Development of the adrenergic phenotype: increase in adrenal messenger RNA coding for phenylethanolamine-N-methyltransferase

Neurotransmitter Switching? No Surprise

Among the many forms of brain plasticity, changes in synaptic strength and changes in synapse number are particularly prominent. However, evidence for neurotransmitter respecification or switching has been accumulating steadily, both in the developing nervous system and in the adult brain, with observations of transmitter addition, loss, or replacement of one transmitter with another. Natural stimuli can drive these changes in transmitter identity, with matching changes in postsynaptic transmitter receptors. Strikingly, they often convert the synapse from excitatory to inhibitory or vice versa, providing a basis for changes in behavior in those cases in which it has been examined. Progress has been made in identifying the factors that induce transmitter switching and in understanding the molecular mechanisms by which it is achieved. There are many intriguing questions to be addressed.

Optic nerve-dependent changes in adult frog tectal cell phenotypes

Optic nerve activity helps determine the placement of retinal ganglion cell terminals in the optic tectum of the frog. We investigated whether the presence of this nerve might also influence a characteristic of its target structure, neurotransmitter biosynthesis. We performed unilateral optic nerve transections on adult animals and assayed the percent and intensity of substance P- and serotoninlike immunoreactive (SP-ir and 5-HT-ir, respectively) cells in the deafferented and afferented tectal lobes. Regeneration of the optic nerve was prevented. The percent of SP-ir cells in the afferented tectal lobes was significantly less than that in the deafferented ones either 6 weeks or 5 months following optic nerve lesion. Comparison to normal animals indicated that the change in SP-ir expression was due to a decrease in the percent of immunoreactive cells in the afferented tecta ipsilateral to the optic nerve lesion. The serotoninlike immunoreactivity of tectal cells was also significantly different in the two lobes following optic nerve lesions. This difference resulted from an increase in the percent of 5-HT-ir cells in the deafferented tectum. In addition, the intensity of 5-HT-ir cells in the deafferented lobe was significantly greater than in the afferented one. The staining intensity of SP-ir cells underwent only a transient, relative decrease in the deafferented tectum. We conclude that the optic nerve does regulate substance P and serotonin expression in the tectum, but that this regulation likely occurs through different pathways.

Steroid-induced inhibition of adenosine transport in cultured chromaffin cells

1. Adenosine transport is subjected to regulation by hormones. Glucocorticoids, sexual steroids, and retinoic acid inhibit adenosine transport in chromaffin cells after a long-term incubation period (24 hr). No effects were observed after a short-term incubation period (10 min). 2. The kinetic parameters of transporters were studied. No significant changes were observed for the affinity constant (Km), whose value remains at 1 +/- 0.2 microM after 24-hr incubation in the presence of these compounds. The maximal velocity (Vmax) was significantly modified, with a decrease of about 20% in all cases. 3. NBTI binding was not modified in its affinity constant or maximal bound capacity (Bmax) by the presence of these compounds for a 24-hr incubation period. Thus the efficiency of transporters (quotient Vmax/Bmax) changed from 10.9 +/- 0.08 adenosine molecules transported per transporter per sec in the control cells to 9.1 +/- 0.07 in hormone-treated cultured cells. 4. The thyroid hormone (T3) significantly increased adenosine transport in a long-term incubation period in chromaffin cells (24 hr). This activatory effect is antagonized by steroid hormones and retinoic acid.

Glucocorticoids stimulate transcription of the rat phenylethanolamine N-methyltransferase (PNMT) gene in vivo and in vitro

1. Phenylethanolamine N-methyltransferase (PNMT) is regulated by glucocorticoid hormones. This study investigates the ability of glucocorticoids to modulate transcription of the rat PNMT gene in vivo and in vitro. 2. In the adrenal glands of hypophysectomized (HPX'd) rats, the synthetic glucocorticoid dexamethasone (DEX) stimulates production of PNMT mRNA. Quantitative hybridization reveals that the levels of PNMT mRNA increase approximately threefold in total and poly(A)+RNA after 4 days of DEX treatment of HPX'd rats, a level which is maximal for this treatment. 3. ACTH, the hormonal stimulus of glucocorticoid biosynthesis in the adrenal cortex, enhances PNMT mRNA production to levels comparable to that achieved with DEX in this system. The steroid responsiveness of PNMT message production is specific for glucocorticoids. DEX also increases PNMT mRNA in the brain stem, although the magnitude and speed of response are lower than observed in the adrenal gland. 4. Additional confirmation of the inductive ability of glucocorticoids is demonstrated by the increase in PNMT immunoprecipitated following translation in vitro of adrenal RNAs from DEX-treated rats. Furthermore, the PNMT mRNA signal obtained by in situ hybridization histochemistry in adrenal sections and in primary cultures of dispersed rat adrenal medullae reveals that DEX effects on PNMT mRNA can be elicited both in vivo and in vitro. 5. Specifically, glucocorticoids exert their effects on expression of PNMT mRNA by elevating the rate of PNMT gene transcription: a 2.3-fold increase in PNMT transcription persists for 18 hr following DEX treatment of HPX'd rats. In summary, this study establishes that glucocorticoids directly and rapidly stimulate transcription of the rat PNMT gene.

