Neural mechanisms of behavior

Repair and Regeneration of Functional Synaptic Connections: Cellular and Molecular Interactions in the Leech

A major problem for neuroscience has been to find a means to achieve reliable regeneration of synaptic connections following injury to the adult CNS. This problem has been solved by the leech, where identified neurons reconnect precisely with their usual targets following axotomy, re-establishing in the adult the connections formed during embryonic development. It cannot be assumed that once axons regenerate specific synapses, function will be restored. Recent work on the leech has shown following regeneration of the synapse between S-interneurons, which are required for sensitization of reflexive shortening, a form of non-associative learning, the capacity for sensitization is delayed. The steps in repair of synaptic connections in the leech are reviewed, with the aim of understanding general mechanisms that promote successful repair. New results are presented regarding the signals that regulate microglial migration to lesions, a first step in the repair process. In particular, microglia up to 900 microm from the lesion respond within minutes by moving rapidly toward the injury, controlled in part by nitric oxide (NO), which is generated immediately at the lesion and acts via a soluble guanylate cyclase (sGC). The cGMP produced remains elevated for hours after injury. The relationship of microglial migration to axon outgrowth is discussed.

Physiological bases for parasite-induced alterations of host behaviour

Parasitism is defined in various ways as an intimate relationship in which one partner, the parasite, lives on or in another, the host, generally at the expense of the latter. Parasitism commonly results in a unique array of host physiological responses and adaptations. Most studies of the physiological effects of parasitism have focused on the pathological consequence of infection and disease. While many physiological changes contribute to pathogenesis, it is now recognized that parasitic infections at sub-clinical levels also produce physiological effects that either ameliorate or may not contribute to the disease process. Moreover, these physiological changes are often manifested by altered host behaviour. Behavioural studies have enabled an ecological- and evolutionary-oriented evaluation of host responses. In this fashion, physiological effects may be assessed as to whether they affect fitness and confer benefit or harm to one or both of the symbionts involved. We briefly examine how these physiological responses, specifically neural, endocrine, neuromodulatory, and immunomodulatory components, may interact to modify host behaviors. We consider the adaptiveness of these responses and how the behavioural patterns elicited may simultaneously appear adaptive for the parasite as well as the host. In addition, we address how parasite-host physiological and behavioural interactions may be altered during the course of parasitism.