).

RATIONALE: Previous work has shown that clozapine suppressed tacrine-induced jaw movements at lower doses than those required for suppression of lever pressing. OBJECTIVE: The novel atypical antipsychotic olanzapine was assessed in these behavioral tests. METHODS: The effect of acute olanzapine on the suppression of tacrine-induced tremulous jaw movements was examined. In order to determine the relative potency of this effect compared with other behavioral effects of olanzapine, suppression of lever pressing also was studied. In a second series of experiments, rats received olanzapine for 14 consecutive days to study the effects of repeated injections of this drug on jaw movements and lever pressing. RESULTS: Acute olanzapine administration decreased tacrine-induced jaw movements (ED50: 0.4 mg/kg), and also reduced lever pressing (ED50: 1.12 mg/kg). The ratio of the ED50 for suppression of jaw movements to that for suppression of lever pressing was used as an index of liability to produce extrapyramidal side effects, and the present results demonstrate that olanzapine has a ratio similar to that previously shown for clozapine. In the repeated administration studies, rats were observed on day 13 of drug treatment for the ability of olanzapine to induce jaw movements, and olanzapine failed to induce jaw movements. On day 14, olanzapine reduced tacrine-induced tremulous jaw movements (ED50: 1.12 mg/kg). In a separate experiment, olanzapine significantly suppressed lever pressing, and this effect showed sensitization with repeated administration (day 14, ED50: 0.76 mg/kg). Thus, repeated injections of olanzapine reduced tacrine-induced jaw movements in a dose range similar to or slightly higher than that which suppressed lever pressing. CONCLUSIONS: On tests of jaw-movement activity and lever pressing after both acute and repeated drug administration, olanzapine demonstrated a profile somewhat similar to clozapine, and both of these drugs differ substantially from the typical antipsychotic haloperidol.

Nucleus accumbens dopamine depletions and time-constrained progressive ratio performance: effects of different ratio requirements.

Two experiments were conducted to determine the effects of accumbens dopamine (DA) depletions on progressive ratio responding for food reinforcement. In one version of this schedule, ratio requirement increased by one response after each reinforcer was obtained (PROG1). In the other version, ratio requirement increased by five responses after each reinforcer was obtained (PROG5). For both versions, 60-min sessions were conducted. Accumbens DA depletions produced by local injections of 6-OHDA substantially decreased the number of responses on both schedules. The deficits in the response number induced by DA depletions persisted through the two weeks of postsurgical testing for both the PROG1 and PROG5 schedules. However, there were differences between the effects of DA depletions on the two schedules in terms of the time to complete the last ratio. Although time to complete the last ratio was significantly reduced by DA depletions only in the first week of testing on the PROG1 schedule, rats recovered on this measure by the second week after surgery. In contrast, DA-depleted rats on the PROG5 schedule showed a more persistent suppression of the time to complete the last ratio, which lasted through both weeks of postsurgical testing. Performance on schedules that generate low baseline rates of responding (e.g., continuous, fixed, and variable interval) is relatively unaffected by accumbens DA depletions; nevertheless, accumbens DA depletions substantially impair progressive ratio response output. The high work output necessary for responding on the PROG5 schedule may make these animals more sensitive to the effects of accumbens DA depletions.