Long-term regulation of tyrosine hydroxylase activity in the superior cervical ganglion in organ culture: effects of nerve stimulation and dexamethasone

Limited recovery of pineal function after regeneration of preganglionic sympathetic axons: evidence for loss of ganglionic synaptic specificity

The cervical sympathetic trunks (CSTs) contain axons of preganglionic neurons that innervate the superior cervical ganglia (SCGs). Because regeneration of CST fibers can be extensive and can reestablish certain specific patterns of SCG connections, restoration of end organ function would be expected. This expectation was examined with respect to the pineal gland, an organ innervated by the two SCGs. The activity of pineal serotonin N-acetyltransferase (NAT) exhibits a large circadian rhythm that is dependent on the sympathetic input of the gland, with high activity at night. Thirty-six hours after the CSTs were crushed bilaterally, nocturnal NAT was decreased by 99%. Three months later, enzyme activity had recovered only to 15% of control values, a recovery dependent on regeneration of CST fibers. Nevertheless, a small day/night rhythm was present in lesioned animals. Neither the density of the adrenergic innervation of the gland nor the ability of an adrenergic agonist to stimulate NAT activity was reduced in rats with regenerated CSTs. In addition, stimulation of the regenerated CST at a variety of frequencies was at least as effective in increasing NAT activity as seen with control nerves. These data suggest that the failure of pineal function to recover is not attributable to a quantitative deficit in the extent of reinnervation or synaptic efficacy. Rather, we suggest that there is some loss of specificity in the synaptic connections made in the SCG during reinnervation, resulting in a loss of the central neuronal information necessary for directing a normal NAT rhythm and thus normal pineal function.

Activity-dependent regulation of dopamine content in the olfactory bulbs of naris-occluded rats

Several lines of evidence strongly suggest that reduced olfactory nerve activity results in decreased bulb dopamine content. In the present study, high performance liquid chromatography with electrochemical detection was used to assess catecholamine levels in bulbs from postnatal day 60 rats that had undergone either unilateral naris cautery or a sham surgery on day 30. Thirty days of odor deprivation dramatically reduced dopamine and dihydroxyphenylacetic acid levels in functionally-deprived bulbs (ipsilateral to occluded nares) as compared to contralateral controls, while norepinephrine and dihydroxyphenylglycol levels were unchanged. The loss of dopamine was more severe in medial as compared to lateral aspects of experimental bulbs, while the loss of dihydroxyphenylacetic acid was similar on the two sides. To test directly the hypothesis that afferent activity regulates dopamine and dihydroxyphenylacetic acid content, 1 h of high frequency tetanic nerve stimulation was provided to the rostral-medial olfactory nerve layer in deprived olfactory bulbs, and catecholamine levels were assessed from 6 to 192 h later. Partial and temporary recovery of dopamine was observed in medial aspects of the bulb when rats were examined 96 h later, while consistent recovery of dihydroxyphenylacetic acid content was not apparent. These data corroborate evidence that olfactory nerve activity is a potent regulator of bulb dopamine and indicate that continued afferent input is necessary to maintain dopamine levels.

Plasticity in adult and ageing sympathetic neurons

The nature of neural plasticity and the factors that influence it vary throughout life. Adult neurons undergo extensive and continual adaptation in response to demands that are quite different from those of early development. We review the main influences on the survival, growth and neurotransmitter expression in adult and ageing sympathetic neurons, comparing these influences to those at work in early development. This "developmental" approach is proposed because, despite the contrasting needs of different phases of development, each phase has a profound influence on the mechanisms of plasticity available to its successors. Interactions between neurons and their targets, whether effector cells or other neurons, are vital to all of these aspects of neural plasticity. Sympathetic neurons require access to target-derived diffusible neurotrophic factors such as NGF, NT3 and GDNF, as well as to bound elements of the extracellular matrix such as laminin. These factors probably influence plasticity throughout life. In adult life, and even in old age, sympathetic neurons are relatively resistant to cell death. However, they continue to require target-derived diffusible and bound factors for their maintenance, growth and neurotransmitter expression. Failure to maintain appropriate neuronal function in old age, for example in the breakdown of homeostasis, may result partly from a disturbance of the dynamic, trophic relationship between neurons and their targets. However, there is no clear evidence that this is due to a failure of targets to synthesize neurotrophic factors. On the neural side of the equation, altered responsiveness of sympathetic neurons to neurotrophic factors suggests that expression of the trk and p75 neurotrophin receptors contributes to neuronal survival, maintenance and growth in adulthood and old age. Altered receptor expression may therefore underlie the selective vulnerability of some sympathetic neurons in old age. The role of neural connectivity and activity in the regulation of synthesis of target-derived factors, as well as in neurotransmitter dynamics, is reviewed.

