0.12–0.90 mg/kg/day

Rat midbrain slices were used to make intracellular recordings from dopaminergic neurons of the substantia nigra and the ventral tegmental area. Gamma-aminobutyric acid (GABA)-mediated synaptic transmission was assessed from the inhibitory postsynaptic potentials (IPSPs) mediated by GABAA and GABAB receptors. Intracellular recordings from midbrain SNpc and VTA dopaminergic neurons were performed at 33.0 +/- 0.5°C in a recording chamber submerged with aCSF flowing at a rate of 2.5–3 mL•min-1 and continuously oxygenated, on the stage of an upright (inverted) microscope (Axioscope FS, Zeiss, Gottingen, Germany), equipped for infrared video microscopy (Hamamatsu, Tokyo, Japan) in order to allow a direct visualization of the recorded cells. Neurons, selected for their morphology, were identified as dopaminergic by their electrophysiological properties such as the presence of a regular spontaneous firing activity (0.5–4 Hz), a large inward current (Ih) in response to hyperpolarizing voltages and a membrane hyperpolarization due to dopamine (10–30 mmol•L-1) application . The recording electrodes were filled with 2 mol•L-1 KCl and had a tip resistance of 30–80 MW. GABAB synaptic potentials were evoked using a bipolar tungsten stimulating electrode with a tip separation of 300–700 mkm. Representative recordings showing the effect of fenfluramine in reducing the GABAB-mediated IPSP. Such effects were evident at concentrations below 1 mmol•L-1. The amplitude of synaptic potentials was halved by fenfluramine at concentration around 10 mmol•L-1 and was completely blocked by CGP (1 mmol•L-1), a selective GABAB receptor antagonist. Fenfluramine and sibutramine induce a concentration-dependent reduction of the GABAB-mediated inhibitory postsynaptic potentials (IPSPs).