The expression of gamma-aminobutyric acid and Leu-enkephalin immunoreactivity in primary monolayer cultures of rat striatum

Optogenetic Interrogation of Electrophysiological Dendritic Properties and Their Effect on Pacemaking Neurons from Acute Rodent Brain Slices

Understanding dendritic excitability is essential for a complete and precise characterization of neurons' input-output relationships. Theoretical and experimental work demonstrates that the electrotonic and nonlinear properties of dendrites can alter the amplitude (e.g., through amplification) and latency of synaptic inputs as viewed in the axosomatic region where spike timing is determined. The gold-standard technique to study dendritic excitability is using dual-patch recordings with a high-resistance electrode used to patch a piece of distal dendrite in addition to a somatic patch electrode. However, this approach is often impractical when distal dendrites are too fine to patch. Therefore, we developed a technique that utilizes the expression of Channelrhodopsin-2 (ChR2) to study dendritic excitability in acute brain slices through the combination of a somatic patch electrode and optogenetic activation. The protocol describes how to prepare acute slices from mice that express ChR2 in specific cell types, and how to use two modes of light stimulation: proximal (which activates the soma and proximal dendrites in a ~100 µm diameter surrounding the soma) with the use of a high-magnification objective and full-field stimulation through a low-magnification objective (which activates the entire somato-dendritic field of the neuron). We use this technique in conjunction with various stimulation protocols to estimate model-based spectral components of dendritic filtering and the impact of dendrites on phase response curves, peri-stimulus time histograms, and entrainment of pacemaking neurons. This technique provides a novel use of optogenetics to study intrinsic dendritic excitability through the use of standard patch-clamp slice physiology. Key features • A method for studying the effects of electrotonic and nonlinear dendritic properties on the sub- and suprathreshold responses of pacemaking neurons. • Combines somatic patch clamp or perforated patch recordings with optogenetic activation in acute brain slices to investigate dendritic linear transformation without patching the dendrite. • Oscillatory illumination at various frequencies estimates spectral properties of the dendrite using subthreshold voltage-clamp recordings and studies entrainment of pacemakers in current clamp recordings. • This protocol uses Poisson white noise illumination to estimate dendritic phase response curves and peri-stimulus time histograms.

GABAergic inputs from direct and indirect striatal projection neurons onto cholinergic interneurons in the primate putamen

Striatal cholinergic interneurons (ChIs) are involved in reward-dependent learning and the regulation of attention. The activity of these neurons is modulated by intrinsic and extrinsic γ-aminobutyric acid (GABA)ergic and glutamatergic afferents, but the source and relative prevalence of these diverse regulatory inputs remain to be characterized. To address this issue, we performed a quantitative ultrastructural analysis of the GABAergic and glutamatergic innervation of ChIs in the postcommissural putamen of rhesus monkeys. Postembedding immunogold localization of GABA combined with peroxidase immunostaining for choline acetyltransferase showed that 60% of all synaptic inputs to ChIs originate from GABAergic terminals, whereas 21% are from putatively glutamatergic terminals that establish asymmetric synapses, and 19% from other (non-GABAergic) sources of symmetric synapses. Double pre-embedding immunoelectron microscopy using substance P and Met-/Leu-enkephalin antibodies to label GABAergic terminals from collaterals of "direct" and "indirect" striatal projection neurons, respectively, revealed that 47% of the indirect pathway terminals and 36% of the direct pathway terminals target ChIs. Together, substance P- and enkephalin-positive terminals represent 24% of all synapses onto ChIs in the monkey putamen. These findings show that ChIs receive prominent GABAergic inputs from multiple origins, including a significant contingent from axon collaterals of direct and indirect pathway projection neurons.

Combination of group I mGlu receptors antagonist with dopaminergic agonists strengthens the synaptic transmission at corticostriatal synapses in culture

Restoring synaptic plasticity in neurodegenerative diseases could prevent neuronal degeneration, as well as motor and cognitive disorders. In Parkinson's disease, synaptic plasticity at corticostriatal synapses is altered. Dendrites of striatal medium spiny neurons (MSNs) receive dopaminergic inputs from the substantia nigra and glutamatergic cortical afferents. Because both glutamate and dopamine are required to induce and sustain MSNs plasticity, the particular molecular mechanisms involved at this synaptic triad are difficult to understand. In the present work, we established a convenient in vitro model of the corticostriatal synapse to study synaptic plasticity. We focused on long-term depression involving group I metabotropic glutamate (mGlu) receptors. We found that in striatal neurons co-cultured with cortical neurons, the absence of dopaminergic stimuli favored the excess of glutamatergic drive from cortical neuron terminals, thus resulting in a constitutive depression of the corticostriatal glutamatergic transmission. Indeed, concomitant blockade of group I mGlu receptors and activation of dopaminergic receptors stably reduced the depression of the synaptic transmission. Thus the dependence on glutamate and dopamine balance of the corticostriatal synapse responsiveness validates the accuracy of this manageable in vitro model to depict the molecular pathways involved in the plasticity at corticostriatal synapses and to test restorative therapeutic approaches in Parkinson's disease. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

