Detection and regulation of tyrosine hydroxylase mRNA in catecholaminergic terminal fields: possible axonal compartmentalization.

Reverse transcriptase coupled with nested polymerase chain reaction amplification (RT/nested-PCR) was used to detect mRNA encoding tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, in adult rat cerebellum, striatum, and pituitary neurointermediate lobe (NIL). These regions receive catecholaminergic innervation from the locus coeruleus, substantia nigra, and arcuate and periventricular nuclei of the hypothalamus, respectively, but are devoid of catecholamine-synthesizing cells. The RT/nested-PCR products, which were generated using primers located on different exons of the tyrosine hydroxylase gene, indicate that the tyrosine hydroxylase mRNA detected is devoid of introns and, hence, is processed. These findings raise the possibility that tyrosine hydroxylase mRNA may be axonally transported. Using the same RT/nested-PCR protocol, we were unable to detect mRNA encoding dopamine beta-hydroxylase, a different catecholaminergic biosynthetic enzyme, in either cerebellum, striatum, or NIL pituitary tissue. Thus, the detection of tyrosine hydroxylase mRNA in catecholamine terminal regions is biochemically specific. We were unable to detect tyrosine hydroxylase mRNA in optic nerve, indicating some degree of anatomical specificity as well. Expression of tyrosine hydroxylase mRNA in the cerebellum was markedly increased by subcutaneous administration of the catecholamine-depleting agent, reserpine, suggesting that tyrosine hydroxylase mRNA in catecholamine terminal regions may be functionally important. This finding also indirectly supports the hypothesis that tyrosine hydroxylase mRNA can be axonally transported since the ability of reserpine to induce expression of this transcript in conventional catecholamine cell groups is considered secondary to catecholamine depletion, and cerebellar cells do not synthesize catecholamines. Finally, lesions of the nigrostriatal pathway significantly decreased levels of tyrosine hydroxylase mRNA in the striatum, providing strong additional support for this hypothesis.