ORIGINAL PAPER E. R. Wood á D. E. Wiel á J. C. Weeks Neural correlates of habituation of the proleg withdrawal re¯ex in larvae of the hawk moth, Manduca sexta Accepted: 20 December 1996 Abstract The larval proleg withdrawal re¯ex of the hawk moth, Manduca sexta, exhibits robust habituation. This re¯ex is evoked by de¯ecting one or more mec- hanosensory planta hairs on a proleg tip. We examined neural correlates of habituation in an isolated proleg preparation consisting of one proleg and its segmental ganglion. Repeated de¯ection of a single planta hair caused a signi®cant decrease in the number of action potentials evoked in the proleg motor nerve (which carries the axons of proleg retractor motor neurons). Signi®cant response decrement was seen for interstimu- lus intervals of 10 s, 60 s and 5 min. Response decrement failed to occur in the absence of repetitive stimulation, the decremented response recovered spontaneously fol- lowing a rest, and electrical stimulation of a body wall nerve facilitated the decremented response (a neural correlate of dishabituation). Adaptation of sensory neuron responses occurred during repeated hair de¯ec- tions. However, when adaptation was eliminated by di- rect electrical stimulation of sensory neurons, the re- sponse in the proleg motor nerve still decreased signi®- cantly. Muscle recordings indicated that the response of an identi®ed proleg retractor motor neuron decreased signi®cantly during habituation training. Thus, habitu- ation of the proleg withdrawal re¯ex includes a central component that is apparent at the level of a single motor neuron. Key words Habituation á Dishabituation á Manduca sexta á Learning á Invertebrate Abbreviations a-PH a planta hair á APR Accessory planta retractor motoneuron á APRM Accessory planta retractor muscle á CNS Central nervous system á DN Dorsal nerve á DN L Lateral branch of the dorsal nerve á EPSP Excitatory postsynaptic potential á ISI Interstimulus interval á PH Planta hair á PPR Principal planta retractor motoneuron á PPRM Principal planta retractor muscle á PWR Proleg withdrawal re¯ex á VN Ventral nerve á VN A Anterior branch of the ventral nerve á VN AABr3 Third branch of the anterior division of the anterior branch of the ventral nerve á VN L Lateral branch of the ventral nerve Introduction One of the most ubiquitous forms of learning is habit- uation, a decrease in the behavioral response to a stim- ulus with repeated presentation of the stimulus. The neural mechanisms underlying habituation have been investigated in many systems, including the gill and si- phon withdrawal re¯exes of the mollusc, Aplysia cali- fornica (Castellucci et al. 1970; Byrne 1982), the tail ¯ip escape response of cray®sh (Krasne 1969; Zucker 1972), and the hindlimb ¯exor re¯ex of the spinal cat (Groves and Thompson 1973). Activity-dependent changes in the intrinsic properties and/or synaptic connections of sen- sory neurons, motor neurons, or interneurons could potentially contribute to habituation, as could changes in muscle properties. Several neural correlates of habituation in the central nervous system (CNS) have been described. One well- described mechanism is homosynaptic depression, as is seen at the excitatory synapses from sensory neurons to interneurons and motor neurons in the Aplysia gill- withdrawal re¯ex circuit (Bailey and Chen 1988; Byrne 1982; Castellucci et al. 1970; Castellucci and Kandel 1974; Edmonds et al. 1990; Klein et al. 1980). Other mechanisms, both intrinsic and extrinsic to re¯ex cir- cuits, may also contribute to response decrement (Zucker 1972; Davis and File 1984). For example, in the J Comp Physiol A (1997) 180: 639±657 Ó Springer-Verlag 1997 E.R. Wood 1 á D.E. Wiel á J.C. Weeks (&) Institute of Neuroscience, University of Oregon, Eugene, Oregon 97403-1254, USA, Tel.: +1-541/346-4517, Fax: +1-541/346-4548, e-mail: weeks@uoneuro.uoregon.edu 1 Present address: Department of Psychology, Boston University, 64 Cummington St., Boston, MA 02215, USA