Distribution of AMPA receptor subunit mRNAs in the human basal ganglia: an in situ hybridization study

Galanin (1-15) Enhances the Behavioral Effects of Fluoxetine in the Olfactory Bulbectomy Rat, Suggesting a New Augmentation Strategy in Depression

BACKGROUND

Selective serotonergic reuptake inhibitors, including fluoxetine (FLX), are the most commonly used for the treatment of major depression. However, they are effective for remission in only 30% of patients. Recently, we observed that Galanin (1-15) [GAL(1-15)] enhanced the antidepressant effects of FLX in naïve animals, suggesting a new augmentation strategy in depression.

METHODS

We have analyzed in an animal model of depression, the olfactory bulbectomy (OBX) rats, the effect of GAL(1-15) on FLX-mediated responses in the forced swimming test and the sucrose preference test and the involvement of GAL receptor 2 with its antagonist, M871. We have also studied the corticosterone levels in OBX after the coadministration of GAL(1-15) with FLX. Moreover, we studied whether the effects of GAL(1-15) on FLX actions were mediated via auto- and heteroreceptor 5-HT1A (5-HT1AR), analyzing the binding characteristics, mRNA levels, and functionality of 5-HT1AR in the dorsal hippocampus.

RESULTS

GAL(1-15) enhances the antidepressant-like effects induced by FLX in OBX animals in the forced swimming test and the sucrose preference test. The involvement of the GALR2 was demonstrated with M871. Importantly, the mechanism underlying the GAL(1-15)/FLX interactions in the OBX animals involves the 5-HT1AR in the hippocampus at the plasma membrane (increase of affinity and density of 5HT1AR in the DG) and transcriptional (increase of 5HT1AR mRNA levels in DG and CA1) levels. Besides, the coadministration of GAL(1-15) and FLX also reduced OBX-increased corticosterone levels.

CONCLUSIONS

The results open the possibility to use GAL(1-15) in combination with FLX as a novel strategy for the treatment of depression.

Comparative study of the expression of cholinergic system components in the CNS of experimental autoimmune encephalomyelitis mice: Acute vs remitting phase

Acetylcholine (ACh) is involved in the modulation of the inflammatory response. ACh levels are regulated by its synthesizing enzyme, choline acetyltransferase (ChAT), and by its hydrolyzing enzymes, mainly acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE). A more comprehensive understanding of the cholinergic system in experimental autoimmune encephalomyelitis (EAE) disease progression could pave the path for the development of therapies to ameliorate multiple sclerosis (MS). In this work, we analyzed possible alterations of the CNS cholinergic system in the neuroinflammation process by using a MOG-induced EAE mice model. MOG- and vehicle-treated animals were studied at acute and remitting phases. We examined neuropathology and analyzed mRNA expression of ChAT, AChE and the α7 subunit of the nicotinic acetylcholine receptor (α7nAChR), as well as AChE and BuChE enzyme activities, in brain and spinal cord sections during disease progression. The mRNA expression and enzyme activities of these cholinergic markers were up- or down-regulated in many cholinergic areas and other brain areas of EAE mice in the acute and remitting phases of the disease. BuChE was present in a higher proportion of astroglia and microglia/macrophage cells in the EAE remitting group. The observed changes in cholinergic markers expression and cellular localization in the CNS during EAE disease progression suggests their potential involvement in the development of the neuroinflammatory process and may lay the ground to consider cholinergic system components as putative anti-inflammatory therapeutic targets for MS.

Galanin (1-15) enhancement of the behavioral effects of Fluoxetine in the forced swimming test gives a new therapeutic strategy against depression

The pharmacological treatment of major depression is mainly based on drugs elevating serotonergic (5-HT) activity. Specifically, selective 5-HT reuptake inhibitors, including Fluoxetine (FLX), are the most commonly used for treatment of major depression. However, the understanding of the mechanism of action of FLX beyond its effect of elevating 5-HT is limited. The interaction between serotoninergic system and neuropeptides signaling could be a key aspect. We examined the ability of the neuropeptide Galanin(1-15) [GAL(1-15)] to modulate the behavioral effects of FLX in the forced swimming test (FST) and studied feasible molecular mechanisms. The data show that GAL(1-15) enhances the antidepressant-like effects induced by FLX in the FST, and we demonstrate the involvement of GALR1/GALR2 heteroreceptor complex in the GAL(1-15)-mediated effect using in vivo rat models for siRNA GALR1 or GALR2 knockdown. Importantly, 5-HT1A receptors (5HT1A-R) also participate in the GAL(1-15)/FLX interactions since the 5HT1AR antagonist WAY100635 blocked the behavioral effects in the FST induced by the coadministration of GAL(1-15) and FLX. The mechanism underlying GAL(1-15)/FLX interactions affected the binding characteristics as well as the mRNA levels of 5-HT1A-R specifically in the dorsal hippocampus while leaving unaffected mRNA levels and affinity and binding sites of this receptor in the dorsal raphe. The results open up the possibility to use GAL(1-15) as for a combination therapy with FLX as a novel strategy for treatment of depression.

Expression of 5-HT2A receptors in prefrontal cortex pyramidal neurons projecting to nucleus accumbens. Potential relevance for atypical antipsychotic action

The prefrontal cortex (PFC) is involved in higher brain functions altered in schizophrenia. Classical antipsychotic drugs modulate information processing in cortico-limbic circuits via dopamine D2 receptor blockade in nucleus accumbens (NAc) whereas atypical antipsychotic drugs preferentially target cortical serotonin (5-HT) receptors. The brain networks involved in the therapeutic action of atypical drugs are not fully understood. Previous work indicated that medial PFC (mPFC) pyramidal neurons projecting to ventral tegmental area express 5-HT2A receptors suggesting that atypical antipsychotic drugs modulate dopaminergic activity distally, via 5-HT2A receptor (5-HT2A-R) blockade in PFC. Since the mPFC also projects heavily to NAc, we examined whether NAc-projecting pyramidal neurons also express 5-HT2A-R. Using a combination of retrograde tracing experiments and in situ hybridization we report that a substantial proportion of mPFC-NAc pyramidal neurons in rat brain express 5-HT2A-R mRNA in a layer- and area-specific manner (up to 68% in layer V of contralateral cingulate). The functional relevance of 5-HT2A-R to modulate mPFC-NAc projections was examined in dual-probe microdialysis experiments. The application of the preferential 5-HT2A-R agonist DOI into mPFC enhanced glutamate release locally (+66 ± 18%) and in NAc (+74 ± 12%) indicating that cortical 5-HT2A-R activation augments glutamatergic transmission in NAc. Since NAc integrates glutamatergic and dopaminergic inputs, blockade of 5-HT2A-R by atypical drugs may reduce cortical excitatory inputs onto GABAergic neurons of NAc, adding to dopamine D2 receptor blockade. Together with previous observations, the present results suggest that atypical antipsychotic drugs may control the activity of the mesolimbic pathway at cell body and terminal level.

