Muneo Matsunaga

Role of serotonergic neurons in L-DOPA-derived extracellular dopamine in the striatum of 6-OHDA-lesioned rats.

The effect of L-dihydroxyphenylalanine (L-DOPA) on extracellular dopamine (DA) in the striatum was determined by microdialysis in 6-hydroxydopamine (6-OHDA)-lesioned rats treated with and without the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT). At the same time the intensity of L-DOPA-induced rotational behavior was assessed. In 6-OHDA-lesioned rats treated with 5,7-DHT, L-DOPA (50 mg/kg, i.p.) increased extracellular DA only to 20% of that measured in animals not treated with 5,7-DHT. Likewise, 6-OHDA-lesioned rats treated with 5,7-DHT exhibited a significantly lower number of L-DOPA-induced rotations. These results suggest that serotonergic terminals in the striatum can convert exogenously administered L-DOPA into DA that can be released into the extracellular space.

Activation of 5-HT(1A) but not 5-HT(1B) receptors attenuates an increase in extracellular dopamine derived from exogenously administered L-DOPA in the striatum with nigrostriatal denervation.

In order to determine whether l‐DOPA‐derived extracellular dopamine (DA) in the striatum with dopaminergic denervation is affected by activation of serotonin autoreceptors (5‐HT1A and 5‐HT1B receptors), we applied in vivo brain microdialysis technique to 6‐hydroxydopamine‐lesioned rats and examined the effects of the selective 5‐HT1A receptor agonist 8‐hydroxy‐2‐(di‐n‐propylamino)tetralin (8‐OH‐DPAT) and the selective 5‐HT1B receptor agonist CGS‐12066 A on l‐DOPA‐derived extracellular DA levels. Single l‐DOPA injection (50 mg/kg i.p.) caused a rapid increase and a following decrease of extracellular DA, with a peak value at 100 min after l‐DOPA injection. Pretreatment with both 0.3 mg/kg and 1 mg/kg 8‐OH‐DPAT (i.p.) significantly attenuated an increase in l‐DOPA‐derived extracellular DA and the times of peak DA levels were prolonged to 150 min and 225 min after l‐DOPA injection, respectively. These 8‐OH‐DPAT‐induced changes in l‐DOPA‐derived extracellular DA were antagonized by further pretreatment with WAY‐100635, a selective 5‐HT1A antagonist. In contrast, intrastriatal perfusion with the 5‐HT1B agonist CGS‐12066 A (10 nm and 100 nm) did not induce any changes in l‐DOPA‐derived extracellular DA. Thus, stimulation of 5‐HT1A but not 5‐HT1B receptors attenuated an increase in extracellular DA derived from exogenous l‐DOPA. These results support the hypothesis that serotonergic neurons are primarily responsible for the storage and release of DA derived from exogenous l‐DOPA in the absence of dopaminergic neurons.

Reserpine pretreatment prevents increases in extracellular striatal dopamine following L-DOPA administration in rats with nigrostriatal denervation

Abstract: The influence of L‐DOPA and reserpine on extracellular dopamine (DA) levels in the striatum of intact and dopaminergic denervated rats was studied using the brain microdialysis technique. In intact rats, reserpine (5 mg/kg s.c.) reduced extracellular DA levels to 4% of basal values. L‐DOPA (50 mg/kg i.p.) had no effect on extracellular DA levels in reserpine‐pretreated rats. In rats with 6‐hydroxydopamine‐induced lesion of the nigrostriatal dopaminergic system, basal levels of extracellular DA were low but markedly increased by L‐DOPA (50 mg/kg i.p.). In 6‐hydroxydopamine‐lesioned rats, pretreatment with reserpine (5 mg/kg s.c.) diminished L‐DOPA (50 mg/kg i.p.)‐induced increases in extracellular DA levels to 16% of those obtained in denervated animals not pretreated with reserpine (p < 0.01). These results suggest that in the intact striatum, extracellular DA stems mainly from vesicular storage sites and that in the striatum with dopaminergic denervation, a large part of the L‐DOPA‐derived extracellular DA is also derived from a vesicular pool that is released by an exocytosis mechanism.

