Electrical coupling synchronizes subthreshold activity in locus coeruleus neurons in vitro from neonatal rats

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Abstract
Locus coeruleus neurons in brain slices prepared from neonatal rats have rhythmic oscillations in membrane potential at frequencies ranging from 0.3 to 3 Hz. Recordings from pairs of neurons separated by 50–300 microns showed that this oscillatory activity was synchronized at ages less than 24 d. Slices cut from rats 24–27 d old showed rhythmic activity which was only partially synchronous between cell pairs, but full synchrony could be restored by superfusion with tetraethylammonium (2 mM) and/or barium (2 mM). No synchronous rhythmic activity was observed between neurons in slices from rats 40 d old, even in the presence of tetraethylammonium and barium. In those cells in which rhythmic potential oscillations were synchronous, action potentials occurring in one cell were not observed in the second cell. Electrotonic coupling was directly demonstrated between 41% of neurons (12 of 29 pairs) in slices from rats less than 10 d old but not in tissue from older rats (4 pairs). The input resistance of neurons from neonatal rats (less than 15 d old) was about half (81 M omega) that measured under identical conditions from neurons from adult rats (213 M omega). The electrotonic potential in cells from rats less than 15 d old was best fit by a double exponential, whereas that from adults was best fit by a single exponential. The results demonstrate that significant electrical coupling occurs among locus coeruleus neurons from neonatal rats; this appears to decline with age. The coupling serves as a low-pass filter and causes the synchronized occurrence of membrane potential oscillations. Such a rhythmic background activity within the entire nucleus may be important for the widespread trophic role of the noradrenergic neurons during development.