Critical role for PDE4 subfamilies in the development of experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is an animal model of multiple sclerosis exhibiting neuroinflammation, axonal damage and demyelination, further characterized by T- and B-cell responses to myelin oligodendrocyte glycoprotein. Pharmacological manipulation of phosphodiesterases (PDEs) provokes profound anti-inflammatory responses through modulation of cAMP levels. The PDE4B subfamily has been related to the inflammatory immune response in mice and PDE4 inhibition produces amelioration of the clinical signs and delayed onset in the EAE model. Analyses of the expression of the mRNA coding for PDE4B splice variants revealed an upregulation of PDE4B2 in the brainstem and spinal cord of EAE mice which correlated with forkhead box P3 (FoxP3) and transforming growth factor beta (TGF-β) mRNAs expression in a score-dependent manner. The increase observed for the PDE4B protein was mainly found in antigen-presenting cells (APCs) such as dendritic cells and microglia/macrophages, in areas with high cellular infiltration. Unexpectedly, PDE4B(-/-) mice showed an earlier onset of the disease compared to wildtype mice. The results point to a possible role of the PDE4B enzyme and in particular the PDE4B2 splice variant during EAE pathogenesis, probably by modulating cAMP levels in APCs, consequently influencing the cytokine environment important for T-cell differentiation.

Sex-related differences of cAMP-specific PDE4B3 mRNA in oligodendrocytes following systemic inflammation

Sex-related differences have been observed in the incidence and severity of several neurological diseases and in sepsis in humans. Cyclic adenosine monophosphate (cAMP) has been shown to play an important role in modulating the inflammatory environment during neuroinflammation and importantly in protecting myelin from excitotoxic cell death. Considering the sexual dimorphism in the functional properties of oligodendrocytes and the importance of a systemic inflammation in the progression of multiple sclerosis, we focused on identifying possible sex-related differences in the alterations previously reported for the two phosphodiesterase4B (PDE4B) splice-variants (PDE4B2 and PDE4B3) mRNA expression during innate neuroinflammation. PDE4A, PDE4B, and PDE4D are present in oligodendrocytes and we have previously reported that PDE4B3 mRNA is readily expressed in both oligodendrocytes and neurons. In this study, we analyzed the influence of an intraperitoneal lipopolysaccharide injection on the distribution pattern and expression levels of the PDE4B mRNA splicing variants in both male and female mice brains. Clear differences were observed in PDE4B2 and PDE4B3 mRNA expression levels in males compared with females in a time-dependent manner. Furthermore, we observed that the clear downregulation of PDE4B3 mRNA was reflected in a lower percentage of oligodendrocytes positive for this transcript which correlated with a decrease in inducible cAMP early repressor expression in female corpus callosum.

Expression of parvalbumin and glutamic acid decarboxylase-67 after acute administration of MK-801. Implications for the NMDA hypofunction model of schizophrenia

RATIONALE

A reduction of GABAergic markers in postmortem tissue is consistently found in schizophrenia. This is generally mediated by a decreased expression of the calcium-binding protein, parvalbumin (PV), and the 67-kDa isoform of the GABA synthesizing enzyme glutamic acid decarboxylase (GAD(67)). Similar reductions of PV or GAD(67) are observed after repeated exposure to N-methyl-D-aspartate (NMDA) receptor antagonists but less attention has been paid to what occurs after their acute administration.

OBJECTIVES

Here, we have used in situ hybridization to examine the expression of PV and GAD(67) mRNAs at 4 h and 24 h after an acute administration of MK-801 (1 mg/kg).

RESULTS

Four hours after MK-801, the expression of PV mRNA decreased only in dentate gyrus of the hippocampus. Twenty four hours after this treatment, a reduction of the levels of PV mRNA was found in the medial prefrontal, orbitofrontal and entorhinal cortices, hippocampus and the basolateral nucleus of the amygdala. In contrast, no changes in the expression of GAD(67) were observed in any of the brain regions examined. Interestingly, the reduction in PV mRNA expression is observed in discrete corticolimbic subregions that have been implicated in schizophrenia, which is coincident with changes observed in postmortem tissue of schizophrenia brain.

CONCLUSIONS

These findings indicate that acute administration of a NMDA antagonist delineate a pattern of changes in GABAergic markers different from those observed in postmortem tissue in schizophrenia inasmuch as only deficits in parvalbumin (but not GAD(67)) were seen.

Neuroanatomical distribution and neurochemical characterization of cells expressing adenylyl cyclase isoforms in mouse and rat brain

Transmembrane adenylyl cyclases (Adcy) are involved in the regulation of multiple brain processes such as synaptic plasticity, learning and memory. They synthesize intracellular cyclic adenosine monophosphate (cAMP) following activation by G-protein coupled receptors. We examined the neuroanatomical distribution of the nine Adcy isoforms in rat and mouse brain by in situ hybridization, as well as their location in glutamatergic, GABAergic and cholinergic neurons in several mouse brain areas by double in situ hybridization. The Adcys are widely distributed throughout the brain in both rat and mouse, being especially abundant in cortex, hippocampus, thalamic nuclei, the olfactory system and the granular layer of the cerebellum. Double-labeling experiments showed that Adcy isoforms are differently expressed in glutamatergic, GABAergic and cholinergic neuronal cell populations. We report the neuroanatomical distribution of the nine known Adcy isoforms in rat and mouse brain and their cellular localization.

