A Transcranial Magnetic Stimulation Study of the Effects of Cannabis Use on Motor Cortical Inhibition and Excitability

Abstract

Active compounds in cannabis such as tetrahydrocannabinol (THC) interact with the inhibitory neurotransmitter d-aminobutyric acid (GABA) but little is known about the functional effects of cannabis on human cortical brain processes. Therefore, the aim of the study was to investigate whether patients with chronic cannabis use demonstrate abnormalities in cortical inhibition or excitability. In all, 42 chronic cannabis using subjects (divided into heavy and light using subjects) and 19 controls were included in the study. Single and paired pulse transcranial magnetic stimulation were used to assess a number of parameters of cortical inhibition and cortical excitability. In addition, psychomotor function and THC plasma levels were measured. Both cannabis using groups (heavy and light use) demonstrated a reduction in short interval cortical inhibition compared with healthy controls, but there was no difference in other measures of cortical inhibition or cortical excitability. There was also no difference between the two groups on measures of psychomotor performance. Chronic cannabis use is associated with a reduction in cortical inhibition potentially related to activity at the GABA A receptors. Further research is required to explore whether this results from chronic cannabis use or reflects an underlying predisposition to developing chronic substance use problems.

Inhibition of the cortex using transcranial magnetic stimulation in psychiatric populations: current and future directions

“…Comorbid cannabis abuse was suggested to potentiate the reduced short interval cortical inhibition and enhanced intracortical facilitation observed in patients with first-episode schizophrenia. This finding is consistent with a those of a previous study by Fitzgerald and colleagues 55 reporting that heavy and light users of cannabis demonstrated significantly decreased short interval cortical inhibition compared with healthy controls. Taken together, these studies provide evidence that schizophrenia is associated with cortical inhibition deficits in the motor cortex.…”

Exogenous and Endogenous Cannabinoids Suppress Inhibitory Neurotransmission in the Human Neocortex

“…SICI was attenuated in cannabis users, and one of the interpretations was that GABA A receptor-mediated synaptic transmission was inhibited by cannabinoids in the cortex (Fitzgerald et al, 2009). Inhibition of GABA A receptor-mediated synaptic transmission by a cannabinoid in the human cortex in vitro, as demonstrated in our study at the monosynaptic level, may be the basis of the effect observed in vivo by Fitzgerald et al (2009).…”

Disrupted Gamma-Band Neural Oscillations During Coherent Motion Perception in Heavy Cannabis Users

“…Such decreases in CB1Rs could lead to alterations in GABA release, thus disrupting the ability of neural networks to generate synchronized neural oscillations. Indeed, a transcranial magnetic stimulation (TMS) study by Fitzgerald et al (2009) found that both heavy and light users of cannabis exhibit disruptions in short interval cortical inhibition that is indicative of altered GABA function. A final potential mechanism relates to the effect of cannabis on neurodevelopment.…”