Peptidergic and adrenergic regulation of the intracellular 3',5'-cyclic adenosine monophosphate content in cultured rat Schwann cells

Adrenoceptors Modulate Cholinergic Synaptic Transmission at the Neuromuscular Junction

Adrenoceptor activators and blockers are widely used clinically for the treatment of cardiovascular and pulmonary disorders. More recently, adrenergic agents have also been used to treat neurodegenerative diseases. Recent studies indicate a location of sympathetic varicosities in close proximity to neuromuscular junctions. The pressing question is whether there could be any effects of endo- or exogenous catecholamines on cholinergic neuromuscular transmission. It was shown that the pharmacological stimulation of adrenoceptors, as well as sympathectomy, can affect both acetylcholine release from motor nerve terminals and the functioning of postsynaptic acetylcholine receptors. In this review, we discuss the recent data regarding the effects of adrenergic drugs on neurotransmission at the neuromuscular junction. The elucidation of the molecular mechanisms by which the clinically relevant adrenomimetics and adrenoblockers regulate quantal acetylcholine release from the presynaptic nerve terminals and postsynaptic sensitivity may help in the design of highly effective and well-tolerated sympathomimetics for treating a number of neurodegenerative diseases accompanied by synaptic defects.

Sympathomimetics regulate neuromuscular junction transmission through TRPV1, P/Q- and N-type Ca2+ channels

Increasing evidence indicates that, first, the sympathetic nervous system interacts extensively with both vasculature and skeletal muscle fibers near neuromuscular junctions (NMJs) and, second, its neurotransmitter, noradrenaline, influences myofiber molecular composition and function and motor innervation. Since sympathomimetic agents have been reported to improve NMJ transmission, we examined whether two in clinical use, salbutamol and clenbuterol, affect the motor axon terminal via extracellular Ca2+ and molecular targets, such as TRPV1 and P/Q- and N-type voltage-activated Ca2+ channels. Electrophysiological recordings in ex-vivo preparations of peroneal nerves and lumbricalis muscles from young adult mice focused on spontaneous miniature end-plate potentials and singly and repetitively evoked end-plate potentials. Adding one dose of salbutamol or clenbuterol to the nerve/muscle preparation or repeatedly administering salbutamol to a mouse for 4 weeks increased spontaneous and evoked synaptic vesicle release but induced a steep decline in EPP amplitude in response to repetitive nerve stimulation. These effects were mediated primarily by ω-agatoxin IVA-sensitive P/Q-type and secondarily by ω-conotoxin GVIA-sensitive N-type Ca2+ channels. Presynaptic arvanil-sensitive TRPV1 channels seem to regulate Ca2+ at the motor neuron terminal at rest, while putative presynaptic β-adrenergic receptors may mediate sympathomimetic and catecholamine effects on presynaptic Ca2+ channels during NMJ activation.

Schwann cells express IP prostanoid receptors coupled to an elevation in intracellular cyclic AMP

We have shown previously that prostaglandin E(2) (PGE(2)) and prostaglandin I(2) (PGI(2)) are each produced in an explant model of peripheral nerve injury. We report that IP prostanoid receptor mRNA and protein are present in primary rat Schwann cells. IP prostanoid receptor stimulation using prostacyclin produced an elevation in intracellular cyclic AMP concentration ([cAMP](i)) in primary Schwann cells. Peak [cAMP](i) was observed between 5-15 min of stimulation followed by a gradual recovery toward basal level. Phosphorylation of cyclic AMP-response element binding protein (CREB) on Ser(133) was also detected after IP prostanoid receptor stimulation and CREB phosphorylation was inhibited completely by the protein kinase A inhibitor, H-89. Intracellular calcium levels were not affected by IP prostanoid receptor stimulation. Unlike forskolin, IP prostanoid receptor stimulation did not significantly augment Schwann cell proliferation in response to growth factor treatment. However, IP prostanoid receptor stimulation increased the number of Schwann cells that were able to generate a calcium transient in response to P2 purinergic receptor activation. These findings suggest that signaling via the IP prostanoid receptor may by relevant to Schwann cell biology in vivo.

