Biomedical subjects

F J Seidler

Publications and source records attributed to F J Seidler.

At least 73 records · Page 4Linked to original sources

PubMed1997

Expression of mRNAs coding for the transforming growth factor-beta receptors in brain regions of euthyroid and hypothyroid neonatal rats and in adult brain.

The TGF-beta family of peptides has been postulated to play a role in control of the cell cycle but also may act in the developing brain to influence neuronal differentiation and survival. Because reception of TGF-beta signals requires the simultaneous expression of all three known receptor subtypes, we examined two neonatal rat brain regions in which neurogenesis has been largely completed. mRNA coding for all three receptors was detectable in both the forebrain and brainstem but only the type II receptor in brainstem showed a difference from adult levels of expression. Animals given perinatal PTU treatment to achieve congenital cretinism did not show significant differences in expression of any of the receptor subtypes in either of the regions, despite the fact that the treatment is known to cause anomalies of neuronal differentiation. These results indicate that regions in which neurons are undergoing axonogenesis and synaptogenesis rather than neurogenesis, nevertheless express the mRNAs coding for TGF-beta receptors and are thus likely to be receptive to trophic signals mediated through TGF-beta. However, synthesis and release of TGF-beta, rather than receptor expression per se, is more likely to be the major point for regulation of signaling. The potential roles of TGF-beta in developmental events outside of the cell cycle, such as synaptogenesis and apoptosis, need to be examined.

Animals↗

PubMed1997

Cryptic brain cell injury caused by fetal nicotine exposure is associated with persistent elevations of c-fos protooncogene expression.

Neurobehavioral teratogenesis caused by prenatal nicotine exposure is associated with deficiencies in brain cell numbers that reflect, in part, effects on cell replication but that also involve delayed cell loss. In the current study, pregnant rats were given nicotine by implanted minipump infusion either from gestational days 4-12 or 4-21 and fetal and neonatal brain regions were examined for expression of the mRNA encoding c-fos, a nuclear transcription factor that becomes chronically elevated when cell injury or apoptosis are occurring. Fetuses exposed to nicotine on gestational days 4-12 did not show elevations of c-fos mRNA on gestational day 18 whereas animals undergoing exposure through day 21 did. In the latter group, elevated c-fos expression was still present on postnatal day 2 despite the cessation of nicotine exposure on gestational day 21. In contrast to the elevation of c-fos seen with prenatal nicotine, postnatal nicotine injections given to 2-day-old rats did not cause acute stimulation of c-fos expression. The ability of injected nicotine to evoke acute rises in c-fos emerged by postnatal day 8 and initially displayed regional specificity paralleling the concentration of nicotinic cholinergic receptors. With increasing maturity, regional selectivity of the c-fos response to acute nicotine was lost, consistent with indirect actions that could be mediated through nicotine-induced hypoxia/ischemia. These results indicate that prenatal nicotine exposure causes chronic elevations of c-fos expression in fetal and neonatal brain that are distinguishable from the later onset of the ability of acute nicotine to cause short-term stimulation of c-fos. The critical period and dose threshold for these effects correspond to those of subsequent cell damage and cell loss identified in previous studies with fetal nicotine exposure. Given that chronic elevations of c-fos are known to be associated with cell injury and to evoke apoptosis in otherwise healthy cells, these results suggest that prenatal nicotine exposure evokes delayed neurotoxicity by altering the program of neural cell differentiation, and that elevated c-fos expression provides an early marker of the eventual deficits.

Aging↗

PubMed1997

Serotonin transporter expression in rat brain regions and blood platelets: aging and glucocorticoid effects.

Hyperactivity of the hypothalamus-pituitary-adrenal axis is more common in elderly depression than in younger cohorts and glucocorticoids are known to influence serotonergic systems. The current study explores the interaction of glucocorticoids with aging on serotonin transporter expression and function. Continuous infusions of dexamethasone (26 days) reduced transporter expression in the aged brain but the ability of imipramine to inhibit synaptosomal [3H]serotonin uptake was unimpaired. These effects were unique to aged animals, as prior work with young adults found no effects of dexamethasone on transporter expression. In contrast to the effects in the brain, there were no differences in platelet transporter expression between young and old rats nor did dexamethasone treatment affect the values in the aged group: thus, the platelet may not reliably model these aspects of CNS function. The results suggest that there are basic biologic differences in the effects of glucocorticoids in aged vs. young brain that could contribute to lowered effectiveness to antidepressants in elderly depression; if transport capacity is already reduced by the effects of increased glucocorticoids, further inhibition of transport by antidepressants would have proportionally less impact on synaptic serotonin concentrations.

