Lesion-induced DA supersensitivity in aging C57BL/6J mice

Unilateral nigrostriatal 6-hydroxydopamine lesions in mice I: motor impairments identify extent of dopamine depletion at three different lesion sites

The unilateral 6-hydroxydopamine mouse lesion models of Parkinson's disease have received increasing attention in recent years, but comparison of the different lesion models was largely focused at a histological level. An extensive behavioural comparison between different mouse models on tests of motor function has yet to be carried out, to pin point tests that accurately discriminate between different extents of dopaminergic depletion. In the present study we examine the consequences of injection of the toxin at three sites along the nigrostriatal tract (substantia nigra, medial forebrain bundle, and striatum) on a broad range of simple motor tasks, and on the dopaminergic pathology. All lesion groups demonstrated marked behavioural deficits and displayed distinct profiles of degeneration along the nigrostriatal dopamine pathway. Tests that correlated closely with the level of substantia nigra cell loss included the corridor, cylinder and balance beam tests, the rotarod, inverted cage lid and three types of rotational assessment (spontaneous, amphetamine-induced and apomorphine-induced). Specific tasks are identified which are capable of distinguishing a near-complete lesion, with amphetamine rotation, corridor and cylinder tests showing the highest correlations with levels of nigral cell loss. Performance in the different behavioural tests was associated with distinct profiles of cell loss in the SN and VTA. We provide a comprehensive behavioural assessment of lesion-induced deficits in mouse models of PD, which should facilitate selection of the most appropriate lesion model and most sensitive behavioural tests for use in future studies investigating therapeutic interventions.

Unilateral nigrostriatal 6-hydroxydopamine lesions in mice II: predicting l-DOPA-induced dyskinesia

In the 6-hydroxydopamine (6-OHDA) lesioned rodent the location of the lesion produces significantly different behavioural phenotypes, responses to the dopamine precursor l-3,4-dihydroxyphenylalanine (l-DOPA) and neuropathology. Lesion extent is commonly determined by a series of motor tests, but whether any of these tests have a relationship to the development and predictability of dyskinesia is unknown. We used mice with 6-OHDA lesions of the striatum, medial forebrain bundle and substantia nigra to examine the relationship between a range of tests used to determine motor function in the absence of l-DOPA: rotarod, cylinder, corridor, the balance beam, locomotor activity, psycho-stimulant and spontaneous rotational behaviour. The mice were subsequently treated with l-DOPA in progressively increasing doses and the development of l-DOPA-induced dyskinesia assessed. Most of these tests predict dopamine depletion but only rotarod, spontaneous rotations, apomorphine-induced rotations and locomotor activities were significantly correlated with the development of dyskinesia at 6mg/kg and 25mg/kg l-DOPA. The losses of dopaminergic neurons and serotonergic density in the ventral and dorsal striatum were dependent upon lesion type and were also correlated with l-DOPA-induced dyskinesia. The expression of FosB/ΔFosB was differentially affected in the striatum and nucleus accumbens regions in dyskinetic mice according to lesion type.

Autoimmune-induced damage of the midbrain dopaminergic system in lupus-prone mice

Spontaneous development of lupus-like disease is accompanied by impaired dopamine catabolism and degenerating axon terminals in the mesencephalon of MRL-lpr mice. We presently examine the hypothesis that systemic autoimmunity affects the central dopaminergic system in behaviorally impaired animals. The functional damage of the nigrostriatal pathway was assessed from rotational behavior after a single injection of the D1/D2-receptor agonist apomorphine. Neurodegeneration in the midbrain was estimated by Fluoro Jade B (FJB) staining. The causal role of autoimmunity was tested by comparing asymptomatic and diseased MRL-lpr mice, and by employing the immunosuppressive drug cyclophosphamide. Damage of dopaminergic neurons was assessed by tyrosine-hydroxylase (TH) staining of the midbrain. Apomorphine induced significant asymmetry in limb use, which lead to increased circling in the diseased MRL-lpr group. While FJB-positive somas were not seen in the striatum, increased staining in the substantia nigra (SN) and ventral tegmental area (VTA) were detected in behaviorally impaired MRL-lpr mice, but not in age-matched controls. Reduced brain mass and increased levels of TNF-alpha in their cerebrospinal fluid (CSF) suggested cerebral atrophy and inflammation. In addition, CSF was neurotoxic to a dopaminergic progenitor cell line. Immunosuppression attenuated CSF cytotoxicity, TNF-alpha levels, and midbrain neurodegeneration. Supportive of the notion that dying neurons were dopaminergic, the SN of autoimmune mice showed approximately a 35% reduction in the number of TH-positive cells. A three-fold increase in serum brain-reactive antibodies accompanied this loss. Although the source of toxic mediator(s) remains unknown, present results are consistent with the hypothesis that autoimmunity-induced destruction of mesonigral and mesolimbic dopaminergic pathways contributes to the etiology of aberrant behavior in an animal model of neuropsychiatric lupus.

