Dopamine-depleted Rats Research Articles

D1/D2 DOPAMINE RECEPTOR STIMULATION BY L-DOPA: A[14C]-2-DEOXYGLUCOSE AUTORADIOGRAPHIC STUDY

In rats with unilateral 6-hydroxydopamine substantia nigra lesions, the effects of selective D1 and D2 dopamine receptor antagonists on L-DOPA-induced rotation and regional cerebral glucose utilization (RCGU) changes were examined. Contralateral rotation induced by L-DOPA (25 mg/kg) was effectively blocked by D1 (SCH 23390, 1.0 mg/kg) and D2 (eticlopride, 2.0 mg/kg) antagonists, in combination, but not by either antagonist alone. This suggests that in the dopamine-depleted rat, L-DOPA administration results in the stimulation of both D1 and D2 receptor systems, each capable of independently eliciting a full motor response, L-DOPA altered RCGU in the following brain regions ipsilateral to the lesion: entopeduncular nucleus (EP, + 105%), substantia nigra pars reticulata (SNr, + 121%), subthalamic nucleus (STN, + 32%), deep layers of the superior colliculus (DLSC, + 35%), and lateral habenula nucleus (LHN, -52%). The effects in the EP and SNr were blocked completely by D1 antagonist pretreatment but only partially attenuated by D2 antagonist pretreatment, indicating the critical dependence of these changes on D1 stimulation. In contrast, combined D1 and D2 antagonist pretreatment, but neither drug alone, blocked the L-DOPA-induced increases in the STN and DLSC. The effects of L-DOPA in the LHN were attenuated by either SCH 23390 or eticlopride, and blocked completely by the antagonist combination. These results provide evidence that dopamine formed following the decarboxylation of L-DOPA stimulates both D1 and D2 receptors in vivo and that stimulation of each receptor contributes uniquely to its physiological effects. Neural mechanisms of action of L-DOPA are discussed in the context of these findings.

Characterization of the impaired feeding behavior in rats given haloperidol or dopamine-depleting brain lesions

Systemic administration of 0.2 mg/kg of the dopamine antagonist haloperidol reduced the rate of feeding in food-deprived rats, whereas a dose of 0.4 mg/kg suppressed both the rate of feeding and time spent feeding. Pre-feeding a group of rats decreased the time spent feeding but had minimal effects on the rate of feeding. Regression analysis showed that food intake, time spent feeding, and rate of feeding were related to each other in different ways in haloperidol-treated and pre-fed rats. Near-total depletion of brain dopamine by injection of the neurotoxin 6-hydroxydopamine into the medial forebrain bundle initially decreased both time spent feeding and feeding rate. When rats that were initially aphagic after dopamine depletion recovered feeding behavior, their rate of feeding did not return to control values, and they spent more time feeding than control rats. Similarly, 6-hydroxydopamine-treated rats with smaller dopamine depletions, which were not initially aphagic after the lesions, nevertheless had a prolonged reduction in the rate of feeding. Although control rats typically held the food pellets in both forepaws while feeding, dopamine-depleted rats often fed with one or no paws holding the pellet. The suppression of food intake and rate of feeding in 6-hydroxydopamine-treated rats was correlated with the depletion of dopamine in the lateral striatum, but not the medial striatum. These results indicate that dopamine in the striatum is involved in aspects of motor control that are necessary for efficient feeding behavior in rats.

Paradoxical Kinesia in Parkinsonism Is Not Caused by Dopamine Release

Rats become akinetic after large dopamine-depleting brain lesions, yet they show an activation-induced restoration of motor function. In this study, rats were given intraventricular injections of the neurotoxin 6-hydroxydopamine to permanently reduce the dopamine content of the corpus striatum by 98%. Although the rats were akinetic in their home cages, they swam effectively when placed in deep water and escaped from a shallow floating ice bath. These behaviors were not abolished by pretreating the animals with the dopamine antagonists haloperidol and SCH-23390. In contrast, haloperidol completely blocked the brain-damaged animals' behavioral responses to amphetamine. These results suggest that the paradoxical kinesia of dopamine-depleted rats is not a consequence of dopamine release from residual dopaminergic fibers.

