Increase in thyroid follicular cell tumors in nelfinavir-treated rats observed in a 2-year carcinogenicity study is consistent with a rat-specific mechanism of thyroid neoplasia.

The carcinogenic potential of nelfinavir mesylate (nelfinavir) was evaluated in a 2-year oral (gavage) study on Sprague-Dawley rats at dose levels of 0 (control), 0 (vehicle control), 100, 300 and 1000 mg/kg per day. At the end of the treatment, increased incidences of thyroid follicular cell hyperplasia and neoplasms were observed at 300 (males) and 1000 mg/kg per day (both sexes). There were no other treatment-related effects and no tumors at other sites. Results from previous studies indicated a number of effects in the liver and thyroid, as well as metabolic profiles that suggested nelfinavir might cause thyroid hyperplasia/neoplasia secondary to hormone imbalance by altering thyroid hormone disposition. To investigate this hypothesis, the effects of nelfinavir on gene expression in rat hepatocytes and liver slices (in vitro), thyroxine plasma clearance, and thyroid gland function were evaluated. Compared to controls, gene expression analyses demonstrated an increased expression of glucuronyltransferase (UDPGT) and CYP450 3A1 in nelfinavir-treated rat hepatocytes and liver slices. In rats treated with nelfinavir (1000 mg/kg per day) for 4 weeks, liver weights and centrilobular hepatocellular hypertrophy were increased and minimal to mild diffuse thyroid follicular cell hypertrophy and follicular cell hyperplasia were evident in the thyroid gland. Thyroid-stimulating hormone (TSH) levels were significantly increased (three-fold), while tri-iodothyronine (T3)/tetra-iodothyronine (T4) and reverse T3(rT3) levels were unchanged, indicating that a compensated state to maintain homeostasis of T3/T4 had been achieved. Plasma 125I-thyroxine clearance was increased and the plasma thyroxine AUC0-48 was decreased (24%) compared to control. In conclusion, these data indicate that thyroid neoplasms observed in the nelfinavir-treated rats were secondary to thyroid hormone imbalance. Increased thyroxine clearance contributes to the effects of nelfinavir on thyroid gland function and is probably a result of UDPGT induction that leads to elevated TSH levels in the rat and eventual thyroid neoplasia. These results are consistent with a well-recognized rat-specific mechanism for thyroid neoplasms.

Neonatal mouse model: review of methods and results.

The neonatal mouse model, in various forms, has been used experimentally since 1959 and a large number of chemicals have been tested. The neonatal model is known to be very sensitive for the detection of carcinogens that operate via a genotoxic mode of action. In contrast, it is known not to respond to chemicals that act via epigenetic mechanisms, commonly observed in the two-year carcinogenicity studies. As such, the model has a high sensitivity and specificity in its response. Dose selection for the neonatal model is based on the maximum tolerated or feasible dose. Traditionally, compounds have been tested via the IP route of administration in this model. In some cases, this has limited the amount of material that can be administered because of the low dosing volumes (10 to 20 microL) that can be administered IP. For the ILSI project, the neonatal model was adapted for oral administration, which has the advantages of being the same route for which most pharmaceuticals are administered. In addition, a 10-fold increase in the volume of administration (100 to 200 microL) and the ability to dose drugs in suspension, permits much higher doses to be used as compared to the IP route of administration. The spontaneous tumors in the neonatal model occurred mainly in the liver of male mice and lung of male and female mice with a few tumors observed in the Harderian gland. The positive control, DEN produced a robust, uniform, and reproducible tumor response with the target organs essentially limited to liver and lung. A total of 13 compounds out of the 21 ILSI ACT compounds were evaluated in the neonatal model involving 18 studies with duplicate studies for some compounds. The genotoxic carcinogens including those used as positive controls were clearly positive (cyclophosphamide, diethylnitrosamine, 6-nitrochrysene). The non-genotoxic rodent carcinogens were clearly negative (chlorpromazine, sulfisoxazole, sulfamethoxazole, clofibrate, DEHP, haloperidol, metaproteranol, and phenobarbital). The non-genotoxic human carcinogen (cyclosporin) was clearly negative. The two other human carcinogens phenacetin and DES were negative and interestingly estradiol was negative in one of the two oral studies, but was clearly positive in the other. Considering the mode of action for three of the human carcinogens (DES, cyclosporin and phenacetin), which were negative in this model, the mode of action in humans is likely to be epigenetic. Overall, for the 3 clearly genotoxic chemicals, all were positive. For the 9 clearly non-genotoxic chemicals, all 9 were negative. The two human carcinogens for which genotoxicity may or may not play a role (DES and phenacetin) were negative and estradiol was positive in I of the two oral studies. Overall, the extensive database for compounds tested in the neonatal mouse model would support its use as an alternative model for the assessment of the carcinogenic potential of a chemical. The model responds to chemicals that act via a genotoxic mode of action that represent a greater concern for human cancer risk.

