Nicotinic receptor stimulation activates enkephalin release and biosynthesis in adrenal chromaffin cells

Structure of the gene encoding VGF, a nervous system-specific mRNA that is rapidly and selectively induced by nerve growth factor in PC12 cells

Nerve growth factor (NGF) plays a critical role in the development and survival of neurons in the peripheral nervous system. Following treatment with NGF but not epidermal growth factor, rat pheochromocytoma (PC12) cells undergo neural differentiation. We have cloned a nervous system-specific mRNA, NGF33.1, that is rapidly and relatively selectively induced by treatment of PC12 cells with NGF and basic fibroblast growth factor in comparison with epidermal growth factor. Analysis of the nucleic acid and predicted amino acid sequences of the NGF33.1 cDNA clone suggested that this clone corresponded to the NGF-inducible mRNA called VGF (A. Levi, J. D. Eldridge, and B. M. Paterson, Science 229:393-395, 1985; R. Possenti, J. D. Eldridge, B. M. Paterson, A. Grasso, and A. Levi, EMBO J. 8:2217-2223, 1989). We have used the NGF33.1 cDNA clone to isolate and characterize the VGF gene, and in this paper we report the complete sequence of the VGF gene, including 853 bases of 5' flank revealed TATAA and CCAAT elements, several GC boxes, and a consensus cyclic AMP response element-binding protein binding site. The VGF promoter contains sequences homologous to other NGF-inducible, neuronal promoters. We further show that VGF mRNA is induced in PC12 cells to a greater extent by depolarization and by phorbol-12-myristate-13-acetate treatment than by 8-bromo-cyclic AMP treatment. By Northern (RNA) and RNase protection analysis, VGF mRNA is detectable in embryonic and postnatal central and peripheral nervous tissues but not in a number of nonneural tissues. In the cascade of events which ultimately leads to the neural differentiation of NGF-treated PC12 cells, the VGF gene encodes the most rapidly and selectively regulated, nervous-system specific mRNA yet identified.

Nicotine-induced alterations in peripheral tissue concentrations of native and cryptic Met- and Leu-enkephalin

This study examined the peripheral tissue distribution of native and cryptic Met- and Leu-enkephalin, and regulation of tissue enkephalins by nicotine. Met- and Leu-enkephalin concentrations showed widespread variation in tissue concentration and degree of processing. HPLC characterization of homogenate of spleen revealed that both native and cryptic immunoreactive Met-enkephalin are comprised of two peaks, one representing authentic Met-enkephalin pentapeptide and the other its sulfoxide. Subacute repeated administration of nicotine 0.1 mg/kg ip, six times at 30 min intervals, increased native Met- and Leu-enkephalin in adrenal medulla without affecting cryptic Met- and Leu-enkephalin concentrations, consistent with increased processing of larger peptides to Met- and Leu-enkephalin. Subacute nicotine decreased splenic concentrations of native and cryptic Met-enkephalin and native Leu-enkephalin, consistent with increased release of Met- and Leu-enkephalin from spleen and decreased synthesis of proenkephalin A or inadequate processing of larger peptides to enkephalin pentapeptides in spleen to compensate for the increased release during this period. HPLC characterization revealed that nicotine-induced decrease in native Met-enkephalin in spleen resulted from reductions in both pentapeptide and its sulfoxide. Nicotine also increased native Met-enkephalin in jejunum, decreased cryptic Met-enkephalin in heart atrium, increased native Leu-enkephalin in anterior pituitary and decreased cryptic Leu-enkephalin in jejunum. Nicotine may produce some of its effects through alterations in release of enkephalins from peripheral tissues.

Structure of the gene encoding VGF, a nervous system-specific mRNA that is rapidly and selectively induced by nerve growth factor in PC12 cells

Nerve growth factor (NGF) plays a critical role in the development and survival of neurons in the peripheral nervous system. Following treatment with NGF but not epidermal growth factor, rat pheochromocytoma (PC12) cells undergo neural differentiation. We have cloned a nervous system-specific mRNA, NGF33.1, that is rapidly and relatively selectively induced by treatment of PC12 cells with NGF and basic fibroblast growth factor in comparison with epidermal growth factor. Analysis of the nucleic acid and predicted amino acid sequences of the NGF33.1 cDNA clone suggested that this clone corresponded to the NGF-inducible mRNA called VGF (A. Levi, J. D. Eldridge, and B. M. Paterson, Science 229:393-395, 1985; R. Possenti, J. D. Eldridge, B. M. Paterson, A. Grasso, and A. Levi, EMBO J. 8:2217-2223, 1989). We have used the NGF33.1 cDNA clone to isolate and characterize the VGF gene, and in this paper we report the complete sequence of the VGF gene, including 853 bases of 5' flank revealed TATAA and CCAAT elements, several GC boxes, and a consensus cyclic AMP response element-binding protein binding site. The VGF promoter contains sequences homologous to other NGF-inducible, neuronal promoters. We further show that VGF mRNA is induced in PC12 cells to a greater extent by depolarization and by phorbol-12-myristate-13-acetate treatment than by 8-bromo-cyclic AMP treatment. By Northern (RNA) and RNase protection analysis, VGF mRNA is detectable in embryonic and postnatal central and peripheral nervous tissues but not in a number of nonneural tissues. In the cascade of events which ultimately leads to the neural differentiation of NGF-treated PC12 cells, the VGF gene encodes the most rapidly and selectively regulated, nervous-system specific mRNA yet identified.

Regulation of proenkephalin A gene expression in aggregating fetal rat brain cells

1. Aggregating fetal rat brain cells express a significant amount of proenkephalin A (PENK) mRNA, a selective radioimmunoassay shows that this mRNA is also translated into enkephalins. 2. Depolarization with potassium chloride (KCl) or veratridine increases the expression of PENK mRNA in a time-dependent fashion, with a maximal increase of sixfold. It is interesting, however, that depolarization of the same cultures with KCl has no effect on the expression of prodynorphin mRNA. 3. An increase in PENK mRNA levels has been also observed in cultures treated with 8-Br-cAMP, phorbol 12-myristate-13-acetate (TPA), or dexamethasone. 4. However, incubation of the cultures with the opioid agonist etorphine or the antagonist naltrexone did not alter PENK gene expression, suggesting that there is not feedback control of opioids on PENK biosynthesis in these cells. 5. The increase in PENK mRNA in depolarized and in TPA-dexamethasone-, or 8-Br-cAMP-treated cultures was not accompanied by a significant increase in the amount of free immunoreactive met-enkephalin. Fetal brain cell cultures are therefore a useful neuronal model system for studying the mechanism that regulated the expression of PENK mRNA.

