Effects of synaptic and antidromic stimulation on tyrosine hydroxylase activity in the rat superior cervical ganglion

Neurotransmitter Switching? No Surprise

Among the many forms of brain plasticity, changes in synaptic strength and changes in synapse number are particularly prominent. However, evidence for neurotransmitter respecification or switching has been accumulating steadily, both in the developing nervous system and in the adult brain, with observations of transmitter addition, loss, or replacement of one transmitter with another. Natural stimuli can drive these changes in transmitter identity, with matching changes in postsynaptic transmitter receptors. Strikingly, they often convert the synapse from excitatory to inhibitory or vice versa, providing a basis for changes in behavior in those cases in which it has been examined. Progress has been made in identifying the factors that induce transmitter switching and in understanding the molecular mechanisms by which it is achieved. There are many intriguing questions to be addressed.

Combination therapies prevent the neuropathic, proinflammatory characteristics of bone marrow in streptozotocin-induced diabetic rats

We previously showed that peripheral neuropathy of the bone marrow was associated with loss of circadian rhythmicity of stem/progenitor cell release into the circulation. Bone marrow neuropathy results in dramatic changes in hematopoiesis that lead to microvascular complications, inflammation, and reduced endothelial repair. This series of events represents early pathogenesis before development of diabetic retinopathy. In this study we characterized early alterations within the bone marrow of streptozotocin (STZ)-induced diabetic rats following treatments that prevent experimental peripheral neuropathy. We asked whether bone marrow neuropathy and the associated bone marrow pathology were reversed with treatments that prevent peripheral neuropathy. Three strategies were tested: inhibition of neutral endopeptidase, inhibition of aldose reductase plus lipoic acid supplementation, and insulin therapy with antioxidants. All strategies prevented loss of nerve conduction velocity resulting from STZ-induced diabetes and corrected the STZ-induced diabetes-associated increase of immunoreactivity of neuropeptide Y, tyrosine hydroxylase, and somatostatin. The treatments also reduced concentrations of interleukin-1β, granulocyte colony-stimulating factor, and matrix metalloproteinase 2 in STZ-induced diabetic bone marrow supernatant and decreased the expression of NADPH oxidase 2, nitric oxide synthase 2, and nuclear factor-κB1 mRNA in bone marrow progenitor cells. These therapies represent novel approaches to attenuate the diabetic phenotype within the bone marrow and may constitute an important therapeutic strategy for diabetic microvascular complications.

Ca2+ influx through both L- and N-type Ca2+ channels increases c-fos expression by electrical stimulation of sympathetic neurons

During direct membrane depolarization, Ca2+ influx primarily through L-type Ca2+ (L-) channels initiates activity-dependent gene transcription. This is surprising given that in most neurons a minority of the total Ca2+ current arises from L-channel activity. However, many studies have stimulated Ca2+ influx with unphysiological stimuli such as chronic membrane depolarization using high K+ medium. Few studies have tested whether other Ca2+ channels stimulate gene transcription in adult neurons as a consequence of direct electrical stimulation. Therefore, we evaluated the role of L- and N-type Ca2+ (N-) channel activity in regulating mRNA levels of c-fos, an activity-dependent transcription factor, in adult rat superior cervical ganglion (SCG) neurons as the majority of Ca2+ channels are N-type, while only a minority are L-type. Changes in c-fos mRNA levels were measured using semi-quantitative and single-cell RT-PCR. Phosphorylation of CREB (pCREB) and changes in c-Fos levels were visualized in dissociated cells by immunocytochemistry. Increases in pCREB, c-fos mRNA and c-Fos protein with either K+ or electrical depolarization required Ca2+ influx. These results support previous findings that elevated c-fos levels result from pCREB stimulating c-fos transcription. Elevation of pCREB, c-fos and c-Fos with K+ depolarization depended on L-channel activity. By contrast, antagonizing either channel at 10-Hz stimulation minimized these increases despite unequal numbers of the two channel types. Transition to exclusive L-channel involvement occurred with increasing frequency of stimulation (from 10 to 20 to 50 Hz). Our results demonstrate that N- and L-channel participation in regulating c-fos expression is encoded in the pattern of electrical stimulation.

