Acetylcholine-mediated modulation of striatal function

Activated phosphorylation of cyclic AMP response element binding protein is associated with preservation of striatal neurons after focal cerebral ischemia in the rat

Phosphorylation of the DNA-binding transcription factor, cyclic AMP response element binding protein, has recently been suggested to provide neuroprotective signals in times of cellular stress. Medium-sized striatal neurons are among the cells that are most vulnerable to ischemic stress in the brain. In the present study, phosphorylation of cyclic AMP response element binding protein was immunohistochemically evaluated in rat striatum in order to examine the ischemic vulnerability of each striatal region from the standpoint of cyclic AMP response element binding protein. Rats were subjected to 90-min focal cerebral ischemia followed by various periods of recirculation. Focal ischemia was induced by occlusion of the middle cerebral artery by the intraluminal suture method. Local cerebral blood flow measured by the 14C-iodoantipyrine method in the lateral and the medial striatal regions during occlusion was 5.0+/-7. 1 and 42.5+/-8.1ml/100g/min, respectively. Cerebral blood flow in each region was restored to the control level during the recirculation period. The lateral and the medial regions of the striatum in the sham animals showed hardly any immunoreactivity with the specific antibody against phosphorylated cyclic AMP response element binding protein. By contrast, at 3.5h of recirculation, a number of phosphorylated cyclic AMP response element binding protein-positive neurons were detected in the medial striatal region on the occluded side, and the increase in the number of immunopositive cells continued until two weeks of recirculation with gradual decline. The lateral striatal region on the ischemic side showed only a mild increase in phosphorylated cyclic AMP response element binding protein-positive cells at 3.5h of recirculation, and the immunoreactivity rapidly disappeared during the subsequent recirculation period. Appreciable increase in immunoreactive cells was also noted in the contralateral striatum during the early phase of recirculation, and this increase seemed to be associated with spontaneous circling movements of the animals. Cresyl Violet staining revealed that striatal neurons in the medial region remained intact until two weeks of recirculation, whereas neurons in the lateral striatal region soon showed ischemic damage, followed by complete neuronal loss, and evolution of a frank infarct. Immunoreactivity for bcl-2, apoptosis-suppressive protein, was clearly detected in many neurons in the medial striatal region, but no such immunoreactivity was detected in the lateral striatal region. These findings suggest that persistently activated phosphorylation of cyclic AMP response element binding protein in the striatum during post-ischemic recirculation may be closely associated with protection of striatal neurons on the ischemic side, while it may be associated with spontaneous circling movements on the contralateral side.

Dopamine D5 receptors of rat and human brain

In contrast to dopamine D1 receptors, the anatomical distribution of D5 receptors in the CNS is poorly described. Therefore, we have studied the localization of dopamine D5 receptors in the brain of rat and human using our newly prepared subtype-specific antibody. Western blot analysis of brain tissues and membranes of cDNA transfected cells, and immunoprecipitation of brain dopamine receptors suggest that this antibody is highly selective for native dopamine D5 receptors. The D5 antibody labeled dopaminergic neurons of mesencephalon, and cortical and subcortical structures. In neostriatum, the D5 receptors were localized in the medium spiny neurons and large cholinergic interneurons. The D5 labeling in caudate nucleus was predominantly in spines of the projection neurons that were frequently making asymmetric synapses. Occasionally, the D5 receptors were also found at the symmetric synapses. Within the cerebral cortex and hippocampus, D5 antibody labeling was prominent in the pyramidal cells and their dendrites. Dopamine D5 receptors were also prominent in the cerebellum, where dopamine innervation is known to be very modest. Differences in the localization of D5 receptors between both species were generally indistinguishable except in hippocampus. In rat, the hippocampal D5 receptor was concentrated in the cell body, whereas in human it was also associated with dendrites. These results show that D5 receptors are localized in the substantia nigra-pars compacta, hypothalamus, striatum, cerebral cortex, nucleus accumbens and olfactory tubercle. Furthermore, the presence of D5 receptors in the areas of dopamine pathways suggests that this receptor may participate actively in dopaminergic neurotransmission.

