Characterization of enkephalins in rat adrenal medullary explants

Comparison of tyrosine hydroxylase and preproenkephalin expression in rat adrenal medullary explants in vitro and transplanted into subarachnoid space

When adrenal medullary cells are cultured in vitro, tyrosine hydroxylase (TH) mRNA, preproenkephalin (PPEnk) mRNA, and methionine enkephalin (Mek) immunoreactivity was markedly increased compared with intact adrenal medullary cells in situ, suggesting an increased biosynthesis of catecholamines and enkephalin-containing peptides. In transplanted adrenal medullary cells in vivo, TH mRNA and TH immunoreactivity are still apparent for at least 1 year after transplantation, indicating continued capacity for catecholamine biosynthesis. PPEnk mRNA levels in surviving adrenal medullary grafted cells increased, particularly in the first week after transplantation, and remained above levels found in the intact adrenal gland for at least 1 year after transplantation. These results support other studies in our laboratory, suggesting that adrenal medullary transplants reduce pain by synthesis and secretion of both catecholamines and enkephalin-containing peptides. The differences in expression of TH mRNA and PPEnk mRNA in the adrenal medulla in situ, in explants in culture and in transplants in the spinal subarachnoid space, indicate that the mechanisms regulating the expression of neurohumoral factors depend upon environmental factors extrinsic to the medullary cells themselves.

Preproenkephalin mRNA and enkephalin levels in the adult Syrian hamster: the influence from glucocorticoids

Proenkephalin (Penk) gene structure in hamsters and humans are similar but they differ from rats. In this study hamster Penk gene expression was examined after hypophysectomy+/-glucocorticoid receptor blockade with RU 486 (mifepristone). In contrast to rats, basal Penk gene expression in hamster adrenals did not change after treatments that reduced both the influence from glucocorticoids and phenylethanolamine-N-methyltransferase mRNA levels. Meanwhile, striatal preproenkephalin mRNA levels increased under these conditions.

Changes in proenkephalin gene expression in the developing hamster

Proenkephalin (Penk) gene expression is high in the adult hamster adrenal medulla and it is comparable to that found in both the hamster and rat striatum. In addition, Penk gene expression in the hamster adrenal medulla is more typical of adult mammalian adrenals than the rat. Since the nature of Penk gene expression in the developing hamster adrenal is not known, it was examined and compared to that found in the striatum were adult levels in the adrenal and striatum are similar. The results show that Penk gene expression progressively increases in the developing hamster adrenal to peak on postnatal day 4. There is then a small decline to adult levels by postnatal day 12 when the morphology of the developing adrenal resembles the adult. Functional splanchnic nerve activity, as assessed by the ability of reserpine to induce increases in adrenal tyrosine hydroxylase mRNA, is not present until after postnatal day 4. Therefore, early increases in Penk gene expression are independent of splanchnic nerve activity. Adrenal EC peptides resulting from the developmental increases in Penk gene expression appear to be unprocessed and proenkephalin-like. This is based on the very low levels of free enkephalin (met-enkephalin) detected in the adrenals from both newborn and adult hamsters (1-5% of total EC peptide levels). In the developing hamster striatum, Penk gene expression remains low and unchanged until postnatal day 4 and increases six-fold by adulthood. Free enkephalin (met-enkephalin) levels remain high (between 36 and 88% of total EC peptide levels) in the developing and adult hamster striatum. Therefore the results show early increases in adrenal Penk gene expression in the developing hamster that are independent of splanchnic nerve activity and adult Penk gene expression which is high and dependent on splanchnic nerve activity. This differs from what is observed in the frequently studied rat. However, developmental changes in the hamster striatum are similar to those in the rat.

Superfluous neurotransmitters?

