We found that both ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate autoreceptors were present on the glutamate-releasing terminals of cerebellar parallel/climbing fibers and that they functioned as facilitatory autoreceptors. Extracellular cGMP inhibited the neurotransmitter release evoked by presynaptic kainate receptor activation; the inhibitory effect of extracellular cGMP was selective for the kainate autoreceptor-mediated response and did not affect the AMPA autoreceptor-mediated response. Endogenously synthesized cGMP might be the physiological source for the extracellular cGMP modulating the response to kainate autoreceptor activation.It has long been known that intracellular cGMP, whose production is linked through nitric oxide formation to the activation of cerebellar glutamatergic transmission, functions as a crucial cerebellar intracellular messenger, e.g., in the long-term depression of excitatory transmission at parallel fiber-Purkinje cell synapses (Ito and Karachot, 1992;Boxall and Garthwaite, 1996;Daniel et al., 1998;Hartell, 2001, and references therein). It is also known that cGMP can be actively transported from the intracellular compartment to extracellular fluid through membrane transport proteins (including a probenecid-sensitive transporter and members of the multidrug-resistance protein family; Sager, 2004). Activation of glutamate receptors has been shown to increase intracellular cGMP levels and to cause cGMP release from the intracellular to the extracellular compartment in cultured cerebellar neurons (Montoliu et al., 1999) or in cerebellar slices (in a probenecid-sensitive way) (Tjörnhammar et al., 1986), with the extracellular cGMP levels matching the intracellular levels. Indeed, extracellular cGMP has been exploited in in vivo models as a reliable measure of activation of cerebellar glutamatergic transmission (Wood and Rao, 1991;Luo et al., 1994;Fedele and Raiteri, 1999). Nevertheless, the roles of extracellular cGMP in the cerebellum have hardly been studied. Interference of cGMP with the response to activation of ionotropic glutamate receptors has been occasionally reported in cultured cerebellar cells; extracellular cGMP has been reported to inhibit glutamate responses at ␣-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/ kainate receptors in Purkinje cells and in granules (Linden et al., 1995;Poulopoulou and Nowak, 1998) and to prevent glutamate neurotoxicity in cerebellar neurons (Montoliu et al., 1999). It has also been reported to inhibit binding to a kainate low-affinity site in rat cerebellum membranes (Migani et al., 1997).We investigated the ability of extracellular cGMP to interfere with AMPA/kainate receptor responses in the adult cerebellum, by using superfused purified synaptosomes, which allow direct functional evidence of presynaptic release-regulating receptors. Two aspects were investigated: 1) the presence and functioning of AMPA and kainate autoreceptors on the glutamatergic terminals of rat cerebellar parallel/climbin...