Release of γ-aminobutyrate from the external plexiform layer of the rat olfactory bulb: Possible dendritic involvement

There is biochemical, electrophysiological and immunocytochemical evidence which indicates that the granule cell dendrites of the olfactory bulb contain and synthesize y-aminobutyric acid. The possible existence of mechanisms for the uptake and release of [³H]y-aminobutyrate from these dendrites was investigated by using microdissected samples from the external plexiform layer of the rat olfactory bulb. A high affinity uptake system for [³H]γ-aminobutyrate was shown in this tissue preparation which was temperature and sodium dependent and greatly reduced by L-2,4 diaminobutyric acid. The release of exogenously applied [^H]γ-aminobutyrate from the external plexiform layer was determined in vitro and found to be very similar to that observed in the γ-aminobutyrate-containing nerve terminals of the substantia nigra. High molarity potassium and μmolar amounts of veratridine were able to elicit a reproducible release of [³H]γ-aminobutyrate from the external plexiform layer. Both responses were largely calcium dependent. The veratridine effect was completely blocked by tetrodotoxin.These results would favour the view that dendrites of the granule cell neurones take up and release γ-aminobutyrate from dendritic terminals.     

Dopamine release from the rat substantia nigra in vitro. Effect of raphe lesions and veratridine stimulation

In order to eliminate the 5-hydroxytryptaminergic input to the substantia nigra lesions were placed in the dorsal and medial raphe nuclei in a number of rats. The release of exogenously applied [³H]dopamine from the partially denervated substantia nigra was determined in vitro and found to be very similar to the release observed from slices of control substantia nigra. These results lend further support to the theory that the release of exogenously applied [³H]dopamine at the level of the substantia nigra occurs mainly from dopaminergic dendrites, rather than from terminals of 5-hydroxytryptamine-containing neurons.A veratridine-induced release of [³H]dopamine from the pars reticulata of the substantia nigra is also described. An almost complete blockade of veratridine (3.0 μM) stimulation was observed with 100 nM tetrodotoxin. Similar effects of veratridine and tetrodotoxin were also observed on [³H]dopamine release from slices of corpus striatum. These results suggest that dendrites of the dopaminergic neurones in the substantia nigra contain fast, tetrodotoxin-sensitive sodium channels.     

Effects of nigral application of muscimol on release of [3H]γ-aminobutyrate and on multi-unit activity in various cat thalamic nuclei

The release of [³H]γ-aminobutyrate (GABA) neosynthesized from [³H]glutamine was estimated in one substantia nigra and in the ipsilateral thalamus of halothane-anesthetized cats by perfusing a [³H]glutamine-enriched physiological medium through a push-pull cannula implanted in the two structures under investigation. After two hours of superfusion, muscimol (10^?6 M) was delivered through the nigral push-pull cannula for 50–60 min and local- and distal-evoked changes of [³H]GABA release were analyzed. In some experiments, changes of global neuronal activity induced by muscimol application were recorded in different thalamic nuclei, using a bipolar electrode. In a few of the above experiments, biochemical and electrophysiological determinations were simultaneously performed in the substantia nigra and the thalamus. The nigral application of muscimol (10^?6 M) induced locally an activation of the substantia nigra reticulata cells, as well as an increase in release of [³H]GABA.Distally, in the thalamus, two types of biochemical and electrophysiological responses were observed according to the localization of the tip of the push-pull cannula or the electrode. (1) An increased release of [³H]GABA and a depression of the global multi-unit cellular activity were obtained in the ventralis medialis-ventralis lateralis, the centralis lateralis and the paracentralis nuclei. These effects could reflect an activation of the GABAergic nigrothalamic neurons projecting to these different thalamic nuclei. (2) In contrast, in the medialis dorsalis paralamellar zone adjacent to the intralaminar nuclei of the thalamus, a decrease of [³H]GABA release and an activation of the multi-unit activity were obtained. These latter results may suggest either a polysynaptic response or the non-GABAergic nature of the nigrothalamic neurons afferent to the medialis dorsalis paralamellar zone.     

