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PERSPECTIVES IN PHARMACOLOGY

Ontogenesis of -Adrenoceptor Signaling: Implications for Perinatal Physiology and for Fetal Effects of Tocolytic Drugs

Department of Pharmacology and Cancer Biology, Duke University Medical Center, Durham, North Carolina

Received for publication March 7, 2003
Accepted April 4, 2003.

	Abstract

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G-Protein-coupled receptors play an instrumental role in cellular development and function. In the mature organism, receptor signaling is controlled through the processes of desensitization and down-regulation. Recent evidence suggests that these regulatory mechanisms are not inherent properties, however, but rather are acquired during ontogenesis. This review focuses on -adrenoceptors (ARs), which are found in fetal and neonatal tissues and are effectively linked through adenylyl cyclase (AC) to the production of cAMP. Agonist-induced stimulation of ARs in the immature organism fails to produce desensitization, and instead, responsiveness increases. The unique mechanisms underlying this anomalous response involve induction of AC, a switch to more catalytically efficient AC isoforms, an increase in the ratio of stimulatory to inhibitory G-proteins, and interference with the expression and/or function of other G-protein-linked receptors that provide offsetting, inhibitory inputs. These adjustments are thus heterologous, influencing signaling mediated by a host of other G-protein-coupled neurotransmitter and hormone receptors. The net effect is to maintain and augment AR signaling in the face of continued stimulation, properties that disappear with maturation. The unique regulatory mechanisms for AR signaling in the fetus and neonate provide the necessary physiological adjustments required for the perinatal transition from intrauterine to extrauterine life. At the same time, however, the inability to restrict AR function may underlie adverse effects of AR-agonist tocolytics that are used in the treatment of preterm labor.

	Regulation of -Adrenoceptor Signaling in the Mature Organism

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When excessive AR stimulation is maintained for a prolonged period, the concentration of receptors at the cell surface declines (down-regulation). This also proceeds through receptor phosphorylation and association with arrestins, events followed by endocytosis and internalization of the receptors, and finally by their degradation in lysosomes (Kohout and Lefkowitz, 2003). In addition, down-regulation can occur by decreasing the synthesis of new receptors and/or increasing the rate of receptor degradation. Together with desensitization, receptor down-regulation thus serves to curtail cell signaling in the face of prolonged or excessive input.

The close regulatory relationships among stimulatory input, receptor concentrations, and receptor coupling thus maintain a carefully-controlled balance. It is therefore critical to note that receptor desensitization and down-regulation are not inherent properties of cells containing the receptors but rather are mechanisms that are acquired during development. As shall be reviewed here, there is substantial evidence that the fetus and neonate are resistant to both receptor desensitization and down-regulation, unusual properties that are critical for maintaining physiological responsiveness during the transition from intrauterine to neonatal life. At the same time, the absence of the normal regulatory mechanisms may render the immature organism vulnerable to disruption of cell and organ development by AR agonist tocolytic drugs that are used in the treatment of preterm labor.