Dopamine and cAMP-regulated phosphoprotein 32 kDa controls both striatal long-term depression and long-term potentiation, opposing forms of synaptic plasticity

A complex chain of intracellular signaling events, critically important in motor control, is activated by the stimulation of D1-like dopamine (DA) receptors in striatal neurons. At corticostriatal synapses on medium spiny neurons, we provide evidence that the D1-like receptor-dependent activation of DA and cyclic adenosine 3',5' monophosphate-regulated phosphoprotein 32 kDa is a crucial step for the induction of both long-term depression (LTD) and long-term potentiation (LTP), two opposing forms of synaptic plasticity. In addition, formation of LTD and LTP requires the activation of protein kinase G and protein kinase A, respectively, in striatal projection neurons. These kinases appear to be stimulated by the activation of D1-like receptors in distinct neuronal populations.

Upregulation of preprodynorphin and preproenkephalin mRNA expression by selective activation of group I metabotropic glutamate receptors in characterized primary cultures of rat striatal neurons

Group I metabotropic glutamate receptors (mGluRs) are positively coupled to phosphoinositide hydrolysis, and are expressed in medium spiny neurons of rat striatum in vivo. By modifying intracellular activities, this group of mGluRs is involved in the regulation of gene expression important for neuroplasticity. To characterize the regulatory role of group I receptors in opioid peptide mRNA expression in vitro, primary cultures of striatal cells were prepared from neonatal day-1 rat pups. Cells were cultured in the presence of a mitotic inhibitor, cytosine arabinoside, which generated predominant neuronal cell cultures after 12-14 days in culture as demonstrated by dense immunostaining of more than 90% of cultured cells to a specific marker for neurons (microtubule-associated protein) but not for astroglial cells (glial fibrillary acidic protein). The vast majority of neurons (>90%) were also verified as GABAergic neurons according to their positive immunoreactivity to GABA and glutamic acid decarboxylase-65/67 antibodies. A few large neurons (<5%) showed high levels of choline acetyltransferase immunoreactivity, presumably cholinergic neurons. To confirm group I mGluR expression in cultured neurons, both in situ hybridization and immunocytochemistry were performed, which detected moderate levels of mGluR1 and mGluR5 mRNAs and protein products in most neurons (>70%), respectively. On this culture system, quantitative in situ hybridization was then performed to quantify changes in preprodynorphin (PPD) and preproenkephalin (PPE) mRNA levels in response to mGluR stimulation. Acute incubation of a non-subgroup selective agonist, 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), increased PPD and PPE mRNA levels in a concentration-dependent manner (176 and 189% over control for PPD and PPE after 100 microM ACPD incubation, respectively). Application of a selective group I agonist, 3,5-dihydroxyphenylglycine (DHPG), produced much greater induction of either mRNA (285 and 289% over control for PPD and PPE after 100 microM DHPG incubation, respectively). Co-incubation of a selective group I antagonist, n-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide (PHCCC), blocked both ACPD- and DHPG-induced PPD/PPE expression. These data demonstrate the validity of a neuronal cell culture model for studying the molecular regulation of opioid gene expression in vitro. Selective activation of identified group I mGluRs facilitates constitutive expression of PPD and PPE mRNAs in cultured striatal neurons.

Role of tonically-active neurons in the control of striatal function: cellular mechanisms and behavioral correlates

1. The striatum is primarily involved in motor planning and motor learning. Human diseases involving its complex circuitry lead to movement disorders such as Parkinson's disease (PD) and Huntington's disease (HD). Moreover the striatum has been involved in processes linked to reward, cognition and drug addiction. 2. The high content of acetylcholine (ACh) found in the striatum is due to the presence of cholinergic interneurons. The intrinsic electrical and synaptic properties of these interneurons have been recently characterized. However, their functional significance is far from being fully elucidated. 3. In vivo electrophysiological experiments from behaving monkeys have identified these cholinergic interneurons as "Tonically Active Neurons" (TANs). They are activated by presentation of sensory stimuli of behavioral significance or linked to reward. 4. Experimental evidence showed that integrity of the nigrostriatal dopaminergic system is essential for TANs to express learned activity. 5. PD is known to be due to the loss of the nigrostriatal dopaminergic pathway and the ensuing imbalance between the content of dopamine and acetylcholine in the striatum. This evidence supports the hypothesis that cholinergic interneurons, or TANs, play a key role in the modulation of striatal function.

