Optogenetic brain‐stimulation reward: A new procedure to re‐evaluate the rewarding versus aversive effects of cannabinoids in dopamine transporter‐Cre mice

Most cannabinoids reduce rather than enhance reward-seeking behavior in mice, according to an optogenetic self-stimulation procedure. Cocaine dose-dependently increased responding, shifting stimulation–response curves upward. In contrast, Δ9-tetrahydrocannabinol (THC), WIN55,212-2, and ACEA dose-dependently decreased responding and shifted curves downward; cannabidiol had no effect. Among newer synthetic cannabinoids, XLR-11 produced a cocaine-like increase, AM-2201 produced a THC-like reduction, and 5F-AMB had no effect. CB1 receptors were expressed mainly in VTA GABA and glutamate neurons, while CB2 receptors were expressed mainly in VTA dopamine neurons. These findings suggest most cannabinoids are reward attenuating or aversive in mice.

Study at a glance

Abstract

Abstract Despite extensive research, the rewarding effects of cannabinoids are still debated. Here, we used a newly established animal procedure called optogenetic intracranial self‐stimulation (ICSS) (oICSS) to re‐examine the abuse potential of cannabinoids in mice. A specific adeno‐associated viral vector carrying a channelrhodopsin gene was microinjected into the ventral tegmental area (VTA) to express light‐sensitive channelrhodopsin in dopamine (DA) neurons of transgenic dopamine transporter (DAT)‐Cre mice. Optogenetic stimulation of VTA DA neurons was highly reinforcing and produced a classical “sigmoidal”‐shaped stimulation–response curve dependent upon the laser pulse frequency. Systemic administration of cocaine dose‐dependently enhanced oICSS and shifted stimulation–response curves upward, in a way similar to previously observed effects of cocaine on electrical ICSS. In contrast, Δ 9 ‐tetrahydrocannabinol (Δ 9 ‐THC), but not cannabidiol, dose‐dependently decreased oICSS responding and shifted oICSS curves downward. WIN55,212‐2 and ACEA, two synthetic cannabinoids often used in laboratory settings, also produced dose‐dependent reductions in oICSS. We then examined several new synthetic cannabinoids, which are used recreationally. XLR‐11 produced a cocaine‐like increase, AM‐2201 produced a Δ 9 ‐THC‐like reduction, while 5F‐AMB had no effect on oICSS responding. Immunohistochemistry and RNAscope in situ hybridization assays indicated that CB 1 Rs are expressed mainly in VTA GABA and glutamate neurons, while CB 2 Rs are expressed mainly in VTA DA neurons. Together, these findings suggest that most cannabinoids are not reward enhancing, but rather reward attenuating or aversive in mice. Activation of CB 1 R and/or CB 2 R in different populations of neurons in the brain may underlie the observed actions.