Rethinking Remapping: Circuit Mechanisms of Recovery after Stroke.

Stroke is one of the most common causes of disability, and there are few treatments that can improve recovery after stroke. Therapeutic development has been hindered because of a lack of understanding of precisely how neural circuits are affected by stroke, and how these circuits change to mediate recovery. Indeed, some of the hypotheses for how the CNS changes to mediate recovery, including remapping, redundancy, and diaschisis, date to more than a century ago. Recent technological advances have enabled the interrogation of neural circuits with ever greater temporal and spatial resolution. These techniques are increasingly being applied across animal models of stroke and to human stroke survivors, and are shedding light on the molecular, structural, and functional changes that neural circuits undergo after stroke. Here we review these studies and highlight important mechanisms that underlie impairment and recovery after stroke. We begin by summarizing knowledge about changes in neural activity that occur in the peri-infarct cortex, specifically considering evidence for the functional remapping hypothesis of recovery. Next, we describe the importance of neural population dynamics, disruptions in these dynamics after stroke, and how allocation of neurons into spared circuits can restore functionality. On a more global scale, we then discuss how effects on long-range pathways, including interhemispheric interactions and corticospinal tract transmission, contribute to post-stroke impairments. Finally, we look forward and consider how a deeper understanding of neural circuit mechanisms of recovery may lead to novel treatments to reduce disability and improve recovery after stroke.

Pharmacological targeting of G protein-coupled receptor heteromers.

A main rationale for the role of G protein-coupled receptor (GPCR) heteromers as targets for drug development is the putative ability of selective ligands for specific GPCRs to change their pharmacological properties upon GPCR heteromerization. The present study provides a proof of concept for this rationale by demonstrating that heteromerization of dopamine D1 and D3 receptors (D1R and D3R) influences the pharmacological properties of three structurally similar selective dopamine D3R ligands, the phenylpiperazine derivatives PG01042, PG01037 and VK4-116. By using D1R-D3R heteromer-disrupting peptides, it could be demonstrated that the three D3R ligands display different D1R-D3R heteromer-dependent pharmacological properties: PG01042, acting as G protein-biased agonist, counteracted D1R-mediated signaling in the D1R-D3R heteromer; PG01037, acting as a D3R antagonist cross-antagonized D1R-mediated signaling in the D1R-D3R heteromer; and VK4-116 specifically acted as a ß-arrestin-biased agonist in the D1R-D3R heteromer. Molecular dynamics simulations predicted potential molecular mechanisms mediating these qualitatively different pharmacological properties of the selective D3R ligands that are dependent on D1R-D3R heteromerization. The results of in vitro experiments were paralleled by qualitatively different pharmacological properties of the D3R ligands in vivo. The results supported the involvement of D1R-D3R heteromers in the locomotor activation by D1R agonists in reserpinized mice and L-DOPA-induced dyskinesia in rats, highlighting the D1R-D3R heteromer as a main pharmacological target for L-DOPA-induced dyskinesia in Parkinson's disease. More generally, the present study implies that when suspecting its pathogenetic role, a GPCR heteromer, and not its individual GPCR units, should be considered as main target for drug development.

Coexistence of D3 R typical and atypical signaling in striatonigral neurons during dopaminergic denervation. Correlation with D3 nf expression changes.

Dopamine D3 R are widely expressed in basal ganglia where interact with D1 R. D3 R potentiate cAMP accumulation and GABA release stimulated by D1 R in striatonigral neurons through "atypical" signaling. During dopaminergic denervation, D3 R signaling changes to a "typical" in which antagonizes the effects of D1 R, the mechanisms of this switching are unknown. D3 nf splice variant regulates membrane anchorage and function of D3 R and decreases in denervation; thus, it is possible that D3 R signaling switching correlates with changes in D3 nf expression and increases of membranal D3 R that mask D3 R atypical effects. We performed experiments in unilaterally 6-hydroxydopamine lesioned rats and found a decrease in mRNA and protein of D3 nf, but not of D3 R in the denervated striatum. Proximity ligation assay showed that D3 R-D3 nf interaction decreased after denervation, whereas binding revealed an increased Bmax in D3 R. The new D3 R antagonized cAMP accumulation and GABA release stimulated by D1 R; however, in the presence of N-Ethylmaleimide (NEM), to block Gi protein signaling, activation of D3 R produced its atypical signaling stimulating D1 R effects. Finally, we investigated if the typical and atypical effects of D3 R modulating GABA release are capable of influencing motor behavior. Injections of D3 R agonist into denervated nigra decreased D1 R agonist-induced turning behavior but potentiated it in the presence of NEM. Our data indicate the coexistence of D3 R typical and atypical signaling in striatonigral neurons during denervation that correlated with changes in the ratio of expression of D3 nf and D3 R isoforms. The coexistence of both atypical and typical signaling during denervation influences motor behavior.

High-Sensitivity Intrinsic Optical Signal Imaging Through Flexible, Low-Cost Adaptations of an Upright Microscope.

Intrinsic optical signal imaging (IOSI) is a staple technique in modern neuroscience. Pioneered >30 years ago, IOSI allows macroscopic mapping of neuronal activity throughout the cortex. The technique has been used to study sensory processing and experience-dependent plasticity, and is often used as an adjunctive procedure to localize cortical areas for subsequent targeting by other imaging or physiology techniques. Despite the ubiquity of IOSI in neuroscience, there are few commercially available turn-key IOSI systems. As a result, investigators have typically resorted to building their own imaging systems. Over the years, simplified systems built either as dedicated rigs or incorporated into existing microscope platforms have been developed. Here we present a straightforward set of adaptations that can be applied to any standard upright microscope, using readily available, inexpensive, commercial parts for illumination, optics, and signal detection, that enables high-sensitivity IOSI. Using these adaptations, we are able to readily map sensory-evoked signals across the somatosensory and visual cortex, including single-whisker barrel cortical activity maps in mice. We show that these IOSI maps are highly reproducible across animals and can be used to study plasticity mechanisms in the somatosensory cortex. We also provide open-source applications to control illumination and analyze raw data to generate activity maps. We anticipate that these resources will be useful for neuroscience investigators looking to add IOSI capabilities to an existing microscope in the laboratory on a budget.

Effects of chronic solvent abuse exposure on umami taste perception.

Inhalant abuse constitutes an important public health problem in Mexico that is more prevalent among children and adolescents. Commercial products that are abused are complex mixtures of solvents containing mainly toluene, in association with other solvents like benzene and xylene. Epidemiological evidence indicates that chronic solvent abuse exposure can cause loss of appetite among other unwanted effects. The mechanisms by which loss of appetite is produced are unknown. It is a matter of interest to determine if loss of appetite is related to changes in taste perception. One of the basic flavors detected by the taste system is umami taste (monosodium glutamate) and it has been proposed that glutamatergic receptors can play an important role in umami taste transduction and perception. It is unknown however, if chronic solvent abuse exposure can induce alterations in umami taste perception. The purpose of this work was to determine if chronic solvent exposure in rats causes alterations in glutamate solution consumption. Rats were exposed to solvents (6000 ppm) in a static chamber for 2 months, as follows: a toluene group, a benzene group, a xylene group and a control group. During the treatment, glutamate solution (120 mM) consumption, food intake and rat weights were measured. The results show that glutamate solution intake was increased in rats chronically exposed to solvents, with differences in consumption patterns between solvents. In addition, chronically exposed animals had a lower weight increase compared with unexposed rats. These data suggest that solvent inhalation originates feeding behavior alteration in rats.