Dopamine D1 receptor-expressing neurons activity is essential for locomotor and sensitizing effects of a single injection of cocaine

Dopamine D1 receptors play an important role in the effects of cocaine. Here, we investigated the role of neurons which express these receptors (D1-neurons) in the acute locomotor effects of cocaine and the locomotor sensitization observed after a second injection of this drug, using the previously established two-injection protocol of sensitization. We inhibited D1-neurons using double transgenic mice conditionally expressing the inhibitory Gi-coupled designer receptor exclusively activated by designer drugs (Gi-DREADD) in D1-neurons. Chemogenetic inhibition of D1-neurons by a low dose of clozapine (0.1 mg/kg) decreased the cocaine-induced expression of Fos in striatal neurons. It diminished the basal locomotor activity and acute hyper-locomotion induced by cocaine (20 mg/kg). Clozapine 0.1 mg/kg had no effect by itself and did not alter cocaine effects in wild-type mice. Inhibition of D1-neurons during the first cocaine administration prevented the sensitization of the locomotor response in response to a second cocaine administration 10 days later. On Day 11, inhibition of D1-neurons by clozapine stimulation of Gi-DREADD blocked cocaine-induced locomotion including in sensitized mice, whereas on Day 12, in the absence of clozapine and D1-neurons inhibition, all mice displayed a sensitized response to cocaine. These results show that chemogenetic inhibition of D1-neurons decreases spontaneous and cocaine-induced locomotor activity. It prevents sensitization induction and blocks sensitized locomotion in a two-injection protocol of sensitization but does not reverse established sensitization. Our study further supports the central role of D1-neurons in mediating the acute locomotor effects of cocaine and its sensitization.

Long-lasting tagging of neurons activated by seizures or cocaine administration in Egr1-CreERT2 transgenic mice

Permanent tagging of neuronal ensembles activated in specific experimental situations is an important objective to study their properties and adaptations. In the context of learning and memory, these neurons are referred to as engram neurons. Here, we describe and characterize a novel mouse line, Egr1-CreER^T2 , which carries a transgene in which the promoter of the immediate early gene Egr1 drives the expression of the CreER^T2 recombinase that is only active in the presence of tamoxifen metabolite, 4-hydroxy-tamoxifen (4-OHT). Egr1-CreER^T2 mice were crossed with various reporter mice, Cre-dependently expressing a fluorescent protein. Without tamoxifen or 4-OHT, no or few tagged neurons were observed. Epileptic seizures induced by pilocarpine or pentylenetetrazol in the presence of tamoxifen or 4-OHT elicited the persistent tagging of many neurons and some astrocytes in the dentate gyrus of hippocampus, where Egr1 is transiently induced by seizures. One week after cocaine and 4-OHT administration, these mice displayed a higher number of tagged neurons in the dorsal striatum than saline/4-OHT controls, with differences between reporter lines. Cocaine-induced tagging required ERK activation and tagged neurons were more likely than others to exhibit ERK phosphorylation or Fos induction after a second injection. Interestingly neurons tagged in saline-treated mice also had an increased propensity to express Fos, suggesting the existence of highly responsive striatal neurons susceptible to be re-activated by cocaine repeated administration, which may contribute to the behavioral adaptations. Our report validates a novel transgenic mouse model for permanently tagging activated neurons and studying long-term alterations of Egr1-expressing cells.

Astrocytic BDNF and TrkB regulate severity and neuronal activity in mouse models of temporal lobe epilepsy

Astrocytes have emerged as crucial regulators of neuronal network activity, synapse formation, and underlying behavioral and cognitive processes. Despite some pathways have been identified, the communication between astrocytes and neurons remains to be completely elucidated. Unraveling this communication is crucial to design potential treatments for neurological disorders like temporal lobe epilepsy (TLE). The BDNF and TrkB molecules have emerged as very promising therapeutic targets. However, their modulation can be accompanied by several off-target effects such as excitotoxicity in case of uncontrolled upregulation or dementia, amnesia, and other memory disorders in case of downregulation. Here, we show that BDNF and TrkB from astrocytes modulate neuronal dysfunction in TLE models. First, conditional overexpression of BDNF from astrocytes worsened the phenotype in the lithium-pilocarpine mouse model. Our evidences pointed out to the astrocytic pro-BDNF isoform as a major player of this altered phenotype. Conversely, specific genetic deletion of BDNF in astrocytes prevented the increase in the number of firing neurons and the global firing rate in an in vitro model of TLE. Regarding to the TrkB, we generated mice with a genetic deletion of TrkB specifically in hippocampal neurons or astrocytes. Interestingly, both lines displayed neuroprotection in the lithium-pilocarpine model but only the mice with genetic deletion of TrkB in astrocytes showed significantly preserved spatial learning skills. These data identify the astrocytic BDNF and TrkB molecules as promising therapeutic targets for the treatment of TLE.