Rapid down-regulation of tyrosine hydroxylase expression in the olfactory bulb of naris-occluded adult rats

In most sensory systems, afferent innervation regulates morphological and biochemical characteristics of target cells for a limited time during development. Sensory deprivation experiments in adult rats also have suggested a critical period for afferent influences on olfactory bulb structure and function. Previous odorant deprivation studies that employed unilateral naris closure in neonatal rats demonstrated down-regulation of the catecholamine biosynthetic enzyme tyrosine hydroxylase (TH) in dopamine neurons intrinsic to the olfactory bulb. Accompanying the altered biochemical parameters was a decrease in bulb size. To distinguish between deprivation-induced alterations in TH expression secondary to developmental sequelae and those occurring in mature neurons, the consequences of unilateral naris closure were assessed in young adult rats. In agreement with previous studies significant postnatal increases occurred in TH expression and total protein, an indication of bulb size. At 30 days post-closure, total protein was unaltered in the ipsilateral olfactory bulb but showed a small (12.9%), significant decline at 60 days. In contrast to the limited morphological consequences of odor deprivation, profound reductions occurred in TH expression. TH activity ipsilateral to the closure decreased significantly by 14 days post-closure and remained depressed for up to 6 months. In parallel with enzyme activity, TH immunoreactivity did not decline in the first few days post-closure. In situ hybridization revealed that TH mRNA levels decreased rapidly, i.e., by 2 days post-closure, reached a nadir at 1 month, and remained depressed for at least 6 months. The capacity of odor deprivation in the adult rat olfactory system to down-regulate TH expression suggests that this phenotypic alteration occurs independently of a presumed critical period.

Differential regulation of tyrosine hydroxylase protein and activity in rabbit sympathetic neurones after long-term cold exposure: altered responses in ageing

The aim of this study was to investigate the response of sympathetic neurones to prolonged neural stimulation, using cold exposure as a non-invasive experimental paradigm. We examined the effects of prolonged (8 days and 4 wk) cold exposure on tyrosine hydroxylase (TH) protein and activity and neuropeptide Y (NPY) levels in sympathetic neurones of the superior cervical ganglion (SCG), together with NPY levels in the ear artery from young and aged rabbits. The main findings were as follows. In young rabbits, TH levels and TH activity were differentially regulated in response to prolonged cold exposure. TH levels rose whilst TH activity tended to decline. Decentralization of SCG from young animals before cold exposure abolished the rise in TH levels. TH activity in SCG from young rabbits was reduced by decentralization whilst cold exposure resulted in an increase in TH activity. Thus, TH activity was induced in the SCG in the absence of pre-ganglionic input, demonstrating a non-synaptic regulatory mechanism. In old rabbits, cold-induced changes were either delayed or failed to occur, indicating that the responses of sympathetic neurones to cold stress are impaired in old age.

Vasoactive intestinal peptide and secretin produce long-term increases in tyrosine hydroxylase activity in the rat superior cervical ganglion

Electrical stimulation of the preganglionic fibers innervating the rat superior cervical ganglion (SCG) produces both short-term and long-term increases in tyrosine hydroxylase (TH) activity that are not completely blocked by nicotinic antagonists. Vasoactive intestinal peptide (VIP) and secretin, two neuropeptides known to produce short-term increases in TH activity, were examined for their ability to produce long-term increases in this enzyme activity. Culturing the SCG in the presence of either peptide produced a 30-50% increase in TH activity measured 2 days later. The results raise the possibility that one of these peptides or a related molecule participates in the transsynaptic induction of ganglionic TH.

