Neural mechanism underlying re-depression of crayfish tailflip habituation

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Crayfish escape up and forwards by flipping its tail when intense stimulus is applied to their tailfan. This escape behavior is mediated by a pair of giant fiber, lateral giants (LGs). The LGs receive mechanosensory information via direct electrical and chemical synapses from the sensory afferents and disynaptically via ascending interneurones. Bath application of serotonin (5-HT) increased the EPSP amplitude of LGs in response to the sensory stimulation though their resting potential and membrane resistance remained consistent. To analyze the nature of 5-HT modulation, sensory responses of LG escape circuit neurones were analyzed under the mixture of membrane permeable cAMP analog, CPT-cAMP and cAMP phosphodiesterase inhibitor IBMX. As 5-HT superfusion, EPSP amplitude of LGs was increased by cAMP analog superfusion. The increase of response of LGs was reduced after about 30 min of washing with normal saline. The sensory responses of interneurones A and C also increased by cAMP analog. These results suggest 5-HT increases intrinsic level of cAMP in LG escape circuit.Bath application of biogic amines, serotonin (5-HT) or octopamine (OA) increased amplitude of LG EPSPs in response to sensory stimulation. To investigate the action of these biogic amines, LG activity was analyzed under the mixture of membrane permeable cAMP analogue (CPT-cAMP) and cAMP phosphodiesterase inhibitor (IBMX) or cGMP analogue (8-Br-cGMP) alone. The cAMP analogue increased LG activity while cGMP analogue had no effect. Adenylate cyclase inhibitor SQ22536 inhibited the effect of 5-HT but had no effect on OA. Iontophoretic injection of cAMP analogue, Sp-cAMP, directly into LG mimicked the modulatory effect of 5-HT. These results suggest 5-HT increases LG activity by promoting intrinsic level of cAMP in LG itself.