Determination of transmitter function by neuronal activity

Neurotransmitter Switching in the Developing and Adult Brain

Neurotransmitter switching is the gain of one neurotransmitter and the loss of another in the same neuron in response to chronic stimulation. Neurotransmitter receptors on postsynaptic cells change to match the identity of the newly expressed neurotransmitter. Neurotransmitter switching often appears to change the sign of the synapse from excitatory to inhibitory or from inhibitory to excitatory. In these cases, neurotransmitter switching and receptor matching thus change the polarity of the circuit in which they take place. Neurotransmitter switching produces up or down reversals of behavior. It is also observed in response to disease. These findings raise the possibility that neurotransmitter switching contributes to depression, schizophrenia, and other illnesses. Many early discoveries of the single gain or loss of a neurotransmitter may have been harbingers of neurotransmitter switching.

Aquaporins: Multiple Roles in the Central Nervous System

Aquaporins (AQPs) represent a diverse family of membrane proteins found in prokaryotes and eukaryotes. The primary aquaporins expressed in the mammalian brain are AQP1, which is densely packed in choroid plexus cells lining the ventricles, and AQP4, which is abundant in astrocytes and concentrated especially in the end-feet structures that surround capillaries throughout the brain and are present in glia limitans structures, notably in osmosensory areas such the supraoptic nucleus. Water movement in brain tissues is carefully regulated from the micro- to macroscopic levels, with aquaporins serving key roles as multifunctional elements of complex signaling assemblies. Intriguing possibilities suggest links for AQP1 in Alzheimer's disease, AQP4 as a target for therapy in brain edema, and a possible contribution of AQP9 in Parkinson's disease. For all the aquaporins, new contributions to physiological functions are likely to continue to be discovered with ongoing work in this rapidly expanding field of research. NEUROSCIENTIST 13(5):470—485, 2007.

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.

Non-cell-autonomous mechanism of activity-dependent neurotransmitter switching

Activity-dependent neurotransmitter switching engages genetic programs regulating transmitter synthesis, but the mechanism by which activity is transduced is unknown. We suppressed activity in single neurons in the embryonic spinal cord to determine whether glutamate-gamma-aminobutyric acid (GABA) switching is cell autonomous. Transmitter respecification did not occur, suggesting that it is homeostatically regulated by the level of activity in surrounding neurons. Graded increase in the number of silenced neurons in cultures led to graded decrease in the number of neurons expressing GABA, supporting non-cell-autonomous transmitter switching. We found that brain-derived neurotrophic factor (BDNF) is expressed in the spinal cord during the period of transmitter respecification and that spike activity causes release of BDNF. Activation of TrkB receptors triggers a signaling cascade involving JNK-mediated activation of cJun that regulates tlx3, a glutamate/GABA selector gene, accounting for calcium-spike BDNF-dependent transmitter switching. Our findings identify a molecular mechanism for activity-dependent respecification of neurotransmitter phenotype in developing spinal neurons.

Activity-dependent neurotransmitter respecification

For many years it has been assumed that the identity of the transmitters expressed by neurons is stable and unchanging. Recent work, however, shows that electrical activity can respecify neurotransmitter expression during development and in the mature nervous system, and an understanding is emerging of the molecular mechanisms underlying activity-dependent transmitter respecification. Changes in postsynaptic neurotransmitter receptor expression accompany and match changes in transmitter specification, thus enabling synaptic transmission. The functional roles of neurotransmitter respecification are beginning to be understood and appear to involve homeostatic synaptic regulation, which in turn influences behaviour. Activation of this novel form of plasticity by sensorimotor stimuli may provide clinical benefits.

Stages of neurotransmitter development in autonomic neurons

The ontogeny of neurotransmitters in autonomic neurons proceeds through the successive stages of early expression, definitive expression, modulation, and regulation, extending from embryonic life to maturity. Although different extracellular signals influence development at different stages, a number of signals that influence development continue to govern transmitter function during maturity. The sequential ontogenetic stages parallel the progressive restriction of mutability of phenotypic expression; however, some degree of neuronal mutability appears to persist through maturity.