Pharmacological adaptations and muscarinic receptor plasticity in hypothalamus of senescent rats treated chronically with cholinergic drugs

Receptor plasticity is an important compensatory process by which the central nervous system adapts to pathological insult or long-term exposure to drugs. Senescent animals may show an age-related impairment of muscarinic receptor up- or down-regulation after chronic exposure to cholinergic drugs. The purpose of this study was to assess biochemical and pharmacological endpoints of muscarinic receptor plasticity in young, adult and senescent animals. Male, Fischer 344 rats (ages 3, 9, and 27 months) were administered methylatropine or oxotremorine intracerebroventricularly (IVT) for 3 weeks and tested for their functional response to a muscarinic agonist. The density of hypothalamic, muscarinic receptors was also estimated from analysis of 3H-QNB binding isotherms. In young rats, parallel changes in muscarinic receptors and response were noted, but chronic administration of cholinergic drugs to senescent animals had no effect. Thus, 3H-QNB binding in hypothalamus of young and adult rats was increased (31% and 17%) after chronic IVT methylatropine and decreased (20% and 15%) after IVT oxotremorine. Also, young rats treated with IVT methylatropine were supersensitive to the hypothermic effects of a muscarinic agonist (oxotremorine), while young and adult animals administered chronic IVT oxotremorine exhibited marked tolerance. In contrast, identically treated senescent rats showed no changes in 3H-QNB binding or oxotremorine-induced hypothermia. These results demonstrate the impaired ability of senescent rats to up- or down-regulate brain muscarinic receptors and to exhibit functional adaptations seen in young animals treated chronically with cholinergic drugs.

Nerve growth factor released by transgenic astrocytes enhances the function of adrenal chromaffin cell grafts in a rat model of Parkinson's disease

Previous studies have demonstrated that astrocytes genetically modified to express recombinant nerve growth factor (NGF) support the survival and neuronal transdifferentiation of intrastriatal adrenal chromaffin cell grafts at 2 weeks post-transplantation [15]. The present study was performed to determine whether these effects would be maintained at longer times post-transplantation and, if so, whether the co-grafts would reduce rotational behavior in the unilateral 6-hydroxydopamine-lesioned rat. In the present study, we have demonstrated that primary type I rat astrocytes infected with a replication-defective retrovirus conferring expression of a mouse beta-NGF cDNA sequence secrete NGF at a rate that is approximately 40-fold higher than that of controls (i.e., 8.0 vs. 0.2 pg NGF/h/10(5) cells, respectively). The genetically modified astrocytes were also found to express recombinant NGF following intrastriatal transplantation, as indicated by a 23% increase in striatal NGF content compared with controls, measured at 4 weeks post-transplantation. When NGF-producing astrocytes and adrenal chromaffin cells were co-grafted into the dopamine-denervated striatum of the unilateral 6-hydroxydopamine-lesioned rat, the chromaffin cells displayed extensive neurite outgrowth and a 5-12-fold increase in survival compared to controls at 10 weeks post-grafting. These effects were paralleled by a 60% reduction of apomorphine-induced rotational behavior, suggesting a partial normalization of striatal function. These results suggest that genetically modified astrocytes promote the prolonged survival and function of adrenal chromaffin cell grafts in a rat model of Parkinson's disease.

Impaired supersensitivity to morphine following chronic naltrexone treatment in senescent rats

Chronic administration of opiate antagonists produces an increase in the density of opiate receptors, as well as an enhanced sensitivity to the analgesic and locomotor depressant effects of morphine. The present study assessed whether aging alters these regulatory processes. Young (3-4 months), middle-aged (10-11 months), and senescent (25-30 months) rats were implanted subdermally with slow-release naltrexone pellets or were given sham surgery. The pellets were removed 10 days later. Twenty-four hours after pellet removal, morphine-induced (5 mg/kg, SC) analgesia and locomotor activity were assessed. Young and middle-aged rats treated with naltrexone showed enhanced sensitivity to the analgesic and locomotor activity depressant effects of morphine relative to age-matched controls. In contrast, senescent rats treated with naltrexone did not differ from age-matched controls in their response to morphine. The density of opiate receptors labeled with 3H-naloxone was measured in the anterior striatum. Both young and senescent rats treated with naltrexone exhibited an increase in opiate receptor density relative to age-matched controls. The results indicate that senescent rats are capable of up-regulating opiate receptors following chronic naltrexone treatment but do not exhibit the corresponding functional supersensitivity to morphine.