D2 and D4 dopamine receptor mRNA distribution in pyramidal neurons and GABAergic subpopulations in monkey prefrontal cortex: implications for schizophrenia treatment

D2 and D4 dopamine receptors play an important role in cognitive functions in the prefrontal cortex and they are involved in the pathophysiology of neuropsychiatric disorders such as schizophrenia. The eventual effect of dopamine upon pyramidal neurons in the prefrontal cortex depends on which receptors are expressed in the different neuronal populations. Parvalbumin and calbindin mark two subpopulations of cortical GABAergic interneurons that differently innervate pyramidal cells. Recent hypotheses about schizophrenia hold that the root of the illness is a dysfunction of parvalbumin chandelier cells that produces disinhibition of pyramidal cells. In the present work we report double in situ hybridization histochemistry experiments to determine the prevalence of D2 receptor mRNA and D4 receptor mRNA in glutamatergic neurons, GABAergic interneurons and both parvalbumin and calbindin GABAergic subpopulations in monkey prefrontal cortex layer V. We found that around 54% of glutamatergic neurons express D2 mRNA and 75% express D4 mRNA, while GAD-positive interneurons express around 34% and 47% respectively. Parvalbumin cells mainly expressed D4 mRNA (65%) and less D2 mRNA (15-20%). Finally, calbindin cells expressed both receptors in similar proportions (37%). We hypothesized that D4 receptor could be a complementary target in designing new antipsychotics, mainly because of its predominance in parvalbumin interneurons.

Immunohistochemical localization of AMPA-type glutamate receptor subunits in the striatum of rhesus monkey

Corticostriatal and thalamostriatal projections utilize glutamate as their neurotransmitter. Their influence on striatum is mediated, in part, by ionotropic AMPA-type glutamate receptors, which are heteromers composed of GluR1-4 subunits. While the cellular localization of AMPA-type subunits in the basal ganglia has been well characterized in rodents, the cellular localization of AMPA subunits in primate basal ganglia is not. We thus carried out immunohistochemical studies of GluR1-4 distribution in rhesus monkey basal ganglia in conjunction with characterization of each major neuron type. In striatum, about 65% of striatal neurons immunolabeled for GluR1, 75%-79% immunolabeled for GluR2 or GluR2/3, and only 2.5% possessed GluR4. All neurons the large size of cholinergic interneurons (mean diameter 26.1 microm) were moderately labeled for GluR1, while all neurons in the size range of parvalbuminergic interneurons (mean diameter 13.8 microm) were intensely rich in GluR1. Additionally, somewhat more than half of the neurons in the size range of projection neurons (mean diameter 11.6 microm) immunolabeled for GluR1, and about one third of these were very rich in GluR1. About half of the neurons the size of cholinergic interneurons were immunolabeled for GluR2, and the remainder of the neurons that were immunolabeled for GluR2 coincided with projection neurons in size and shape (GluR2 diameter=10.7 microm), indicating that the vast majority of striatal projection neurons possess immunodectible GluR2. Similar results were observed with GluR2/3 immunolabeling. Half of the neurons the size of cholinergic interneurons immunolabeled for GluR4 and seemingly all neurons in the size range of parvalbuminergic interneurons possessed GluR4. These results indicate that AMPA receptor subunit combinations for striatal projection neurons in rhesus monkey are similar to those for the corresponding neuron types in rodents, and thus their AMPA responses to glutamate are likely to be similar to those demonstrated in rodents.

Dopamine D1, D2 and mu-opioid receptors are co-expressed with adenylyl cyclase 5 and phosphodiesterase 7B mRNAs in striatal rat cells

Intracellular cAMP levels are regulated by cAMP synthesis and degradation rate. Nine isoforms of cAMP-synthesizing enzymes called adenylyl-cyclases (ACs) and eleven phosphodiesterases (PDEs) that degrade cyclic nucleotides have been identified. Both types of enzymes exhibit variations not only in their expression pattern distribution throughout the brain, but also in their regulatory characteristics. Different isoforms of ACs and PDEs may be co-expressed in a single cell, thus a gradient of cAMP intracellular levels is formed, which accounts for the diversity of cell responses. Among these isoforms, AC5 and PDE7B are highly expressed in striatum, where the cAMP pathway is implicated in diverse behavioural functions. Striatal AC5 is involved in drug reinforcing actions and motor activity. Less is known about the role of the PDE7B isoenzyme. We performed a double in situ hybridization analysis of the co-expression patterns of AC5 and PDE7B with mu-opioid-receptor (MOR), D1- and D2-receptor mRNAs to contribute to a better understanding in the regulation of cAMP levels under dopamine or opioidergic pathway activation in striatum. We found co-expression of AC5 and PDE7B mRNAs in caudate-putamen and nucleus accumbens; we also encountered that more than 50% of MOR, D2- and D1-expressing cells contained AC5 and PDE7B mRNAs. The presence of AC5 and PDE7B mRNAs in D1- and D2-containing cells suggests the participation of these enzymes in striatal functions involving dopaminergic pathways. Co-localization of both isoenzyme mRNAs with MOR expressing cells suggests their involvement in opioid reinforcing effects.

Pyramidal neurons in rat prefrontal cortex projecting to ventral tegmental area and dorsal raphe nucleus express 5-HT2A receptors

The prefrontal cortex (PFC) is involved in higher brain functions altered in schizophrenia. Classical antipsychotics modulate cortico-limbic circuits mainly through subcortical D2 receptor blockade, whereas second generation (atypical) antipsychotics preferentially target cortical 5-HT receptors. Anatomical and functional evidence supports a PFC-based control of the brainstem monoaminergic nuclei. Using a combination of retrograde tracing experiments and in situ hybridization we report that a substantial proportion of PFC pyramidal neurons projecting to the dorsal raphe (DR) and/or ventral tegmental area (VTA) express 5-HT(2A) receptors. Cholera-toxin B application into the DR and the VTA retrogradely labeled projection neurons in the medial PFC (mPFC) and in orbitofrontal cortex (OFC). In situ hybridization of 5-HT(2A) receptor mRNA in the same tissue sections labeled a large neuronal population in mPFC and OFC. The percentage of DR-projecting neurons expressing 5-HT(2A) receptor mRNA was approximately 60% in mPFC and approximately 75% in OFC (n = 3). Equivalent values for VTA-projecting neurons were approximately 55% in both mPFC and ventral OFC. Thus, 5-HT(2A) receptor activation/blockade in PFC may have downstream effects on dopaminergic and serotonergic systems via direct descending pathways. Atypical antipsychotics may distally modulate monoaminergic cells through PFC 5-HT(2A) receptor blockade, presumably decreasing the activity of neurons receiving direct cortical inputs.

