Abstract

Norepinephrine-³H was introduced into the cerebroventricular system of the rabbit by a simple intracisternal approach that is described in detail. The size of the rabbit brain and spinal cord permitted ready investigation of the fate of injected norepinephrine-³H not only in the whole brain and five of its subdivisions (telencephalon, hypothalamus, midbrain, medulla-pons and cerebellum) but also in three regions of the spinal cord (cervical, thoracolumbar and lumbosacral). The initial distribution of injected norepinephrine-³H was not as closely correlated with the distribution of endogenous norepinephrine as has been reported for the rat. Instead, the distance of each region from the injection site appeared to be the major factor determining the relative uptake. For example, uptake was progressively lower in the cervical, thoracolumbar and lumbosacral segments of the spinal cord even though these three areas contain similar concentrations of endogenous norepinephrine. There was a multiphasic decline in norepinephrine-³H concentration in whole brain, whole spinal cord and in each of the brain and cord regions examined. Initial rapid disappearance in the first hour was followed by a slower, single-exponential decay phase observed between 6 and 12 hours after norepinephrine-³H injection. The half-lives for this second phase of norepinephrine-³H disappearance were longest in the hypothalamus, thoracolumbar cord and midbrain and shortest in the cerebellum, medulla-pons and cervical cord.