Changes in behavioral sensitivity to SKF-38393 and quinpirole following withdrawal from continuous cocaine administration in rats

Object memory impairment at post-drug Day 15 but not at Day 1 after a regimen of repeated treatment with oral methylphenidate

Methylphenidate (MPH) is a dopamine and norepinephrine reuptake inhibitor that is widely used for the treatment of attention-deficit/hyperactivity disorder in children and adults. Its similarity to other psychostimulants suggests that, at certain doses, the drug may generate lasting neuroadaptations that can be detrimental to the recipient. Some investigators have found that, in rats, the residual effects of the drug (i.e., following a 10-14 day washout period) can interfere with memory for objects when the retention interval is 3h or more. The present experiment replicated this result and demonstrated the critical importance of the washout period. Long-Evans male rats treated with MPH (5.0mg/kg po b.i.d.) on 21 days (during a post-natal period ranging from Day 29 to Day 57) and then twice-assessed for their performance in an object recognition task were able to recognize a familiar object on Day 1 after the last dose of the drug had been administered, behaving the same as the untreated control group. However, on Day 15 post-drug, the same MPH group failed to distinguish between a familiar and a novel object, exploring both nearly equally, while the control group continued to investigate the novel object to a greater extent than the familiar one. This suggests that, if a test for object recognition is conducted too early after the MPH treatment period ceases, a memory impairment may not be detected. In general, this finding has implications for studies of other behavioral or neurophysiological consequences of MPH that may appear following a drug-free withdrawal period.

Behavioral sensitization to cocaine in rats: evidence for temporal differences in dopamine D3 and D2 receptor sensitivity

RATIONALE

Cocaine-induced changes in D(2) receptors have been implicated in the expression of sensitized behavioral responses and addiction-like behaviors; however, the influence of D(3) receptors is less clear.

OBJECTIVES

To characterize the effects of repeated cocaine administration on the sensitivity of rats to D(2)- and D(3)-mediated behaviors, as well as the binding properties of ventral striatal D(2)-like and D(3) receptors.

METHODS

Pramipexole was used to assess the sensitivity of rats to D(3)/D(2) agonist-induced yawning, hypothermia, and locomotor activity, 24 h, 72 h, 10, 21, and 42 days after repeated cocaine or saline administration. The locomotor effects of cocaine (42 day) and the binding properties of ventral striatal D(2)-like and D(3) receptors (24 h and 42 days) were also evaluated.

RESULTS

Cocaine-treated rats displayed an enhanced locomotor response to cocaine, as well as a progressive and persistent leftward/upward shift of the ascending limb (72 h-42 day) and leftward shift of the descending limb (42 days) of the pramipexole-induced yawning dose-response curve. Cocaine treatment also decreased B (max) and K (d) for D(2)-like receptors and increased D(3) receptor binding at 42 days. Cocaine treatment did not change pramipexole-induced hypothermia or locomotor activity or yawning induced by cholinergic or serotonergic agonists.

CONCLUSIONS

These studies suggest that temporal differences exist in the development of cocaine-induced sensitization of D(3) and D(2) receptors, with enhancements of D(3)-mediated behavioral effects observed within 72 h and enhancements of D(2)-mediated behavioral effects apparent 42 days after cocaine. These findings highlight the need to consider changes in D(3) receptor function when thinking about the behavioral plasticity that occurs during abstinence from cocaine use.

Psychostimulant withdrawal as an inducing condition in animal models of depression

A large body of evidence indicates that the withdrawal from high doses of psychostimulant drugs in humans induces a transient syndrome, with symptoms that appear isomorphic to those of major depressive disorder. Pharmacological treatment strategies for psychostimulant withdrawal in humans have focused mainly on compounds with antidepressant properties. Animal models of psychostimulant withdrawal have been shown to demonstrate a wide range of deficits, including changes in homeostatic, affective and cognitive behaviors, as well as numerous physiological changes. Many of these behavioral and physiological sequelae parallel specific symptoms of major depressive disorder, and have been reversed by treatment with antidepressant drugs. These combined findings provide strong support for the use of psychostimulant withdrawal as an inducing condition in animal models of depression. In the current review we propound that the psychostimulant withdrawal model displays high levels of predictive and construct validity. Recent progress and limitations in the development of this model, as well as future directions for research, are evaluated and discussed.

