The effect of acute and chronic nicotine consumption on intra-cortical inhibition and facilitation: A transcranial magnetic stimulation study

Abstract

ObjectiveThe aim of the present study was to explore the impact of acute and chronic nicotine consumption on measures of intracortical inhibition and facilitation. MethodsThis study involved 50 chronic heavy cigarette smokers and 40 healthy subjects matched for age, sex and educational level, with no history of chronic nicotine intake. Intracortical inhibition and facilitation were assessed using transcranial magnetic stimulation (TMS) measures of motor threshold (MT), short- and long-interval intra-cortical inhibition (SICI, LICI), cortical silent period (CSP) and intra-cortical facilitation (ICF). Basal serum levels of cotinine were measured in the healthy group and at ½ and 2 h after smoking a single cigarette in the chronic smokers. ResultsThere was enhanced SICI and reduced ICF in smokers (independent of time after smoking) compared with non-smokers. The former suggests a chronic effect of increased nicotine levels on GABA-A neurotransmission whereas the latter suggests an additional effect on glutamatergic transmission. There were no significant differences between smokers and non-smokers in other TMS parameters. There was a significant negative correlation between cotinine levels at ½ h after smoking and SICI at 3 ms ISI (P < 0.001). There were no significant differences in any of the neurophysiological measures between smokers at ½ h versus 2 h after smoking a single cigarette. ConclusionChronic nicotine consumption enhances SICI, and reduces ICF, supporting the hypothesis that nicotine acts as a neuromodulator of GABA-A and glutamate neurotransmission.