Neonatal dopamine-rich grafts and 6-OHDA lesions independently provide partial protection from the adult nigrostriatal lesion syndrome

Previous studies have shown that neonatally dopamine-depleted rats are subsensitive to dopamine antagonists and do not respond to homeostatic imbalances as adults. This suggests that these animals maintain themselves independent of the dopamine system. If this is so, they should be insensitive to treatment with adult 6-hydroxydopamine (6-OHDA) lesions. Other experiments have shown that dopamine-rich grafts in the neonatal brain will provide some protection from the severe ingestive deficits induced by bilateral 6-OHDA lesions in adulthood. Three groups of animals received either nigra grafts into the intact neonatal brain, neonatal 6-OHDA lesions, or both neonatal 6-OHDA lesions and nigra grafts. A fourth group served as sham-operated controls. Methylamphetamine and haloperidol challenges showed that the neonatally lesioned animals regulated locomotor activity, eating and drinking independent of the dopamine system. Remarkably, however, 80% of these nevertheless showed the full syndrome of aphagia, adipsia and akinesia in response to adult lesions. The grafts into intact group showed enhanced survival in that 36% of the rats were able to maintain themselves following the adult lesion. The graft into neonatally lesioned rats restored their activational response to pharmacological challenges but did not provide any additional protection from the adult lesion. This suggests that different mechanisms underlie the protection against adult nigrostriatal lesions provided by neonatal grafts and neonatal lesions.

Central depletion of dopamine in rats by 1-methyl-4-phenylpyridine

Effects of 1-methyl-4-phenylpyridine (MPP +), a putative neurotoxic metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), on the contents of dopamine were examined in the various regions of the rat brain. Under anesthesia with pentobarbital sodium and flunitrazepam, MPP + 150 μg/rat was intracerebroventricularly infused for 5 hours, at 30 μg/ 100 μ1/hr. Seven days later, the contents of dopamine, but not those of noradrenaline and activities of choline acetyl transferase in the brain were found to be significantly decreased, as compared to findings in the respective controls. The MPP +-induced depletion of dopamine was most evident in the striatum (38% of control). Contents of dopamine in the substantia nigra and ventral tegmental area were not significantly affected by MPP +. These results are interpreted to mean that intracerebroventricular continuous infusion of MPP +, in a relatively low concentration, induces a moderate but relatively specific disruption of central dopaminergic nerve terminals in rats, presumably by the selective accumulation of this neurotoxic agent into these nerve terminals.

Development of dopamine innervation and turning behavior in dopamine-depleted infant rats receiving unilateral nigral transplants

Three-day-old rats were bilaterally dopamine-depleted with 6-hydroxydopamine and 3 days later cell suspensions derived from the dopamine-rich ventral mesencephalic area were injected into the right rostral striatum. The transplants rapidly developed a substantial innervation of one striatum, so that by 15 days after transplantation (21 days of age) animals rotated away from the reinnervated side in response to amphetamine. The amount of turning correlated with the extent of innervation of the striatum as determined by tyrosine hydroxylase immunocytochemistry. By 25 days post-transplantation (31 days of age), animals turned in response to stress as well as amphetamine, although this later-developing phenomenon was not associated with any significant change in the extent of dopamine innervation. A second group of animals was bilaterally dopamine-depleted at 3 days of age, but transplantation with nigral cell suspensions was delayed until maturity. Partial reinnervation of the rostral striatum occurred with this delayed transplant paradigm, and turning to both amphetamine and stress commenced at 15 days post-transplantation. In contrast to animals receiving transplants shortly after lesioning, these animals began to turn spontaneously at about 20 days post-transplantation. The serotonin hyper-innervation that occurs following dopamine depletion in infancy was not altered by dopamine transplants made at either time. Results from both groups of transplant animals suggest that dopamine-rich transplants can provide substantial innervation that exerts some functional control over the striatum. This occurs despite the fact that neonatally dopamine-depleted rats, unlike adult-lesioned animals, survive quite well in the absence of striatal dopamine. However, the degree of incorporation into existing circuitry, and the types of regulation that result, may vary considerably depending on the age at which the tissue is transplanted.