MEF-2 regulates activity-dependent spine loss in striatopallidal medium spiny neurons

Striatal dopamine depletion profoundly reduces the density of spines and corticostriatal glutamatergic synapses formed on D(2) dopamine receptor expressing striatopallidal medium spiny neurons, leaving D(1) receptor expressing striatonigral medium spiny neurons relatively intact. Because D(2) dopamine receptors diminish the excitability of striatopallidal MSNs, the pruning of synapses could be a form of homeostatic plasticity aimed at restoring activity into a preferred range. To characterize the homeostatic mechanisms controlling synapse density in striatal medium spiny neurons, striatum from transgenic mice expressing a D(2) receptor reporter construct was co-cultured with wild-type cerebral cortex. Sustained depolarization of these co-cultures induced a profound pruning of glutamatergic synapses and spines in striatopallidal medium spiny neurons. This pruning was dependent upon Ca(2+) entry through Cav1.2 L-type Ca(2+) channels, activation of the Ca(2+)-dependent protein phosphatase calcineurin and up-regulation of myocyte enhancer factor 2 (MEF2) transcriptional activity. Depolarization and MEF2 up-regulation increased the expression of two genes linked to synaptic remodeling-Nur77 and Arc. Taken together, these studies establish a translational framework within which striatal adaptations linked to the symptoms of Parkinson's disease can be explored.

Toxic effects of methoxychlor in rat striatum: modifications in several neurotransmitters

Neurotoxic effects of methoxychlor (MTX) are poorly understood at present. This study was undertaken to evaluate the possible effects of MTX in norepinephrine, dopamine and amino acid contents and serotonin turnover in rat striatum. For this purpose, adult male Sprague-Dawley rats were administered 25 mg/kg/day of MTX in sesame oil or vehicle only for 30 days. The neurotransmitters of interest were measured in the striatum by HPLC. MTX decreased norepinephrine and 5-hydroxyindole acetic acid (5-HIAA) content and serotonin turnover (measured as 5-HIAA/serotonin ratio), and increased glutamate and GABA concentrations. However, the content of serotonin, aspartate, glutamine and taurine was not modified by MTX exposure. These data suggest that MTX exposure inhibits norepinephrine synthesis and serotonin metabolism. The inhibitory effect on norepinephrine could be explained, at least in part, by the increase of both GABA and glutamate contents. Further studies are needed to understand the effects of MTX on serotonin. Also a disruptive effect of MTX on the metabolisms of glutamate, aspartate, glutamine and GABA emerges.

Dissociation of retinal ganglion cells without enzymes

We describe here methods for dissociating retinal ganglion cells from adult goldfish and rat without proteolytic enzymes, and show responses of ganglion cells isolated this way to step-wise voltage changes and fluctuating current injections. Taking advantage of the laminar organization of vertebrate retinas, photoreceptors and other cells were lifted away from the distal side of freshly isolated goldfish retinas, after contact with pieces of membrane filter. Likewise, cells were sliced away from the distal side of freshly isolated rat retinas, after these adhered to a membrane filter. The remaining portions of retina were incubated in an enzyme-free, low Ca2+ solution, and triturated. After aliquots of the resulting cell suspension were plated, ganglion cells could be identified by dye retrogradely transported via the optic nerve. These cells showed no obvious morphological degeneration for several days of culture. Perforated-patch whole-cell recordings showed that the goldfish ganglion cells spike tonically in response to depolarizing constant current injections, that these spikes are temporally precise in response to fluctuating current injections, and that the largest voltage-gated Na+ currents of these cells were larger than those of ganglion cells isolated with a neutral protease.

Agonist stimulation provokes dendritic and axonal dopamine D(1) receptor redistribution in primary cultures of striatal neurons

To investigate the influence of neurotransmitter on G-protein-coupled receptor trafficking and compartimentalization in neurons, we have developed a model of primary neuronal cultures from fetal rat striatum on which we have studied the cellular and subcellular distribution and trafficking of the D(1) dopaminergic receptor. This receptor is known to be somatodendritic and axonal targeted in vivo, mostly to extrasynaptic locations. Immunohistochemical studies at the light and electron microscopic levels showed that, in cultures, the D(1) dopaminergic receptor is expressed in the absence of dopamine stimulation. The pattern of D(1) dopaminergic receptor immunostaining after stimulation by the D(1) dopaminergic receptor agonist SKF 82958 (1 microM) is dramatically modified with a decrease of the number of labeled D(1) dopaminergic receptor puncta (-40%) and an increase of their size in both dendrites (+120%) and axons (+240%). Seven hours after removal of the agonist, return to normal pattern was observed. The D(1) dopaminergic receptor antagonist SCH 23390 (2 microM) abolishes the effect of SKF 82958. Electron microscopy demonstrated, in dendrites, a translocation of the labeling from the plasma membrane to endosomes. Axonal D(1) dopaminergic receptor redistribution after acute stimulation indicates that the D(1) dopaminergic receptor is membrane targeted and responsive to stimulation. These results validate primary culture of striatal neurons to study subcellular localization and intraneuronal trafficking of G-protein-coupled receptors. This preparation will be useful to address various questions concerning the behavior and the trafficking of these receptors in neurons in relation to the neurotransmitter environment.