Regulation of proenkephalin synthesis in adrenal medullary chromaffin cells

The synthesis of proenkephalin was assessed in primary cultures of bovine adrenal medullary chromaffin cells by incubation of the cells with [35S]methionine, digestion of proenkephalin-derived peptides with trypsin and carboxy-peptidase B, and quantitation of radioactivity incorporated into Met-enkephalin following reversed-phase HPLC. Nicotine, histamine, and vasoactive intestinal peptide each enhanced the rate of proenkephalin synthesis approximately 10-fold when examined between 16 and 32 h after the drug or hormone addition. Inclusion of nifedipine (1 microM) partially blocked the stimulatory effect of nicotine, but not that of vasoactive intestinal peptide or histamine, or proenkephalin synthesis. Theophylline, tetrabenazine, and angiotensin II also increased the rate of proenkephalin synthesis (three- to eight-fold). These increases in the apparent rate of proenkephalin synthesis were not attributable to altered [35S]methionine specific radioactivity or rates of turnover and did not reflect similar increases in total protein synthesis. The half-life for turnover of Met-enkephalin sequences was 3-4 days in the cultured chromaffin cell. These studies directly show that proenkephalin synthesis is the primary regulatory step in control of chromaffin cell opioid peptide content.

Coordinate and differential regulation of proenkephalin A and PNMT mRNA expression in cultured bovine adrenal chromaffin cells: responses to cAMP elevation and phorbol esters

The expression of proenkephalin A (ProEnk A) mRNA and phenylethanolamine N-methyltransferase (PNMT) mRNA in response to cAMP analogues, forskolin and phorbol esters was examined in cultures of bovine adrenal chromaffin cells. Exposure of chromaffin cells to 1 mM dibutyryl cAMP for 24 h increased significantly the levels of ProEnk A mRNA, with no significant effect on the levels of PNMT mRNA. Cells exposed to the tumor promoting phorbol esters (phorbol 12-myristate 13-acetate, phorbol 12,13-dibutyrate or 4-beta-phorbol 12,13-didecanoate) for 12 h differentially activated PNMT mRNA and ProEnk A mRNA expression. The levels of PNMT mRNA were dramatically elevated in response to low concentrations (10(-9) to 10(-8)M) of these phorbol esters, but these increases were diminished at higher concentrations (10(-7) to 10(-6) M) of the phorbol esters. These responses were synergistically potentiated by dexamethasone (1 microM), a synthetic glucocorticoid. None of these effects was seen with the biologically inactive phorbol ester, 4-alpha-phorbol 12,13-didecanoate. By contrast, the expression of ProEnk A mRNA was activated by the tumor promoting phorbol esters in a concentration-dependent manner. The results of this study demonstrate a differential stimulatory effect of second messenger mechanisms in the control of PNMT and ProEnK A mRNA expression and provide further evidence for an independent control for the enkephalin and adrenaline synthesis in these cells.

Coordinate and differential regulation of proenkephalin A and PNMT mRNA expression in cultured bovine adrenal chromaffin cells: responses to secretory stimuli

The expression of proenkephalin A (ProEnk A) mRNA and phenylethanolamine N-methyltransferase (PNMT) mRNA in response to nicotine and to a number of secretagogues was examined in cultured bovine adrenal chromaffin cells. Prolonged incubation with nicotine (10 microM) resulted in a 2-fold increase in ProEnk A mRNA but had no significant effect on the level of PNMT mRNA. Similarly, prolonged stimulation with high K+ (56 mM) induced a time-dependent elevation in the level of ProEnk A mRNA reaching 4-fold basal level after 24 h incubation. By contrast, the level of PNMT mRNA was not changed by treatment with high K+. The increase in the level of ProEnk A mRNA by high K+ was abolished by the presence of 10 microM D600, a calcium channel blocker. Unlike the effects of high K+, treatment of the cells with the sodium channel activator veratridine significantly elevated the levels of both ProEnk A and PNMT mRNA. This increase in ProEnk A and PNMT mRNA levels was however less affected by D600. Stimulation of the cells with Ba2+ (1.1 mM) also stimulated the levels of ProEnk A and PNMT mRNA and this action required the presence of extracellular Ca2+. This was in contrast to the effect of Ba2+ in stimulating catecholamine secretion, which was inhibited by Ca2+ and enhanced in Ca2(+)-free buffer. The results of the present study indicate that membrane depolarization and entry of extracellular Ca2+ play an important role on the regulation of ProEnk A and PNMT mRNAs, in addition to their well-known actions on hormone secretion. Furthermore, these results suggest that the expression of ProEnk A mRNA and PNMT mRNA are under independent regulation in response to secretory stimulation.

Adrenal medullary implants in the rat spinal cord reduce nociception in a chronic pain model

Previous work in this laboratory has indicated that the transplantation of adrenal medullary tissue into the subarachnoid space of the rat spinal cord can reduce pain sensitivity to acute noxious stimuli, particularly following stimulation by nicotine. This most likely results from the stimulated release of opioid peptides and catecholamines from the transplanted chromaffin cells. However, chronic pain models may more closely resemble human clinical pain, and the arthritic rat model has been used for screening potential therapeutic strategies. The purpose of the present study was to assess the potential for adrenal medullary tissue implanted into the spinal subarachnoid space to alleviate chronic pain. Adrenal medullary tissue was implanted into adjuvant-induced arthritic rats, and changes in body weight and vocalization responses were monitored over the 10 week course of the disease. Results indicate that the severe weight reduction normally associated with this inflammatory arthritis was attenuated by adrenal medullary, but not control, implants. In addition, vocalizations were reduced in animals implanted with adrenal medullary, but not control tissue following nicotine stimulation. This reduction was blocked by the opiate antagonist, naloxone, and partially attenuated by the alpha-adrenergic antagonist, phentolamine. Together, these results suggest that the transplantation of adrenal medullary tissue into the subarachnoid space of the spinal cord may provide a local source of opioid peptides and catecholamines for the reduction of chronic pain.

