Synchronous activity in locus coeruleus results from dendritic interactions in pericoerulear regions

Locus coeruleus (LC) neurons in brain slices from adult rats were studied using intracellular and extracellular recordings to investigate synchronous activity. Spontaneous field potentials were recorded with extracellular electrodes in solutions containing tetraethylammonium chloride (10 mM) and BaCl₂ (1 mM). These field potentials were found throughout but not outside the LC cell body region. No field potentials were observed in control solutions. Paired recordings showed that field potentials were synchronous in all areas of the LC. The synchronous activity was resistant to tetrodotoxin (1 μM) and to the neurotransmitter receptor blockers D-2- amino-5-phosphonopentanoic acid, bicuculline, 6-cyano-7-nitroquinoxaline- 2,3-dione, idazoxan, and strychnine, suggesting that this activity was not synaptically driven. Field potentials were also synchronous with oscillations in membrane potential recorded with intracellular electrodes. The oscillations in membrane potential were 5-30 mV in amplitude and had a biphasic waveform. Neither the frequency nor the waveform of the oscillations was dependent on the membrane potential. The glycyrrhetinic acid derivative carbenoxolone and intracellular acidification with CO₂ disrupted synchronous activity, suggesting a role of electrotonic coupling. When the cell body region of the LC was isolated from the pericoerulear dendritic regions by sectioning the slice rostral and caudal to the cell body region, synchronous activity was reduced or abolished. Dendritic interaction in the pericoerulear region was also indicated by improved voltage control of the opioid-induced potassium current, as indicated by a shift in the reversal potential to the potassium equilibrium potential. The results suggest that electrical interactions between dendrites outside the cell body region can account for synchronous activity within the nucleus.