D(1) dopamine receptor activation reduces GABA(A) receptor currents in neostriatal neurons through a PKA/DARPP-32/PP1 signaling cascade

Dopamine is a critical determinant of neostriatal function, but its impact on intrastriatal GABAergic signaling is poorly understood. The role of D(1) dopamine receptors in the regulation of postsynaptic GABA(A) receptors was characterized using whole cell voltage-clamp recordings in acutely isolated, rat neostriatal medium spiny neurons. Exogenous application of GABA evoked a rapidly desensitizing current that was blocked by bicuculline. Application of the D(1) dopamine receptor agonist SKF 81297 reduced GABA-evoked currents in most medium spiny neurons. The D(1) dopamine receptor antagonist SCH 23390 blocked the effect of SKF 81297. Membrane-permeant cAMP analogues mimicked the effect of D(1) dopamine receptor stimulation, whereas an inhibitor of protein kinase A (PKA; Rp-8-chloroadenosine 3',5' cyclic monophosphothioate) attenuated the response to D(1) dopamine receptor stimulation or cAMP analogues. Inhibitors of protein phosphatase 1/2A potentiated the modulation by cAMP analogues. Single-cell RT-PCR profiling revealed consistent expression of mRNA for the beta1 subunit of the GABA(A) receptor-a known substrate of PKA-in medium spiny neurons. Immunoprecipitation assays of radiolabeled proteins revealed that D(1) dopamine receptor stimulation increased phosphorylation of GABA(A) receptor beta1/beta3 subunits. The D(1) dopamine receptor-induced phosphorylation of beta1/beta3 subunits was attenuated significantly in neostriata from DARPP-32 mutants. Voltage-clamp recordings corroborated these results, revealing that the efficacy of the D(1) dopamine receptor modulation of GABA(A) currents was reduced in DARPP-32-deficient medium spiny neurons. These results argue that D(1) dopamine receptor stimulation in neostriatal medium spiny neurons reduces postsynaptic GABA(A) receptor currents by activating a PKA/DARPP-32/protein phosphatase 1 signaling cascade targeting GABA(A) receptor beta1 subunits.

The dopaminergic innervation of the avian telencephalon

The present review provides an overview of the distribution of dopaminergic fibers and dopaminoceptive elements within the avian telencephalon, the possible interactions of dopamine (DA) with other biochemically identified systems as revealed by immunocytochemistry, and the involvement of DA in behavioral processes in birds. Primary sensory structures are largely devoid of dopaminergic fibers, DA receptors and the D1-related phosphoprotein DARPP-32, while all these dopaminergic markers gradually increase in density from the secondary sensory to the multimodal association and the limbic and motor output areas. Structures of the avian basal ganglia are most densely innervated but, in contrast to mammals, show a higher D2 than D1 receptor density. In most of the remaining telencephalon D1 receptors clearly outnumber D2 receptors. Dopaminergic fibers in the avian telencephalon often show a peculiar arrangement where fibers coil around the somata and proximal dendrites of neurons like baskets, probably providing them with a massive dopaminergic input. Basket-like innervation of DARPP-32-positive neurons seems to be most prominent in the multimodal association areas. Taken together, these anatomical findings indicate a specific role of DA in higher order learning and sensory-motor processes, while primary sensory processes are less affected. This conclusion is supported by behavioral findings which show that in birds, as in mammals, DA is specifically involved in sensory-motor integration, attention and arousal, learning and working memory. Thus, despite considerable differences in the anatomical organization of the avian and mammalian forebrain, the organization of the dopaminergic system and its behavioral functions are very similar in birds and mammals.

NMDA and non-NMDA receptor-mediated excitotoxicity are potentiated in cultured striatal neurons by prior chronic depolarization