Vitamin D3-induced proliferative lesions in the rat adrenal medulla.

Adrenal medullary hyperplasia and pheochromocytomas are induced in rats by a variety of non-genotoxic agents, and we have hypothesized that these agents induce lesions indirectly by stimulating chromaffin cell proliferation. Vitamin D3, which has not been previously associated with adrenal medullary proliferative lesions, is the most potent in vivo stimulus to chromaffin cell proliferation yet identified. The present investigation utilized the vitamin D3 model to prospectively test the relationship between mitogenicity and focal proliferative lesions in the adrenal medulla and to determine early events in the pathogenesis of these lesions. Charles River Crl:CD BR rats were treated with 0; 5000; 10,000; or 20,000 IU/kg/day of vitamin D3 in corn oil (5 ml/kg) by oral intubation. Rats were killed after 4, 8, 12, or 26 weeks of treatment, following a final week of labeling with bromodeoxyuridine (BrdU) using a mini-pump. Adrenal sections were double-stained for BrdU and phenylethanolamine-N-methyl transferase (PNMT) to discriminate epinephrine (E) from norepinephrine (NE) cells or for vesicular acetylcholine transporter (VAchT) to identify cholinergic nerve endings. Vitamin D3 caused a 4-5-fold increase in BrdU labeling at week 4, diminishing to a 2-fold increase by week 26. An initial preponderance of labeled E cells gave way to a preponderance of labeled NE cells. By week 26, 17/19 (89%) animals receiving the 2 highest doses of vitamin D3 had focal adrenal medullary proliferative lesions, in contrast to an absence of lesions in control rats. The lesions encompassed a spectrum including BrdU-labeled "hot spots" not readily visible on H and E sections, hyperplastic nodules, and pheochromocytomas. Lesions were usually multicentric, bilateral, and peripheral in location, and almost all were PNMT-negative. The lesions were not cholinergically innervated, suggesting autonomous proliferation. Hot spots, hyperplastic nodules, and pheochromocytomas appear to represent a continuum rather than separate entities. Their development might involve selective responses of chromaffin cell subsets to mitogenic signals, influenced by both innervation and corticomedullary interactions. A number of non-genotoxic compounds that induce pheochromocytomas in rats are known to affect calcium homeostasis. The results of this study provide further evidence to support the hypothesis that altered calcium homeostasis is indirectly involved in the pathogenesis of pheochromocytomas, via effects on chromaffin cell proliferation.

Absence of DNA adduct formation by phenobarbital, polychlorinated biphenyls, and chlordane in mouse liver using the 32P-postlabeling assay.

Phenobarbital (PB), polychlorinated biphenyls (PCBs), and chlordane (CLD) increase liver tumor incidences in rodents, and all are tumor promoters. Most indirect tests for DNA reactivity, including mutagenicity and chromosomal damage, have been negative with these agents. Consequently, the modes of action for tumorigenesis by these compounds are not believed to involve direct DNA reactivity; however, only limited information from direct tests is available for the lack of DNA adduct formation. PB, PCBs, and CLD were tested for DNA adduct formation in the liver of male and female B6C3F1 mice after either single or 2-week dietary exposures. Single gavage dose levels were as follows: PB, 200 mg/kg; PCBs, 50 mg/kg; and CLD, 50 mg/kg. Dietary dose levels were as follows: PB, 1000 ppm; PCBs, 200 ppm and CLD, 200 ppm. Animals were killed 24 h following the end of test-substance administration. DNA was extracted from the liver, and DNA adduct concentrations were enriched using either 1-butanol extraction of adducted nucleotides or nuclease P1 digestion of unadducted nucleotides. Using this protocol, none of the three test compounds produced DNA adducts detected by 32P-postlabeling. Similar negative results were obtained for DNA from the livers of both male and female mice receiving either single or 2-week exposures. The two positive controls, benzidine for the 1-butanol extraction procedure and 2-acetylaminofluorene for the nuclease P1 procedure, showed the expected patterns of DNA adducts. These results support the conclusion that the carcinogenicity of PB, PCBs, and CLD in experimental animals is not the result of direct DNA reactivity, but involves epigenetic mechanisms.