A serotonin 5-HT1A receptor agonist prevents behavioral sensitization to L-DOPA in a rodent model of Parkinson's disease.

Marked fluctuation of dopamine concentration in the striatum following long-term L-DOPA administration contributes to the development of L-DOPA-induced motor complications including L-DOPA-induced dyskinesias and wearing-off in patients with Parkinsons disease. We have shown that pretreatment with 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A (5-hydroxytryptamine) receptor agonist, alleviates fluctuation of dopamine levels in the dopamine-denervated striatum of 6-hydroxydopamine-lesioned (hemiparkinsonian) rats after L-DOPA treatment. To determine whether co-administration of 8-OH-DPAT with L-DOPA prevents L-DOPA-induced motor complications, we examined rotation behavior and levels of messenger RNAs coding for dynorphin and glutamic acid decarboxylase in the striatum of 6-hydroxydopamine-lesioned rats treated with L-DOPA alone or L-DOPA + 8-OH-DPAT, twice daily, for 2 weeks. Co-administration of 8-OH-DPAT inhibited an increase of rotation behavior to L-DOPA and L-DOPA-induced increases in levels of messenger RNAs coding for dynorphin and glutamic acid decarboxylase in the dopamine-denervated striatum, both of which are established indices of L-DOPA-induced motor complications. These results suggest that pharmaceutical products that stimulate 5-HT1A receptors could prove useful in prevention of the development of L-DOPA-induced motor complications in patients with Parkinsons disease.

Rapid induction of serotonergic hyperinnervation in the adult rat striatum with extensive dopaminergic denervation

The aim of our study was to determine whether serotonergic hyperinnervation is rapidly induced in the striatum of adult rats with extensive dopaminergic denervation. Immunohistochemical study was performed on the brain sections obtained at 2 and 8 weeks after injection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle. The extent of dopaminergic denervation was evaluated as a percentage loss of tyrosine hydroxylase immunopositive neurons in the substantia nigra pars compacta. The immunopositive areas for serotonin (5-HT) in the striatum were measured. In the lesioned rats 97.5+/-0.6% of dopamine neurons were lost. 5-HT immunopositive areas in the striatum were significantly increased both at 2 and 8 weeks after 6-OHDA injection (P<0.01). These results suggest that extensive dopaminergic denervation in adult rats induces rapid serotonergic hyperinnervation in the striatum as early as 2 weeks after lesioning.

High frequency oscillations in early cortical somatosensory evoked potentials

OBJECTIVE To evaluate the characteristics of high frequency (HF) components of the early cortical somatosensory evoked potentials (SEPs). METHODS We recorded 8-channel SEPs from the frontal and left centro-parietal scalp after right median nerve stimulation with a wide band-pass (0.5-2000 Hz) and digitized at 40 kHz sampling rate in 12 healthy subjects. HF components were analyzed after digital band-pass filtering (300-1000 Hz). The power spectrum was obtained by a maximum entropy method. RESULTS HF oscillations (maximum power at 600-800 Hz) consisting of 5 to 8 peaks were discriminated from the preceding P14 far-field in all cases and their phases were reversed between the frontal and contralateral parietal regions. In addition, in subjects with a high amplitude central P22 potential in original wide-band recordings, a single HF oscillation with a maximum at the central region was present. Furthermore, this component showed no phase reversal over the centro-parietal area. CONCLUSION We therefore conclude that HF oscillations are superimposed not only on the tangential N20-P20 but on the radial P22 potential, and are generated from both tangential (area 3b) and radial (area 1) current sources.

Fluoxetine reduces L-DOPA-derived extracellular DA in the 6-OHDA-lesioned rat striatum.

We investigated the effect of fluoxetine, a selective serotonin reuptake inhibitor (SSRI), on L-DOPA-derived extracellular dopamine (DA) levels in the striatum of rats with nigrostriatal dopaminergic denervation using in vivo microdialysis. Treatment with fluoxetine (10 mg/kg, i.p.) induced a 41% reduction in the cumulative amount of extracellular DA during 300 min following L-DOPA administration (50 mg/kg, i.p.;p < 0.01). This effect was antagonized by pretreatment with WAY-100635, a potent 5-HT1A antagonist, indicating that this effect of fluoxetine is due to its indirect 5-HT1A agonistic property. These results suggest that SSRIs may impair motor functions in patients with Parkinsons disease by reducing efflux of exogenous L-DOPA-derived DA.