Chronic nicotine and smoking treatment increases dopamine transporter mRNA expression in the rat midbrain

Previous pharmacokinetics and electrophysiological results indicated an important role of nicotine in the modulation of dopamine transporter (DAT). To elucidate the expression changes of DAT on chronic nicotine and smoke administration, the effects of nicotine and passive cigarette smoke on DAT mRNA expression in the ventral tegmental area (VTA) and the substantia nigra (SN) area were examined using in situ hybridization and RNase protection assay. The results showed that chronic nicotine and smoke exposure highly unregulated DAT mRNA in the VTA and SN areas, including the dorsal part of substantia nigra pars compacta. Smoke for 30 min showed the highest increasing effect, whereas nicotine and smoke for 10 min only had slightly increasing effects. However, smoke for 1 h showed an increasing effect to a lesser extent than 30 min. These results revealed a new aspect of nicotine's modulation on the DAT, and may have important roles in neuropsychological disorders related to the midbrain abnormalities such as drugs addiction.

Signaling from synapse to nucleus: the logic behind the mechanisms

Signaling from synapse to nucleus is vital for activity-dependent control of neuronal gene expression and represents a sophisticated form of neural computation. The nature of specific signal initiators, nuclear translocators and effectors has become increasingly clear, and supports the idea that the nucleus is able to make sense of a surprising amount of fast synaptic information through intricate biochemical mechanisms. Information transfer to the nucleus can be conveyed by physical translocation of messengers at various stages within the multiple signal transduction cascades that are set in motion by a Ca(2+) rise near the surface membrane. The key role of synapse-to-nucleus signaling in circadian rhythms, long-term memory, and neuronal survival sheds light on the logical underpinning of these signaling mechanisms.

Nicotinic alpha 7 receptors: synaptic options and downstream signaling in neurons

Nicotinic receptors are cation-ion selective ligand-gated ion channels that are expressed throughout the nervous system. Most have significant calcium permeabilities, enabling them to regulate calcium-dependent events. One of the most abundant is a species composed of the alpha 7 gene product and having a relative calcium permeability equivalent to that of NMDA receptors. The alpha 7-containing receptors can be found presynaptically where they modulate transmitter release, and postsynaptically where they generate excitatory responses. They can also be found in perisynaptic locations where they modulate other inputs to the neuron and can activate a variety of downstream signaling pathways. The effects the receptors produce depend critically on the sites at which they are clustered. Instructive preparations for examining alpha 7-containing receptors are the rat hippocampus, where they are thought to play a modulatory role, and the chick ciliary ganglion, where they participate in throughput transmission as well as regulatory signaling. Relatively high levels of alpha 7-containing receptors are found in the two preparations, and the receptors display a variety of synaptic options and functions in the two cases. Progress is starting to be made in understanding the mechanisms responsible for localizing the receptors at specific sites and in identifying components tethered in the vicinity of the receptors that may facilitate signal transduction and downstream signaling.

Voltage-gated channels block nicotinic regulation of CREB phosphorylation and gene expression in neurons

Synaptic activation of the transcription factor CREB and downstream gene expression usually depend on calcium influx aided by voltage-gated calcium channels. We find that nicotinic signaling, in contrast, activates CREB and gene expression in ciliary ganglion neurons both in culture and in situ only if voltage-gated channels are silent. The nicotinic response requires calcium influx and release from internal stores and acts through CaMK and MAPK pathways to sustain activated CREB. Voltage-gated channels mobilize CaMK to activate CREB initially, but they also enable calcineurin and PP1 to terminate the activation before transcription is affected. L-type voltage-gated channels dominate the outcome and block the effects of nicotinic signaling on transcription. This demonstrates a novel aspect of activity-dependent gene regulation.

Physiological patterns of electrical stimulation can induce neuronal gene expression by activating N-type calcium channels

Activity-dependent neuronal gene expression is thought to require activation of L-type calcium channels, a view based primarily on studies in which chronic potassium (K(+)) depolarization was used to mimic neuronal activity. However, N-type calcium channels are primarily inactivated during chronic depolarization, and their potential contribution to gene expression induced by physiological patterns of stimulation has not been defined. In the present study, electrical stimulation of dissociated primary sensory neurons at 5 Hz, or treatment with elevated K(+), produced a large increase in the percentage of neurons that express tyrosine hydroxylase (TH) mRNA and protein. However, blockade of L-type channels, which completely inhibited K(+)-induced expression, had no effect on TH expression induced by patterned stimulation. Conversely, blockade of N-type channels completely inhibited TH induction by patterned stimulation, whereas K(+)-induced expression was unaffected. Similar results were obtained for depolarization-induced expression of the immediate early genes Nurr1 and Nur77. In addition, TH induction by patterned stimulation was significantly reduced by inhibitors of PKA and PKC but was unaffected by inhibition of the mitogen-activated protein kinase (MAPK) pathway. On the other hand, K(+)-induced TH expression was significantly reduced by inhibition of the MAPK pathway but was unaffected by inhibitors of PKA or PKC. These results demonstrate that N-type calcium channels can directly link phasic membrane depolarization to gene expression, challenging the view that activation of L-type channels is required for nuclear responses to physiological patterns of activity. Moreover, our data show that phasic and chronic depolarizing stimuli act through distinct mechanisms to induce neuronal gene expression.

