Real‐time decoding of dopamine concentration changes in the caudate–putamen during tonic and phasic firing

Abstract

The fundamental process that underlies volume transmission in the brain is the extracellular diffusion of neurotransmitters from release sites to distal target cells. Dopaminergic neurons display a range of activity states, from low-frequency tonic firing to bursts of high-frequency action potentials (phasic firing). However, it is not clear how this activity affects volume transmission on a subsecond time scale. To evaluate this, we developed a finite-difference model that predicts the lifetime and diffusion of dopamine in brain tissue. We first used this model to decode in vivo amperometric measurements of electrically evoked dopamine, and obtained rate constants for release and uptake as well as the extent of diffusion. Accurate predictions were made under a variety of conditions including different regions, different stimulation parameters and with uptake inhibited. Second, we used the decoded rate constants to predict how heterogeneity of dopamine release and uptake sites would affect dopamine concentration fluctuations during different activity states in the absence of an electrode. These simulations show that synchronous phasic firing can produce spatially and temporally heterogeneous concentration profiles whereas asynchronous tonic firing elicits uniform, steady-state dopamine concentrations. Keywords: amperometry, caudate-putamen, cocaine, diffusion, steady state, volume transmission. J. Neurochem. (2003Neurochem. ( ) 87, 1284Neurochem. ( -1295 Dopaminergic neurons fire in a low-frequency tonic mode and periodically exhibit bursts of high-frequency action potentials . Microdialysis studies have revealed that tonic dopamine concentrations are in the low nanomolar range (Justice 1993). Recently, naturally occuring increases in dopamine concentration have been detected on a subsecond time scale with carbon-fiber electrodes Phillips et al. 2003). These transients appear to arise from phasic firing because they are mimicked by high-frequency electrical stimulation of dopaminergic cell bodies. Like the phasic firing of dopaminergic neurons that occurs during salient stimuli (Schultz 1998), the dopamine transients can be matched to specific behaviors such as interaction with another animal (Robinson et al. 2001 or lever pressing to self-administer cocaine (Phillips et al. 2003).It is well documented that dopamine in the striatum communicates via volume transmission (Garris et al. 1994) which differs from the classic synaptic mode of wiring transmission because neurotransmitter can diffuse to target cells distant from release sites (Zoli et al. 1998;Vizi 2000). The existence of naturally occuring dopamine concentration transients in the brain raises several questions. Are there temporal differences in dopamine concentrations at release and target sites? Is dopamine volume transmission differentially affected by tonic and phasic firing patterns? To answer such questions, mathematical models that consider the rates of release, uptake and coupled diffusion are required. Although several models have been d...

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