Vasoactive intestinal polypeptide-related peptides modulate tyrosine hydroxylase gene expression in PC12 cells through multiple adenylate cyclase-coupled receptors

We investigated the receptor mechanisms by which vasoactive intestinal polypeptide (VIP) and related peptides exert their effects on tyrosine hydroxylase (TH) gene expression. VIP, secretin, and peptide histidine isoleucine (PHI) each produced increases in TH gene expression, as measured by increases in TH mRNA levels and TH activity. The concentrations at which the effects of these peptides were maximal differed for TH activity and TH mRNA. Moreover, maximal increases in TH activity were 130-140% of control, whereas maximal increases in TH mRNA were 250% of control. The concentration dependence of the increases in TH mRNA in response to the three peptides was analyzed by fitting the data to nonlinear regression models that assume either one or two components to the response. The data for secretin fit best to a model that assumes a single component to the increase in TH mRNA levels. The data derived for PHI and VIP fit best to models that assumed two components to the TH mRNA response. These data suggested that there may be more than one receptor or signal transduction mechanism involved in the response to the various peptides. We examined whether the peptides exerted their effects through common or multiple second messenger systems. The ability of maximally active concentrations of these peptides to stimulate increases in TH mRNA was not additive, indicating that the peptides work through a common receptor or signal transduction pathway. Each peptide stimulated increases in protein kinase A (PKA) activity. Secretin and VIP were ineffective in increasing TH mRNA levels in a PKA-deficient mutant PC12 cell line (A126-1B2). Moreover, the adenylate cyclase antagonist 2',5'-dideoxyadenosine prevented the increase in TH mRNA produced by each peptide. Thus, each peptide requires an intact cyclic AMP second messenger pathway to produce changes in TH gene expression, suggesting that the complex pattern of response to VIP and PHI revealed by concentration-response analysis was due to the actions of these peptides at multiple receptors. To evaluate this possibility, we examined the effect of several peptide receptor antagonists on the increase in TH gene expression elicited by VIP, PHI, and secretin. The secretin antagonist secretin (5-27) (20 microM) had no significant effect on VIP or PHI stimulation of TH gene expression, but reduced the effect of secretin. The VIP antagonist VIP (10-28) (20 microM) reduced the effect of VIP on increasing TH mRNA, but had no significant effect on the response of TH mRNA to secretin or PHI.(ABSTRACT TRUNCATED AT 400 WORDS)

Catecholaminergic traits of chick sympathetic neurons may be differentially regulated by a cGMP-dependent pathway

The purine metabolites inosine and adenosine selectively increase the catecholamine, but not the acetylcholine production in cultured chick superior cervical ganglion neurons via an as yet unknown intracellular pathway. In order to elucidate some of the molecular events involved in this differential regulation of neurotransmitter production by purines, the SCG neurons were cultured in the presence of cyclic nucleotide analogs and activators of adenylate and guanylate cyclase. Neither 8-bromo-cyclic AMP (8-Br-cAMP), 8-bromo-cyclic GMP (8-Br-cGMP), or forskolin, an activator of adenylate cyclase, could mimic the effect of inosine, i.e. differentially increase catecholamine production. Sodium nitroprusside, an activator of guanylate cyclase, however, has a strong potentiating action on the effect of inosine. The noradrenergic properties of chick sympathetic neurons may thus be differentially modulated by a cGMP-dependent pathway.

GABA receptor modulation of tyrosine hydroxylase gene expression in the rat adrenal gland

Chromaffin cell gamma-aminobutyric acid (GABA) receptors play a role in modulating catecholamine secretion. The present experiments examined the role of GABA receptors in modulation of tyrosine hydroxylase (TH) induction in rat adrenal gland. Administration of bicuculline, a GABA antagonist, had no effect on TH activity or TH mRNA. However, bicuculline potentiated reserpine's effect on TH activity and TH mRNA induction. These data suggest that GABA receptors modulate induction of TH and TH mRNA in the adrenal gland.

