BDNF in the Aged Brain: Translational Implications for Parkinson's Disease

Brain Derived Neurotrophic Factor (BDNF) is a member of the neurotrophin family of secreted growth factors. BDNF signaling is known to exert both chronic, pro-survival effects related to gene expression and protein synthesis ("canonical signaling"), and acute effects as a modulator of neurotransmission ("non-canonical signaling"). BDNF has received a great deal of attention for its role in neurodegenerative diseases including Huntington's Disease (HD), Alzheimer's Disease (AD), and Parkinson's Disease (PD) and has been extensively reviewed elsewhere in this regard (e.g., [1-6]). However aging-related changes in BDNF function and expression have been studied only rarely, with the majority of studies characterizing changes in structures such as the hippocampus and neocortex. In this review, we attempt to briefly summarize the extent of the existing literature on age-related BDNF changes, and discuss the relevance of these changes as a factor potentially impacting therapeutics in aged parkinsonian subjects.

Comparison of Neurotoxicity following Repeated Administration of L-Dopa, D-Dopa, and Dopamine to Embryonic Mesencephalic Dopamine Neurons in Cultures Derived from Fisher 344 and Sprague-Dawley Donors

Levodopa is the most efficacious and widely used symptomatic drug for Parkinson's disease (PD). There is currently, however, a great deal of interest focused on the possibility that levodopa-induced increases in dopamine (DA) turnover may increase oxidative damage derived from the breakdown of DA. Increased oxidative damage following levodopa may contribute to the progressive degeneration of remaining host nigral neurons as well as interfere with development and function of embryonic nigral neurons in neural grafting trials. There is abundant evidence that levodopa is toxic to embryonic nigral DA neurons in both cell culture and neural grafting models. These findings have prompted a number of studies on mechanisms of levodopa toxicity to identify effective means of ameliorating potential oxidative stress related to levodopa in PD. In the current study we have utilized cultures of embryonic nigral DA neurons to address the fundamental question of whether levodopa-induced toxicity is related to DA production or whether dopa itself contributes to cell death. We compared the degree of nigral DA cell death following chronic administration of: 1) levodopa (e.g.: l-dopa); 2) its less active stereoisomer d-dopa; and 3) DA. We examined the rank order of toxicity of these compounds in two species of rats, Fisher 344 (F344) and Sprague-Dawley (SD). Results indicate a toxicity profile of: DA > l-dopa > > d-dopa. In addition, although there was no difference in response of F344 and SD cultures to l-dopa, the SD cultures were significantly more susceptible to the neurotoxic effects of DA. Taken collectively, these results suggest that levodopa-induced toxicity is related primarily to DA production rather than oxidation of dopa to toxic metabolites, and that some strain related differences do exist.

Neuroprotective potential of pleiotrophin overexpression in the striatonigral pathway compared with overexpression in both the striatonigral and nigrostriatal pathways

Intrastriatal injection of recombinant adeno-associated viral vector serotype 2/1 (rAAV2/1) to overexpress the neurotrophic factor pleiotrophin (PTN) provides neuroprotection for tyrosine hydroxylase immunoreactive (THir) neurons in the substantia nigra pars compacta (SNpc), increases THir neurite density in the striatum (ST) and reverses functional deficits in forepaw use following 6-hydroxydopamine (6-OHDA) toxic insult. Glial cell line-derived neurotrophic factor (GDNF) gene transfer studies suggest that optimal neuroprotection is dependent on the site of nigrostriatal overexpression. The present study was conducted to determine whether enhanced neuroprotection could be accomplished via simultaneous rAAV2/1 PTN injections into the ST and SN compared with ST injections alone. Rats were unilaterally injected in the ST alone or injected in both the ST and SN with rAAV2/1 expressing either PTN or control vector. Four weeks later, all rats received intrastriatal injections of 6-OHDA. Rats were euthanized 6 or 16 weeks relative to 6-OHDA injection. A novel selective total enumeration method to estimate nigral THir neuron survival was validated to maintain the accuracy of stereological assessment. Long-term nigrostriatal neuroprotection and functional benefits were only observed in rats in which rAAV2/1 PTN was injected into the ST alone. Results suggest that superior preservation of the nigrostriatal system is provided by PTN overexpression delivered to the ST and restricted to the ST and SN pars reticulata and is not improved with overexpression of PTN within SNpc neurons.

