Figure 6. Possible model of the reduction of Ca²⁺ responses in the MB calyx and α lobe.

A, in the wild‐type, the projection neurons of the antennal lobe (AL‐PN) release acetylcholine (black dots) following repetitive AL stimulation, and nAChRs (blue) are activated in the MB calyx. The increase in [Ca²⁺]_i through nAChRs stimulates Rut‐AC (red), and cAMP is produced. The increase in cAMP level induces synaptic depression in the MB calyx. B, in the rut¹ mutant, repetitive AL stimulation induces Ca²⁺ entry through nAChRs. However, Rut‐AC‐dependent cAMP production does not occur. Thus, synaptic depression also does not occur. C, in the wild‐type, action potentials generated by repetitive AL stimulation may induce Ca²⁺ influx through voltage‐gated calcium channels (VGCCs; green) in the α lobe. The increase in [Ca²⁺]_i activates Rut‐AC, and cAMP is produced. The increase in cAMP level induces the reduction of Ca²⁺ responses in the α lobe. D, in the rut¹ mutant, repetitive AL stimulation induces Ca²⁺ influx into the α lobe. However, Rut‐AC‐dependent cAMP production does not occur. Thus, the reduction of Ca²⁺ responses also does not occur. E, forskolin treatment or dnc¹ mutation increases the cAMP level. In this situation, the reduction of Ca²⁺ responses in the α lobe is promoted. [Color figure can be viewed at http://wileyonlinelibrary.com]