Adenosine and dopamine are two important modulators of glutamatergic neurotransmission in the striatum. However, conflicting reports exist about the role of adenosine and adenosine receptors in the modulation of striatal dopamine release. It has been previously suggested that adenosine A 1 receptors localized in glutamatergic nerve terminals indirectly modulate dopamine release, by their ability to modulate glutamate release. In the present study, using in vivo microdialysis, we provide evidence for the existence of a significant glutamate-independent tonic modulation of dopamine release in most of the analyzed striatal compartments. In the dorsal, but not in the ventral, part of the shell of the nucleus accumbens (NAc), blockade of A 1 receptors by local perfusion with the selective A 1 receptor antagonist 8-cyclopentyl-1,3-dimethyl-xanthine or by systemic administration of the non-selective adenosine antagonist caffeine induced a glutamate-dependent release of dopamine. On the contrary, A 1 receptor blockade induced a glutamate-independent dopamine release in the core of the NAc and the nucleus caudate-putamen. Furthermore, using immunocytochemical and functional studies in rat striatal synaptosomes, we demonstrate that a fraction of striatal dopaminergic terminals contains adenosine A 1 receptors, which directly inhibit dopamine release independently of glutamatergic transmission.

No abstract

stores, and fatty acid metabolism. AEA was recently reported to block TASK-3 potassium channels thereby depolarizing membranes. Common inhibitors of TASK-3, Zn 2 þ , Ruthenium Red, and low pH mimicked the excitatory effects of AEA and NADA, suggesting that their effects on [Ca 2 þ ] i and transmitter levels may be attributable to membrane depolarization upon TASK-3 blockade. The K þ -evoked Ca 2 þ entry and Ca 2 þ -dependent transmitter release were inhibited by nanomolar concentrations of the CB 1 receptor agonist WIN55212-2; this action was sensitive to the selective CB 1 receptor antagonist AM251. However, in the low micromolar range, WIN55212-2, NADA and AEA inhibited the K þ -evoked Ca 2 þ entry and transmitter release independently of CB 1 receptors, possibly through direct Ca 2 þ channel blockade. Conclusions and implications: We report here for hybrid endocannabinoid/endovanilloid ligands novel dual functions which were qualitatively similar to activation of CB 1 or TRPV 1 receptors, but were mediated through interactions with different targets.

1. Train-of-four fade (TOF(fade) ) is a clinically useful parameter to monitor the degree of block of neuromuscular transmission in curarized patients. Experimentally, TOF(fade) has been attributed to the blockade of facilitatory nicotinic receptors on motor nerve terminals. There is less information regarding the involvement of coexistent presynaptic receptors (e.g. muscarinic M(1) and M(2) , adenosine A(1) and A(2A) ) in the TOF(fade) produced by antinicotinic agents. 2. In the present study, we evaluated the TOF(fade) caused by antinicotinic neuromuscular relaxants (hexamethonium, d-tubocurarine, vecuronium and rocuronium) as the ratio of the muscle tension produced in the rat diaphragm by the fourth to the first stimulus (T(4) /T(1) ) of a train-of-four stimuli delivered to the phrenic nerve trunk at a frequency of 2 Hz. 3. All antinicotinic agents, except hexamethonium, decreased the amplitude of muscle tension during the first stimulus. Hexamethonium, (5.47 mmol/L), d-tubocurarine- (1.1 μmol/L), vecuronium (4.7 μmol/L)- and rocuronium (9.8 μmol/L)-induced TOF(fade) was attenuated by 10 nmol/L pirenzepine (an M(1) receptor antagonist), 1 μmol/L methoctramine (an M(2) receptor antagonist) and 2.5 nmol/L 1,3-dipropyl-8-cyclopentylxanthine (an A(1) receptor antagonist). Blockade of the A(2A) receptor with 10 nmol/L ZM241385 partially reversed the TOF(fade) induced by d-tubocurarine, vecuronium and rocuronium, but not that caused by the 'pure' neuronal nicotinic receptor antagonist hexamethonium, unless one increased the concentration of ZM241385 to 50 nmol/L. 4. The data indicate that presynaptic M(1) , M(2) , A(1) and A(2A) receptors play a role in neuromuscular TOF(fade) caused by antinicotinic neuromuscular relaxants. Such interplay depends on adenosine tonus and on the affinity of neuromuscular blocking agents for neuronal versus muscular nicotinic receptors.

Diadenosine pentaphosphate (Ap(5)A) and ATP stimulate an intracellular free calcium concentration ([Ca(2+)](I)) increase in rat hippocampal synaptosomes via different receptors as demonstrated by the lack of cross-desensitization between Ap(5)A and ATP responses. The ATP response was inhibited by P2 receptor antagonists and not by the dinucleotide receptor antagonist, diinosine pentaphosphate (Ip(5)I). In contrast, the Ap(5)A response was inhibited by Ip(5)I but not by P2 receptor antagonists. Studies in single hippocampal synaptic terminals showed that 31% of them responded to Ap(5)A by a [Ca(2+)](i) increase. Adenosine receptors (A(1), A(2A), and A(3)) were also present in isolated terminals as demonstrated by immunohistochemistry. The activation of A(1) or A(2A) receptors by specific agonists changed the sigmoid concentration-response curve for Ap(5)A (EC(50) = 33.5 +/- 4.5 microM) into biphasic curves. When the high-affinity adenosine receptors A(1) or A(2A) were activated, the Ap(5)A dose-response curves showed a high-affinity component with EC(50) values of 41.1 +/- 1.9 pM and 99.9 +/- 10.2 nM, respectively. The low-affinity component showed EC(50) values of 17.1 +/- 0.8 and 21.6 +/- 1.4 microM for A(1) and A(2A) receptor activation, respectively. However, the adenosine A(3) receptor activation induced a right shift of the dinucleotide concentration-response curve, showing an EC(50) value of 331.4 +/- 54.6 microM. In addition, in the presence of the A(2A) agonist, the Ap(5)A calcium influx responses were increased up to 300% of the control values. These results clearly demonstrate that the activation of presynaptic adenosine receptors is able to modulate the dinucleotide response in hippocampal nerve terminals.