Addressing sufficiency of the CB1 receptor for endocannabinoid-mediated functions through conditional genetic rescue in forebrain GABAergic neurons

Floortje Remmers,Beat Lutz,Hans-Christian Pape,Martina Hamann,Sabine Ruehle,Maren D Lange

doi:10.1007/s00429-017-1411-5

Abstract

Genetic inactivation of the cannabinoid CB1 receptor gene in different cell types in the brain has previously revealed necessary functions for distinct synaptic plasticity processes and behaviors. Here, we sought to identify CB1 receptor expression sites that are minimally required to reconstruct normal phenotypes. In a CB1-null background, we re-expressed endogenous CB1 receptors in forebrain GABAergic neurons, thereby assessing the sufficiency of CB1 receptors. Depolarization-induced suppression of inhibitory, but not excitatory, transmission was restored in hippocampal and amygdalar circuits. GABAergic CB1 receptors did not convey protection against chemically induced seizures, but prevented the spontaneous mortality observed in CB1 null mutants. Rescue of GABAergic CB1 receptors largely restored normal anxiety-like behavior but improved extinction of learned fear only marginally. This study illustrates that the approach of genetic reconstruction of complex behaviors is feasible. It also revealed distinct degrees of modulation for different emotional behaviors by the GABAergic population of CB1 receptors.

Highlights

In the brain, the cannabinoid type 1 (CB1) receptor is the major conveyor of cannabinoid signaling (Kano et al 2009; Katona and Freund 2012)
In the elevated plus-maze (EPM) and LD tests, we found a large degree of sufficiency of the CB1 receptor in forebrain GABAergic neurons for appropriate anxiety-like behavior; in GABA-CB1 receptor was rescued either globally (CB1RS) mice, the increase in anxiety-like behavior observed in Stop-CB1 mice was reversed
We used a Cre-mediated cell-type-specific CB1 receptor rescue strategy in a CB1-null background to attempt to restore normal behaviors by rescuing the receptor to its endogenous levels solely in forebrain GABAergic neurons. This subpopulation of the CB1 receptor was found to convey no substantial protection against kainic acid-induced epileptiform seizures, but abolished the spontaneous seizures observed in mice lacking the CB1 receptor

Summary

Introduction

The cannabinoid type 1 (CB1) receptor is the major conveyor of cannabinoid signaling (Kano et al 2009; Katona and Freund 2012). Conditional knockout mice for the CB1 receptor have proven useful to assess the necessity of the receptor present in certain neuronal subpopulations (e.g., Monory et al 2006; MetnaLaurent et al 2012; Dubreucq et al 2012; Rey et al 2012; Llorente-Berzal et al 2015). With this strategy, a conditional knockout of a subpopulation of the receptor that is necessary results in a “breakdown” of the normal behavior to the phenotype seen in CB1 null-mutants, whereas sufficiency of subpopulations can be assessed by investigating which subpopulations are required to “reconstruct” the normal behavior. The reported sufficiency of the dorsal telencephalic glutamatergic CB1 receptor population

Methods

Results

Discussion

Conclusion