Interaction between the second messengers cAMP and Ca²⁺ 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 Ca²⁺ levels in identified taste cells. Stimulating taste buds with forskolin (Fsk; 1 μm) + isobutylmethylxanthine (IBMX; 100 μm), which elevates cellular cAMP, triggered Ca²⁺ transients in 38% of presynaptic cells (n = 128). We used transgenic GAD-GFP mice to show that cAMP-triggered Ca²⁺ 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 Ca²⁺. Thus, the response to elevated cAMP is a PKA-dependent influx of Ca²⁺. This Ca²⁺ influx was blocked by nifedipine (an inhibitor of L-type voltage-gated Ca²⁺ 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_v1.2 (an L-type subunit) is expressed in cells that display cAMP-triggered Ca²⁺ influx, while Ca_v2.1 (a P/Q subunit) is expressed in all presynaptic cells, and underlies depolarization-triggered Ca²⁺ influx. Collectively, these data demonstrate cross-talk between cAMP and Ca²⁺ signalling in a subclass of taste cells that form synapses with gustatory fibres and may integrate tastant-evoked signals.