The excitatory input from cortex and/or thalamus to striatum appears to promote the maturation of glutamate receptors on striatal neurons, but the mechanisms by which it does so have been uncertain. To explore the possibility that the excitatory input to striatum might influence glutamate receptor maturation on striatal neurons, at least in part, by its depolarizing effect on striatal neurons, we examined the influence of chronic KCl depolarization on the development of glutamate receptor-mediated excitotoxic vulnerability and glutamate receptors in cultured striatal neurons. Dissociated striatal neurons from E17 rat embryos were cultured for 2 weeks in Barrett's medium containing either low (3 mM) or high (25 mM) KCl. The vulnerability of these neurons to NMDA receptor agonists (NMDA and quinolinic acid), non-NMDA receptor agonists (AMPA and KA), and a metabotropic glutamate receptor agonist (trans-ACPD) was examined by monitoring cell loss 24 h after a 1-h agonist exposure. We found that high-KCl rearing potentiated the cell loss observed with 500 microM NMDA or 250 microM KA and yielded cell loss with 250 microM AMPA that was not evident under low KCl rearing. In contrast, neither QA up to 5 mM nor trans-ACPD had a significant toxic effect in either KCl group. ELISA revealed that chronic high KCl doubled the abundance of NMDA NR2A/B, AMPA GluR2/3, and KA GluR5-7 receptor subunits on cultured striatal neurons and more than doubled AMPA GluR1 and GluR4 subunits, but had no effect on NMDA NR1 subunit levels. These receptor changes may contribute to the potentiation of NMDA and non-NMDA receptor-mediated excitotoxicity shown by these neurons following chronic high-KCl rearing. Our studies suggest that membrane depolarization produced by corticostriatal and/or thalamostriatal innervation may be required for maturation of glutamate receptors on striatal neurons, and such maturation may be important for expression of NMDA and non-NMDA receptor-mediated excitotoxicity by striatal neurons. Striatal cultures raised under chronically depolarized conditions may, thus, provide a more appropriate culture model to study the role of NMDA or non-NMDA receptor subtypes in excitotoxicity in striatum.

Modulation of the voltage-gated sodium current in rat striatal neurons by DARPP-32, an inhibitor of protein phosphatase

DARPP-32 is a cyclic adenosine monophosphate-regulated inhibitor of protein phosphatase 1, highly enriched in striatonigral neurons. Stimulation of dopamine D1 receptors increases phosphorylation of DARPP-32, whereas glutamate acting on N-methyl-D-aspartate receptors induces its dephosphorylation. Yet, to date, there is little direct evidence for the function of DARPP-32 in striatal neurons. Using a whole cell patch-clamp technique, we have studied the role of DARPP-32 in the regulation of voltage-gated sodium channels in rat striatal neurons maintained in primary culture. Injection of phospho-DARPP-32, but not of the unphosphorylated form, reduced the sodium current amplitude. This effect was similar to those induced by okadaic acid, with which there was no additivity and by tautomycin. Our results indicate that, in striatal neurons, sodium channels are under dynamic control by phosphorylation/dephosphorylation, and that phospho-DARPP-32 reduces sodium current by stabilizing a phosphorylated state of the channel or an associated regulatory protein. We propose that the DARPP-32-mediated modulation of sodium channels, via inhibition of phosphatase 1, contributes to the regulation of these channels by D1 receptors and other neurotransmitters which influence the state of phosphorylation of DARPP-32.

Induction of c-fos and zif/268 gene expression in rat striatal neurons, following stimulation of D1-like dopamine receptors, involves protein kinase A and protein kinase C

Changes in the level of dopaminergic activity in the rat striatum lead to the induction of a number of immediate-early genes, including c-fos and zif/268. These immediate-early genes are thought in turn to alter the rate of transcription of downstream genes. There is evidence that the dopaminergic activation of the c-fos and zif/268 genes in the striatum in vivo is linked to stimulation of D1-like dopamine receptors. We have used primary cultures of embryonic rat striatal neurons to identify the intracellular pathways involved in this response. Dopamine (10 nM-5 microM) caused a marked increase in the levels of c-fos mRNA and zif/268 mRNA in cultured striatal neurons, an effect that was reproduced by the D1-like dopamine receptor agonist SKF38393 (10 nM-5 microM). These actions were attenuated by the D1-like antagonist SCH23390 (1 microM) but not by the D2-like antagonist eticlopride (1 microM). The D2-like agonist quinpirole did not increase zif/268 mRNA above basal levels at concentrations up to 5 microM, but caused a slight increase in the levels of c-fos mRNA. The stimulation of c-fos mRNA levels caused by 1 microM SKF38393 was reduced by 45% following pretreatment with the selective protein kinase A inhibitor KT5720, and by 87% following pretreatment with the selective protein kinase C inhibitor calphostin C. The stimulation of zif/268 mRNA levels caused by 1 microM SKF38393 was reduced by 90% following pretreatment with KT5720, but was not significantly affected by pretreatment with calphostin C. In addition, the actions of SKF38393 to stimulate the expression of both immediate-early genes were attenuated by coadministration of quinpirole. These results suggest that SKF38393 acts on striatal neurons to stimulate c-fos expression predominantly through protein kinase C, but also partially through protein kinase A. Conversely, SKF38393 induces zif/268 expression through protein kinase A. The ability of quinpirole to antagonize the actions of SKF38393 on cultured neurons is consistent with the presence of both D1-like receptors on the same neuronal population.

The neuritogenic effect of myenteric plexus on striatal neurones in co-culture involves nitric oxide

We reported previously that myenteric plexus explants promoted striatal neurite elongation in co-culture and that this effect was abolished by tetrodotoxin (TTX). Here we demonstrate that the nitric oxide synthase blocker N-nitro-L-arginine methyl ester significantly reduced the neuritogenic effect of the myenteric plexus whereas the nitric oxide donor, sodium nitroprusside (SNP), partially reversed the blocking effect of TTX. 2-Chloroadenosine (2-CA), a stable analogue of adenosine, which is produced following release of ATP from enteric neurones, further enhanced the effect of SNP. Basic fibroblast growth factor or neurotrophin-3 in combination with 2-CA and SNP were only marginally neuritogenic in striatal cultures alone. These results suggest that NO is involved in the trophic effects of myenteric plexus explants on striatal neurones.

