OCTOPAMINE INDUCES STEADY-STATE REFLEX REVERSAL IN CRAYFISH THORACIC GANGLIA

1. This paper investigates the effect of octopamine on spontaneous and reflex motor output of crayfish leg motor neurons. Octopamine modulat ed spontaneous activity, both rhythmic and tonic, and dramatically mod ulated the pattern of reflex motor output elicited by stimulating iden tified proprioceptors of the basal limb. 2. Spontaneous reciprocal mot or patterns, involving alternating bursts of promotor and remoter moto r neuron activity, were reversibly abolished by octopamine. The thresh old concentration for this effect was similar to 1 mu M. 3. At concent rations greater than similar to 10 mu M octopamine inhibited spontaneo us promotor nerve activity in both bursting and nonbursting preparatio ns. In some experiments promotor inhibition was correlated with the in duction of tonic remoter nerve activity. The ECS(50) for complete inhi bition of promotor nerve activity by octopamine was 20-30 mu M. 4. Ref lexes mediated by two basal limb proprioceptors, the thoracocoxal musc le receptor organ(TCMRO; which signals (eg promotion) and the thoracoc oxal chordotonal organ CTCCO; which signals leg remotion) were analyze d in a number of promotor and remoter motor neurons. In both cases ass istance reflexes (excitation of promoters by the TCCO and remotors by the TCMRO) were restricted to subgroups of the motor pool. Among remot er motor neurons, the first two units recruited during bursts of spont aneous activity were members of the assistance reflex group (group 1). A third unit, sometimes recruited during more intense spontaneous bur sts, was excited by TCCO stimulation and was therefore a member of the resistance reflex group (group 2). Other resistance group remotors we re also excited by the TCCO, but this input normally remained subthres hold. 5. Stimulation of the TCCO afferent nerve elicited excitatory po stsynaptic potentials (EPSPs) in group 2 (resistance group) remotor mo tor neurons at a latency compatible with a monosynaptic connection. Th e same stimulation excited group 1 (assistance group) promotor motor n eurons, but at a greater and more variable latency. Thus the remoter r esistance reflex from the TCCO is probably monosynaptic, but the promo tor assistance reflex, also elicited by TCCO stimulation, is likely to be di- or polysynaptic. Assistance group (group 1) remoter motor neur ons are inhibited by mechanical stimulation of the TCCO, or electrical stimulation of its nerve. 6. Octopamine had selective effects on indi vidual remoter units. First, assistance group remotor motor neurons we re affected in two ways. One unit was inhibited, so that reflex spikin g in response to TCMRO stimulation was abolished. A second unit was no t inhibited, but its reflex response mode changed, so that instead of responding to TCMRO input with an assistance reflex, it responded to T CCO input with a resistance reflex. Second, among motor neurons that n ormally respond to TCCO input with resistance reflexes, these response s were enhanced by octopamine. 7. Promotor motor neurons were inhibite d by octopamine and reflex responses were also affected selectively. R esponses to TCCO input (assistance reflexes) were abolished; whereas, responses to TCMRO input (resistance reflexes) were relatively less af fected. 8. Intracellular recordings revealed that the majority of remo ter motor neurons depolarized in the presence of octopamine. In prepar ations where these could be classified on the basis of TCMRO/ TCCO inp uts, all were identified as group 2 (resistance group). A minority of remoter motor neurons were hyperpolarized by octopamine. All of these were identified as group 1 (assistance group), with strong TCMRO input . 9. The majority of promotor motor neurons were depolarized by octopa mine. This depolarization was nevertheless inhibitory since it reverse d slightly positive to rest and was associated with a substantial fall in input resistance.