IDENTIFIED OCTOPAMINERGIC NEURONS PROVIDE AN AROUSAL MECHANISM IN THELOCUST BRAIN

1. Habituation is the declining responsiveness of a neural circuit (or behavior) to repetitive stimulation. Dishabituation (or arousal) can be brought about by the sudden presentation of an additional, novel st imulus. A clear example of arousal in the locust is provided by the vi sual system: the habituated response of the descending contralateral m ovement detector (DCMD) interneuron to repetitive visual stimuli can b e dishabituated by a variety of other visual and tactile stimuli. 2. A pplication of octopamine to the locust brain and optic lobes dishabitu ates the DCMD in a manner similar to the effect of visual and tactile stimulation.3. The locust CNS contains two pairs of octopamine-immunor eactive cells, the protocerebral medulla 4 (PM4) neurons, that could p otentially mediate this dishabituation effect; PM4 neurons arborize in the optic lobe, they contain octopamine, and they respond to the same visual and tactile stimuli that dishabituate the DCMD. 4. To investig ate whether PM4 activity dishabituates the DCMD, we recorded intracell ularly from one of the PM4 neurons while recording extracellularly fro m the DCMD. When the PM4 neuron is injected with hyperpolarizing curre nt to render it completely inactive, the DCMD exhibits its characteris tic habituation to a moving visual stimulus. However, depolarizing the PM4 neuron, to produce action potentials at similar to 20 Hz, signifi cantly increases the number of DCMD action potentials per stimulus. 5. The PM4 neurons may therefore play an important role in dishabituatin g the DCMD to novel stimuli; This effect is presumably mediated by PM4 neurons releasing endogenous octopamine within the optic lobe.