Nicotine protects against mu-opioid receptor antagonism by beta-funaltrexamine: evidence for nicotine-induced release of endogenous opioids in brain

We have hypothesized that some effects of nicotine are mediated through endogenous opioids. This study was designed to demonstrate in rats that nicotine releases endogenous opioids in brain. In the control group, subcutaneous morphine (8 mg/kg) produced analgesia or antinociception as measured by prolongation of tail flick latency. Intracerebroventricular administration 24 h earlier of beta-funaltrexamine (beta-FNA, 2.5 micrograms), an antagonist which irreversibly alkylates opioid receptors, markedly reduced (66%) morphine analgesia. Subcutaneous administration of nicotine (0.1 mg/kg) prior to beta-FNA attenuated (31%) the inhibitory effect of beta-FNA on morphine analgesia. These data support our hypothesis that endogenous opioids released by nicotine bind to mu-opioid receptors in brain and protect them against inactivation by beta-FNA.

Opioid and nicotine receptors affect growth regulation of human lung cancer cell lines

Using specific ligands, we find that lung cancer cell lines of diverse histologic types express multiple, high-affinity (Kd = 10(-9)-10(-10) M) membrane receptors for mu, delta, and kappa opioid agonists and for nicotine and alpha-bungarotoxin. These receptors are biologically active because cAMP levels decreased in lung cancer cells after opioid and nicotine application. Nicotine at concentrations (approximately 100 nM) found in the blood of smokers had no effect on in vitro lung cancer cell growth, whereas mu, delta, and kappa opioid agonists at low concentrations (1-100 nM) inhibited lung cancer growth in vitro. We also found that lung cancer cells expressed various combinations of immunoreactive opioid peptides (beta-endorphin, enkephalin, or dynorphin), suggesting the participation of opioids in a negative autocrine loop or tumor-suppressing system. Due to the almost universal exposure of patients with lung cancer to nicotine, we tested whether nicotine affected the response of lung cancer cell growth to opioids and found that nicotine at concentrations of 100-200 nM partially or totally reversed opioid-induced growth inhibition in 9/14 lung cancer cell lines. These in vitro results for lung cancer cells suggest that opioids could function as part of a "tumor suppressor" system and that nicotine can function to circumvent this system in the pathogenesis of lung cancer.

Membrane depolarization and calcium induce c-fos transcription via phosphorylation of transcription factor CREB

The mechanism by which the calcium influx signal, triggered by membrane depolarization, is transduced to the nucleus to activate c-fos proto-oncogene transcription has been characterized. A calcium response element (CaRE) that is indistinguishable from a cAMP response element (CRE) mediates transcriptional inducibility by depolarization. Its cognate transcription factor CREB is the target for both calcium and cAMP signals. CREB is rapidly phosphorylated in response to depolarization or cAMP, at a site known to be important for the transcriptional activating function of this protein. The convergent effects of calcium and cAMP on CREB activation are mediated by distinct protein kinase signaling pathways. CREB and its binding site, the Ca/CRE, can thus function as a regulatory element that integrates both calcium and cAMP signals in the control of gene expression.

Coordinate and differential regulation of phenylethanolamine N-methyltransferase, tyrosine hydroxylase and proenkephalin mRNAs by neural and hormonal mechanisms in cultured bovine adrenal medullary cells

Primary cultures of bovine adrenal medullary cells (AM) in a chemically defined media were used to examine the role of neural and hormonal factors in the expression of proenkephalin A (pEK), phenylethanolamine N-methyltransferase (PNMT) and tyrosine hydroxylase (TH) genes. Acetylcholine or nicotine reduced cellular content of catecholamines by 30% and increased the relative abundance of pEK, TH, and PNMT mRNAs. The increases produced by acetylcholine were +129%, +147%, and +43% for pEK, TH, and PNMT mRNA, respectively. The kinetics of increases produced by nicotine were different for the 3 mRNAs, with pEK and TH showing enhanced levels over 48 h incubation, while PNMT showed increase during the initial 18 h (+90%) followed by decline to control levels at 48 h. 8-Br cAMP and forskolin elicited a similar pattern of changes as nicotine, suggesting that cyclic AMP may be involved in the mediation of the nicotinic effects. To examine the role of depletion of cellular catecholamines in the regulation of mRNA levels, cells were exposed to tetrabenazine or reserpine. Decreases in cellular catecholamine contents were accompanied by increases in TH and pEK mRNA levels, while the expression of PNMT gene exhibited a transient 4-fold increase and then profound inhibition (60-95%) over a 48-h period. The tetrabenazine effect on TH and pEK mRNA was reduced by alpha-amanitin, suggesting transcriptionally-mediated regulation. Inductions of pEK but not TH or PNMT mRNAs were inhibited by cycloheximide. Hormonal regulation of TH, PNMT, and pEK mRNAs was examined by incubation of cells with dexamethasone. Low concentrations of dexamethasone (0.1, 10 nM) were effective to increase PNMT (+35%, +90%) and pEK (+27%, 45%) mRNA levels. TH mRNA was not affected by similar concentrations of dexamethasone, however, there was a 45% increase at 1 microM. Dexamethasone-elicited increases in PNMT mRNA levels were observed at 48 h and persisted up to 7 days, suggesting that hormonal mechanisms may be distinct from those mediating effects of nicotine, cAMP or tetrabenazine. Taken together, these results indicate that (1) the level of TH, PNMT, and pEK mRNAs are regulated by direct neural (acetylcholine) and hormonal (glucocorticoid) inputs to adrenal medullary cells; (2) effects of acetylcholine could be mediated by cyclic AMP and alterations in catecholamine content; and (3) expression of individual genes is regulated differentially. Such differential regulation of TH, PNMT, and pEK mRNAs may contribute to the long-term selective control of hormonal output from adrenomedullary cells.

Light inhibits the release of both [Met5]enkephalin and [Met5]enkephalin-containing peptides in chicken retina, but not their syntheses

The levels of native and cryptic [Met5]enkephalin in the chicken retina were found to vary during a 12:12 h light-dark cycle, both rising in the light and falling during the dark. Such variations could conceivably arise from (a) changes in the rate of release and subsequent degradation of native and/or cryptic [Met5]enkephalin, (b) changes in the rate of proenkephalin A synthesis, or (c) changes in the rate of proenkephalin A processing. Measurement of the rate of release of native and cryptic [Met5]enkephalin in vitro indicated that the increased rate of release of both of these forms of [Met5]enkephalin during the dark quantitatively accounted for the fall in their retinal levels during the dark. This indicated that the biosynthesis of proenkephalin A was not activated during the light-dark cycle. Molecular weight fractionation of retinal extracts also supported this idea, since the pool of high molecular weight precursors did not vary in size, suggesting that processing was not modulated during the light-dark cycle. Instead, the fall in both cryptic and native [Met5]enkephalin during the dark was due to their increased rate of release together with a rate-limiting conversion of high molecular weight [Met5]enkephalin-containing peptides to low molecular weight [Met5]enkephalin-containing peptides. The enkephalinergic cells of the retina seem to cope with physiological variations in demand by accumulating a large pool of peptide during periods of low stimulation (light), so that when stimulation and release is high (dark), the decrease in pool levels does not compromise the function of the cells and their postsynaptic targets.

