l\Ilechanislns of synaptic modulation by neuropeptides in crayfish and DrosopJlila

Modlliation of chemical synapses is an important function in the nervous systelTI because it underlies learning, memory, development and alteration of behaviour. Understanding the intracellular Inechanisms of synaptic modlliation by neuropeptides is important for understanding their functional roles. This thesis examines the mechanisms underlying synaptic modulation by neuropeptides in two invertebrates, fruitflies (Drosophila n1elal10gaster) and crayfisll (Procambarus clarkii). In crayfish, DRN.FLRFalnide (DF2), a FMRFalnide-like peptide, increases transmitter release from nerve terminals at neuromuscular synapses (Skerrett et al. 1995) in a temperature-dependent manner (Friedrich et aL 1994). This thesis examines the mechanisms underlying such telnperature-dependence. The enhancement of translnitter output by DF2 increases as telnperature decreases. Reducing telnperature lowers tral1S111itter output, vvhicll could potelltially il1crease the store oftranSl11ittera'vailable for release. Thus, the peptide Ini&~t be more effective at lo\v temperature because there is more transmitter available to be mobilized and released~ This possibility is addressed by reducing transmitter release \vith other Inethods. One alternative method of reducing transmitter output, specifically reducing extracellular calcium concentration, also increases the effectiveness of the oentide. Data from these exneriments_ in which .L ~ ~-' temperature and extracellular calcium were varied, show that the degree of modulation by DF2 is inversely related to the strength of synaptic transmission. However, a third method of reducing transmitter release, induction of "low frequency depression", failed to enhance the ability of the peptide to modulate chemical synapses. Thus, the