A quantitative estimate of the role of striatal D-2 receptor proliferation in dopaminergic behavioral supersensitivity: the contribution of mesolimbic dopamine to the magnitude of 6-OHDA lesion-induced agonist sensitivity in the rat

Rats with unilateral depletions of neostriatal dopamine display increased sensitivity to dopamine agonists estimated to be 30 to 100 x in the 6-hydroxydopamine (6-OHDA) rotational model. Given that mild striatal dopamine D-2 receptor proliferation occurs (20-40%), it is difficult to explain the extent of behavioral supersensitivity by a simple increase in receptor density. This study was designed to investigate the quantitative aspects of the rotational behavior model utilizing constrained non-linear curve fitting routines. A dose-response curve for the rotational response arising from apomorphine stimulation of the normosensitive striatum was obtained in animals bearing unilateral lesions of striatal efferents (predominantly the striato-nigral pathway as previously described). After the control dose-response experiment, rats received a dopamine- (DA) depleting lesion in the contralateral hemisphere. In one group, 6-OHDA was infused into the medial forebrain bundle (MFB), a placement which is common in the literature and is known to deplete DA in both the striatum and nucleus accumbens. In a second group of rats, 6-OHDA was infused into the globus pallidus at a site which depletes caudate DA, but leaves n. accumbens DA relatively intact. The two experimental groups were tested in identical apomorphine-induced rotation dose-response experiments. The ED50's of the MFB- and caudate-lesioned rats were reduced by 36 and 5.8 fold, respectively, as compared to the control dose-response curve. The MFB and caudate lesions depleted striatal DA and produced a 30 and 36% increase in striatal D-2 binding sites, respectively. Modeling the behavioral and biochemical data with the null model for receptor occlusion indicated that increased striatal D-2 receptor density could account for the magnitude of behavioral supersensitivity in neither the MFB-lesioned group, nor even in the caudate-lesioned group. Thus simple up-regulation or D-2 receptors is unlikely to account for supersensitization as measured in the rotational model. Further, we suggest that quantitative modeling of such hypotheses is a valuable experimental technique for assessing relationships between biochemical and behavioral variables.

Cryopreservation and storage of embryonic rat mesencephalic dopamine neurons for one year: comparison to fresh tissue in culture and neural grafts

Blocks of embryonic rat ventral mesencephalic tissue containing the developing A8-A10 dopamine (DA) cell groups were cryopreserved and stored for approximately 1 year, at which time this tissue was thawed, dissociated into a cell suspension, and compared to a similar preparation of fresh mesencephalic tissue for viability in tissue culture and neural grafts. Estimates of total cell number immediately prior to plating in culture indicated that cryopreserved tissue yields fewer cells, but when this reduced cell number is compensated for, and equal numbers of cells were plated in culture, approximately equal total numbers of neurons, as well as tyrosine hydroxylase (TH)-positive neurons, were present in cultures from cryopreserved and fresh tissue. Grafting of equal numbers of fresh and cryopreserved mesencephalic cells into the striatum of adult rats with large unilateral lesions of the nigrostriatal DA pathway tended to yield smaller grafts with fewer surviving TH-positive cells with less extensive neuronal processes when tissue was previously cryopreserved. However, grafts derived from freeze-stored tissue provided a similar time-course and extent of behavioral recovery in amphetamine-induced rotational tests to that provided by fresh tissue grafts. Taken together, our findings indicate that while cryopreservation of mesencephalic tissue has its costs--reduced cell yield in cultures and grafts, and compromised morphology in grafts--sufficient numbers of cryopreserved neurons survive the grafting procedure to ameliorate behavioral signs of DA depletion in the lesioned rat model.

Co-grafts of embryonic dopamine neurons and adult sciatic nerve into the denervated striatum enhance behavioral and morphological recovery in rats

We have recently demonstrated that a diffusible factor(s) derived from explanted adult rat sciatic nerve can increase the number and neurite outgrowth of embryonic rat dopamine (DA) neurons in culture. The present study extends this finding to compare DA neuron-sciatic nerve co-grafts to grafts of DA-rich neural tissue alone for behavioral and morphological effects in rats with unilateral nigrostriatal lesions of the DA pathway. Our results indicate that the presence of a co-grafted segment of sciatic nerve increased the likelihood of rapid behavioral recovery and promoted complete recovery mediated by small grafts that yielded only modest behavioral changes in the absence of co-grafted nerve. These behavioral effects were accompanied by a modest increase in survival of grafted tyrosine hydroxylase-positive neurons in the striatum and a more pronounced increase in the area and density of striatal reinnervation provided by grafted DA neurons in co-grafted animals. This evidence supports the view that a diffusible product of explanted peripheral nerve acts as a growth-promoting factor for embryonic DA neurons and that the presence of this factor augments the behavioral efficacy of grafted DA neurons.