Stimulation of zif/268 gene expression by basic fibroblast growth factor in primary rat striatal cultures

The presence of basic fibroblast growth factor (bFGF) in the basal ganglia, and its known neurotrophic activity, has created interest in its possible role as an agent to attenuate striatal neurodegeneration. However, little information is available on the mechanisms through which bFGF might exert a long-term influence on striatal function. Primary cultures of embryonic rat striatal neurones were used to ascertain whether bFGF can alter the pattern of striatal gene expression. Treatment of cultures with bFGF (500 pM) resulted in a dramatic increase in the levels of zif/268 mRNA within 45 min. This induction was attenuated by the tyrosine kinase inhibitor genistein (100 microM), but not by its inactive structural analogue genistin (100 microM). The induction of zif/268 mRNA was found to occur in non-neuronal cells, with no increase in mRNA levels being observed in neurones. A similar induction was noted for another putative transcription factor, jun B, although no induction of the related factor jun D could be detected. These results show that bFGF can induce immediate-early gene expression in striatal cultures, and therefore that this may provide a mechanism, mediated by non-neuronal cells, which allows bFGF to cause a long-term change in striatal neurochemistry.

Dopamine D1 receptor modulates the voltage-gated sodium current in rat striatal neurones through a protein kinase A

1. Whole-cell recordings were made from striatal neurones obtained from neonatal rats and maintained in primary cultures. The effects of dopamine D1 receptor activation were studied on the voltage-gated sodium current. 2. Bath application of a specific D1 agonist, SKF38393 (1 microM), reduced the neuronal excitability recorded in current-clamp by increasing the threshold for generation of action potentials. 3. In voltage-clamp recordings, SKF38393 (1 microM) reversibly reduced the peak amplitude of the sodium current by 37.8 +/- 4.95%. This effect was reversed by the D1 antagonist SCH23390 and was blocked by the intracellular loading of GDP-beta-S (2 mM) suggesting GTP-binding protein involvement. 4. The D1 agonist reduced the peak amplitude of the sodium current without significantly affecting (i) the voltage dependence of the current-voltage relationship, (ii) the voltage dependence of the steady-state activation and inactivation, (iii) the kinetics of the time-dependent inactivation, and (iv) the kinetics of recovery from inactivation. 5. The peak amplitude of the sodium current was progressively reduced by intracellular loading of cyclic AMP-dependent protein kinase (100 U ml-1). 6. Diffusion of a specific peptide inhibitor of the cyclic AMP-dependent protein kinase (PKI; 10 microM) into the cytosol of neurones blocked the effect of the D1 agonist on the sodium current amplitude. 7. These results demonstrate that dopamine acting at the D1 receptor reduces the amplitude of the sodium current without modifying its voltage- and time-dependent properties. This effect involves activation of the cyclic AMP-dependent protein kinase and results in a depression of the striatal neuronal excitability by increasing the threshold for generation of action potentials.

Emergence of a synaptic neuronal network within primary striatal cultures seeded in serum-free medium

In order to investigate the basic cellular mechanisms involved in neuronal interactions within the striatum, we prepared a primary striatal cell culture from rat fetal brain in chemically defined medium. Using morphological and whole-cell recording methods, we observed that an intensive neuritic elongation with a progressive build up of a sodium-dependent electrogenesis occurred during the first week of culture. Morphologically mature synapses began to develop after 10 days in vitro. By this time, most of the neurons (82 +/- 9%) received spontaneously synaptic potentials, which led them to fire (71 +/- 11%). The spontaneous firing was prevented by cadmium (200 microM) and tetrodotoxin (5 microM), which suggested that a Ca(2+)-dependent release of neurotransmitters was involved in the synaptic activation. We further obtained evidence that GABA, and to a lesser extent acetylcholine, contributed to these spontaneous synaptic potentials. At 15 days in vitro, it was possible to observe up to four synaptic contacts on a given dendrite. By this time, whole-cell recordings performed on pairs of neurons showed that the mature neurons were interconnected by excitatory synapses. As the number of synapses increased, the striatal neurons gradually formed a large network in which spontaneous activity developed, which tended to be organized into synchronized bursting patterns.

Modulation of calcium currents by a D1 dopaminergic protein kinase/phosphatase cascade in rat neostriatal neurons

In rat neostriatal neurons, D1 dopamine receptors regulate the activity of cyclic AMP-dependent protein kinase (PKA) and protein phosphatase 1 (PP1). The influence of these signaling elements on high voltage-activated (HVA) calcium currents was studied using whole-cell voltage-clamp techniques. The application of D1 agonists or cyclic AMP analogs reversibly reduced N- and P-type Ca2+ currents. Inhibition of PKA antagonized this modulation, as did inhibition of PP1, suggesting that the D1 effect was mediated by a PKA enhancement of PP1 activity directed toward Ca2+ channels. In a subset of neurons, D1 receptor-mediated activation of PKA enhanced L-type currents. The differential regulation of HVA currents by the D1 pathway helps to explain the diversity of effects this pathway has on synaptic integration and plasticity in medium spiny neurons.

