| Pulsatile release of gonadotropin-releasing hormone (GnRH) is requisite for fertility. The rhythms generated by GnRH neurons, however, are not well characterized, nor are the interaction among GnRH neurons that result in secretory patterns. To describe the rhythmic output of GnRH neurons at the individual cell and network levels, extracellular recordings of firing patterns were made using two model systems. The first series of experiments utilized immortalized GnRH neurons (GT1 cells) grown onto multi-micro electrode arrays to observe artificial network activity among multiple cells. Results indicated that GT1-7 cells at several different locations within a culture displayed low frequency rhythms. When combined, these low frequency component rhythms produced an overall pattern that was consistent with previous reports of secretory pulse intervals, suggesting network interactions produce appropriate secretory patterns. For the second set of experiments, targeted extracellular recordings were made from green-fluorescent-protein-expressing GnRH neurons in coronal brain slices. Results demonstrated that GnRH neurons in display low frequency rhythms consistent with secretory patterns of the reproductive neuroendocrine axis in rodents. Blockade of the dominant ionotropic neurotransmitters gamma-aminobutyric acid and glutamate did not change patterns in neurons recorded from ovariectomized (OVX) mice. Blockade in OVX plus estradiol implanted mice, however, resulted in OVX-like patterns in 50% of GnRH neurons; these were located in the medial preoptic area. This suggests that estradiol effects can be conveyed to GnRH neurons by transmitters that gate ionotropic receptors. In addition, Fourier spectral analysis identified high frequency burst firing rhythms and low frequency rhythms in the range of secretory intervals. Peaks and nadirs in the low frequency rhythm formed from changes in the interval between bursts of action currents (high frequency rhythm), thus demonstrating a possible link between these rhythms. Collectively, these results suggest at least three types of rhythms in GnRH neurons: (1) a low frequency rhythm occurring at secretory intervals, (2) a high frequency burst firing rhythm, and (3) a rhythm that arises from the integrated activity of many GnRH neurons. The role of each of these rhythms in generating secretory patterns in GnRH neurons is discussed, and a model of rhythm interactions is presented. |