Stimulation of the rat subthalamic nucleus is neuroprotective following significant nigral dopamine neuron loss

Deep brain stimulation of the subthalamic nucleus (STN-DBS) is efficacious in treating the motor symptoms of Parkinson's disease (PD). However, the impact of STN-DBS on the progression of PD is unknown. Previous preclinical studies have demonstrated that STN-DBS can attenuate the degeneration of a relatively intact nigrostriatal system from dopamine (DA)-depleting neurotoxins. The present study examined whether STN-DBS can provide neuroprotection in the face of prior significant nigral DA neuron loss similar to PD patients at the time of diagnosis. STN-DBS between 2 and 4 weeks after intrastriatal 6-hydroxydopamine (6-OHDA) provided significant sparing of DA neurons in the SN of rats. This effect was not due to inadvertent lesioning of the STN and was dependent upon proper electrode placement. Since STN-DBS appears to have significant neuroprotective properties, initiation of STN-DBS earlier in the course of PD may provide added neuroprotective benefits in addition to its ability to provide symptomatic relief.

The synaptic impact of the host immune response in a parkinsonian allograft rat model: Influence on graft-derived aberrant behaviors

Graft-induced dyskinesias (GIDs), side-effects found in clinical grafting trials for Parkinson's disease (PD), may be associated with the withdrawal of immunosuppression. The goal of this study was to determine the role of the immune response in GIDs. We examined levodopa-induced dyskinesias (LIDs), GID-like behaviors, and synaptic ultrastructure in levodopa-treated, grafted, parkinsonian rats with mild (sham), moderate (allografts) or high (allografts plus peripheral spleen cell injections) immune activation. Grafts attenuated amphetamine-induced rotations and LIDs, but two abnormal motor syndromes (tapping stereotypy, litter retrieval/chewing) emerged and increased with escalating immune activation. Immunohistochemical analyses confirmed immune activation and graft survival. Ultrastructural analyses showed increases in tyrosine hydroxylase-positive (TH+) axo-dendritic synapses, TH+ asymmetric specializations, and non-TH+ perforated synapses in grafted, compared to intact, striata. These features were exacerbated in rats with the highest immune activation and correlated statistically with GID-like behaviors, suggesting that immune-mediated aberrant synaptology may contribute to graft-induced aberrant behaviors.

A clonal line of mesencephalic progenitor cells converted to dopamine neurons by hematopoietic cytokines: a source of cells for transplantation in Parkinson's disease

Neural progenitor cells potentially provide a limitless, on-demand source of cells for grafting into patients with Parkinson's disease (PD) if the signals needed to control their conversion into dopamine (DA) neurons could be identified. We have recently shown that cytokines which instruct cell division and differentiation within the hematopoeitic system may provide similar functions in the central nervous system. We have shown that mitotic progenitor cells can be isolated from embryonic rat mesencephalon and that these cells respond to a combination of interleukin-1, interleukin-11, leukemia inhibitory factor, and glial cell line-derived neurotrophic factor yielding a tyrosine hydroxylase-immunoreactive (THir) phenotype in 20-25% of total cells. In the present study, 24 clonal cell lines derived from single cells of mesencephalic proliferation spheres were examined for their response to the cytokine mixture. The clone yielding the highest percentage of THir neurons (98%) was selected for further study. This clone expressed several phenotypic characteristics of DA neurons and expression of Nurr1. The response to cytokines was stable for several passages and after cryopreservation for several months. When grafted into the striatum of DA-depleted rats, these cells attenuated rotational asymmetry to the same extent as freshly harvested embryonic DA neurons. These data demonstrate that mesencephalic progenitor cells can be clonally expanded in culture and differentiated in the presence of hematopoietic cytokines to yield enriched populations of DA neurons. When transplanted, these cells provide significant functional benefit in the rat model of PD.