Serotonin 1A receptors in human and monkey prefrontal cortex are mainly expressed in pyramidal neurons and in a GABAergic interneuron subpopulation: implications for schizophrenia and its treatment

Serotonin 1A (5-HT(1A)) receptors are found in high densities in prefrontal cortex. However, their distribution within cortical cell populations is unknown in both humans and primates. We used double in situ hybridization histochemistry to quantify the percentage of glutamatergic and GABAergic neurons expressing 5-HT(1A) receptors in human and monkey prefrontal cortex. Moreover, in the case of the monkey, we also quantified the parvalbumin and calbindin GABAergic subpopulations expressing this receptor. 5-HT(1A) receptor mRNAs were expressed in about 80% of glutamatergic neurons in external layers II and upper III, and in around 50% in layer VI; they were also present in approximately 20% of GABAergic neurons in both species. Although they were found in up to 43% of the calbindin cell subpopulation they were rarely present in parvalbumin cells in monkey prefrontal cortex. The knowledge of the phenotype of the prefrontal cortex (PFC) cells expressing 5-HT(1A) will help understanding serotonin actions in PFC.

Quantitative analysis of glutamatergic and GABAergic neurons expressing 5-HT(2A) receptors in human and monkey prefrontal cortex

5-hydroxytryptamine (5-HT) or serotonin 2A receptors play an important role in modulation of prefrontal cortex (PFC) activity and have been implicated in the physiopathology of psychiatric disorders. There is no quantitative information on the percentage of glutamatergic and GABAergic cells that express 5-HT(2A) receptors in human and monkey PFC. We have used double in situ hybridization to quantify the mRNA co-localization of 5-HT(2A) receptor with the glutamatergic transporter vesicular glutamate transporter 1, and with the GABAergic marker glutamic acid decarboxylase 65/67 and in parvalbumin and calbindin GABAergic cell populations. Our results show that nearly every glutamatergic cell (86-100%) in layers II-V expressed 5-HT(2A) receptor mRNA in both species. This percentage was lower in layer VI (13-31%). In contrast, not all the GABAergic interneurons (13-46%) expressed 5-HT(2A) receptor mRNA. This receptor was expressed in 45-69% of parvalbumin and in 61-87% of calbindin positive cells. These results indicate that, while the majority of glutamatergic neurons can be sensitive to 5-HT action via 5-HT(2A) receptors, this modulation occurs only in a limited population of GABAergic interneurons and provides new neuroanatomical information about the role played by serotonin through 5-HT(2A) receptors in the PFC and on the sites of action for drugs such as antipsychotics and antidepressants used in treatment of psychiatric disorders.

Species differences in the localization of soluble guanylyl cyclase subunits in monkey and rat brain

Nitric oxide (NO) exerts most of its physiological effects through activation of a predominantly soluble guanylyl cyclase (sGC). In mammalian cells sGC exists as a heterodimer of alpha and beta subunits. Currently, four subunits (alpha1, alpha2, beta1, and beta2) have been characterized. We used in situ hybridization with subunit-specific 33P-labeled oligonucleotide probes to compare the anatomical distribution of sGC subunit mRNAs in rat and monkey brains. Message for all subunits except beta2 was detected in both species. The sGC subunit showing the highest expression and widest distribution was beta1. High expression for all subunits was found in basal ganglia, olfactory system, hippocampus, cortex, and cerebellum. Minor species differences in the relative distribution of alpha subunits were observed. In general, the alpha1 message was more prominent in monkey brain and the alpha2 message in rat brain. This was more evident in limbic areas and cerebellar cortex. Some differences were also observed in subunit laminar distribution in cerebral cortex. The limited species differences in sGC subunit distribution suggest that in primates, as occurs in rodents, the NO-cGMP signaling pathway will be involved in important brain functions such as memory formation, sensory processing, and behavior.

Neuronal expression of cAMP-specific phosphodiesterase 7B mRNA in the rat brain

cAMP plays an important role as second messenger molecule controlling multiple cellular processes in the brain. cAMP levels depend critically on the phosphodiesterases (PDE) activity, enzymes responsible for the clearance of intracellular cAMP. We have examined the regional distribution and cellular localization of mRNA coding for the cAMP-specific phosphodiesterase 7B (PDE7B) in rat brain by in situ hybridization histochemistry. PDE7B mRNA is specifically distributed in rat brain, preferentially in neuronal cell populations. The highest levels of hybridization are observed in olfactory tubercle, islands of Calleja, dentate gyrus, caudate-putamen and some thalamic nuclei. Positive hybridization signals are also detected in other areas, such as cerebral cortex, Purkinje cells of the cerebellum and area postrema. By double in situ hybridization histochemistry, we found that 74% and 79% of the cells expressing PDE7B mRNA in striatum and olfactory tubercle, respectively, were GABAergic cells (expressing glutamic acid decarboxylase mRNA), in contrast with the lack of expression in the few cholinergic cells (expressing choline acetyltransferase mRNA) present in those two areas (around 0.4% in olfactory tubercle). In the thalamic nuclei, a majority of cells containing PDE7B mRNA also expresses a glutamatergic marker (76.7% express vesicular glutamate transporter vGluT1 and 76% express vGluT2 mRNAs). Almost all PDE7B expressing cells in dentate gyrus (93%) were glutamatergic. These results offer a neuroanatomical and neurochemical base that will support the search for specific functions for cAMP dependent PDEs and for the development of specific PDE7 inhibitors.