Aging↗

PubMed1997

Impaired cardiac function during postnatal hypoxia in rats exposed to nicotine prenatally: implications for perinatal morbidity and mortality, and for sudden infant death syndrome.

Maternal smoking correlates highly with parturitional/neonatal death including SIDS; nicotine exposure of fetal rats reproduces the increased mortality when animals are tested postnatally with hypoxia. In the current study, pregnant rats received nicotine infusions simulating smokers' plasma nicotine levels. At 1-2 days postpartum, the nicotine group displayed normal heart rates, EKG waveforms, and respiratory rates in normoxia. With hypoxia (5% O2, 10 min), controls showed initial tachycardia and a subsequent slight decline in heart rate; atrioventricular conduction was gradually impaired and repolarization abnormalities also appeared. The nicotine group showed no tachycardia and heart rate declined rapidly and precipitously within a few minutes after commencing hypoxia; otherwise, EKG alterations mimicked the controls'. Changes in respiration were identical in the two groups: initial tachypnea and subsequent decline. These results suggest that prenatal nicotine affects sinoatrial reactivity to hypoxia without impairing cardiac conduction per se. These mechanisms explain increased hypoxia-induced mortality in animals exposed to prenatal nicotine, and in man could account for increased morbidity/mortality and SIDS. Our results also indicate the need to test adverse effects of fetal drug exposure using conditions that challenge any given physiological system rather than relying solely on changes under basal conditions.

Animals↗

PubMed1997

Ontogeny of regulatory mechanisms for beta-adrenoceptor control of rat cardiac adenylyl cyclase: targeting of G-proteins and the cyclase catalytic subunit.

Fetal and neonatal tissues are resistant to catecholamine-induced desensitization of essential physiological responses. We examined the mechanisms underlying the ontogeny of desensitization in neonatal rat heart for the beta-adrenergic receptor/adenylyl cyclase signaling cascade. Animals of different ages received isoproterenol daily or 4 days and cardiac membrane preparations were evaluated on the 5th day (6, 15, 25 days old and adults). Measurements were made of basal activity, activity stimulated by two agonists (isoproterenol or glucagon) that operate at different receptors but that share Gs as the transduction intermediate, or by forskolin-Mn' to assess total catalytic capacity of the cyclase subunit; we also assessed inhibition of activity by carbachol which acts via muscarinic cholinergic receptors and G. Adult rats exhibited robust desensitization of the adenylyl cyclase response but the effect was heterologous in that equivalent loss of activity was seen for basal, isoproterenol- and glucagon-stimulated activity forskolin-Mn(2+)-stimulated activity was also decreased. Two factors contributed to desensitization; generalized reduction in membrane protein concentrations caused by cell enlargement (reduced surface-to-volume ratio), and specific interference with the G-protein component that couples receptors to the cyclase. Thus, after adjustment for changes in membrane protein, the desensitization of the forskolin-Mn2, response was no longer evident, but the effects on the other measures were still present. In addition, isoproterenol treatment produced crosstalk with the carbachol/Gi signaling pathway, with significant reductions in the ability of carbachol to inhibit adenylyl cyclase activity. Heterologous desensitization by isoproterenol was also present in 15 and 25 day old rats, but involved only selective components of the effects seen in adults. At 25 days, uncoupling of signals operating through Gs and Gi was obtained without a reduction in forskolin-Mn(2+)-stimulated activity. At 15 days, only the effect on Gs coupling was seen. At 6 days, agonist-induced desensitization was not detectable and instead, heterologous sensitization was found. In these youngest animals, isoproterenol treatment produced a parallel increase in basal, isoproterenol-, glucagon- and forskolin-Mn(2+)-stimulated activity, unaccompanied by changes in membrane protein concentrations, indicating an increase in adenylyl cyclase catalytic activity. These results indicate that the ability to elicit desensitization is not an inherent property of cardiac cells but rather is acquired in distinct stages during development. Sensitization by agonists early in development may be important in preserving physiological responsiveness during ontogenetic surges of adrenergic activity.

