Zaprinast stimulates extracellular adenosine accumulation in rat pontine slices

Adenosine appears to be an endogenous somnogen. The lateral dorsal tegmental/pedunculopontine nucleus (LDT/PPT) located in the mesopontine tegmentum is important in the regulation of arousal. Neurons in this nucleus are strongly hyperpolarized by adenosine and express neuronal nitric oxide synthase. Zaprinast is a cyclic nucleotide phosphodiesterase inhibitor, and has been shown in the hippocampal slice to inhibit the field excitatory postsynaptic potential. This action could be blocked by an adenosine receptor antagonist, and therefore is presumably due to adenosine release stimulated by zaprinast. In the present study we tested the effect of zaprinast on extracellular adenosine accumulation in pontine slices containing the LDT. Zaprinast at 10 μM evoked an increase in extracellular adenosine concentration. This effect was blocked by impermeant inhibitors of 5′-nucleotidase, indicating that the extracellular adenosine was derived from extracellular AMP. However, inhibitors of cAMP degradation had little or no effect on zaprinast-evoked adenosine accumulation, suggesting that extracellular cAMP was not the source. Removal of extracellular calcium inhibited the effect of zaprinast. These results demonstrate that a pathway exists by which zaprinast stimulates extracellular adenosine accumulation, and the presence of this pathway in the pontine slice suggests the possibility that it may be relevant for the regulation of behavioral state.