The blood-brain barrier for catecholamines - revisited.

Although it is well-recognized that catecholamines are generally unable to penetrate the developed blood-brain barrier (BBB) to gain entry into brain, except at circumventricular sites where the BBB is absent or deficient, ontogenetic development of this barrier seems to have escaped systematic study. To explore BBB development, several approaches were used. In the first study rats were treated once on a specific day of postnatal ontogeny, as early as the day of birth, with the neurotoxin 6-hydroxydopamine (6-OHDA; 60 mg/kg), and then terminated in adulthood for regional analysis of endogenous norepinephrine (NE) content of brain. In another study, rats were treated once, on a specific day of postnatal ontogeny, with the BBB-permeable neurotoxin 6-hydroxydopa (6-OHDOPA; 60 mg/kg) following pretreatment with the BBB-impermeable amino acid decarboxylase inhibitor carbidopa (100 mg/kg IP), then terminated in adulthood for regional analysis of endogenous NE content of brain. In the third study rats were treated once, on a specific day of postnatal ontogeny, with the analog [3H]metaraminol, and terminated 1 hour later for determination of regional distribution of tritium in brain. On the basis of [3H]metaraminol distribution and NE depletions after neurotoxin treatments, it is evident that the BBB in neocortex, striatum, cerebellum and other brain regions forms in stages over a period of at least 2 weeks from birth. Moreover, because the BBB consists of several element (physical-, ion-restrictive-, and enzymatic-barrier), the method employed will derive data mainly applicable to the targeted aspect of the barrier, which may or may not necessarily coincide with elements of the barrier that have a different rate of ontogenetic development. Accordingly, it is evident that some aspects of physical- and ion-restrictive elements of the BBB form within approximately the first week after birth in rat neocortex and striatum, while enzymatic elements of the BBB form more than than 2 weeks later. Regardless, the BBB forms at earlier times in forebrain vs hindbrain regions.