Nanomolar propofol stimulates glutamate transmission to dopamine neurons: a possible mechanism of abuse potential?

Anesthesiologists among physicians are on the top of the drug abuse list, and the mechanism is unclear. Recent studies suggest occupation-related second-hand exposure to i.v. drugs, including propofol, may play a role. Growing evidence indicates that propofol is one of the choices of drugs being abused. In this study, we show that propofol at minute concentrations increases glutamatergic excitatory synaptic transmission and discharges of dopamine neurons in the ventral tegmental area (VTA). We found that acute application of propofol (0.1-10 nM) to the VTA in midbrain slices of rats increased the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (EPSCs) mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors. We observed that propofol increased the amplitude but decreased the paired-pulse ratio of EPSCs evoked by stimulation in the absence and the presence of gabazine (SR 95531), a GABA(A) receptor antagonist. Moreover, the propofol-induced facilitation of EPSCs was mimicked by 6-phenyl-4-azabicyclo[5.4.0]undeca-7,9,11-triene-9,10-diol (SKF38393), an agonist of dopamine D(1) receptor, and by 1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine dihydrochloride (GBR 12935), a dopamine reuptake inhibitor, but blocked by (+/-)-7-bromo-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine hydrochloride (SKF83566), a D(1) antagonist, or by depleting dopamine stores with reserpine. Finally, 1 nM propofol increased the spontaneous discharge rate of dopamine neurons. These findings suggest that propofol at minute concentrations enhances presynaptic D(1) receptor-mediated facilitation of glutamatergic synaptic transmission and the excitability of VTA dopamine neurons, probably by increasing extracellular dopamine levels. These changes in synaptic plasticity in the VTA, an addiction-related brain area might contribute to the development of propofol abuse and the increased susceptibility to addiction of other drugs.