Behavioral evidence for supersensitivity after chronic bromocriptine administration

Behavioral sensitization and tolerance to the D2 agonist RU 24213: dissociation between several behavior patterns in mice

Previous studies have shown that different components of behavioral effects of indirect agonists given chronically to laboratory rodents can follow different courses during treatment. Whether repeated injections of a D2 agonist can lead to the same phenomenon was investigated in mice using the D2 agonist N-n-propyl-N-phenylethyl-p-(3-hydroxy-phenyl)-ethylamine (RU 24213). Five mutually exclusive behaviors were examined over seven intermittent administrations (every other day over 13 daily injections) of RU 24213 (2.5 mg/kg SC) in mice. Rapid tolerance to the clearest initial effect of RU 24213, stillness, was found. Suppression of grooming also showed tolerance later in the treatment regimen (from the fourth test). From the third test, parallel time courses of sensitization were obtained for ambulation and rearing. Sleeping position was strongly depressed throughout the chronic treatment. These results show that the development in time of the behavioral effects of RU 24213 injected chronically strongly depend upon the behavioral measure. This supports the use of multiple measures in the same animal in the behavioral analysis of chronically injected dopaminergic drugs.

Lack of cross-sensitization between the locomotor-activating effects of bromocriptine and those of cocaine or heroin

Rats were given daily injections of bromocriptine (5.0 mg/kg IP) or vehicle either in the home cage or in a test box equipped with photocells to measure locomotion. The animals were then tested in the photocell boxes for their locomotor response to cocaine (10.0 mg/kg IP), heroin (0.5 mg/kg IP), or quinpirole (0.1 mg/kg IP). Repeated bromocriptine in the test box but not in the home cage caused progressive increases in sensitivity to the locomotor-stimulating effects of bromocriptine and increases in the subsequent sensitivity to quinpirole but caused only trivial signs of cross-sensitization to cocaine or heroin. Cross-sensitization to quinpirole was temporary; responsiveness to quinpirole decreased with further quinpirole injections. Lack of significant cross-sensitization between bromocriptine and either cocaine or heroin and lack of permanence of the cross-sensitization between bromocriptine and quinpirole raise questions as to the biological basis of psychomotor stimulant sensitization.

Locomotor-activating effects of the D2 agonist bromocriptine show environment-specific sensitization following repeated injections

Biphasic effects of bromocriptine (2.0, 5.0, 10.0, and 20.0 mg/kg IP) on locomotion were quantified in photocell activity boxes in rats. Following early suppression of activity, bromocriptine produced a clear, dose-dependent increase in locomotion that lasted several hours. When a low dose of bromocriptine (5.0 mg/kg) was administered daily over a 3-week period, the locomotor-activating effects of the drug showed progressive enhancement over days. The sensitization was environment specific; rats administered bromocriptine six times in the home cage showed no sign of a sensitized response to bromocriptine when subsequently tested in the activity box. Thus, selective stimulation of D2 receptors stimulates locomotion and sensitizes animals to subsequent injections, just as do the indirect-acting dopamine agonists cocaine and amphetamine.

Receptor changes during chronic dopaminergic stimulation

The underlying cause of the long term complications of L-DOPA or dopamine agonist therapy in Parkinson's disease remains unknown. Previous studies of repeated administration of L-DOPA or bromocriptine to rodents have shown increases, decreases or no change in brain dopaminergic activity. For this reason we have re-examined the effects of chronic L-DOPA or dopamine agonist administration on brain dopamine receptor function in rats. Repeated intraperitoneal administration of L-DOPA to rats for 21 days followed by 3 days drug withdrawal caused an enhancement of apomorphine-induced stereotypy but no apparent alteration in striatal dopamine receptor numbers or affinity (as judged by 3H-spiperone; 3H-NPA and 3H-piflutixol binding). Chronic oral administration of L-DOPA plus carbidopa to rats for one year was without effect on apomorphine-induced stereotypy or striatal D-2 dopamine receptors. Similarly, no effects were observed on striatal dopamine function following one year's administration of bromocriptine. Pergolide produced an enhancement of apomorphine-induced stereotypy but a decrease in D-2 receptor density as judged by 3H-spiperone binding. In rats with a unilateral 6-OHDA lesion of the medial forebrain bundle the oral administration of L-DOPA plus carbidopa for 4 weeks, followed by 4 days withdrawal, enhanced the rate of apomorphine-induced contraversive rotation. It appears difficult, at least in rats, to manipulate striatal dopamine receptors with L-DOPA or dopamine agonist drugs. An enhancement of motor behaviour can occur in the presence of no change or a decrease in dopamine receptor numbers identified by in vitro ligand binding to tissue homogenates.

