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VOL. 73 (4), 1127-1157, 2007 PHYSIOLOGICAL ROLE OF EXTRACELLULAR...

release induced by nucleotides. What is the mechanism responsible
for the effect? This is a question with relevant physiological
implications. In our experimental model of cholinergic synaptic
terminals the inhibitory effect of nicotinic receptors on P2X receptors
appears to be mediated by the activation of Calcium calmoduline
kinase II, CaMKII, as this enzyme becomes activated by the
intrasynaptosomal calcium increase upon receptor activation. The
key role played by this enzyme in nerve terminals is confirmed by
using the enzyme inhibitor KN62 wich completely prevents the
inhibitory effect induced through nicotinic receptors agonists. This
undoubtedly will allow us to take another step in the synaptic
knowledge and avoid over-stimulation (60, 68). Figure 5 sumarises
the modulation of P2X receptors by ionotropic (nicotinic) and
metabotropic (GABAB) receptors.

Ionotropic nucleotide receptors modulation by metabotropic
receptors

Presynaptic ionotropic nucleotide receptors are also under the
influence of a large variety of metabotropic receptors located in the
presynaptic area. To this respect we have described dramatic effects
caused by protein kinase and phosphatase action, when they are
directly activated or inhibited, or through a more physiological way
of presynaptic receptors stimulation. The receptor’s affinity for their
nucleotide agonists can be decreased in situations where kinase
activity is increased, both by cAMP dependent kinases, PKA, or
members of the PKC family (69). The oposite situation also occurs
and an inhibition of protein kinases, or agonistic action on Gi
coupled metabotropic receptors, results in a bigger affinity of P2X
receptors, and also an increase in calcium maximal response. There
are remarkable examples such as those reported for A1 adenosine
receptors, and also GABAB receptors, both coupled to Gi. In the
presence of these receptors agonists, the affinity values of nucleotides
for P2X are increased in 3-5 orders of magnitude, going from mM
to nM or pM (70). An especial mention should be given to GABAB
receptors, since they are ubiquitous in CNS and especially at a
presynaptic level, given the abundance of nucleotide signalling and
presence of P2X3 subunits in GABAergic terminals, the study of the

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