Characterization of HTT inclusion size, location, and timing in the zQ175 mouse model of Huntington's disease: an in vivo high-content imaging study.

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the huntingtin gene. Major pathological hallmarks of HD include inclusions of mutant huntingtin (mHTT) protein, loss of neurons predominantly in the caudate nucleus, and atrophy of multiple brain regions. However, the early sequence of histological events that manifest in region- and cell-specific manner has not been well characterized. Here we use a high-content histological approach to precisely monitor changes in HTT expression and characterize deposition dynamics of mHTT protein inclusion bodies in the recently characterized zQ175 knock-in mouse line. We carried out an automated multi-parameter quantitative analysis of individual cortical and striatal cells in tissue slices from mice aged 2-12 months and confirmed biochemical reports of an age-associated increase in mHTT inclusions in this model. We also found distinct regional and subregional dynamics for inclusion number, size and distribution with subcellular resolution. We used viral-mediated suppression of total HTT in the striatum of zQ175 mice as an example of a therapeutically-relevant but heterogeneously transducing strategy to demonstrate successful application of this platform to quantitatively assess target engagement and outcome on a cellular basis.

Antinociceptive effect of peripheral serotonin 5-HT2B receptor activation on neuropathic pain.

Serotonin is critically involved in neuropathic pain. However, its role is far from being understood owing to the number of cellular targets and receptor subtypes involved. In a rat model of neuropathic pain evoked by chronic constriction injury (CCI) of the sciatic nerve, we studied the role of 5-HT(2B) receptor in dorsal root ganglia (DRG) and the sciatic nerve. We showed that 5-HT(2B) receptor activation both prevents and reduces CCI-induced allodynia. Intrathecal administration of 5-HT(2B) receptor agonist BW723C86 significantly attenuated established mechanical and cold allodynia; this effect was prevented by co-injection of RS127445, a selective 5-HT(2B) receptor antagonist. A single application of BW723C86 on the sciatic nerve concomitantly to CCI dose-dependently prevented mechanical allodynia and significantly reduced cold allodynia 17 days after CCI. This behavioral effect was accompanied with a marked decrease in macrophage infiltration into the sciatic nerve and, in the DRG, with an attenuated abnormal expression of several markers associated with local neuroinflammation and neuropathic pain. CCI resulted in a marked upregulation of 5-HT(2B) receptor expression in sciatic nerve and DRG. In the latter structure, it was biphasic, consisting of a transient early increase (23-fold), 2 days after the surgery and before the neuropathic pain emergence, followed by a steady (5-fold) increase, that remained constant until pain disappeared. In DRG and sciatic nerve, 5-HT(2B) receptors were immunolocalized on sensory neurons and infiltrating macrophages. Our data reveal a relationship between serotonin, immunocytes, and neuropathic pain development, and demonstrate a critical role of 5-HT(2B) receptors in blood-derived macrophages.

High-novelty-preference rats are predisposed to compulsive cocaine self-administration.

Sensation/novelty-seeking is amongst the best markers of cocaine addiction in humans. However, its implication in the vulnerability to cocaine addiction is still a matter of debate, as it is unclear whether this trait precedes or follows the development of addiction. Sensation/novelty-seeking trait has been identified in rats on the basis of either novelty-induced locomotor activity (high-responder (HR) trait) or novelty-induced place preference (high-novelty-preference trait (HNP)). HR and HNP traits have been associated with differential sensitivity to psychostimulants. However, it has recently been demonstrated that HR rats do not develop compulsive cocaine self-administration (SA) after protracted exposure to the drug, thereby suggesting that at least one dimension of sensation/novelty seeking in the rat is dissociable from the vulnerability to switch from controlled to compulsive cocaine SA. We therefore investigated whether HNP, as measured as the propensity to choose a new environment in a free choice procedure, as opposed to novelty-induced locomotor activity, predicts the vulnerability to, and the severity of, addiction-like behavior for cocaine. For this, we identified HR/LR rats and HNP/LNP rats before any exposure to cocaine. After 60 days of cocaine SA, each rat was given an addiction score based on three addiction-like behaviors (persistence of responding when the drug is signaled as not available, high breakpoint under progressive ratio schedule and resistance to punishment) that resemble the clinical features of drug addiction, namely inability to refrain from drug seeking, high motivation for the drug and compulsive drug use despite adverse consequences. We show that, as opposed to HR rats, HNP rats represent a sub-population predisposed to compulsive cocaine intake, displaying higher addiction scores than LNP rats. This study thereby provides new insights into the factors predisposing to cocaine addiction, supporting the hypothesis that addiction is sustained by two vulnerable phenotypes: a 'drug use prone' phenotype such as HR which brings an individual to develop drug use and an 'addiction prone' phenotype, such as HNP, which facilitates the shift from sustained to compulsive drug intake and addiction.

Transition to addiction is associated with a persistent impairment in synaptic plasticity.

Chronic exposure to drugs of abuse induces countless modifications in brain physiology. However, the neurobiological adaptations specifically associated with the transition to addiction are unknown. Cocaine self-administration rapidly suppresses long-term depression (LTD), an important form of synaptic plasticity in the nucleus accumbens. Using a rat model of addiction, we found that animals that progressively develop the behavioral hallmarks of addiction have permanently impaired LTD, whereas LTD is progressively recovered in nonaddicted rats maintaining a controlled drug intake. By making drug seeking consistently resistant to modulation by environmental contingencies and consequently more and more inflexible, a persistently impaired LTD could mediate the transition to addiction.