The discovery that a neurotransmitter can be present within a neuron that does not synthesize it has led to the hypothesis that neurons may borrow transmitters through uptake from extracellular sources. A borrowed transmitter is one that is not synthesized by a neuron but is instead acquired through uptake. It is subsequently released and acts on a target organ or neuron. This study will use the hindgut neurons (HGNs) of crayfish to test the borrowed transmitter hypothesis. 1) 5-HT synthesis. The HGNs exhibit labile levels of 5-HT that can be attributed to either uptake of extrinsically controlled levels of 5-HT or facultative synthesis of 5-HT. 2) Regulation of 5-HT levels. Sensory input may stimulate facultative synthesis, local release of 5-HT or alter the effectiveness of the serotonin transporter. The circuitry and mechanisms involved in controlling levels of 5-HT in the HGNs will e examined. 3) The role of 5- HT in the HGNs. The lability of 5-HT in HGNs may also be controlled by variable release patterns. The lability of 5-HT in HGNs may also be controlled by variable release patterns. This study will also microvoltammetry and [H3]5-HT to trace release of 5-HT from the HGNs. If release is detected, electrophysiological techniques will be used to test whether released 5-HT affects hindgut motility. If release is detected, electrophysiological techniques will be used to test whether 5- HT affects hindgut motility. If release is not detected, electrophysiological techniques will be used to test whether released 5- HT affects hindgut motility. If release is not detected then 5-HT may affect levels of endogenous transmitter. This study will use electrophysiological techniques to examine exteroceptive and proprioceptive sensory input to the HGNs. Quantitative immunohistochemical techniques and HPLC will be used to examine changes in 5-HT levels under different stimulation paradigms. The indication that neurons can borrow a transmitter, particularly 5-HT, may have broad clinical implications since the use of selective serotonin re- uptake inhibitors, often used to treat depression and anxiety, would have a profound effect on borrowing effect on borrowing neurons and their targets.