Increased levels of Met-enkephalin-like immunoreactivity in the spinal cord CSF of rats with adrenal medullary transplants

Recent work in our laboratory has demonstrated that the transplantation of adrenal medullary tissue into the spinal cord subarachnoid space can reduce pain sensitivity, particularly following nicotinic stimulation. This analgesia most likely results from the release of opioid peptides from the implanted chromaffin cells since it is blocked by the opiate antagonist naloxone. The purpose of the present study was to more directly measure opioid peptide release from adrenal medullary implants in the spinal cord using spinal cord superfusions. Basal levels of Met-enkephalin-like immunoreactivity (MELI) in spinal cord superfusates of animals with adrenal medullary implants was twice that in animals with control implants. The injection of nicotine further increased MELI release in adrenal medullary, but not control implanted animals. Both the basal MELI levels and the MELI levels following nicotine were correlated with reduced pain sensitivity in animals with adrenal medullary implants. Morphological studies revealed good long-term survival of grafted chromaffin cells. Results of this study suggest that it is possible to increase opioid peptide levels and concomitantly decrease pain sensitivity by the transplantation of adrenal medullary tissue into the spinal cord subarachnoid space.

Mechanisms involved in the respiratory depressant actions of nicotine in anesthetized rats

In the urethane-pentobarbital anesthetized rat, the respiratory depressant and lethal effects of intravenously infused (-)-nicotine (120 micrograms/kg/min) or (+)-nicotine (600 micrograms/kg/min) were effectively prevented by pretreatment with the opioid antagonist, naltrexone, whereas the lethal effect of (-)-nicotine (120 micrograms/kg/min) was not altered by bilateral adrenalectomy. Further, pretreatment with either the nicotinic ganglion-blocker, mecamylamine, a secondary amine, or the quarternary nicotinic ganglion-blocker, hexamethonium, completely prevented the lethal effects of (-)-nicotine (120 micrograms/kg/min). These data suggest that central opioidergic and nicotinic processes are involved in nicotine's respiratory depressant and lethal effects.

Pharmacology of neuronal gene expression

Pharmacological treatments were used to estimate trans-synaptic regulation of opioid peptide gene expression occuring at specific neurotransmitter receptors. In vitro and in vivo studies have shown that different signal-transduction mechanisms regulate the transcription of proenkephalin, proopiomelanocortin and nerve growth factor mRNA. The activation of receptors coupled to adenylate cyclase elicited the increase of proenkephalin and nerve growth factor gene expression. Therefore, a cAMP-dependent mechanism was suggested to be involved in such regulation. However, the temporal delay between the elevation of the intracellular cAMP content and the increase in nerve growth factor and proenkephalin mRNAs prompted us to investigate whether additional mechanisms associated with the second messenger were operative in the regulation of the expression of these two genes. We report evidence that a protein(s), probably functioning as a trans-acting factor, might be involved in the regulation of nerve growth factor gene transcription. The characterization and isolation of these DNA regulatory proteins will provide the pharmacologist with valuable information for the development of new compounds in the therapy of mental disorders.

Effect of secretagogues on chromogranin A synthesis in bovine cultured chromaffin cells. Possible regulation by protein kinase C

Chromogranin A is a major component of storage granules in many different secretory cell types. After [35S]methionine labelling of proteins from cultured bovine chromaffin cells, chromogranin A was immunoprecipitated with specific antibodies, and the radioactivity incorporated into chromogranin A was determined and used as an index of its synthesis rate. Depolarization of cells with nicotine or high K+ evoked a Ca2+-dependent increase in chromogranin A synthesis, whereas muscarine, which does not evoke significant Ca2+ influx from bovine chromaffin cells, had no effect on chromogranin A synthesis. Forskolin, an activator of adenylate cyclase, affected neither the basal nor the nicotine-stimulated rate of chromogranin A synthesis. In contrast, 12-O-tetradecanoylphorbol 13-acetate (TPA), an activator of protein kinase C, significantly enhanced the incorporation of radioactivity into chromogranin A. Sphingosine, an inhibitor of protein kinase C, abolished both nicotine-stimulated and TPA-induced chromogranin A synthesis. In addition, long-term treatment of chromaffin cells with TPA decreased protein kinase C activity and inhibited the nicotine-stimulated chromogranin A synthesis. These results suggest that protein kinase C may play an important role in the control of chromogranin A synthesis.

Neuropeptides and other secretagogues in bovine chromaffin cells: their effect on opioid peptide metabolism

Bovine adrenal medullary cells released significant amounts of opioid pentapeptides (Methionine- and Leucine-enkephalin), when stimulated with neuropeptides like angiotensin II (AT), bradykinin (BK) or neurotensin (NT). Dose-response curves for angiotensin II and bradykinin were shown to be double-sigmoidal, with the first EC50 in the nM range; much lower than that for acetylcholine, nicotine or histamine. Secretory effects were dependent on extracellular CA++ and impaired by the Ca++ channel blockers D600. Moreover, these peptides produced an increase in opioid peptide cell content after 48 h of incubation, as did nicotine-, histamine- and K+ treated cells. The compound's capacity to induce long-term Met-enk secretion was similar to their efficacy in raising intracellular opioid peptide levels, suggesting compensatory increases. Besides affecting opioid peptide metabolism, AT, BK and NT produced a significant accumulation of inositol-1-phosphates (IP1), the role of which remains to be clarified. The physiological significance of vasoactive compounds in the adrenal medulla will be discussed.