Phenotypic characterisation of rat striatal neurones in primary culture

The aim of this study was to determine to what extent the neuronal phenotypes present in primary cultures of rat striatal neurones correspond to those present in vivo. A large percentage of cultured striatal neurones contained relatively high levels of proenkephalin mRNA. In addition, a high level of expression was found for the prosomatostatin mRNA. Protachykinin mRNA and proneuropeptide Y mRNA were also expressed, but at a comparatively low level. No prodynorphin mRNA could be detected. Considerable numbers of neurones were also found to express NADPH-diaphorase activity, while a smaller number of neurones were positive for acetylcholinesterase. The NADPH-diaphorase and the acetylcholinesterase could be detected both in cell bodies, and in neuronal processes contacting groups of neighbouring neurones. Since nitric oxide does not require synaptic specialisations to exert its intercellular actions, this provides strong evidence that NADPH-positive neurones communicate with other cells in primary culture. These observations demonstrate that when striatal neurones are grown in primary culture, a range of neurochemical phenotypes are present which correspond closely to those present in the mature striatum in vivo. Together with the evidence for cell-cell interactions, this suggests that primary striatal cultures will provide a suitable model to study the molecular mechanisms controlling striatal function.

Myenteric plexus explants promote neurite elongation and survival of striatal neurons in vitro

Dissociated striatal neurons exhibited increased neurite outgrowth when co-cultured with myenteric plexus explants. Enriched enteric neurons or enriched enteric glia produced a less marked response; non-ganglionic cells had no effect. Increases in striatal neuron and glial cell numbers were seen in all co-cultures. Tetrodotoxin abolished the neuritogenic response of myenteric plexus explants but did not affect increases in cell numbers. These observations suggest that spontaneous neuronal activity within the myenteric plexus is involved in the release of a neuritogenic factor(s), possibly from glial cells, and that this is distinct from the factor(s) affecting striatal cell numbers.

Basic fibroblast growth factor induces c-fos expression in primary cultures of rat striatum

Basic fibroblast growth factor (bFGF) is present in the rat striatum in vivo, where evidence suggests it may have a long-term trophic role in supporting the survival of striatal neurones. To examine the possibility that these effects of bFGF might be mediated by induction of neuronal gene expression, we have investigated the ability of bFGF to stimulate expression of the immediate-early gene c-fos in primary cultures of embryonic rat striatum. The basal levels of c-fos mRNA were low in both neurones and glia in culture. Application of 500 pM bFGF resulted, within 45 min, in a 11-fold increase in the c-fos hybridisation signal in the non-neuronal cells. No significant induction of c-fos mRNA was detected in the striatal neurones at this time. The induction in non-neuronal cells was blocked by the tyrosine kinase inhibitor genistein (100 microM), but not by its inactive structural analogue genistin (100 microM). These results represent a novel mechanism whereby bFGF can exert prolonged effects on striatal function, and indicate that the increases in striatal c-fos gene expression induced by bFGF occur primarily in non-neuronal cells.

High- and low-voltage activated calcium currents are expressed by neurons cultured from embryonic rat neostriatum

Current-clamp studies have shown that voltage-dependent Ca currents are present in rat neostriatal neurons. Although these studies have provided evidence for the presence of high-voltage activated Ca channels, it has been unclear whether low-voltage activated channels are also present. Using the whole-cell variant of the patch-clamp technique, we have studied isolated Ca currents in an attempt to answer this question. We have found that both high- and low-voltage activated calcium currents are expressed by neostriatal neurons cultured from embryonic rat brain. These currents are similar in voltage-dependence and pharmacology to those found in other brain neurons.

Developmental regulation of a slowly-inactivating potassium conductance in rat neostriatal neurons

In late embryonic and early post-natal rat neostriatal neurons, the voltage-dependent potassium currents activated by depolarization are largely attributable to a rapidly inactivating A-current and a delayed rectifier current. Over the first 4 weeks of post-natal life, a third potassium current emerges in most cells. This slowly inactivating conductance is distinct from the A-current and delayed rectifier in voltage-dependence, kinetics and pharmacology. The properties of this conductance suggest that it may be of central importance to the integrative behavior of neostriatal neurons by controlling such features as first spike latency and interspike interval.