Chronic levodopa impairs the recovery of dopamine agonist-induced rotational behavior following neural grafting

The effect of chronic levodopa treatment on the function of embryonic mesencephalic tissue grafts was assessed in rats by monitoring rotational behavior elicited by dopamine (DA) agonists before and after neural grafting. Rats were given unilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal pathway and baseline measures of rotational behavior induced by D1 receptor stimulation, D2 receptor stimulation, or amphetamine were determined. Subsequently, DA grafts were implanted into the lesioned striatum and chronic regimens of either saline or levodopa began one day after neural grafting and were continued for 7 weeks. Rotational behavior elicited by the D1 agonist, SKF 38393, was completely attenuated throughout the six-week-period following the commencement of levodopa treatment, regardless of the absence or presence of a DA graft. Conversely, rotational behavior elicited by the D2 agonist, quinpirole, was significantly elevated in ungrafted animals receiving chronic levodopa. Grafted animals receiving chronic levodopa did not show a significant reduction in rotational behavior, whereas grafted animals receiving chronic saline showed a significant 67% reduction in quinpirole-induced rotational behavior. Amphetamine-induced rotational behavior was reduced in both levodopa and saline treated grafted animals, however grafted animals receiving chronic levodopa treatment showed a reduction of rotational behavior that was uncharacteristic and less compensatory than that observed in grafted animals receiving chronic saline treatment. Morphology of grafts indicate that there were areas of impaired neurite outgrowth of TH-positive fibers in animals treated with levodopa. The results of the present study suggest that the impaired recovery in quinpirole- and amphetamine-induced rotational behavior in grafted animals receiving chronic levodopa treatment may be related to (1) impaired graft function, (2) an alteration in pre- and postsynaptic mechanisms in the host DAergic system, or (3) a combined effect of (1) and (2).

Behavioral demonstration of a reciprocal interaction between dopamine receptor subtypes in the mouse striatum: possible involvement of the striato-nigral pathway

It is well known that stimulation of the D-2 dopamine receptor in vitro inhibits the increased efflux of cyclic adenosine monophosphate caused by D-1 receptor agonists. Furthermore, behavioral data suggest that the striato-nigral pathway is more involved with the dopamine agonist-induced expression of oral behaviors, which are, in turn, mediated by stimulation of the D-1 receptor. We examined an in vivo model to determine whether this D-1/D-2 reciprocal interaction is detectable at a behavioral level. First, mice were pretreated with wide range of doses of the D-2 antagonist, spiperone, and then injected with a behaviorally active dose of apomorphine (a nonspecific direct dopamine agonist) and were observed for incidence of oral behavior and rated for stereotypic behavior. A biphasic effect of spiperone pretreatment was observed, at some low doses both stereotypy and oral behavior were enhanced, while at high doses, both agonist-induced behaviors were progressively inhibited. To test the specificity of this effect for the striato-nigral pathway, mice were administered discrete electrolytic lesions in the ventral portion of the internal capsule in one hemisphere. The animals that responded to apomorphine by rotating ipsilaterally to the lesion were used in two, five-point apomorphine dose-response curves, one with, and one without, pretreatment with the dose of spiperone which most enhanced stereotypic behavior and incidence of oral behavior. The spiperone pretreatment caused a clear increase in the maximum rotational response to apomorphine without affecting the ED50. These data suggest that behavior associated with the striato-nigral efferent from striatum is marked by the opposition of D-1 and D-2 receptors.

Bromocriptine-induced rotation: characterization using a striatal efferent lesion in the mouse

Two lesion techniques were used to elucidate the mode of action of bromocriptine (BRC)-induced behavior in mice. With the first lesion, a unilateral 6-hydroxydopamine (6-OHDA) preparation, BRC administration resulted in contralateral rotation which was blocked by alpha-methyl-para-tyrosine (AMPT), comparable to previous reports using rats. After striatonigral/entopeduncular lesion, mice did not rotate in response to doses of BRC up to 30 mg/kg but did show general activation which was also inhibited by AMPT pretreatment. It is concluded that BRC does not elicit rotation when there is no dopaminergic asymmetry such as that caused by a unilateral 6-OHDA lesion or no asymmetry in the striatonigral or striatoentopeduncular efferents. Since BRC-induced behaviors are dependent on intact presynaptic dopamine and BRC is predominantly a D-2 agonist, behaviors elicited in response to BRC must be the result of coactivation of D-1 receptors by endogenous dopamine. Thus, the behavioral effects of BRC, and perhaps D-2 agonists in general, must be mediated by efferents other than the striatonigral and striatoentopeduncular pathways.

