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.

There is a limited critical period for dopamine's effects on D1 receptor expression in the developing rat neostriatum.

Neonatal lesions of the dopamine (DA) system have different behavioral and neurochemical effects than lesions made in adulthood. Previous data from this laboratory have indicated that in the early postnatal period, lesions to the DA system induced by instrastriatal 6-hydroxydopamine (6-OHDA) result in a rapid and permanent loss of striatal D1 binding sites, but D2 receptor binding is unaffected. The length of the postnatal period within which neonatal instrastriatal 6-OHDA administration is effective in modulating D1 receptor binding is not known. To determine when D1 and D2 receptors are vulnerable to lesions of the DA system, we administered 6-OHDA intrastriatally to damage the DA innervation at different ages in the early postnatal period, at day of birth/postnatal day 1 (P0/1), P7 or P15 and examined DA receptor binding at P90 with quantitative autoradiography. Using [3H]mazindol binding to DA transporters (DAT) to verify the extent of the lesion, we then quantified the number of D1 binding sites using [3H]SCH23390 and D2 sites with [3H]spiroperidol. There were significant reductions in DAT sites at P0/1 (78 to 88%) and P7 (67 to 81%) but less significant changes at P15 (34 to 50% losses). The lesions were most effective for the dorsal caudate-putamen than more ventrally or in the nucleus accumbens. Our results demonstrate a significant reduction in D1 sites in all regions of the neostriatum following lesions at P0/1. The dorsal caudate-putamen was affected the most (51% loss, and the nucleus accumbens (41%) and ventral caudate-putamen less so (31%). No significant changes in D1 receptors were found at P7 or P15 and D2 receptors were unaffected with lesions in any of the age groups. The results indicate that there is a critical period for affecting expression of D1 receptors and this effect may, in addition, be related to the pattern of DA loss. Additionally, regulation of D2 receptors by this degree of loss of DA innervation does not occur during the first two weeks postnatally.

Behavioral responsitivity to dopamine receptor agonists after extensive striatal dopamine lesions during development.

Dopamine (DA) receptor responsitivity was investigated in adult rats that received intrastriatal (i.s.) injections of 6-OHDA (20 micrograms per striatum) on day of birth or postnatal Day 1 (Day 0/Day 1). Neonatally lesioned rats exhibited self-biting behavior and increases in stereotypic gnawing following treatment with the mixed D1/D2 receptor agonist apomorphine (0.32-3.2 mg/kg) or the D1-like receptor agonist SKF38393 (10 mg/kg). Increases in locomotor activity, rearing, and paw treading were also observed in the lesioned rats after SKF38393 (1-10 mg/kg) treatment. The incidences of the prototypical D1 receptor-mediated behaviors, grooming and abnormal perioral movements (i.e., oral dyskinesias) were not increased in the lesioned rats. However, the low dose (0.32 mg/kg) of apomorphien as well as all doses of the D2-like receptor agonist quinpirole (0.32-3.2 mg/kg) induced grooming in the lesioned rats, which was not observed in nonlesioned control rats. Autoradiographs of [3H]mazindol binding to high affinity DA uptake sites revealed an extensive loss of DA terminals in the striata of the neonatally lesioned rats. These data suggest that near-total (> or = 95%) DA depletions on Day 0/Day 1 result in long-term alterations in the functional sensitivity of DA receptors, as well as possible changes in the interactions between D1 and D2 receptors. Comparisons of these results with those seen following lesions of the early-developing DA system ("patch-selective" lesions) and lesions made at other time points will be discussed.

Delayed plasticity of the mesolimbic dopamine system following neonatal 6-OHDA lesions.