5-methylcytosine is present in the 5' flanking region of Ha-ras in mouse liver and increases with ageing.

Modifications to DNA-5-methylcytosine (5MeC) content (i.e., alterations in the level of 5MeC) constitute epigenetic events. In general, hypomethylation of a gene is necessary but not sufficient for expression, while methylated genes typically are quiescent. Ha-ras is an oncogene commonly implicated in murine liver tumorigenesis, often, though not always, involving mutation. A PCR-based approach using pre-PCR digestion with methylation-sensitive enzymes was employed to determine the 5MeC content of the 5' flanking region of this gene in (i) B6C3F1 and C57BL/6 mouse liver from young animals (4 months old) and (ii) B6C3F1 mouse liver from aged animals (24 months old). Two segments of the 5' flanking region of Ha-ras were examined. We demonstrate the presence of 5MeC in a portion of the 5' flanking region of Ha-ras that does not share characteristics of a CpG island, while a region that shares CpG island characteristics is primarily unmethylated. Differences in methylation status in these areas of Ha-ras were not observed between B6C3F1 and C57BL/6 mouse livers. Increases in methylation status were observed with ageing in B6C3F1 mouse liver. These data provide a role for methylation in regulating Ha-ras expression in mouse liver. Ha-ras in human liver has been reported to be unmethylated. There are substantial sequence differences in a key region of the 5' flanking region of Ha-ras in mice as compared to humans. These differences in DNA methylation and sequence may, in part, provide a basis for the frequent involvement of Ha-ras in mouse liver tumors and its virtual lack of involvement in human tumors.

Comparison of effect of tumor promoter treatments on DNA methylation status and gene expression in B6C3F1 and C57BL/6 mouse liver and in B6C3F1 mouse liver tumors.

The effects of different liver tumor-promoting treatments (i.e., a choline-devoid, methionine-deficient (CMD) diet, phenobarbital (PB), or both) on Ha-ras and raf methylation status and expression were determined in mouse strains with different susceptibilities to liver tumor formation: the relatively sensitive B6C3F1 and the relatively resistant C57BL/6. Additionally, B6C3F1 mouse liver tumors, spontaneous or PB induced, were assessed for alterations in global DNA methylation status and expression of Ha-ras and raf. The CMD diet led to hypomethylation of Ha-ras and raf after 12 wk of administration in B6C3F1 and C57BL/6 mice. At this early phase of tumor promotion, the frequency of increased expression of both Ha-ras and raf mRNAs was higher in the B6C3F1 but not the C57BL/6 mice. This is a mechanism that may, in part, underlie the heightened sensitivity of the B6C3F1 mouse to liver tumorigenesis. Subpopulations of B6C3F1 mouse liver tumors displayed altered global methylation status, with both hypomethylation and hypermethylation evident. Carcinomas were significantly more hypomethylated than adenomas. The level of raf mRNA was not changed in spontaneous or PB-induced B6C3F1 mouse liver tumors. Increased expression of Ha-ras was evident in some spontaneous B6C3F1 liver tumors and in most of the PB-induced liver tumors. These experiments support the concept that altered DNA methylation plays a key role in tumorigenesis and indicate that the high propensity of the B6C3F1 mice to liver tumorigenesis may be due, in part, to a decreased ability to maintain normal methylation status.

Comparative studies of chromaffin cell proliferation in the adrenal medulla of rats and mice.

