Species and strain differences in the expression of a novel glutamate‐modulating cannabinoid receptor in the rodent hippocampus

Abstract

A novel, non‐CB1 cannabinoid receptor has been defined by the persistence of inhibition of glutamatergic EPSPs by the cannabinoid receptor agonist WIN55,212‐2 in mice lacking the cloned CB1 receptor (CB1^–/–) ( Hajos et al., 2001 ). This novel receptor was also distinguished from CB1 by its sensitivity to the antagonist SR141716A and its insensitivity to the antagonist AM251 ( Hajos & Freund, 2002 ). We have chosen to refer to this putative receptor as CBsc due to its identification on Schaffer collateral axon terminals in the hippocampus. We examined properties of CBsc receptors in Sprague Dawley (SD) rats and two strains of wild‐type (WT) mice (C57BL/6J and CD1) used as backgrounds for two independent lines of CB1^–/– mice ( Ledent et al., 1999 ; Zimmer et al., 1999 ). The inhibition of synaptic glutamate release by WIN55,212‐2 was observed in hippocampal slices from WT CD1 mice and SD rats but was absent in WT C57 mice. We also found that AM251 and SR141716A antagonized the effect of WIN55,212‐2 in hippocampal slices from CD1 mice and SD rats demonstrating a lack of selectivity of these ligands for CB1 and CBsc receptors in these animals. The results indicate that the glutamate‐modulating CBsc cannabinoid receptor is present in the hippocampi of CD1 mice and SD rats but not in C57BL/6J mice. Thus, we have identified animal models that may permit the study of cannabinoids independently of the novel CBsc receptor (C57^CB1+/+), the CBsc receptor independently of the cloned CB1 receptor (CD1^CB1–/–), or in the absence of both receptors (C57^CB1–/–).