Habituation of the Proleg Withdrawal Reflex in Manduca sexta Does Not Involve Changes in Motoneuron Properties or Depression at the Sensorimotor Synapse

• Published in: Neurobiology of learning and memory Vol. 76; no. 1; pp. 57 - 80
• Main Authors: Wiel, Devon E, Wood, Emma R, Weeks, Janis C
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 01.07.2001; Elsevier
• Subjects: Animals; Behavioral psychophysiology; Biological and medical sciences; cholinergic; excitatory postsynaptic potential; Extremities - physiology; Fundamental and applied biological sciences. Psychology; habituation; Habituation, Psychophysiologic; insect; invertebrate; Manduca; Manduca sexta; Mechanoreceptors - physiology; Membrane Potentials - physiology; monosynaptic; Motor Neurons - physiology; neural circuit; Neural Inhibition - physiology; nonassociative learning; paired-pulse facilitation; Psychology. Psychoanalysis. Psychiatry; Psychology. Psychophysiology; Reflex - physiology; reflex, sensorimotor; Reflexes; Synapses - physiology; synaptic depression; neural circuit; reflex, sensorimotor; excitatory postsynaptic potential; habituation; cholinergic; insect; Manduca sexta; invertebrate; monosynaptic; paired-pulse facilitation; synaptic depression; nonassociative learning; Reflex; Insecta; Electrophysiology; Motor neuron; Sphingidae; Habituation; Acquisition process; Pest; Larva; Evoked potential; Arthropoda; Animal; Lepidoptera; Synaptic plasticity; Sensory neuron; Developmental stage; Invertebrata; Excitatory postsynaptic potential
• ISSN: 1074-7427; 1095-9564
• DOI: 10.1006/nlme.2000.3982

Larvae of the hawkmoth, Manduca sexta, exhibit a defensive proleg withdrawal reflex in which deflection of mechanosensory hairs on the proleg tip (the planta) evokes retraction of the proleg. A previous behavioral study showed that this reflex habituates in response to repeated planta hair deflection and exhibits several other defining features of habituation. In a semi-intact preparation consisting of a proleg and its associated segmental ganglion, repeated deflection of a planta hair or electrical stimulation of its sensory neuron causes a neural correlate of habituation, manifested as a decrease in the number of action potentials evoked in the proleg motor nerve. Monosynaptic connections from planta hair sensory neurons to the principal planta retractor motoneuron exhibit several forms of activity-dependent plasticity. In the present study we recorded intracellularly from this motoneuron during repetitive electrical stimulation of a planta hair sensory neuron. The number of action potentials evoked in the motoneuron decreased significantly, representing a neural correlate of habituation. The motoneuron's resting membrane potential, input resistance, and spike threshold measured before and after repetitive stimulation did not differ between the stimulated group and a control group. Furthermore, the amplitude of the monosynaptic excitatory postsynaptic potential, as well as the magnitude of paired-pulse facilitation, evoked in the motoneuron by the sensory neuron did not change after repetitive stimulation. These results suggest that depression at the sensorimotor synapse does not contribute to reflex habituation. Rather, other mechanisms in the ganglion of the stimulated segment, such as changes in polysynaptic reflex pathways, appear to be responsible.