	The Fetus and Neonate: Agonist-Induced Sensitization, Not Desensitization

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ARs arise extremely early in embryonic life, even before the formation of the neural tube (Fujinaga and Scott, 1997), and their presence during organogenesis is essential to embryonic survival. Cardiac development provides a particularly prominent example. Physiological responses to AR stimulation are demonstrable as early as the 16-somite embryo (Robkin et al., 1976). Knockouts of tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, result in embryolethality because of cardiac malformations, and the catecholamine-deficient animals can be rescued by supplying norepinephrine precursors or AR agonists (Thomas et al., 1995; Portbury et al., 2003). In terms of regulation of receptor function, the subsequent developmental period presents us with a paradox. Initially, postsynaptic cells do not receive functional innervation so that, as synapses develop, neural input climbs with age. If receptors were regulated as they are in the adult, we would expect to see high receptor numbers and sensitivity early in development and a subsequent fall-off with the onset of innervation. ARs are expressed in a widespread fashion throughout the fetus, even in tissues or cell types that ultimately will be relatively sparse for the receptor, and the receptors are effectively coupled to cAMP generation through AC (Slotkin et al., 1994b). Nevertheless, in most tissues, ARs increase in numbers and responsiveness during the postnatal period in which innervation develops (Lau et al., 1982; Friedhoff and Miller, 1983; Slotkin, 1986; Hou et al., 1989; Maier et al., 1989; Nathanson, 1989; Kidokoro, 1993). It would thus appear that the standard forms of receptor regulation are not present during development. Indeed, repeated administration of AR agonists to fetal or neonatal animals fails to cause receptor down-regulation or desensitization of physiological responses such as heart rate, blood flow, lung liquid reabsorption, lung compliance, or surfactant synthesis (Lau et al., 1982; Kudlacz and Slotkin, 1990; Habib et al., 1991; Stein et al., 1992), the physiological targets of AR stimulation that are essential to the perinatal transition (Lagercrantz and Slotkin, 1986). Even more surprisingly, early exposure of the receptors to neurotransmitter agonists actually enhances net physiological responses instead of desensitizing them (Friedhoff and Miller, 1983; Kudlacz and Slotkin, 1990; Slotkin et al., 1994a), implying a "programming" function of the initial ontogenetic phases of cell signaling.

What mechanisms underlie the resistance to desensitization or down-regulation? Is there simply something "missing" or is there a unique set of active mechanisms that offset desensitization to produce sensitization instead? What role does early input play in determining the future reactivity of the cell to AR input? A number of studies have appeared that answer those questions using the approach shown in Fig. 1. AR signaling through AC involves a series of control points that can be probed with the appropriate stimulants acting at each step in the pathway (Zeiders et al., 1997, 1999a,b, 2000; Auman et al., 2001b, 2002; Garofolo et al., 2002). The concentration of ARs can be assessed with radioligands to provide a measure of receptor up- or down-regulation. Isoproterenol acts through ARs to promote AC activity with the participation of G_s. In turn, the ability of G_s to activate AC can be evaluated with the direct G-protein stimulant NaF. Two separate stimulants, forskolin and Mn²⁺, activate AC directly, bypassing the requirement for receptor or G-protein participation; however, they stimulate AC at different epitopes so that shifts in the AC isoform expressed by the cell lead to differential responses to the two stimulants. Heterologous changes for other receptors participating in the AC pathway can be evaluated, and examples are provided in Fig. 1. Glucagon, which operates through a totally different receptor, shares the same requirement for G_s as does the AR, so that downstream changes in G_s and AC will be shared by glucagon-mediated responses. Inhibitory receptors, such as the m₂AChR, counter the effects of ARs on AC and operate through the inhibitory protein G_i. Evaluations of the number of m₂AChRs (ligand binding) and the AC response to an agonist such as carbachol can thus characterize this component. Finally, as NaF stimulates both G_s and G_i, a change in the net response to NaF reflects a shift in the balance of expression or function of these two opposing signaling proteins.

View larger version (32K): [in this window] [in a new window]   Fig. 1. Schematic example of stimulatory and inhibitory receptor mechanisms controlling AC activity in developing rat cardiac cells. Both ARs and glucagon receptors enhance AC activity through the stimulatory G-protein, Gs, whereas m2AChRs diminish AC activity through the inhibitory protein, Gi. Each step in the pathway can be probed with the appropriate stimulant: isoproterenol for ARs, glucagon for the glucagon receptor, carbachol for the m2AChR, NaF for the G-proteins, and forskolin and Mn2+ for AC itself. In the adult, prolonged AR stimulation leads to desensitization because of AR down-regulation, impaired expression/function of Gs, reduced catalytic activity of AC, and enhanced Gi expression/function. In the fetus and neonate, ARs are usually not down-regulated, Gs expression/function is enhanced, m2AChR and Gi expression/function are reduced, and AC activity is augmented by a switch to a more catalytically active isoform and by increased AC expression. Because the alterations involve signaling elements shared by receptors other than the AR, agonist-induced changes are heterologous and influence the response to mediators, like glucagon, that operate through other Gs-linked receptors.