Differential colocalization of neuropeptide Y- and methionine-enkephalin-Arg6-Gly7-Leu8-like immunoreactivity in catecholaminergic neurons in the rat brain stem

The present study, using a combination of catecholamine (CA) histofluorescence and peptide immunocytochemistry in the same tissue sections, investigated the coexistence of neuropeptide Y (NPY) and methionine-enkephalin-Arg6-Gly7-Leu8 (MEAGL)-like immunoreactivity (LI) in catecholaminergic neurons of colchicine-treated rat brain stems. Of the total number of catecholaminergic neurons in the A1/C1, A2/C2, A3, A4, and A6 regions approximately 83, 28, 98, 76, and 36%, respectively, contained both NPY-LI and CA. Of the total number of catecholaminergic neurons in A1/C1, A2/C2, A3, and A5 regions, approximately 47, 4, 8, and 17%, respectively, contained both MEAGL-LI and CA. Moreover, about 24% of the catecholaminergic neurons in the A1/C1 region contained both NPY- and MEAGL-LI. Neither the noradrenergic neurons (A7) in the pons nor any of the dopaminergic neurons in the midbrain (A8, A9, A10) contained NPY- or MEAGL-LI. Neurons containing both NPY- and MEAGL-like immunoreactive peptides without CA were not found in the rat brain stem. These findings indicate that catecholaminergic neurons in the brain stem of the rat can be subdivided into distinct subgroups on the basis of the coexistence of specific peptides.

Activity-dependent regulation of gene expression in muscle and neuronal cells

In both the central and the peripheral nervous systems, impulse activity regulates the expression of a vast number of genes that code for synaptic proteins, including neuropeptides, enzymes involved in neurotransmitter biosynthesis and degradation, and membrane receptors. In recent years, the mechanisms involved in these regulations became amenable to investigation by the methods of recombinant DNA technology. The first part of this review focuses on the activity-dependent control of nicotinic acetylcholine receptor biosynthesis in vertebrate muscle, a model case for the regulation of synaptic protein biosynthesis at the postsynaptic level. The second part summarizes some examples of neuronal proteins whose biosynthesis is under the control of transsynaptic impulse activity. The first, second, and third intracellular messengers involved in membrane-to-gene signaling are discussed, as are possible posttranscriptional control mechanisms. Finally, models are proposed for a role of neuronal activity in the genesis and stabilization of the synapse.

Developmental expression of proenkephalin A mRNA and phenylethanolamine N-methyltransferase mRNA in foetal sheep adrenal medulla

The ontogenic expression of proenkephalin A (ProEnk A) mRNA and phenylethanolamine N-methyltransferase (PNMT) mRNA was examined in the foetal sheep adrenal medulla by the use of specific oligodeoxyribonucleotide probes. Northern blot analysis of RNA extracts from foetal adrenals demonstrated that ProEnk A mRNA was expressed as early as 60 days of gestation, a time at which the foetal adrenal is not functionally innervated. In situ hybridization on sections of foetal adrenals revealed that at 110-140 days gestation ProEnk A mRNA was expressed in chromaffin cells at the outer margin of the adrenal medulla but at earlier stages of gestation (e.g. 95 days) appeared to be expressed homogeneously throughout the whole of the adrenal medulla. In comparison, PNMT mRNA was expressed preferentially in cells at the outer margin of the adrenal medulla from the earliest stage detectable. Both PNMT mRNA and ProEnk A mRNA co-localized in cells at the outer margin of foetal adrenal of late gestations (110-140 days), a similar pattern to that seen in the adult adrenal medulla. These results indicate that, as with adult animals, in foetuses of late gestation, adrenal enkephalins are co-stored within adrenaline cells. It is likely therefore that enkephalins are co-released from the foetal adrenal with adrenaline in response to intra-uterine stress.

Neurotensin affects metabolism of opioid peptides, catecholamines and inositol phospholipids in bovine chromaffin cells

Bovine adrenal medullary cells released significant amounts of opioid pentapeptides Met- and Leu-enkephalin and catecholamines, when stimulated by neurotensin (NT). Maximal release induced by this peptide was about 40-50% of that seen after nicotinic activation of cholinergic receptors. Dose-response curves for neurotensin-induced secretion revealed an EC50 of 1x10(-6)M, thereby being in the range of that for acetylcholine or nicotine. Secretory effects were dependent on extracellular Ca++ and impaired by the Ca++ channel blocker D 600. Moreover NT produced an increase in opioid peptide cell content after 48 and 72 hrs of incubation. Besides affecting opioid peptide metabolism, NT significantly produced accumulation of inositol- 1-phosphates (IP1), the significance of which remains to be clarified in the observed metabolic effects.

Differences in the effects of membrane depolarization on levels of preprosomatostatin mRNA and tyrosine hydroxylase mRNA in rat sympathetic neurons in vivo and in culture

Regulation of preprosomatostatin mRNA and tyrosine hydroxylase mRNA were examined in sympathetic neurons of the rat superior cervical ganglion (SCG). Surgical denervation of the adult SCG increased ganglion levels of preprosomatostatin (SS) mRNA more than 11-fold, and levels of the mRNA remained elevated 14 days after surgery. By contrast, denervation decreased levels of tyrosine hydroxylase (TH) mRNA. Potassium- or veratridine-induced membrane depolarization of cultured neonatal sympathetic neurons decreased levels of SS mRNA but elevated levels of TH mRNA. Sodium channel blockade with tetrodotoxin prevented the effects of veratridine on SS and TH mRNAs. In toto these observations suggest that transsynaptic nerve impulse activity and sympathetic neuron membrane depolarization decrease SS synthesis but increase TH synthesis at the mRNA level. Thus nerve impulse activity may alter the relative levels of different transmitters co-expressed in the same neuronal population by inhibiting levels of some species of mRNA while simultaneously stimulating levels of others.

Ultrastructural evidence suggests variations in biosynthesis and processing within LH-RH neurons as a function of ovariectomy in rats

We have demonstrated that populations of luteinizing hormone-releasing hormone (LH-RH)-immunopositive neuronal perikarya change following gonadectomy of male and female rats in a sex-dependent manner related to rises in plasma luteinizing hormone (LH). In this study we characterize the ultrastructural state of organelles involved in protein synthesis, primarily within perikarya of rostral preoptic area LH-RH neurons surrounding the organum vasculosum of the lamina terminalis (OVLT). One day following ovariectomy little rough endoplasmic reticulum (RER) was evident, however, the cisternae were heavily laden with ribosomes and numerous polysomes were present free in the cytoplasm. Six days and 3 weeks post-ovariectomy the cisternae of RER were progressively more abundant and dilated; the multiple Golgi apparati, located in close proximity to the RER, were composed of many lamelae and extensive associated vesicles. We propose that increased pools of messenger ribonucleic acid (mRNA) are generated by 1 day post-ovariectomy prior to increased synthesis of precursor, 6 days and 3 weeks post-ovariectomy. Axodendritic synaptic profiles in the neuropil surrounding LH-RH perikarya increased in number in the rostral medial preoptic area and lateral anterior hypothalamic area. We conclude that removal of gonadal steroids results in greater biosynthetic activity in LH-RH neurons, and suggest that the enhanced biosynthesis is related to increases in afferent activity.