Adrenergic-cholinergic dual function in cultured sympathetic neurons of the rat

Sympathetic principal neurons, dissociated from the superior cervical ganglia of newborn rats and put into culture, exhibit plasticity with respect to the choice between noradrenaline (norepinephrine) and acetylcholine as transmitter. The neurons shift from an initial, immature adrenergic state to a cholinergic state in certain culture conditions, e.g in co-culture with a variety of non-neuronal cells or after exposure to a medium conditioned by such cells. To study the transition directly, we have grown single neurons in "microcultures" with cardiac myocytes, which provide a sensitive assay for the transmitters secreted by the neurons. We have shown previously that during the transition from adrenergic to cholinergic status such neurons secrete both transmitters and have terminals of mixed fine structure (dual function). We describe here experiments in which identified neurons were serially assayed over periods of 9-45 days. Partial transitions were observed, always in the direction adrenergic to cholinergic function, and one complete transition was observed from apparently purely adrenergic function to dual function and then to apparently purely cholinergic function. We also report observation of adrenergic-cholinergic dual function, in preliminary single and serial assays, in sympathetic principal neurons from the superior cervical ganglia of adult rats.

Growth and development of sympathetic neurons in tissue culture

Adrenergic neurons from the superior cervical ganglion of the neonatal rat, when studied under certain culture conditions, develop cholinergic properties including hexamethonium-sensitive synaptic interactions, choline acetyltransferase activity and synaptic endings containing clear vesicles. Evidence from correlative biochemical, physiological and morphological studies on populations of neurons indicates that cholinergic function is acquired by the majority of neurons and not by subpopulation. The factors that influence the development of cholinergic function in culture include the presence of non-neuronal cells, the addition of human placental serum and chick embryo extract to the culture medium as well as the stage of development at which the neurons are placed in culture. Neurons from mature rats, maintained as explants in culture, develop low choline acetyltransferase activity and the synaptic endings containing dense-cored vesicles. In contrast, if dissociated, these adult neurons develop several cholinergic characteristics. Studies to determine which adrenergic properties are retained in neurons expressing cholinergic characteristics have shown an increase in the activities of tyrosine hydroxylase and dopamine beta-hydroxylase in both explanted and dissociated perinatal neurons. In addition, tyrosine hydroxylase has been localized immunocytochemically in neurons identified as cholinergic by electrophysiological methods.

Postinfarct sympathetic hyperactivity differentially stimulates expression of tyrosine hydroxylase and norepinephrine transporter

The balance between norepinephrine (NE) synthesis, release, and reuptake is disrupted after acute myocardial infarction, resulting in elevated extracellular NE. Stimulation of sympathetic neurons in vitro increases NE synthesis and the synthetic enzyme tyrosine hydroxylase (TH) to a greater extent than it increases NE reuptake and the NE transporter (NET), which removes NE from the extracellular space. We used TGR(ASrAOGEN) transgenic rats, which lack postinfarct sympathetic hyperactivity, to test the hypothesis that increased cardiac sympathetic nerve activity accounts for the imbalance in TH and NET expression in these neurons after myocardial infarction. TH and NET mRNA levels were identical in the stellate ganglia of unoperated TGR(ASrAOGEN) rats compared with Sprague Dawley (SD) controls, but the threefold increase in TH and twofold increase in NET mRNA seen in the stellate ganglia of SD rats 1 wk after ischemia-reperfusion was absent in TGR(ASrAOGEN) rats. Similarly, the increase in TH and NET protein observed in the base of the SD ventricle was absent in the base of the TGR (ASrAOGEN) ventricle. Neuronal TH content was depleted in the left ventricle of both genotypes, whereas NET was unchanged. Basal heart rate and cardiac function were similar in both genotypes, but TGR(ASrAOGEN) hearts were more sensitive to the β-agonist dobutamine. Tyramine-induced release of endogenous NE generated similar changes in ventricular pressure and contractility in both genotypes, but postinfarct relaxation was enhanced in TGR(ASrAOGEN) hearts. These data support the hypothesis that postinfarct sympathetic hyperactivity is the major stimulus increasing TH and NET expression in cardiac neurons.