Figure 2 summarizes studies on the effects of repeated AR stimulation evoked by isoproterenol administration given to neonatal rats on postnatal days (PN) 2 to 5, with evaluations conducted on PN 6, 24 h after the last dose (Zeiders et al., 1997, 1999a,b, 2000; Auman et al., 2001b, 2002; Garofolo et al., 2002). The determinations were then conducted on cardiac membrane preparations isolated after the in vivo treatments. Instead of evoking AR desensitization, the neonatal isoproterenol treatment instead produced sensitization, evidenced by a significant increase in the membrane response to isoproterenol (Fig. 2A). Sensitization was heterologous since the same effects were seen when the membranes were tested with glucagon, NaF, or forskolin; indeed, the pattern suggested that sensitization specifically involved changes at the level of G-proteins and AC itself. In contrast, the same treatment given to adult rats produced desensitization, with reductions in the response to all the signaling components. In the adult, there was a homologous component, as the greatest decrease was seen for the AR-mediated response, but clearly the more important effects were heterologous. Indeed, one of the essential differences between the adult and neonate was the fact that adults, but not neonates, showed pronounced cardiac hypertrophy upon repeated AR stimulation. As already discussed, cell enlargement decreases the effectiveness of membrane-based signaling because of the reduction in the cell surface-to-volume ratio (Zeiders et al., 1997).

View larger version (35K): [in this window] [in a new window]   Fig. 2. Mechanisms contributing to heterologous sensitization of AR signaling in the neonatal rat heart. In each case, animals were given daily s.c. treatment of saline or isoproterenol (1.25 mg/kg) on PN 2 to 5 or in adulthood, and samples were taken 24 h after the last dose. A, neonatal treatment produces heterologous sensitization of cardiac AC signaling for stimulants acting at the AR (isoproterenol), the glucagon receptor (glucagon), G-proteins (NaF), or AC itself (forskolin), whereas the same treatments elicit desensitization in the adult. B, neonatal isoproterenol treatment increases the concentration of membrane-associated Gs and decreases the major subtype (Gi1,2) of Gi. C, shows the resultant increase in the ability of Gs to associated with ARs. D, the isoproterenolinduced increase in the number of binding sites for [3H]forskolin is indicative of induction of additional AC molecules, and the shift in the affinity (slope) suggests a change in AC isoform. E, the isoform shift is confirmed by a change in the preference ratio for AC responses to Mn2+ compared with forskolin, with isoproterenol moving the response on PN 6 to one midway between that of the neonate and more mature animals. F, shows the heterologous down-regulation and loss of responsiveness to m2AChRs, receptors that mediate inhibition of AC. Data were compiled from previously published studies (Zeiders et al., 1997, 1999a,b, 2000; Auman et al., 2001b, 2002; Garofolo et al., 2002).

The heterologous sensitization to glucagon and NaF seen after neonatal AR agonist treatment suggests that there was a specific shift in signaling to favor G_s compared with G_i. This was verified by studies assessing G-protein expression and function. As shown in Fig. 2B, neonatal sensitization involved an increase in membrane-associated G_s and a decrease in G_i1,2, the major neonatal G_i subtype (Zeiders et al., 2000; Auman et al., 2002). In addition, the treatment elevated the concentration of the more active splice-variant of G_s (Auman et al., 2002). As a result, the interaction of ARs with G_s was enhanced (Fig. 2C), whereas the same treatment of adults reduced the interaction (Zeiders et al., 2000). The net pattern of AR-mediated effects on G-protein expression and function are thus completely opposite to those seen in mature cells (Reithmann et al., 1989). Indeed, the unique suppression of G_i seen in the neonate is probably responsible for the inability of isoproterenol to cause cardiac hypertrophy. Recent studies indicate that it is the specific cross talk of ARs through G_i that elicits this adaptive response (Zou et al., 1999).

