Plasma cortisol and hypothalamic monoamine responses in yellow perch Perca flavescens after intraperitoneal injection of lipopolysaccharide.

The concentrations of monoamines in the hypothalamus were determined in yellow perch Perca flavescens before and after injection with lipopolysaccharide (LPS; 3 mg kg⁻¹ fish weight) or saline to test for the presence of neurochemical changes potentially associated with changes in plasma cortisol characteristic of intraperitoneal (ip) challenge with LPS. In the first experiment, yellow perch were injected with saline or LPS and the hypothalamus removed and plasma sampled before and at 0.5, 1.5, 3.0, and 6 h after injection. Plasma cortisol was elevated in both saline- and LPS-injected fish through 1.5 h after injection and returned to levels resembling pre-injection by 3 h after injection. Significantly higher amounts of cortisol in plasma from LPS-injected relative to saline-injected fish were observed 6 h following injection. A significant decrease relative to levels observed 0.5-3 h after handling was observed in serotonin concentrations at 6 h following LPS and saline injection with a concomitant increase in the ratio of 5-hydroxyindoleacetic acid:serotonin. In the second experiment, hypothalamic monoamines were sampled before and at 3, 6, 9, 12, and 24 h after injection with LPS or saline. Significant increases from pre-injection levels were observed in the ratio 5-hydroxyindoleacetic acid:serotonin at 9, 12, and 24 h after injection, but no differences were detected between LPS- and saline-injected fish. These results support a model linking serotonergic system activation following handling stress, but no correlations with the sustained elevations of plasma cortisol associated with inflammatory challenge were observed.