D. M. Jackson

A functional effect of dopamine in the nucleus accumbens and in some other dopamine-rich parts of the rat brain

Dopamine (5 to 50 Μg) applied bilaterally to the nucleus accumbens of reserpine-nialamide pretreated rats produced a marked dose-dependent rise in coordinated locomotor activity, devoid of stereotypies such as gnawing, rearing and licking seen after dopamine application (50 Μg) to the neostriatum. The locomotor activity was completely blocked by pimozide, but not by phenoxybenzamine. The effects of apomorphine or d-noradrenaline were similar to those of dopamine. In contrast, l-noradrenaline produced a “convulsive” syndrome devoid of coordinated locomotor activity, and this convulsive syndrome could be completely blocked by phenoxybenzamine but not by pimozide. Release of endogenous dopamine by d- or l-amphetamine (10 and 50 Μg) in the nucleus accumbens produced a rise in coordinated activity, the d-isomer was about 4 times as potent as the l-isomer, and the effect of the d-isomer was blocked completely by α-methyltyrosine. Bilateral application of trifluoperazine (2.5 Μg) to the nucleus accumbens completely blocked the effect of systemically administered d-amphetamine (1.5 and 3.0 mg/kg), but similar application to the area of the central nucleus of the amygdala or the neostriatum was much less effective. Partial protection of the endogenous dopamine stores against the depleting action of reserpine by local application of metatyramine to the nucleus accumbens resulted in a higher level of basal activity than in control animals. Application of dopamine or noradrenaline to the area of the central nucleus of the amygdala or to the olfactory tubercles did not lead to any consistent changes in locomotor activity.The nucleus accumbens and olfactory tubercles contained most of the dopamine in the limbic forebrain, with noradrenaline more evenly distributed.These data suggest that the nucleus accumbens plays an important role in the locomotor activity in rats.

Role of D1 and D2 dopamine receptors in mediating locomotor activity elicited from the nucleus accumbens of rats

The present study examined the role of D1 and D2 receptors in mediating locomotor activity induced by dopamine (DA) agonists after injection into the nucleus accumbens (Acb). The D1 receptor agonist SKF38393 (as the racemic mixture) induced a dose-related increase in activity when injected bilaterally (1-10 micrograms/side). At a dose of 1 microgram/side, only the R-enantiomer was active. The SKF38393 (10 micrograms/side)-induced activity was antagonized by the D1 receptor antagonist SCH23390 (0.5 mg/kg i.p.), by the D2 receptor antagonist spiperone (0.1 mg/kg, i.p.), but not by the 5-HT2 antagonist ketanserin (1 mg/kg, i.p.). Another D1 agonist, CY208 243, also induced a moderate increase in activity when injected into the Acb (2 and 8 micrograms/side), but this was of much less intensity and of shorter duration than that produced by SKF38393. The D2 receptor agonist quinpirole slightly increased activity when administered into the Acb (0.3-3 micrograms/side), with the magnitude and duration of the response, however, being much less than that produced by SKF38393. The locomotor stimulant effects of SKF38393 (5 micrograms/side), CY208 243 (2 micrograms/side) and quinpirole (1 microgram/side) were blocked by the depletion of catecholamines with reserpine (5 mg/kg s.c., 24 h pretreatment) and alpha-methyl-p-tyrosine (200 mg/kg, i.p.). However, when SKF38393 and quinpirole were injected concurrently into the Acb at doses of 5 and 1 microgram/side respectively, a marked locomotor stimulation occurred in catecholamine-depleted rats. Furthermore, SKF38393 (1 microgram/side) or CY208 243 (2 micrograms/side), injected concurrently with quinpirole (0.3 microgram/side), into the Acb of rats with intact DA stores produced an at least additive effect on locomotor activity. These results suggest that both D1 and D2 receptor stimulation in the Acb is required for the expression of locomotor effects. Furthermore, D1 and D2 receptors in this nucleus appear to interact positively with each other, and may mediate the additive locomotor stimulatory effects induced by concurrent systemic administration of selective D1 and D2 agonists.

Bromocriptine induces marked locomotor stimulation in dopamine-depleted mice when D-1 dopamine receptors are stimulated with SKF38393

In mice pretreated with reserpine plus alphamethyl-p-tyrosine, neither the D-2 selective agonist bromocriptine, nor the D-1 selective agonist SKF38393, produced any measurable increase in locomotion in mice. However, the combination of the two agonists produced a marked and dose-dependent increase in co-ordinated locomotor activity. In mice with their dopamine stores and dopamine synthesis intact, SKF38393 was inactive by itself, but significantly enhanced the stimulant effect produced by bromocriptine. The data suggest that bromocriptine requires concomitant stimulation of D-1 receptors for the full expression of its behavioural stimulant effects.