Brain-derived neurotrophic factor modulates dopaminergic deficits in a transgenic mouse model of Huntington's disease

Dysfunction of dopaminergic neurons may contribute to motor impairment in Huntington's disease. Here, we study the role of brain-derived neurotrophic factor (BDNF) in alterations of the nigrostriatal system associated with transgenics carrying mutant huntingtin. Using huntingtin-BDNF+/- double-mutant mice, we analyzed the effects of reducing the levels of BDNF expression in a model of Huntington's disease (R6/1). When compared with R6/1 mice, these mice exhibit an increased number of aggregates in the substantia nigra pars compacta. In addition, reduction of BDNF expression exacerbates the dopaminergic neuronal dysfunction seen in mutant huntingtin mice, such as the decrease in retrograde labelling of dopaminergic neurons and striatal dopamine content. However, mutant huntingtin mice with normal or lowered BDNF expression show the same decrease in the anterograde transport, number of dopaminergic neurons and nigral volume. In addition, reduced BDNF expression causes decreased dopamine receptor expression in mutant huntingtin mice. Examination of changes in locomotor activity induced by dopamine receptor agonists revealed that, in comparison with R6/1 mice, the double mutant mice exhibit lower activity in response to amphetamine, but not to apomorphine. In conclusion, these findings demonstrate that the decreased BDNF expression observed in Huntington's disease exacerbates dopaminergic neuronal dysfunction, which may participate in the motor disturbances associated with this neurodegenerative disorder.

Expression of serotonin 5-HT2C receptors in GABAergic cells of the anterior raphe nuclei

We have used double in situ hybridization to examine the cellular localization of 5-HT2C receptor mRNA in relation to serotonergic and GABAergic neurons in the anterior raphe nuclei of the rat. In the dorsal and median raphe nuclei 5-HT2C receptor mRNA was not detected in serotonergic cells identified as those expressing serotonin (5-HT) transporter mRNA. In contrast, 5-HT2C receptor mRNA was found in most GABAergic cells, recognized by the presence of glutamic acid decarboxylase mRNA. Such 5-HT2C receptor-positive GABAergic neurons were mainly located in the intermediolateral and lateral portions of the dorsal raphe and lateral part of the median raphe. The present data give anatomical support to a previous hypothesis that proposed a negative-feedback loop involving reciprocal connections between GABAergic interneurons bearing 5-HT2A/2C receptors and 5-HT neurons in the dorsal raphe and surrounding areas. According to this model, the excitation of GABAergic interneurons through these 5-HT2C (and also 5-HT2A) receptors would result in the suppression of 5-HT cell firing.

The kappa opioid agonist U50,488 potentiates 6-hydroxydopamine-induced neurotoxicity on dopaminergic neurons

Several observations support the hypothesis that kappa opioid (kappa-opioid) receptor agonism may contribute to neurotoxicity, but other reports have suggested that certain kappa-agonists can attenuate neurological dysfunction. Degeneration of dopaminergic neurons in the substantia nigra is the pathological hallmark of Parkinson's disease. Therefore, it is of particular interest to study whether kappa-opioid receptor agonism has an influence on the progressive degeneration of dopaminergic neurons. We have investigated the effect exerted by the selective kappa-agonist U50,488 on the neurotoxicity induced by intrastriatal 6-hydroxydopamine (6-OHDA) administration on dopaminergic neurons. Male Sprague-Dawley rats received an acute (0.5 mg/kg) or subacute (0.5 mg/kg, twice at day, for 7 days) administration of U50,488, receiving the last dose 30 min before intrastriatal 6-OHDA administration. Acute or subacute U50,488 pretreatment potentiated the 6-OHDA-induced decrease in the number of nigral tyrosine hydroxylase immunoreactive neurons (P < 0.05). Acute U50,488 pretreated animals showed a tendency, although not statistically significant to increase striatal mRNA encoding for enkephalin (PPE mRNA). Subacute U50,488 significantly potentiated the increase in PPE mRNA induced by 6-OHDA (P < 0.05). The present results show a neurotoxic effect of the kappa agonist U50,488 on dopaminergic neurons in rats with a striatal lesion induced by 6-OHDA. This neurotoxic effect is associated to an increase in striatal PPE mRNA levels, suggesting that an increase in the indirect pathway activity and consequently an increase in the activity of the subthalamo-nigral pathway might be involved in this phenomenon.

5-ht5BReceptor mRNA in the raphe nuclei: Coexpression with serotonin transporter

We used double-label in situ hybridization to examine the cellular localization of 5-ht(5B) receptor mRNA in relation to serotonin transporter mRNA in the rat dorsal raphe (DR) and central superior nucleus (CS, median raphe nucleus). 5-ht(5B) receptor mRNA hybridization signal was often found on serotonin transporter mRNA-positive neuron profiles. The degree of cellular colocalization of these mRNAs notably varied among the different regions of the raphe nuclei. In the DR, cell bodies showing 5-ht(5B) receptor mRNA expression were abundant in the medial portions of the nucleus, all of them being also labeled for serotonin transporter mRNA. In contrast, in the ventrolateral regions (lateral wings) of the DR, we observed serotonin transporter mRNA-positive cells, but they were devoid of 5-ht(5B) receptor mRNA signal. In the CS, the level of coexpression of 5-ht(5B) receptor mRNA with serotonin transporter mRNA was high in the intermediate portions of the nucleus; however, we were unable to detect specific 5-ht(5B) receptor mRNA hybridization signal in its caudal extent. Our results support the presence of 5-ht(5B) receptor in serotonergic neurons in the DR and CS, suggesting an autoreceptor role for this receptor subtype.

In vivo modulation of 5-hydroxytryptamine release in mouse prefrontal cortex by local 5-HT(2A) receptors: effect of antipsychotic drugs

