Organization of Afferents to the LC

The organization of afferents to LC has been a subject of controversy for a long time. Early retrograde tracing studies (using the horseradish peroxidase-diamino-benzidine technique) led to the conclusion that LC receives afferents from more than 30 regions (including the dorsal prefrontal cortex, amygdala, ventral teg-mental area, raphe nucleus, contralateral LC and deep cerebellar nuclei, and dorsal horn of the spinal cord). However, more recent experiments using new tracers allowing more focal injections restricted the source of afferents to only two structures: the nucleus paragi-gantocellularis (PGi) and the nucleus prepositus hypoglossi (PrH) in the medulla oblongata. Antero-grade analysis further confirmed that the ventral tegmental area, the dorsal horn of the spinal cord, the rostral solitary tractus, and the prefrontal cortex do not project to the LC but to adjacent structures.

Electrophysiological studies did not confirm, however, that PGi and PrH are the sole sources of afferents to the LC. First, the destruction of both PGi and PrH does not prevent LC responses to sensory stimuli. Second, it remains that stimulation of the ventral tegmental area, prefrontal cortex, and raphe nucleus alters LC firing, a finding suggesting that their projections to structures adjacent to the LC might be implicated.

The answer to this paradox came from light and electronic microscopic analyses, which revealed that many LC neurons extend their dendrites to pericer-ulear regions (rostromedial and caudal juxtaependy-mal regions). These pericerulear regions are sometimes denoted as the shell of the LC and distinguished from the core of the LC that represents the LC proper. These shell dendrites are selectively targeted by a number of extrinsic afferent inputs that do not project appreciably into the LC core.

Although it is not the primary LC afferent, PGi remains an important afferent to the LC and is a key sympathoexcitatory region in the brain. There are widespread afferents to the PGi from diverse brain areas. The PGi may be an integrative pathway for activating the LC by a variety of mechanisms because it is involved in the control of arterial blood pressure, cardiopulmonary reflexes, and sympathetic function. It has been suggested that the peripheral sympathetic nervous system is activated in parallel with the LC by projections to both areas from the PGi.

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