In recent years, studies have suggested that the complexity of eukaryotic gene regulation, with its recurring and interacting motifs of cis and trans-acting regulatory elements, might result in superfluous gene expression. This conclusion is supported by a variety of experimental results that suggest that non-adaptive gene expression might be common. However, with few exceptions, the practical ramifications of unnecessary gene expression for cell biologists have not been addressed directly; this is particularly true for peptidergic neurophysiology, a field that might be plagued more than most with the consequences of this phenomenon. In this article, Chauncey W. Bowers discusses the superfluous expression of neuropeptides in the nervous system in the context of gene regulation extrapolated from studies in Drosophila.

Cloning and characterization of hamster proenkephalin gene

Our previous studies have shown that the hamster adrenal, like the human, contains high levels of preproenkephalin (PPenk) mRNA and enkephalin peptides, and may serve as a mammalian model for the in vivo study of proenkephalin (Penk) gene expression, peptide biosynthesis, and release. To define further the factors that may regulate hamster Penk gene expression, the hamster Penk gene was isolated from a genomic library prepared from Syrian hamster liver. The hamster Penk gene contains four exons and three introns and encodes 268 amino acids including six copies of Met-enkephalin containing peptides and one copy of Leu-enkephalin. In the 5' upstream region, there are TATA and GC boxes and multiple putative regulatory elements including the cAMP response element, AP-1, AP-2, AP-4, and the glucocorticoid response element (GRE). Possible GREs are also present in the introns. A comparison with the human and the rat Penk genes indicates that both the human and hamster Penk gene contain three introns, while the rat Penk gene has two introns. The intron missing from the rat Penk gene is short and separates the first and second exons of the hamster and human genes. In addition, the hamster and human genes share a region (100 bases) in the 5' upstream sequence that is 98% homologous. It is of interest that Penk gene expression is high in the adrenal medulla of both human and hamster, but is much lower in the rat. These homologous regions and the extra intron may contain regulatory features responsible for a high level of expression in the human and hamster adrenal medulla.

Developmental changes in rat adrenal enkephalin precursor: peptide ratio

Developmental changes in rat adrenal [Met5]enkephalin (ME) immunoreactivity and [Met5]enkephalinargininephenylalanine (MERF) immunoreactivity (which presumably reflects proenkephalin) were examined. The concentration of MERF appeared highest at the end of the first week postnatally and decreased fourfold to near adult values by day 21, while ME levels decreased only about twofold. This indicates a fall in the adrenal proenkephalin:ME ratio during the transition from the suckling to the weanling period in the rat. The elevated MERF levels during the suckling period may result from deficient processing of proenkephalin to free ME. An increase in the activity of the processing enzyme carboxypeptidase E was observed at day 15, which may contribute to the developmental fall in the adrenal proenkephalin to free ME ratio. Mimicking sympathetic activity by nicotine treatment (3 mg/kg i.p. twice daily from birth) produced a precocious decrease in the MERF:ME ratio at day 8, and also increased the activity of the processing enzyme. Denervation at day 10 markedly decreased adrenal ME and produced a three-fold increase in MERF:ME ratio when measured at day 30. Adrenal carboxypeptidase E activity was also decreased after denervation. In summary, these results suggest that increasing preganglionic sympathetic nerve activity during the third week postnatally stimulates proenkephalin processing and leads to the developmental decrease in MERF:ME ratio in the rat adrenal.

