Both phasic sensory stimulation and tonic pharmacological activation increase Fos-like immunoreactivity in the rat locus coeruleus.

Neuronal activation increases levels of Fos protein, the product of the early immediate gene c-fos. Since most studies used stimuli that evoke sustained elevations in activity; the present study examined Fos-like immunoreactivity (Fos-LI) in the nucleus locus coeruleus (LC). Halothane-anesthetized rats were given either footshock to elicit phasic activation of LC neurons or yohimbine (10 mg/kg, i.p.) to induce a tonic increase in firing. Both treatments markedly increased Fos-LI in a subpopulation of LC neurons. These results demonstrate that c-fos induction does not require high tonic levels of neuronal activity and that Fos-LI may underestimate the proportion of LC neurons neurophysiologically activated by a given stimulus and suggest that factors beyond neuronal activity per se contributes to c-fos expression.