Striatal proenkephalin turnover and gene transcription are regulated by cyclic AMP and protein kinase C-related pathways

Preproenkephalin metabolism, in the rat, was studied in primary striatal neurons maintained in a chemically defined medium. Acute treatment (30 min) with forskolin (10(-5) M) or phorbol 12 myristate 13 acetate (10(-7) M) resulted, respectively, in a two- and seven-fold increase in methionine-enkephalin secretion. Chronic treatment with forskolin or phorbol 12 myristate 13 acetate (24 h) induced a 100% increase in methionine-enkephalin content (forskolin) and on the other hand a 50% decrease in methionine-enkephalin (phorbol 12 myristate 13 acetate). Both treatments increased preproenkephalin mRNA levels in a time-dependent manner, this augmentation being observable after 180 min by Northern blot analysis and in situ hybridization. These data indicate that under chronic stimulation, with either forskolin or phorbol 12 myristate 13 acetate, proenkephalin turnover is accelerated. However, after stimulation with phorbol 12 myristate 13 acetate, the more potent methionine-enkephalin secretagogue, increased peptide synthesis is not sufficient to replenish methionine-enkephalin intracellular stores. Preproenkephalin gene transcription was analysed by introducing the preproenkephalin gene promoter fused to the bacterial acetyl chloramphenicol transferase reporter gene into primary neurons. Chronic stimulation (48 h) by forskolin (10(-5) M) or phorbol 12 myristate 13 acetate (10(-7) M) of striatal neurons transfected with this fusion gene increased chloramphenicol acetyltransferase activity six-fold and the two effects were additive. These data suggest that the cyclic AMP and the protein kinase C pathways directly activate preproenkephalin gene transcription.

Muscarinic regulation of cyclic AMP metabolism in rat neostriatal cultures

Muscarinic receptor expression and function were investigated in cultured rat neostriatum. Muscarinic receptor levels were determined from saturation binding experiments performed on intact cultures using [3]N-methylscopolamine. In cultures maintained for 3, 7 and 12-14 days in vitro, the Bmax was 2.3, 5.4 and 10.9 fmol/culture. The average number of receptors per neuron increased during the 2nd week in vitro. Carbachol (100 microM) had no significant effect on basal cAMP levels but reduced cAMP levels elevated by forskolin. Carbachol significantly reduced cAMP levels stimulated with dopamine only in cultures untreated with a phosphodiesterase inhibitor. Comparing equimolar doses, the carbachol response was more sensitive to the M1 selective antagonist pirenzepine than the cardioselective M2 antagonist AF-DX 116. These results suggest that the muscarinic receptors regulate cAMP levels in neostriatal neurons and, in so doing, provide a post-synaptic substrate for the interaction of dopamine and acetylcholine.

Factors regulating the expression of acetylcholinesterase-containing neurons in striatal cultures: effects of chemical depolarization

The influence of chemical depolarization on the survival and differentiation of acetylcholinesterase (AChE)-containing neurons was examined in primary rat striatal cultures, maintained in different types of media (serum-free and serum-supplemented) and substrate (poly-ornithine and astrocyte monolayer). Chronic application of 5 microM veratridine resulted in a significant loss of neurites by AChE-positive cells, while a higher concentration (20 microM) reduced the number of stained cell bodies. These effects appeared to be selective with regard to AChE-positive cells, as indicated by morphological observations of the cells in the treated cultures and receptor binding measurements. Similarly, elevation of extracellular KCl levels (20-60 mM) produced a dose-dependent neurite loss by AChE-containing cells. Blockers of voltage-sensitive Ca2+ channels--verapamil (1 microM) and nifedipine (1 microM)--did not affect the veratridine-induced neurite loss, while tetrodotoxin (0.1 microM) had a partial effect. When cultures treated with 5 microM veratridine were allowed to recuperate for several days, the number of AChE-positive cells possessing neurites returned close to control values, thus indicating the reversibility of the effect of chemical depolarization. The possibility that chronic neuronal depolarization in the striatum might play a role in regulation of the neuronal processes outgrowth by AChE-containing cells is discussed.

Quinolinate and kainate neurotoxicity in neostriatal cultures is potentiated by co-culturing with neocortical neurons

It has been suggested that a disorder in the regulation of excitatory amino acids (EAA) may underlie the loss of neostriatal neurons seen in Huntington's disease. The role of neocortical afferent fibers in determining the EAA sensitivity of neostriatal neurons was assessed by comparing EAA toxicity in co-cultures of neocortex and neostriatum with that of neostriatum alone. In cultures of neostriatum alone, EAAs produced only modest neuronal losses. Kainate, which tended to be the most potent excitotoxin, produced a loss of approximately 30% of the neurons after a 5-min exposure at a 1-mM concentration. In co-cultures, the sensitivity of neostriatal neurons to EAA toxicity was dramatically enhanced; toxicity was increased about two-fold for kainate and quinolinate at millimolar concentrations and as much as 8-fold for quinolinate at micromolar concentrations. The effects of EAA co-incubation with the N-methyl-D-aspartate (NMDA) receptor antagonist, 2-amino-5-phosphonovaleric acid, suggested that the toxic actions of quinolinate, but not kainate, were mediated largely by NMDA receptors.

