Striatal interneurons in dissociated cell culture

Central Cholinergic Synapse Formation in Optimized Primary Septal-Hippocampal Co-cultures

Septal innervation of basal forebrain cholinergic neurons to the hippocampus is critical for normal learning and memory and is severely degenerated in Alzheimer's disease. To understand the molecular events underlying physiological cholinergic synaptogenesis and remodeling, as well as pathological loss, we developed an optimized primary septal-hippocampal co-culture system. Hippocampal and septal tissue were harvested from embryonic Sprague-Dawley rat brain and cultured together at varying densities, cell ratios, and in the presence of different growth factors. We identified conditions that produced robust septal-hippocampal synapse formation. We used confocal microscopy with primary antibodies and fluorescent ligands to validate that this system was capable of generating developmentally mature cholinergic synapses. Such synapses were comprised of physiological synaptic partners and mimicked the molecular composition of in vivo counterparts. This co-culture system will facilitate the study of the formation, plasticity, and dysfunction of central mammalian cholinergic synapses.

Cell adhesion molecule close homolog of L1 binds to the dopamine receptor D2 and inhibits the internalization of its short isoform

Cell adhesion molecule close homolog of L1 (CHL1) and the dopamine receptor D2 (DRD2) are associated with psychiatric and mental disorders. We here show that DRD2 interacts with CHL1 in mouse brain, as evidenced by co-immunostaining, proximity ligation assay, co-immunoprecipitation, and pull-down assay with recombinant extracellular CHL1 domain fused to Fc (CHL1-Fc). Direct binding of CHL1-Fc to the first extracellular loop of DRD2 was shown by ELISA. Using HEK cells transfected to co-express CHL1 and the short (DRD2-S) or long (DRD2-L) DRD2 isoforms, co-localization of CHL1 and both isoforms was observed by immunostaining and proximity ligation assay. Moreover, CHL1 inhibited agonist-triggered internalization of DRD2-S. Proximity ligation assay showed close interaction between CHL1 and DRD2 in neurons expressing dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP32) or tyrosine hydroxylase (TH) in tissue sections of adult mouse striatum. In cultures of striatum or ventral midbrain, CHL1 was also closely associated with DRD2 in DARPP32- or TH-immunopositive cells, respectively. In the dorsal striatum of CHL1-deficient mice, lower levels of DRD2 and phosphorylated TH were observed, when compared to wild-type littermates. In the ventral striatum of CHL1-deficient mice, levels of phosphorylated DARPP32 were reduced. We propose that CHL1 regulates DRD2-dependent presynaptic and postsynaptic functions.

Tubulin post-translational modifications in developing dog primary neurons obtained with methods according to the 3Rs principles

Microtubules play a crucial role during neuronal morphogenesis regulating many functions. In the study of these phenomena in vitro cellular models have been employed, mainly resorting to housed experimental animals. Among alternative models in neurobiological study, recently dog caught particular attention. In fact, the complexity of the canine brain, the life long span and the neurodegenerative pathologies render the dog a species more close to humans than rodents. Lately, growing interest in the limitation of the use of experimental animals, has stimulated the search for alternative experimental protocols. Starting from fetal dog brain, obtained by alternative way of sampling, we set neuronal primary cultures. Through immunofluorescence, we examined the presence and the cellular distribution of tubulin post-translational modifications as tyrosinated and acetylated α-tubulin, as markers of dynamic and stable microtubule respectively. In addition, we evaluated the pattern of two associated proteins which may slide on these two tubulin modifications, i.e. CLIP-170 and Kinesin-1. A clear positivity for tyrosinated and acetylated α-tubulin, was found. As far as the motor proteins are concerned, we detected a prevalence of CLIP-170 compared to kinesin-1 with a better overlapping between tyrosinated α-tubulin and CLIP-170. Our findings highlighted some original data about the role of the microtubular network during early phases of canine neuronal morphogenesis. In addition, the experimental protocol underlined the utility of this alternative model that allows to bypass both the scarcity of commercial canine neuronal cell lines and the need to resort to experimental dogs, respecting the 3Rs principles (reduction, refinement, and replacement).