Distribution of γ-aminobutyrate in the rat thalamus: Specific decreases in thalamic γ-aminobutyrate following lesion or electrical stimulation of the substantia nigra

Various subnuclei of the rat's thalamus were punched out or finely microdissected from frozen coronal sections of brain and assayed for γ-aminobutyrate by microdansylation. Highest levels were recorded in the ventromedial and parafascicular regions, corresponding to the sites of termination of nigrothalamic neurones. One week after placing a unilateral electrolytic lesion in the zona reticulata of the substantia nigra there were significant decreases in γ-aminobutyrate levels in the ipsilateral ventromedial (?14%) and parafascicular (?11%) nuclei, but not in other nuclei tested. Falls in the level of γ-aminobutyrate of 19–26% were found to be confined to the ventromedial region in microdissections of brain slices containing this nucleus, two weeks after injecting 1 μg kainic acid into the substantia nigra. The concentrations of glutamate, aspartate, glutamine, glycine and alanine were not altered in the deafferented ventromedial thalamus. Pathological effects of kainate were registered as a complete loss of neuronal perikarya at the injection site coupled with intense gliosis. Mild gliosis, but no postsynaptic neuronal damage, was visible in the corresponding ventromedial and parafascicular nuclei in the thalamus, although degenerating electron-dense terminal boutons were occasionally seen in electron micrographs of this tissue.In rats in which thalamic glutamate decar?ylase activity was inhibited by systemic injection of 3-mercaptopropionic acid, trains of biphasic electrical pulses delivered to the nigra (100 Hz, 0.1 mA, width 0.6 ms for 15 min) evoked a stimulus-dependent decrease in ventromedial (?17%) and parafascicular (?10%) γ-aminobutyrate content, but not in that of other thalamic nuclei. Similar post-stimulus falls in the level of γ-aminobutyrate in the ventromedial nucleus were noted after direct stereotaxic injection of this nucleus with the enzyme inhibitor.These findings are consistent with the existence of γ-aminobutyrate-containing nigrothalamic fibres whose cell bodies lie in the medial part of the substantia nigra zona reticulata and which terminate in the ventromedial and parafascicular nuclear regions of the thalamus.     

成年大鼠脑内ATP敏感性钾通道亚型mRNA表达的研究

为探讨ATP敏感性钾通道(KATP)亚型在大鼠脑组织中的表达,本研究采用半定量逆转录聚合酶链反应 (RT PCR)检测了大鼠小脑、大脑皮质、海马、纹状体、黑质的KATP亚型mRNA的表达。结果显示Kir6. 1、Kir6. 2、Sur1、Sur2B在小脑、大脑皮质、海马、纹状体、黑质中均有表达,Sur2A在脑组织中未见表达。Kir6. 1mRNA在海马和黑质的相对表达水平明显高于小脑、大脑皮质和纹状体(P<0.01);Kir6. 2和Sur1mRNA在黑质的相对表达水平明显高于小脑、大脑皮质、海马和纹状体 (P<0.01 );Sur2BmRNA在黑质、海马和纹状体的相对表达水平明显高于小脑和大脑皮质(P<0.01 )。以上结果提示KATP在脑内具有广泛表达,其表达水平在不同部位存在着差异性。     

K+-stimulated release of labeled γ-aminobutyrate,glycine and taurine in slices of several regions of rat central nervous system

The effect of depolarization by high K⁺ concentration (68.5 mm) on the release of [³H] γ-aminobutyrate (GABA), [¹⁴C]glycine and [³⁵S]taurine was studied in superfused slices of rat cerebellum, striatum, hypothalamus, colliculi, cerebral cortex and ventral and dorsal halves of spinal cord. The release of [³H]GABA was notably stimulated by K⁺-depolarization in all regions studied, particularly in the cerebral cortex and the hypothalamus. The Ca²⁺-dependence of this phenomenon was studied in the cortex and ventral spinal cord; in both regions the K⁺-stimulated release was abolished when Ca²⁺ was omitted from the superfusing medium. The release of [¹⁴C]glycine was also stimulated in all regions, except the cerebellum, but to a lesser extent than that of GABA. This stimulation was Ca²⁺-dependent in the ventral spinal cord but not in the cerebral cortex. The release of [³⁵S]taurine was not affected by K⁺-depolarization in any of the regions studied.These results are consistent with a widely distributed neurotransmitter role for GABA. The Ca²⁺-dependence of glycine release in the spinal cord is in agreement with a role of this amino acid as a transmitter in this region. The finding that [³⁵S]taurine release was not stimulated by K⁺-depolarization in any of the regions studied, under experimental conditions identical to those resulting in an enhancement of [³H]GABA and [¹⁴C]glycine release, argues against a neurotransmitter role of this amino acid in brain and spinal cord.     