The motor effects of bromocriptine — a review

For many years, bromocriptine has proven to be a useful treatment for some of the disabling motor effects seen in Parkinsons disease. As such, it has been the only commonly used directly acting D2 agonist available. But its mechanism of action has been obscure because many animal models indicated an absolute requirement for the presence of endogenous DA for bromocriptine to have any efficacy, despite its undoubted occupation of the D2 receptor with high affinity. Several scattered reports indicated, however, that bromocriptine could potentiate the effects of a number of other dopamine agonists (such as apomorphine and l-dopa) in a variety of pharmacological models and in the clinic. With the availability of SKF38393 and SCH23390, it soon became clear that bromocriptine, while a selective D2 agonist, depended in an absolute sense on the integrity of the D1 receptors. Thus, if SKF38393 was administered together with bromocriptine to rodents depleted of dopamine, marked locomotor excitation was produced, despite either drug alone being inactive. The present review explores the literature on the motor effects of bromocriptine and endeavours to integrate its behavioural, biochemical and electrophysiological effects into a coherent whole. It closes with a consideration of several remaining unsolved problems associated with the pharmacology of bromocriptine and suggests some future studies.

The Effect of Long-Term Penfluridol Treatment on the Sensitivity of the Dopamine Receptors in the Nucleus accumbens and in the Corpus striatum

The effect of local application of dopamine to the nucleus accumbens or corpus striatum on locomotor activity was studied in rats 4 days after withdrawal from a 6 weeks term of penfluridol medication. The bilateral application of dopamine into the nucleus accumbens of penfluridol-treated rats produced a very marked increase in coordinated locomotor activity which was 3–5 times higher than that of rats not treated with penfluridol. This effect of dopamine in both penfluridol-treated and control rats was antagonized by intraperitoneally administered haloperidol. The bilateral application of dopamine into the corpus striatum of penfluridol-treated animals produced a marked stereotyped behavioural syndrome in all rats studied, whereas no signs of stereotyped behaviour were observed in any of the rats not treated with penfluridol. The results indicate that long-term treatment of rats with the dopamine receptor blocking agent penfluridol produces an increase in the sensitivity of the dopamine receptors in the nucleus accumbens and corpus striatum and that the nucleus accumbens may play a role in locomotor activity.

A pharmacological study of changes in central nervous system receptor responsiveness after long-term dexamphetamine and apomorphine administration.

Mice administered dexamphetamine (4 mg/kg i.p.) once daily for 20 days displayed an enhanced locomotor response (compared to that of vehicletreated mice) to dexamphetamine when challenged 4 to 16 days but not when challenged 32 days after withdrawal.In the experiments described a 20-day dexamphetamine administration followed by an 8-day withdrawal period was used. Pimozide or haloperidol not only completely antagonised the enhanced response to dexamphetamine (2 mg/kg i.p.) in dexamphetamine-treated mice, but also antagonised all dexamphetamine-induced stimulation. Reserpine, in contrast, preferentially blocked the difference in the response to dexamphetamine of dexamphetamine-and vehicle-treated mice, without antagonising all the dexamphetamine-induced locomotion. The stimulation produced in dexamphetamine-(but not in vehicle-) treated mice by dexamphetamine was partially blocked by phentolamine, phenoxybenzamine, and propranolol. FLA-63 did not significantly influence the response to dexamphetamine in either group. That a dopaminergic mechanism plays a major role in the enhanced response to dexamphetamine was shown by the significantly greater response in dexamphetamine-treated mice to apomorphine challenge.The treatment of mice with apomorphine (10 mg/kg/day i.p. for 20 days), produced a greater response to apomorphine challenge in the apomorphine-treated mice than in vehicle-treated mice 8 days after withdrawal.The data show that with long-term administration both dexamphetamine and apomorphine are able to produce in mice what appear to be supersensitive dopamine receptors. Moreover, the enhanced response to dexamphetamine after withdrawal from long-term dexamphetamine treatment appears to require the presence of reserpine-sensitive amine stores and, to a lesser extent, the presence of unblocked α-adrenergic receptors.