Heterologous sensitization also involves adaptations at the level of AC, as evidenced by the enhanced response to forskolin, a direct stimulant. Here, too, specific mechanisms have been identified (Zeiders et al., 1999b, 2000). Repeated neonatal isoproterenol treatment elicited an increase in the capacity for [³H]forskolin binding, reflecting the presence of more AC molecules (Fig. 2D); in addition, the change in ligand-protein affinity indicated that the shape of the binding pocket was different after the AR agonist treatment, a finding that in turn, suggests a shift in the AC isoform. That conclusion was confirmed by studies comparing the ratio of AC responses to Mn²⁺ and forskolin (Fig. 2E). Ordinarily, the isoform in the immature heart shows a bigger relative response to Mn²⁺, and as this isoform is replaced, the preference is reversed. Isoproterenol administration hastened the ontogenetic shift in AC isoform so that the treated animals displayed a response pattern midway between the immature and mature values.

Finally, changes in the expression and function of inhibitory receptors also contribute to the heterologous sensitization seen in neonates given AR agonists (Fig. 2F). The concentration of m₂AChRs and their ability to inhibit AC were both obtunded after repeated isoproterenol administration, effects that were again unique to the immature organism (Garofolo et al., 2002).

Turning to the ability of AR agonists to elicit receptor down-regulation, the picture is less straightforward. In most, but not all immature tissues, ARs are resistant to agonistinduced down-regulation, with the typical adult pattern emerging postnatally (Lau et al., 1982; Auman et al., 2001a, 2001b). The mechanisms underlying the resistance of the developing organism to AR down-regulation have been little explored, but there does not seem to be a deficiency in the activity of G-protein coupled receptor kinases (Zeiders et al., 1999a); the development of arrestins and their function remain to be evaluated. Nevertheless, some tissues do display down-regulation even in the fetus, partially dependent on the AR subtype (Auman et al., 2001a, 2001b); as in the adult, ₂ARs tend to show greater down-regulation than ₁ARs. Whether or not down-regulation is present, AR signaling is sustained, largely as a result of the heterologous sensitization occurring at G-proteins and AC (Auman et al., 2001a, 2001b). The net effect is thus more dependent on the unique downstream signaling sensitization rather than on receptor regulation per se. These conclusions are bolstered by studies with mutant mice overexpressing G-protein or AC subtypes, which also indicate that these postreceptor elements are the primary determinants of AR signaling (Vatner et al., 1998; Gao et al., 1999).

The major unanswered question is to identify the events that trigger the transition from the immature pattern of agonist-induced sensitization to the mature response, desensitization. Because the timing of the transition coincides with the development of innervation, it is tempting to speculate that neural input itself is responsible for initiating the maturational changes that convert the neonatal pattern to that of the adult. Nevertheless, neither neonatal denervation nor treatments that reduce neural input to AR targets prevents the loss of the unique neonatal response, although they do modulate the timing of the shift (Slotkin et al., 1996). Accordingly, the transition from sensitization to desensitization may be an autochthonous property of cell differentiation. In support of that view, studies of AR signaling conducted on isolated, immature myocytes typically display the adult pattern of agonist-induced desensitization (reviewed in Zeiders et al., 1999a); myocyte preparation and in vitro culturing induce cell differentiation and specifically alter the expression and function of G-proteins toward maturity in the absence of functional neural connections. Thus, differentiation itself seems to be sufficient to convert the unique, immature response pattern to mimic that of the adult.

