MODULATION OF SPONTANEOUS AND REFLEX ACTIVITY OF CRAYFISH LEG MOTOR-NEURONS BY OCTOPAMINE AND SEROTONIN

1. We compared the effects of octopamine and serotonin on the activity of crayfish leg motor neurons in an isolated preparation of the 4th t horacic ganglion. Spontaneous activity of leg promotor (swing phase in a forward walking crayfish) and remoter (stance phase) motor neurons consisted either of continuous promotor activity (with the remoter ner ve silent) or alternating bursts of promotor and remoter activity. Oct opamine and serotonin, at high concentrations (less than or equal to 1 00 and less than or equal to 20 mu M, respectively), abolished spontan eous promotor activity and rhythmic bursting (if ongoing). Both amines induced tonic remoter nerve activity, but each amine activated differ ent identified remoter motor neurons. 2. Reflex responses of remoter m otor neurons to stimulation of thoracocoxal (TC) joint proprioceptors were modulated by octopamine and serotonin in characteristic ways. The muscle receptor (TCMRO) that signals joint remotion excited a subset of remoter motor neurons in an assistance reflex. The chordotonal orga n (TCCO) that signals joint promotion excited different remoter motor neurons in a resistance reflex. Octopamine abolished assistance reflex es and facilitated resistance reflexes. One assistance group unit was inhibited, whereas reflex reversal was induced in another: this unit w as now excited in a resistance reflex, rather than in an assistance re flex. The responses of resistance group remoter units were enhanced. S erotonin had the opposite effect on assistance group remotors: one uni t was excited and generated a stronger assistance reflex. The effect o f serotonin on resistance group remoter units was similar (but quantit atively different) to that of octopamine. 3. Both octopamine and serot onin modulated spontaneous motor output at concentrations below those required to inhibit promotor nerve activity. Rhythmic promotor and rem oter bursting was abolished, and replaced with continuous promotor act ivity, by serotonin at 1 mu M and octopamine at 1-10 mu M. In nonburst ing preparations, promotor activity could be excited (instead of inhib ited)by either amine at lower concentrations. 4. Octopaminergic inhibi tion of spontaneous promotor activity was antagonized by mianserin (10 mu M). Phentolamine at the same concentration was less effective as a n antagonist. Serotonergic inhibition of promotor activity was not blo cked by mianserin. Mianserin also antagonized inhibitory, but not exci tatory, effects of octopamine on remoter reflex responses. Serotonergi c modulation of these reflexes was not affected. 5. An intersegmental difference was found in aminergic inhibition of promotor nerve activit y. Whereas the effect (at the higher concentrations used) was inhibiti on of promotor activity from T-4, simultaneous recordings from promoto r nerves of the more rostral ganglia T-3 and T-2 showed either promoto r excitation, or inhibition that was significantly weaker than in T-4. This may relate to the known postural effects of these amines in inta ct crayfish and lobsters. 6. We conclude that octopamine and serotonin are modulators of segmental reflexes in the crayfish walking system. Each amine ''assembles'' a unique remoter nerve reflex response from d ifferent combinations of remoter units. In the case of octopamine, inh ibitory effects are mediated by a mianserin-sensitive receptor, wherea s excitatory effects are mediated by a mianserin-insensitive receptor.