Axoplasmic transport of norepinephrine in the locus coeruleus-hypothalamic system in the rat

The axoplasmic transport of norepinephrine (NE) between the nucleus locus coeruleus (LC) and the hypothalamus has been measured by three methods which employ the stereotaxic injection of [3H]catecholamines in the rat brain. [3H]NE is synthesized from [3H]dopamine (DA) injected into the LC and transported at a rate of 0.8-0.9 mm/h. This rate is probably an underestimate since it includes the time required for uptake and conversion of [3H]DA to [3H]NE prior to transport. Measurement of transport of [3H]NE between the posterior and anterior hypothalamus gives a rate of 2 mm/h which is independent of [3H]DA uptake and [3H]NE synthesis. This rate is in good agreement with the 1.9 mm/h figure calculated for [3H]NE transport in the system after LC injection of [3H]NE and represents a closer estimate of the true rate of axoplasmic transport than the 0.8-0.9 mm/h estimate. Transport of [3H]NE occurs through primarily ipsilateral NE fibers running in the median forebrain bundle and is blocked by 6-hydroxydopamine lesions in the bundle. Early appearing [3H]NE in the hypothalamus forms a relatively stable pool which is unaffected by median forebrain bundle lesions and is essentially unchanged by [3H]NE transported from the LC. This pool appears to be derived from non-specific spread of [3H]DA after injection, probably through the cerebrospinal fluid.