Adenylyl Cyclases↗

PubMed1997

Cellular mechanisms for developmental toxicity of chlorpyrifos: targeting the adenylyl cyclase signaling cascade.

Developmental neurotoxicity caused by chlorpyrifos exposure is generally thought to target cholinesterase but chlorpyrifos may also act on cellular intermediates, such as adenylyl cyclase, that serve global functions in the coordination of cell development. In the current study, neonatal rats were exposed to apparently subtoxic doses of chlorpyrifos (no weight loss, no mortality) either on Postnatal Days 1-4 or on Postnatal Days 11-14, and the effects on components of the adenylyl cyclase cascade were evaluated in brain regions that are enriched (forebrain) or sparse (cerebellum) in cholinergic innervation, as well as in a nonneural tissue (heart). In all three, chlorpyrifos evoked deficits in multiple components of the adenylyl cyclase cascade: expression and activity of adenylyl cyclase itself, functioning of G-proteins that link neurotransmitter and hormone receptors to cyclase activity, and expression of neurotransmitter receptors that act through this cascade. Disruption of signaling function was not restricted to transduction of cholinergic signals but rather extended to adrenergic signals as well. In most cases, the adverse effects were not evident during the immediate period of chlorpyrifos administration, but appeared after a delay of several days. These results suggest that chlorpyrifos can affect cell development by altering the activity and reactivity of the adenylyl cyclase signaling cascade, a major control point for trophic regulation of cell differentiation. The effects are not restricted to cholinergic targets, nor even to the central nervous system. Hence, disruption of cell development by chlorpyrifos is likely to be more widespread than previously thought.

Adenylyl Cyclases↗

PubMed1997

Chlorpyrifos interferes with cell development in rat brain regions.

Chlorpyrifos, one of the most widely used pesticides, exhibits greater toxicity during development than in adulthood. We administered chlorpyrifos to neonatal rats in doses spanning the threshold for systemic toxicity and examined developing brain regions (brainstem, forebrain, cerebellum) for signs of interference with cell development using markers for cell packing density and cell number (DNA concentration and content) and cell size (protein/DNA ratio). Neonatal rats given 5 mg/kg of chlorpyrifos on postnatal days 1-4 showed significant mortality and the survivors exhibited severe cell loss in the brainstem; brainstem growth was maintained by enlargement of the remaining cells. This effect was not seen at 1 mg/kg, a dose that did not compromise survival or growth, nor was there any adverse effect at either dose in the forebrain, despite the fact that both brainstem and forebrain possess comparable cholinergic projections. When chlorpyrifos was administered later, on days 11-14, the major target for cell loss shifted from the brainstem to the forebrain and in this case, effects were seen at doses that did not compromise survival or growth. The loss of forebrain cell number occurred between 15 and 20 days of age rather than during the chlorpyrifos treatment. The cerebellum differed from the other regions in that it showed short-term elevations of DNA after chlorpyrifos exposure in either early or late postnatal periods; nevertheless, values then regressed to subnormal in parallel with the loss of cells in other regions. Thus, chlorpyrifos likely causes delayed cell death. Although regions rich in cholinergic projections, such as brainstem and forebrain, may be more affected than noncholinergic regions (cerebellum), the maturational timetable of each region (brainstem earliest, forebrain intermediate, cerebellum last) appears to be more important in setting the window of vulnerability. These results indicate that, even when growth or survival are unaffected, chlorpyrifos produces cellular deficits in the developing brain that could contribute to behavioral abnormalities.

Age Factors↗

PubMed1997

Neuronal control of cardiac and hepatic macromolecule synthesis in the neonatal rat: effects of sympathectomy.