Spontaneous and drug-induced pheochromocytomas are common in rats and rare in mice. The antihypertensive drug reserpine has been shown to both induce pheochromocytomas and stimulate chromaffin cell proliferation in rats, leading to the hypothesis that reserpine causes pheochromocytomas indirectly by providing a proliferative setting in which DNA damage may occur. The present investigation was undertaken to obtain baseline information on the relationship across species between chromaffin cell proliferation and pheochromocytomas. Basal chromaffin cell proliferation was compared in age-matched young adult mice and rats. In addition, mice were studied for adrenal medullary responses to reserpine, and mouse chromaffin cells in vitro were studied for responses to agents that are mitogenic for cultured rat chromaffin cells. Concurrently maintained F-344 rats and several strains of mice showed no significant difference in basal BrdU incorporation over a 1-week period. Mice also showed an adrenal medullary proliferative response to reserpine that was comparable to the response previously reported for rats. However, there was a marked disparity between rat and mouse chromaffin cells in vitro, and cultured mouse chromaffin cells did not respond to any mitogens. The in vivo data indicate that interspecies differences in basal- or reserpine-stimulated chromaffin cell proliferation sufficient to account for different frequencies of pheochromocytomas are not detectable at a single time point in young adult animals. However, the possibility that such differences might emerge with aging has not been ruled out. These data further suggest either that stimulation of chromaffin cell proliferation might be necessary but not sufficient for development of pheochromocytomas or that stimulated proliferation in mice might not be sustained. The inability of cultured mouse chromaffin cells to respond to mitogens raises the speculation of whether mechanisms that prevent proliferation of normal chromaffin cells in vitro might also help to protect mice from developing pheochromocytomas.

Vitamin D3, lactose, and xylitol stimulate chromaffin cell proliferation in the rat adrenal medulla.

Chronic consumption by rats of diets rich in sugars or sugar alcohols leads to an increased incidence of pheochromocytomas. This relationship is hypothesized to be based on altered Ca2+ homeostasis due to increased intestinal Ca2+ absorption. Other agents associated with pheochromocytomas in rats in long-term toxicity studies have been shown to increase chromaffin cell proliferation, leading to the suggestion that the tumors occur secondarily to increased chromaffin cell turnover. We have demonstrated marked stimulation of chromaffin cell proliferation by vitamin D3, a potent stimulus to Ca2+ absorption not previously associated with adrenal medullary toxicity. This effect is detectable during the first week of dietary supplementation and persists throughout a 4-week time course. Lactose and xylitol, representative of sugars and sugar alcohols associated with pheochromocytomas, are also mitogenic but to a lesser extent, with their effects first detectable during Week 4 of dietary supplementation. Vitamin D3, its active metabolite calcitriol, lactose, and xylitol all fail to stimulate proliferation of rat chromaffin cells in vitro. The mitogenic effects of these agents may be mediated presynaptically in vivo. The data suggest that altered Ca2+ homeostasis may increase chromaffin cell proliferation and support the hypothesis that diets containing high concentrations of sugars and sugar alcohols cause pheochromocytomas in rats secondarily by this mechanism.

Cell proliferation and global methylation status changes in mouse liver after phenobarbital and/or choline-devoid, methionine-deficient diet administration.

Our laboratory is testing the hypothesis that hypomethylation of DNA [a decreased content of 5-methylcytosine (5MeC) compared with cytosine] facilitates aberrant oncogene expression involved in tumorigenesis, using a model system of mouse strains with differing susceptibilities to liver tumorigenesis. The B6C3F1 (C57BL/6 x C3H/He) mouse serves as the relatively susceptible strain and C57BL/6 serves as the relatively resistant strain. Phenobarbital (PB) and/or administration of a choline-devoid, methionine-deficient diet (CMD) were employed as non-genotoxic hepatocarcinogens. We have examined hepatocyte and nonhepatocyte proliferation in conjunction with an assessment of global methylation changes in liver DNA of B6C3F1 and C57BL/6 mice following these promoter treatments. Bromodeoxyuridine incorporation into DNA, used to measure cell proliferation indirectly, was visualized by immunohistochemistry and quantified by a Macintosh-based image analysis system. Increased hepatocyte proliferation was demonstrated following all three treatments. This increase was larger in C57BL/6 (the relatively resistant strain) as compared with B6C3F1. In contrast, global hypomethylation was evident to a larger extent in the B6C3F1 mouse, as compared with C57BL/6. PB led to hypomethylation (>20% decrease as compared with controls) at weeks 1, 2 and 4 in B6C3F1, but not in C57BL/6 at the same time points. CMD diet administration led to hypomethylation in both strains. At week 1, 21 and 9% decreases in global methylation status were observed in B6C3F1 and C57BL/6 respectively. Evaluation of these data suggests that the heightened sensitivity of the B6C3F1 mouse compared with the C57BL/6 is due, in part, to a decreased capacity for, or fidelity of, maintaining normal methylation status. The relatively resistant strain is better able to maintain the normal methylation status of DNA in the face of a higher level of cell proliferation.

