Department of Biology
Master of Science
Circadian rhythms have been shown to exist in all studied species and play an important role in their survival. The cockroach is an excellent model organism in which to study circadian systems because of the wealth of background information regarding both its anatomy and circadian system. Clock activity measured electrophysiologically in the cockroach system is 180 degrees out of phase with locomotor activity, with the maximum neural output occurring in the morning, twelve hours prior to the onset of locomotor activity. This inverse relationship of locomotor and neural activity suggests that the clock output may be inhibitory. In vertebrates, the suprachiasmatic nuclei (SCN) is the site of the biological clock, and it has been shown that gamma-aminobutyric acid (GABA) plays an important role in the vertebrate circadian system. The importance of GABA in the vertebrate SCN and its presence in the brain of the cockroach made the inhibitory neurotransmitter GABA a prime candidate for related studies. To determine if GABA was expressed according to a daily rhythm, a brain extract was collected from individual cockroaches at different times of the day and the levels of GABA were determined using HPLC coupled with fluorometric detection. o-Phthalaldehyde was used to make fluorescent derivatives of GABA and the surrogate internal standard, AVA, that could be detected by the system. The levels of GABA determined ranged from 530 ±49 ng/brain at 1800 h, the time when locomotor activity is initiated and neural output from the isolated clock is low (Page 1989), to 793 ± 50 ng/brain at 1200 h, the time of maximum neural output from the clock measured electrophysiologically. These GABA levels and the times at which they occur suggest that the clock may be inhibitory in its action. However, an ANOVA performed on the data did not show a significant difference between the GABA levels at the 95% confidence level, but did at the 84% confidence level (P < 0.162). The sample set was small (n=3 for each time period), and it is possible that with a larger sample set a significant difference at the 95% confidence level could be detected.
Newton, Jonathan, "Temporal Changes in the Levels of Gamma-Aminobutyric Acid in the Brain of the Cockroach, Leucophaea Maderae" (1995). Masters Theses & Specialist Projects. Paper 917.