Dopaminergic inhibition of striatal GABA release after 6-hydroxydopamine

α-synucleinopathy exerts sex-dimorphic effects on the multipurpose DNA repair/redox protein APE1 in mice and humans

Lewy body disorders are characterized by oxidative damage to DNA and inclusions rich in aggregated forms of α-synuclein. Among other roles, apurinic/apyrimidinic endonuclease 1 (APE1) repairs oxidative DNA damage, and APE1 polymorphisms have been linked to cases of Lewy body disorders. However, the link between APE1 and α-synuclein is unexplored. We report that knockdown or inhibition of APE1 amplified inclusion formation in primary hippocampal cultures challenged with preformed α-synuclein fibrils. Fibril infusions into the mouse olfactory bulb/anterior olfactory nucleus (OB/AON) elicited a modest decrease in APE1 expression in the brains of male mice but an increase in females. Similarly, men with Lewy body disorders displayed lower APE1 expression in the OB and amygdala compared to women. Preformed fibril infusions of the mouse OB/AON induced more robust base excision repair of DNA lesions in females than males. No fibril-mediated loss of APE1 expression was observed in male mice when the antioxidant N-acetylcysteine was added to their diet. These findings reveal a potential sex-biased link between α-synucleinopathy and APE1 in mice and humans. Further studies are warranted to determine how this multifunctional protein modifies α-synuclein inclusions and, conversely, how α-synucleinopathy and biological sex interact to modify APE1.

Effects of 2,3-benzodiazepine AMPA receptor antagonists on dopamine turnover in the striatum of rats with experimental parkinsonism

Although levodopa is the current "gold standard" for treatment of Parkinson's disease, there has been disputation on whether AMPA receptor antagonists can be used as adjuvant therapy to improve the effects of levodopa. Systemic administration of levodopa, the precursor of dopamine, increases brain dopamine turnover rate and this elevated turnover is believed to be essential for successful treatment of Parkinson's disease. However, long-term treatment of patients with levodopa often leads to development of dyskinesia. Therefore, drugs that feature potentiation of dopamine turnover rate and are able to reduce daily levodopa dosages might be used as adjuvant in the treatment of patients suffering from Parkinson's disease. To investigate such combined treatment, we have examined the effects of two non-competitive AMPA receptor antagonists, GYKI-52466 and GYKI-53405, alone or in combination with levodopa on dopamine turnover rate in 6-hydroxydopamine-lesioned striatum of the rat. We found here that repeated administration of levodopa, added with the peripheral DOPA decarboxylase inhibitor carbidopa, increased dopamine turnover rate after lesioning the striatum with 6-hydroxydopamine. Moreover, combination of levodopa with GYKI-52466 or GYKI-53405 further increased dopamine turnover enhanced by levodopa administration while the AMPA receptor antagonists by themselves failed to influence striatal dopamine turnover. We concluded from the present data that potentiation observed between levodopa and AMPA receptor antagonists may reflect levodopa-sparing effects in clinical treatment indicating the therapeutic potential of such combination in the management of Parkinson's disease.

Effects of adrenal medulla graft on recovery of GABAergic and dopaminergic neuron deficits in mice: behavioural, pharmacological and immunohistochemical study

We studied the capacity of adrenal medullary transplant to restore the deficits of GABAergic and dopaminergic neurons in mice injected with quinolinic acid (QA), using an open field test as well as pharmacological and immunohistochemical techniques. We analysed behavioural traits-total locomotor activity, peripheral and central activities, grooming, leaning and rearing in the QA-lesioned mice and mice that had undergone adrenal medulla (AM) transplantation. We found that the adrenal transplant recovered a loss of GABAergic neurons. It reduced QA-induced hyperactivity in locomotion and improved emotional indices. In addition, immunohistochemical studies of catecholaminergic markers-tyrosine hydroxylase (TH), dopamine (DA) and neuronal vesicular monoamine transporter type 2- and a single post-trial injection of tetrabenazine (TBZ; 5 mg/kg) indicated that catecholamines-synthesising chromaffin cells in the AM grafts were also involved in the beneficial effects. A likely interpretation of this behavioural pattern of results is that adrenal medullary transplants set into play an interaction between GABAergic and DAergic factors. Our results may contribute to the clarification of the beneficial effects of AM transplants in striatal function.

In vivo extracellular recording of striatal neurons in the awake rat following unilateral 6-hydroxydopamine lesions

