The olfactory receptor neurons in frogs resemble those of mammals, in that they

Question

The olfactory receptor neurons in frogs resemble those of mammals, in that they express olfactory receptors that are coupled to a G protein. When the G protein is activated, it activates an adenylyl cyclase to produce cAMP, which then opens cyclic-AMP gated cation channels in the plasma membrane. The opening of these channels depolarizes the membrane, leading to the production of an action potential. Your friend is interested in why neurons stop responding to an odor after prolonged exposure to it, a process called adaptation. He has conducted experiments examining the depolarization of the olfactory receptor neuron, the binding of odorant to the receptor, the activation of the G protein, the levels of cAMP in the cell, and the phosphorylation of adenylyl cyclase. His results are summarized in Table 1.

           

            From these results, what step in the odor transduction pathway is altered to elicit adaptation? Propose a possible molecular mechanism to explain why olfactory neurons no longer respond to an odor after prolonged exposure to it.

brief exposure after prolonged or exposure to odor depolarization of neuron? no yes no no yes yes odorant binding to receptor? no yes yes activation of the G protein? high adenylylcyclase phosphorylation? no no yes

Explanation / Answer

Answer:

The step that is altered to elicit the adaptation is the failure to open cyclic AMP (cAMP) gated ion channels. This happens because adenylyl cyclase is phosphorylated which does not produce the cAMP needed to create an action potential.

The prolonged binding of the odorant to the receptor causes the phosphorylation of adenylyl cyclase by presenting an ATP for use. This causes the cAMP levels to decrease and adaptation to occur.

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