Impairment of brain neurotransmitter receptors in aged rats

Dopamine and gamma-aminobutyric acid (GABA) receptor functions have been measured in various brain areas of aged rats. [³H] Spiroperidol binding is decreased in various dopaminergic brain areas, particularly in striatum and tuberculum olfactorium. In striatum the number of binding sites for [³H] spiroperidol is similar in both groups of animals, while the affinity is reduced in senescent rats. Moreover, in the pituitary a 50% increase of [³H] spiroperidol binding was detected in the group of senescent animals. On the other hand, [³H]GABA binding is significantly decreased in substantia nigra and hypothalamus of aged rats, while it is unmodified in cerebral cortex, cerebellum, striatum and nucleus accumbens.     

Evaluation of glutamate as a neurotransmitter of cerebellar parallel fibers.

Glutamate release, in response to excess K⁺, from synaptosomal preparations from rat cerebellum was studied in relationship to the characteristics of stimulus-secretion coupling processes used by known transmitters and compared to those of γ-aminobutyrate (GABA), a known cerebellar neurotransmitter. The properties of glutamate release were indistinguishable from those of GABA: the release of both substances is dependent on Ca²⁺, antagonized by Mg²⁺ and stimulated by K⁺ depolarization and therefore mimics the essential characteristics for the release of a neurotransmitter.Evidence that parallel fiber boutons are a source of evoked glutamate release is supported by the fact that synaptosomal preparations from the cerebellar molecular layer released twice as much glutamate per tissue content as did similar preparations from whole cerebellum. Furthermore, neonatal X-irradiation of the cerebellum, which decreases the granule cell and the parallel fiber populations, results in a reduction of Ca²⁺-dependent glutamate release indicating that Ca²⁺-dependent release originates in part from parallel fiber boutons. Under these conditions neither glutamate levels nor high affinity glutamate uptake are significantly changed. These data indicate that glutamate is either a neurotransmitter of the parallel fibers or a molecule released by the stimulus-secretion coupling process along with the neurotransmitter.Depolarizing K⁺ concentrations in the absence of external Ca²⁺ also induced an increase in the efflux of glutamate and [³H]GABA. This effect was much more prominent for exogenously loaded amino acids than for endogenous glutamate. Accumulated glutamate appears to be stored in at least two separate compartments, which differed in their storage capacity and sensitivity to K⁺ and Ca²⁺. One pool, highly sensitive to K⁺ in the absence of Ca²⁺, showed a poor retention of glutamate. The second pool, sensitive to Ca²⁺ in the presence of a depolarizing agent, stored glutamate more stably. Since the K⁺-induced efflux of glutamate independent of external Ca²⁺ decreased after loss of the granule cells, it is likely that both glutamate compartments are present, at least in part, in neurons.     

The effect of cochlear nerve lesion on the release of glutamate,aspartate, and GABA from cat cochlear nucleus,in vitro

Summary Pool studies of glutamate and aspartate have suggested a transmitter role for these amino acids in cochlear nerve endings. As further evidence. the K⁺-evoked release of glutamate, aspartate and GABA was measured in cat cochlear nucleus slices in vitro and compared to the release following a cochlear nerve lesion. Using [³H]glutamine as precursor, the K⁺-evoked release of glutamate and -aminobutyric acid (GABA) was respectively 4.1 and 7.2 times the spontaneous release. Using [¹⁴C]glutamate as a marker, the K⁺-evoked release of glutamate and GABA was respectively 7.1 and 2.8 times the basal release. All K⁺-evoked releases were Ca⁺⁺-dependent. Nine days after unilateral lesion of the cochlear nerve in the cat, the glutamate release decreased by about 75% on the lesioned side compared to the intact one, whereas the GABA release was not decreased. The labelled tissue glutamate, either synthesized from [³H]glutamine or labelled with [¹⁴C]glutamate, was also markedly decreased on the lesioned side. In comparison, the evoked release of aspartate, newly synthesized from [¹⁴C]glutamate, remained low and was only decreased by about 45% after cochlear nerve lesions. Comparing cat with rat cochlear nucleus, the glutamate release was similar in both animals, whereas the GABA release was much higher in the rat.It is concluded that glutamate and to a lesser extent aspartate are likely to be released from cochlear nerve terminals, supporting a transmitter role in these nerve fibres for both amino acids.     