Tumor necrosis factor alpha is toxic to embryonic mesencephalic dopamine neurons

Levels of the proinflammatory cytokine tumor necrosis factor alpha (TNFalpha) are increased in postmortem brain and cerebral spinal fluid from patients with Parkinson's disease (PD). This observation provides a basis for associating TNFalpha with neurodegeneration, but a specific toxicity in dopamine (DA) neurons has not been firmly established. Therefore, we investigated TNFalpha-induced toxicity in DA neurons by utilizing primary cultures of embryonic rat mesencephalon. Exposure to TNFalpha resulted in a dose-dependent decrease in DA neurons as evidenced by decreased numbers of tyrosine hydroxylase-immunoreactive (THir) cells. TNFalpha toxicity was selective for DA neurons in that neither glial cell counts nor the total number of neurons was decreased and no general cytotoxicity was evidenced by lactate dehydrogenase assay. Many of the cells which remained immunoreactive for TH had shrunken and rounded cell bodies with broken, blunted, or absent processes. However, TNFalpha-treated cultures also contained some THir cells which appeared to be undamaged and possibly resistant to TNFalpha-induced toxicity. Additionally, immunocytochemistry revealed basal expression of TNFalpha receptor 1 (p55, R1) and TNFalpha receptor 2 (p75, R2) on all cells within the mesencephalic cultures to some degree, even though only DA neurons were affected by TNFalpha treatment. These data strongly suggest that TNFalpha mediates cell death in a sensitive population of DA neurons and support the potential involvement of proinflammatory cytokines in the degeneration of DA neurons in PD.

Diminished survival of mesencephalic dopamine neurons grafted into aged hosts occurs during the immediate postgrafting interval

The survival rate of dopamine (DA) neurons in mesencephalic grafts to young adult rats is poor, estimated at 5-20%, and even poorer in grafts to the aged striatum. Grafted cells die in young adult rats during the first 4 days after implantation. The present study was undertaken to determine whether the decreased survival of DA neurons in grafts to aged rats is (1) due to additional cell death during the immediate postgrafting interval or (2) due to protracted cell loss during longer postgrafting intervals. We compared survival rates of tyrosine hydroxylase-immunoreactive (THir) neurons in cell suspension grafts to young adult (3 months) and aged (24 months) male Fischer 344 rats at 4 days and 2 weeks after transplantation. At 4 days after grafting, mesencephalic grafts within the aged rat striatum contain approximately 25% of the number of THir neurons in the same mesencephalic cell suspension grafted to young adult rats. This corroborates the decreased survival of grafted DA neurons we have demonstrated previously at 10 weeks postgrafting. THir neurons in grafts to the intact striatum possessed a significantly shorter "long axis" than their counterparts on the lesioned side. No significant differences in the number of apoptotic nuclear profiles or total alkaline phosphatase staining between mesencephalic grafts to young and aged rats were detectable at 4 days postgrafting. In summary, the present study indicates that the exaggerated cell death of grafted DA neurons that occurs following implantation to the aged striatum occurs during the immediate postgrafting interval, timing identical to that documented for young adult hosts.

Oligodendrocyte-type 2 astrocyte-derived trophic factors increase survival of developing dopamine neurons through the inhibition of apoptotic cell death

Survival of embryonic dopamine (DA) neurons is extremely low (5-20%) following transplantation. Strategies to increase this survival are critical to the future of transplantation for Parkinson's disease. We demonstrate here that a factor(s) released from striatal oligodendrocyte-type 2 astrocytes (SO2A) greatly improves the survival and phenotype expression of mesencephalic DA neurons in culture while simultaneously decreasing the presence of apoptotic nuclear profiles, as detected by the TUNEL method and bisbenzamide/tyrosine hydroxylase double labeling. This SO2A-derived trophic factor(s) has minimal effects on glia and no effect on nondopaminergic mesencephalic neurons. The developmental period during which this SO2A trophic effect occurs (E14-18) coincides with the period when mesencephalic grafts are undergoing the highest rates of apoptosis, i.e., immediately following implantation. Therefore, SO2A-derived trophic factor(s) offers great potential for the augmentation of grafted DA neuron survival.

