NEURAL MECHANISMS OF BEHAVIORAL PLASTICITY - METAMORPHOSIS AND LEARNING IN MANDUCA-SEXTA

This review summarizes our current understanding of the neural circuit underlying the larval proleg withdrawal reflex (PWR) of Manduca sexta and describes how PWR function changes in two contexts: metamorphosis and learning. The first form of PWR plasticity occurs during the larv al-pupal transformation, when the reflex is lost. One mechanism that c ontributes to this loss is the weakening of monosynaptic excitatory co nnection from proleg sensory neurons to proleg retractor motor neurons . This change is associated with the hormonally-mediated regression of proleg motor neuron dendrites, which may break synaptic contacts betw een the sensory and motor neurons. After pupation, some of the proleg motor neurons die in a segment-specific pattern that persists even aft er individual motor neurons are isolated from the nervous system and e xposed to hormones in vitro. The second form of PWR plasticity involve s short-term, activity-dependent changes in neural function during the larval stage. The nicotinic cholinergic connections from proleg senso ry neurons to motor neurons exhibit several forms of plasticity includ ing facilitation, depression, post-tetanic potentiation and two types of muscarinic modulation. Larval PWR behavior exhibits two simple form s of learning - habituation and dishabituation - which involve alterat ions in the central PWR circuit. These studies of a simple circuit ill ustrate neural mechanisms by which behaviors undergo both short- and l ong-term modifications.