The Role of Adenosine Receptors and L-Type Calcium Channels in the Regulation of the Mediator Secretion in Mouse Motor Synapses

The changes in spontaneous and evoked neurotransmitter release caused by agonists and antagonists of the A1- and A2A-subtypes of adenosine receptors combined with inhibition of some enzymes and voltage-dependent calcium channels of L-type were studied in mouse diaphragm motor synapses using intra-cellular microelectrode recordings of miniature endplate potentials (MEPPs) and evoked endplate potentials (EPPs). Simultaneous activation of presynaptic A1 and A2A receptors by endogenous adenosine during short-term rhythmic activity of motor synapses was shown for the first time. Activation of receptors A1 prevails and is followed by downregulation of the ACh release due to inhibition of intracellular cascade, which involves protein kinase A (PKA) and L-type voltage-dependent calcium channels. Activation of receptors A2A with their agonist CGS-21680 caused upregulation of the ACh secretion due to enhancement of PKA activity followed by activation of L-type voltage-dependent calcium channels. The mechanism of the evoked release potentiation, when A1 receptors are blocked or the activity of A2A receptors prevails over that of A1 receptors, involves calcium release from ryanodine-sensitive intracellular calcium stores coupled with PKA and activation of L-type voltage-dependent calcium channels. It was found that protein phosphatase calcineurin participates in downregulation of the L-type voltage-dependent calcium channels irrespective of the A1 receptors. It was shown for the first time that disinhibition of L-type voltage-dependent calcium channels caused by the calcineurin inhibition requires participation of the activity of A2A receptors and PKA. In conclusion, reciprocal interactions between presynaptic receptors A1 and A2A and their effect on the ACh release were shown in motor synapses. These interactions are mediated by the following cascade: PKA -> L-type voltage-dependent calcium channels -> ryanodine receptors of calcium stores. Final effect on the neurotransmitter release depends on conditions of coactivation of these receptors and interplay of enzymes and L-type voltage-dependent calcium channels within synaptic terminals.