Dynamic neuronal Na + /K + pumps normally only respond to intense action potential firing owing to their low affinity for intracellular Na + . Recruitment of these Na + pumps produces a post-activity ultraslow afterhyperpolarization (usAHP) up to $10 mV in amplitude and $60 s in duration, which influences neuronal properties and future network output. In spinal motor networks, the usAHP underlies short-term motor memory (STMM), reducing the intensity and duration of locomotor network output in a manner dependent on the interval between locomotor bouts. In contrast to tonically active Na + pumps that help set and maintain the resting membrane potential, dynamic Na + pumps are selectively antagonized by low concentrations of ouabain, which, we show, blocks both the usAHP and STMM. We examined whether dynamic Na + pumps and STMM can be influenced by neuromodulators, focusing on 5-HT and nitric oxide. Bath-applied 5-HT alone had no significant effect on the usAHP or STMM. However, this is due to the simultaneous activation of two distinct 5-HT receptor subtypes (5-HT7 and 5-HT2a) that have opposing facilitatory and suppressive influences, respectively, on these two features of the locomotor system. Nitric oxide modulation exerts a potent inhibitory effect that can completely block the usAHP and erase STMM. Using selective blockers of 5-HT7 and 5-HT2a receptors and a nitric oxide scavenger, PTIO, we further provide evidence that the two modulators constitute an endogenous control system that determines how the spinal network self-regulates the intensity of locomotor output in light of recent past experience.
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The present protocol describes a method to induce tissue-specific and highly reproducible injuries in zebrafish larvae using a laser lesion system combined with an automated microfluidic platform for larvae handling.
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