Peripheral and central conduction abnormalities in diabetes mellitus

Objectives: To investigate peripheral and central somatosensory conduction in patients with diabetes. Methods: The authors recorded sensory nerve action potentials and 5-channel somatosensory evoked potentials (SEPs) with noncephalic reference after median nerve stimulation in 55 patients with diabetes and 41 age- and height-matched normal subjects. The authors determined onset or peak latencies of the Erb’s potential (N9) and the spinal N13-P13 and the cortical N20-P20 components, and obtained the central conduction time (CCT) by onset-to-onset and peak-to-peak measurements. Results: Both onset and peak latencies of all SEP components were prolonged in patients with diabetes. The mean onset CCT in the diabetic group was 6.3 ± 0.5 msec (mean ± SD)—significantly longer than that in the control group (6.1 ± 0.2 msec)—whereas no significant difference was found in the peak CCT. The amplitudes of N9 and N13-P13 components (but not N20-P20) were significantly smaller in the diabetic group. The peripheral sensory conduction velocity was also decreased in the diabetic group, but there was no significant correlation between peripheral conduction slowing and the onset of CCT prolongation. Conclusions: Diabetes affects conductive function in the central as well as peripheral somatosensory pathways. The CCT abnormality does not coincide with lowering of the peripheral sensory conduction. The current results do not favor a hypothesis that a central–peripheral distal axonopathy plays an important role in development of diabetic polyneuropathy.

Loss of regulation by presynaptic dopamine D2 receptors of exogenous l-DOPA-derived dopamine release in the dopaminergic denervated striatum

To determine whether dopamine release derived from exogenous l-DOPA is under inhibitory control of presynaptic dopamine D2 receptors in the dopaminergic denervated striatum, extracellular dopamine levels were measured in the striatum of 6-hydroxydopamine-lesioned rats using in vivo brain microdialysis. Quinpirole, a D2 agonist, dose-dependently (0.01-3 mg/kg s.c.) inhibited endogenous dopamine release both in the intact and dopaminergic denervated striatum. The dose-response curve obtained from the denervated striatum showed a shift to the right. Administration of l-DOPA (30 mg/kg i.p.) with carbidopa (25 mg/kg i.p.) increased dopamine release to 130% of basal levels in the intact striatum and 770% of basal levels in the denervated striatum. In the intact striatum, dopamine release was continuously inhibited by quinpirole pretreatment (1 mg/kg s.c.) even after l-DOPA administration. In the denervated striatum, l-DOPA-derived dopamine release was not affected by quinpirole pretreatment. These results suggest that, in the striatum with dopaminergic denervation, regulation by presynaptic D2 receptors is still operative on endogenous dopamine release but it does not work on dopamine release derived from exogenously administered l-DOPA.

Expression of metabotropic glutamate receptor mRNAs in the human spinal cord: implications for selective vulnerability of spinal motor neurons in amyotrophic lateral sclerosis.

To elucidate the relevance of metabotropic glutamate receptors (mGluRs) to the selective vulnerability of motor neurons in the spinal cord in patients with amyotrophic lateral sclerosis (ALS), we investigated the distribution of mRNAs coding mGluR1-5 in the normal human spinal cord. The mRNAs for mGluR1, 4 and 5 were observed in the spinal gray matter, whereas mGluR2 mRNA was absent in the spinal cord and mGluR3 mRNA was displayed only on glial cells in the white matter. Signals for mGluR1 and mGluR5 were enriched in the dorsal horn, while mGluR4 mRNA was abundant in the ventral horn. Since agonists to group I mGluRs (mGluR 1 and 5) have been demonstrated to have neuroprotective effects on spinal motor neurons, less expression of mRNAs coding mGluR1 and mGluR5 in the ventral horn than in the dorsal horn may be implicated in the selective susceptibility of spinal motor neurons in ALS.