Sustained stimulation of rat adrenal chromaffin cell proliferation by reserpine.

Chronic administration of reserpine is associated with the development of pheochromocytomas in rats. Short-term administration of reserpine to rats has been shown to stimulate chromaffin cell proliferation, leading to the hypothesis that reserpine causes pheochromocytomas indirectly by providing a proliferative backdrop on which genetic damage may occur. However, it is not known whether the proliferative effects of reserpine persist long enough for this model to be tenable. In the present investigation, the effects of reserpine on bromodeoxyuridine (BrdU) incorporation into epinephrine (E)- and norepinephrine (NE)-type chromaffin cells were studied after 1, 4, and 12 weeks of reserpine administration. Reserpine administered in the diet at 10 or 50 ppm was shown to result in a persistent mitogenic stimulation of the rat adrenal medulla. Cells that incorporated BrdU at all time points appeared to be typical E- and NE-type chromaffin cells, and the ratio of BrdU-labeled E cells to BrdU-labeled NE cells was not altered by reserpine. An additional observation was that the ratio of all E cells to all NE cells declined after Week 1 and that the decline could be accelerated by administration of reserpine. This finding suggests that neural stimulation of chromaffin cells might play a role in age-related functional changes of the adrenal medulla during early adult life. The present observations support the hypothesis that reserpine induces pheochromocytomas indirectly by increasing chromaffin cell proliferation. They also decrease the likelihood that rat pheochromocytomas arise from preferential stimulation of proliferation of a particular cell type.

Mechanistic considerations for the relevance of animal data on thyroid neoplasia to human risk assessment.

There are two basic mechanisms whereby chemicals produce thyroid gland neoplasia in rodents. The first involves chemicals that exert a direct carcinogenic effect in the thyroid gland and the other involves chemicals which, through a variety of mechanisms, disrupt thyroid function and produce thyroid gland neoplasia secondary to hormone imbalance. These secondary mechanisms predominantly involve effects on thyroid hormone synthesis or peripheral hormone disposition. There are important species differences in thyroid gland physiology between rodents and humans that may account for a marked species difference in the inherent susceptibility for neoplasia to hormone imbalance. Thyroid gland neoplasia, secondary to chemically induced hormone imbalance, is mediated by thyroid-stimulating hormone (TSH) in response to altered thyroid gland function. The effect of TSH on cell proliferation and other aspects of thyroid gland function is a receptor mediated process and the plasma membrane surface of the follicular cell has receptors for TSH and other growth factors. Small organic molecules are not known to be direct TSH receptor agonists or antagonists; however, various antibodies found in autoimmune disease such as Graves' disease can directly stimulate or inhibit the TSH receptor. Certain chemicals can modulate the TSH response for autoregulation of follicular cell function and thereby increase or decrease the response of the follicular cell to TSH. It is thus important to consider mechanisms for the evaluation of potential cancer risks. There would be little if any risk for non-genotoxic chemicals that act secondary to hormone imbalance at exposure levels that do not disrupt thyroid function. Furthermore, the degree of thyroid dysfunction produced by a chemical would present a significant toxicological problem before such exposure would increase the risk for neoplasia in humans.

Comparative pharmacodynamics of hepatic cytochrome P450 2B induction by 5,5-diphenyl- and 5,5-diethyl-substituted barbiturates and hydantoins in the male F344/NCr rat.

To explore further the structural requirements for ligand interaction with the putative phenobarbital receptor, the pharmacodynamics of CYP2B induction by 5,5-diphenylbarbituric acid, phenytoin (5,5-diphenylhydantoin), barbital (5,5-diethylbarbituric acid) and 5,5-diethylhydantoin were investigated in the male F344/NCr rat. Steady-state total (free plus protein-bound) serum drug concentration, measured after 14 days of administration of the compounds in the diet, was used as an approximation of intrahepatocellular drug concentration. The serum concentrations associated with half-maximal hepatic CYP2B induction (EC50 values) were 6 to 11 microM and 15 to 18 microM for the diphenyl-substituted barbiturate and hydantoin, respectively, based on measurement of pentoxy- or benzyloxyresorufin O-dealkylation activities, or immunoreactive CYP2B1 protein. The corresponding potency values for the diethyl-substituted barbiturate and hydantoin were 16 to 20 microM and > or = 500 microM, respectively. The magnitudes of the maximal CYP2B induction responses elicited by the diphenyl-substituted congeners and by barbital were 94 to 122% of the responses resulting from phenobarbital itself. In contrast, the maximum induction responses elicited by 5,5-diethylhydantoin were only 24% as great as those elicited by phenobarbital. The finding of a CYP2B inducer with a potency value 2 to 3 orders of magnitude lower than those for certain other prototype CYP2B inducers is suggestive but not proof of receptor mediation in the induction process.