An increase in striatal Met-enkephalin-like immunoreactivity in neonatally dopamine-depleted rats

A pronounced decrease in dopamine content in both the striatum and the mesolimbic area was observed up to 120 days of age after treatment with 6-hydroxydopamine (100 μg, intracisternally) following desipramine pretreatment (20 mg/kg, i.p.) at the 5th postnatal day. In the 6-hydroxydopamine-treated rats, Met-enkephalin-like immunoreactivity in the striatum was found to have increased at 35 and 70 days old, although it remained unchanged at 25 days old. The neonatal administration of 6-hydroxydopamine, however, did not affect the peptide content in the mesolimbic area at any age. These results indicate that the neonatal dopamine reduction changes the Met-enkephalinergic neuronal activity in the striatum at the adult age.

Adrenal medullary cells transmute into dopaminergic neurons in dopamine-depleted rat caudate and ameliorate motor disturbances

Adrenal medullary cell suspensions, derived from newborn rats (postnatal day 1–6), were implanted into the head of the caudate nucleus in 35 rats with unilateral 6-hydroxydopamine (6-OHDA) lesions in the nigrostriatal dopamine (DA) pathway. Behavioral recovery from Met-amphetamine induced circling, cell growth morphological features (tyrosine hydroxylase positive cells), and release of adrenaline (Ad), noradrenaline (NA), DA, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were investigated for 40 weeks after transplantation. Met-amphetamine induced circling decreased significantly in 43% (15/35) of the rats. The decrease was concurrent with transmutation of the tyrosine hydroxylase-like immunopositive (THLI) cells into mature neurons that had abundant elongated neurites with varicosities and synapses on neuronal elements in the host caudate. In the absence of behavioral recovery (57%, 20/35) THLI cells were very scant. DA, DOPAC and HVA were reduced more than 90% in perfusates collected by in vivo dialysis from the striata of the animals that were not improved by transplant. These levels recovered to 20–50% of controls in animals whose behavior recovered. Ad and NA were not detected in the perfusates of either recovered or non-recovered animals. The results suggest that some grafted adrenal medullary cells transform into dopaminergic neurons and the release of DA from these grafted cells functionally affects behavior improvement for at least 40 weeks.

Food wrenching and dodging: a neuroethological test of cortical and dopaminergic contributions to sensorimotor behavior in the rat.

Sensorimotor behavior in unilateral decorticate and unilateral dopamine-depleted rats was studied in a naturally occurring social interaction in which rats compete for food with relatively stereotyped species-typical responses. In the interaction a "robber" attempts to wrest food from a feeding "victim," which in turn protects the food by making rapid contralateral dodges. Hemidecortication abolished dodging to food wrenching attempts made by a rat approaching contralaterally to the lesion, so that the food was stolen, but recovery occurred between 15 and 60 days after surgery. Use of the side contralateral to the lesion to wrest food was moderately impaired, and recovery of food wresting was more rapid than recovery of dodging. There were no impairments in dodging to approaches to the side of the body ipsilateral to the lesion, nor were there impairments in using that side of the body to wrest food from other rats. Unilateral dopamine depletion produced dodging impairments to food-wresting attempts that were made both ipsilateral and contralateral to the side of the lesion, and the deficits endured over 60 test days. The food was frequently lost to food-wrenching attempts made contralateral to the lesion, whereas the direction of dodges to approaches ipsilateral to the lesion was reversed. Food wresting was also bilaterally impaired. In conclusion, the study shows (a) there are differences in the sensorimotor impairments that follow unilateral neocortical damage and unilateral dopamine depletion, (b) the neural organization of orienting and dodging is different, and (c) the form of the sensorimotor impairments as well as the processes of recovery can be usefully analyzed by using species-typical social interactions.

Amphetamine- and stress-induced turning after nigral transplants in neonatally dopamine-depleted rats

Neonatally dopamine-depleted rats fail to show the deficits seen in similarly lesioned adults, and no longer appear to require dopamine. Nevertheless, unilateral nigral transplants provided extensive reinnervation and were capable of modifying motor patterns. Both amphetamine and external stressors elicited contraversive turning. This raises the question of whether extrinsic dopamine innervation supplements or overrides alternative compensatory mechanisms in the neonate.

