of tonic activity in motoneurons during stick 1 insect walking

18 Stick insect middle leg (mesothoracic) motoneurons receive tonic excitatory input during front leg 19 stepping on a treadmill. We studied the pharmacology of this excitatory input to the motoneurons 20 during single-legged treadmill walking (in situ). During bath application of drugs restricted to the 21 mesothoracic ganglion activity in motoneurons contralateral to the stepping front leg was recorded 22 from neuropilar processes. Application of the cholinergic antagonist atropine reduced the tonic 23 depolarization amplitude. These results were compared to findings in acutely dissociated 24 motoneuron cell bodies (in vitro) under whole-cell voltage-clamp conditions. The presence of an 25 acetylcholine-induced current in situ was supported by the finding of an acetylcholine evoked 26 biphasic inward current with a sustained component that could be blocked by atropine. In situ the 27 tonic depolarization was generally increased by application of the neuro-modulator octopamine and 28 decreased by its antagonist mianserin. In vitro, however, octopamine reduced the inward current 29 evoked by acetylcholine application to motoneurons. Intracellular application of BAPTA into 30 motoneurons in situ revealed a dependence of the tonic depolarization on Ca and application of 31 the membrane permeable cAMP analogue 8-bromo-cAMP increased the tonic depolarization. In 32 contrast, 8-bromo-cAMP reduced the inward current evoked by acetylcholine application to 33 motoneurons in vitro. We conclude that, during walking, acetylcholine contributes to mediating the 34 tonic depolarization possibly by acting on atropine sensitive receptors on motoneurons. Octopamine 35 that is released during walking increases the tonic depolarization. This increase, however, is not 36 based on modulation of cholinergic action on motoneurons but rather on effects on premotor 37 neurons. Both, Ca and cAMP are likely 2 messengers involved in mediating the tonic 38 depolarization, but while Ca acts in motoneurons, cAMP does not appear to mediate a cholinergic 39 depolarization in motoneurons. 40