In this study, we determined the ontogenetic profile (at postnatal days 7, 14, 35, and 90) of tyrodine hydroxylase (TH) mRNA in the ventral mesencephalon, and the levels of TH immunoreactivity (TH-IR) and dopamine (DA) transporter (DAT) sites in the striatum of rats that had received intrastriatal 6-hydroxy dopamine (6-OHDA) or vehicle lesions on day of birth (DO) or postnatal day 1 (P1). TH-IR was significantly decreased in all quadrants of the caudate-putamen at all time points, while TH-IR in the nucleus accumbens was unchanged, as compared to controls. Relative to the earliest time point (P7 lesion group), TH-IR recovered significantly in the medial caudate-putamen (CPu) of the P14, P35 and P90 6-OHDA-lesioned groups. Quantitative autoradiography of [3H]-mazindol binding to DAT sites showed significant, lesion-induced losses throughout the caudate-putamen of the 6-OHDA-lesioned groups at all time points and did not show appreciable recovery. Using in situ hybridization, significant (P < .05) decreases in TH mRNA levels were found at all time points in the lateral and medial substantia nigra pars compacta of 6-OHDA-lesioned animals. TH mRNA levels in the rostral ventral tegmental area (VTA), which were significantly decreased at P7, P14 and P35, returned to control levels at P90. TH mRNA levels in the caudal VTA were unchanged through P35 and became significantly elevated as compared to controls (+22%, P < .05) by P90. Thus, recovery of TH-IR in the medial caudate-putamen occurred prior to the elevation in levels of TH mRNA of the VTA. Our findings suggest that compensation exists in early development in certain subpopulations of mesostriatal DA neurons that differs from that in the adult.

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.

Neonatal 6-hydroxydopamine lesions lead to opposing changes in the levels of dopamine receptors and their messenger RNAs.

Previous studies have established that selective damage to the early-developing components of the mesostriatal dopamine system produces profound changes in dopamine D1 receptor-mediated behaviors, while decreasing D1 receptor density. In order to better understand the effects of early intrastriatal 6-hydroxydopamine lesions, we studied the ontogenetic expression (postnatal days 7, 14, 35 and 90) of D1 and D2 receptors, and their corresponding messenger RNAs, in rats that had received intrastriatal 6-hydroxydopamine or vehicle lesions on postnatal day 1. Using receptor autoradiography, significant (P < 0.05) decreases in [3H]SCH 23390 binding to D1 receptors in the rostral and caudal dorsomedial and ventromedial caudate of 6-hydroxydopamine-lesioned animals were evident by postnatal day 7, and remained depressed at all future time points. A significant decrease in D1 receptor concentration occurred in the dorsolateral caudate at later time points (postnatal days 35 and 90). [3H]Spiperone binding to D2 receptor sites was unchanged throughout the entire study. In situ hybridization for D1 and D2 messenger RNA expression showed contrasting results. 6-Hydroxydopamine induced significant decreases of D1 messenger RNA levels in the dorsolateral and dorsomedial caudate by postnatal day 7. By postnatal day 14, messenger RNA expression was significantly elevated in the dorsomedial and ventromedial caudate of the 6-hydroxydopamine group, and remained elevated thereafter. D1 messenger RNA levels became elevated in the lateral caudate at later time points (postnatal days 35 and 90). The opposing changes in D1 receptor concentrations and the messenger RNA encoding the protein did not occur as a consequence of increased transport of D1 receptors to striatonigral terminals. D2 messenger RNA levels in the dorsal caudate were significantly decreased on postnatal day 7, and became higher than controls at postnatal day 14, but were unchanged from controls at later time points. Together, the D1 receptor and D1 messenger RNA findings suggest that the normal relationship between levels of D1 receptor transcript and D1 receptor protein is permanently altered following the early loss of dopamine. In contrast, the results indicate that dopamine plays a minor role in the early postnatal development of the D2 receptor protein and transcript. These findings suggest that dopamine may be involved in the coordinated expression of some dopamine receptors and their corresponding messenger RNAs during development.

D1 and D2 dopamine receptors do not up-regulate in response to neonatal intrastriatal 6-hydroxydopamine lesions.