The N-methyl-D-aspartate neurotransmitter receptor is a mammalian brain target for the dinoflagellate Pfiesteria piscicida toxin

Blooms of Pfiesteria piscicida, a dinoflagellate in eastern U.S. coastal rivers, are believed to secrete toxins that kill fish and produce short-term memory loss in humans. Only one or two of Pfiesteria's multiple stages secrete the toxin, and only under certain environmental conditions. Thus, neither the presence of Pfiesteria nor fish kill alone can be indicative of toxin presence. The objective of this study was to identify the mammalian molecular brain target for the toxin that is associated with decrements in memory. Seven rat brain neurotransmitter receptors were selected to study because of their reported roles in cognitive function: receptors for nicotine, muscarine, AMPA/kainate, N-methyl-D-aspartate (NMDA), gamma-aminobutyric acid, and dopamine 1 and 2. The effects of 17 environmental and laboratory samples on radioactive ligand binding to these receptors were studied. Of the seven receptors, binding only to the NMDA receptor was inhibited by only the two Pfiesteria-containing waters (identified by PCR) that also killed fish, and not by any of the other 15 samples tested. It is suggested that inhibition of NMDA-receptor binding is the cause of memory loss in exposed humans. Thus, it could be a useful biomarker for the toxin's presence in rivers for decisions on closures and for identification of the fractions containing the toxin during its purification. Knowledge of the toxin's molecular target, and how it affects its function, also leads to suggestions for therapeutics to use in animal models.

Substance P modulation of striatal dopamine outflow is determined by M(1) and M(2) muscarinic receptors in male wistar rats

Substance P (SP) stimulates striatal dopamine outflow through a cholinergic muscarinic link. SP-induced increase in acetylcholine (Ach) is concentration-dependent, whereas the stimulation of dopamine outflow is seen only over a limited concentration range. M(1) and M(2) receptor stimulation has opposite effects on dopamine outflow. We postulated that the effect of SP on dopamine outflow depends on the M(1)/M(2) balance. We show that Ach (10-2500 microM) stimulates dopamine outflow in striatal slices in a biphasic manner, similar to SP (0.01-100 nM). An inactive SP concentration (10 nM) which was higher than the active concentration range, became active in the presence of the M(2) antagonist methoctramine (100 microM). Conversely, the effect of 1 nM SP was reversed by the M(1) antagonist pirenzepine (1 microM). Our observations show that SP modulation of dopamine outflow is determined by a balance between M(1) and M(2) receptors.

Molecular cloning of a human, hemicholinium-3-sensitive choline transporter

Under many physiological circumstances, Na(+)- and Cl(-)-dependent, hemicholinium-3 (HC-3)-sensitive, high-affinity choline uptake (HACU) in cholinergic neurons is thought to be rate-limiting in the biosynthesis of acetylcholine (ACh). Based on sequence information provided by the Human Genome Project and the recently reported rat CHT1 (rCHT1), we cloned a human CHT cDNA from spinal cord. The hCHT cDNA encodes a protein of 580 amino acids having 93% identity to rCHT1 and 51% identity to the Caenorhabditis elegans homolog CHO-1, and is distantly related to members of the Na(+)-coupled glucose transporter (SGLT) gene family of Na(+)-coupled glucose (SGLT), nucleoside and iodide transporters. Northern blot analysis reveals the expression of a approximately 5 kb transcript in human brain regions rich in cholinergic neurons including the putamen, spinal cord, and medulla. Expression of hCHT cDNA in COS-7 cells results in saturable, Na(+)/Cl(-)-dependent choline uptake (K(m) = 1.2 microM) in membrane vesicles and [(3)H] HC-3 binding (K(d) = 4 nM) in membrane fractions, consistent with characteristics reported in mammalian cholinergic neurons. Using radiation hybrid mapping techniques, we localized the hCHT gene to human chromosome 2q12. These studies elucidate the primary structure and chromosomal localization of hCHT and provide a basis for mechanistic analysis of HACU regulation and an investigation of the role of hCHT in disease states.