In the rat, postsynaptic 5-hydroxytryptamine2A receptors medial prefrontal cortex control the activity of the serotonergic system through changes in the activity of pyramidal neurons projecting to the dorsal raphe nucleus. Here we extend these observations to mouse brain. The prefrontal cortex expresses abundant 5- hydroxytryptamine2A receptors, as assessed by immunohistochemistry, Western blots and in situ hybridization procedures. The application of the 5-hydroxytryptamine2A/2C agonist DOI (100 microm) by reverse dialysis in the medial prefrontal cortex doubled the local release of 5-hydroxytryptamine. This effect was reversed by coperfusion of tetrodotoxin, and by the selective 5-hydroxytryptamine2A receptor antagonist M100907, but not by the 5-hydroxytryptamine2C antagonist SB-242084. The effect of DOI was also reversed by prazosin (alpha1-adrenoceptor antagonist), BAY x 3702 (5-hydroxytryptamine1A receptor agonist), NBQX (alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate/kainic acid antagonist) and 1S,3S-ACPD (mGluR II/III agonist), but not by dizocilpine (N-methyl-d-aspartate antagonist). alpha-Amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate mimicked the 5-hydroxytryptamine elevation produced by DOI, an effect also reversed by BAY x 3702. Likewise, the coperfusion of classical (chlorpromazine, haloperidol) and atypical antipsychotic drugs (clozapine, olanzapine) fully reversed the 5-hydroxytryptamine elevation induced by DOI. These observations suggest that DOI increases 5-hydroxytryptamine release in the mouse medial prefrontal cortex through the activation of local 5-hydroxytryptamine2A receptors by an impulse-dependent mechanism that involves/requires the activation of local alpha-amino-3-hydroxy-5-methyl-4-isoxazole-4-propionate receptors. This effect is reversed by ligands of receptors present in the medial prefrontal cortex, possibly in pyramidal neurons, which are involved in the action of antipsychotic drugs. In particular, the reversal by classical antipsychotics may involve blockade of alpha1-adrenoceptors, whereas that of atypical antipsychotics may involve 5-hydroxytryptamine2A receptors and alpha1-adrenoceptors.

GABAB receptor mRNA in the raphe nuclei: co-expression with serotonin transporter and glutamic acid decarboxylase

We have used double-label in situ hybridization techniques to examine the cellular localization of GABAB receptor mRNA in relation to serotonin transporter mRNA and glutamic acid decarboxylase mRNA in the rat dorsal raphe, median raphe and raphe magnus nuclei. The degree of cellular co-localization of these markers notably varied among the different nuclei. In the dorsal raphe, cell bodies showing GABAB receptor mRNA were very abundant, the 85% being also labelled for serotonin transporter mRNA, and a low proportion (5%) showing glutamic acid decarboxylase mRNA. In the median raphe, the level of co-expression of GABAB receptor mRNA with serotonin transporter mRNA was significantly lower. Some cells were also identified that contained GABAB receptor mRNA in the absence of either one of the other mRNA species studied. Our results support the presence of GABAB receptors in serotonergic as well as GABAergic neurones in the dorsal and median raphe, providing the anatomical basis for the reported dual inhibitory/disinhibitory effect of the GABAB agonist baclofen on serotonergic function.

AMPA receptor function is altered in GLUR2-deficient mice

The GluR2 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) receptor determines many of the biophysical properties of native AMPA receptors, including Ca++ permeability. Genetically engineered mice unable to edit the Q to R site of the GluR2 subunit die within 3 wk postpartum, presumably due to toxicity associated with enhanced Ca++ influx through AMPA receptors. In contrast, disruption of the gene encoding GluR2 is not necessarily lethal. The objective of this study was to explore potential mechanisms that permit survival of GluR2 (-/-) mice despite AMPA receptors that are highly Ca++ permeable. Whole-cell, patch-clamp recording of AMPAreceptor responses in cortical pyramidal cells revealed that the kinetics of recovery from desensitization were significantly slower for receptors from GluR2 (-/-) mice compared to receptors from GluR2 (+/+) mice. The recovery time constants for AMPA receptors from GluR2 (-/-) and GluR2 (+/+) mice were 109.8 +/- 17 ms and 54.4 +/- 7.1 ms, respectively. The slower recovery kinetics would be expected to reduce Ca++ influx during repetitive stimulation. Because both RNA editing at the R/G site and alternative splicing of the flip and flop module affect AMPA receptor desensitization recovery rates, the possibility that these mechanisms were changed in GluR2 (-/-) mice was investigated. On a macroscopic level, neither editing nor splicing of the GluR-1, 3 or 4 subunits were changed in GluR2 (-/-) mice compared to GluR2 (+/+) mice. In summary, an increase in the time constant for recovery from desensitization may contribute to the ability of GluR2 (-/-) to survive.

Ionotropic and metabotropic GABA and glutamate receptors in primate basal ganglia

The functions of glutamate and GABA in the CNS are mediated by ionotropic and metabotropic, G protein-coupled, receptors. Both receptor families are widely expressed in basal ganglia structures in primates and nonprimates. The recent development of highly specific antibodies and/or cDNA probes allowed the better characterization of the cellular localization of various GABA and glutamate receptor subtypes in the primate basal ganglia. Furthermore, the use of high resolution immunogold techniques at the electron microscopic level led to major breakthroughs in our understanding of the subsynaptic and subcellular localization of these receptors in primates. In this review, we will provide a detailed account of the current knowledge of the localization of these receptors in the basal ganglia of humans and monkeys.

Differential distribution of cAMP-specific phosphodiesterase 7A mRNA in rat brain and peripheral organs

We investigated the regional distribution and cellular localization of mRNA coding for the cAMP-specific phosphodiesterase 7A (PDE7A) in rat brain and several peripheral organs by in situ hybridization histochemistry. The regional expression of two splice variants, PDE7A1 and PDE7A2, was examined by RT-PCR using RNA extracted from several brain regions. PDE7A mRNA was found to be widely distributed in rat brain in both neuronal and nonneuronal cell populations. The highest levels of hybridization were observed in the olfactory bulb, olfactory tubercle, hippocampus, cerebellum, medial habenula nucleus, pineal gland, area postrema, and choroid plexus. Positive hybridization signals were also detected in other areas, such as raphe nuclei, temporal and entorhinal cortex, pontine nuclei, and some cranial nerve motor nuclei. Both mRNA splice forms were differentially distributed in several areas of the brain with the striatum expressing only PDE7A1 and the olfactory bulb and spinal cord expressing PDE7A2 exclusively. In peripheral organs the highest levels of PDE7A hybridization were seen in kidney medulla, although testis, liver, adrenal glands, thymus, and spleen also presented high hybridization signal. These results are consistent with PDE7A being involved in the regulation of cAMP signaling in many brain functions. The consistent colocalization with PDE4 mRNAs suggests that PDE7A could have an effect on memory, depression, and emesis. The results offer clear anatomical and functional systems in which to investigate future specific PDE7 inhibitors.