Physiological patterns of electrical stimulation can induce neuronal gene expression by activating N-type calcium channels

Activity-dependent neuronal gene expression is thought to require activation of L-type calcium channels, a view based primarily on studies in which chronic potassium (K(+)) depolarization was used to mimic neuronal activity. However, N-type calcium channels are primarily inactivated during chronic depolarization, and their potential contribution to gene expression induced by physiological patterns of stimulation has not been defined. In the present study, electrical stimulation of dissociated primary sensory neurons at 5 Hz, or treatment with elevated K(+), produced a large increase in the percentage of neurons that express tyrosine hydroxylase (TH) mRNA and protein. However, blockade of L-type channels, which completely inhibited K(+)-induced expression, had no effect on TH expression induced by patterned stimulation. Conversely, blockade of N-type channels completely inhibited TH induction by patterned stimulation, whereas K(+)-induced expression was unaffected. Similar results were obtained for depolarization-induced expression of the immediate early genes Nurr1 and Nur77. In addition, TH induction by patterned stimulation was significantly reduced by inhibitors of PKA and PKC but was unaffected by inhibition of the mitogen-activated protein kinase (MAPK) pathway. On the other hand, K(+)-induced TH expression was significantly reduced by inhibition of the MAPK pathway but was unaffected by inhibitors of PKA or PKC. These results demonstrate that N-type calcium channels can directly link phasic membrane depolarization to gene expression, challenging the view that activation of L-type channels is required for nuclear responses to physiological patterns of activity. Moreover, our data show that phasic and chronic depolarizing stimuli act through distinct mechanisms to induce neuronal gene expression.

Critical dependence of cAMP response element-binding protein phosphorylation on L-type calcium channels supports a selective response to EPSPs in preference to action potentials

Activity-dependent gene expression in neurons shows a remarkable ability to differentiate between different types of stimulation: orthodromic inputs that engage synaptic transmission are much more effective than antidromic stimuli that do not. We have studied the basis of such selectivity in cultured hippocampal neurons in which nuclear cAMP response element-binding protein (CREB) phosphorylation is induced by synaptic activity but not by action potential (AP) stimulation in the absence of EPSPs, although spikes by themselves generate large elevations in intracellular Ca(2+). Previous work has shown that Ca(2+) entry through L-type Ca(2+) channels plays a dominant role in triggering calmodulin mobilization and activation of calmodulin-dependent kinases that phosphorylate CREB, raising the possibility that L-type channels contribute to the selective response to EPSPs rather than APs. Accordingly, we performed voltage-clamp experiments to compare the currents carried by L-type channels during depolarizing waveforms that approximated APs or dendritic EPSPs. The integrated current generated by L-type channels was significantly less after mock APs than with EPSP-like depolarizations. The difference was traced to two distinct factors. Compared with other channels, L-type channels activated at relatively negative potentials, favoring their opening with EPSP stimulation; they also exhibited relatively slow activation kinetics, weighing against their contribution during an AP. The relative ineffectiveness of APs as a stimulus for CREB phosphorylation could be overcome by exposure to the agonist Bay K8644, which potentiated the AP-induced influx through L-type channels by approximately 10-fold. Under normal conditions, the unique biophysical properties of L-type channels allow them to act as a kinetic filter to support spike-EPSP discrimination.