Adulthood nicotine treatment alleviates behavioural impairments in rats neonatally treated with quinpirole: possible roles of acetylcholine function and neurotrophic factor expression

Increases in dopamine D(2) receptor sensitivity are known to be common in drug abuse and neurological disorders. Past data from this laboratory have shown that long-term increases in D(2) sensitivity can be produced by quinpirole treatment (a D(2)/D(3) agonist) during early development. The present investigation was designed to test the hypothesis that nicotine administration in adulthood would reduce both cognitive and skilled reaching impairments produced by increases in D(2) sensitivity. Female Sprague-Dawley rats were treated with quinpirole (1 mg/kg) or saline from postnatal day 1 (PD 1) to PD 21. Beginning in adulthood (PD 61), rats were treated with nicotine (0.3 mg/kg free base) or saline twice daily for 14 consecutive days before behavioural testing commenced. Animals neonatally treated with quinpirole demonstrated performance deficits on the Morris water task and a skilled reaching task compared to controls. Deficits on both tasks were completely alleviated by adulthood nicotine treatment. Animals neonatally treated with quinpirole demonstrated a significant 36% decrease of ChAT in the hippocampus compared to saline controls that was partially eliminated by nicotine. Additionally, neonatal quinpirole produced a significant decrease in hippocampal NGF content compared to controls, however, nicotine failed to alleviate this decrease in NGF. The results of this investigation demonstrate that long-term increases in dopamine D(2) receptor sensitivity produce significant decreases in hippocampal cholinergic and NGF expression that may result in cognitive impairment. Nicotine alleviates both cognitive and skilled reaching impairments caused by increases in D(2) sensitivity, but the mechanism through which nicotine is acting is currently unknown.

Further evidence for the mediation of both subtypes of dopamine D1/D2 receptors and cerebral neuropeptide Y (NPY) in amphetamine-induced appetite suppression

Amphetamine (AMPH) is known as an anorectic agent. Repeated treatments of AMPH for several days induced a marked anorexia on day 1 followed by a gradual return of food intake to normal level. Previously, using daily food intake (DFI) as an indicator, we found that both dopamine (DA) D1 and D2 receptors were involved in AMPH anorexia. In the present study, using DFI and body weight change (BWC) as indicators, we took a further step to examine whether repeated co-administrations of D1 and D2 agonists might produce an AMPH-like action. Results revealed that repeated co-administrations of D1 and D2 agonists, but not D1 or D2 agonist alone, produced an AMPH-like action (i.e., tolerant DFI and BWC), confirming the previous study. In addition, we examined whether cerebral neuropeptide Y (NPY), an orexigenic neurotransmitter reported to mediate AMPH anorexia, was involved in the anorectic action of D1/D2 co-administration. Our result revealed that injection of NPY antisense into brain could modify the anorectic action of repeated D1/D2 agonists, indicating the involvement of NPY. Taken together, the present results confirmed that both subtypes of D1 and D2 receptors and cerebral NPY were involved in the anorectic action of AMPH.

Behavioral depression during cocaine withdrawal is associated with decreased spontaneous activity of ventral tegmental area dopamine neurons

Withdrawal from an escalating-dose, bingelike regimen of cocaine administration in rats produced significantly depressed levels of locomotor activity during the nocturnal portion of the day-night cycle. This effect was observed during the first 48 hr of testing. Extracellular single-unit recordings of ventral tegmental area (VTA) dopamine (DA) neurons revealed no differences between saline- and cocaine-treated rats with respect to basal firing rates. However, significantly fewer spontaneously active VTA DA neurons were encountered in rats withdrawn from binge cocaine. As with the nocturnal hypoactivity, this effect was observed only during the first 48 hr of withdrawal. These findings suggest that short-term DA neuron dysfunction during cocaine withdrawal temporally corresponds to behavioral disruptions that are similar to those described in human addicts.