Cyclosporin A protects striatal neurons in vitro and in vivo from 3-nitropropionic acid toxicity

The neuroprotective properties of cyclosporin A (CsA) are mediated by its ability to prevent mitochondrial permeability transition during exposure to high levels of calcium or oxidative stress. By using the mitochondrial toxin 3-nitropropionic acid (3NP), the present study assessed whether CsA could protect striatal neurons in vitro and in vivo. In vitro, 3NP produced a 20-30% reduction of striatal glutamic acid decarboxylase-immunoreactive (GAD-ir) neurons. A single treatment with CsA protected GAD-ir neurons from 3NP toxicity at lower (0.2 or 1.0 microM), but not at higher (5.0 microM) doses. Similar findings were seen when the cultures were treated twice with cyclosporin. In vivo experiments used the Lewis rat model of Huntington's disease (HD) in which a low 3NP dose was delivered subcutaneously through an osmotic minipump. Rats received unilateral or bilateral intrastriatal saline injections to disrupt the blood-brain barrier (BBB) and facilitate CsA reaching vulnerable neurons. In the first experiment, CsA treated 3NP-lesioned rats displayed significantly more dopamine-and adenosine-3;, 5;-monophosphate-regulated phosphoprotein (DARPP32-ir) neurons ipsilateral to BBB disruption compared to the contralateral intact striatum, indicating that disruption of the BBB maybe necessary for CsA's neuroprotective effects. In the second experiment, stereological counts of DARPP32-ir neurons revealed that CsA protected striatal neurons in a dose-dependent manner following bilateral disruption of the striatal BBB. Rats treated with the higher (15 or 20 mg/kg) but not lower (5 mg/kg) doses of CsA displayed greater numbers of DARRP32-ir striatal neurons relative to vehicle-treated 3NP-lesioned rats. Thus, under conditions in which CsA can gain access to striatal neurons, significant protection from 3NP toxicity is observed. Therefore, CsA or more lipophilic analogues of this compound, may be of potential therapeutic benefit by protecting vulnerable neurons from the primary pathological event observed in HD.

Time course of apoptotic cell death within mesencephalic cell suspension grafts: implications for improving grafted dopamine neuron survival

The vast majority ( congruent with 90%) of embryonic mesencephalic dopamine (DA) neurons die following transplantation to the striatum. Recent reports indicate that at least a subpopulation of grafted cells undergo apoptotic cell death at early times following implantation. This study examines the temporal pattern and magnitude of apoptotic cell death following the implantation of mesencephalic cell suspension grafts. Two techniques, a modified terminal deoxynucleotide-mediated nucleotide end labeling (TUNEL) technique and cresyl violet staining, are used to assess apoptotic cell death by detection of its biochemical and morphological identifiers, respectively. Male, Fischer 344 rats were examined at 1, 4, 7, and 28 days following implantation of embryonic day 14 (E14) ventral mesencephalic cells to the DA-denervated striatum. Results indicate that the overwhelming majority of apoptotic cell death occurs within the first 7 days after transplantation. However, the impact of the apoptosis that occurs over the first week following grafting only appears to limit grafted tyrosine hydroxylase-immunoreactive (THir) neuron survival during the first 4 days. No significant differences between the survival rates of THir neurons at 4 days after grafting and at 28 days after grafting were found. Therefore, it appears that the critical interval during which an estimated 90% of grafted DA neurons die is during the first 4 days postimplantation and that a major contributor to this cell death is apoptosis.

Striatal trophic activity is reduced in the aged rat brain

Our previous studies demonstrated that the survival of a mesencephalic graft was reduced in aged animals suggesting an age-related decline in target-derived neurotrophic activity. We tested this hypothesis by examining dopamine (DA) and trophic activities from the striatum of intact or unilateral 6-hydroxydopamine (6-OHDA) lesioned rats of increasing age. Fisher 344 rats were 4, 12, 18, and 23 months old (m.o.) at sacrifice. Half the animals had received unilateral 6-OHDA lesions of the mesostriatal DA pathway 8 weeks earlier. Striatal tissue punches were analyzed for DA, homovanillic acid (HVA), and DA activity (HVA/DA) using HPLC. The remainder of the striatal tissue was homogenized to generate tissue extracts which were added to E14.5 ventral mesencephalic cultures to test trophic activity. In the non-lesioned animals, striatal DA was reduced and striatal DA activity was increased in the 18 and 23 m.o. animals relative to the 4 and 12 m.o. animals. Striatal trophic activity was inversely related to age. In the lesioned animals, striatal DA ipsilateral to 6-OHDA infusion was below detection limits while the contralateral striatum exhibited age-related changes in DA similar to those seen in the non-lesioned animals. In 4 m.o. lesioned rats, striatal trophic activity ipsilateral to 6-OHDA infusion was elevated by 26% relative to the contralateral side. The ipsi/contra-lateral differences in striatal trophic activity were reduced in 12 m.o. animals and absent in the 18 and 23 m.o. groups. These data suggest that advancing age is associated with a reduction in striatal DA as well as trophic activity. Moreover, the aged striatum loses its ability to biochemically and trophically compensate for DA reduction and therefore may represent a more challenging environment for the survival, growth, and function of a fetal graft.