Proliferative and morphological effects of endothelins in Schwann cells: roles of p38 mitogen-activated protein kinase and Ca(2+)-independent phospholipase A2

The regulation of Schwann cell (SC) proliferation and morphology is critical to nerve homeostasis. We have previously reported that endothelins (ETs) regulate the activity of different effectors in SC including adenylyl cyclase, phospholipases C and A2 and mitogen-activated protein kinases (MAPKs). These effects imply a possible participation of ETs in the regulation of SC phenotype. We have now investigated the effects of endothelins on the proliferation and morphology of SC, and compared them with the responses to platelet-derived growth factor (PDGF), a known mitogen in these cells. Both endothelin-1 (ET-1) and PDGF increased the incorporation of [3H]thymidine and the proportion of SC in S and G2/M, with a concomitant decrease in the G0/G1 stage cells. Treatment with ET-1 produced rapid changes in the morphology of the SC, characterized by the appearance of cell spreading with shorter processes. The response to ET-1 was considered to represent a proliferative phenotype, in contrast to the effects of forskolin, which decreased [3H]thymidine incorporation in immortalized SC (iSC) and lead to a differentiated morphology with longer extensions. While both ET-1 and PDGF displayed a proliferative effect on SC, treatment with PDGF did not affect the morphology of these cells to a significant extent. A role for p38 MAPK and Ca(2+)-independent phospholipase A2 in the changes in morphology and proliferation of iSC driven by ET-1 was suggested by the effects of selective inhibitors of these pathways [SB202190 and HELSS, respectively]. The unique pattern of signaling pathways recruited by ET-1 and its combined effects on regulation of phenotype and proliferation of SC suggest an important role for this peptide during nerve degeneration/regeneration.

Identification and functional characterization of thromboxane A2 receptors in Schwann cells

Previous reports have demonstrated the presence of functional thromboxane A2 (TP) receptors in astrocytes and oligodendrocytes. In these experiments, the presence and function of TP receptors in primary rat Schwann cells (rSC) and a neurofibrosarcoma-derived human Schwann cell line (T265) was investigated. Immunocytochemical and immunoblot analyses using polyclonal anti-TP receptor antibodies demonstrate that both cell types express TP receptors. Treatment with the stable thromboxane A2 mimetic U46619 (10 microM) did not stimulate intracellular calcium mobilization in rSC, whereas T265 cells demonstrated a calcium response that was inhibited by prior treatment with TP receptor antagonists. U46619 also stimulated CREB phosphorylation on Ser133 in T265 cells and, to a lesser extent, in rSC. To identify potential mechanisms of CREB phosphorylation in rSC, we monitored intracellular cAMP levels following U46619 stimulation. Elevated levels of cAMP were detected in both rSC (20-fold) and T265 (15-fold) cells. These results demonstrate that TP receptor activation specifically stimulates CREB phosphorylation in T265 cells, possibly by a calcium- and/or cAMP-dependent mechanism. In contrast, TP receptor activation in rSC stimulates increases in cAMP and CREB phosphorylation but does not elicit changes in intracellular calcium.

Agonists calcitonin, corticotropin-releasing hormone, and vasoactive intestinal peptide, but not prostaglandins or beta-adrenergic agonists, elevate cyclic adenosine monophosphate levels in oligodendroglial cells

Although 3',5'-cyclic AMP (cAMP) is known to regulate oligodendrocyte development in vitro, little is known about the identity of agonists that induce cAMP synthesis in oligodendroglia. To identify such agonists, we used a novel immunohistochemical method of visualizing cAMP within single cells to screen compounds that are known to activate cAMP synthesis in other cellular systems. Calcitonin, corticotropin-releasing hormone, and vasoactive intestinal peptide elevated cAMP in oligodendroglial cells but not in other cell types present in the cultures (i.e., astrocytes and microglia). In contrast, prostaglandins and the beta-adrenergic agonist isoproterenol, which have previously been reported to induce modest increases in oligodendroglial cell cAMP from biochemical assay of cell homogenates, did not induce a detectable cAMP response in individually identified oligodendroglial cells but instead induced a robust cAMP response in nonoligodendroglial cells.

Colocalization of neurotransmitter receptors on astrocytes in explant cultures of rat CNS

In recent years evidence has accumulated that astrocytes express functional receptors for a variety of neurotransmitters/neuromodulators. By means of electrophysiological and combined autoradiographic and immunohistochemical methods we have demonstrated the colocalization of cholinergic, adrenergic and peptidergic receptors on astrocytes in explant cultures from various regions of rat central nervous system. A great number of biochemical and electrophysiological studies from other laboratories have shown that most of the neurotransmitters exert their effects on second messenger systems and on Ca2+-activated K+-channels. Furthermore, certain neurotransmitters are involved in the regulation of energy metabolism by stimulating enzymatic breakdown of glycogen in astrocytes. It was suggested that there is a cross-talk between the various neurotransmitter receptors on the glial membrane and that these receptors act in a synergistic or antagonistic way. The coexistence of cholinergic and peptidergic receptors on astrocytes is of great interest since both neurotransmitter systems are involved in cognitive functions and are impaired in patients with Alzheimer's dementia. The question is therefore raised whether not only neurones but also astrocytes might be involved in neurodegenerative disorders such as Alzheimer's disease.

