Inhibition of Protease-Activated Receptor 1 Does not Affect Dendritic Homeostasis of Cultured Mouse Dentate Granule Cells.

Protease-activated receptors (PARs) are widely expressed in the central nervous system (CNS). While a firm link between PAR1-activation and functional synaptic and intrinsic neuronal properties exists, studies on the role of PAR1 in neural structural plasticity are scarce. The physiological function of PAR1 in the brain remains not well understood. We here sought to determine whether prolonged pharmacologic PAR1-inhibition affects dendritic morphologies of hippocampal neurons. To address this question we employed live-cell microscopy of mouse dentate granule cell dendrites in 3-week old entorhino-hippocampal slice cultures prepared from Thy1-GFP mice. A subset of cultures were treated with the PAR1-inhibitor SCH79797 (1 µM; up to 3 weeks). No major effects of PAR1-inhibition on static and dynamic parameters of dentate granule cell dendrites were detected under control conditions. Granule cells of PAR1-deficient slice cultures showed unaltered dendritic morphologies, dendritic spine densities and excitatory synaptic strength. Furthermore, we report that PAR1-inhibition does not prevent dendritic retraction following partial deafferentation in vitro. Consistent with this finding, no major changes in PAR1-mRNA levels were detected in the denervated dentate gyrus (DG). We conclude that neural PAR1 is not involved in regulating the steady-state dynamics or deafferentation-induced adaptive changes of cultured dentate granule cell dendrites. These results indicate that drugs targeting neural PAR1-signals may not affect the stability and structural integrity of neuronal networks in healthy brain regions.

NMDA-receptor inhibition restores Protease-Activated Receptor 1 (PAR1) mediated alterations in homeostatic synaptic plasticity of denervated mouse dentate granule cells.

A common feature of neurological diseases is the loss of central neurons, which leads to deafferentation of connected brain regions. In turn, the remodeling of denervated neuronal networks is considered to play an important role for the postlesional recovery, but has also been linked to maladaptive plasticity resulting in disease-related complications such as memory dysfunction or epilepsy. Recent work has indicated that Protease-Activated Receptor 1 (PAR1), which can be activated by thrombin that enters the brain under pathological conditions, alters synaptic plasticity and neuronal excitability. However, the role of PAR1 in lesion-induced synaptic plasticity remains incompletely understood. Here, we used entorhinal denervation of organotypic hippocampal slice cultures to study the effects of PAR1 on denervation-induced homeostatic synaptic plasticity. Our results disclose that PAR1 activation counters the ability of denervated dentate granule cells to increase their excitatory synaptic strength in a compensatory, i.e., homeostatic manner. Furthermore, we demonstrate that this PAR1 effect is rescued by pharmacological inhibition of N-methyl-d-aspartate receptors (NMDA-R). Thus, NMDA-R inhibitors may restore the ability of denervated neurons to express homeostatic synaptic plasticity under conditions of increased PAR1-activity, which may contribute to their beneficial effects seen in the context of neurological diseases.