Neurotransmitters are thought to influence cell development in their target tissues. In the current study, neonatal rats were given 6-hydroxydopamine to produce permanent sympathetic denervation, and the effects on cardiac and hepatic DNA and protein synthesis were assessed. Lesioned animals showed deficits in cardiac DNA synthesis over the first 8 d postpartum, a period in which sympathetic innervation is sparse and synaptic norepinephrine concentrations are low; the effect of lesioning was also evident for protein synthesis. Subsequently, DNA synthesis in control animals declined precipitously during the second to third postnatal week, the phase associated with ingrowth of the majority of sympathetic terminals and sympathetic hyperactivity. Neonatal lesioning delayed the ontogenetic decline in DNA synthesis: this effect was not shared by protein synthesis. In the liver, a tissue whose cells, unlike the heart, maintain the ability to divide into adulthood, there was no effect of 6-hydroxydopamine on DNA synthesis and only minor changes in protein synthesis. These results suggest that neural input provides two distinct trophic signals to the developing heart: an early promotion of cell replication associated with low levels of stimulation, and a subsequent promotion of the switchover from cell replication, to cell differentiation and enlargement, associated with high levels of stimulation. In light of the precipitous rise in circulating catecholamines at parturition, and of the subsequent development of sympathetic innervation, catecholamines are likely to play a trophic role in the establishment of the proper pattern of cardiac cell development.

Animals↗

PubMed1997

A critical period for the role of thyroid hormone in development of renal alpha-adrenergic receptors.

Adrenergic input influences renal cell replication/differentiation and the development of excretory function. Kidney cells make adrenoceptors before the arrival of the majority of nerve terminals, and the current study examines whether thyroid hormone plays a role in receptor development. Propylthiouracil (PTU) was given to pregnant and neonatal rats from gestational d 17 through postnatal d 5, a treatment that obtunds thyroid hormone levels throughout the first 2-3 wk postpartum. The PTU group showed significant deficits in the number of alpha1-receptors, and values resolved to normal in parallel with hormone level recovery. The effects were not secondary to alterations in cell differentiation or growth. as the period of receptor abnormalities did not correspond to that of growth inhibition. Similarly, the effects were selective for the alpha1-receptor, as no comparable effects were seen for total membrane protein or for alpha2-receptors. The role of thyroid hormone in alpha1-receptor ontogeny involved a critical developmental window; later in development neither treatment with PTU nor with large doses of thyroid hormone had any impact on alpha1-receptors. Studies of mRNAs encoding the alpha1-receptor subtypes indicated that hypothyroidism targets the alpha1a-subtype, which has been implicated in the transduction of neurotrophic signals; alpha1a-receptor mRNA also showed the largest proportional developmental increase compared with those encoding other alpha1-subtypes. Accordingly, thyroid hormone is likely to set the stage for the subsequent trophic control of renal development by neural input, and hypothyroidism during this critical window can be expected to result in abnormal renal functional development and increased perinatal risk.

Animals↗

PubMed1996

Ontogeny of beta-adrenoceptor/adenylyl cyclase desensitization mechanisms: the role of neonatal innervation.

The ability of adrenergic stimulation to elicit desensitization of the beta-receptor/adenylyl cyclase signaling cascade is not an inherent property of cells but rather is acquired during the period in which sympathetic innervation develops. This study examines whether innervation provides the signal that enables target cardiac and hepatic cells to learn to desensitize their responses. Neonatal rats were sympathectomized with 6-OHDA on postnatal day 1 and were treated at various ages with a regimen of isoproterenol known to elicit desensitization in adults. In control rats, desensitization first appeared between days 6 and 15. Desensitization was heterologous, involving changes in the efficiency of G-protein coupling, as there were parallel decreases in isoproterenol-stimulated adenylyl cyclase activity, basal activity and fluoride-stimulated activity (maximal G-protein activation) without changes in forskolin-Mn2+-stimulated activity (total cyclase catalytic activity). The lesioned animals showed a delay in the onset of desensitization as isoproterenol did not evoke decreased responsiveness until day 25 in the heart; the liver did not display agonist-induced desensitization even at day 25. The effects of lesioning on development of desensitization were entirely separable from those on regulation of beta-receptors themselves: agonist-induced decreases in receptor binding appeared by day 15 in both control and lesioned animals. Uniquely in the youngest animals (6 days old), isoproterenol treatment produced heterologous sensitization of adenylyl cyclase responses rather than desensitization, with a parallel increase in basal, isoproterenol-, fluoride- and forskolin-Mn2+-stimulated activity; the latter indicates induction of total catalytic activity as the primary mechanism of sensitization. The lesioned neonates did not show sensitization, despite the fact that during this period, sympathetic pathways are not functionally competent. Our results indicate that innervation provides a timing signal for the onset of desensitization capabilities of sympathetic target cells, but is not absolutely required for the cells to learn how to desensitize. Prior to the onset of desensitization, agonists induce sensitization that may be important in preserving physiological responsiveness during ontogenetic surges of adrenergic activity. The absence of sensitization in lesioned animals implies that, before physiological function is completely established, early pioneer synapses provide a trophic signal that enables cells to increase their sensitivity to stimulation during the perinatal transition period.