Vasoactive intestinal peptide enhances its own expression in sympathetic neurons after injury

Neurons in the adult rat superior cervical sympathetic ganglion (SCG) dramatically increase their content of vasoactive intestinal peptide (VIP) and its mRNA after axotomy in vivo and after explantation. Because the VIP gene contains a functional cAMP response element, the effects of cAMP-elevating agents on VIP expression were examined. VIP, forskolin, or isoproterenol increased cAMP accumulation in explanted ganglia. Secretin, a peptide chemically related to VIP, or forskolin increased VIP levels above those seen in ganglia cultured in control medium, whereas treatment with VIP or secretin increased the level of peptide histidine isoleucine (PHI), a peptide coded for by the same mRNA that encodes VIP. VIP or forskolin also increased VIP-PHI mRNA. In contrast, isoproterenol did not alter levels of VIP, PHI, or VIP-PHI mRNA. Although VIP or forskolin increased cAMP levels in both dissociated neurons and in non-neuronal cells, isoproterenol significantly stimulated cAMP accumulation only in the latter. VIP6-28 was an effective antagonist of the actions of exogenous VIP on cAMP and VIP-PHI mRNA in neuron-enriched cultures. When adult SCG explants were cultured in defined medium, endogenous VIP immunoreactivity was released. When VIP6-28 was added to such cultures, it significantly inhibited the increase in VIP-PHI mRNA that normally occurs. These data indicate that VIP, or a closely related molecule, produced by adult neurons after injury can enhance the expression of VIP. Such a mechanism may prolong the period during which VIP is elevated after axonal damage. The possibility is also discussed that, because VIP is present in preganglionic neurons in normal animals, its release during periods of increased sympathetic nerve activity could alter VIP expression in the SCG.

Regulation of neuropeptide expression in sympathetic neurons. Paracrine and retrograde influences

Sympathetic neurons and other peripheral neurons exhibit a great deal of plasticity in their neuropeptide phenotype in adulthood. In this review, two phenotypes have been described in detail: that of normal sympathetic neurons and that of axotomized neurons. Two factors produced by nonneuronal cells, LIF and NGF, determine which of these phenotypes is expressed. Under normal conditions, the neurons receive NGF primarily, if not exclusively, from the target tissues they innervate. Prior to surgery, the nonneuronal cells within the ganglion and nerve tract express little, if any, LIF. This milieu favors the expression of NPY and suppresses the expression of VIP, galanin, and substance P (Fig. 6). After axotomy, however, this situation is reversed. The neuronal cell bodies are deprived of target-derived NGF and are exposed to LIF both within the ganglion and at the site of the injury (Fig 6). Both the removal of NGF and the exposure to LIF inhibit NPY expression, while promoting the expression of VIP and galanin. Expression of substance P after axotomy occurs primarily, if not entirely, because of the effects of LIF, with the removal of NGF playing no obvious role in the regulation of this peptide.

The cellular and molecular basis of peripheral nerve regeneration

Functional recovery from peripheral nerve injury and repair depends on a multitude of factors, both intrinsic and extrinsic to neurons. Neuronal survival after axotomy is a prerequisite for regeneration and is facilitated by an array of trophic factors from multiple sources, including neurotrophins, neuropoietic cytokines, insulin-like growth factors (IGFs), and glial-cell-line-derived neurotrophic factors (GDNFs). Axotomized neurons must switch from a transmitting mode to a growth mode and express growth-associated proteins, such as GAP-43, tubulin, and actin, as well as an array of novel neuropeptides and cytokines, all of which have the potential to promote axonal regeneration. Axonal sprouts must reach the distal nerve stump at a time when its growth support is optimal. Schwann cells in the distal stump undergo proliferation and phenotypical changes to prepare the local environment to be favorable for axonal regeneration. Schwann cells play an indispensable role in promoting regeneration by increasing their synthesis of surface cell adhesion molecules (CAMs), such as N-CAM, Ng-CAM/L1, N-cadherin, and L2/HNK-1, by elaborating basement membrane that contains many extracellular matrix proteins, such as laminin, fibronectin, and tenascin, and by producing many neurotrophic factors and their receptors. However, the growth support provided by the distal nerve stump and the capacity of the axotomized neurons to regenerate axons may not be sustained indefinitely. Axonal regenerations may be facilitated by new strategies that enhance the growth potential of neurons and optimize the growth support of the distal nerve stump in combination with prompt nerve repair.