Modulation of tyrosine hydroxylase gene expression in the rat adrenal gland by age and reserpine

Tyrosine hydroxylase (TH), TH messenger RNA (TH mRNA) and dopamine (DA) were measured simultaneously in adrenal glands of individual Fischer 344 rats aged 2, 6, 13 and 23 months. Between 2 and 23 months TH activity rose 2-fold as compared to the youngest group. TH mRNA content of the adrenal gland rose 3-fold between 2 and 23 months. A 3-fold increase in adrenal DA content, the first catecholamine product of TH, provides evidence that the increases in TH gene expression are functionally significant. To determine if mechanisms that regulate gene expression are altered by aging, the effects of reserpine on induction of TH mRNA and TH activity were compared in another group of rats aged 2, 12 and 27 months. Consistent with the results of the first experiment, there were age-related increases in both TH activity and TH mRNA in the age-matched control groups. TH activity rose 2-fold and TH mRNA rose more than 6-fold between 2 and 27 months. The discrepancy in the relative magnitudes of increases in TH mRNA and TH protein suggest an uncoupling of regulation of TH mRNA and TH protein levels. Moreover, there were significant age-related differences with respect to modulation of TH gene expression by reserpine treatment. TH activity was induced by reserpine in the youngest group, but not in the two older age-groups. In contrast, reserpine caused significant induction of TH mRNA in all age groups. These results provide evidence that aging is accompanied by alterations in transcriptional and post-transcriptional mechanisms involved in regulation of TH gene expression.

Increases in the activity of tryptophan hydroxylase from rat cortex and midbrain in response to acute or repeated sound stress are blocked by adrenalectomy and restored by dexamethasone treatment

Exposure of male Sprague-Dawley rats to acute sound stress (2 s, 110 dB sound pulses presented randomly every minute for 1 h) increases the in vitro activity of cortical and midbrain tryptophan hydroxylase by an alkaline phosphatase-reversible mechanism. Repeated exposure to sound stress on three separate days produces a stable increase in enzyme activity that persists 24 h after the termination of the stress and is insensitive to alkaline phosphatase. Adrenalectomy abolishes both increases in enzyme activity to acute or repeated sound stress but does not change baseline levels of enzyme activity. The synthetic glucocorticoid, dexamethasone, (500 micrograms/day i.p.) given for 3 days or 5 out of 6 days, starting day 3 after adrenalectomy, restores the increases in enzyme activity in adrenalectomized rats exposed, respectively, to acute or repeated sound stress. The mineralocorticoid, aldosterone (5 micrograms/day s.c.), does not substitute for dexamethasone in acutely sound-stressed, adrenalectomized rats. Dexamethasone does not alter control levels of enzyme activity in either adrenalectomized rats or rats with intact adrenals (sham-adrenalectomized), but is required to allow the increase in enzyme activity in response to acute or repeated sound stress to be expressed. The effect of the glucocorticoid, thus, appears to be a permissive one.

Preganglionic nerve stimulation increases mRNA levels for tyrosine hydroxylase in the rat superior cervical ganglion

Increased synaptic stimulation of sympathetic neurons in vivo causes a delayed increase in the activity and the amount of tyrosine hydroxylase (TH). To determine whether these changes result from an increase in the messenger RNA for TH, the rat preganglionic cervical sympathetic trunk was electrically stimulated unilaterally for 90 min, and 48 h later RNA was extracted from stimulated and contralateral control superior cervical ganglia. Northern blots probed with a cDNA for TH demonstrated that nerve stimulation produced about a 2.5-fold increase in the amount of TH mRNA in the ganglion. These results indicate that synaptic stimulation leads to an increase in TH mRNA, either by increasing the rate of transcription of the TH gene or by increasing the stability of its mRNA.