Critical dependence of cAMP response element-binding protein phosphorylation on L-type calcium channels supports a selective response to EPSPs in preference to action potentials

Activity-dependent gene expression in neurons shows a remarkable ability to differentiate between different types of stimulation: orthodromic inputs that engage synaptic transmission are much more effective than antidromic stimuli that do not. We have studied the basis of such selectivity in cultured hippocampal neurons in which nuclear cAMP response element-binding protein (CREB) phosphorylation is induced by synaptic activity but not by action potential (AP) stimulation in the absence of EPSPs, although spikes by themselves generate large elevations in intracellular Ca(2+). Previous work has shown that Ca(2+) entry through L-type Ca(2+) channels plays a dominant role in triggering calmodulin mobilization and activation of calmodulin-dependent kinases that phosphorylate CREB, raising the possibility that L-type channels contribute to the selective response to EPSPs rather than APs. Accordingly, we performed voltage-clamp experiments to compare the currents carried by L-type channels during depolarizing waveforms that approximated APs or dendritic EPSPs. The integrated current generated by L-type channels was significantly less after mock APs than with EPSP-like depolarizations. The difference was traced to two distinct factors. Compared with other channels, L-type channels activated at relatively negative potentials, favoring their opening with EPSP stimulation; they also exhibited relatively slow activation kinetics, weighing against their contribution during an AP. The relative ineffectiveness of APs as a stimulus for CREB phosphorylation could be overcome by exposure to the agonist Bay K8644, which potentiated the AP-induced influx through L-type channels by approximately 10-fold. Under normal conditions, the unique biophysical properties of L-type channels allow them to act as a kinetic filter to support spike-EPSP discrimination.

Localization, regulation and functions of neurotransmitters and neuromodulators in cervical sympathetic ganglia

Cervical sympathetic ganglia represent a suitable model for studying the establishment and plasticity of neurochemical organization in the nervous system since sympathetic postganglionic neurons: (1) express several neuromediators, i.e., short acting transmitters, neuropeptide modulators and radicals, in different combinations; (2) receive synaptic input from a limited number of morphologically and neurochemically well-defined neuron populations in the central and peripheral nervous systems (anterograde influence on phenotype); (3) can be classified morphologically and neurochemically by the target they innervate (retrograde influence on phenotype); (4) regenerate readily, making it possible to study changes in neuromediator content after axonal lesion and their possible influence on peripheral nerve regeneration; (5) can be maintained in vitro in order to investigate effects of soluble factors as well as of membrane bound molecules on neuromediator expression; and (6) are easily accessible. Acetylcholine and noradrenaline, as well as neuropeptides and the recently discovered radical, nitric oxide, are discussed with respect to their localization and possible functions in the mammalian superior cervical and cervicothoracic (stellate) paravertebral ganglia. Furthermore, mechanisms regulating transmitter synthesis in sympathetic neurons in vivo and in vitro, such as soluble factors, cell contact or electrical activity, are summarized, since modulation of transmitter synthesis, release and metabolism plays a key role in the neuronal response to environmental influences.

Increases in tyrosine hydroxylase messenger RNA in the locus coeruleus after a single dose of nicotine are followed by time-dependent increases in enzyme activity and noradrenaline release

We have utilized biochemical, molecular biological, and functional neurochemical measurements to investigate the integrated and long-term effects of a single dose of nicotine on the noradrenergic system in the central nervous system of the rat, from enzyme induction to transmitter release. We have found that a single systemic injection of nicotine (0.8 mg/kg) increases messenger RNA for the rate-limiting enzyme in the synthesis of catecholamines, tyrosine hydroxylase, two to six days later in the noradrenergic cell body region, the locus coeruleus (and not in the dopaminergic cell body regions, substantia nigra and ventral tegmental area). This was then followed by a time-dependent increase in enzyme activity, measured in vitro, in terminal regions of the ascending dorsal noradrenergic bundle up to four weeks later. Functionally, the increase in tyrosine hydroxylase activity in the terminals four weeks after a single administration was associated with an increase in the capacity to release noradrenaline in the hippocampus, measured using in vivo microdialysis in freely moving animals. This occurred in response to an acute systemic nicotine injection (0.4 mg/kg) but not to a local, intrahippocampal, challenge with 250 microM nicotine. These experiments have revealed a long-term effect of nicotine on noradrenergic activity in the central nervous system, associated with induction of tyrosine hydroxylase. This is accompanied by a time-dependent increase in terminal tyrosine hydroxylase activity and an increase in noradrenaline release.

