RAPID COMMUNICATION GABAA-Mediated Local Synaptic Pathways Connect Neurons in the Rat Suprachiasmatic Nucleus

the synaptic circuitry of the SCN is largely unexplored. Most SCN Sprague-Dawley rats were housed under a 12-h light:dark cycle neurons use the neurotransmitter g-aminobutyric acid (GABA), and recordings were made predominantly during the light phase. and anatomic studies indicate many GABAergic synapses and local Coronal slices (150–400 mm) of hypothalamus were cut and mainaxon collaterals; however, physiological evidence for synaptic tained in artificial cerebrospinal fluid (ACSF) containing (in mM) communication among SCN neurons is indirect. We have used 125 NaCl, 2.5 KCl, 1 CaCl2 , 1 MgCl2 , 24 NaHCO3, 1.25 NaH2PO4, three approaches to investigate local circuitry in the SCN in acute and 10 glucose, bubbled with 5% CO2-95% O2, pH 7.4. Recordings hypothalamic slices from rat. First, tetrodotoxin was used to block were made at room temperature (20–227C) from 2to 4-week-old action-potential-dependent synaptic release, which resulted in a rats. decrease in the frequency of spontaneous synaptic currents in SCN Patch pipettes (3–5 MV) contained (in mM) 140 cesium gluconeurons, suggesting that spontaneously active neurons in the slice nate (or 130 KCl in photostimulation experiments) , 1 CaCl2 , 1 connect synaptically to SCN neurons. Postsynaptic currents in SCN MgCl2 , 1 NaCl, 5 bis-(o-aminophenoxy)-N,N,N *,N *-tetraacetic neurons were also evoked by the selective stimulation of other acid, or 5–10 ethylene glycol-bis(b-aminoethyl ether)SCN neurons with glutamate, which avoids direct activation of N,N,N *,N *-tetraacetic acid, and 2–4 magnesium ATP, pH 7.2. axons that might originate outside the SCN. Two different methods Whole cell currents were amplified, filtered (corner frequency 5 of glutamate microapplication (i.e., pressure ejection and ultraviokHz, Axopatch 1D), and stored digitally. Recordings were refillet photolysis of caged glutamate) indicated that SCN neurons tered (1 or 2 kHz), redigitized (5 kHz), and analyzed off-line receive GABAA-receptor-mediated synaptic input from other SCN (pClamp software) . neurons. In contrast, glutamate-receptor-mediated synaptic connecThe frequencies and amplitudes of spontaneous PSCs were meations between SCN neurons were not detected. The GABAergic sured before and after the addition of tetrodotoxin (TTX) to block synapses that comprise the network described here could conceivaction-potential-dependent synaptic activity. Cumulative amplitude ably be a substrate for the synchronization and amplification of the distributions of inhibitory PSCs (IPSCs) were compared with the circadian rhythm of SCN firing. Alternatively, this circuitry might use of the Kolmogorov-Smirnov test. The one-sample x 2 test was mediate other aspects of clock function such as the integration of used to assess the effect of TTX on IPSC frequency. Data are environmental and physiological information. generally expressed as means { SE.