470 2008) and chronic (Wobrock et al. 2009) effects of alcohol. Low doses of alcohol already reduce excitability of the frontal cortex as evidenced by transcranial magnetic stimulation (TMS) and electroencephalography (EEG) (Ziemann et al. 1995; Kähkönen et al. 2001, 2003). This reduction in frontal cortical excitability has been argued to relate to binding of alcohol to gamma-amino butyric acid (GABA)-a receptors in addition to alcohol-related inhibi- tion of N-methyl D-aspartate (NMDA) currents (Kähkönen et al. 2003). However, the precise mechanisms that underlie the behavioral effects of alcohol and possible sex differ- ences remain unclear. One possible mechanism may involve (transient) frontal lobe impairments arguably due to reductions in effective frontal interhemispheric connec- tivity (Kähkönen et al. 2001). Frontal interhemispheric connectivity refers to callosal signal transfer between the left and right frontal cortex (Aboitiz and Montiel 2003). This signal transfer can be measured with paired-pulse TMS wherein the connectivity between the left and right primary motor cortex (M1) is operationalized by applying a conditioning pulse to ipsilateral M1 followed by a test pulse to contralateral M1 10 ms later (Ferbert et al. 1992). Compared to the motor evoked potential (MEP) elicited by the test pulse alone, the conditioning pulse inhibits the MEP of the test pulse by about 50% (Daskalakis et al. 2002). This so-called transcallosal inhibition (TCI) reflects an important physiological mechanism by which the frontal lobes interact and contribute to behavior by providing a noninvasive way to study frontal interhemispheric connec- tivity (Hofman and Schutter 2009). The aim of the present placebo-controlled counterbal- anced cross-over design was to examine sex differences in frontal interhemispheric connectivity in response to alcohol intake as indexed by TCI. We hypothesized that alcohol will reduce TCI in both males and females. In addition, it is expected that reductions in TCI will be more pronounced in females than in males. Materials and methods Participants Twenty-two healthy volunteers (ten males, mean ± SD, 22.5 ±0.85 years of age) participated in this study. All participants were right-handed, nonsmoking, and free of any psychiatric or neurological disorders and had more than 12 years of education. All female participants used oral contraceptives. Subjects had no history of alcohol abuse or dependence as measured with the Alcohol Use Disorder Identification Test (AUDIT; Berks and McCormick 2008). Written informed consent was obtained, and volunteers were paid for participation. The study was approved by the Psychopharmacology (2010) 208:469–474 medical ethical committee of the University Medical Centre Utrecht and in accordance with the Declaration of Helsinki. All participants were naïve to the aim of the study. Alcohol administration and monitoring The amount of alcohol which would induce a blood alcohol concentration (BAC) of 0.5‰ was estimated with Widmark’s formula: BAC ( ) = --------------------------- - 0.085 grams of pure alcohol m ×r×(1/1.055) where m is body weight and r refers to the distribution ratio which for men is on average 0.68 and for women 0.55 (Friel et al. 1995). The alcoholic solution (300 ml) consisted of alcohol mixed with orange juice and a few drops of peppermint oil to mask the alcohol. Placebo (300 ml) consisted of orange juice and peppermint oil. Participants’ BAC was measured every 5 min using an AlcoMate CORE AL-600 Pro Alcohol Breath Analyzer (AK Solutions USA LLC, Palisades Park, NJ, USA). Measurements were started when BAC approximated 0.5‰. Importantly, TCI was measured at the start of the downward limb of the BAC curve because at this point, BAC values are suggested to be relatively stable (Verster et al. 2002). Transcranial magnetic stimulation TMS was performed using two biphasic magnetic brain stimulators (maximum output 4,160 A peak/1,750 VAC peak) and an iron core coil (Neotonus, Atlanta, GA, USA) over the left and right primary motor cortex (M1). Bilateral paired-pulse TMS was applied to measure TCI. In this paradigm, the MEP following the single test pulse (unconditioned MEP, uMEP) to M1 is compared to the MEP evoked by the test pulse that is preceded by a conditioning pulse to the contralateral M1 10 ms earlier (conditioned MEP, cMEP; Ferbert et al. 1992). The mean percentage reduction between the unconditioned and conditioned MEP was calculated. Stimulation intensity was set at 120% of the individual motor threshold (MT). To minimize anticipation effects, TMS was applied at a frequency of 0.18–0.2 Hz. The following conditions were applied in random counterbalanced order: (1) a single test pulse to the left M1 (uMEPleft), (2) a conditioning pulse to the right M1 followed by a test pulse to the left M1 (cMEPleft), (3) a single test pulse to the right M1 (uMEPright), and (4) a conditioning pulse to the left M1 followed by a test pulse to the right M1 (cMEPright; Ferbert et al. 1992).