Adenylyl Cyclases↗

PubMed1996

Do glucocorticoids contribute to the abnormalities in serotonin transporter expression and function seen in depression? An animal model.

Adrenocorticosteroids and serotonergic neurons exert reciprocal regulatory actions, and both are abnormal in depression. We evaluated whether glucocorticoids influence the serotonin transporter in rat platelets and brain by infusing dexamethasone for 26 days, sufficient for replacement of the entire platelet population. Effectiveness was verified by measurement of plasma dexamethasone levels, adrenal atrophy, and growth inhibition. At the end of the infusion, we examined [3H]paroxetine binding to platelet, hippocampal, and cerebrocortical membranes, and [3H]serotonin uptake into platelets and synaptosomes. Dexamethasone slightly reduced platelet [3H]paroxetine binding (12%) and had no effect on binding in brain. Platelet [3H]serotonin uptake was unaffected, but synaptosomal uptake was significantly reduced. In neither platelets nor synaptosomes did dexamethasone alter imipramine's ability to inhibit uptake. Thus, elevated glucocorticoids are not responsible for reduced platelet serotonin transporter expression in depression, nor for resistance to imipramine's effect in platelets in elderly depression; however, reduced synaptosomal [3H]serotonin uptake indicates that glucocorticoids can affect transport efficiency, even when the number of transporter molecules is unaltered.

Animals↗

PubMed1996

Programming of brainstem serotonin transporter development by prenatal glucocorticoids.

Prenatal stress or exposure to excess glucocorticoids are known to alter central nervous system function and to result in lasting changes in reactions to stress. The potential involvement of specific elements of brainstem serotonergic neurons was examined in the current study. Pregnant rats were given 0.05, 0.2 or 0.8 mg/kg of dexamethasone on gestational days 17, 18 and 19, and the effects on development of the serotonin presynaptic transporter were assessed from birth to young adulthood by measurement of [3H]paroxetine binding to membrane preparations. Dexamethasone produced a dose-dependent retardation of body and brainstem growth but evoked a significant elevation of [3H]paroxetine binding that persisted into adulthood. Effects on [3H]paroxetine binding were robust even at the lowest dose, which did not suppress growth, indicating that the programming of this transporter is more sensitive to glucocorticoids than is general development. At the highest dose, promotional effects on serotonin transporter expression were offset by impaired growth, so that the peak effect was seen at the intermediate dose of dexamethasone. There were no comparable effects on serotonin transmitter levels, indicating selectivity toward promotion of transporter expression as distinct from simply increasing the number of serotonergic nerve terminals or all nerve terminal components. As the effect of prenatal dexamethasone treatment on the serotonin transporter is more persistent than those on other monoamine transporters, and is not shared by postnatal treatment or by treatment in adulthood, it likely represents specific programming by glucocorticoids during the prenatal period. Aberrant serotonergic transporter expression may contribute to alterations of synaptic function that ultimately produce the physiological abnormalities seen after prenatal stress or glucocorticoid treatment.

Animals↗

PubMed1996

Expression of mRNA coding for the serotonin transporter in aged vs. young rat brain: differential effects of glucocorticoids.

Serotonin transporter mRNA expression in midbrain of young and aged rats was measured after long-term infusion of dexamethasone (0.01 and 0.05 mg/kg/day). Aging alone had no effect. Dexamethasone significantly decreased expression in both young and old rats but the effect was greater in the aged group. Adrenocortical dysregulation is common in elderly depression; our results suggest that glucocorticoids interact with aging to exacerbate abnormalities of serotonergic function, contributing to reduced antidepressant effectiveness.

