Date of Award

7-22-1999

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Willis K. Samson

Abstract

The targeting of oxytocin (OT) receptive cells with a novel cytoxin (ricin A-chain oxytocin conjugated to oxytocin; rAOT) has revealed a potential role for OT in the osmolality-related inhibition of salt appetite seen in two model systems. The experiments in this study were designed to test the hypothesis that intraventricular (icv) treatment with rAOT results in the loss of [I 125]-OVTA binding sites in the central amygdala in these two model systems (Experiment 1: Mannitol Challenge; Experiment 2 PEG/Mannitol Challenge). Experiment 1 (Mannitol Challenge; produces hyperosmotic/hyponatremic conditions in the rat) and Experiment 2 (PEG/Mannitol Challenge; produces hypovolemic/hyperosmotic/hyponatremic conditions in the rat) were conducted to evaluate the effects of these conditions on water and saline intakes in rAOT- and rA-treated (unconjugated ricin A-chain, toxin control) rats. On the day of Experiment 2, both groups were given subcutaneous injections of PEG and deprived of food and fluids for 24 hours. On the following day both groups of PEG-treated animals were injected with 2 M mannitol (ip). The rats were allowed access to both water and 0.3 M NaCl solutions 30 minutes after the injections, and subsequent fluid intakes were measured every hour for five hours. The results of cumulative fluid intakes in Experiment 1 and Experiment 2 indicate that the rAOT-treated group did not drink significantly more water during the five hour period when compared to the rA-treated rats. However the cumulative 0.3 M saline intakes in the rAOT treated rats in both experiments were significantly elevated (p < 0.0001) when compared to rA-treated rats. Following the five hour measurements of fluid intake in the above experiments (Experiment 1 and Experiment 2), the rats were sacrificed and the brains were immediately removed, frozen on dry ice, then stored at −20 C prior to sectioning. In Experiment 1, tissue sections including the central amygdala (CeA), the ventromedial nucleus of the hypothalamus (VMH) and the subiculum were collected. In Experiment 2, the CeA and the VMH were the regions obtained. These slides were then used to examine [I125]-OVTA binding using autoradiographic analysis (phosphor imaging). These results indicated that in both experiments [I125]-OVTA mean average binding in the VMH of the rAOT-treated groups were significantly elevated (p < 0.05) when compared to the rA-treated rats. The data also indicates that in Experiment 1 [I125]-OVTA mean volume binding in the CeA of the rAOT-treated group was significantly elevated when compared to the rA-treated control group. We hypothesize that the observed increase in [I125]-OVTA binding in the CeA and VMH of the rAOT-treated rats was due to astrogliosis following neuronal destruction and that the invading astrocytes brought with them a new population of OT-binding sites.

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