Effects of cold exposure on rat adrenal tyrosine hydroxylase: an analysis of RNA, protein, enzyme activity, and cofactor levels

Long-term cold exposure (5-7 days) is known to induce concomitant increases in the levels of adrenomedullary tyrosine hydroxylase (TH) RNA, protein, and enzyme activity. In this report, we compare the time courses of these changes and investigate the effects of cold exposure on the levels of biopterin, the cofactor required for tyrosine hydroxylation. After only 1 h of cold exposure, TH mRNA abundance increased 71% compared with nonstressed controls. Increases in total cellular TH RNA levels were maximal (threefold over control values) within 3-6 h of cold exposure and remained elevated throughout the duration of the experiment (72 h). TH protein levels increased rapidly after 24 h of cold exposure and reached a maximal value threefold above that of controls at 48-72 h. Despite the relatively rapid and large elevations in TH RNA and protein content, only modest increases in TH activity were detected during the initial 48 h of cold exposure. Adrenomedullary biopterin increased rapidly after the onset of cold exposure, rising to a level approximately twofold that of the nonstressed controls at 24 h, and remained at this level throughout the duration of the stress period. Taken together, the results of this time course study indicate that cold-induced alterations in adrenal TH activity are mediated by multiple cellular control mechanisms, which may include pre- and posttranslational regulation. Our findings also suggest that cold stress-induced increases in the levels of the TH cofactor may represent another key event in the sympathoadrenal system's response to cold stress.

Regulation of phenylethanolamine N-methyltransferase (PNMT) mRNA in the rat adrenal medulla by corticosterone

In the adrenal medulla of adult rat, physiological levels of glucocorticoid hormones are required to maintain the catalytic activity of the epinephrine-synthesizing enzyme, phenylethanolamine N-methyltransferase (PNMT). The present study was undertaken to determine whether glucocorticoid regulation of PNMT occurs at the level of mRNA coding for PNMT. Adult male Sprague-Dawley rats were hypophysectomized (HPX) and killed after 2 weeks; pellets of corticosterone were implanted for 1, 3 or 7 days prior to killing. Determinations were made of plasma corticosterone levels, adrenal PNMT activity and PNMT mRNA levels by Northern gel analysis. HPX resulted in a decrease in plasma corticosterone to undetectable levels, and decreases in PNMT activity and PNMT mRNA levels to 1 and 18% of the levels observed in sham rats, respectively. Corticosterone replacement produced high prolonged plasma levels of corticosterone which were 10 times those of sham rats, and significantly increased levels of PNMT activity and mRNA. However, corticosterone replacement failed to restore PNMT activity and mRNA levels fully. These results suggest that the maintenance of PNMT mRNA levels is dependent on maintaining corticosterone levels and supports the hypothesis that PNMT gene expression in the adrenal medulla is directly regulated by glucocorticoids produced by the adrenal cortex. However, the results also suggest that in the chronically HPX rat, factors in addition to naturally produced glucocorticoids are required for full restoration of PNMT mRNA levels.

The complete nucleotide sequence and structure of the gene encoding bovine phenylethanolamine N-methyltransferase

A cDNA clone for bovine adrenal phenylethanolamine N-methyltransferase (PNMT) was used to screen a Charon 28 genomic library. One phage was identified, designated lambda P1, which included the entire PNMT gene. Construction of a restriction map, with subsequent Southern blot analysis, allowed the identification of exon-containing fragments. Dideoxy sequence analysis of these fragments, and several more further upstream, indicates that the bovine PNMT gene is 1,594 base pairs in length, consisting of three exons and two introns. The transcription initiation site was identified by two independent methods and is located approximately 12 base pairs upstream from the ATG translation start site. The 3' untranslated region is 88 base pairs in length and contains the expected polyadenylation signal (AATAAA). A putative promoter sequence (TATA box) is located about 25 base pairs upstream from the transcription initiation site. Computer comparison of the nucleotide sequence data with the consensus sequences of known regulatory elements revealed potential binding sites for glucocorticoid receptors and the Sp1 regulatory protein in the 5' flanking region of the gene. Additionally, comparison of the sequence of the exons of the PNMT gene with cDNA sequences for other enzymes involved in biogenic amine synthesis revealed no significant homology, indicating that PNMT is not a member of a multigene family of catecholamine biosynthetic enzymes.

Sequence homology between tyrosine hydroxylase mRNA and a rat adrenal medullary cDNA

A rat adrenal medullary recombinant clone has been isolated by cross-hybridization with a tyrosine hydroxylase (TH) cDNA. The new clone has a mRNA size of 5.6 kb and also hybridizes to the 1.9-kb tyrosine hydroxylase message. Southern blot analysis reveals several hybridizing bands in common between the TH cDNA and the adrenal medullary clone. These results demonstrate that the adrenal clone shares sequences in common with the TH gene and/or is closely linked to it in the genome. Hybrid-selected mRNA translation products of the adrenal clone can be immunoprecipitated with dopamine beta hydroxylase antisera. This suggests that the adrenal medullary cDNA may code for another catecholamine pathway enzyme.

Subcellular site of biosynthesis of the catecholamine biosynthetic enzymes in bovine adrenal medulla

The subcellular site of biosynthesis of the catecholamine biosynthetic enzymes was examined. Free and membrane-bound polysomes were prepared from bovine adrenal medulla and mRNA was isolated from these polysomes. Both were active in directing cell-free translations. Immunoprecipitation of cell-free products with specific antisera localized the biosynthesis of the subunits of tyrosine hydroxylase (TH) (apparent Mr = 61,000) and of phenylethanolamine N-methyltransferase (PNMT) (apparent Mr = 32,000) on free polysomes, compared with biosynthesis of subunits of dopamine beta-hydroxylase (DBH) (apparent Mr = 67,000) on membrane-bound polysomes. Cross-reactivity between translation products was observed. Antibodies for DBH recognized a polypeptide with electrophoretic mobility identical to newly synthesized PNMT. However increasing concentrations of antibodies to DBH recognized at most 1/20 of the PNMT formed. The results of this study show the subcellular distribution of the catecholamine synthesizing enzymes is determined by their site of biosynthesis.