Supersensitive endocrine response to physostigmine in dopamine-depleted rats: A model of depression?

Depressed patients exhibit an abnormal “supersensitive” increase in the plasma concentration of several pituitary hormones following intravenous injection of the acetyl cholinesterase inhibitor physostigmine (PHY). In the present study, we examined the effects of PHY treatments on the plasma concentrations of prolactin (PRL) and adrenocorticotrophic hormone (ACTH) in the rat. Physostigmine (0–0.6 mg/kg, s.c.) produced a dose-dependent increase in PRL and ACTH immunoreactivity in unoperated animals. Neurotoxin-induced depletion of brain dopamine (DA) or norepinephrine (NE) did not significantly alter baseline plasma PRL or ACTH values. Following depletion of brain DA, but not NE, animals exhibited a “supersensitive” increase in plasma ACTH values, which was evidenced by a sixfold left shift in the dose-response properties of PHY. These results suggest that there are intriguing parallels between the abnormal endocrine response to PHY demonstrated by depressed patients and that demonstrated by rats following depletion of central nervous system (CNS) DA levels.

Roles of catecholamine terminals and intrinsic neurons of the ventral tegmentum in self-stimulation investigated in neonatally dopamine-depleted rats

Three series of experiments were undertaken to determine whether the residual catecholamine (CA) terminals or intrinsic neurons of ventral tegmentum (VT) in rats given 6-hydroxydopamine (6-OHDA) after desmethylimipramine (DMI) in the lateral ventricles at birth, mediated VT self-stimulation (SS). In Experiment I, male pups were injected bilaterally on days 3 and 5 with 6-OHDA (total dose 200 μg) or with the vehicle after pretreatment with DMI (50 mg/kg, IP) 30 min earlier. Each subject, 150 days old, was implanted bilaterally in the VT with electrode-cannula units. Both the dopamine (DA)-depleted and control groups yielded similar percentages of self-stimulators. The rate of responding was, however, slightly but significantly lower in the DA-depleted group than in the controls. In experiment II, 8 DA-depleted and 7 control rats were pretreated with pargyline (50 mg/kg, IP) and then given unilateral injections of 6-OHDA in the VT, in the tissue below the SS electrode. These intracerebral injections of 6-OHDA had no effect on VT SS in both groups. Seventeen controls and 12 DA-depleted rats, in Experiment III, were given injections of kainic acid (KA; 5 nM) either ipsilaterally or contralaterally. The ipsilateral injection abolished SS (14 days of testing), whereas the contralateral injection had no effects on ipsilateral SS in both groups. Histochemical fluorescence study in Experiment I and II showed that the neonatal treatment with DMI + 6-OHDA had reduced the number of DA-containing perikarya in the VT and that reinjection of 6-OHDA into the VT caused the disappearance of the residual CA terminals in tissue surrounding the electrode tip. Conventional histology for rats in Experiment III showed the destruction of cell bodies in tissue below the tip of the SS electrode after KA. These findings suggest that the intrinsic neurons in the VT play a critical role in SS of the VT.

Rats given dopamine-depleting brain lesions as neonates do not respond to acute homeostatic imbalances as adults.

Rats given near-total dopamine-depleting brain lesions as neonates exhibit none of the severe sensorimotor or ingestive dysfunctions that result when adult rats are given comparable lesions. These experiments showed that the apparent sparing of function is incomplete; when tested as adults, the brain-damaged animals did not eat in response to acute glucoprivation, nor did they drink water and saline in response to hypovolemia. Indeed, after receiving these homeostatic challenges, the animals became much less able to simply move about. In contrast to these marked effects, the dopamine-depleted rats ate normally after glucoprivation when they were handled hourly instead of being left alone. Similarly, administration of caffeine permitted them to drink while hypovolemic. These and other observations suggest that some central neurons other than those containing dopamine are responsible for maintaining behavioral activation in rats after neonatal destruction of dopaminergic fiber systems in the brain and that the homeostatic challenges compromise their function and thereby disrupt behavior.

