24 August 2008

Ode to rewarding brain stimulation

"After sending a single electric shock to
the axons of dopamine neurons,
extracellular dopamine concentrations
in the dorsal and ventral striatum
within 1.3-10.0 ms
from baselines of 5-10 nM
via intrasynaptic peaks of 500-3000 nM
to extrasynaptic concentrations of 250-500 nM in rats and guinea pigs
and 500-1600 nM in monkeys
(Kawagoe et al. 1992, Dugast et al. 1994, Garris et al. 1994, Cragg et al. 2000)

Concentrations quickly become homogenous
at ~80 nM
within a sphere
of 3.5-4 micrometers in diameter
(Gonon 1997, Cragg & Rice 2004),
which is the average distance
between the dopamine-releasing varicosities
(Doucet et al. 1986).
Maximal diffusion is reached within
75 ms after release onset
and extends to 7-12 micrometers,
even with intact reuptake transport.

Multiple electrical shocks
at intervals of 16.66 to 500 ms (2-60 Hz)
induce peaks
exceeding 4000 nM after 200-300 ms,
which are higher
than those obtained
with the same number
of more widely spaced impulses
(Garris & Wightman 1994, Gonon 1988, 1997)

Owing to the action
of the extrasynaptic dopamine reuptake transporter,
concentrations come back to baseline
within 200 ms
after singel pulses
and within 500-600 ms
after multiple pulses."

- Wolfram Schultz (2007) Multiple Dopamine Functions at Different Time Courses
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