Unilateral up-regulation of glutamate receptors in limbic regions of amygdaloid-kindled rats

Summary Quantitative autoradiography was used to examine central binding sites for L-[³H]glutamate in amygdaloid-kindled rats since receptors for excitatory amino acids have been implicated in epileptiform activity and seizure behaviors. In tissue from rats killed five days after two kindled seizures, the ipsilateral hippocampus, entorhinal, perirhinal and parietal cortices had significantly (35–100%) greater densities of binding sites for L-[³H]glutamate than the opposite, contralateral side or operated, unstimulated controls. These regions receive excitatory inputs from the amygdala via the entorhinal cortex. Dissociation constants were not altered and significant differences were not observed in the binding parameters for L-[³H]glutamate between control and kindled rats or ipsilateral and contralateral sides of the amygdala, corpus striatum, nucleus accumbens or substantia nigra. The proportion and affinity of N-methylD-aspartate (NMDA)-sensitive binding sites for L-[³H]glutamate was unchanged after kindling, as were the relative proportions of kainate- and AMPA- (DL-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) sensitive sites. However, the density of NMDA and non-NMDA receptor subtypes was increased in the ipsilateral hippocampus, entorhinal, perirhinal and parietal cortices of kindled rats. These findings of specific, unilateral glutamate receptor up-regulation may indicate adaptive responses to the enhanced excitation found in kindling, and are consistent with other neuronal changes reported in early kindling.     

Sequential changes of [3H]forskolin, [3H]cyclohexyladenosine and [3H]PN200‐110 binding sites in the brain of 6‐hydroxydopamine‐lesioned rats

Receptor autoradiographic technique was studied to investigate sequential changes in adenylyl cyclase, adenosine A₁ receptors and L ‐type calcium channels in the striatum and substantia nigra 1–8 weeks after unilateral 6‐hydroxydopamine injection of the medial forebrain bundle in rats. [³H]Forskolin, [³H]cyclohexyladenosine (CHA) and [³H]PN200‐110 were used to label adenylyl cyclase, adenosine A₁ receptors and L ‐type calcium channels, respectively. The degeneration of the nigrostriatal pathway caused a significant increase in [³H]forskolin binding in the striatum of both the ipsilateral and contralateral sides from 2 to 4 weeks post‐lesion. The ipsilateral substantia nigra showed a transient increase in [³H]forskolin binding 4 weeks post‐lesion. In contrast, [³H]CHA binding showed no significant change in most brain areas after lesioning. On the other hand, a conspicuous decrease in [³H]PN200‐110 binding was observed in the dorsolateral striatum of ipsilateral side 4 weeks post‐lesion. Thereafter, the striatum of both the ipsilateral and contralateral sides showed a significant decrease in [³H]PN200‐110 binding 8 weeks post‐lesion. These results demonstrate that unilateral 6‐hydroxydopamine into the medial forebrain bundle of rats can experimentally cause a significant increase in adenylyl cyclase binding sites in the striatum and substantia nigra, whereas no conspicuous change in adenosine A₁ receptors is observed in these areas during post‐lesion. In contrast, L ‐type calcium channels were progressively damaged in the striatum after unilateral 6‐hydroxydopamine treatment. These findings suggest that adenylyl cyclase and calcium system may contribute to the degeneration processes of the dopaminergic neurones.     