Further studies of the distribution of CDF/LIF mRNA

Differentiation choices in the haemopoietic and nervous systems are controlled in part by instructive factors. The cholinergic differentiation factor (CDF, also known as leukaemia inhibitory factor, LIF) affects the development of cultured cells from both systems. To understand the role of CDF/LIF during normal development in vivo, we have begun to localize its mRNA in the late fetal and postnatal rat. Application of reverse transcriptase-polymerase chain reaction and RNase protection methods reveals that CDF/LIF mRNA levels are developmentally modulated in both haemopoietic and neural tissues. A target tissue of cholinergic sympathetic neurons, the footpads that contain the sweat glands, express high levels of this mRNA (relative to mRNA for actin and beta 2-microglobulin). Levels in targets of noradrenergic neurons are lower, but do undergo significant changes during development. Signals are also detected in selective regions of the adult brain, and in embryonic skeletal muscle. This finding in muscle may be significant for motor neurons, because CDF/LIF is a trophic factor for these neurons in culture. Embryonic liver, neonatal thymus and postnatal spleen express CDF/LIF mRNA, and expression in gut is the highest of all tissues examined. The selective tissue distribution and developmental modulation of CDF/LIF mRNA expression support a role for this factor in the normal development of several organ systems.

A multielectrode microcompartment culture platform for studying signal transduction in the nervous system

This paper describes the design, fabrication, and characterization of a microfabricated compartmented culture system (micro-CCS) useful for electrophysiological signaling studies in cultured neurons. The focus of the paper is the process of interfacing the micro-CCS with cultured neurons and to demonstrate the applicability of the system for biochemical-mediated electrophysiological studies. Moreover, we show that we can record action potentials from cultured neurons through the extracellular compartmented application of elevated levels of K(+) ions. Finally, we show that we can isolate the electrophysiological effects of the sodium channel blocker tetrodotoxin in one of the compartments of a two compartment culture while recording electrophysiological data from both compartments.

A compartmented neuronal culture system in microdevice format

This paper describes a microfabricated compartmented culture system (mu-CCS) for studying the effects of drugs on cultured neurons. We describe the fabrication of the microsystem and show the ability to culture DRG neurons in the microsystem. Furthermore, we demonstrate the ability to culture neurons with extensions growing into adjoining compartments while maintaining fluid isolation between compartments. The axonal growth pattern was controlled along the surface of the glass microelectrode substrate using a micropatterned collagen film. The ability to isolate fluids to selected compartments while simultaneously allowing intercompartmental growth of the axons enables various studies in which selected segments of neurons or populations of neurons can be selectively exposed to biochemical treatment. The neurotoxin vincristine was used as the test vehicle to demonstrate the functionality of the mu-CCS. Vincristine was applied to the axonal compartment to show that the interaction of drugs with DRG neurons progresses in a way similar to neurons cultured/exposed using conventional techniques.

Chronic depolarization stimulates norepinephrine transporter expression via catecholamines

Chronic depolarization increases norepinephrine (NE) uptake and expression of the norepinephrine transporter (NET) in sympathetic neurons, but the mechanisms are unknown. Depolarization of sympathetic neurons stimulates catecholamine synthesis, and several studies suggest that NET can be regulated by catecholamines. It is not clear if the depolarization-induced increase in NET is because of nerve activity per se, or is secondary to elevated catecholamines. To determine if induction of NET mRNA was a result of increased catecholamines, we used pharmacological manipulations to (i) inhibit tyrosine hydroxylase activity in neurons depolarized with 30 mm KCl, thereby preventing increased catecholamines, or (ii) stimulate tyrosine hydroxylase activity in the absence of depolarization. Inhibiting the depolarization-induced increase in catecholamines prevented the up-regulation of NET mRNA, but did not block the increase in tyrosine hydroxylase (TH) mRNA. Furthermore, stimulating catecholamine production in the absence of depolarization elevated NE uptake, NET protein, and NET mRNA in sympathetic neurons. Similarly, elevating endogenous catecholamines in SK-N-BE2M17 neuroblastoma cells increased NE uptake and NET expression. These data suggest that chronic depolarization of sympathetic neurons induces NET expression through increasing catecholamines, and that M17 neuroblastoma cells provide a model system in which to investigate catechol regulation of NET expression.