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)

N-acetylaspartylglutamate immunoreactivity in neurons of the monkey's visual pathway

The acidic dipeptide N-acetylaspartylglutamate (NAAG) was identified immunohistochemically within neurons of the visual pathways of two adult macaque monkeys which had undergone midsagittal sectioning of the optic chiasm 6 or 9 years earlier. In both temporal and nasal retinae, amacrine cells, including some displaced amacrine cells, expressed NAAG immunoreactivity. In temporal but not nasal retina, retinal ganglion cells were stained, as were their dendrites in the inner plexiform layer, and their axons in the optic nerve fiber layer. In nasal retina, the ganglion cells had degenerated because they were axotomized by the optic chiasm section. In the target regions of the retinal ganglion cells, the superior colliculus and the lateral geniculate nucleus (LGN), both neuropil and cell bodies were stained. In LGN, staining was confined to layers 2, 3, and 5, that is, to the layers innervated by the intact ipsilateral pathway. Immunoreactivity was also seen in the cells of layers 2, 3A, 4B, 5, and 6 of area 17 and layers 3 and 5 of area 18. The neuropil was stained in all layers of area 17, but more heavily in layers 1, 2, 4B, the bottom of 4C beta, 5B, and 6B. Within 4C the staining was patchy; in tangential sections there were alternating bands of light and dark label which matched the ocular dominance bands demonstrated by cytochrome oxidase histochemistry in adjacent sections. This banding pattern is consistent with the presence of NAAG in geniculocortical terminals of the intact ipsilateral pathway and the absence of such terminals for the contralateral pathway, which had undergone transneuronal degeneration due to the optic chiasm sectioning. Overall, our results for monkey are very similar to those in cat and suggest that NAAG or a structurally related molecule may have a prominent role in the communication of visual signals at retinal, thalamic, and cortical levels.

Morphological changes in the geniculocortical pathway associated with monocular deprivation

To summarize (Fig. 10), the structural consequences of monocular deprivation include the following changes: the relay cells in the binocular segments of the deprived geniculate layers shrink and contain less of the possible neurotransmitter NAAG. These changes appear to be secondary to a loss of terminal synaptic arbor. Certainly, deprived geniculocortical cells project to smaller ocular dominance patches in layer IV of visual cortex, where they make fewer and abnormal synapses. As a result, they activate ocular activation columns that, in addition to being small, are faint and usually fail to extend into extragranular layers. This failure to extend to other layers probably results from a failure of the poorly activated deprived-eye cells in layer IV to compete successfully with neighboring experienced-eye cells in layer IV, resulting in a loss of connections from layer IV to other layers (Fig. 11). Thus, the primary effect of monocular deprivation is probably the disruption of the geniculocortical synapse, with the other changes, such as cell size, and possibly the change in neurotransmitter content, being secondary. The disrupted synapse would result in poorly driven cortical cells and faint ocular activation columns, which in turn would bias a secondary competition for access to cells in extragranular layers. There are certain general principles that unite the findings presented in this chapter with the others in this session. First, there are similarities in the types of morphological changes observed, for example, changes in the number and size of synaptic terminals, as well as mitochondrial changes. This implies that there are similar changes during development and adult plasticity and also similar changes in vertebrates and invertebrates. Second, it is not so much the amount of activity that determines these changes, but the pattern of activity. In my results, the relative imbalance in activity is important, but not the absolute amount (for example, the columns activated by the 8-hr eye of an AME 8/1 are different from those activated by the 8-hr eye of an AME 8/8). Similarly, the binocular segment, where there was an imbalance and competition could occur, was affected, whereas the monocular segment, where there was no imbalance and competition could not occur, was not. Finally, the recent results of Reiter and Stryker suggest that monocular deprivation produces changes only when the activity of the presynaptic cell and the postsynaptic cell are correlated.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of neuropeptide expression in avian embryonic sympathetic cultures

Two distinct neuropeptide-related phenotypes are found in avian paravertebral sympathetic ganglia, corresponding to somatostatin- (SS) and vasoactive intestinal polypeptide- (VIP) expressing cells. We have detected the same cell phenotypes in cultures of embryonic quail sympathetic ganglia and have used this system to study the modulation of their expression by the environment. The cell phenotypes were identified using immunocytochemistry and induced catecholamine fluorescence and quantitative data were obtained by radioimmunoassay. Dissociation of the ganglia caused a profound increase in the expression of VIP but had no effect on SS levels. Addition of corticosterone (10(-6) M) increased the expression of SS without modifying VIP levels. In contrast, depolarization of the cells induced changes in levels of both neuropeptides. The modulation of VIP correlates with the modulation of cholinergic properties. The regulation of neuropeptide expression in the avian system shows both similarities and differences to what has been found in the mammalian system.

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.