Dopamine depletion, stimulation or blockade in the rat disrupts spatial navigation and locomotion dependent upon beacon or distal cues

Rats depleted of dopamine by intraventricular or nigrostriatal bundle 6-hydroxydopamine injection were compared with normal rats on acquisition and retention of place and cue navigation in the Morris swimming pool test and on a battery of sensorimotor tests. Rats with extensive bilateral dopamine depletions were able to swim vigorously, but were unable to acquire either the place or cue task. Rats with unilateral lesions, although impaired in the rate of acquisition were eventually able to learn both tasks to close to normal levels. Animals pretrained on the tasks prior to the lesions displayed retention deficits that were related to the extent of dopamine depletion: after extensive depletions, performance on both tasks deteriorated until successful navigation was abolished, whereas incomplete depletions impaired but did not abolish performance on either task. In separate groups of pretrained animals, both dopamine antagonists (haloperidol, α-flupenthixol) and agonists (apomorphine, metamphetamine) blocked performance on both place and cue tasks, although there were individual differences in sensitivity of the rats. Performance on the place task was more sensitive to disruption than the cue task both by the lesions and by haloperidol, α-flupenthixol or apormorphine but not by metamphetamine. On the sensorimotor tests dopamine-depleted rats were impaired at visual but not contact placing, they oriented weakly to snout touches and surfaces but not to distal stimuli, and they were akinetic on a number of tests of motor function but when wet they displayed as many grooming movements and groomed as long as did normal rats. The results suggest that dopamine depletion may impair spatial navigation by a disruption of their ability to use distal cues for guidance.

Monoamine release from dopamine-depleted rat caudate nucleus reinnervated by substantia nigra transplants: An in vivo electrochemical study

Previous studies have shown that fetal substantia nigra (SN) transplanted into a cavity overlying a dopamine (DA)-denervated caudate nucleus can reverse a number of the behavioral abnormalities induced by the denervation. While some histochemical and physiological evidence suggests that this reversal is the result of a functional DA input from the transplant to the host brain, there is little direct evidence for transmitter release from ingrowing graft-derived nerve fibers. In the present work in vivo electrochemistry was used to analyse the magnitude, time course and spatial distribution of neurotransmitter releases evoked by local application of potassium (K +) from DA-depleted, SN transplant-reinnervated striatum. Animals were injected unilaterally with 6-hydroxydopamine (6-OHDA) into the SN and screened by measuring apomorphine-induced rotation. Some were then given SN grafts, which were placed in a ‘delayed cavity’ just dorsal to the lesioned striatum. Nafion-coated graphite epoxy capillary (GEC) electrodes were employed for the electrochemistry to minimize signals derived from ascorbate or acidic DA metabolites. The GEC electrode was fixed to a K +-filled micropipette and this assembly was used to map the caudate nucleus of control, 6-OHDA-treated, and 6-OHDA-treated, grafted animals. The morphometric relationships between striatal recording sites and transplant location were subsequently verified histologically. Releases from striatal sites within 1.0 mm of the SN grafts were slightly, but not significantly, less than those obtained from control caudate. By contrast, releases from striatal sites further distal from transplants were markedly reduced to about 30% of control; similar values were obtained from 6-OHDA-treated striata which did not received SN grafts. The time course of the K +-evoked releases was also reduced following 6-OHDA treatment, but returned to control values in striatal sites proximal to a transplant. The data thus lend further evidence to the postulate that SN grafts ameliorate lesion-induced behavioral dysfunctions by providing specific DA input to the host brain.

Radial arm maze performance in rats following neonatal dopamine depletion.

Neonatal dopamine (DA) depletion produces learning impairments both during development and throughout adulthood in the rat. The present experiment further investigated the memory capabilities of the dopamine-depleted rat by assessing performance in the radial arm maze. Results showed that, following neonatal injection of 6-hydroxydopamine and desmethylimipramine, lesioned rats per-performed more accurately than controls. In this paradigm, DA-depleted rats tended to enter each arm to obtain a food pellet and not enter unbaited, incorrect arms. The difference in performance of control and treated rats could not be accounted for by differences in locomotor activity, body weights, or motivational factors. A computer analysis of the data revealed that DA-depleted animals adopted a strategy of choosing adjacent arms consecutively, which probably accounted for their superior performance. Results are discussed in terms using algorithms versus extra-maze cues to complete the maze following early brain injury.