Muscarinic modulation of a transient K+ conductance in rat neostriatal neurons

Neurons of the neostriatum are richly innervated by cholinergic neurons of intrinsic origin. Both pre- and post-synaptic muscarinic receptors mediate the effects of acetylcholine (ACh). Activation of these receptors is functionally significant, particularly in Parkinson's disease. Current-clamp studies indicate that muscarinic receptors serve to decrease the responsiveness of neostriatal neurons to excitatory inputs. Here we present evidence that this effect is caused, in part, by the muscarinic modulation of the A-current, a transient outward potassium current. The voltage dependence of this current suggests that normally it enhances spike repolarization and slows discharge rate, but does not affect 'synaptic integration'. We find that under the influence of muscarinic agonists, the voltage dependence of A-current activation and inactivation is shifted towards more negative membrane potentials and the peak conductance is increased. Therefore, at relatively hyperpolarized resting potentials, ACh transiently alters the functional role of the A-current, allowing it to suppress excitatory inputs and further slow the discharge rate. But at relatively depolarized resting potentials, ACh increases excitability by removing the A-current through inactivation.

Establishment of a long-term primary culture of striatal neurons

A new method of obtaining long-term primary cultures (lasting more than 8 weeks) of striatal neurons is described in this paper. The originality of the method consists of: (1) starting the culture for 3 days in a serum-free medium which allows attachment and neurite proliferation of neurons as well as the death of non-neuronal cells (mainly consisting of astrocytes); (2) introducing a limited amount of fetal calf serum (FCS) (2-5%) after 3 days in vitro (3 DIV), which likely provides optimal neuronal survival and attachment factors, and a limited amount of astrocyte proliferating factors. The period of introduction of serum, as well as the amount of serum introduced are critical factors. By phase contrast and transmission electron microscopy, we observed that neurons continued to develop neurite extensions, synaptic vesicles and synapse formations up to 50 DIV. Neuronal membranes, and synaptic contacts were particularly healthy up to 50 DIV. Interestingly, the number of astrocytes was constant between 30-50 DIV and limited to about 10%. We therefore obtained an equilibrium between neuronal and astrocyte differentiation and proliferation. It is likely that the small population of astrocytes, plus the low percentage of FCS added, provide essential factors for neuronal survival and differentiation, whereas a high density of differentiated neurons inhibited astrocyte cell proliferation. The clear-cut stability of these neuronal cultures goes in parallel with the stability of the pharmacological responses studied here: the coupling of carbachol and quisqualate receptors with the inositol phosphate production system. The culture method described here could be of particular interest to pursue biochemical, pharmacological and biological studies on neurons as well as on reciprocal interactions between neurons and astrocytes.

M1 muscarinic acetylcholine receptor in cultured rat neostriatum regulates phosphoinositide hydrolysis

Muscarinic acetylcholine receptor expression and function in cultured rat neostriatal neurons were examined. All experiments were performed on intact neurons grown in vitro for 12-14 days. The muscarinic antagonist N-[3H]methylscopolamine [( 3H]NMS) binds to a single site in cultures with a KD of 89 pM and a Bmax of 187 fmol/mg of protein, or 32,000 sites/neuron. Competition studies using [3H]NMS were performed to determine what receptor subtypes were present. Nonlinear analysis of competition curves was best described with a single binding site for atropine, pirenzepine, and AF-DX 116 (11-[[2-[(diethylamino)-methyl]-1-piperidinyl]acetyl]-5,11-dihydro- 6H-pyrido[2,3-b][1,4]benzodiazepine-6-one), with Ki values of 0.6, 62, and 758 nM, respectively. These results indicate that the muscarinic receptors present in neostriatal cultures are of the M1 subtype, having high affinity for pirenzepine and low affinity for AF-DX 116. In contrast with antagonists, carbachol displaced [3H]NMS from two sites with Ki values of 6.5 and 147 microM, with the higher-affinity form predominant (83% of sites). The M1 receptor subtype was linked to phosphoinositide turnover. Carbachol stimulated the formation of phosphoinositides with an EC50 of 37 microM and was antagonized by atropine. At equimolar doses, pirenzepine was more potent than AF-DX 116 at antagonizing the response.

Two types of A-current differing in voltage-dependence are expressed by neurons of the rat neostriatum

Transient potassium currents of the A type are thought to be important in a number of physiological processes of excitable cells, including spike repolarization and synaptic integration. This functional diversity may reflect the contribution of distinct subtypes of A channel to cellular behavior. Using the whole-cell variant of the patch clamp technique, we have found that two types of A-current are expressed in rat neostriatal neurons, one that is similar to previous descriptions in mammals and a second that is activated at considerably more depolarized potentials.

Voltage-clamp analysis of a transient potassium current in rat neostriatal neurons

Whole cell voltage-clamp recordings were made from cultured rat neostriatal neurons. Depolarizing voltage commands evoked transient and sustained outward K-currents. The transient K-current was activated by depolarizing commands beyond -50 mV; peak current was dependent upon holding potential. Bath application of 4-aminopyridine, but not inorganic calcium channel blockers (Cd, Co, Mn), attenuated the transient current. Reversal was near the K-equilibrium potential. These properties suggest that this transient K-current is similar to the A-current described in a number of other neurons.