Phosphodiesterase type 4 isozymes expression in human brain examined by in situ hybridization histochemistry and[3H]rolipram binding autoradiography. Comparison with monkey and rat brain

We have examined the distribution of four different cyclic AMP-specific phosphodiesterase isozyme (PDE4A, PDE4B, PDE4C and PDE4D) mRNAs in the brain of different species by in situ hybridization histochemistry and by autoradiography with [3H]rolipram. We have compared the localization of each isozyme in human brain with that in rat and monkey brain. We have found that the four PDE4 isoforms display a differential expression pattern at both regional and cellular level in the three species. PDE4A, PDE4B and PDE4D are widely distributed in human brain, with the two latter appearing more abundant. In contrast, PDE4C in human brain, presents a more restricted distribution, limited to cortex, some thalamic nuclei and cerebellum. This is at variance with the distribution of PDE4C in rat brain, where it is found exclusively in olfactory bulb. In monkey brain, the highest expression for this isoform is found in the claustrum, and at lower levels in cortical areas and cerebellum. PDE4B presented a broad distribution, being expressed in both neuronal and non neuronal cell populations. In general, the distribution of binding sites visualized with [3H]rolipram correlated well with the expression of each PDE4 isozyme.

Age-related loss of the AMPA receptor subunits GluR2/3 in the human nucleus basalis of Meynert

Magnocellular cholinergic neurons in the basal forebrain have long been recognized as vulnerable to the pathology of Alzheimer's disease. Despite numerous anatomical, pharmacological, behavioral, and physiological investigations of these neurons the cellular mechanism that underlines their selective vulnerability remains unclear. As part of an ongoing investigation into the molecular mechanism(s) underlying neuronal vulnerability in Alzheimer's disease and normal aging, we employed immunocytochemical techniques and examined the cellular localization of the alpha-amino-3-hydroxy-5-methyl-4-isoaxolepropionate (AMPA) glutamate receptor subunits GluR1 and GluR2/3 in the basal forebrain of eight nondemented elderly human subjects (66-102 years). For each case we observed GluR1-positive magnocellular cells darkly labeled within all main divisions of the basal forebrain (Ch1-Ch4). Double-labeling immunohistochemical techniques confirmed that the overwhelming majority (94%) of these neurons were also positive for the p75NGFr antibody, thus substantiating the cholinergic nature of these neurons. In contrast, GluR2/3 immunolabeling upon magnocellular neurons was relatively faint or nonexistent. The latter observations were most apparent in cases of advanced age and in the posterior part of the nucleus basalis of Meynert (NBM) (i.e., Ch4). In contrast, in adjacent structures (e.g., globus pallidus), a number of robustly labeled GluR2/3-positive cells were observed. In addition to the eight elderly subjects, we examined GluR1 and GluR2/3 immunostaining in the NBM of five younger cases, 5, 33, 36, 47, and 48 years of age. Although practical considerations limited our observations to the Ch4 region, we observed both GluR1 and GluR2/3 labeling upon NBM neurons in this latter region. On average, the distribution of labeled cells and intensity of immunoreaction were comparable between GluR1 and GluR2/3. The presence of GluR2/3- and GluR1-labeled neurons in the Ch4 region of younger cases but primarily GluR1 in cases of advanced age suggests an age-related decrease in GluR2/3. Functionally, the loss of GluR2 from the AMPA receptor complex results in ion channels highly permeable to Ca(2+). These alterations in cation permeability of the AMPA receptor together with the occurrence of a number of other intrinsic and extrinsic events (i.e., decrease Ca(2+)-binding protein) likely contribute to the vulnerability of these neurons in aging and in AD.

Chemical anatomy of striatal interneurons in normal individuals and in patients with Huntington's disease

This paper reviews the major anatomical and chemical features of the various types of interneurons in the human striatum, as detected by immunostaining procedures applied to postmortem tissue from normal individuals and patients with Huntington's disease (HD). The human striatum harbors a highly pleomorphic population of aspiny interneurons that stain for either a calcium-binding protein (calretinin, parvalbumin or calbindin D-28k), choline acetyltransferase (ChAT) or NADPH-diaphorase, or various combinations thereof. Neurons that express calretinin (CR), including multitudinous medium and a smaller number of large neurons, are by far the most abundant interneurons in the human striatum. The medium CR+ neurons do not colocalize with any of the known chemical markers of striatal neurons, except perhaps GABA, and are selectively spared in HD. Most large CR+ interneurons display ChAT immunoreactivity and also express substance P receptors. The medium and large CR+ neurons are enriched with glutamate receptor subunit GluR2 and GluR4, respectively. This difference in AMPA GluR subunit expression may account for the relative resistance of medium CR+ neurons to glutamate-mediated excitotoxicity that may be involved in HD. The various striatal chemical markers display a highly heterogeneous distribution pattern in human. In addition to the classic striosomes/matrix compartmentalization, the striosomal compartment itself is composed of a core and a peripheral region, each subdivided by distinct subsets of striatal interneurons. A proper knowledge of all these features that appear unique to humans should greatly help our understanding of the organization of the human striatum in both health and disease states.

Pharmacology of AMPA/kainate receptor ligands and their therapeutic potential in neurological and psychiatric disorders