Roles of the pituitary-adrenocortical axis in control of the native and cryptic enkephalin levels and proenkephalin mRNA in the sympathoadrenal system of the rat

The effects of hypophysectomy (HPX) and dexamethasone (DEX) on the levels of Met5-enkephalin (ME), ME precursors, and the abundance of proenkephalin (pEK) mRNA, were examined in the adrenal medulla (AM) and superior cervical ganglia (SCG). To assess possible changes in enkephalin processing, both cryptic (after trypsin and carboxypeptidase B digestions) and native (without enzyme digestions) ME-like immunoreactivity (ME-LI) was measured. Three weeks after HPX the proportion of pEK mRNA to the total RNA content in the AM was not significantly changed when compared to sham-operated (SO) animals. Total (native + cryptic) ME-LI was decreased by 45% in the AM of HPX rats. This decrease was paralleled by a 58% depletion of AM proteins. Cryptic ME-LI was also reduced by 43%. In contrast, native ME-LI was not altered after HPX, indicating enhanced processing of ME precursors. Treatment with DEX (5 daily injections--1 mg/kg, i.p.) increased the relative abundance of pEK mRNA (+27%) and total ME-LI in the AM of HPX group, but not in SO group. Native ME-LI, cryptic ME-LI, and their ratio were not significantly affected by DEX in the AM of HPX or SO rats. In SCG, the relative abundance of pEK mRNA decreased by 25% after hypophysectomy. Total and cryptic ME-LI in the SCG of HPX rats were not changed when compared to SO rats. In contrast, HPX reduced native ME-LI suggesting decreased processing of ME precursors. Similarly, as in AM, DEX produced increase in the SCG pEK mRNA only in HPX (+68%) and not in the SO rats. In SCG, DEX produced decreases in total ME-LI which could be attributed to an increased enkephalin release. An overall reduction of cryptic ME-LI was also observed after DEX, whereas native ME-LI remained unchanged suggesting increased processing of enkephalins. Our findings indicate that the pituitary adrenocortical axis controls the relative proportions of ME to its precursors, and that this control involves both glucocorticoid-dependent (SCG) and glucocorticoid-independent (AM) mechanisms. In contrast, our studies do not suggest specific control of pEK synthesis by the pituitary adrenocortical axis. The pituitary adrenocortical axis may also influence the relative contents of ME and catecholamines in the AM and SCG. The ratio of ME/catecholamines increased after HPX (AM and SCG) and after DEX (SCG). Such regulation may contribute to the control of co-transmitter output in the sympathoadrenal system.

Characterization of enkephalins in rat adrenal medullary explants

In the rat, removal of depolarizing stimuli to the adrenal medulla by surgical denervation in vivo or by explanting adrenal medullae has been shown to dramatically increase preproenkephalin mRNA, and enkephalin-containing (EC) peptides. To further elucidate the cellular basis of these effects and the role of transsynaptic influences on post-translational processing, we have defined the time course, and characterized EC peptides in rat adrenal medullary explants in control and depolarized states. The rise in EC peptides begins after 1 day in culture and reaches a peak at 4-7 days. Although the onset of the increase in EC peptides in culture is delayed by 12-24 h compared to the changes seen in vivo, following surgical denervation, the time course of peak and duration is remarkably similar. Size exclusion chromatography (SEC) revealed that the major species of newly appearing EC peptides in explanted glands is a high molecular weight peptide of approximately 18,000 with a Met-/Leu-enkephalin ratio of approximately 6. These results suggest that proenkephalin, the initial precursor of the EC peptide family, is the major EC peptide that accumulates in rat adrenal medullary explants. A low-molecular weight EC peptide, found by high-performance liquid chromatography to be free Met-enkephalin, is a minor component of the culture induced increase in EC peptides. Culturing of medullae in the presence of depolarizing concentrations of K+ prevents the accumulation of the proenkephalin-like EC peptides and free enkephalins.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal TRH synthesis: developmental and circadian TRH mRNA levels

Peptide biosynthesis within a neuron involves several steps occurring at the soma and during its travel to the nerve terminal, where it accumulates to be released under stimulatory conditions. We have measured hypothalamic TRH and TRH mRNA during ontogeny and circadian cycle and observed that TRH mRNA variations are more prominent than TRH ones. On the basis of these results and in vitro release experiments, we propose a compensatory mechanism working at the nerve terminal which is activated after release.

Calcium requirements for barium stimulation of enkephalin and vasoactive intestinal peptide biosynthesis in adrenomedullary chromaffin cells

The divalent cation barium was used to study the role of calcium in coupling neuropeptide secretion and biosynthesis following secretagogue stimulation of bovine chromaffin cells. Barium chloride (0.1-2.5 mM) stimulated in a dose-dependent manner the secretion of met-enkephalin (up to 20% of intracellular peptide content) and increased the total amount (cell plus medium content) of met-enkephalin and vasoactive intestinal polypeptide (VIP) 2- to 3-fold after 72 hours. A greater than six-fold increase in proenkephalin mRNA (mRNA(enk)) was observed by 24 hours following barium stimulation. The voltage-sensitive calcium channel blocker D600 inhibited the barium-stimulated secretion of enkephalin and blocked the stimulation of VIP biosynthesis and mRNA(enk). Reducing calcium in the medium resulted in an enhancement of barium-stimulated release of both peptides, but blocked the induction of their biosynthesis. The data indicate that calcium targets involved in secretion can be activated by barium or calcium while calcium targets involved in biosynthesis specifically require calcium. It is therefore proposed that pathways leading to peptide secretion and biosynthesis in the adrenal diverge just after secretagogue-stimulated calcium influx.