Selective effects of buspirone and molindone on dopamine metabolism and function in the striatum and frontal cortex of the rat

The hypothesis that the nerve endings of the dopamine projection of the frontal cortex lack autoreceptors for regulation of tyrosine hydroxylase was tested by using the preferential inhibitors of dopamine autoreceptors, molindole and buspirone. In contrast to haloperidol, which elevates dopamine metabolism in the striatum and frontal cortex, both molindone and buspirone elicited little change in dopamine metabolism in the frontal cortex at doses up to 3.0 mg/kg, which cause the same maximal response in the corpus striatum as does haloperidol. Thus, the lack of autoreceptors in the frontal cortex is of pharmacological importance. That preferential inhibition of striatal dopamine autoreceptors may reverse catalepsy by enhancing synthesis and release of dopamine was tested by first inducing catalepsy with different drugs and then administering molindone or buspirone. Only buspirone (1.0 mg/kg) reversed catalepsy. This effect does not require presynaptic dopamine as catalepsy was reversed by buspirone in the dopamine-depleted rat (with 2.0 mg/kg R04-1284) as well as after postsynaptic dopamine receptor blockade by haloperidol of cis-flupenthixol. Thus, the mechanism for the reversal of catalepsy appears to be located efferent from the dopamine neuron. Buspirone, a non-benzodiazepine anti-anxiety drug, may prove useful for treatment of extrapyramidal motor disorders of either iatrogenic or idiosyncratic origin.

Open-field behavior in dopamine-depleted rat pups and their mothers

Possible changes in the behavior of rat mothers and their pups were investigated by administering intracisternal injections of 6-hydroxydopamine (6-OHDA) or its vehicle to 5-day-old rats. Administration of the neurotoxin resulted in a significant depletion of whole-brain dopamine levels to 23% of control levels, whereas norepinephrine levels were reduced to 83% of controls. Open-field behavior revealed that the 6-OHDA-treated rat pups were hypoactive, in terms of decreased square crossings, at 15 days of age, yet were hyperactive at 30 days of age. Toxin-treated pups also showed lower urination scores at 25 and 30 days of age. Mothers' open-field behavior was virtually unaffected by the treatment status of their offspring (i.e., 6-OHDA vs. vehicle-treated), although several of the mothers' behaviors decreased with repeated measures over days.

Animal Models and Childhood Behavioral Disturbances: Dopamine Depletion in the Newborn Rat Pup

Abstract Abnormalities in catecholamine metabolism have been implicated in the pathogenesis of various psychiatric disturbances of childhood, including minimal brain or cerebral dysfunction (MBD), childhood psychosis, affective disturbances, and the syndrome of chronic multiple tics. Developmental research on the relationship between brain function and behavior can be facilitated by appropriate animal models, interpreted conservatively. When rat pups are selectively depleted of brain dopamine by treatment with 6-hydroxydopamine in the newborn period, they display a constellation of behaviors similar to those seen in childhood MBD: hyperactivity, disturbed habituation, cognitive deficits, and “normalization” with stimulant medication. As adults, dopamine-depleted rats exhibit aberrations in mothering. This and similar models may help clarify certain aspects of the developmental associations between alterations in brain physiology, abnormal psychological mechanisms, and the emergence of behavioral deviance.

Meal patterns in rats with nigrostriatal dopamine-depleting lesions, subdiaphragmatic vagotomy, and their combination

Rats which had sustained 97% bilateral striatal dopamine depletions after intranigral 6-hydroxydopamine recovered feeding and drinking to show normal meal patterns on both solid and liquid food. The motor acts of eating were somewhat slow but, in contrast to lateral hypothalamic lesioned animals, there were no interruptions for other activities. Vagotomy produced the expected decrease in liquid meal size, and an identical change was observed when the dopamine-depleted rats were similarly vagotomized. The fragmented meal patterns seen after far-lateral hypothalamic lesions do not arise from damage to either of the systems here investigated.