Diminished viability, growth, and behavioral efficacy of fetal dopamine neuron grafts in aging rats with long-term dopamine depletion: an argument for neurotrophic supplementation

We examined the behavioral and morphological correlates of the response to a single intrastriatal dispersed cell graft of fetal rat ventral mesencephalic tissue in male Fischer-344 rats of varying age (4, 17, and 24-26 months old) and history of mesostriatal dopamine (DA) depletion (1 or 14 months). Our goal was to determine the impact of advancing age and duration of DA depletion in the host on DA graft viability and function. The findings can be summarized as follows. (1) Fetal DA neuron grafts that were effective in completely ameliorating amphetamine-induced rotational behavior in young rats with short-term lesions were virtually without effect in aged rats with long-term lesions. Middle-aged rats with long-term lesions responded to these grafts with partial behavioral recovery. (2) Age of the host at the time of transplantation, and not duration of DA depletion, was the primary determinant of response to DA grafts. (3) Diminished efficacy of grafts in lesioned aging rats was related to decreased survival and neurite extension of transplanted DA neurons. (4) Co-grafts of DA neurons with Schwann cells as a source of neurotrophic support improved the behavioral outcome of grafts in aged lesioned rats. These findings support the view that the DA-depleted striatum of aged rats is an impoverished environment for survival, growth, and function of DA grafts. Consistent with this view, local supplementation of the neurotrophic environment of grafted DA neurons with products of co-grafted Schwann cells, a demonstrated source of neurotrophic activity for embryonic DA neurons, improved graft outcome.

Therapeutic potential of nerve growth factors in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative syndrome which primarily affects dopamine-producing neurons of the substantia nigra, resulting in poverty and slowness of movement, instability of gait and posture, and tremor at rest in individuals with the disease. While symptoms of the disease can be effectively managed for several years with available drugs, the syndrome is progressive and the efficacy of standard drugs wanes with time. One experimental approach to therapy is to use natural and synthetic molecules which promote survival and growth of dopaminergic neurons, so-called 'neurotrophic factors', to stabilise the diminishing population of dopaminergic neurons and stimulate compensation and growth in these cells. In this review, we examine the available evidence on 29 molecules with neurotrophic properties for dopaminergic neurons. The properties of these molecules provide ample reasons for optimism that a neurotrophic strategy can be developed that would provide a significant treatment option for patients with PD. While the search continues for even more specific, potent and long lasting agents, the single greatest challenge is the development of techniques for targeted delivery of these molecules.

Co-grafted embryonic striatum increases the survival of grafted embryonic dopamine neurons

To enhance the current therapeutic benefit of dopamine (DA) neuron grafts in Parkinson's disease, strategies must be developed that increase both DA neuron survival and fiber outgrowth into the denervated striatum. Previous work in our laboratory has demonstrated that dopaminergic neurons grow to greater size when co-grafted with striatal cell suspensions and display extensive tyrosine hydroxylase-positive (TH+) projections, but no conclusion could be reached concerning enhancement of survival of grafted DA neurons. The aim of the present study was to characterize further the potential trophic effects of striatal co-grafts on grafted mesencephalic DA neuron survival. Unilaterally lesioned male Fischer 344 rats were grafted with either a suspension of mesencephalic cells or with both mesencephalic and striatal cell suspensions. Co-grafts were either mixed together or placed separately into the striatum. Lesioned rats receiving no graft served as controls. Rotational behavior was assessed following amphetamine challenge at 2 weeks prior to grafting and at 4 and 8 weeks following grafting. Only rats receiving co-grafts of nigral and striatal suspensions separated by a distance of 1 mm showed significant behavioral recovery from baseline rotational asymmetry. Both mixed and separate striatal co-grafts were associated with a doubling of DA neuron survival compared with solo mesencephalic grafts. In the mixed co-graft experiment, DA neurite branching appeared enhanced and TH-rich patches were observed, whereas with co-grafts that were separated, TH+ innervation of the intervening host striatum was increased significantly. These results provide the first evidence suggesting that nigral-striatal co-grafts, particularly those placed separately and in proximity to each other, increase both DA neuron survival and neurite extension from the mesencephalic component of the grafts.