5-Hydroxytryptamine2A receptors on cultured rat Schwann cells

Intracellular calcium responses of cultured rat Schwann cells to 5-hydroxytryptamine (5-HT) were examined using the calcium indicator dye fluo-3. Consistent changes in [Ca2+]i were observed with bath application of 5-HT and the basis of these responses was characterized. Application of 5-HT elicited a transient increase in intracellular calcium in a subpopulation of cultured Schwann cells. In many responding cells, the response recurred at approximately regular intervals following the initial transient. In some cases, these oscillations lasted for hours following removal of 5-HT from the bath. The increase in intracellular calcium evoked by 5-HT still occurred in the absence of extracellular calcium, suggesting that 5-HT induces calcium release from intracellular stores. Consistent with this hypothesis, the response to 5-HT was prevented by depletion of inositol trisphosphate-sensitive intracellular calcium stores with thapsigargin. Bath application of caffeine, known to activate Ca2+ release from ryanodine receptor-mediated stores, did not elicit an increase in [Ca2+]i. These results also suggested that 5-HT acted by stimulating a member of the 5-HT2 receptor family since this family employs inositol trisphosphate as a second messenger. In agreement with this interpretation, it was found that the 5-HT-induced intracellular calcium transients could be reversibly blocked by both ketanserin and spiperone, suggesting that the transients are mediated by 5-HT2A receptors. Additional support for this conclusion was obtained by immunocytochemistry using an anti-idiotypic antibody that recognizes a subset of 5-HT receptors.

P2-purigenic receptors regulate phospholipase C and adenylate cyclase activities in immortalized Schwann cells

Schwann cells play an important role in both the development and regeneration of peripheral nerves. Proliferation and differentiation of Schwann cells are critically dependent on changes in the levels of cAMP. ATP is a fast excitatory transmitter in the peripheral nervous system, inducing depolarization of the vagus nerve through occupancy of P2-purinergic receptors. In the present study we demonstrate that extracellular ATP stimulates phospholipase C and inhibits adenylate cyclase activities in cultured Schwann cells. Addition of ATP inhibited, in a concentration-dependent manner, forskolin- or isoprenaline-stimulated adenylate cyclase activity. The rank order of potency corresponding to different purinergic receptor agonists was 2-methylthio-ATP > ATP = ADP > or = adenosine 5'-[gamma-thio]triphosphate (ATP[S]) > UTP, consistent with the involvement of a P2y subtype. Adenosine and adenosine 5'-[alpha,beta-methylene]-triphosphate (pp[CH2pA) were ineffective. Preincubation with pertussis toxin completely blocked this inhibitory effect. When Schwann cells were pre-labelled with myo-[3H]inositol and incubated in Hanks' balanced salt solution containing Ca2+ and Mg2+, addition of ATP[S] resulted in a concentration-dependent increase in the release of InsP with a concomitant increase in intracellular free [Ca2+] ([Ca2+]i). Under these conditions, the effects of both ATP and UTP were of lower magnitude. Removal of Ca2+ and Mg2+ from the assay medium resulted in a significant increase in the effects of ATP[S], ATP and UTP. The decreased response observed in the presence of both bivalent cations (1.2 mM Ca2+ and 1 mM Mg2+) could not be explained either by increased degradation of ATP by Ca2+/Mg2+-dependent nucleotidases or by cation influx. The rank order of potency for the effects of agonists on phospholipase C activity was ATP[S] = adenosine 5'[gamma-imido]triphosphate > ATP -UTP > ADP, indicating the involvement of a P(2U) receptor subtype in this response. Adenosine, AMP and pp[CH2]pA were ineffective. These results demonstrate that immortalized Schwann cells express P(2U) and P(2Y) purinoceptors, which are coupled to stimulation of phospholipase C and inhibition of adenylate cyclase, respectively. Our observations unveil signal-transduction pathways that may be used by ATP to regulate proliferation and differentiation of Schwann cells, and ultimately to influence nerve homeostasis.