Interactions between dopamine and γ-aminobutyrate in the substantia nigra: Implications for the striatonigral output hypothesis

Experiments employing a rodent circling model were conducted to test the predictive capacity of the theory which states that striatonigral γ-aminobutyrate neurones transmit striatal information influencing the animal's locomotion and orientation. In agreement with this proposal, blocking nerve conduction in one substantia nigra with procaine, or nigral γ-aminobutyrate receptors with bicuculline administered stereotaxically, frequently forced rats to move ipsiversively to systemic apomorphine, as though the treatment had impaired striatonigral transmission on that side of the brain. Attempts to reverse the direction of apomorphine circling by stimulating γ-aminobutyrate receptors with muscimol, by facilitating the amino acid's action with flurazepam, or by increasing its synaptic concentration either with a breakdown inhibitor (ethanolamine O-sulphate or 4-amino-hex-5-enoic acid) or an uptake blocker (cis-1,3-aminocyclohexane carboxylic acid) in one nigra, proved unsuccessful. In fact, ethanolamine O-sulphate, flurazepam and muscimol all gave the appearance of hindering rather than enhancing the passage of striatal-derived motor information through the nigra. Broadly speaking, these drugs gave predictable behavioral responses from the ventromedial thalamus, suggesting they were acting in accordance with known mechanisms.The anomalous behaviour with ethanolamine O-sulphate may be attributed to its elevating γ-aminobutyrate levels in other brain areas, since similar ipsiversive rotations occurred if γ-aminobutyrate catabolism was prevented at a wide variety of extranigral sites. A simple explanation for the paradoxical ipsiversive behaviours produced by intranigral flurazepam or muscimol in combination with systemic or intracerebral injection of dopamine agonists, is that they act via presynaptic receptors to inhibit the release of endogenous γ-aminobutyrate and thereby impede striatonigral outflow ipsilaterally.     

Involvement of the thalamus in the reciprocal regulation of the two nigrostriatal dopaminergic pathways

The effects of various sagittal sections on the asymmetric changes in [³H]dopamine release from nerve terminals and dendrites of the two nigrostriatal dopaminergic pathways induced by the application of α-methylparatyrosine ord-amphetamine in the left substantia nigra were examined in halothane-anaesthetized cats implanted with push-pull cannulae in both caudate nuclei and both substantiae nigrae. [³H]tyrosine was continuously delivered to each push-pull cannula and [³H]dopamine was estimated in serial fractions of superfusates. In cats without a lesion, α-methylparatyrosine (10^?4M) reduced the release of [³H]dopamine in the left substantia nigra and induced an opposite effect in the left caudate nucleus. In contrast, [³H]dopamine release was enhanced in the right substantia nigra and reduced in the right caudate nucleus. Opposite asymmetric responses were observed in the four structures during application ofd-amphetamine (10^?6M) into the left substantia nigra. The sagittal section of the mesencephalic decussations was without effect on the changes in [³H]dopamine release induced either by α-methylparatyrosine ord-amphetamine. The section of the corpus callosum and of the commissura anterior potentiated, whereas the section of the thalamic massa intermedia prevented thed-amphetamine-induced reduction of [³H]dopamine release in the left caudate nucleus. This could indicate that neuronal circuits under the control of contralateral structures may contribute to the regulation of the activity of ipsilateral dopaminergic neurons. Only the section of the thalamic massa intermedia prevented the contralateral responses evoked by the unilateral nigral application of α-methylparatyrosine ord-amphetamine.These results suggest that nigrothalamic neurons and thalamic nuclei are involved in the reciprocal regulation of both nigrostriatal dopaminergic pathways.     

In vitro release of 5-[3H]hydroxytryptamine from rat spinal trigeminal nucleus

A potassium-evoked release of 5-[³H]hydroxytryptamine (5-HT), newly synthesized from [³H]tryptophan, was obtained from slices of the subnucleus caudalis of rat spinal trigeminal nucleus by means of an in vitro perfusion system. This release was dose-dependent and calcium-sensitive and quantitatively comparable to the 5-HT release in substantia nigra. The putative transmitter of the primary sensory afferents, substance P, stimulates the spontaneous 5-HT release, whereas another peptide, neurotensin, has no effect.     

Age-related alteration in excitatory amino acid neurotransmission in rat brain

The excitatory amino acids as neurotransmitters in the neocortex, hippocampus, striatum, thalamus, amygdala, nucleus basalis of Meynert and cerebellum from rats aged 4 months, 12 months and 24 months have been examined by measuring sodium-dependent high affinity uptake of D-[3H]-aspartate into preparations containing synaptosomes. Calcium-dependent K(+)-stimulated release of endogenous glutamate from the nucleus basalis was also measured. The hippocampus and cerebellum failed to show significant age-related changes in uptake of D-[3H]-aspartate. D-[3H]-aspartate uptake decreased significantly in the neocortex (29%), striatum (29%), nucleus basalis (26%), amygdala (19%) and thalamus (16%) in the middle-aged rats as compared to the young rats, but the changes were not progressive with age. The release of glutamate from the nucleus basalis was unaltered during the aging process.     

