In vivo measurement of somatodendritic release of dopamine in the ventral tegmental area.

The mesolimbic dopamine pathway projects from the ventral tegmental area (VTA) to the nucleus accumbens and plays an important role in reward processing (Pan et al., 2005). It runs parallel to another dopaminergic pathway, the nigrostriatal pathway, which regulates locomotion and originates in the substantia nigra (SN). Dopamine release at the terminals of both of these pathways has been extensively characterized. Terminal release is evoked by action potentials, is highly regulated, and has a release capacity that is regulated by dopamine stores in the active pool (Garris et al., 1993; Montague et al., 2004; Nicolaysen et al., 1988). Following release, dopamine is cleared from the extracellular space by the dopamine transporter (DAT), which transports it back into the neuron (Gainetdinov and Caron, 2003). Because uptake is time dependent, whereas release is dependent on the frequency of action potentials, dopamine overflow into the extracellular space is highly dependent on the frequency of action potentials. At high frequencies, uptake has little time between pulses to sequester dopamine leading to greater overflow than at low frequencies (Heien and Wightman, 2006; Wightman et al., 1988). As well as interacting with postsynaptic receptors, dopamine binds to presynaptic autoreceptors that serve to inhibit subsequent dopamine release (Garris et al., 2003; Phillips et al., 2002). Dopamine is also stored in the dendrites of the SN and VTA (Bjorklund and Lindvall, 1975; Cuello and Kelly, 1977). In the SN, Ca2+ dependent somatodendritic dopamine release was first reported in brain slices (Geffen et al., 1976). This finding was confirmed by in vivo measurement with a push-pull canula (Cheramy et al., 1981). Subsequent microdialysis studies also demonstrated dopamine release in the VTA (Kalivas and Duffy, 1988). Voltammetric measurements in VTA containing slices from guinea pig brain (Rice et al., 1994), from rat (Iravani et al., 1996), and from mouse (John et al., 2006) also confirm somatodendritic dopamine release evoked by local electrical stimulation. While dopamine release was suggested to occur via reverse transport via the DAT (Falkenburger et al., 2001), further research has shown that dendritic DA release is exocytotic in both the SN and VTA (Chen and Rice, 2001; John and Jones, 2006). Somatodendritic dopamine release has a presynaptic regulatory role as well as a postsynaptic role (Beckstead et al., 2007; Beckstead et al., 2004), although it does not appear to play a role in regulating release in the VTA (Cragg and Greenfield, 1997). While neurons normally transmit action potentials from the cell bodies to the terminals, it has been demonstrated that midbrain dopaminergic neurons can also backpropagate action potentials (Gentet and Williams, 2007; Hausser et al., 1995) to the dendritic field. In this work we have tested the hypothesis that back-propagation of action potentials in dopaminergic neurons can evoke dopamine release. To achieve this goal, a stimulating electrode was lowered in an anesthetized rat to the level of the MFB, and a carbon-fiber microelectrode was placed in the VTA. Dopamine release in the VTA was measured during stimulations with fast-scan cyclic voltammetry (Robinson et al., 2008). The stimulated release of dopamine resembled its evoked release in terminal regions except it was lower, uptake was slower, and it was insensitive to drugs that inhibit dopamine autoreceptor functioning.