Time course of enkephalin mRNA and peptides in cultured rat adrenal medulla

Explantation of rat adrenal medullae to organ culture results in dramatic changes in enkephalins and catecholamines that are similar to the changes seen in vivo in response to denervation, which eliminates transsynaptic impulse activity. We have used rapid and sensitive solution hybridization methods to measure preproenkephalin (PPenk) mRNA and total cellular RNA in samples from rat tissues and adrenal medullary explants. The profiles of adrenal medullary PPenk mRNA, enkephalin-containing (EC) peptides, total cellular RNA and catecholamines [epinephrine (epi) and norepinephrine (norepi)] were measured during 14 days of organ culture. After 8 h in culture, total RNA had declined by 60%, epi and norepi declined 80 to 85% and EC peptides by 50% while the amount of PPenk mRNA per gland increased by 400%. Between 8 h and 14 days total RNA and catecholamine levels remained constant while PPenk mRNA increased to a peak of 85 +/- 10 (S.E.M.) pg/gland at 2-4 days, a value that was 80 times greater than the zero time (preculture) values. EC peptide levels lagged behind the increase in PPenk mRNA and reached a peak of 25 +/- 4 (S.E.M.) pmol Met-enkephalin equivalents/gland at 4 days that was 80 times greater than zero time values. Both PPenk mRNA and EC peptides declined in parallel between 4 and 14 days. The ratio of the copies of proenkephalin (Penk) peptide to PPenk mRNA was estimated to be 25,000 at the time of explantation and after 4 days in culture. From steady-state kinetics half-life estimates of 9.6 h for PPenk mRNA and 14.7 h for Penk peptide were obtained.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of adrenomedullary preproenkephalin mRNA: effects of hypoglycemia during development

To further evaluate whether transsynaptic mechanisms account for stress-induced changes in adrenomedullary preproenkephalin mRNA (ppEnk mRNA), neonatal rats were made hypoglycemic at a time when synapses are non-functional (less than 10 days postnatal age). While ppEnk mRNA in medullae from adult rats increased as much as 60-fold in this paradigm (insulin 10 U/kg), ppEnk mRNA levels in the newborn increased only 1.6-fold (insulin 20 U/kg). To evaluate whether postsynaptic cholinergic pathways of the neonatal adrenal medulla were functional, we treated 5-day-old pups with cholinergic agonists (nicotine [1 mg/kg, s.c., q 12 h] + carbachol [1.7 mumol/kg, s.c., q 12 h x 4 days]). Combined cholinergic agonist treatment augmented enkephalin prohormone and peptide levels up to 3-fold (P less than 0.05). To determine whether the blunted response to hypoglycemia in the newborn resulted from a deficiency in functional transsynaptic activity, synapses were matured using thyroid hormone pretreatment (postnatal days 2 and 3) before hypoglycemic stress. Hypoglycemia now caused a 40-fold increase in adrenomedullary ppEnk mRNA levels only in the T3/insulin treated group. To exclude other secondary effects of hypoglycemia (eg. hormonal, or insulin treatment-dependent), intracellular glycopenia was produced in the presence of secondary hyperglycemia by injecting adult rats or pups with 2-deoxyglucose (500 mg/kg). Similar to the insulin-hypoglycemia group, a large increase in adrenomedullary ppEnk mRNA resulted in the adult but not in the 5-day-old neonatal adrenal medullae. We conclude that enkephalin biosynthesis, like co-stored catecholamines, is induced by a transsynaptic process.(ABSTRACT TRUNCATED AT 250 WORDS)

Cultured astrocytes release proenkephalin

Astrocytes as well as neurons express the mRNA encoding the opioid peptide precursor, proenkephalin. In neurons proenkephalin is cleaved intracellularly to yield smaller, bioactive peptides such as Met-enkephalin and Leu-enkephalin. By contrast, utilizing a combination of radioimmunoassay and chromatographic analysis, we report here that astrocytes cultured from neonatal rat brain contain primarily unprocessed proenkephalin and only small amounts of Met-enkephalin. Further, similar experiments with and without the inclusion of several peptidase inhibitors indicate that cultured astrocytes release proenkephalin itself into the medium where it may be subsequently cleaved to smaller peptide products. The release of intact proenkephalin by astrocytes suggests that the glial propeptide subserves a different function than neuronal proenkephalin and that opioid peptides may play novel roles in the central nervous system.

