1. ¹Alfred Psychiatry Research Centre, The Alfred and Monash University School of Psychology, Psychiatry and Psychological Medicine, Commercial Rd Melbourne, Victoria, Australia
2. ²Monash University Department of Epidemiology and Preventive Medicine, The Alfred Hospital, Commercial Rd Melbourne, Victoria, Australia
3. ³Centre for Addiction and Mental Health, University of Toronto, Toronto, Ontario, Canada

Correspondence: Professor PB Fitzgerald, Alfred Psychiatry Research Centre, First Floor, Old Baker Building, The Alfred, Commercial Rd Melbourne, Victoria 3004, Australia, Tel: +61 3 9076 6552, Fax: +61 3 9076 6588, E-mail: paul.fitzgerald@med.monash.edu.au

Received 26 February 2009; Revised 14 May 2009; Accepted 22 May 2009; Published online 1 July 2009.

Abstract

Active compounds in cannabis such as tetrahydrocannabinol (THC) interact with the inhibitory neurotransmitter -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.