[Effects of diazepam and levodopa single doses on motor cortex plasticity modulation in healthy human subjects: a TMS study].

INTRODUCTION: Administration of pharmacological agents with specific actions on neurotransmitter systems is a powerful driver of functional cortical reorganization. Plastic reorganization of the motor cortex in humans studies by the use of non-invasive stimulation protocols, which mimic the Hebbian model of associative plasticity.

OBJECTIVE: Aiming to explore pharmacological modulation on human motor cortex plasticity, we tested healthy subjects after each dosage of diazepam, levodopa i placebo administration, using paired associative stimulation protocol (PAS) that induce fenomena similar to a long-term potentiation and depression, as defined on the synaptic level.

METHODS: We analyzed effects of benzodiazepines (10 mg), levodopa (200 mg) and placebo on PAS protocol in 14 healthy volunteers, using a double-blind placebo-controlled study design. PAS consisted of electrical stimuli pairs at n.medianus and magnetic pulses over the scalp (transcranial magnetic stimulation) in precisely defined intervals (ISI was 10 and 25 ms) for a total of about 15 minutes (200 pairs). MEP amplitudes before and after (0, 10, 20 and 30 minutes later) interventional protocols were compared.

RESULTS: When protocols were applied with placebo depending on ISI (10 ms--inhibitory, 25 ms--facilitatory effects), MEP amplitudes decreased or increased, while values in the post-interventional period (0, 10, 20 and 30 min) were compared with initial values before the use of SAS. The use of benzodiazepines caused the occlusion of LTP-like effect, in contrast to amplification effects recorded after the administration of levodopa. With respect to the LTD-like protocol, the reverse was true (ANOVA for repeat measurements p < 0.001).

CONCLUSION: Administration of GABA-ergic agonist diazepam interferes with the induction of associative plasticity in the motor cortex of healthy individuals, as opposed to the use of levodopa, which stimulates these processes. The observed effects point at a potential role of pharmacological modulation of plasticity in humans.