Cocaine-induced synaptic structural modification is differentially regulated by dopamine D1 and D3 receptors-mediated signaling pathways

Synaptic plasticity plays a critical role in cocaine addiction. The dopamine D1 and D3 receptors differentially regulate the cocaine-induced gene expression, structural remodeling and behavioral response. However, how these two receptors coordinately mediate the ultra-structural changes of synapses after cocaine exposure and whether these changes are behaviorally relevant are still not clear. Here, using quantitative electron microscopy, we show that D1 and D3 receptors have distinct roles in regulating cocaine-induced ultra-structural changes of synapses in the nucleus accumbens and caudoputamen. Pre-treatment of cocaine-treated mice with D3 receptor antagonist NGB2904 resulted in an increase in the ratio of total and asymmetric synapse to neuron and in the length of postsynaptic densities, compared with cocaine treatment alone. In contrast, pre-treatment of cocaine-treated mice with D1 receptor antagonist SCH23390 caused a reduction in synapse-to-neuron ratio and in postsynaptic densities length. Similarly, NGB2904 and SCH23390 showed opposite/differential effects on cocaine-induced structural plasticity, conditioned place preference and locomotor activity and signaling activation, including the activation of ERK, CREB and NR1 and the expression of c-fos and Cdk5. Therefore, we provide direct electron microscopy evidence that dopamine D1 and D3 receptors reciprocally regulate the ultra-structural changes of synapses following chronic exposure to cocaine. In addition, our data suggest that D1 and D3 receptors may regulate cocaine-induced ultra-structural changes and behavior responses by impact on structural plasticity and signaling transduction.

Rebuilding a realistic corticostriatal "social network" from dissociated cells

Many of the methods available for the study of cortical influences on striatal neurons have serious problems. In vivo the connectivity is so complex that the study of input from an individual cortical neuron to a single striatal cell is nearly impossible. Mixed corticostriatal cultures develop many connections from striatal cells to cortical cells, in striking contrast to the fact that only connections from cortical cells to striatal cells are present in vivo. Furthermore, interneuron populations are over-represented in organotypic cultures. For these reasons, we have developed a method for growing cortical and striatal neurons in separated compartments that allows cortical neurons to innervate striatal cells in culture. The method works equally well for acutely dissociated or cryopreserved neurons and allows a number of manipulations that are not otherwise possible. Either cortical or striatal compartments can be transfected with channel rhodopsins. The activity of both areas can be recorded in multielectrode arrays or individual patch recordings from pairs of cells. Finally, corticostriatal connections can be severed acutely. This procedure enables determination of the importance of corticostriatal interaction in the resting pattern of activity. These cultures also facilitate development of sensitive analytical network methods to track connectivity.

Extrasynaptic glutamate NMDA receptors: key players in striatal function

N-methyl-D-aspartate receptors (NMDAR) are crucial for the function of excitatory neurotransmission and are present at the synapse and on the extrasynaptic membrane. The major nucleus of the basal ganglia, striatum, receives a large glutamatergic excitatory input carrying information about movements and associated sensory stimulation for its proper function. Such bombardment of glutamate synaptic release results in a large extracellular concentration of glutamate that can overcome the neuronal and glial uptake homeostatic systems therefore allowing the stimulation of extrasynaptic glutamate receptors. Here we have studied the participation of their extrasynaptic type in cortically evoked responses or in the presence of NMDARs stimulation. We report that extrasynaptic NMDAR blocker memantine, reduced in a dose-dependent manner cortically induced NMDA excitatory currents in striatal neurons (recorded in zero-Mg(++) plus DNQX 10 μM). Moreover, memantine (2-4 μM) significantly reduced the NMDAR-dependent membrane potential oscillations called up and down states. Recordings of neuronal striatal networks with a fluorescent calcium indicator or with multielectrode arrays (MEA) also showed that memantine reduced in a dose-dependent manner, NMDA-induced excitatory currents and network behavior. We used multielectrode arrays (MEA) to grow segregated cortical and striatal neurons. Once synaptic contacts were developed (>21DIV) recordings of extracellular activity confirmed the cortical drive of spontaneous synchronous discharges in both compartments. After severing connections between compartments, active striatal neurons in the presence of memantine (1 μM) and CNQX (10 μM) were predominantly fast spiking interneurons (FSI). The significance of extrasynaptic receptors in the regulation of striatal function and neuronal network activity is evident.