Distribution of DT diaphorase in the rat brain: biochemical and immunohistochemical studies

DT diaphorase [NAD(P)H:quinone oxidoreductase] activity was measured in subcellular fractions from homogenates of striatum, frontal cortex, hippocampus, cerebellum, hypothalamus and substantia nigra. This flavoprotein, which by definition oxidizes dihydronicotinamide adenine dinucleotide and dihydronicotinamide adenine dinucleotide phosphate at equal rates and is completely inhibited by 10(-5) M dicoumarol, was found to constitute 80-90% of the total dihydronicotinamide adenine dinucleotide- and dihydronicotinamide adenine dinucleotide phosphate-reductase activities in all brain regions studied. Antibodies raised against purified cytosolic DT diaphorase from the rat liver cross-reacted with the brain enzyme and inhibited soluble DT diaphorase from striatum and cerebellum to 80-90%. Immunohistochemical studies with the same antibodies demonstrated the occurrence of DT diaphorase immunoreactivity in a population of neurons in the substantia nigra and ventral tegmental area. In some neurons there was a colocalization of DT diaphorase and tyrosine hydroxylase-like immunoreactivity. The dense network of DT diaphorase-immunoreactive fibres in the striatum disappeared along with the dopaminergic innervation after 6-hydroxydopamine lesion. DT diaphorase immunoreactivity was also found in Bergmann glia, astrocytes and tanycytes. No correlation appeared to exist between the localization of neuronal DT diaphorase immunoreactivity and the dihydronicotinamide adenine dinucleotide phosphate-diaphorase-like activity, as defined by tetrazolium salt staining, used as a marker for certain peptidergic and cholinergic neurons. However, in, for example, glial cells in the cerebellum, DT diaphorase might contribute or be responsible for the histochemical dihydronicotinamide adenine dinucleotide phosphate-diaphorase activity.     

Selective uptake of [3H]glutamine and [3H]glutamate into neurons and satellite cells of dorsal root ganglia in vitro

The uptake of [³H]glutamate and [³H]glutamine into rat dorsal root ganglia has been examined by autoradiography and thin-layer chromatography. [³H]glutamate was selectively accumulated by satellite glial cells and after 10 min, 53% of this had been converted to [³H]glutamine. [³H]glutamine, on the other hand, entered neuronal perikarya and 40% was converted to [³H]glutamate. It is suggested that these selective uptake processes provide supporting evidence for the existence of a neuronal-glial glutamine cycle in dorsal root ganglia. Small dark (B) cells accumulated 6 times as much [³H]glutamine as did large light (A) cells. The reasons for this marked difference in the metabolism of the two main types of dorsal root ganglion neurone are discussed.     

The effect of some putative neurotransmitters on the release of 5-hydroxytryptamine and gamma-aminobutyric acid from slices of the rat midbrain raphe area.

The effect of various putative transmitters has been studied on the efflux of [³H]5-hydroxytryptamine and [³H]γ-aminobutyric acid from small superfused slices of the midbrain raphe area of the rat. The slices apparently contained functionally intact terminals for these transmitters since a depolarizing stimulus (50 mM KCl) stimulated the rate of efflux of [³H]5-hydroxytryptamine and [³H]γ-aminobutyric acid in a calcium-dependent fashion. Furthermore, slices accumulated radioactivity with apparent high affinity mechanisms. γ-Aminobutyric acid (50 and 100 μ m) and substance P (100 and 500μ m) stimulated the efflux of [³H]5-hydroxytryptamine. The effect of γ-aminobutyrate was blocked by picrotoxin (50 μ m) but not by strychnine (1 μ m). Other inhibitory amino acids (β-alanine, taurine and glycine) were without effect on the release of [³H]5-hydroxytryptamine. l-Noradrenaline (0.2–1 mM) stimulated the efflux of [³H]γ-aminobutyric acid but not that of [3H]5-hydroxytryptamine. Phentolamine (10μ m) but not (±)-propranolol (10 μ m) abolished the effect of 1 mM noradrenaline. Neither dopamine nor 5-hydroxytryptamine influenced the efflux of [3H]γ-aminobutyric acid.It is possible that interactions in the midbrain raphe area proposed from other studies can be mediated through presynaptic influences on transmitter release.     