Tolerance to the effects of Δ9-tetrahydrocannabinol in mice on intestinal motility, temperature and locomotor activity

The onset and duration of tolerance to three effects of δ9-tetrahydrocannabinol (δ9-THC) given orally to mice were compared. The effects of δ9-THC studied were: hypothermia, the depression of intestinal motility and the effect on spontaneous locomotor activity. When mice were dosed and tested at 24 hrs intervals it was apparent that tolerance was complete to its hypothermic and locomotor depressant effects after the first doses and to depression of intestinal motility after the fourth dose. Duration of tolerance also differed so that the normal hypothermic response had returned after 12 dose-free days, but not after 5 drug-free days; the effect on locomotor activity had returned within 4 days; and, apparent partial tolerance to the depressant effect of an acute challenging dose of δ9-THC on intestinal motility still existed after 19 dose-free days.It is apparent that the time of onset and the duration of tolerance to δ9-THC in mice showed a different pattern in the three parameters studied. It seems unlikely therefore that any one mechanism, such as metabolic tolerance, explains all the results observed and that several mechanisms should be explored to explain the phenomenon of tolerance to δ9-THC.

Locomotor activity stimulation in rats produced by dopamine in the nucleus accumbens: potentiation by caffeine

The increased motor activity of reserpine‐nialamide pretreated rats given dopamine into the nucleus accumbens was potentiated in a dose‐dependent manner by systemically administered caffeine. Similarly, the increase in motor activity seen when the endogenous dopamine was released by intraperitoneally administered amphetamine was potentiated by systemically given caffeine. These effects might be due to an increase in the dopamine‐induced accumulation of cyclic AMP in the nucleus accumbens after inhibition of the phosphodiesterase by caffeine.

Anticonvulsant effects of cannabinoids in mice: Drug interactions within cannabinoids and cannabinoid interactions with phenytoin

The anticonvulsant activity of orally administered δ9-tetrahydrocannabinol (δ9-THC), δ8-THC, cannabidiol (CBD) and cannabinol (CBN) was tested in mice utilizing electroshock and chemoshock methods. In doses tested δ9-THC afforded no protection to mice from chemoshock seizures and was effective against electroshock only in high doses (160–200 mg/kg). CBD and CBN (150–200 mg/kg) were without effect in both tests.An interaction between cannbinoids was apparent when all three were administered simultaneously (each at 50 mg/kg) because this combination produced a significant reduction in the duration of the hind-limb extensor phase of the electroshock seizures.The administration of δ9-THC significantly potentiated the anticonvulsant effectiveness of phenytoin against electroshock seizures and this effect was further potentiated by the concurrent administration of CBD. Whilst the potentiation of phenytoin by δ9-THC (50 mg/kg) was of the order of 1.5 times, the combination of δ-9THC and CBD (each 50 mg/kg) produced a four-fold potentiation.Neither within-cannabinoid interaction nor cannabinoid potentiation of phenobarbitone effectiveness could be demonstrated in chemoshock tests.The mechanism of the cannabinoid facilitation of phenytoin is unknown but it possibly involves activity at central nervous system level rather than being a metabolic interaction. This drug interaction may have potential clinical significance.

The demonstration of a change in adrenergic receptor sensitivity in the central nervous system of mice after withdrawal from long-term treatment with haloperidol

Mice, administered haloperiodl (3 mg/kg/d) in their drinking water for 21 days, displayed, 4 days after cessation of the haloperidol-treatment, marked locomotor stimulation to clonidine (100 or 500 μg/kg) which lasted for about 6 h. 25 μg clonidine/kg was inactive. Premedication with FLA-63 (25 mg/kg) blocked the difference in stimulation after clonidine between the haloperidol- and vehicle-treated animals, but locomotor activity was still present in both groups. Haloperidol-treated animals displayed a supersensitive response to dexamphetamine. The difference in stimulation produced by dexamphetamine in the two groups was completely blocked by phenoxybenzamine (2.5 mg/kg), phentolamine (10 mg/kg), which drugs did not, however, block the locomotor stimulation produced by dexamphetamine in vehicle-treated animals. Pimozide (3 mg/kg) blocked all locomotor stimulation produced by dexamphetamine in both vehicle- and haloperidol-treated groups, while 1 mg/kg completely blocked the dexamphetamine response in vehicle-treated animals but not in haloperidol-treated animals. FLA-63 (25 mg/kg) blocked the difference in response between the haloperidol- and vehicletreated groups to dexamphetamine, but did not antagonise the stimulation in the vehicle-treated animals. The data suggest that long-term haloperidol treatment leads to the development of “supersensitive” adrenergic receptors in the central nervous system, which, appropriately stimulated, effect an increase in locomotor activity. Moreover, the results indicate that a large component of the supersensitive response to dexamphetamine observed after long-term haloperidol-treatment is due to adrenergic receptor supersensitivity. However, the dopamine receptor (which was shown to be supersensitive to apomorphine) is of fundamental importance because phenoxybenzamine and phenolamine, while blocking the supersensitive response to dexamphetamine, failed to block the response to dexamphetamine in vehicletreated animals, which was, however, blocked by pimozide.