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The Journal of Neuroscience, July 1, 1998, 18(13):4854-4860
Quantitative Evaluation of 5-Hydroxytryptamine (Serotonin)
Neuronal Release and Uptake: An Investigation of Extrasynaptic
Transmission
Melissa A.
Bunin and
R. Mark
Wightman
Curriculum in Neurobiology and Department of Chemistry, University
of North Carolina at Chapel Hill, Chapel Hill, North Carolina
27599-3290
Whether neurotransmitters are restricted to the synaptic cleft
(participating only in hard-wired neurotransmission) or diffuse to
remote receptor sites (participating in what has been termed volume or
paracrine transmission) depends on a number of factors. These include
(1) the location of release sites with respect to the receptors, (2)
the number of molecules released, (3) the diffusional rate away from
the release site, determined by both the geometry near the release site
as well as binding interactions, and (4) the removal of transmitter by
the relevant transporter. Fast-scan cyclic voltammetry allows for the
detection of extrasynaptic concentrations of many biogenic amines,
permitting direct access to many of these parameters. In this study the
hypothesis that 5-hydroxytryptamine (5-HT) transmission is primarily
extrasynaptic in the substantia nigra reticulata, a terminal region
with identified synaptic contacts, and the dorsal raphe nucleus, a
somatodendritic region with rare synaptic incidence, was tested in
brain slices prepared from the rat. Using carbon fiber microelectrodes,
we found the concentration of 5-HT released per stimulus pulse in both
regions to be identical when elicited by single pulse stimulations or
trains at high frequency. 5-HT efflux elicited by a single stimulus
pulse was unaffected by uptake inhibition or receptor antagonism. Thus,
synaptic efflux is not restricted by binding to intrasynaptic receptors
or transporters. The number of 5-HT molecules released per terminal was
estimated in the substantia nigra reticulata and was considerably less
than the number of 5-HT transporter and receptor sites, reinforcing the
hypothesis that these sites are extrasynaptic. Furthermore, the
detected extrasynaptic concentrations closely match the affinity for
the predominant 5-HT receptor in each region. Although they do not
disprove the existence of classical synaptic transmission, our results
support the existence of paracrine neurotransmission in both
serotonergic regions.
Key words:
5-hydroxytryptamine; volume transmission; substantia
nigra reticulata; dorsal raphe; fast-scan cyclic voltammetry; transporter kinetics
Copyright © 1998 Society for Neuroscience 0270-6474/98/18134854-07$05.00/0
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