Alteration of calmodulin distribution does not accompany dopaminergic supersensitization of the mouse striatum

Membrane-bound calmodulin increases following dopaminergic supersensitization of the rat striatum. To assess the generality of this relationship, mice were treated with two different supersensitization paradigms. Calmodulin levels and subcellular distribution were determined by radioimmunoassay. Chronic haloperidol treatment increased striatal D2 dopamine receptor density by 25% but had no effect on membrane-bound calmodulin levels. Similarly, 6-hydroxy-dopamine (6-OHDA) lesions depleted striatal dopamine content greater than 95% without affecting membrane-bound calmodulin. In contrast, soluble calmodulin levels decreased by 15% in the 6-OHDA-lesioned striatum, suggesting that soluble calmodulin is enriched in presynaptic dopaminergic terminals. We conclude that dopaminergic supersensitization can occur in the mouse striatum in the absence of any change in calmodulin distribution.

Reduced muscarinic receptor plasticity in frontal cortex of aged rats after chronic administration of cholinergic drugs

Age-related differences in muscarinic receptor plasticity were observed in young, adult and senescent Fischer 344 rats (3, 9 and 27 months old, respectively) following the chronic, intracerebroventricular (ivt) administration of a cholinergic agonist, oxotremorine, or antagonist, methylatropine. After three weeks treatment of young rats with ivt oxotremorine, the maximum number (Bmax) of 3H-QNB binding sites in frontal cortex, determined by saturation experiments, was reduced by 27%, with no apparent change in the affinity (Kd) of 3H-QNB for the muscarinic receptor. Conversely, chronic ivt methylatropine administered to 3 month old animals caused a 29% increase in Bmax with no significant change in Kd. Adult animals showed a somewhat lesser degree of muscarinic receptor plasticity (16% down-regulation after oxotremorine, 22% up-regulation after methylatropine). However, 3H-QNB binding parameters in frontal cortex from senescent rats were not significantly altered following identical treatments with oxotremorine or methylatropine. Thus, muscarinic receptor adaptation to chronic, cholinergic drug administration was impaired in aged animals. This reduced receptor plasticity with aging could have important implications for the long-term drug treatment of elderly patients and for the therapeutic efficacy of cholinergic drugs in age-related neurological disorders, such as Alzheimer's disease.

Effect of duration of haloperidol treatment on DA receptor supersensitization in aging C57BL/6J mice

Apomorphine-induced behavior, striatal [3H]spiperone binding, and striatal choline acetyltransferase (ChAT) activity were assessed in 6 1/2, 13, and 27-30 month-old male C57BL/6J mice following 0, 30, 60 or 90 days treatment with the dopaminergic (DA) antagonist haloperidol. Both apomorphine-induced behavior and [3H]spiperone binding (Bmax) increased linearly with duration of haloperidol treatment, with no detectable age difference in the degree of supersensitization, although basal receptor density declined with age. Middle- and old-aged mice showed prolonged stereotypic behavior relative to young mice, suggesting slower apomorphine clearance. No differences in ChAT activity were detected with either age or duration of haloperidol treatment. Although the group means of binding and behavior were highly related, the within group correlations were poor. Overall, the results suggest that aged animals are capable of DA receptor supersensitization when given a sufficient stimulus--in this case, relatively long treatment regimes. Previously reported deficits in neuroleptic-induced supersensitization in old mice may be confined to relatively short treatment periods at low doses.

Quantification of lesion-induced dopaminergic supersensitivity using the rotational model in the mouse

A dual lesion technique was used to determine the degree of supersensitivity resulting from nigrostriatal lesions in C57BL/6J mice. Internal capsule lesions encroaching on globus pallidus resulted in reliable ipsilateral rotation both to apomorphine and amphetamine. Dose-response curves to apomorphine were determined before and 21 days after 6-hydroxydopamine lesion of the contralateral nigrostriatal pathway. A 31.5-fold shift to the left was observed following the nigrostriatal lesion, with no change in slope. The extent and placement of the internal capsule lesion, as well as the magnitude of supersensitization correspond closely to those previously reported in the rat.