Characterization of 1,1-dimethyl-4-phenylpiperazinium induced increased proenkephalin processing in bovine chromaffin cells

Acute stimulation of bovine adrenal chromaffin cells in culture with 1,1-dimethyl-4-phenylpiperazinium (DMPP) gives rise to a significant increase in secretion of [Met5]-enkephalin immunoreactive material (ME-IRM) into the culture medium (1). Following this secretion the cellular ME-IRM levels do not decrease, suggesting the replenishment of the peptides. The repletion of the cellular ME-IRM appears to result from an increase in processing of large molecular weight peptides containing [Met5]-enkephalin and [Leu5]-enkephalin. Gel filtration chromatography on Bio-Gel P-10 was used to fractionate the enkephalin-like peptides (ELPs) present in the culture media and chromaffin cell extracts. Fractionation was done for samples before and after nicotinic receptor stimulation by DMPP to demonstrate the secretion and repletion of the ELPs. Gel chromatographic profiles of ELPs present in the culture media after DMPP stimulation revealed the presence of 4 peaks, representing different molecular forms of these peptides (Peaks 1-4), with a selective increase in secretion of Peaks 3 and 4. The chromatograms of ELPs extracted from cultured chromaffin cells showed similar patterns to those obtained from ELPs present in the culture medium after stimulation. Analyses of individual peaks after fractionation of cell culture extracts showed an increase in the amount of immunoreactive material found in Peak 4 with a concomitant decrease in the immunoreactivity found in the higher molecular weight peaks (Peaks 1-3). Further purification of Peak 4 from cell extracts on reversed-phase HPLC (RP-HPLC) showed a significant amount of ELPs existed as the sulfoxide derivative of [Met5]-enkephalin. The content of [Met5]-enkephalin sulfoxide (ME-O-enk) did not decrease following DMPP stimulation. We conclude that acute stimulation of nicotinic receptors in the chromaffin cells enhances the processing of proenkephalin precursors to keep pace with the secretion of low molecular weight peptides.

Histamine affects release and biosynthesis of opioid peptides primarily via H1-receptors in bovine chromaffin cells

Histamine is a potent secretagogue for opioid pentapeptides (Met- and Leu-enkephalin) in adrenal chromaffin cells in vitro. This effect is dependent on extracellular Ca2+ and is reduced by Ca2+ channel blockers such as Co2+, D 600, and nifedipine. Moreover, histamine also produced a profound compensatory increase in cellular peptide content after 48 h of exposure, most likely caused by a four- to fivefold increase in the mRNA levels coding for the proenkephalin A precursor. All the histamine-induced effects (acute release, changes in peptide cell content, proenkephalin A mRNA levels) are antagonized by the H1-receptor antagonist, clemastine, whereas the H2-receptor antagonists, ranitidine and cimetidine, were less effective (approximately 20% inhibition).

Substance P, nicotinic acetylcholine receptors and antinociception in the rat

Intracerebroventricular injections of both nicotine and substance P (SP) induced antinociception in the tail-flick test in rats. The antinociceptive effect was blocked in both cases by mecamylamine and by naloxone, suggesting that central nicotinic acetylcholine receptors and endogenous opioids are implicated in the action of these drugs. A link between substance P neurones and central cholinergic systems, involving nicotinic receptors, was also suggested by the quickly developed cross-tolerance between the antinociceptive effect of substance P and nicotine. A smaller, subeffective dose of substance P was able to block, on acute administration, the antinociceptive action of nicotine, an effect not shared by the two other mammalian tachykinins, neurokinin A or neurokinin B. The results obtained in the present study appear to indicate a dual action of substance P on central nicotinic cholinoceptors.

The enkephalin-containing cell: strategies for polypeptide synthesis and secretion throughout the neuroendocrine system

1. Enkephalinergic cells are found throughout the diffuse neuroendocrine system, in the adrenal medulla, brain, spinal cord, peripheral and enteric nervous systems, and endocrine pancreas. 2. In each of these diverse cell types, the enkephalin phenotype is (i) established during development, (ii) modified by the particular environment in which the cell is located, and (iii) maintained by ongoing biosynthesis at a rate consistent with loss of enkephalins from the cell during periods of secretion. 3. Enkephalin expression and biosynthesis have been studied in several neuroendocrine cell types and tumor cell lines. Transcriptional, translational, and posttranslational factors can play a role at all three stages (establishment, modification, and maintenance) in the regulation of enkephalin expression during the lifetime of the cell. 4. Cyclic nucleotides, glucocorticoids, and calcium may all act to control the overall level of enkephalin biosynthesis pretranslationally, while regulation of posttranslational processing of proenkephalin seems to be important in determining the pattern of proenkephalin-derived opiate peptides produced in a given tissue. 5. The themes (and variations) of cell regulation that apply to enkephalin expression may be similar for other bioactive peptides produced in neural and endocrine tissues.

Molecular studies of the neuronal nicotinic acetylcholine receptor family

Nicotinic acetylcholine receptors on neurons are part of a gene family that includes nicotinic acetylcholine receptors on skeletal muscles and neuronal alpha bungarotoxin-binding proteins that in many species, unlike receptors, do not have an acetylcholine-regulated cation channel. This gene superfamily of ligand-gated receptors also includes receptors for glycine and gamma-aminobutyric acid. Rapid progress on neuronal nicotinic receptors has recently been possible using monoclonal antibodies as probes for receptor proteins and cDNAs as probes for receptor genes. These studies are the primary focus of this review, although other aspects of these receptors are also considered. In birds and mammals, there are subtypes of neuronal nicotinic receptors. All of these receptors differ from nicotinic receptors of muscle pharmacologically (none bind alpha bungarotoxin, and some have very high affinity for nicotine), structurally (having only two types of subunits rather than four), and, in some cases, in functional role (some are located presynaptically). However, there are amino acid sequence homologies between the subunits of these receptors that suggest the location of important functional domains. Sequence homologies also suggest that the subunits of the proteins of this family all evolved from a common ancestral protein subunit. The ligand-gated ion channel characteristic of this superfamily is formed from multiple copies of homologous subunits. Conserved domains responsible for strong stereospecific association of the subunits are probably a fundamental organizing principle of the superfamily. Whereas the structure of muscle-type nicotinic receptors appears to have been established by the time of elasmobranchs and has evolved quite conservatively since then, the evolution of neuronal-type nicotinic receptors appears to be in more rapid flux. Certainly, the studies of these receptors are in rapid flux, with the availability of monoclonal antibody probes for localizing, purifying, and characterizing the proteins, and cDNA probes for determining sequences, localizing mRNAs, expressing functional receptors, and studying genetic regulation. The role of nicotinic receptors in neuromuscular transmission is well understood, but the role of nicotinic receptors in brain function is not. The current deluge of data using antibodies and cDNAs is beginning to come together nicely to describe the structure of these receptors. Soon, these techniques may combine with others to better reveal the functional roles of neuronal nicotinic receptors.