Pattern of synaptophysin immunoreactivity within mesencephalic grafts following transplantation in a parkinsonian primate model

The majority of investigations into the degree of restoration of neural circuitry following transplantation of the embryonic ventral mesencephalon to the striatum have focused upon the particular neurochemical subtypes of the fibers exchanged between graft and host. Visualization of neurites of specific neurotransmitter type while informative regarding the specificity of graft-host interactions, vastly underrepresents overall synaptogenesis as it may occur in the grafting situation. The present approach of using a molecular marker characteristic of all normal, functional synapses provides broader information about the synaptic remodeling that occurs after tissue grafting. Synaptophysin (SY), an integral membrane protein of the synaptic vesicle, is a reliable marker of nerve terminal differentiation. Immunohistochemical staining with antibodies directed against SY and the dopamine synthetic enzyme tyrosine hydroxylase (TH) was used to assess overall synaptic differentiation as well as the relationship between SY immunoreactivity and the distribution of grafted dopamine (DA) neurons and processes in mesencephalic grafts and mesencephalic-striatal co-grafts implanted in the striatum of MPTP-treated African green monkeys. Grafted embryonic cerebellar tissue was used as a comparison graft type that does not normally exchange prominent direct projections with striatum. Dense pericellular arrays of SY-positive terminals were associated with TH-positive neurons in mesencephalic grafts. In mixed mesencephalic-striatal co-grafts, TH-positive fiber patches within the striatal portion of the graft demonstrated a high degree of correspondence with SY immunoreactivity. In contrast, grafts of cerebellar tissue did not display the same pattern of prominent pericellular arrays of SY staining. These observations suggest that functional synapses are abundantly present within grafted mesencephalon, and that these contacts are enriched in areas of the graft occupied by DA neurons. Implantation of an inappropriate striatal target, the cerebellum, results in visibly diminished innervation. The pattern of SY labeling observed suggests that tissue grafts are extensively innervated, probably both from extrinsic and intrinsic sources, and that the pattern and density of this innervation corresponds to the appropriateness of the graft-host interaction.

Agonist-induced morphologic decrease in cellular D1A dopamine receptor staining

The distribution of D1A dopamine (DA) receptor proteins was assessed by using subtype specific antireceptor antisera after acute DA exposure. The immunofluorescent staining of D1A DA receptor protein expression was examined in (1) stably transfected Chinese hamster ovary (CHO) cells, (2) primary striatal cell cultures, and (3) rat striatal brain slices. After agonist exposure as brief as 2 min and as long as 60 min, profound loss of immunofluorescent D1A receptor protein staining occurred in each paradigm. Additionally in the tissue slice, immunofluorescent neuropil staining for the receptor protein also was attenuated. The DA-induced alteration in receptor protein staining was blocked by the antagonist (+)-butaclamol and by the selective D1-family antagonist SCH 23390. Receptor staining patterns reverted back to the control immunofluorescent distribution within 15 min after removing the agonist from the bath. Immunofluorescence for the second-messenger cyclic AMP increased at all DA exposure times in the three experimental paradigms, was blocked by D1-family antagonists, and decreased to basal staining after brief recovery periods. This demonstrated the functional integrity of the D1A receptor in target cells. Pretreatment with the mitogenic plant lectin concanavalin A blocked the immunofluorescent decrease in receptor staining but not the elevation of the second messenger, indicating a morphologic distinction in these two events, parallel to other biochemical reports. The data suggested that a morphologic basis of acute homologous D1A DA receptor desensitization may be transposition of membrane-surface receptors to a transiently unavailable, intracellular compartment. This finding is supported by specific fluorescence incorporation of FM1-43, used as a marker of endocytosis, in CHO cells treated with DA.