Enhanced stereotypic behavior by chronic treatment with bromocriptine accompanies increase of D-1 receptor binding

Different effects of chronic treatment with bromocriptine (BRO) on D-1 and D-2 receptors in the rat were studied through behavioral observation and DA receptor binding assays. Chronic BRO led to enhancement of stereotypy to apomorphine (APO), at the same time it increased the density of D-1 receptor binding by 43% and decreased that of D-2 receptor binding by 21%. Our data suggest that BRO has different effects on D-1 and D-2 receptors and the behavioral hypersensitivity caused by BRO may be relevant to the proliferation of D-1 receptor.

The effects of chronic bromocriptine treatment on behaviour and dopamine receptor binding in the rat striatum

Agonist-induced rotation and striatal binding of [3H]spiperone ([3H]SPIP) were assessed in rats with unilateral lesions of the substantia nigra during and after a period of chronic bromocriptine administration. Agonist-induced rotation significantly increased over a three week period of daily administration of bromocriptine (10 mg/kg i.p.); control animals were tested for agonist-induced rotation at one week intervals, which remained constant. Rotation was increased by chronic bromocriptine administration in response to either of two DA agonists, apomorphine (APO) and bromocriptine, suggesting that increased agonist sensitivity did not reflect a reduction in the metabolism of bromocriptine. Striatal binding of the dopamine D2 radioligand, [3H]SPIP, was significantly increased in the denervated striata of nigra-lesioned rats. Chronic bromocriptine administration decreased binding in denervated striata to levels not significantly different from control values. [3H]SPIP binding in intact striata was significantly reduced by bromocriptine to below control values. Differences in receptor levels reflected changes in the maximum density of binding sites with no change in affinities. Paradoxical behavioural hypersensitivity developing during chronic bromocriptine levels is not apparently mediated by changes in striatal D2 binding sites.

The comparative long-term effects of ciladopa (AY-27,110), a chemically novel dopaminergic agonist, in 6-OHDA-lesioned and intact rats

The effect of the chemically novel dopaminergic agonist, ciladopa (AY-27,110) was studied on rotational behavior in unilaterally 6-OHDA-lesioned rats and on stereotyped behavior in normal animals during and after chronic treatment. Ciladopa, at a 1.25 mg/kg SC daily dose, was administered for 4 and 6 weeks to the lesioned and normal animals, respectively. This dose was threshold for turning but subthreshold with respect to stereotypy. Both behaviors were evaluated weekly during chronic treatment and biweekly for 11 weeks after its discontinuation in response to doses previously shown to elicit turning and stereotypy. The chronic administration of ciladopa enhanced rotational behavior considerably more and with an earlier onset than stereotypy, although the duration of behavioral supersensitivity after cessation of treatment was the same. These results indicate that behaviors mediated by supersensitive or intact dopamine receptors are differently affected by chronic dopamine agonist treatment. The results are discussed in relation to their potential therapeutic implications and current concepts of agonist-induced supersensitivity.

