Schematic illustrating possible pathways involved in LC activation by different stimuli. The top coronal section is at the level of LC. The bottom coronal section is at the level of the PGi. Substantial evidence suggests that LC activation produced by nicotine, opiate withdrawal, and footshock is mediated by excitatory amino acid afferents (open circle, dotted line) from PGi. Although LC activation by bladder distention requires excitatory amino acid afferents to the LC, the source of these afferents have not been determined (indicated by "?"). LC activation by hemodynamic stress requires CRF input to the LC (solid circles, solid line), although the source of these afferents has not been established (indicated by "?"). Potential sources of CRF input to LC that may mediate this activation include the PGi, pericoerulear CRF neurons such as Barrington's nucleus (B), or the dorsal cap of the paraventricular nucleus of the hypothalamus (not shown). Breaks in the lines leading from the stimulus to the brainstem indicate that the circuitry underlying the activation of LC afferents by the different stimuli is unknown.