Age-dependent neurobehavioral plasticity following forebrain dopamine depletions

Preadolescent dopamine receptor antagonism increases postadolescent reward-related operant behaviors that may depend on dopamine receptor hypersensitivity

The dopaminergic system has a long history of being associated with reward-related activities but the developmental consequences of blocking dopamine receptor function on reward-based associative learning has been less studied. To this end, male, Long Evans rats were systemically (i.p.) treated with the dopamine receptor (DAr) antagonist, flupenthixol (0.25 mg/kg), or saline, from postnatal day (P)18 - 24 (preadolescence) then trained on an operant conditioning task from P41 - P45 (postadolescent) without drug treatment. The preadolescent flupenthixol group showed elevated active lever responses and locomotor activity during the drug-free test. Another group of rats was given flupenthixol prior to each acquisition session from P41 - 45 which significantly suppressed both active lever presses and locomotor activity. Separate groups of rats were treated with flupenthixol or saline from P18 - 24 then treated with apomorphine or saline on P41 followed by assessment of c-Fos labeling in the nucleus accumbens. Early flupenthixol treatment was associated with more apomorphine-induced c-Fos labeling in the nucleus accumbens shell than the early saline-apomorphine group, indicating a sensitized response. These findings suggest that preadolescent dopamine receptor blockade may lead to a sensitized postadolescent dopaminergic response that underlies enhanced behavioral responses in the presence of rewarding stimuli.

Functional analysis of dopaminergic systems in a DYT1 knock-in mouse model of dystonia

The dystonias are a group of disorders characterized by involuntary twisting movements and abnormal posturing. The most common of the inherited dystonias is DYT1 dystonia, which is due to deletion of a single GAG codon (ΔE) in the TOR1A gene that encodes torsinA. Since some forms of dystonia have been linked with dysfunction of brain dopamine pathways, the integrity of these pathways was explored in a knock-in mouse model of DYT1 dystonia. In DYT1(ΔE) knock-in mice, neurochemical measures revealed only small changes in the content of dopamine or its metabolites in tissue homogenates from caudoputamen or midbrain, but microdialysis studies revealed robust decreases in baseline and amphetamine-stimulated extracellular dopamine in the caudoputamen. Quantitative stereological methods revealed no evidence for striatal or midbrain atrophy, but substantia nigra neurons immunopositive for tyrosine hydroxylase were slightly reduced in numbers and enlarged in size. Behavioral studies revealed subtle abnormalities in gross motor activity and motor coordination without overt dystonia. Neuropharmacological challenges of dopamine systems revealed normal behavioral responses to amphetamine and a minor increase in sensitivity to haloperidol. These results demonstrate that this DYT1(ΔE) knock-in mouse model of dystonia harbors neurochemical and structural changes of the dopamine pathways, as well as motor abnormalities.

Dopamine-dependent periadolescent maturation of corticostriatal functional connectivity in mouse

Altered corticostriatal information processing associated with early dopamine systems dysfunction may contribute to attention deficit/hyperactivity disorder (ADHD). Mice with neonatal dopamine-depleting lesions exhibit hyperactivity that wanes after puberty and is reduced by psychostimulants, reminiscent of some aspects of ADHD. To assess whether the maturation of corticostriatal functional connectivity is altered by early dopamine depletion, we examined preadolescent and postadolescent urethane-anesthetized mice with or without dopamine-depleting lesions. Specifically, we assessed (1) synchronization between striatal neuron discharges and oscillations in frontal cortex field potentials and (2) striatal neuron responses to frontal cortex stimulation. In adult control mice striatal neurons were less spontaneously active, less responsive to cortical stimulation, and more temporally tuned to cortical rhythms than in infants. Striatal neurons from hyperlocomotor mice required more current to respond to cortical input and were less phase locked to ongoing oscillations, resulting in fewer neurons responding to refined cortical commands. By adulthood some electrophysiological deficits waned together with hyperlocomotion, but striatal spontaneous activity remained substantially elevated. Moreover, dopamine-depleted animals showing normal locomotor scores exhibited normal corticostriatal synchronization, suggesting that the lesion allows, but is not sufficient, for the emergence of corticostriatal changes and hyperactivity. Although amphetamine normalized corticostriatal tuning in hyperlocomotor mice, it reduced horizontal activity in dopamine-depleted animals regardless of their locomotor phenotype, suggesting that amphetamine modified locomotion through a parallel mechanism, rather than that modified by dopamine depletion. In summary, functional maturation of striatal activity continues after infancy, and early dopamine depletion delays the maturation of core functional capacities of the corticostriatal system.