The effect of chronic bromocriptine and L-dopa on spiperone binding and apomorphine-induced stereotypy

Chronic treatment with dopamine (DA) agonists has been reported in various paradigms to cause supersensitivity of DA receptors or, contradictorily, subsensitivity of DA receptors. The present study administered 15 mg/kg bromocriptine for 7 days and measured both striatal spiperone binding and apomorphine (AP)-induced stereotypy. A significant decrease in AP-induced stereotypies was observed after chronic bromocriptine treatment, but without a significant parallel decrease in striatal spiperone binding. These results probably do not represent a true agonist-induced subsensitivity, but possibly show that residual bromocriptine in vivo may antagonize AP-induced stereotypy. Since some reports have suggested that L-Dopa may specifically reverse the increases in DA receptor number induced by chronic haloperidol, we also studied the effect of 7 days of L-Dopa treatment after 6-week chronic haloperidol treatment of mice. While chronic haloperidol significantly increased striatal spiperone binding, subsequent L-Dopa treatment did not reverse this biochemical supersensitivity. It is concluded that agonist induction of subsensitivity in the DA system is difficult to reproduce and may depend on highly specific dosage conditions and treatment schedules.

Chronic L-DOPA treatment of mice: a behavioural and biochemical study

Mice were pretreated once daily with L-DOPA (200 mg/kg) plus benserazide (B) (50 mg/kg) for ten days and challenged with various doses of L-DOPA + B on the first, fourth or sixteenth days of withdrawal. L-DOPA + B-pretreated mice were more sensitive the locomotor stimulant effect of L-DOPA + B challenge one and four days, but not sixteen days after withdrawal. The enhanced response was most marked on the first day of withdrawal. Other mice, pretreated once daily with B (50 mg/kg), responded one day after the tenth dose with a slightly enhanced response to L-DOPA + B challenge compared to the response to vehicle-pretreated animals. Moreover, vehicle-pretreated mice challenged with B alone, were significantly less active than those challenged with vehicle. On the first day of withdrawal, the L-DOPA + B-pretreated animals were supersensitive to locomotor stimulant effects of apomorphine but subsensitive to dexamphetamine (Bailey et al., 1979). On the fourth day of withdrawal, there were no differences in the responses of the L-DOPA + B-pretreated mice compared to the vehicle-pretreated mice, to apomorphine or apomorphine plus clonidine, but L-DOPA + B-pretreated mice were still subsensitive to the locomotor stimulant effects of dexamphetamine. Clonidine produced a dose-dependent, but similar, degree of hypothermia in both pretreatment groups. On the first and fourth days of withdrawal L-DOPA + B-pretreated mice exhibited higher brain levels of dopamine (DA) and DOPA than vehicle-pretreated mice in response to an acute dose of L-DOPA + B. The biochemical results suggest that the enhanced locomotor response to L-DOPA + B in L-DOPA + B-pretreated mice is probably dependent on changes in the amount of L-DOPA (and DA) available in the brain. Moreover, it is not ruled out that some of the effects of L-DOPA + B pretreatment were due to the B alone. Some of the enhanced response to L-DOPA + B on the first day of withdrawal may have been dependent on the same mechanism as that underlying the apparent supersensitivity to apomorphine. The subsensitive response to dexamphetamine would appear to be independent of changes in post-synaptic DA and alpha-adrenergic receptor sensitivity.

The effect of amygdala kindling on spontaneous and cocaine-induced motor activity and lidocaine seizures

Interactions of amygdala kindling and drug effects were explored in two experiments. Pretreatment with cocaine (40 mg/kg, IP) for 10 days did not significantly affect the rate of amygdala kindling compared to saline or non-kindled controls. In contrast, daily amygdala kindling with 200 microA for 0.5 s for 20 days substantially altered subsequent behavioral responses in a long-lasting fashion. Animals showed decreased spontaneous vertical rearing activity, as well as decreased cocaine-induced vertical activity. In contrast, they were more reactive to the direct dopamine receptor agonist apomorphine. Eighteen days following completion of amygdala kindling, kindled animals were more sensitive to lidocaine-induced convulsions; 88% of kindled animals, but only 24% of the implanted sham-stimulated controls, had seizures. These data suggest that amygdala kindling may produce long-lasting changes in selected spontaneous and drug-induced behaviors, as well as convulsive thresholds. Possible physiological and neurological changes underlying this altered responsivity are discussed.