Up-regulated dopamine D1 receptor binding can be detected in vivo following repeated SCH 23390, but not SKF 81297 or 6-hydroxydopamine, treatments

Three different pharmacological treatments, previously shown to cause dopamine D1 receptor supersensitivity in rats, were studied for changes in the binding of R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH 23390) labeled with carbon-11. Rats treated subchronically with the full dopamine D1 receptor agonist R/S-(+/-)-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SKF 81297) showed no significant difference in dopamine D1 receptor binding. Similarly, unilateral 6-hydroxydopamine lesioning, followed by apomorphine screening for contralateral rotation, failed to cause significant differences in the rat brain distribution of [11C]SCH 23390 in the lesioned versus the nonlesioned striatal sides. In contrast, repeated exposure with the dopamine D1 receptor antagonist SCH 23390 significantly enhanced the uptake of [11C]SCH 23390 in the dopamine D1 receptor-rich striatum and olfactory tubercles. These results demonstrate that [11C]SCH 23390 can significantly detect enhanced binding in rat brain regions expected to have up-regulated dopamine D1 receptors. The failure of [11C]SCH 23390 to reveal any differences after subchronic agonist or 6-hydroxydopamine treatments suggests that the behavioural supersensitization induced by these treatments is possibly due to changes to the high-affinity state or to components downstream of dopamine D1 receptors in the signal transduction pathway. The present study has implications for studies imaging dopamine D1 receptors in neuropsychiatric disorders with abnormal dopamine stimulation using positron emission tomography.

Co-administration of dopamine D1 and D2 agonists additively decreases daily food intake, body weight and hypothalamic neuropeptide Y level in rats

This study investigated whether co-administration of dopamine D1 and D2 agonists might additively inhibit the feeding effect and whether this effect was mediated by the action on hypothalamic neuropeptide Y (NPY). The D1 agonist SKF 38393 (SKF) and D2 agonists apomorphine (APO) or quinpirole (QNP) were administered, alone or in combination, to examine this possibility. In single administration, decreases of daily food intake were observed only in rats treated twice a day with a higher dose of SKF, APO or QNP. However, combined administration of D1 and D2 agonists, with each agent at a dose that alone did not induce anorexia in one daily treatment, exerted a significant effect. These results reveal that co-activation of D1 and D2 receptors can additively reduce daily food intake and body weight. The same treatment also decreased the level of hypothalamic NPY 24 h post-treatment. These results suggest an additive effect during combined activation of D1 and D2 receptor subtypes to decrease food intake and body weight that are mediated by the action of hypothalamic NPY. Similar to the effects seen in healthy rats, combined D1/D2 administration was also effective in the reduction of food intake in diabetic rats, revealing the efficiency of D1/D2 agonist in the improvement of hyperphasia in diabetic animals.

Amphetamine reinstates polydipsia induced by chronic exposure to quinpirole, a dopaminergic D2 agonist, in rats

The hypothesis that the combined activation of D1 and D2 dopaminergic receptors is instrumental in inducing amphetamine (AMPH)-mediated hyperdipsia was tested in rats. The D1 agonist SKF-38393 (SKF) and the D2 agonist quinpirole (QNP) were i.p. injected, alone or in combination, to male rats for 10 days. After 2 days of wash-out, a single dose of AMPH (3 mg/kg) was administered. Intake of water and food and diuresis were daily measured at 2, 5 and 24 h. In two further experiments the higher dose of QNP (0.56 mg/kg) was given with two different doses of the D1 antagonist SCH-23390 (SCH), or, respectively, of the peripheral D2 antagonist domperidone (DMP). In a fourth experiment, the possibility that QNP, given alone or in combination with SKF, produces an AMPH-like internal state was evaluated by using a drug-discrimination paradigm. Results show that chronic administration of QNP produced a significant increase of 24 h water intake that was reinstated by AMPH. This QNP effect was only partially prevented by DMP, suggesting a main central mechanism of action. By itself D1 receptor manipulation did not affect water intake, but influenced QNP polydipsia that, accordingly, was enhanced by the lower dose of SKF (0.3 mg/kg) and inhibited by the lower dose of SCH (0.01 mg/kg). In rats trained to discriminate AMPH from solvent, QNP partially generalized for the AMPH stimulus, an effect that was potentiated by SKF. In conclusion, a D1-modulated sensitization of D2 dopaminergic mechanisms is probably involved in AMPH-induced hyperdipsia.