Ontogeny and dopaminergic regulation in brain of Ras homolog enriched in striatum (Rhes)

Rhes is one of several signaling molecules preferentially expressed in the striatum. This GTP-binding protein affects dopamine-mediated signaling and behavior. Denervating the striatum of its dopaminergic inputs in adulthood reduces rhes mRNA expression. Here we show that dopamine depletion in adult rats by 6-hydroxydopamine caused a significant decrease in striatal Rhes protein levels as measured by Western blotting. The role of dopamine input on rhes mRNA induction during ontogeny was also examined. Rhes mRNA was measured on postnatal days 4, 6, 8, 10, 15, and 24 with in situ hybridization to determine its normal ontogeny. Signal in striatum was detectable, but very low, on postnatal day 4 and increased gradually to peak levels at days 15 and 24. Outside of the striatum, rhes mRNA was expressed at high levels in hippocampus and cerebellum during the postnatal period. Hippocampal signal was initially highest in CA3 and dentate gyrus, but shifted to higher expression in CA1 by the late postnatal period. Several other nuclei showed low levels of rhes mRNA during ontogeny. Depletion of dopamine by 6-hydroxydopamine injection on postnatal day 4 did not affect the ontogenetic development of rhes mRNA, such that expression did not differ statistically in lesioned versus vehicle-treated animals tested in adulthood. These findings suggest that although dopamine input is not necessary for the ontogenetic development of rhes mRNA expression, changes in both rhes mRNA and Rhes protein are integral components of the response of the adult striatum to dopamine depletion.

Commentary: Dopaminergic dysfunction in DYT1 dystonia

A three-base-pair deletion in the torsinA gene leads to generalized torsion dystonia (DYT1) in humans, an often devastating movement disorder in which voluntary movements are disrupted by sustained muscle spasms and abnormal limb posturing. In a recent issue of Experimental Neurology, Zhao et al. (2008) have provided a thorough behavioral, anatomic, and biochemical characterization of a mouse line that over-expresses human mutant torsinA, with particular emphasis on the possible role of dopaminergic dysfunction in these animals. This commentary provides an overview of the clinical and genetic features of the human disease and of the available transgenic mouse models for DYT1 dystonia, and discusses the evidence favoring the role of dopamine in the clinical manifestations of the disease.

Neonatal dopamine depletion induces changes in morphogenesis and gene expression in the developing cortex

The mesocorticolimbic dopamine (DA) system is implicated in mental health disorders affecting attention, impulse inhibition and other cognitive functions. It has also been involved in the regulation of cortical morphogenesis. The present study uses focal injections of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle of BALB/c mice to examine morphological, behavioral and transcriptional responses to selective DA deficit in the fronto-parietal cortex. Mice that received injections of 6-OHDA on postnatal day 1 (PND1) showed reduction in DA levels in their cortices at PND7. Histological analysis at PND120 revealed increased fronto-cortical width, but decreased width of somatosensory parietal cortex. Open field object recognition suggested impaired response inhibition in adult mice after 6-OHDA treatment. Transcriptional analyses using 17K mouse microarrays showed that such lesions caused up-regulation of 100 genes in the cortex at PND7. Notably, among these genes are Sema3A which plays a repulsive role in axonal guidance, RhoD which inhibits dendritic growth and tubulin beta-5 microtubule subunit. In contrast, 127 genes were down-regulated, including CCT-epsilon and CCT-zeta that play roles in actin and tubulin folding. Thus, neonatal DA depletion affects transcripts involved in control of cytoskeletal formation and pathway finding, instrumental for normal differentiation and synaptogenesis. The observed gene expression changes are consistent with histological cortical and behavioral impairments in the adult mice treated with 6-OHDA on PND1. Our results point towards specific molecular targets that might be involved in disease process mediated by altered developmental DA regulation.

