Spatiotemporal distribution of dopamine

How are the individual microspheres of influence of each dopaminergic release site related to one another This depends on the degree of divergence of the individual axons, and the volume in which the terminals are distributed, from which the nearest-neighbor distances between synapses of the same presynaptic neuron can be estimated. 

Dopamine cells in the pars compacta number about 7200 (Oorschot, 1996) whereas we estimate 2.7 x 109 dopamine synapses in the striatum, suggesting that on an average, each dopamine cell must contribute about 370,000 synapses (Table 1). This is a high degree of divergence of individual axons. Failures are thought to be uncommon based on measures of the dopamine efflux after macroscopic stimulation (Garris et al., 1994) but this has not been directly measured for unitary synaptic events. 

The distance between dopamine synapses of a given cell is remarkably similar to the 7 pm diffusion distance at which at least 50% of the released dopamine can diffuse (Gonon et al., 2000). Thus, individual action potentials are likely to produce gradients of dopamine concentration, varying as a function of distance from release sites. However, the difference in concentration within the field of influence of a single dopamine cell may be as little as two-fold. 

DOPAMINE NEURONE FIRING PATTERNS AND DOPAMINE RELEASE 

1984b) was not seen in vitro, except after some rather drastic manipulations of the external environment of the slices (Johnson et al., 1992). 

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What is the likely effect on target structures of the different modes of firing of dopamine cells outlined above To address this question it is necessary to estimate the spatiotemporal distribution of the dopamine concentration increases produced by different firing patterns, and combine this with data on the affinities and potencies of target receptors. 

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