Although the data discussed above were derived from studies of cardiac ARs in the neonatal rat, it is important to note that similar effects have been seen in other tissues, including the central nervous system (Auman et al., 2001b; Slotkin et al., 2001) and in all species studied to date (Habib et al., 1991; Stein et al., 1992; Sun, 1999; Zeiders et al., 1999a, 2000). It would be extremely worthwhile to examine whether the unique attributes of AR signaling in the immature organism are shared by other G-protein-coupled receptors. Certainly, the fact that sensitization occurs via induction of downstream signaling elements would suggest that similar activation of other G_s-coupled receptors might elicit the same outcome, but this has not yet been tested. There is, however, indirect evidence that suggests the same "reversal" of regulation for other receptors in the fetal/neonatal period; inhibition of -noradrenergic or dopaminergic inputs in the perinatal period produces desensitization of responses instead of sensitization (Deskin et al., 1981; Friedhoff and Miller, 1983). If the unusual features of AR regulation in the fetus and neonate are indeed shared by other G-protein-coupled receptors, these factors are likely to play an important role in adverse outcomes of exposures to drugs and environmental factors that act through promotion or inhibition of neurotransmission, an issue that will be discussed in the next two sections.

	Neural Input and the Programming of Future Cellular Responsiveness

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As shown above, the resistance to desensitization of ARs in the fetus and neonate involves changes in signaling elements downstream from the receptors: induction of AC, a shift toward more catalytically efficient AC isoforms, promotion of stimulatory responses mediated by G_s, and suppression of inhibitory signaling mediated by G_i. Accordingly, sensitization is heterologous, rendering supersensitive all receptors that activate AC. Equally important, fetal/neonatal AR stimulation seems to reduce the expression of receptors that oppose AR actions (Garofolo et al., 2002), raising the possibility that early receptor input influences the repertoire of other receptors expressed on the surface of a developing cell. How does the developing cell "know" what complement of receptors to manufacture? One possibility is that the first few, "pioneer" synapses provide trophic information that dictates the subsequent expression and reactivity of the cell, an issue whose ramifications will be discussed in greater detail below. From the point of view of ARs and the unique pattern of heterologous sensitization in the fetus and neonate, it seems that receptor stimulation actively suppresses the expression of at least one of the inhibitory receptors (m₂AChRs) that would otherwise offset the excitatory input (Garofolo et al., 2002). The down-regulation of m₂AChRs by AR stimulation represents true intracellular cross talk because it occurs only in tissues where the ARs and mAChRs are located on the same cells. We also have preliminary evidence (Kreider et al., 2003) that similar suppression occurs for ₂ARs, another receptor that inhibits AC activity through G_i.

If this is the case, then changes in neural input early in development may alter the set-point for reactivity to future stimuli, "programming" the responsiveness of the cell to its environment. Until this point, we have considered only the issue of the unusual mechanisms called into play upon fetal or neonatal AR stimulation. What about the opposite case, namely interference with the function or appearance of AR signals? Here again, there is strong evidence indicating a unique response pattern. In the adult, AR blockade or the destruction of neuronal inputs to ARs result in compensatory supersensitivity of receptor signaling and eventually to up-regulation of the number of receptors on the cell surface (Kohout and Lefkowitz, 2003). When these events are triggered in the developing organism, however, there is little or no increase in sensitivity or receptor numbers (Slotkin et al., 1996; Garofolo et al., 2002). Interestingly, the receptors then never acquire some of their essential properties. For example, neonatal denervation of the heart with the neurotoxin 6-hydroxydopamine fails to evoke AR up-regulation or supersensitivity (Slotkin et al., 1996; Garofolo et al., 2002), actually impairs the development of the heart rate response to AR stimulation (Hou et al., 1989), and renders receptor stimulation permanently incapable of transducing stimulatory input into growth signals (Hou et al., 1989). Accordingly, after neonatal denervation, AR agonists administered in adulthood cannot elicit the cardiac hypertrophy that normally occurs from receptor stimulation (Hou et al., 1989). The same types of defects can be elicited by central nervous system lesions that reduce peripheral sympathetic neuronal activity without physically disrupting cardiac sympathetic innervation. Again, because the deficiencies involve signaling intermediates that are shared by multiple types of receptors, the interference with the proper programming of cell responses compromises the effects of other neurotransmitters and hormones at the level of cell signaling and a host of physiological responses besides hypertrophy (Hou et al., 1989; Gray et al., 1991; Slotkin et al., 1996). Accordingly, the arrival of AR signals at the developmentally appropriate time helps to program the set-point of reactivity of the end organ so that, if innervation is absent during that period, the target cells do not learn how to produce supersensitivity and become incapable of transducing receptor signals into cellular responses.

