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B. B. FREDHOLM ANAL. REAL ACAD. NAL. FARM.

ATP levels are some 100,000 times higher, it is obvious that substantial
changes in adenosine levels can occur without any major changes in ATP
levels. Indeed, extracellular adenosine can be formed without any
measurable change in the energy status of CNS preparations (Mitchell,
Lupica and Dunwiddie; 1993, Doolette; 1997). Given that the energy
demanded – and hence the utilization of ATP – is substantial in localized
neuronal compartments as a consequence of work needed to maintain ion
balance, it is clear that substantial local adenosine formation could occur
via this mechanism. Nevertheless, it is difficult to explain the time course
of extracellular adenosine build-up, which may become sufficient to
modulate synaptic transmission in less than 20 ms (Mitchell et al.; 1993),
a time course much lower than the Kcat of most enzymes and adenosine
transporters. Also, this release of adenosine through nucleoside
transporters is at odds with the ability of inhibitors of nucleoside transport
to potentiate the effects of endogenous adenosine (for references, see
Cunha; 2001a, Latini and Pedata; 2001), which indicates that the role of
nucleoside transporters is mostly to clear up rather than to mediate
adenosine release, at least in non-stressful situations. Nevertheless, and as
discussed previously, this finding may be explained if in any
compartment there are cells that produce adenosine, and others that
release itsee (Fredholm, Lindström and Wallman-Johansson; 1994).

The last hypothesis concerning the origin of extracellular
adenosine is based on the extracellular catabolism of released adenine
nucleotides, mainly of released ATP, through the ecto-nucleotidase
pathway (for reviews, see Zimmermann; 2000, Cunha; 2001b). In fact, in
contrast to adenosine, ATP is present in synaptic vesicles and the
vesicular release of ATP on stimulation of nerve terminals is well
documented (see references in Cunha; 2001a). However, only a few
studies have so far gathered direct evidence for the role of ecto-
nucleotidases in forming extracellular adenosine (see references in Cunha;
2001a-b). This might be due to the difficulty to block completely an
enzyme system that is extremely efficient and where the spatial
relationship between enzyme and effector system is very close
(Dunwiddie, Diao and Proctor; 1997, Cunha, Sebastião and Ribeiro;
1998, Cunha; 2001b). It should also be pointed out that neurochemical
studies have concluded that the extracellular adenosine, which is relevant

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