5-HT modulates protein synthesis and the expression of specific proteins during long-term facilitation in Aplysia sensory neurons

Long-term, but not short-term, facilitation induced by 5-HT in Aplysia sensory neurons is blocked by application of inhibitors of protein synthesis or RNA synthesis during the 1.5 hr training period. These findings suggest that genes and proteins not needed for the short-term process are required for long-term facilitation. To identify these proteins, we examined changes in overall and specific protein synthesis in sensory neurons. During the 1.5 hr period of stimulation, 5-HT initiated three temporally distinct changes in total protein synthesis: a small decrease at 0.5 hr, a small increase at 1 hr, and a second and larger increase at 3 hr. 5-HT also produced three temporally distinct changes in the synthesis of individual proteins that could be resolved on 2-D gels. First, there was an increase at 30 min in the rate of synthesis of 10 proteins and a decrease in the synthesis of 5 proteins. These changes in protein expression are rapid and transient, subsiding within 1-3 hr, and are transcription-dependent. Second, at 3 hr, there is a transient increase in 4 different proteins that also is transcription-dependent and lasts approximately 5 hr. Finally, at 24 hr, during the retention phase of facilitation, there was a sustained increase in the expression of two proteins.

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)

Biochemistry of information storage in the nervous system

The use of molecular biological approaches has defined new mechanisms that store information in the mammalian nervous system. Environmental stimuli alter steady-state levels of messenger RNA species encoding neurotransmitters, thereby altering synaptic, neuronal, and network function over time. External or internal stimuli alter impulse activity, which alters membrane depolarization and selectively changes the expression of specific transmitter genes. These processes occur in diverse peripheral and central neurons, suggesting that information storage is widespread in the neuraxis. The temporal profile of any particular molecular mnemonic process is determined by specific kinetics of turnover and by the geometry of the neuron resulting in axonal transport of molecules to different synaptic arrays at different times. Generally, transmitters, the agents of millisecond-to-millisecond communication, are subject to relatively long-lasting changes in expression, ensuring that ongoing physiological function is translated into information storage.

Comparison of the effects of elevated K+ ions and muscle-conditioned medium on the neurotransmitter phenotype of cultured sympathetic neurons

Neuronal depolarization and culture media conditioned by certain nonneuronal cells (CM) are known to exert opposite effects on the expression of cholinergic and noradrenergic traits in cultured rat sympathetic neurons. We have compared their effects on the developments of choline acetyltransferase (CAT), tyrosine hydroxylase (TOH), dopa decarboxylase (AADC) and acetylcholinesterase (AcChE) in these cultures. A macromolecular factor which was partially purified from CM increased CAT development in a dose-dependent manner and depressed the development of TOH and AADC by 5- to 10-fold. In the presence of intermediate concentrations of this partially purified factor, both CAT and catecholamine synthesizing enzymes developed to high levels, whereas high concentrations caused a long-lasting, but not total, impairment of TOH development. The effects of CM on both CAT and AADC activities resulted from variations in the number of immunotitratable enzyme molecules. Conversely, K+ ions (30-40 mM) depressed the development of CAT by 90% and stimulated TOH development 2.5-fold. Cultures grown with CM in high K+ medium had similar CAT and TOH activities as compared to those cultures grown without CM in low K+ medium suggesting that CM and K+ ions had antagonistic effects on the expression of these enzymes. However, K+ ions did not affect the development of AADC in these cultures. CM suppressed in a reversible manner the development of the 16 S form of AcChE. In the presence of 40 mM K+, the rate of development of AcChE was reduced. In particular, the development of 16 S AcChE was strikingly impaired, although not totally suppressed. The effect of elevated K+ ions on the percentage of 16 S AcChE was rapidly reversible. It is concluded that CM and elevated K+ ions have antagonistic effects on CAT and TOH, but not on AADC development; AcChE, in particular its asymmetric 16 S form, is regulated independently of the cholinergic/noradrenergic status of sympathetic neurons.

