Development of uncoupling between D1- and D2-mediated motor behavior in rats depleted of dopamine as neonates

Increased sensitivity in the interaction of the dopaminergic/adenosinergic system at the level of the adenylate cyclase activity in the striatum of the "weaver" mouse

The specific antagonistic interaction between dopamine D1 and adenosine A1 receptors (D1/A1), as well as between dopamine D2 and adenosine A2a receptors (D2/A2a) exist not only at the receptor/receptor level, but also at the level of the secondary messengers. In this study, we examined the possible changes in these interactions at the level of cAMP formation in membrane preparation from "weaver" mouse striatum (a genetic model of Parkinson disease), by using specific agonists of these receptors. We also examined in the striatum of the "weaver" mouse the interaction between D1 and D2 dopamine receptors. Our results showed that in the striatum of "weaver" mice: a) the cAMP synthesis induced by D1 receptor activation (SKF 38393), was significantly reduced compared to control mice, while A1 receptor activation (L-PIA) leaded to a more intense inhibition of the D1-induced cAMP-formation compared to the controls, b) the cAMP synthesis which was induced by A2a receptor activation (CGS 21680), was significantly increased compared to the control mice. The specific D2 receptor agonist Quinpirole, added in low concentrations, caused a significant reduction of the A2a-induced cAMP formation, which was not observed in the control mouse. Furthermore, the D1 receptor induced cAMP synthesis was significantly higher in control compared to "weaver" striatum, which was more efficiently downregulated by D2 receptor agonist Quinpirole. These results suggest that the sensitivity to D1 and A2a receptor agonists is altered and that the interaction between D1/A1 and D2/A2a receptors is enhanced in the striatum of the "weaver" mutation, while an uncoupling between D1 and D2 receptors was observed. Since the adenylate cyclase basal activity did not differ between "weaver" and control striatum, the above-mentioned changes seem to be due to alterations in the function of the adenosine/dopamine receptors and their coupling to the G-proteins.

Importance of D1 and D2 receptors in the dorsal caudate-putamen for the locomotor activity and stereotyped behaviors of preweanling rats

Dopaminergic compounds often affect the unlearned behaviors of preweanling and adult rats differently, although the brain regions underlying these age-dependent behavioral effects have not been specified. A candidate brain region is the dorsal caudate-putamen (CPu); thus, a goal of the present study was to determine whether D1 and D2 receptors in the dorsal CPu are capable of modulating the unlearned behaviors of preweanling rats. In Experiments 1 and 2, selective and nonselective dopamine agonists were bilaterally microinjected into the dorsal CPu on postnatal day (PD) 18 and both locomotor activity and stereotypy were measured. In Experiment 3, the functional coupling of D1 and D2 receptors was assessed by microinjecting the D1 agonist SKF-82958 and the D₂/D₃ agonist quinpirole either alone or in combination. In Experiments 4 and 5, quinpirole and the D1 receptor antagonist SCH-23390, or SKF-82958 and the D2 receptor antagonist raclopride, were co-administered into the dorsal CPu to further assess whether a functional D1 or D2 receptor system is necessary for the expression of quinpirole- or SKF-82958-induced behaviors. Results showed that selective stimulation of D1 or D2 receptors in the dorsal CPu increased both the locomotor activity and stereotypy of preweanling rats. Receptor coupling was evident on PD 18 because co-administration of a subthreshold dose of SKF-82958 and quinpirole produced more locomotor activity than either agonist alone. Lastly, the dopamine antagonist experiments showed that both D1 and D2 receptor systems must be functional for SKF-82958- or quinpirole-induced locomotor activity to be fully manifested. When the present data are compared to results from non-ontogenetic studies, it appears that pharmacological manipulation of D1 and D2 receptors in the dorsal CPu affects the behavior of preweanling and adult rats in a generally similar manner, although some important age-dependent differences are apparent. For example, D1 and/or D2 agonists preferentially induce locomotor activity, and not intense stereotypy, in younger animals.

Timing: A critical determinant of the functional consequences of neonatal 6-OHDA lesions

Previous data have indicated that intrastriatal (IS) lesions of the dopamine (DA) system early in development result in a selective effect on D1 receptor expression and sensitivity, which is not seen with adult lesions or lesions made later in development. The purpose of the present study was to test the hypothesis that the timing of the lesion is a critical determinant of the consequences of DA depletion during development. Rats received IS injections of 6-hydroxydopamine (6-OHDA) on day of birth/postnatal day 1 (P0/1) or P7, which resulted in similar decreases in the number of DA uptake sites (> or =70% loss), a measure of DA terminal density. As adults, lesioned rats were challenged with DA receptor agonists to examine the functional sensitivity of D1 and D2 receptors. In adulthood, P0/1-lesioned rats exhibited increases in oral dyskinesias and rearing behavior following treatment with the partial D1 receptor agonists, SKF38393 and SKF77434, whereas rats lesioned on P7 exhibited increases in grooming. P7-lesioned rats also exhibited increases in gnawing, explosive jumping, and self-biting behavior following treatment with the full D1 receptor agonist SKF82958, which were not observed in the other groups. The results support the hypothesis that the timing of DA denervation is of paramount importance for governing the functional consequences of neonatal lesions, as measured by the incidence of DA agonist-induced behaviors in adulthood.