Preproenkephalin mRNA and enkephalin in normal and denervated adrenals in the Syrian hamster: comparison with central nervous system tissues

The distribution and characteristics of preproenkephalin (PPenk) mRNA and enkephalin-containing (EC) peptides are compared in CNS and adrenal tissues from Syrian hamsters and Sprague-Dawley rats. Total cellular RNA extracts from both rat and hamster tissues produce a single hybridization band of PPenk mRNA of approximately 1500 bases when analyzed by Northern blot hybridization. Quantitation by solution hybridization reveals that in the hamster the highest levels of PPenk mRNA are found in adrenal (16.3 +/- 1.4 pg equivalents/micrograms RNA (mean +/- S.E.M.)) and striatum (13.3 +/- 0.7), followed by hypothalamus (0.8 +/- 0.2), and hippocampus (0.4 +/- 0.2). In the rat the highest levels of PPenk mRNA are in the striatum (35 +/- 2 pg/micrograms RNA) followed by the hypothalamus (3.0 +/- 0.5), hippocampus (0.3 +/- 0.1) and adrenal (0.18 +/- 0.04). Thus, the rank order of abundance of PPenk mRNA is similar in these CNS tissues for rat and hamster. The hamster adrenal levels are more than 90-fold greater than those of the rat. The abundance of EC peptides in both hamster and rat tissues mirror the rank order found with PPenk mRNA. Hamster adrenal contains the highest level of EC peptides (441 +/- 37 pmol/mg protein (mean +/- S.E.M.)) which is more than 400-fold greater than that of the rat adrenal and 8- to 12-fold greater than that found in rat and hamster striatum or hypothalamus. Both size exclusion chromatography and Western blot analysis indicate that EC peptides in hamster adrenal are predominantly large proenkephalin-like peptides with approximately 6 copies of Met- and 1 copy of Leu-enkephalin and that included in their number is a prominent EC peptide with a molecular weight of 34 kDa. Unilateral denervation of the hamster adrenal results in a time-dependent ipsilateral decrease in EC peptide and PPenk mRNA levels. Thus, by day 8 postsurgery, PPenk mRNA levels have declined by an average of 80% while EC peptides are reduced by 68% when compared to the innervated contralateral adrenal. These results demonstrate the great abundance of PPenk mRNA and EC peptides in the hamster adrenal. They also demonstrate the apparent need for transsynaptic impulse activity to maintain the high steady-state levels of PPenk and EC peptides. These characteristics of the hamster adrenal system provide opportunities for physiological and pharmacological investigations of the regulation of proenkephalin gene expression.

Effect of pregnancy and pain on cerebrospinal fluid immunoreactive enkephalins and norepinephrine in healthy humans

Endogenous spinal opioid or noradrenergic system activation may increase pain threshold during pregnancy and following a painful stress. Variation in spinal antinociceptive activity is also postulated to explain in part the large variability in postoperative opioid analgesic requirements. In this study, spinal noradrenergic and opioid activity, as reflected by the CSF concentrations of norepinephrine and immunoreactive enkephalins (total enkephalin-containing peptides), was determined in 58 women prior to surgery. The CSF concentration of these substances did not differ between pregnant and non-pregnant women. CSF norepinephrine tended to be greater in pregnant women who had experienced painful labor than in those who had not (1240 +/- 300 vs. 570 +/- 160 pmol/l; P = 0.056) and these women self-administered less morphine following cesarean section than those without labor pain (64 +/- 4 vs. 86 +/- 7 mg/24 h; P less than 0.01). However, CSF concentration of norepinephrine or immunoreactive enkephalins did not correlate with postoperative morphine use. These results suggest that spinal immunoreactive enkephalin and noradrenergic activity are not increased during pregnancy. However, pain may activate spinal noradrenergic pathways affecting pain sensation.