Co-localization and plasticity of transmitters in peripheral autonomic and sensory neurons

Immunohistochemical studies have shown that most peripheral autonomic and sensory ganglia are heterogeneous, consisting of several populations of neurons which can be distinguished by their content of peptide and non-peptide transmitters, and transmitter-associated enzymes. Many neurons contain several different potential transmitters, especially neuropeptides. Some neuropeptides have been localized in more than one population of autonomic and sensory neurons. However, the peptide often occurs together with a distinctive combination of additional transmitters in each neuronal class. The precise combination of transmitters found in any individual neuron is highly correlated with the peripheral target of the neuron. This indicates that immunohistochemically defined neuronal populations represent distinct functional classes of neurons. In an increasing number of cases, many of the potential transmitters contained in a particular neuron have been shown to be released from the nerve terminals, and to contribute to presynaptic or postsynaptic effects of nerve activation. Despite this association between the combination of potential transmitters contained in a neuron, and the function of the neuron, not all transmitters or transmitter-associated enzymes are expressed equally at all times in the life of a neuron: the levels of some substances change dramatically during development; some are detected only after experimental alteration of the environment of the developing or mature neurons. Taken together, these results indicate that, during development, pathway-specific information influences the differentiation of peripheral autonomic and sensory neurons. Furthermore, the expression of neuropeptides and transmitter-associated enzymes in a particular neuron appears to be under continuous regulation. These phenomena demonstrate the complexity and precision involved in development and maintenance of the peripheral autonomic and sensory nervous systems.

Regulation of tyrosine hydroxylase and phenylethanolamine N-methyltransferase mRNA levels in the sympathoadrenal system by the pituitary-adrenocortical axis

The pituitary-adrenocortical axis plays a complex role in the regulation of the levels of enzymes of the catecholamine biosynthetic pathway. In this report we have explored molecular mechanisms of these regulations, by examining the effects of hypophysectomy (HPX) and dexamethasone (DEX) on tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) mRNA levels in the adrenal medulla (AM) and superior cervical ganglia (SCG). Three weeks after hypophysectomy weights (-48%), total RNA (-49%), and DNA (-22%) contents in AM were significantly reduced, when compared to sham-operated animals (SO). In SCG decreases in weight (-23%) and in the ratio of RNA/DNA (-25%) were also found. TH mRNA contents paralleled decreases in total RNA levels and no significant change in the relative abundance of TH mRNA was found. When HPX rats were injected for 5 days with DEX (1 mg/kg, i.p.), TH mRNA levels in the SCG (+51%) and in the AM (+74%) were significantly increased when compared to saline-treated HPX animals. DEX given to SO rats increased TH mRNA in SCG (+49%); a 27% increase in TH mRNA in the AM was also observed. The relative abundance of PNMT mRNA in the AM was reduced after hypophysectomy (-64%). This decrease was completely reversed by DEX. In contrast, DEX did not affect PNMT mRNA levels in the AM of SO rats. PNMT mRNA was not detected in SCG of saline- or DEX-treated rats. In conclusion, our findings suggest that the pituitary-adrenocortical axis is involved in the regulation of the steady-state levels of TH and PNMT mRNAs. This regulation involves: (1) induction of TH mRNA contents in AM and SCG by increased plasma glucocorticoid levels; and (2) maintenance of the steady-state levels of PNMT mRNA in AM by glucocorticoid-dependent mechanisms.

Both synaptic and antidromic stimulation of neurons in the rat superior cervical ganglion acutely increase tyrosine hydroxylase activity