Alterations in the methylation status and expression of the raf oncogene in phenobarbital-induced and spontaneous B6C3F1 mouse liver tumors.

The liver tumor-prone B6C3F1 mouse (C57Bl/6 female x C3H/He male), in conjunction with the more susceptible C3H/He paternal strain and the resistant C57BL/6 maternal strain, is an excellent model for studying the mechanisms involved in carcinogenesis. The study reported here indicated that the B6C3F1 mouse inherited a maternal raf allele containing a methylated site not present in the paternal allele. Seven days after partial hepatectomy or after administration of a promoting dose of phenobarbital (PB) for 14 d; raf in B6C3F1 mouse liver was hypomethylated. The additional methylated site in the allele inherited from C57BL/6 was not maintained. The methylation status of raf in the liver of the C57BL/6 mouse was not affected by PB treatment. This indicates that the B6C3F1 mouse is less capable of maintaining methylation of raf than the C57BL/6 strain is. In both PB-induced and spontaneous B6C3F1 liver tumors, raf was hypomethylated in a nonrandom fashion. The level of raf mRNA increased in seven of 10 PB-induced tumors but in only one of five spontaneous tumors, whereas the level of Ha-ras mRNA increased in nine of 10 PB-induced tumors and in four of five spontaneous tumors. The results of our investigation (a) support the hypothesis that hypomethylation of DNA is a nongenotoxic mechanism involved in tumorigenesis, (b) support the notion that PB promotes liver tumors that develop along a pathway different from that leading to spontaneous tumors, and (c) indicate that differences in DNA methylation between C57BL/6 and B6C3F1 mice could, in part, account for the unusually high tendency of the latter strain to develop liver tumors.

Thyroid gland neoplasia: non-genotoxic mechanisms.

There are two basic mechanisms whereby chemicals produce thyroid gland neoplasia in rodents. The first involves chemicals that exert a direct carcinogenic effect in the thyroid gland and the other involves chemicals which, through a variety of mechanisms, disrupt thyroid function and produce thyroid gland neoplasia secondary to hormone imbalance. There are important species differences in thyroid gland physiology between rodents and humans which may account for a marked species difference in the inherent susceptibility for neoplasia secondary to hormone imbalance. Thus, it is important to consider mechanism in the evaluation of potential cancer risks. There would be little if any risk for apparently nongenotoxic chemicals that act secondary to hormone imbalance at exposure levels that do not disrupt thyroid function. Further, the degree of thyroid dysfunction produced by a chemical would present a major toxicological problem before such exposure would increase the risk for neoplasia for humans.

Neurogenic signals regulate chromaffin cell proliferation and mediate the mitogenic effect of reserpine in the adult rat adrenal medulla.

The adrenal medulla is innervated by nerve fibers from several sources, which synapse on chromaffin cells and stimulate the secretion of catecholamines. The antihypertensive agent reserpine is known to reflexively increase this neurogenic stimulation by depleting catecholamine stores, and long-term administration of reserpine is associated with adrenal medullary hyperplasia and neoplasia. To determine the role of neurogenic signals in regulating normal and reserpine-stimulated proliferation of chromaffin cells, the incorporation of 5-bromo-2'-deoxyuridine (BrdU) into replicating nuclei was assessed in the adrenal medulla of adult rats. Unilateral adrenal denervation caused a 4-5 fold decrease in chromaffin cell labeling by 5-bromo-2'-deoxyuridine during a 2-week labeling period. Denervation also prevented stimulation of labeling in animals receiving reserpine in their diet. These findings suggest that neurogenic control of cell proliferation may play an important role in the pathogenesis of adrenal medullary hyperplasia and neoplasia, and in the normal development of the peripheral and central nervous systems.