The use of a tyrosine-hydroxylase cDNA probe to study the neurotransmitter plasticity of rat sympathetic neurons in culture

We have compared quantitatively the effects of muscle-conditioned medium (CM) and elevated K+ concentration (40 mM) on the enzymatic activity of tyrosine hydroxylase (TH) and on TH-mRNA levels in primary cultures of rat sympathetic neurons. Northern blot analysis of RNA from cultured neurons with a 32P-labeled rat TH-cDNA probe was performed. The probe hybridized strongly with a single RNA species of 1.9 kb, similar in size to the TH-mRNA from PC12 pheochromocytoma cells. In agreement with earlier data both CM and a partially purified factor from CM increased choline acetyltransferase activity up to 200-fold and depressed TH activity by 2- to 7-fold in cultured sympathetic neurons. These effects were accompanied by a decrease in TH-mRNA level, which correlated with the decrease in TH activity. On the other hand, a culture medium supplemented with 40 mM KCl caused a 1.5- to 5-fold increase in TH activity, which was accompanied by an increase in TH-mRNA level of the same order of magnitude. As a working hypothesis, we suggest that CM and neuronal depolarization control the transcription of the TH gene in an antagonistic manner.

Low-frequency septal stimulation increases tyrosine hydroxylase activity in the hippocampus

Rats were chronically implanted with recording electrodes in the dorsomedial subiculum and stimulating electrodes in the septal area. Low-frequency septal stimulation (regular inter-pulse interval = 130 msec) was used over a period of 10 days to drive the hippocampal theta rhythm at 7.7 Hz (a régime shown previously to proactively facilitate acquisition of barpressing and retard its extinction), and high frequency (77 Hz) stimulation was used to block theta rhythm (a régime shown previously to proactively retard acquisition of barpressing and facilitate its extinction). The activity of soluble tyrosine hydroxylase from hippocampus increased in animals killed 15-33 days after the end of the period of 7.7 Hz septal stimulation but not after high-frequency stimulation. Irregular low-frequency stimulation with a mean inter-pulse of 130 msec produced the same effect on tyrosine hydroxylase activity as regular 7.7 Hz theta-driving. The possible relations between the effects of low-frequency septal stimulation on hippocampal tyrosine hydroxylase and on behavior are discussed.

Differential effects of electrical stimulation of sciatic nerve on metabolic activity in spinal cord and dorsal root ganglion in the rat

Electrical stimulation of the proximal stump of the transected sciatic nerve produces a frequency-dependent activation of glucose utilization, measured with the autoradiographic deoxy [14C]glucose method, in the dorsal horn of the spinal cord but produces no change in glucose utilization in the dorsal root ganglion cells. These results suggest that axon terminals and not the cell bodies are the sites of enhanced metabolic activity during increased functional activity of this pathway.

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

The rat superior cervical ganglion was stimulated in vitro via its preganglionic trunk and then maintained in organ culture for 2-3 days. Following nerve stimulation, the specific activity of tyrosine hydroxylase (TH) increased with a lag period of at least 12 h, and this increased enzyme activity was maintained throughout the culture period. The magnitude of the increase in TH activity depended on the frequency and duration of preganglionic nerve stimulation. This transsynaptic increase in enzyme activity could be completely blocked by the nicotinic antagonist, hexamethonium. The synthetic glucocorticoid dexamethasone, at concentrations above 1 nM, increased TH activity in unstimulated ganglia with a lag period of at least 24 h. The increase in enzyme activity produced by dexamethasone was not affected by nicotinic antagonists or by prior decentralization of the superior cervical ganglion, in contrast to the findings of previous workers. These data indicate that dexamethasone can stimulate ganglionic TH activity in the absence of cholinergic stimulation. In addition, experiments combining dexamethasone with nerve stimulation demonstrate that the steroid can potentiate the effects on TH activity of brief periods of nerve stimulation.

Olfactory bulb dopamine neurons survive deafferentation-induced loss of tyrosine hydroxylase

Peripheral deafferentation of the rodent olfactory bulb results in loss of dopamine content, tyrosine hydroxylase activity and immunocytochemical staining for tyrosine hydroxylase in juxtaglomerular dopamine neurons. Reinnervation of the bulb by afferent neurons results in the return of all parameters to control levels suggesting that the dopamine neurons did not degenerate but that the expression of tyrosine hydroxylase enzyme was transneuronally regulated in a static population of juxtaglomerular cells. To evaluate this possibility, we determined the activity and immunocytochemical localization of the second enzyme in the dopamine biosynthetic pathway, DOPA decarboxylase. At a time when tyrosine hydroxylase activity was reduced to 25% of control values, DOPA decarboxylase activity in the lesioned bulb was maintained at about 65% of that in the unlesioned bulb. Immunocytochemical staining with antibodies to both enzymes, performed sequentially in the same sections, demonstrated that in the unlesioned bulb tyrosine hydroxylase and DOPA decarboxylase are co-localized in the same population of juxtaglomerular neurons. Similar results were obtained in adjacent sections each stained with one of the two antibodies. In contrast, in the deafferented bulb, about three times as many neurons were stained with DOPA decarboxylase as with tyrosine hydroxylase antibodies. The DOPA decarboxylase activity measurements and immunocytochemistry argue for the continued presence, in the lesioned olfactory bulb, of a population of tyrosine hydroxylase deficient dopamine neurons. The data suggest that olfactory receptor cell innervation transneuronally regulates the expression of tyrosine hydroxylase by mechanisms separate from those controlling the levels of DOPA decarboxylase.

