Development of a Novel Technique for Recording Neuronal Activity in the Hypothalamic Suprachiasmatic Nucleus

Introduction The temporal organization of physiological events is a fundamental feature of living things. Nearly all biological events across a broad taxonomic range differ markedly during the day compared to the night. The day-night differences in these processes are thought to have adaptive value to the organisms such as mobilizing energy stores in anticipation of energy-intensive activities like foraging and mating.1 These daily rhythms are not simply passive responses to a changing environment, rather they are programmed changes that are driven by a biological clock. Biological clocks have now been characterized for such a wide range of organisms including plants, yeast, algae, bacteria, insects, amphibians, reptiles, fish, birds, and mammals, that they are now considered to be a fundamental feature of all living things.2 Formal models of biological clocks have shown that all circadian clocks share three fundamental properties. The first and most important property is that the clock oscillates in the absence of all external time cues, thereby demonstrating its endogenous nature.3 The second property requires that a clock can be reset.4 Resetting means that there is a mechanism by which the clock can be synchronized to the external environment (e.g. the daynight cycle). This feature is critical because circadian clocks always oscillate at periods different from 24 hours and, therefore, require a mechanism to synchronize them to the solar day. The third clock property is temperature compensation.5 Whereas most biological processes are highly sensitive to temperature, the biochemical events that comprise a clock must be resistant to temperature so that they can keep time across the broad range of temperature experienced by living things. Time-keeping would be severely compromised if the oscillation of the clock were to change its velocity in response to temperature.5 Even though mammals maintain their temperature within a relatively narrow range, the mammalian clock has preserved this function during its evolution.6 In addition to these three formal properties, recent neurobiological studies have suggested a fourth important property for clocks. This property requires intercellular synchronization for accurate timekeeping. In the mammalian brain, the region that contains the clock is called the hypothalamic suprachiasmatic nuclei (SCN). These nuclei are bilateral structures directly dorsal to the optic chiasm and adjacent to the third ventricle. Each SCN is comprised of approximately 10,000 neurons.7 A wide range of studies have demonstrated conclusively that this structure contains a clock.8 For example, when fetal SCN tissue is transplanted into animals that have been made arrhythmic through lesions of their SCN, circadian rhythms are restored.9 Electrophysiological studies of the SCN have shown that each neuron can function as an independent oscillator.10 Cultured neurons within the isolated mouse SCN express a circadian rhythm in spontaneous firing rate for weeks.11 Therefore, for this structure to keep time accurately and to synchronize all of the major physiological systems in an organism, all of the individual neurons in the SCN must be synchronized to produce a coherent output. Taken together, this means that the generation of circadian rhythms of behavior requires both oscillaNicole D’Arcy