Animals↗

PubMed1996

High affinity choline transporter status in Alzheimer's disease tissue from rapid autopsy.

The degeneration of nucleus basalis cholinergic neurons in Alzheimers disease (AD) has led to therapies that attempt to increase the synaptic availability of acetylcholine in the remaining cholinergic nerve terminals and to thereby reverse or slow the progressive dementia accompanying the disease process. The inadequacy of current choline-replacement therapies suggests that utilization of choline may be disordered and the rate-limiting step in acetylcholine synthesis, the high affinity choline transporter, may be involved. An adequate test of this hypothesis requires the use of fresh, unfrozen tissue, as the transporter activity declines rapidly after death. Using tissue acquired within two hours of death, the activity of the high affinity choline transporter was shown to be increased in cortical brain regions from AD patients compared to non-AD controls. Further studies using frozen tissues with similar short postmortem acquisition times, revealed the expression of the high affinity uptake transporter to be increased in AD cortex as well. When the ratio of regional uptake activity or expression to the regional level of choline acetyltransferase was calculated, the increase in choline transporter activity and expression was clearly statistically significant. Further statistical significance in the choline transporter activity of the AD group was achieved when the putamen, a region without marked pathology in AD, was used as an internal standard to control for agonal state differences in the individual patients contributing tissue to this study. These increases in choline transporter expression and activity in AD indicate disordered regulation of this rate-limiting component of acetylcholine synthesis above and beyond that required to compensate for the reduced cholinergic synaptic availability in AD.

Alzheimer Disease↗

PubMed1996

Cardiac alpha-adrenergic receptor expression is regulated by thyroid hormone during a critical developmental period.

Although thyroid hormone is obligatory for the development of cardiac beta-adrenergic receptors, it is difficult to assign a specific role for the hormone in receptor ontogeny because beta-receptor expression is affected similarly in the adult. We have determined whether thyroid hormone plays a role in receptor development by evaluating alpha 1-adrenergic receptors, which in the adult are downregulated by thyroid hormone. Propylthiouracil given from gestational day 17 through postnatal day 5 caused significant deficits in the number of alpha 1-receptors and values resolved to normal in parallel with hormone level recovery. When propylthiouracil was administered later (postnatal days 11 through 15) only a transient deficit in alpha 1-receptor binding was seen; hyperthyroidism (triiodothyronine) could still evoke stimulation of receptor expression at this stage. The effects on receptor expression were distinguished from general effects on cell differentiation by examining alpha 2-receptors, which disappear over the first three postnatal weeks; delayed differentiation caused by propylthiouracil would slow the decline in alpha 2-receptors, whereas accelerated differentiation caused by triiodothyronine would hasten the decline. Instead, the effects were similar to those on alpha 1-receptors: perinatal propylthiouracil administration reduced, and neonatal triiodothyronine administration enhanced, alpha 2-receptor binding sites. Thus, thyroid hormone plays a role in the control of cardiac adrenergic receptor expression during a critical development period, with conjoint regulation of the multiple receptor subtypes present within the tissue. As adrenergic stimulation is important in maintaining cardiac function in the perinatal period, alterations of thyroid status during this period can be expected to result in abnormal reactivity and increased perinatal risk.

Animals↗

PubMed1996

Atypical regulation of hepatic adenylyl cyclase and adrenergic receptors during a critical developmental period: agonists evoke supersensitivity accompanied by failure of receptor down-regulation.

Ordinarily, beta-adrenergic receptors and responses linked to the receptors increase with development but in the liver, beta-receptors are higher in the fetus and neonate than in adulthood. We examined how hepatic beta-receptor signaling mediated through adenylyl cyclase is regulated in rats of different ages. In each case, animals were pretreated with isoproterenol for 4 d, and on the 5th d, hepatic membrane preparations were examined for adenylyl cyclase activity and receptor binding capabilities. Uniquely in 6-d-old animals, the cyclase response to isoproterenol was enhanced by chronic pretreatment, caused by heterologous sensitization mediated through effects on total catalytic activity (increased response to forskolin-Mn2+) and on G-protein coupling (enhanced effect of fluoride and increased GTP dependence of basal activity). Isoproterenol pretreatment failed to cause beta-receptor down-regulation in 6-d-old animals, but by 15 d of age, down-regulation was detected along with slight desensitization of the cyclase response. However, at 25 d, neither effect was present. In adulthood, repeated isoproterenol administration failed to cause cyclase desensitization but did reduce beta-receptor numbers; the loss of receptors was still unusual in that beta-receptor down-regulation could be achieved with either isoproterenol or with methoxamine, an alpha-receptor agonist. The results indicate that, early in development, hepatic beta-receptor-mediated responses are enhanced, not desensitized, after chronic stimulation. These effects would foster responsiveness of hepatic gluconeogenesis in the face of the massive adrenergic stimulation associated with the transition from fetal to neonatal life. In adulthood, when receptor numbers are far lower than in the neonate, the inability to desensitize the signaling cascade despite receptor down-regulation would serve to maintain the response to catecholamines.

