GABA_A-Mediated Local Synaptic Pathways Connect Neurons in the Rat Suprachiasmatic Nucleus

Abstract

The suprachiasmatic nucleus (SCN) in mammals functions as the biological clock controlling circadian rhythms, but the synaptic circuitry of the SCN is largely unexplored. Most SCN neurons use the neurotransmitter gamma-aminobutyric acid (GABA), and anatomic studies indicate many GABAergic synapses and local axon collaterals; however, physiological evidence for synaptic communication among SCN neurons is indirect. We have used three approaches to investigate local circuitry in the SCN in acute hypothalamic slices from rat. First, tetrodotoxin was used to block action-potential-dependent synaptic release, which resulted in a decrease in the frequency of spontaneous synaptic currents in SCN neurons, suggesting that spontaneously active neurons in the slice connect synaptically to SCN neurons. Postsynaptic currents in SCN neurons were also evoked by the selective stimulation of other SCN neurons with glutamate, which avoids direct activation of axons that might originate outside the SCN. Two different methods of glutamate microapplication (i.e., pressure ejection and ultraviolet photolysis of caged glutamate) indicated that SCN neurons receive GABAA-receptor-mediated synaptic input from other SCN neurons. In contrast, glutamate-receptor-mediated synaptic connections between SCN neurons were not detected. The GABAergic synapses that comprise the network described here could conceivably be a substrate for the synchronization and amplification of the circadian rhythm of SCN firing. Alternatively, this circuitry might mediate other aspects of clock function such as the integration of environmental and physiological information.

Pseudorabies virus expressing enhanced green fluorescent protein: A tool for in vitro electrophysiological analysis of transsynaptically labeled neurons in identified central nervous system circuits

“…Most SCN neurons receive robust local inhibitory input (19). We examined spontaneous IPSCs in PRV-EGFP-labeled SCN neurons to determine if inhibition was also intact in these cells (Fig.…”

Excitatory Actions of GABA in the Suprachiasmatic Nucleus

“…SCN neurons receive a tonic GABA A receptor-mediated synaptic input that, at least partly, originates within the SCN itself and peaks during the night (Kim and Dudek, 1992;Jiang et al, 1997;Strecker et al, 1997;Itri et al, 2004). Early work suggested that GABA could evoke excitatory responses during the day (Wagner et al, 1997), but the generality of this finding has been questioned by other laboratories (Gribkoff et al, 1999;De Jeu and Pennartz, 2002).…”

Excitatory and inhibitory local circuit input to the rat dorsal motor nucleus of the vagus originating from the nucleus tractus solitarius

“…This technique selectively depolarizes soma and dendrites, but not axons. Glutamate photostimulation involves focal release of glutamate using ultraviolet light projected into the slice [6,11,19,29,36]. This method of stimulation avoids physical contact with the slice and affords the advantage of being able to stimulate neurons at multiple, discrete sites within the slice.…”