p75 neurotrophin receptor negatively regulates persistent firing of pyramidal neurons in the entorhinal cortex

JULIEN GIBON; SHANNON BUCKLEY; NICOLAS UNSAIN; VESA KAARTINEN; PHILIPPE SEGUELA; PHILIP A. BARKER

Newport, RI

Congreso; Gordon Reseach Conference, Neurotrophic Factors; 2013

Salve Regina University

The entorhinal cortex (EC) is intimately connected with the hippocampus and plays a crucial role in memory. Persistent activity of EC pyramidal neurons, regulated by cholinergic inputs from basal forebrain through activation of muscarinic receptors, has emerged as a key element in working memory. We are examining the role of neurotrophins and their receptors in initiating and modulating persistent activity within the EC. Here, we report on the role of p75NTR and proneurotrophins in this function. Electrophysiological recordings were performed on layer V pyramidal neurons in acute EC slices lacking their normal cholinergic inputs. In this preparation, persistent firing is induced by addition of carbachol, a cholinergic agonist. We observed that the concentration of carbachol required to elicit persistent activity was significantly less in slices from p75NTR-/- animals. To determine if this effect reflected a loss of neuronal or glial p75NTR, we used a Talpha1:cre driver line to selectively ablate p75NTR in neurons of p75NTRfl/fl animals. To further determine if this effect reflected a developmental defect in p75NTR null mice, we generated p75NTRfl/fl:CMV-ERcre animals and then used tamoxifen to induce p75NTR deletion in adult animals (> 75 days old). In both cases, deletion of p75NTR reduced the threshold for persistent activity in the EC, indicating that p75NTR expressed within adult neurons normally regulates this property. We then asked if p75NTR ligands or a p75NTR-blocking antibody had an effect on persistent activity in EC neurons. Bathing EC slices with a p75NTR-blocking antibody reduced the threshold for persistent activity whereas application of the p75NTR ligand proBDNF rapidly and reversibly blocked persistent firing induced by carbachol. We have previously shown that PIP2 plays a major role in the activation of persistent activity. Therefore, we investigated whether regulation of phosphoinositide levels plays a role in the inhibition of persistent activity induced by proBDNF. Using wortmannin to suppress phospholipid kinase activity, we found that PIP2 depletion abolished the inhibitory effect of proBDNF on persistent activity. This indicates that p75NTR-dependent regulation of PIP2 levels underlies the electrophysiological effects of proBDNF reported here. Together, these results showed that the p75NTR controls neuronal excitability, acting as a negative regulator of persistent firing in pyramidal neurons of EC.