Interaction between the second messengers cAMP and Ca 2+ in mouse presynaptic taste cells

The second messenger, 3′,5′‐cyclic adenosine monophosphate (cAMP), is known to be modulated in taste buds following exposure to gustatory and other stimuli. Which taste cell type(s) (Type I/glial‐like cells, Type II/receptor cells, or Type III/presynaptic cells) undergo taste‐evoked changes of cAMP and what the functional consequences of such changes are remain unknown. Using Fura‐2 imaging of isolated mouse vallate taste cells, we explored how elevating cAMP alters Ca 2+ levels in identified taste cells. Stimulating taste buds with forskolin (Fsk; 1 μ m ) + isobutylmethylxanthine (IBMX; 100 μ m ), which elevates cellular cAMP, triggered Ca 2+ transients in 38% of presynaptic cells ( n = 128). We used transgenic GAD‐GFP mice to show that cAMP‐triggered Ca 2+ responses occur only in the subset of presynaptic cells that lack glutamic acid decarboxylase 67 (GAD). We never observed cAMP‐stimulated responses in receptor cells, glial‐like cells or GAD‐expressing presynaptic cells. The response to cAMP was blocked by the protein kinase A inhibitor H89 and by removing extracellular Ca 2+ . Thus, the response to elevated cAMP is a PKA‐dependent influx of Ca 2+ . This Ca 2+ influx was blocked by nifedipine (an inhibitor of L‐type voltage‐gated Ca 2+ channels) but was unperturbed by ω‐agatoxin IVA and ω‐conotoxin GVIA (P/Q‐type and N‐type channel inhibitors, respectively). Single‐cell RT‐PCR on functionally identified presynaptic cells from GAD‐GFP mice confirmed the pharmacological analyses: Ca v 1.2 (an L‐type subunit) is expressed in cells that display cAMP‐triggered Ca 2+ influx, while Ca v 2.1 (a P/Q subunit) is expressed in all presynaptic cells, and underlies depolarization‐triggered Ca 2+ influx. Collectively, these data demonstrate cross‐talk between cAMP and Ca 2+ signalling in a subclass of taste cells that form synapses with gustatory fibres and may integrate tastant‐evoked signals.