Does MPTP intoxication in mice induce metabolite changes in the nucleus accumbens? A 1H nuclear MRS study

Using in vivo ¹H NMR spectroscopy in a mouse model of Parkinson's disease, we previously showed that glutamate concentrations in the dorsal striatum were highest after dopamine denervation associated with an increase in gamma‐aminobutyric acid (GABA) and (Gln) glutamine levels. The aim of this study was to determine whether the changes previously observed in the motor part of the striatum were reproduced in a ventral part of the striatum, the nucleus accumbens (NAc). This study was carried out on controls and 1‐methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐intoxicated mice. In vivo spectra were acquired for a voxel (8 μL) in the dorsal striatum, and in the NAc (1.56 μL). NMR acquisitions were first performed 10 days after the last MPTP injection in a basal condition [after saline intraperitoneal (i.p.) injection] and then in the same animal the week after basal NMR acquisitions, after acute levodopa administration (200 mg kg^–1, i.p.). Immunohistochemistry was used to determine the levels of (Glu) glutamate, glutamine synthetase (GS) and glutamic acid decarboxylase (GAD) isoform 67 in these two structures. The Glu, Gln and GABA concentrations obtained in the basal state were higher in the NAc of MPTP‐intoxicated mice which have the higher dopamine denervation in the ventral tegmental area (VTA) and in the dorsal striatum. Levodopa decreased the levels of these metabolites in MPTP‐intoxicated mice to levels similar to those in controls. In parallel, immunohistochemical staining showed that glutamate, GS and GAD67 immunoreactivity increased in the dorsal striatum of MPTP‐intoxicated mice and in the NAc for animals with a severe dopamine denervation in VTA. These findings strongly supported a hyperactivity of the glutamatergic cortico‐striatal pathway and changes in glial activity when the dopaminergic denervation in the VTA and substantia nigra pars compacta (SNc) was severe. Copyright © 2012 John Wiley & Sons, Ltd.     

Autoradiographic distribution of the D1 agonist [3H]SKF 38393, in the rat brain and spinal cord. Comparison with the distribution of D2 dopamine receptors

The regional distribution of the specific D1 agonist [3H]SKF 38393 (SKF 38393, 2,3,4,5-tetra-hydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine) has been studied autoradiographically in the rat CNS. The binding of [3H]SKF 38393 to striatal sections was saturable, stereospecific, reversible, of high affinity (Kd = 9.9 nM) and partly sodium sensitive; it occurred at a single population of sites and possessed the pharmacological characteristics of the dopamine D1 receptor. The highest levels of [3H]SKF 38393 binding sites were found in the caudate-putamen, nucleus accumbens, olfactory tubercle and substantia nigra. Moderately high concentrations of the [3H]ligand were observed in the amygdala, endopyriform nucleus, nucleus olfactorius anterior, lateral septum, primary olfactory cortex, cerebellum (molecular layer) and spinal cord. An intermediate labelling was found in the thalamus, habenula, subthalamic nucleus, hypothalamus, ventral tegmental area, superior colliculus, hippocampus and cerebral cortex. Moderate levels of [3H]SKF 38393 binding were observed in the globus pallidus and arcuate nucleus. The autoradiographic distribution of [3H]SKF 38393 overlapped with that of [3H]N,n-propylnorapomorphine, a radioligand which labels the D2 dopamine receptors, in a number of dopamine-rich brain areas but there were several areas which exhibited a high density of [3H]SKF 38393 binding sites but undetectable concentrations of [3H]N,n-propylnorapomorphine. Moreover, in the spinal cord, the subregional localization of these [3H]ligands clearly differed. Intrastriatal injection of ibotenic acid caused a large decrease in [3H]SKF 38393 and [3H]N,n-propylnorapomorphine binding in the striatum and provoked a reduction of [3H]SKF 38393 but not [3H]N,n-propylnorapomorphine binding in the substantia nigra confirming the view that nigral D1 but not D2 receptors are located on striatonigral fibres.