Developmental plasticity in the D1- and D2-mediation of motor behavior in rats depleted of dopamine as neonates

D1- and D2-like antagonist-induced catalepsy and dorsal immobility were studied in pups (Day 10) and weanlings (Days 20, 28, or 35) that received intraventricular injection of 6-OHDA (50 micrograms/hemisphere) or its vehicle solution or postnatal Day 3. The ability of the D1 of D2 antagonists to induce immobility differed as a function of the lesion condition and the age at the time of testing. Moreover, the two behavioral measures exhibited differences in their specific D1 and D2 receptor modulation. Administration of the D1 antagonist SCH 23390 (0.2 or 1.0 mg/kg) or the D2 antagonist clebopride (1.0, 10.0, or 20.0 mg/kg) led to catalepsy and dorsal immobility in intact rats, regardless of test age. Both antagonists induced catalepsy and dorsal immobility in rats depleted of DA when tested on Day 10. However, the effects of each antagonist in DA-depleted rats were ether negligible or significantly less than in controls when animals were tested as weanlings. These data suggest lesion-induced changes in the DA receptor modulation of motor behavior and that this plasticity requires more than a week to become apparent.

Separation of activation and pattern in grooming development of weaver mice

The effects of environmental conditions and age on grooming behavior were examined in weaver mutant mice and control littermates. Due to deficits in both the cerebellum and the dopaminergic system, weaver mice provide an opportunity to investigate how both of these systems are involved in grooming. Although homozygous weaver (wv/wv mice display deficiencies in grooming behavior, our results indicate that these effects are both context and age dependent. Overall wv/wv mice spent less time grooming than did controls. However, during the post-swim period wv/wv, after day 13, reached the grooming levels of pre-swim control mice. After day 15 wv/wv mice showed a higher number of post-swim grooming bouts relative to pre-swim, and in fact exceeded the number of bouts performed by controls in either pre- or post-swim conditions. Although controls displayed longer bouts than mutants overall, during the post-swim period wv/wv mice, after day 13, produced bouts as long as the control animals did pre-swim. This could in part reflect activation by previous swimming. Our data indicate these activational effects can be separated from balance or posture problems. From examination of the individual grooming stroke types used by the two groups, it is evident that the strokes used by mutant animals clustered around the early grooming sequence phase. In contrast, some of the later strokes were never used by the wv/wv mice during the entire developmental period studied. Our results emphasize the importance of using multiple measures of an action sequence and testing under different conditions.

Functional and molecular differentiation of the dopamine system induced by neonatal denervation

The administration of the neurotoxin 6-hydroxydopamine (6-OHDA) to damage the mesostriatal dopamine (DA) system in the neonate results in different neurochemical and behavioral consequences as compared to lesions made in adulthood. There have been few direct data to support the conclusion that the behavioral changes following neonatal 6-OHDA lesions reflect plasticity of the DA system. It is our hypothesis that the plasticity of the developing DA system is fundamentally different from that of the adult. Responses to 6-OHDA lesions can only be understood within the context of the status of the mesostriatal DA system at the time of the lesion. There are stages of development in the early postnatal period when certain components of the mesostriatal DA system are differentially sensitive to 6-OHDA lesions. These "windows" of vulnerability can be predicted from an analysis of the developmental expression of DA receptors and the maturation of the subpopulation of the mesostriatal DA system that innervates them. We review the differences in the behavioral plasticity of the adult and neonate sustaining 6-OHDA lesions to the mesostriatal DA system, the mechanisms responsible for the behavioral plasticity in the adult, and our conceptualization of which mechanisms are affected in the neonate.

Prenatal exposure to cocaine selectively disrupts motor responding to D-amphetamine in young and mature rabbits

Acute administration of D-amphetamine probed the functional effects of prenatal exposure to cocaine on the integrity of monoaminergic systems in preweanling (48-56 days old) and adult (> or = 140 days old) Dutch belted rabbits. D-Amphetamine sulfate (0, 0.3, 1.0, 3.3 and 6.0 mg/kg, s.c.) produced equivalent dose-related reductions in food intake in 180 day-old rabbits that had been exposed in utero on gestational days 8-29 to cocaine or saline. Intrauterine exposure to cocaine also did not alter the incidence of exploratory behaviors stimulated by D-amphetamine during the anorexia test. In contrast, however, prenatal cocaine virtually eliminated stereotyped head bobbing elicited by the highest dose of D-amphetamine. When responses to 5.0 mg/kg D-amphetamine were measured during a 90-min open field test, prenatal cocaine prevented head bobbing in preweanling rabbits and reduced this behavior by 92% in 140 day-old adults. Prenatal cocaine also diminished the intensity of other motor responses in the open field in the adults but not in preweanlings. In normal rabbits, the D1 antagonist R(+)-SCH 23390 (0.01 mg/kg, s.c.) blocked D-amphetamine-induced head bobbing. Thus, prenatal exposure to cocaine produces an early and persistent deficit in behavioral responding to a high dose of D-amphetamine. The deficit is especially selective at the time of weaning, broadens to affect more motor behaviors with maturation and may reveal impaired D,-mediated dopaminergic neurotransmission in the brain.