Peptide E and its products, BAM 18 and Leu-enkephalin, in bovine adrenal medulla and cultured chromaffin cells: release in response to stimulation

Peptide E is a 25 amino acid opioid peptide which, if cleaved at the sole double basic (Lys-Arg) typical processing site, would generate two opioid fragments, the amino-terminal fragment BAM 18 and the carboxy-terminal fragment Leu-enkephalin. We have analysed extracts of bovine adrenal medulla in order to quantify these three opioid peptides (peptide E, BAM 18, and Leu-enkephalin). Here we present evidence that BAM 18 and Leu-enkephalin were present in similar amounts, whereas peptide E was present at a higher concentration. This is consistent with previous observations showing a preferential accumulation of larger peptides in the bovine adrenal, and also with the Lys-Arg bond being the principal site of cleavage of peptide E. However, when bovine adrenal chromaffin cells were maintained in culture for several days, Leu-enkephalin was found to be present in much greater amounts than was BAM 18-like immunoreactivity. The molar amounts of peptide E still exceeded the estimated levels of BAM 18 and Leu-enkephalin. We provide evidence that under conditions of basal release BAM 18 and peptide E were released, whereas Leu-enkephalin was released in much smaller amounts, if at all. On stimulation with nicotine results were consistent with an increased release of all three peptides with a preferential stimulation of Leu-enkephalin release. Under all conditions, the molar amounts of peptide E released apparently exceeded that of the other peptides. The results are discussed in terms of the regulation of partial proteolysis and the fate of peptide E.

Nicotine-induced alterations in brain regional concentrations of native and cryptic Met- and Leu-enkephalin

The distribution of cryptic forms (larger enkephalin-containing peptides) in neostriatum, hypothalamus, spinal cord T3-L1 and neurointermediate lobe of pituitary were determined by radioimmunoassay. Optimal conditions for enzymic hydrolysis of the cryptic enkephalins by trypsin and carboxypeptidase B were established. The proportion of total Met- and Leu-enkephalin represented by native pentapeptide varied markedly among these central nervous system regions. Also, the distributions of native and cryptic Met-enkephalin were distinct from that of Leu-enkephalin. Chromatographic separation by HPLC of immunoreactive Met-enkephalin peptides revealed only two peaks corresponding to Met-enkephalin and Met-enkephalin sulfoxide in rather equal amounts. Hydrolysis of cryptic Met-enkephalin also produced only two HPLC-separable peaks of immunoreactive Met-enkephalin, again corresponding to Met-enkephalin and Met-enkephalin sulfoxide. Bioactivity of cryptic striatal Met-enkephalin after hydrolysis was demonstrated by antinociception and catalepsy in rats following its intracerebroventricular injection. Repeated short-term administration of nicotine, 0.1 mg/kg IP six times at 30 min intervals, produced significant increases in native and cryptic Met-enkephalin in striatum, consistent with an increase in neuronal release of Met-enkephalin together with increases in synthesis and processing of proenkephalin A in this brain region. This regimen of nicotine also decreased levels of native Met-enkephalin and of both native and cryptic Leu-enkephalin in neurointermediate lobe, consistent with nicotine-induced release of both proenkephalin A- and prodynorphin-derived peptides from neurointermediate lobe.

Reserpine increases chromaffin cell enkephalin stores without a concomitant decrease in other proenkephalin-derived peptides

Reserpine increases the levels of enkephalins in adrenal medullary chromaffin cells; however, the origin of the newly apparent pentapeptides has been the subject of debate, because no increase in the levels of proenkephalin mRNA has been observed. The present study was performed for determining if the reserpine-induced increase in context of enkephalins was derived from processing of preexisting fragments of proenkephalin. Bovine chromaffin cell enkephalins and larger enkephalin-containing peptides were separated by reversed-phase HPLC and identified by approximate molecular weight, elution with peptide standards, and enkephalin sequences contained. Treatment of the cells with reserpine increased the levels of enkephalins and of enkephalin-containing peptides of up to approximately 3 kilo-daltons without reducing the levels of larger enkephalin-containing peptides. Similar results were obtained with another catecholamine-depleting drug, tetrabenazine. In contrast, treatment of chromaffin cells with theophylline or forskolin increased the levels of both enkephalins and enkephalin-containing peptides of all sizes. The results suggest that new synthesis of proenkephalin is required for the effects of reserpine, although proenkephalin processing is also altered by this drug.

Synthesis and apical and basolateral secretion of thyroglobulin by thyroid cell monolayers on permeable substrate: modulation by thyrotropin

In the thyroid glands, thyroglobulin (Tg) is specifically synthesized by follicular cells and then secreted into the apical lumen where it is concentrated and used as a substrate for thyroid hormone synthesis. The presence of Tg in the circulation has been reported in normal and pathological situations. To determine the domains of the plasma membrane, apical and/or basolateral, involved in Tg secretion, porcine thyroid epithelial cells were cultured as a monolayer on the porous bottom of a culture chamber in which both apical and basal media are independently accessible. Control experiments using labeled Tg ascertained the tightness of the monolayer and showed that within 48 h only 0.2-0.5% of the Tg introduced in the apical medium was transferred through the cell layer into the basal compartment. For kinetic studies of Tg synthesis and secretion, monolayers were cultured for up to 72 h in the presence of 35S-methionine and with or without 100 microU/ml thyrotropin (TSH) in the basal medium. Labeled Tg was measured by double immunoprecipitation and by fluorography of polyacrylamide gel electrophoresis. We showed that 80-95% of total secreted Tg was recovered in the apical medium. The remainder was secreted through the basolateral membranes in the basal medium. The amount of tg secreted into the apical compartment was stimulated two- to threefold by TSH whereas no TSH effect was observed on secretion in the basal compartment. Moreover, measuring apical and basal volumes, we observed a net water flow from the apical to the basal side. It was stimulated threefold by TSH, increasing the Tg concentration in the apical compartment of the stimulated cell layer. During the culture time, the amount of Tg synthesized and secreted was increased by TSH, as was the Tg mRNA content, as determined by the dot-blot hybridization method.