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(A) Mean number of Fos-like immunoreactive cells in the locus coeruleus of 7-day-old rats that were chronically exposed to morphine (M) or saline (S) (labeled on the X-axis) and then given an acute injection of naltrexone (N) or saline (S) (N = 8 for M-N and M-S, N = 6 for S-S and S-N; mean ± SE). Asterisk (*) denotes significance between the M-N group and the combined controls (M-S, S-S, and S-N). (B) Camera lucida drawing of the locus coeruleus.