It has been postulated, consistent with the ubiquitous presence of glutamatergic neurons in the brain, that defects in glutamatergic neurotransmission are associated with many human neurological and psychiatric disorders. This review evaluates the possible application of ligands acting on glutamate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and kainate (KA) receptors to minimise the pathology and/or symptoms of various diseases. Glutamate activation of AMPA receptors is thought to mediate most fast synaptic neurotransmission in the brain, while transmission via KA receptors contributes only a minor component. Variants of the protein subunits forming these receptors greatly extend the pharmacological and electrophysiological properties of AMPA/KA receptors. Disease and drug use can differentially affect the expression of the subunits and their variants. Ligands bind to AMPA receptors by competing with glutamate at the glutamate binding site, or non-competitively at other sites on the proteins (allosteric modulators). Ligands showing selective competitive antagonist actions at the AMPA/ KA class of glutamate receptors were first reported in 1988, and the systemically active antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX) was first shown to have useful therapeutic effects on animal models of neurological diseases in 1990. Since then, newer antagonists with increased potency, higher specificity, increased water solubility, and a longer duration of action in vivo have been developed. Negative allosteric modulators such as the prototype GYKI-52466 also block AMPA receptors but have little action at KA receptors. Positive allosteric modulators enhance glutamatergic neurotransmission at AMPA receptors. Polyamines and adamantane derivatives bind within the ion channel of calcium-permeable AMPA receptors. The latest developments include ligands selective for KA receptors containing Glu-R5 subunits. Evidence for advantages of AMPA receptor antagonists over N-methyl-D-aspartate (NMDA) receptor antagonists for symptomatic treatment of neurological and psychiatric conditions, and for minimising neuronal loss occurring after acute neurological diseases, such as physical trauma, ischaemia or status epilepticus, have been shown in animal models. However, as yet AMPA receptor antagonists have not been shown to be effective in clinical trials. On the other hand, a limited number of clinical trials have been reported for AMPA receptor ligands that enhance glutamatergic neurotransmission by extending the ion channel opening time (positive allosteric modulators). These acute studies demonstrate enhanced memory capability in both young and aged humans, without any apparent serious adverse effects. The use of these allosteric modulators as antipsychotic drugs is also possible. However, the long term use of both direct agonists and positive allosteric modulators must be approached with considerable caution because of potential adverse effects.

Differential expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor subunits by calretinin-immunoreactive neurons in the human striatum

We recently reported the existence of medium and large intemeurons immunoreactive for the calcium-binding protein calretinin in the human striatum. We also showed a selective sparing of all medium, but not all large, calretinin-immunoreactive striatal neurons in Huntington's disease striatum. Because glutamate receptor-mediated excitotoxicity has been implicated in the massive loss of striatal projection neurons that characterizes Huntington's disease, we have applied a double-antigen localization procedure to post mortem tissue from eight normal human subjects to determine the expression of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate glutamate receptor subunits 1/2/4 by the calretinin-immunoreactive interneurons. The two types of calretinin-immunoreactive neurons were found to display various patterns of glutamate receptor subunit expression and a specific regionalization was also noted in the expression of these glutamate receptor subunits. Approximately half of the large calretinin-immunoreactive neurons displayed immunoreactivity for glutamate receptor subunits 1 and 2, and about the same proportion of medium calretinin-immunoreactive neurons expressed glutamate receptor subunits 1 and 4. These double-labeled neurons were rather uniformly distributed in the caudate nucleus and putamen. In contrast, as much as 70.1% of the large calretinin-immunoreactive neurons displayed glutamate receptor subunit 4 immunoreactivity in the postcommissural portion of the putamen, an area that corresponds to the sensorimotor striatal territory. For their part, the medium calretinin-immunoreactive neurons were markedly enriched with glutamate receptor subunit 2, 76% of them being double labeled in the caudate nucleus, which corresponds to the striatal associative territory, compared with 85.5% in the postcommissural putamen. Receptor subunit composition plays a key role in determining the functional properties of glutamate receptors, including their permeability to calcium and susceptibility to excitotoxic insults. Thus, the differential expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate glutamate receptor subunits reported here may help to explain the selective sparing of certain types of calretinin-immunoreactive striatal interneurons in Huntington's disease, although other factors, such as post-transcriptional editing, are also likely to be involved.

Flip and flop variants of AMPA receptor subunits in the human cerebellum: implication for the selective vulnerability of Purkinje cells

We investigated the distribution of messenger RNAs coding for flip and flop splice variants ofAMPA receptor subunits in the human cerebellum to determine the relevance of AMPA receptors in the selective vulnerability of Purkinje cells to ischemia. Purkinje cells more abundantly expressed transcripts for flip variant of GluR-A, GluR-C, and GluR-D than granule cells, whereas transcripts for flop variants and GluR-B flip were expressed at similar levels on Purkinje cells and granule cells. These results suggest that human Purkinje cells possess AMPA receptors of the slowly desensitizing class as compared to granule cells. This differential distribution may explain the selective vulnerability of Purkinje cells.

[3H]CNQX and NMDA-sensitive [3H]glutamate binding sites and AMPA receptor subunit RNA transcripts in the striatum of normal and weaver mutant mice and effects of ventral mesencephalic grafts

Levels of excitatory amino acid receptors were studied in the weaver mouse model of DA deficiency after unilateral intrastriatal transplantation of E12+/+ mesencephalic cell suspensions. Graft integration was verified by turning behavior tests and from the topographical levels of the DA transporter, tagged autoradiographically with 3 nM [3H]GBR 12935 (average increase in grafted dorsal striatum compared to nongrafted side, 60%). Autoradiography of 80 nM [3H]CNQX and 100 nM NMDA-sensitive [3H]glutamate binding was carried out to visualize the topography of non-NMDA and NMDA receptors, respectively, in +/+ mice and in recipient weaver mutants 3 months after grafting. Increases of 30% or more were found for [3H]CNQX binding in the dorsal nongrafted weaver striatum compared to +/+, and a further 6-9% increase in grafted weaver compared to nongrafted side. The added increase of non-NMDA receptors in the transplanted striatum might be explained by a presence of such receptors on DA presynaptic endings of graft origin. A 20% increase in NMDA-sensitive [3H]glutamate binding was measured in the dorsal nongrafted weaver striatum compared to +/+. NMDA-sensitive [3H]glutamate binding in the transplanted side of weaver mutants tended to be slightly higher in all areas of the striatal complex compared to the nongrafted side, without reaching conventional levels of statistical significance. Using in situ hybridization histochemistry with synthetic 32p labeled oligonucleotide probes, we investigated RNA transcripts encoding the four AMPA receptor subunits. RNA transcripts in the striatum are seen with a decreasing signal intensity in the following order: GluRB > GluRA > GluRC > GluRD. The weaver caudate-putamen shows a 12% increase in GluRA subunit mRNA compared to +/+, whereas mesencephalic neuron transplantation leads to slight increases (3%) in the levels of GluRB mRNA in the nucleus accumbens. The results are placed in the context of the important interaction between the converging glutamatergic corticostriatal and the DAergic nigrostriatal pathways in controlling the functional output of the basal ganglia in Parkinson's disease and in experimental models of DA deficiency.