Neuroanatomical localization of an internal clock: A functional link between mesolimbic, nigrostriatal, and mesocortical dopaminergic systems

The effects of selective dopamine (DA) depleting lesions with 6-hydroxydopamine microinjection into the SN, CPu, and NAS, as well as radiofrequency lesions of the CPu on the performance characteristics of rats trained on a single-valued 20-s peak-interval (PI) timing procedure or a double-valued 10-s and 60-s PI procedure were evaluated. A double dissociation in the performance of duration discriminations was found. Rats with CPu lesions were unable to exhibit temporal control of their behavior suggesting complete insensitivity to signal duration but were able to show discrimination of the relative reward value of a signal by differentially modifying their response rates appropriately. In contrast, rats with NAS lesions were able to exhibit temporal control of their behavior by differentially modifying their response rates as a function of signal duration(s), suggesting no impairment of sensitivity to signal duration, but were unable to show discrimination of the relative reward value of a signal.

Neurochemical correlates of sparing from motor deficits in rats depleted of striatal dopamine as weanlings

The behavioral and neurochemical effects of striatal DA depletions were investigated in rats lesioned as weanlings (Day 27) or as adults (250-300 g). Administration of 6-OHDA into the medial forebrain bundle resulted in comparably large (> or = 95%) depletions of tissue levels of DA in both age groups. As expected, rats depleted of DA as adults exhibited marked deficits in motoric behavior and body weight regulation that persisted for the 8 days of postsurgical observation. In contrast, rats depleted of DA as weanlings were spared from such deficits, and their behavior closely resembled that of age-matched controls. Microdialysis studies revealed dialysate levels of striatal DA that paralleled these age-dependent behavioral differences. At a time when age-related behavioral differences were still quite pronounced (5-6 days postsurgery), basal DA levels were reduced by 80% of control values in rats lesioned as adults whereas basal DA levels in rats lesioned as weanlings were unchanged relative to their controls. Finally, adults depleted of striatal DA as weanlings were no more sensitive to the movement-impairing effects of intrastriatal sulpiride (3.0 or 10.0 micrograms/hemisphere) infusions than were control rats. These data suggest that weanlings compensate for large, but incomplete, denervation of striatal DA with markedly enhanced release and turnover from residual terminals. This developmental plasticity may prevent the occurrence of behavioral deficits soon after the lesion and also the supersensitivity to the challenging effects of DA antagonists as animals grow into adulthood.

Neonatal 6-hydroxydopamine administration to mice is fatal

Depletion of dopamine in adult rats by treatment with the neurotoxin 6-hydroxydopamine (6-OHDA) causes severe deficits in feeding, drinking, and movement that often lead to death. However, when neonatal rats are treated similarly, they survive normally, suggesting that compensatory adaptation to dopamine depletion occurs. In contrast, dopamine-deficient mice that have a selective genetic deficiency in dopamine production die 2-4 weeks after birth. Thus, we tested the hypothesis that killing dopaminergic neurons with 6-OHDA might promote survival of dopamine-deficient mice. Body weights, motor coordination, catecholamine levels, and survival were monitored for several weeks after bilateral administrations of 6-OHDA to 3-day-old mice. Some treated mice were raised in a heated chamber to help them conserve energy. The results demonstrate that regardless of genotype or environmental temperature, bilateral neonatal 6-OHDA lesions are lethal to mice.