Comparison of neurotoxicity following repeated administration of l-dopa, d-dopa and dopamine to embryonic mesencephalic dopamine neurons in cultures derived from Fisher 344 and Sprague-Dawley donors

Levodopa is the most efficacious and widely used symptomatic drug for Parkinson's disease (PD). There is currently, however, a great deal of interest focused on the possibility that levodopa-induced increases in dopamine (DA) turnover may increase oxidative damage derived from the breakdown of DA. Increased oxidative damage following levodopa may contribute to the progressive degeneration of remaining host nigral neurons as well as interfere with development and function of embryonic nigral neurons in neural grafting trials. There is abundant evidence that levodopa is toxic to embryonic nigral DA neurons in both cell culture and neural grafting models. These findings have prompted a number of studies on mechanisms of levodopa toxicity to identify effective means of ameliorating potential oxidative stress related to levodopa in PD. In the current study we have utilized cultures of embryonic nigral DA neurons to address the fundamental question of whether levodopa-induced toxicity is related to DA production or whether dopa itself contributes to cell death. We compared the degree of nigral DA cell death following chronic administration of: 1) levodopa (e.g.: l-dopa); 2) its less active stereoisomer d-dopa; and 3) DA. We examined the rank order of toxicity of these compounds in two species of rats, Fisher 344 (F344) and Sprague-Dawley (SD). Results indicate a toxicity profile of: DA > l-dopa >> d-dopa. In addition, although there was no difference in response of F344 and SD cultures to l-dopa, the SD cultures were significantly more susceptible to the neurotoxic effects of DA. Taken collectively, these results suggest that levodopa-induced toxicity is related primarily to DA production rather than oxidation of dopa to toxic metabolites, and that some strain related differences do exist.

Pharmacologic specificity of antidepressive activity by monoaminergic neural transplants

Previous studies in our laboratory have demonstrated the ability of monoaminergic transplants in the rat frontal cortex to produce antidepressive activity in both the learned helplessness model and the forced swimming test, as well as to increase monoamine levels in the implanted frontal cortex. These findings implicate increased cortical levels of norepinephrine (NE) and serotonin (5-HT) in the antidepressive activity of monoaminergic transplants. The goal of the present study was to characterize the pharmacologic mechanisms involved in the monoaminergic graft-induced antidepressive activity. Immobility scores in the forced swimming test (FST) were assessed after transplantation of 5-HT-containing pineal gland tissue, NE-containing adrenal medullary tissue, a combination of both tissues, or sciatic nerve (control) into the rat frontal cortex and compared to non-transplanted and chronic imipramine-treated rats. Monoaminergic transplants and imipramine treatment significantly reduced immobility scores in the FST in contrast to control transplanted or untreated animals. All groups were assessed pharmacologically with the adrenergic antagonists phentolamine (alpha) and propranolol (beta), and serotonergic antagonists metergoline (5-HT1/5-HT2) and pirenperone (5-HT2). Serotonergic antagonists, particularly the 5HT2 antagonist, blocked the reduction in FST immobility induced by the pineal implants. Adrenergic antagonists not only blocked FST immobility reductions in adrenal medullary grafted animals, but over-compensated for the adrenal transplants, producing a large increase in immobility. The FST reduction induced by pineal and adrenal cografts was blocked by all four monoaminergic antagonists. FST immobility scores in control transplanted and non-transplanted animals were not altered by any of the antagonists. The immobility reduction produced by chronic imipramine treatment was blocked significantly only by propranolol.(ABSTRACT TRUNCATED AT 250 WORDS)

Chromaffin cell xenografts in the rat neocortex can produce antidepressive activity in the forced swimming test