Calcitonin gene-related peptide promotes Schwann cell proliferation

Schwann cells in culture divide in response to defined mitogens such as PDGF and glial growth factor (GGF), but proliferation is greatly enhanced if agents such as forskolin, which increases Schwann cell intracellular cAMP, are added at the same time as PDGF or GGF (Davis, J. B., and P. Stroobant. 1990. J. Cell Biol. 110:1353-1360). The effect of forskolin is probably due to an increase in numbers of PDGF receptors (Weinmaster, G., and G. Lemke. 1990. EMBO (Eur. Mol. Biol. Organ.) J. 9:915-920. Neuropeptides and beta-adrenergic agonists have been reported to have no effect on potentiating the mitogenic response of either PDGF or GGF. We show that the neuropeptide calcitonin gene-related peptide (CGRP) increases Schwann cell cAMP levels, but the cells rapidly desensitize. We therefore stimulated the cells in pulsatile fashion to partly overcome the effects of desensitization and show that CGRP can synergize with PDGF to stimulate Schwann cell proliferation, and that CGRP is as effective as forskolin in the pulsatile regime. CGRP is a good substrate for the neutral endopeptidase 24.11. Schwann cells in vivo have this protease on their surface, so the action of CGRP could be terminated by this enzyme and desensitization prevented. We therefore suggest that CGRP may play an important role in stimulating Schwann cell proliferation by regulating the response of mitogenic factors such as PDGF.

Schwann cell ATP-mediated calcium increases in vitro and in situ are dependent on contact with neurons

Schwann cells freshly isolated from the sciatic nerves of neonatal rats respond to exogenously applied ATP with a rapid increase in cytosolic calcium. This increase in [Ca2+]i is mediated by a P2Y-purinergic pathway (Lyons et al.: J. Neurochem. 63:552-560, 1994) and was measured using the calcium indicator dye, fura-2/AM, and a video-enhanced calcium imaging system. The ability to respond to ATP with increases in intracellular calcium is lost over a period of several days in culture; this loss can be prevented or reversed by application of cAMP analogs in a defined medium. We now demonstrate that the direct contact of Schwann cells with neurons also induces and stabilizes this ATP responsiveness. The induction of ATP responsiveness was observed among all Schwann cells contacting neurites, including those forming myelin, and regardless of whether the source of neurons was dorsal root ganglion neurons or superior cervical ganglion neurons. Approximately 85% of Schwann cells responded to ATP over the time studied (72 d in coculture). Addition of axolemma to Schwann cell cultures did not induce ATP responsiveness. We also examined the ATP responsiveness of Schwann cells in situ (excised nerves) using laser-scanning confocal microscopy and the calcium indicator dye, fluo-3/AM. Schwann cells in intact sciatic nerve segments isolated from neonatal and 16-day-old rats exhibited ATP-mediated [Ca2+]i increases. We conclude that neuronal contact is necessary for the expression of the ATP-mediated calcium responses in Schwann cells and that these responses are independent of myelin formation or maintenance.

cAMP-dependent differential regulation of extracellular matrix (ECM) gene expression in cultured rat Schwann cells

cAMP-dependent regulation of the steady-state mRNA levels for the ECM components, laminin A, B1 and B2 chains, collagen types I, III and IV were examined by Northern blot analysis in cultured rat Schwann cells. ECM mRNAs of laminin B1 chain and collagen types I and IV were expressed at high levels in the control Schwann cells, while laminin B2 chain and collagen type III mRNA levels were low, and laminin A chain mRNA was not detectable. When Schwann cells were treated with forskolin or cAMP derivatives, the gene expression for the ECM molecules constituting the Schwann cell basement membrane, laminin B1 and B2 chains, and collagen type IV, was enhanced in time- and dose-dependent manners for exogenously administered forskolin or cAMP derivatives, while the mRNA levels for the ECM molecules, which are not the major components of the basement membrane, fibrillary collagen types I and III were significantly suppressed. This cAMP-dependent differential regulation of Schwann cell ECM gene expression may be related to the role of each ECM molecule in the peripheral nerve development and regeneration.