Importantly, the consequences of developmental disruption of signaling are likely to be generalized to other neurotransmitter receptors, including those that are linked to signaling cascades other than that mediated by AC (Deskin et al., 1981; Friedhoff and Miller, 1983). A vast number of psychotropic medications or environmental toxicants work directly or indirectly through activation or suppression of receptor-mediated synaptic communication. The current findings thus open the door to studies that may reveal the mechanisms underlying neurobehavioral teratogenesis by a wide variety of drugs and toxicants, as will be discussed in the next section.

	Physiological and Toxicological Implications

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The maintenance of AR-mediated signaling in the fetus and neonate in the face of receptor overstimulation has important physiological ramifications. Circulating catecholamine levels rise substantially at the end of gestation, culminating in an enormous "surge" at parturition in which levels exceed those in the adult by several orders of magnitude (Lagercrantz and Slotkin, 1986). Catecholamine actions at ARs are essential for the cardiovascular, respiratory, and metabolic events that mark the transition from intrauterine to neonatal life, and the lack of desensitization thus subserves an important function, sustaining adrenergic effects during this transitional period. Similarly, the use of AR agonists as tocolytics, including terbutaline and ritodrine, can be expected to have beneficial effects in the event of preterm delivery, as they cross the placenta, and thus, the profound and maintained AR stimulation will initiate many of the necessary physiological adjustments that ordinarily occur with full-term delivery.

In the United States, preterm labor occurs in up to 20% of all pregnancies, with preterm delivery a leading cause of neonatal morbidity and mortality in about half the cases (Berkowitz and Papiernik, 1993); therefore, tocolytic therapy is vital. The lack of desensitization and the presence instead of heterologous sensitization may thus be responsible for adverse neonatal effects noted with AR tocolytics: tachycardia, abnormal glucose metabolism, and an elevated incidence of cardiac abnormalities (reviewed in Garofolo et al., 2003). More recently, neurobehavioral consequences have been identified (reviewed in Garofolo et al., 2003): impaired school performance, cognitive dysfunction, and psychiatric disorders. Sensitization of fetal AC signaling by AR stimulation has an even more sinister implications in light of the role of cAMP in cell differentiation and fate. In virtually all prokaryotic and eukaryotic cells, cAMP controls cell development, particularly governing the switch from cell replication to differentiation. In addition, excessive AR stimulation can elicit both apoptosis and necrosis (Joseph et al., 1983). Mature cells are protected from these adverse effects specifically because of desensitization (Joseph et al., 1983); so by inference, developing cells are likely to be more vulnerable to disruption by AR agonists. In a recent study (Garofolo et al., 2003), we found that terbutaline exposure altered cellular development in the developing brain, particularly in the cerebellum. Earlier work identified specific interference with the maturation of cerebellar noradrenergic neurons (reviewed in Garofolo et al., 2003). Interestingly, abnormalities of cerebellar structure and catecholaminergic function are implicated in the etiology of autism (Bauman and Kemper, 1994; Martineau et al., 1994), and it would be worthwhile to examine a potential relationship to this disorder, parallel to the other neurodevelopmental problems that have already been identified for -agonist tocolytics (reviewed in Garofolo et al., 2003).