Differential enhancement of ERK, PKA and Ca2+ signaling in direct and indirect striatal neurons of Parkinsonian mice

Parkinson's disease (PD) is characterized by severe locomotor deficits due to the disappearance of dopamine (DA) from the dorsal striatum. The development of PD symptoms and treatment-related complications such as dyskinesia have been proposed to result from complex alterations in intracellular signaling in both direct and indirect pathway striatal projection neurons (dSPNs and iSPNs, respectively) following loss of DA afferents. To identify cell-specific and dynamical modifications of signaling pathways associated with PD, we used a hemiparkinsonian mouse model with 6-hydroxydopamine (6-OHDA) lesion combined with two-photon fluorescence biosensors imaging in adult corticostriatal slices. After DA lesion, extracellular signal-regulated kinase (ERK) activation was increased in response to DA D1 receptor (D1R) or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) stimulation. The cAMP-dependent protein kinase (PKA) pathway contributing to ERK activation displayed supersensitive responses to D1R stimulation after 6-OHDA lesion. This cAMP/PKA supersensitivity was specific of D1R-responding SPNs and resulted from Gα_olf upregulation and deficient phosphodiesterase activity. In lesioned striatum, the number of D1R-SPNs with spontaneous Ca²⁺ transients augmented while Ca²⁺ response to AMPA receptor stimulation specifically increased in iSPNs. Our work reveals distinct cell type-specific signaling alterations in the striatum after DA denervation. It suggests that over-activation of ERK pathway, observed in PD striatum, known to contribute to dyskinesia, may be linked to the combined dysregulation of DA and glutamate signaling pathways in the two populations of SPNs. These findings bring new insights into the implication of these respective neuronal populations in PD motor symptoms and the occurrence of PD treatment complications.

Cocaine conditioned place preference: unexpected suppression of preference due to testing combined with strong conditioning

Conditioned place preference (CPP) is widely used for evaluating the rewarding effects of drugs. Like other memories, CPP is proposed to undergo reconsolidation during which it is unstable and sensitive to pharmacological inhibition. Previous studies have shown that cocaine CPP can be apparently erased by extracellular signal-regulated kinase (ERK) pathway inhibition during cocaine reconditioning (re-exposure to the drug-paired environment in the presence of the drug). Here, we show that blockade of D1 receptors during reconditioning prevented ERK activation and induced a loss of CPP. However, we also unexpectedly observed a CPP disappearance in mice that underwent testing and reconditioning with cocaine alone, specifically in strong conditioning conditions. The loss was due to the intermediate test. CPP was not recovered with reconditioning or priming in the short term, but it spontaneously reappeared after a month. When we challenged the D1 antagonist-mediated erasure, we observed that both a high dose of cocaine and a first CPP test were required for this effect. Our results also suggest a balance between D1-dependent ERK pathway activation and an A2a-dependent mechanism in D2 striatal neurons in controlling CPP expression. Our data reveal that, paradoxically, a simple CPP test can induce a complete (but transient) loss of place preference following strong but not weak cocaine conditioning. This study emphasizes the complex nature of CPP memory and the importance of multiple parameters that must be taken into consideration when investigating reconsolidation.

Anatomical and molecular characterization of dopamine D1 receptor-expressing neurons of the mouse CA1 dorsal hippocampus

In the hippocampus, a functional role of dopamine D1 receptors (D1R) in synaptic plasticity and memory processes has been suggested by electrophysiological and pharmacological studies. However, comprehension of their function remains elusive due to the lack of knowledge on the precise localization of D1R expression among the diversity of interneuron populations. Using BAC transgenic mice expressing enhanced green fluorescent protein under the control of D1R promoter, we examined the molecular identity of D1R-containing neurons within the CA1 subfield of the dorsal hippocampus. In agreement with previous findings, our analysis revealed that these neurons are essentially GABAergic interneurons, which express several neurochemical markers, including calcium-binding proteins, neuropeptides, and receptors among others. Finally, by using different tools comprising cell type-specific isolation of mRNAs bound to tagged-ribosomes, we provide solid data indicating that D1R is present in a large proportion of interneurons expressing dopamine D2 receptors. Altogether, our study indicates that D1Rs are expressed by different classes of interneurons in all layers examined and not by pyramidal cells, suggesting that CA1 D1R mostly acts via modulation of GABAergic interneurons.

Haloperidol-induced Nur77 expression in striatopallidal neurons is under the control of protein phosphatase 1 regulation by DARPP-32

Impaired dopaminergic signaling in the striatum is involved in diseases as diverse as Parkinson's disease, addiction, and schizophrenia. An important pathophysiological aspect is the loss of balance between striatopallidal and striatonigral pathways. Nur77 is an orphan nuclear receptor and dopamine-regulated immediate-early gene. Classical antipsychotic drugs widely used in the treatment of schizophrenia, such as haloperidol, increase Nur77 mRNA expression in the striatum. However, little is known about the intracellular signaling pathways involved in Nur77 induction. Here, using pharmacological approaches and transgenic mutant mice, we investigated the mechanisms underlying the up-regulation of Nur77 protein expression in the dorsal striatum after haloperidol injection. In drd1a::EGFP transgenic mice that express GFP in D1 neurons, Nur77 up-regulation induced by haloperidol occurred predominantly in GFP-negative neurons. In Gαolf heterozygous mutant mice, in which cAMP production in response to A2A stimulation is impaired in the striatum, haloperidol effect was not altered. In contrast, in DARPP-32 knock-in mutant mice bearing a T34A point mutation of the site responsible for cAMP-dependent phosphatase 1 inhibition, Nur77 up-regulation by haloperidol was prevented. Haloperidol also induced Nur77 protein in D2 neurons of the nucleus accumbens core of wild type but not T34A knock-in mice. Thus, our results show that expression of Nur77 is induced by haloperidol in D2 receptors-expressing medium-sized spiny neurons, through cAMP-dependent regulation of protein phosphatase 1, which is likely to modulate the effects of other protein kinases. Our results clarify the mechanisms of Nur77 induction by antipsychotic and its possible contribution to extrapyramidal effects.