Bimodal regulation of adrenal opiate peptides by cholinergic mechanisms

Physiologic stressors increase trans-synaptic impulse activity and result in adrenal catecholamine release and biosynthesis. To determine the effects of stress on the co-localized opiate peptide system, rats were cold stressed at 4 degrees C. While cold stress slightly decreased enkephalin levels, a more severe stress (wetting and cold) increased enkephalin levels by 95%. Examining trans-synaptic-cholinergic mechanisms, treatment with either nicotinic or muscarinic agonists alone resulted in no change in adrenal enkephalin content. However, treatment with both nicotinic and muscarinic agonists together resulted in a three-fold rise in enkephalin levels. To further examine cellular mechanisms, medullae were explanted in the presence of agents that increase second messenger cyclic nucleotide levels. Treatments that increase the levels of cAMP, the cyclic nucleotide associated with nicotinic receptor activation, prevented the rise in medullary enkephalin relative to control explants. In contrast, treatments that increased cGMP levels, the cyclic nucleotide associated with muscarinic receptor activation, had no effect on enkephalin content compared to control explants. However, in the presence of both forskolin (10 microM) plus db-cGMP (5 mM), enkephalin content rose three-fold over control explants. These data suggest that, distinct from catecholamine pathways, enkephalin levels can be positively or negatively regulated by the severity of a stressful stimulus, by cholinergic receptor mechanisms and by an interaction of cyclic nucleotide second-messenger pathways.

Transcriptional control of adrenal catecholamine and opiate peptide transmitter genes

In the rat, decreasing transsynaptic activity through adrenal denervation, nicotinic receptor blockade, or explanation is associated with an increase in preproenkephalin mRNA, enkephalin prohormone and peptide. In contrast, catecholamine pathways remain unchanged under similar conditions. Since it is not known whether changes in messenger RNA result from stabilization or increased synthesis, we exploited transcription 'run-on' assays to measure the rate of transmitter gene read out. Tyrosine hydroxylase message (TH-mRNA) was found to be the most abundantly produced transcript in the unmanipulated control rat adrenal medulla. TH-mRNA was produced in excess of twice the rate of transcription of the structural gene beta-actin. In contrast, preproenkephalin transcription occurred at a much lower rate (60% of the actin gene and only 25% of tyrosine hydroxylase gene transcription). All transcripts were inhibited by the polymerase II inhibitor, alpha-amanitin. After two days in explant culture, the rate of enkephalin transcription increased approximately 2-fold (to the same level as actin transcription); while tyrosine hydroxylase transcriptional activity fell to 30% of actin level. To analyze cellular mechanisms, explants were depolarized with potassium chloride. Enkephalin gene transcription was observed to be 2.5-fold less when grown under depolarizing conditions (50 mM KCl) than in control explants. On the other hand, tyrosine hydroxylase gene read-out was unchanged, similar to results obtained when TH catalytic activity was measured. These data indicate that membrane depolarization can selectively regulate expression of a transmitter gene product and are consistent with a proposed transsynaptic regulatory mechanism controlling biosynthesis of adrenal opiate peptides.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogeny of the opiate phenotype: an approach to defining transsynaptic mechanisms at the molecular level in the rat adrenal medulla

Transmitter phenotypic expressions is a dynamic cellular process governed by multiple interactions with the neuronal environment. During sympathoadrenal development the arrival of presynaptic nerve terminals at the adrenal chromaffin cell (in the immediate postnatal period), coincides with the acquisition and subsequent development of a variety of transmitter biosynthetic capacities. Data discussed herein supports the contention that synaptic connections serve a central role in triggering the ontological cascade. Disruption of the normal timing of innervation events is detrimental to subsequent function and results in permanent deficiencies in development. In addition, alteration of transmitter biosynthetic regulatory mechanisms appears to reside at the level of gene expression. In view of this, additional molecular approaches are necessary to further elucidate the fundamental basis of neuronal transmitter phenotypic plasticity. Our approach to this problem represents a logical extension of previous research in this area and ultimately, will involve characterizing transcription activator molecules important in transmitter gene expression at various ontological ages.