Additionally, that fetal/neonatal AR stimulation suppresses the expression and function of inhibitory receptors that ordinarily restrain cellular stimulation opens the door to dysregulation of physiological function. As just one example, vagal control of heart rate and contractility are demonstrable at birth, but parasympathetic input is relatively weak (Mills, 1978) so that even minor effects on the balance between excitatory (AR) and inhibitory (mAChR) inputs may have an adverse functional effect. Indeed, imbalances of muscarinic/adrenergic receptor expression are implicated in perinatal morbidity and mortality, including sudden infant death syndrome (reviewed in Garofolo et al., 2002). It is therefore notable that the changes seen after exposure to AR tocolytics are comparable in magnitude to those seen in infants that died of sudden infant death syndrome or in animal models that recapitulate the cardiovascular changes thought to underlie perinatal hypoxia-induced brain damage (reviewed in Garofolo et al., 2002). Future work should concentrate on whether perinatal exposure to AR agonists compromises neonatal cardiac function, both under basal conditions and in situations, like hypoxia, that may trigger cardiovascular collapse.

Another important area for investigation is the role of AR input in growth control. It has long been known that neurotransmitters act as trophic factors regulating the proliferation, differentiation, and growth of their target cells during critical phases of development of the nervous system and of tissues expressing neurotransmitter receptors (Weiss et al., 1998). In the case of ARs, this role is especially complex. Depending upon the developmental context, the same receptor population and signal transduction cascade can, in sequence, enhance cell replication, repress replication in favor of differentiation, promote postmitotic growth, or evoke apoptosis (Claycomb, 1976; Slotkin et al., 1987; Renick et al., 1997; Tseng et al., 2001). This raises the critical question of how developing cells can produce such different patterns of gene induction and repression in response to a common set of inputs at the cell surface. Only a few studies have investigated the issue, but it seems that differential activation of proto-oncogenes and ribosomal protein kinases provide critical cues that coordinate AR input to the fate of the developing cell (Wagner et al., 1994, 1995; Tseng et al., 2001). Interestingly, a number of different types of cancer cell lines re-express ARs as part of the dedifferentiation associated with malignancy (reviewed in Slotkin et al., 2000), and the use of AR agonists in control of the replication and growth of these cancers is under active investigation (Slotkin et al., 2000).

	Conclusions

Top Abstract Regulation of {beta}... The Fetus and Neonate:... Neural Input and the... Physiological and Toxicological... Conclusions References

 
The developing organism lacks many of the regulatory mechanisms that, in the mature animal, serve to control AR receptor expression and signal transduction. Instead of desensitization, receptor stimulation leads to heterologous sensitization by altering the expression and function of G-proteins and AC itself. The fetus and neonate can thus maintain AR signaling and its essential physiological endpoints in the face of the profound stimulation occurring during the perinatal transition. Because these adjustments involve signaling proteins shared by numerous neurotransmitter and hormone receptors, however, AR input also serves to regulate the cellular response to a wide variety of factors that control cell differentiation and function. Consequently, disruption of the normal timing or intensity of AR signals can lead to permanent changes in the set-point for cellular reactivity. These types of alterations may underlie adverse effects reported for AR tocolytic therapy. As the unique fetal/neonatal mechanisms seem to be shared by other G-proteincoupled receptors, similar processes are likely to provide the underlying mechanisms for neurobehavioral or neurophysiological abnormalities associated with developmental exposure to neuroactive drugs and environmental toxicants.

	Footnotes

 
This work was supported by U.S. Public Health Service Grants R01-HD09713 and T32-ES07031.

DOI: 10.1124/jpet.102.048421.

ABBREVIATIONS: AR, -adrenoceptor; AC, adenylyl cyclase; mAChR, muscarinic acetylcholine receptor; PN, postnatal day.

¹ Present address: NIEHS, P.O. Box 12233, MD D2-04, 111 Alexander Dr., Research Triangle Park, NC 27709.

Address correspondence to: Dr. Theodore A. Slotkin, Dept. of Pharmacology and Cancer Biology, Box 3813 DUMC, Duke University Medical Center, Durham, NC 27710. E-mail t.slotkin{at}duke.edu

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