Adenylyl Cyclases↗

PubMed1996

Combining 'caged-dopamine' photolysis with fast-scan cyclic voltammetry to assess dopamine clearance and release autoinhibition in vitro.

We have developed a methodology for inducing a rapid rise in extracellular dopamine concentrations. The clearance of the applied dopamine, as well as its effect on the endogenous dopamine release (i.e., autoinhibition), was then examined using fast scan cyclic voltammetry. In a recording chamber mounted on a Nikon Optiphot epifluorescence microscope, coronal rat brain slices containing either the caudate nucleus or prefrontal cortex were perfused with ACSF containing 100-200 microM 'caged-DA.' UV illumination (100-200 ms) focused at the tip of the recording electrode produced a peak DA concentration of 1-2 microM within 100-200 ms of terminating the illumination. The caudate nucleus exhibited a faster clearance rate for photo-released DA compared to the prefrontal cortex. Cocaine reduced the clearance rates in both the caudate nucleus and prefrontal cortex. In the prefrontal cortex a combination of desipramine/clomipramine also reduced dopamine clearance, suggesting heterologous uptake of the applied DA by noradrenergic and/or serotonergic terminals. Photo-released dopamine inhibited release of endogenous caudate DA release evoked by single electrical stimulation. The advantages of this methodology are discussed.

Animals↗

PubMed1996

Aging and glucocorticoids: effects on cell signaling mediated through adenylyl cyclase.

Abnormalities of hypothalamus-pituitary-adrenal axis regulation are common in the elderly and excess glucocorticoids have been implicated in the loss of neural function in aging. In the current study, we examined cell signaling mediated through adenylyl cyclase in brain regions, heart and liver of young and aged rats given continuous infusions of dexamethasone (10 or 50 micrograms/kg/day) for 26 days. Aged control animals showed significant deficits in total adenylyl cyclase activity (assessed with forskolin-Mn++) in the brain regions and the heart; superimposed on this change, the striatum and the heart displayed interference with the response mediated either at the level of G-protein coupling to cyclase (striatum) or neurotransmitter receptor coupling to G-proteins (heart). Administration of dexamethasone to young rats did not reproduce the effects of aging on any of the measures of adenylyl cyclase, despite the fact that the higher dose produced Cushingoid effects. The same dexamethasone regimens given to aged rats produced alterations in G-protein coupling mechanisms in the cortex and in serotonergic-mediated cyclase responses in the striatum, and also decreased basal enzyme activity in the heart. In contrast to the brain regions and the heart, the liver showed unique effects of aging and dexamethasone. Total adenylyl cyclase activity, the enzymatic response to beta adrenergic stimulation and the number of beta adrenergic receptors were all elevated in aged animals as compared to the younger cohort. Dexamethasone decreased both hepatic beta receptor numbers and isoproterenol responsiveness in young animals, but increased receptor binding in aged animals. These data indicate that the defects associated with aging in the central nervous system and the cardiac cell signaling mediated through adenylyl cyclase are not a result of glucocorticoid excess; however, central and peripheral tissues respond differently to glucocorticoids in aged vs. young animals. Given the high incidence of hypothalamus-pituitary-adrenal axis dysregulation in the elderly, and particularly in elderly depression, effects of glucocorticoids on cell signaling may contribute to disruption of cell function and to hypo- or hyper-reactivity to drugs, such as antidepressants, that act by altering synaptic transmission.

Adenylyl Cyclases↗