Modulation of Schwann cell phenotype by TGF-beta 1: inhibition of P0 mRNA expression and downregulation of the low affinity NGF receptor

The phenotype of a fully differentiated, mature Schwann cell is apparently largely determined by Schwann cell-axon interactions. In vitro, elevation of intracellular cAMP levels in Schwann cells induces a phenotype which resembles that of a mature, i.e., axon-related, Schwann cell. Therefore, an important role for cAMP as a second messenger of axon-Schwann cell interactions in vivo is assumed. However, the effects of cAMP on Schwann cells are not restricted to induction of features of a mature phenotype. For example, elevation of intracellular cAMP levels results also in a markedly increased synthesis of nerve growth factor (NGF) mRNA, which is barely detectable in intact sciatic nerves of adult animals. Furthermore, since cAMP induces myelin gene expression in cultured Schwann cells, additional regulatory mechanisms have to be postulated for the induction and maintenance of a mature non-myelinating Schwann cell phenotype. Here we show that a soluble protein "growth factor" can partially induce a non-myelinating nature Schwann cell phenotype in vitro. Treatment with transforming growth factor beta 1 (TGF-beta 1) results in a marked and rapid downregulation of the low affinity NGF receptor (NGFR) on cultured Schwann cells without induction of P0 gene expression. In contrast, in agreement with previous studies, an increase in P0 mRNA levels and a reduction in NGFR mRNA levels are observed after cAMP elevation. Downregulation of NGFR mRNA after cAMP elevation is much slower when compared with the effect of TGF-beta 1, suggesting the involvement of different intracellular mechanisms. Consistent with this hypothesis, we did not observe an induction of mRNA coding for TGF-beta isoforms after cAMP elevation in cultured Schwann cells which constitutively synthesize TGF-beta 1 mRNA.

Interaction between cAMP elevation, identified growth factors, and serum components in regulating Schwann cell growth

Most previous studies on Schwann cell proliferation in vitro have used serum-containing media. This complicates the analysis of agents required for cell division since serum contains an ill-defined mixture of hormones and growth factors. Serum-free medium has therefore been used to analyse the response of Schwann cell to previously identified Schwann cell mitogens. Serum factors were not necessary for DNA synthesis in response to platelet-derived growth factor, basic fibroblast growth factor, or glial growth factor, provided they were used in combination with forskolin to elevate intracellular cAMP. Transforming growth factor beta 1, a Schwann cell mitogen in serum, was not mitogenic under these conditions. Neither the growth factors nor forskolin were effective when used alone. Growth control was analysed further using long-term cultured Schwann cells that had spontaneously immortalized. Measurements of endogenous cAMP levels in short- and long-term Schwann cells revealed that long-term cells had two to three times higher basal cAMP levels. As predicted by these findings, platelet-derived growth factor, basic fibroblast growth factor, and glial growth factor stimulated DNA synthesis in long-term cells without requiring costimulation by agents which elevate cAMP (while transforming growth factor beta 1 had no effect).(ABSTRACT TRUNCATED AT 250 WORDS)

Platelet-derived growth factors and fibroblast growth factors are mitogens for rat Schwann cells

Rat sciatic nerve Schwann cells in culture respond to a limited range of mitogens, including glial growth factor, transforming growth factors beta-1 and beta-2 (TGF-beta 1, TGF-beta 2), some cell membrane-associated factors, and to agents such as cholera toxin and forskolin which raise intracellular levels of cAMP. These responses require the presence of FCS, which exhibits little or no mitogenic activity in the absence of other factors. However, we recently found that forskolin greatly potentiates the mitogenic signal from TGFs-beta 1 and beta 2, raising the possibility that cAMP might couple other factors to mitogenesis. We have therefore screened a range of candidate mitogens using DNA synthesis assays. Other than TGFs-beta and glial growth factor, none of the factors tested were mitogenic in the presence of 10% serum alone. With the addition of forskolin, however, porcine PDGF, human PDGF, acidic and basic FGF were potent mitogens for rat Schwann cells, stimulating DNA synthesis and increasing cell number. Cholera toxin and dibutyrylcyclicAMP, but not 1,9-dideoxyforskolin, can substitute for forskolin indicating that the mitogenic effect is mediated via adenylyl cyclase activation. Porcine PDGF gave half-maximal stimulation at 15 pM, and human PGDF an equivalent response at 1 nM. Basic FGF was half maximal at 5 pM, acidic FGF at 1 nM. The recognition of PDGFs and FGFs as mitogens for Schwann cells has many implications for the study of Schwann cell proliferation in the development and regeneration of nerves, and in Schwann cell tumorigenesis.