Electrical stimulation of the preganglionic cervical sympathetic trunk produces an acute increase in the rate of DOPA synthesis in the rat superior cervical ganglion. The present study was designed to test the possibility that this acute transsynaptic stimulation of catechol biosynthesis could be, at least in part, a consequence of an increase in the firing rate of the postganglionic sympathetic neurons. For this purpose, the effect of stimulation in vitro of the preganglionic cervical sympathetic trunk was compared to that of stimulation of the predominantly postganglionic internal and external carotid nerves. Stimulation of the cervical sympathetic trunk at 10 Hz for 30 min produced a 4.6-fold increase in DOPA synthesis, while simultaneous stimulation of the two postganglionic trunks produced a 3.1-fold increase. The internal carotid nerve is known to contain a small population of preganglionic fibers that synapse on principal neurons in the ganglion before entering this nerve trunk. To eliminate the possibility that the effect of stimulation of the internal carotid nerve is mediated by synaptic stimulation via these preganglionic "through fibers", the effect of stimulation of previously decentralized ganglia was examined. While decentralization reduced the magnitude of the effect of stimulation of the internal and external carotid nerves, a 2.0-fold increase in DOPA synthesis was still seen. In addition, when these nerve trunks were stimulated in control ganglia that had been maintained in organ culture for 48 h to allow time for the degeneration of afferent nerve terminals, DOPA synthesis increased 4.1-fold.(ABSTRACT TRUNCATED AT 250 WORDS)

An immunohistochemical study of the acute and long-term effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in the marmoset

Administration of the drug 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine induces a parkinsonian syndrome in primates. Intraperitoneal injections of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in the common marmoset (Callithrix jacchus) produced symptoms of rigidity, akinesia and tremor which persisted for at least one month. However, after this time, considerable behavioural recovery occurred, although animals were still severely bradykinetic compared with controls. Marmosets were allowed to survive for 1, 3 1/2 or 7 months prior to histological and immunocytochemical analysis. Detection of catecholaminergic neurons using antibodies directed against the enzyme tyrosine hydroxylase revealed a profound (80%) loss of dopaminergic cells from the substantia nigra one month after initiation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment. This was accompanied by a severe gliosis. Fewer cells were lost from the adjacent ventral tegmental area (45%), but dopamine-containing cells in other brain areas were not obviously affected. At longer survival times the substantia nigra was less damaged, with a proliferation of glia in the pars compacta and a loss of approximately 20% of the dopaminergic perikarya. Using immunohistochemical techniques, the distribution of neuropeptides substance P, [Met]enkephalin and dynorphin 1-17-like immunoreactivity were examined and found to exhibit distinctive patterns in the marmoset substantia nigra. The integrity of these systems appeared intact at all times after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment. These results support the hypothesis that the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine produces a clinical syndrome, indistinguishable from Parkinson's disease, via a selective destruction only of neurons with perikarya in the substantia nigra pars compacta and the ventral tegmental area. The findings that the peptidergic input to these cells together with most non-nigral dopaminergic cell groups are not damaged, indicate that the selectivity of the lesion produced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine appears greater than that seen in idiopathic Parkinson's disease. The neurotoxic effects of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine in the marmoset may not be permanent since both behavioural and biochemical recovery were observed after several months.

Long-term blockade by toxin F of nicotinic synaptic potentials in cultured sympathetic neurons

The effects of a recently identified blocker of neuronal nicotinic transmission, toxin F, were studied in cultured sympathetic neurons. Single principal neurons, dissociated from superior cervical ganglia of newborn rats, were grown on cardiac myocytes in microculture. The toxin blocked nicotinic synaptic potentials in these cultures but had no effect on muscarinic interactions. When toxin F was applied by addition to the perfusion medium, the concentration required for blocking most of the nicotinic potential was 40 nM, and the recovery from blockade was slow (t1/2 = 95 +/- 64 min). When the toxin was briefly applied by pressure ejection from a pipette, the concentration in the pipette necessary for blockade was 21 microM, and 30-60% of the response recovered within a few minutes while the remainder recovered slowly (t1/2 of the remainder = 105 +/- 82 min). One possible explanation for the difference in recovery time is that toxin F binds initially with low affinity to the nicotinic receptor but with time the toxin receptor complex converts to a high affinity state. The presence of dihydro-beta-erythroidine during perfusion of toxin F prevented the long-lasting blockade by the toxin, suggesting that toxin F and dihydro-beta-erythroidine act through a common binding site. The specificity, potency, and slow reversibility of the effects of toxin F make it useful as a probe for studying neuronal nicotinic receptors of cultured sympathetic neurons.