COUPLING OF EFFERENT NEUROMODULATORY NEURONS TO RHYTHMICAL LEG MOTOR-ACTIVITY IN THE LOCUST

Coupling of efferent neuromodulatory neurons to rhythmical leg motor a ctivity in the locust. J. Neurophysiol. 79: 361-370, 1998. The spike a ctivity of neuromodulatory dorsal unpaired median (DUM) neurons was an alyzed during a pilocarpine-induced motor pattern. in the locust. Pair ed intracellular recordings were made from these octopaminergic neuron s during rhythmic activity in hindleg motor neurons evoked by applying pilocarpine to an isolated metathoracic ganglion. This motor pattern is characterized by two alternating phases: a levator phase, during wh ich levator, flexor, and common inhibitor motor neurons spike, and a d epressor phase, during which depressor and extensor motor neurons spik e. Three different subpopulations of efferent DUM neurons could be dis tinguished during this rhythmical motor pattern according to their cha racteristic spike output. DUM 1 neurons, which in the intact animal do not innervate muscles involved in leg movements, showed no change apa rt from a general increase in spike frequency. DUM 3 and DUM 3,4 neuro ns produced the most variable activity but received frequent and somet imes pronounced hyperpolarizations that were often common to both reco rded neurons. DUM 5 and DUM 3,4,5 neurons innervate muscles of the hin dleg and showed rhythmical excitation leading to bursts of spikes duri ng rhythmic activity of the motor neurons, which innervate these same muscles. Sometimes the motor output was coordinated across both sides of the ganglion so that there was alternating activity between levator s of both sides. In these cases, the spikes of DUM 5 and DUM 3,4,5 neu rons and the hyperpolarization of DUM 3 and DUM 3,4 neurons occurred a t particular phases in the motor pattern. Our data demonstrate a centr al coupling of specific types of DUM neurons to a rhythmical motor pat tern. Changes in the spike output of these particular efferent DUM neu rons parallel changes in the motor output. The spike activity of DUM n eurons thus may be controlled by the same circuits that determine the action of the motor neurons. Functional implications for real walking are discussed.