CB1 Receptor Activation on VgluT2-Expressing Glutamatergic Neurons Underlies Δ9-Tetrahydrocannabinol (Δ9-THC)-Induced Aversive Effects in Mice

Abstract

Cannabis can be rewarding or aversive. Cannabis reward is believed to be mediated by activation of cannabinoid CB1 receptors (CB1Rs) on GABAergic neurons that disinhibit dopaminergic neurons in the ventral tegmental area (VTA). However, little is known about the mechanisms underlying cannabis aversion in rodents. In the present study, CB1Rs are found not only on VTA GABAergic neurons, but also on VTA glutamatergic neurons that express vesicular glutamate transporter 2 (VgluT2). We then used Cre-Loxp transgenic technology to selectively delete CB1Rs in VgluT2-expressing glutamatergic neurons (VgluT2-CB1 −/−) and Cre-dependent viral vector to express light-sensitive channelrhodopsin-2 into VTA glutamatergic neurons. We found that photoactivation of VTA glutamatergic neurons produced robust intracranial self-stimulation (ICSS) behavior, which was dose-dependently blocked by DA receptor antagonists, but enhanced by cocaine. In contrast, Δ9-tetrahydrocannabinol (Δ9-THC), the major psychoactive component of cannabis, produced dose-dependent conditioned place aversion and a reduction in the above optical ICSS in VgluT2-cre control mice, but not in VgluT2-CB1 −/− mice. These findings suggest that activation of CB1Rs in VgluT2-expressing glutamate neurons produces aversive effects that might explain why cannabinoid is not rewarding in rodents and might also account for individual differences in the hedonic effects of cannabis in humans.

β‐Caryophyllene, a dietary terpenoid, inhibits nicotine taking and nicotine seeking in rodents

“…For the optical ICSS experiment, DAT‐cre mice were injected with adeno‐associated virus carrying ChR2‐EGFP (AAV5‐EF1α‐DIO‐ChR2‐EGFP) and implanted with bilateral custom‐made optical fibres (200‐μm inner diameter, N.A. 0.22; Doric Lenses, Quebec, Canada) targeted at the VTA (AP − 3.1, ML ± 0.8, DV − 4.25; Han et al, ; Jordan, Humburg, et al, ; Newman et al, ).…”

“…Mice were gently connected to a cable attached to an optical swivel, which was in turn connected to a 473‐nm laser tuned for channelrhodopsin (ChR2) stimulation. Computer software controlled a pulse generator that controlled the lasers (Han et al, ; Jordan, Humburg, et al, ; Newman et al, ).…”

“…Lastly, to determine whether BCP alters nicotine or BSR through a dopamine‐dependent mechanism, we observed the effects of BCP on BSR maintained by optical stimulation of VTA DA neurons in DAT‐cre mice. The procedures for optical intracranial self‐stimulation (oICSS) were the same as reported previously (Han et al, ). DAT‐cre mice ( n = 6), with ChR2 expression targeting the VTA dopaminergic cells, were connected to a 473‐nm wavelength laser (OEM Laser Systems, UT) by two sheathed optic fibres (200‐μm core diameter, Precision Fiber Products, Chula Vista, CA, USA) and FC/FC fibre rotary joint (Doric Lenses Inc., Quebec, Canada).…”

“…After each test, mice received additional oICSS sessions until a new baseline was established and later were re‐tested with a different dose of BCP. After completion of the above behavioural experiment, immunohistochemistry assay was used to verify AAV‐ChR2‐EGFP expression in VTA dopaminergic neurons in DAT‐cre mice using the methods we reported previously (Han et al, ) and followed the BJP Guidelines (Alexander et al, ).…”

Highway to hell or magic smoke? The dose-dependence of Δ9-THC in place conditioning paradigms

“…Δ 9 -THC shows a comparable affinity to both CB1Rs (K i -values ranging from 5.05 to 80.3 nM) and CB2Rs (K i -values ranging from 3.13 to 75.3 nM; Pertwee 2008), and may also bind to GPR55 (Brown 2007). Furthermore, Δ 9 -THC-associated aversive effects were recently linked not only to CB1R (Monory et al 2007), but also to CB2Rs (Han et al 2017).…”