Regulation of axonal microtubules: effect of sympathetic hyperactivity elicited by reserpine

The density of microtubules in sympathetic postganglionic fibres of the cat was studied with the electron-microscope before and after administration of reserpine. The microtubule density was 56 microtubules per square micron under basal conditions. Six hours after reserpine administration, the density rose by 46%. This change was still present 55 h later. At least 31% of the total microtubular protein in the axoplasm of sympathetic fibres of the unrestrained cat was estimated to be in the soluble form. The increase in microtubule density was prevented by a section of the preganglionic fibres. Microtubules of the unmyelinated fibres of the cutaneous sural nerve were unaffected by reserpine treatment. Since reserpine is known to produce hyperactivity of sympathetic nerves, it is concluded that this hyperactivity is instrumental in the increase of the number of axonal microtubules. It is proposed that the electrical activity of nerves regulates axonal microtubules in the living animal.

Activation of neurosecretory cells enhances their synthesis of secretory protein

We have asked whether neurosecretory cells respond to activation by selectively augmenting their synthesis of secretory protein, using the bag cells of Aplysia, which produce and secrete a peptide egg-laying hormone, ELH. Exposure of bag cell organs to 100 mM K+ for 4 h increased their incorporation of tritiated leucine into ELH precursors by about 25%, an effect which persisted for at least 8 h after the stimulus. Since the high potassium treatment had no effect on the rate of loss of label from previously-synthesized ELH proteins, we conclude that the enhanced labeling represents enhanced synthesis. Elevated external potassium did not enhance ELH synthesis in clusters of bag cell somata which had been surgically isolated from their neurites, suggesting that the augmentation of synthesis is not mediated by direct effects of the high potassium solution. Mediation by a secretion-linked process is indicated by the fact that low Ca2+/high Mg2+ media blocked the effect of high K+. Whether the regulatory signal involves receipt of presynaptic transmitter or hormone secretion remains to be determined.

Tyrosine hydroxylase activity in the superior cervical ganglion during herpes simplex virus infection: correlation with viral titers and viral antigen

The activity of tyrosine hydroxylase (TH) was measured in the superior cervical ganglion (SCG) of the mouse during herpes simplex virus (HSV) infection. TH activity remained at control levels or actually increased during acute infection at a time when viral titers of SCG homogenates were at their peak and viral antigen was detected in from one-third to one-half of ganglionic neurons. A rapid decline in TH activity followed and coincided with falling viral titers, disappearance of viral antigen and replacement of neurons by inflammatory cells. Immunization partially prevented this reduction of TH activity. In addition, when mice were immunosuppressed by cyclophosphamide, TH activity was relatively preserved early in the course of infection despite high viral titers in the ganglion and the presence of viral antigen and histopathological alterations in nearly 100% of neurons. These results suggest that a cellular 'luxury function', in this case TH activity, can be preserved and perhaps even augmented during neuronal HSV infection. Indeed, activity of this enzyme may persist until late into the acute phase of infection, perhaps up to the point of cell death induced either by immune-mediated or direct virus-induced cell lysis.

Preganglionic nerve stimulation increases the amount of tyrosine hydroxylase in the rat superior cervical ganglion

Seventy-two hours after a 90 min period of electrical stimulation of the cervical sympathetic trunk, tyrosine hydroxylase activity was twice as high in homogenates of stimulated ganglia than in homogenates of contralateral control ganglia. When aliquots of the homogenates of the two groups of ganglia were mixed, the tyrosine hydroxylase activity was additive. Immunoprecipitation studies using an antibody prepared against purified tyrosine hydroxylase demonstrated that the doubling in enzyme activity is accompanied by a doubling in the amount of immunoprecipitable material. Thus the increase in tyrosine hydroxylase activity can be totally accounted for by an increase in the amount of tyrosine hydroxylase in the ganglion.