The extent of dopamine (DA) depletion appears to exert important influences on the plasticity of the DA system following lesions made in adulthood. The aim of this study was to determine if the extent of DA depletion has long-term effects on DA receptor regulation after early neonatal lesions. Early intrastriatal injections of 6-hydroxydopamine (6-OHDA) caused a dose-dependent loss of high-affinity DA uptake sites and mu-opioid receptor patches evident in adulthood. DA receptors did not up-regulate in response to any degree of neonatal DA depletion. A patchy loss of D1 binding was evident following the neonatal lesions, although the loss was somewhat more severe and uniform with the highest dose of 6-OHDA (20 micrograms per striatum). There was also a slight decrease in D2 binding which was not dose-dependent. These results suggest that the consequences of early neonatal DA lesions are not dependent upon the degree of DA depletion, as the effects on DA receptor expression were similar regardless of the extent of the lesions.

Neonatal 6-OHDA lesions upregulate adult expression of tyrosine hydroxylase mRNA.

We explored the consequences of postnatal day 1 (P1) intrastriatal 6-hydroxydopamine (6-OHDA) lesions on the developing mesostriatal dopamine system by performing in situ hybridization histochemistry for tyrosine hydroxylase (TH) in the substantia nigra-ventral tegmental area (SN-VTA) complex of neonatally-lesioned and non-lesioned animals at P90. There were significant reductions in the expression of TH mRNA from both the rostral and caudal levels of the lateral and medial SNc of 6-OHDA treated animals. The VTA was preferentially spared from the neurotoxic effects of 6-OHDA and showed elevated levels of TH mRNA in the caudal VTA compared with controls. This heterogeneous pattern of TH mRNA expression suggests that early intrastriatal 6-OHDA lesions selectively damage certain DA systems and affect the development of other systems.

Serotonergic involvement in haloperidol-induced catalepsy.

The ability of serotonin (5-HT)-selective compounds to reverse catalepsy due to blockade of dopamine (DA) receptors was examined in rats treated with the antipsychotic haloperidol (HAL). The 5-HT-releasing drug fenfluramine significantly reversed HAL-induced catalepsy. The 5-HT1A receptor agonists 8-hydroxy-2-(di-n-propylamino)tetralin and buspirone potently and dose-dependently reversed HAL-induced catalepsy. The effect of 8-hydroxy-2-(di-n-propylamino)tetralin was blocked by pretreatment with the 5-HT1A/1B receptor antagonist pindolol, but not the 5-HT2 receptor antagonist ketanserin. The 5-HT1C/2 receptor agonist 2,5-dimethoxy-4-bromoamphetamine also completely reversed HAL-induced catalepsy, an effect blocked by ketanserin, but not pindolol. Neither antagonist alone had any effect. The 5-HT1B/1C receptor agonist trifluoromethylphenylpiperazine only partially reversed HAL-induced catalepsy, and the effect was not dose dependent. The 5-HT1C/2 receptor antagonist mianserin reversed HAL-induced catalepsy on its own; therefore, its ability to block the effect of trifluoromethylphenylpiperazine could not be tested. The nature of the disruption of HAL-induced catalepsy was examined by measuring the ability of increasing doses of HAL to surmount the effects of the serotonergic agonists. The mixed DA-D1/D2 receptor agonist apomorphine shifted the dose-effect curve for HAL to the right in a parallel manner, indicative of a competitive interaction between HAL and apomorphine at the D2 receptor. In contrast, the 5-HT receptor agonists flattened the dose-effect curve for HAL, suggestive of noncompetitive interactions. These data suggest that the 5-HT receptor agonists are not reversing HAL-induced catalepsy by indirectly increasing DA release. Rather, the agonists reverse HAL-induced catalepsy through interactions at their specific 5-HT receptor subtypes. Thus, the 5-HT receptor agonists may provide novel approaches for the development of drugs which can reverse or prevent the extrapyramidal side effects associated with antipsychotic treatment.