Figure 6. Developmental stimulation of nociceptors enhances 5-HT receptor-mediated inhibition of nociceptor-to-target transmission.
(A) Noxious stimulation during development (2.5 mM AITC) does not affect nociceptor-induced responses in serotonergic neurons. n = 33 and 25 larvae for “- AITC” and “+ AITC”, respectively.
(B) Developmental stimulation of C4da neurons sensitizes C4da-to-A08n transmission to ipsapirone. AITC was used to stimulate C4da neurons for calcium imaging. 0, 1, or 10 µM ipsapirone was mixed with AITC and applied sequentially from low to high concentrations. n = 12 neurons (from 6 larvae) for each group.
(C–E) Noxious stimulation during development (2.5 mM AITC) significantly diminishes AITC-elicited increase in cAMP levels in C4da axon terminals without affecting basal cAMP levels. (C) Average traces of cAMP responses. (D) Quantification and statistical analysis of cAMP levels. n = 7 for each group. (E) Quantification and statistical analysis of basal cAMP levels. The value, YFP458/YFP515, was normalized by dividing the average of the “- AITC” group. n = 7 for each group.
(F) Developmental activation of nociceptors does not reduce A08n responses to nicotine. Three different concentrations of nicotine were applied to the brains of mature larvae, and changes in calcium levels in A08n somata were analyzed. Since A08n is specific to nociceptive inputs, A08n responses to nicotine reflect its responses to nociceptive inputs. n = 9 in each group.
(G) A model that explains the experience-dependent sensory-input-induced plasticity in nociceptor-to-SON transmission. Left panel: Acute noxious stimulation in mature larvae increases cAMP levels in the presynaptic terminals of nociceptors (shown in teal) and leads to robust synaptic transmission to SONs (shown in orange). Serotonergic neurons (shown in purple) suppress cAMP levels in nociceptor presynaptic terminals. Right panel: Developmental noxious stimulation enhances the responsiveness of nociceptor presynaptic terminals to 5-HT modulation in mature larvae, leading to further suppression of cAMP production and reduced synaptic transmission to target neurons.