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'.

Development of dissociated cryopreserved rat cortical neurons in vitro

Dissociated neuronal cultures of various brain regions are commonly used to study physiological and pathophysiological processes in vitro. The data derived from these studies are often viewed to have relevance to processes taking place in the mature brain. However, due to the practical challenges associated with lengthy neuronal culture, neurons are often kept for 14 days in vitro (DIV), or less, before being subject to experimentation. Non-proliferative cultures such as primary neuronal cultures can be maintained for more than 42 DIV if water evaporation from culture media is monitored and corrected. To determine appropriate time points corresponding to the stages of cortical development, we compared characteristics of cryopreserved cortical neurons in cultures at various DIV using immunofluorescence, biochemical measurements and multielectrode array recordings. Compared to 21 and 35 DIV, at 14 DIV, cultures are still undergoing developmental changes and are not representative of adult in vivo brain tissue. Specifically, we noted significant lack in immunoreactivity for synaptic markers such as synapsin, vesicular GABA transporter and vesicular glutamate transporter at 14 DIV, relative to 21 and 35 DIV. Moreover, multielectrode array analysis indicated an increase in network firing up to 46 DIV with patterned firing peaking at 35 DIV. Our results provide specific evidence of the maturational stages of neurons in culture that can be used to more successfully plan various types of in vitro experimentation.

The corticostriatal system in dissociated cell culture

The sparse connectivity within the striatum in vivo makes the investigation of individual corticostriatal synapses very difficult. Most studies of the corticostriatal input have been done using electrical stimulation under conditions where it is hard to identify the precise origin of the cortical input. We have employed an in vitro dissociated cell culture system that allows the identification of individual corticostriatal pairs and have been developing methods to study individual neuron inputs to striatal neurons. In mixed corticostriatal cultures, neurons had resting activity similar to the system in vivo. Up/down states were obvious and seemed to encompass the entire culture. Mixed cultures of cortical neurons from transgenic mice expressing green fluorescent protein with striatal neurons from wild-type mice of the same developmental stage allowed visual identification of individual candidate corticostriatal pairs. Recordings were performed between 12 and 37 days in vitro (DIV). To investigate synaptic connections we recorded from 69 corticostriatal pairs of which 44 were connected in one direction and 25 reciprocally. Of these connections 41 were corticostriatal (nine inhibitory) and 53 striatocortical (all inhibitory). The observed excitatory responses were of variable amplitude (−10 to −370 pA, n = 32). We found the connections very secure – with negligible failures on repeated stimulation (approximately 1 Hz) of the cortical neuron. Inhibitory corticostriatal responses were also observed (−13 to −314 pA, n = 9). Possibly due to the mixed type of culture we found an inhibitory striatocortical response (−14 to −598 pA, n = 53). We are now recording from neurons in separate compartments to more closely emulate neuroanatomical conditions but still with the possibility of the easier identification of the connectivity.

Histochemistry and cell biology: the annual review 2010

This review summarizes recent advances in histochemistry and cell biology which complement and extend our knowledge regarding various aspects of protein functions, cell and tissue biology, employing appropriate in vivo model systems in conjunction with established and novel approaches. In this context several non-expected results and discoveries were obtained which paved the way of research into new directions. Once the reader embarks on reading this review, it quickly becomes quite obvious that the studies contribute not only to a better understanding of fundamental biological processes but also provide use-oriented aspects that can be derived therefrom.