The purpose of this study was to further understand the functional effects of dopaminergic input to the dorsal striatum and to compare the effects of dopaminergic lesions in awake and anesthetized animals. We examined the effects of unilateral 6-hydroxydopamine (6-OHDA) lesions of the ascending dopaminergic bundle on the firing properties of dorsal striatal neurons in the awake freely moving rat using chronically implanted microwire electrode arrays. We recorded extracellular activity of striatal neurons under baseline conditions and following the systemic injection of apomorphine in awake and anesthetized subjects. Firing rates were higher in the hemisphere ipsilateral to the 6-OHDA lesion compared to rates of neurons from the contralateral unlesioned hemisphere. Striatal firing rates from sham and no-surgery control rats were, in general, higher than those from the contralateral unlesioned striatum of experimental subjects. Apomorphine (0.05 mg/kg, sc) normalized the differences in firing rates in lesioned animals by increasing firing of neurons within the contralateral unlesioned side, while simultaneously decreasing firing of neurons within the ipsilateral lesioned side. Mean firing rates were substantially higher in awake animals than in subjects anesthetized with chloral hydrate, perhaps reflecting anesthesia-induced decreases in excitatory input to striatal neurons. Chloral hydrate anesthesia decreased firing rates of neurons in the lesioned, unlesioned, and control striata to a similar degree, although absolute firing rates of neurons from the 6-OHDA-lesioned striata remained elevated over all other groups. Unilateral 6-OHDA lesions also altered the pattern of spike output in the awake animal as indicated by an increase in the number of bursts per minute following dopaminergic deafferentation. This and other burst parameters were altered by apomorphine. Our findings show that effects of dopaminergic deafferentation can be measured in the awake behaving animal; this model should prove useful for testing the behavioral and functional effects of experimental manipulations designed to reduce or reverse the effects of dopaminergic cell loss. In addition, these results suggest that the contralateral changes in striatal function which occur in the unilateral dopaminergic lesion model should be considered when evaluating experimental results.

Involvement of adrenal medulla grafts in the open field behavior

Immunohistochemical and behavioral techniques were used to study the effects of adrenal medulla grafts, implanted in striatum after bilateral kainic acid (KA) lesions of this structure, on the open field behavior of mice. KA-induced behavioral changes in leaning, grooming and locomotor activity of the open field test were significantly improved after grafting of the adrenal medulla, and in some respects, fully restored. Immunohistochemical identification showed that grafts contained neuron-like cells with a tyrosine hydroxylase (TH), phenylethanolamine N-methyltransferase, gamma-aminobutyric acid (GABA), choline acetyltransferase (ChAT), and enkephalin-like immunostainings. A likely interpretation of this complex pattern of results is that adrenal medullary grafts may restore the deficits of GABAergic neurons which in turn reverse the abnormalities in emotionality and locomotion. Neurobiologically, these behavioral improvements probably involve GABAergic and catecholaminergic factors of adrenal medulla grafts, although other neuroactive substances, such as acetylcholine and enkephalins, cannot be excluded.

Focal bicuculline increases extracellular dopamine concentration in the nucleus accumbens of freely moving rats as measured by in vivo microdialysis

This study was designed to assess the involvement of GABA(A) receptors in the regulation of in vivo dopamine release in the nucleus accumbens. Extracellular dopamine in the nucleus accumbens was measured using intracerebral microdialysis coupled with a high-performance liquid chromatography with electrochemical detection (HPLC-EC) system in freely moving Sprague-Dawley rats. Bicuculline, a GABA(A) receptor antagonist, and muscimol, a GABA(A) receptor agonist, were administered via a dialysis probe into the nucleus accumbens, respectively. The results showed that perfusion with bicuculline at concentrations of 25, 50, and 100 microM elicited a significant and concentration-dependent increase in extracellular dopamine in the nucleus accumbens. Dopamine levels returned to control values within 40-60 min after the termination of bicuculline perfusion. The increased dopamine produced by perfusion with 100 microM bicuculline was sensitive to sodium channel blockade with tetrodotoxin, and antagonized by co-perfusion with muscimol (25 and 50 microM) in a concentration-related fashion. Perfusion with 25 or 50 microM muscimol alone failed to alter basal levels of dopamine. The results suggest that local application of bicuculline increases dopamine release in the nucleus accumbens via a receptor-mediated process, and are consistent with the concept that basal dopamine release in the nucleus accumbens is under tonic inhibitory control by GABA(A) receptors within this structure.

Do compensatory processes underlie the preclinical phase of neurodegenerative disease? Insights from an animal model of parkinsonism

Lesions of the DA neurons innervating the striatum is accompanied by permanent gross neurological deficits only when the loss of striatal DA is almost complete, a finding reminiscent of Parkinson's disease. This appears to result at least in part from an enhanced capacity of the remaining DA neurons to continue to modulate DA-sensitive targets in the striatum. Among the neurochemical changes that may be responsible for this enhanced capacity are a loss of high-affinity DA uptake sites and time-dependent increases in the synthesis and release of DA. Following very large lesions, an increase in the sensitivity of striatal cells to DA also gradually occurs (Fig. 1). A lesion-induced increase in the functional activity of residual neurons may be a rather general phenomenon. We have made analogous observations in the sympathoadrenal system (Fluharty et al., 1985) and in the noradrenergic (Acheson & Zigmond, 1981; Chiodo et al., 1983; Abercrombie et al., 1989) and serotonergic (Stachowiak et al., 1986) systems of CNS. Thus, during many neurodegenerative diseases, compensatory changes in the affected neural system and its targets may be involved in the extended preclinical stage that often is observed. This hypothesis has several implications. First, many clinical disorders that appear late in life may in fact have their origins in events that had occurred many years earlier, and the emergence of neurological or psychiatric symptoms may represent the end stage of the neurodegenerative-process, rather than its onset. Second, to reverse clinical symptoms one may not need to reverse the entire neurobiological deficit; instead, clinical recovery might be achieved with a relatively modest restoration of the injured projections. Third, it may be possible to achieve recovery even without restoring the connections that have been lost if the capacity of the remaining elements of the injured system can be enhanced further. Finally, in some cases arresting the degenerative process may be sufficient; the natural compensatory processes of the nervous system might then be permitted the time needed to restore function.