Adrenal medullary allografts, as well as other monoaminergic tissues, have been demonstrated in our laboratory to increase antidepressive activity when transplanted into the frontal neocortex of rats. Refinement in the optimal parameters for xenograft viability has indicated that isolated bovine chromaffin cells may be an improved source of graft donor tissue. The aim of the present study was to determine whether isolated bovine chromaffin cell grafts to the rat frontal neocortex could provide an alternative source of catecholamines for antidepressant activity. Isolated bovine chromaffin cells, isolated bovine fibroblasts, or an equal volume of vehicle were unilaterally implanted into the right or left frontal cortex or right visual cortex. All rats were assessed before and 6 weeks after transplantation using the forced swimming test, a popular measure of antidepressant activity. Bovine chromaffin cell grafts in either the right or left frontal cortex produced significant increases in antidepressant activity compared to grafts of bovine fibroblasts and sham-operated or nontransplanted rats. In contrast, bovine chromaffin cells transplanted to the visual cortex did not affect antidepressant activity. Bovine fibroblast grafts in the frontal cortex also induced slight increases in antidepressant activity, although significantly less than chromaffin cell grafts. Morphological analysis revealed robust survival of tyrosine hydroxylase-positive chromaffin cells that retained their in situ ultrastructure and occasionally formed synaptic connections with the host parenchyma. These results suggest that xenografted isolated bovine chromaffin cells can provide a viable source of catecholamines for antidepressive activity.

In vivo release of catecholamines from xenogeneic chromaffin cell grafts with antidepressive activity

Previous studies in our laboratory have demonstrated that allografts of adrenal medullary tissue and xenografts of isolated bovine chromaffin cells to the rat frontal cortex can increase antidepressive activity in two separate animal models. Biochemical and pharmacological evidence suggest that the most likely mechanism of these antidepressive effects is via local release of catecholamines into the surrounding cortical parenchyma. The aim of the present study was to directly characterize the antidepressive mechanism of chromaffin cell xenografts by utilizing in vivo microdialysis to measure extracellular catecholamine levels from bovine chromaffin cell and control implanted rat frontal cortex. Following transplantation, only bovine chromaffin cell grafted rats displayed significant increases in antidepressive activity, as assessed by the forced swimming test, compared to rats with grafts of bovine adrenal medullary fibroblasts or nontransplanted rats. In vivo microdialysis results revealed remarkably elevated levels of epinephrine (EPI) and norepinephrine (NE), but not dopamine, in dialysates from bovine chromaffin cell-transplanted frontal cortex. The most likely source of these enhanced EPI and NE levels is the grafted xenogeneic chromaffin cells. The results of this study directly demonstrate that xenografts of bovine chromaffin cells to the rat frontal cortex provide a releasable pool of catecholamines for antidepressive activity.

Induction of antidepressive activity by monoaminergic transplants in rat neocortex

To assess the ability of monoaminergic transplants to reduce immobility in the forced swimming test (FST), either adrenal medullary tissue, pineal gland tissue, or equal volumes of sciatic nerve were transplanted into the rat frontal neocortex. In the FST the duration of immobility is thought to indicate the level of antidepressant activity, as immobility times are reliably reduced by antidepressant therapies. Immobility times were reduced in rats with adrenal medullary grafts and pineal grafts to the rat frontal neocortex. In contrast, immobility times were not reduced in control sciatic nerve tissue grafts. Biochemical analysis using HPLC revealed that pineal-grafted neocortex contained higher levels of serotonin (5-HT) and adrenal medullary-grafted neocortex contained higher levels of norepinephrine (NE) than sciatic nerve-grafted or nongrafted controls. Immunocytochemical studies showed that the monoaminergic grafts survived well and continue to produce high levels of monoamines. These results support an important role for neocortical 5-HT and NE transmission in antidepressant activity and suggest that transplants of monoaminergic-containing tissue can reduce biochemical deficits in depression.

Monoaminergic neural transplants prevent learned helplessness in a rat depression model

Current theories of the etiology of depression implicate disturbances and imbalances in the function of monoaminergic systems, particularly involving serotonin and norepinephrine. Neural transplantation is a potential approach towards restoring balanced functioning in the central nervous system. The purpose of the present study was to determine the utility of transplanting monoamine-producing cells into the brain to alleviate behavioral depression. Serotonin-containing pineal gland tissue, catecholamine-containing adrenal medullary tissue, a combination of both, and a control of striated muscle tissue were implanted into the frontal neocortex of adult rats. The ability of these grafts to prevent the development of learned helplessness, a widely accepted model for depression, was assessed 6-8 weeks following transplantation. The monoamine-containing transplants, but not the control transplants, were able to prevent the development of learned helplessness. Immunocytochemical and ultrastructural studies revealed that the grafted monoaminergic tissues survived and continued to produce high levels of monoamines. These results suggest that neural transplants may provide a long-term local source of monoamines as a potentially new approach for alleviating some forms of depression.