Adenosine-Mediated Presynaptic Modulation of Glutamatergic Transmission in the Laterodorsal Tegmentum

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Abstract
The laterodorsal tegmentum (LDT) neurons supply most of the cholinergic tone to the brainstem and diencephalon necessary for physiological arousal. It is known that application of adenosine in the LDT nucleus increases sleep in vivo (Portas et al., 1997) and directly inhibits LDT neurons in vitro by activating postsynaptic adenosine A1 receptors (Rainnie et al., 1994). However, adenosine effects on synaptic inputs to LDT neurons has not been previously reported. We found that both evoked glutamatergic EPSCs and GABAergic IPSCs were reduced by adenosine (50 μm). A presynaptic site of action for adenosine A1 receptors on glutamatergic afferents was suggested by the following: (1) adenosine did not affect exogenous glutamate-mediated current, (2) adenosine reduced glutamatergic miniature EPSC (mEPSC) frequency, without affecting the amplitude, and (3) inhibition of the evoked EPSC was mimicked by the A1 agonist N6-cyclohexyladenosine (100 nm) but not by the A2 agonist N6-[2-(3,5-dimethoxyphenyl)-2-(methylphenyl)-ethyl]-adenosine (10 nm). The A1 receptor antagonist 8-cyclopentyltheophylline (CPT; 200 nm) potentiated the evoked EPSCs, suggesting the presence of a tonic activation of presynaptic A1 receptors by endogenous adenosine. The adenosine kinase inhibitor, 5-iodotubercidin (10 μm), mimicked adenosine presynaptic and postsynaptic effects. These effects were antagonized by CPT or adenosine deaminase (0.8 IU/ml), suggesting mediation by increased extracellular endogenous adenosine. Together, these data suggest that the activity of LDT neurons is under inhibitory tone by endogenous adenosine through the activation of both presynaptic A1receptors on excitatory terminals and postsynaptic A1receptors. Furthermore, an alteration of adenosine kinase activity modifies the degree of this inhibitory tone.