Regulation of noradrenergic function by inflammatory cytokines and depolarization

Although the sympathetic neurons innervating the heart are exposed to the inflammatory cytokines cardiotrophin-1 (CT-1), interleukin-6 (IL-6) and tumor necrosis factor alpha (TNFalpha) after myocardial infarction, the effects of these cytokines on noradrenergic function are not well understood. We used cultured sympathetic neurons to investigate the effects of these cytokines on catecholamine content, the tyrosine hydroxylase co-factor, tetrahydrobiopterin (BH4), and norepinephrine (NE) uptake. CT-1, but not IL-6 or TNFalpha, suppressed NE uptake and catecholamines in these neurons, whereas CT-1 and, to a lesser extent, IL-6 decreased BH4 content. CT-1 exerted these effects by decreasing tyrosine hydroxylase, GTP cyclohydrolase (GCH) and NE transporter mRNAs, while IL-6 lowered only GCH mRNA. The neurons innervating the heart are also activated by the central nervous system after myocardial infarction. We examined the combined effect of depolarization and cytokines on noradrenergic function. In CT-1-treated cells, depolarization caused a small increase in BH4 and NE uptake, and a large increase in catecholamines. These changes were accompanied by increased TH, GCH and NE transporter mRNAs. CT-1 and depolarization regulate expression of noradrenergic properties in an opposing manner, and the combined treatment results in elevated cellular catecholamines and decreased NE uptake relative to control cells.

3',5'-cyclic adenosine monophosphate regulates expression of synaptotagmin in neonatal sympathetic ganglia in vitro

The expression of the synaptic vesicle protein, synaptotagmin, in developing rat superior cervical ganglia is influenced by transsynaptic factors associated with membrane depolarization. The present study examines the role of cyclic AMP in the regulation of synaptotagmin in neonatal superior cervical ganglia maintained in explant culture. Ganglia were treated for 48 h in vitro with the Na+-channel ionophore, veratridine, or with pharmacological agents that alter cyclic AMP levels. Levels of cyclic AMP and synaptotagmin were determined by radioimmunoassay. Veratridine treatment significantly increased cyclic AMP in cultured ganglia, with a long time course, and also increased synaptotagmin levels. Drugs that elevate cyclic AMP levels significantly increased synaptotagmin levels, with similar magnitude to that produced by veratridine treatment. These pharmacological agents did not alter neuron survival or total ganglionic protein content. No additive effects were observed after combined treatment with veratridine and pharmacological agents that increased cyclic AMP. Agents that blocked adenylyl cyclase blocked the veratridine-induced increase in synaptotagmin levels. The results suggest that regulation of expression of synaptotagmin in neonatal sympathetic neurons is mediated partially by cyclic AMP.

Gene regulation by patterned electrical activity during neural and skeletal muscle development

Patterned neural activity modifies central synapses during development and the physiological properties of skeletal muscle by selectively repressing or stimulating transcription of distinct genes. The effects of neural activity are mostly mediated by calcium. Of particular interest are the cellular mechanisms that may be used to sense and convert changes in calcium into specific alterations in gene expression. Recent studies have addressed the importance of spatial heterogeneity or of temporal changes in calcium levels for the regulation of gene expression.

A developmental shift from GABAergic to glycinergic transmission in the central auditory system

GABAergic and glycinergic circuits are found throughout the auditory brainstem, and it is generally assumed that transmitter phenotype is established early in development. The present study documents a profound transition from GABAergic to glycinergic transmission in the gerbil lateral superior olive (LSO) during the first 2 postnatal weeks. Whole-cell voltage-clamp recordings were obtained from LSO neurons in a brain slice preparation, and IPSCs were evoked by electrical stimulation of the medial nucleus of the trapezoid body (MNTB), a known glycinergic projection in adult animals. GABAergic and glycinergic components were identified by blocking transmission with bicuculline and strychnine (SN), respectively. In the medial limb of LSO, there was a dramatic change in the GABAergic IPSC component, decreasing from 78% at postnatal day 3 (P3)-P5 to 12% at P12-P16. There was an equal and opposite increase in the glycinergic component during this same period. Direct application of GABA also elicited significantly larger amplitude and longer duration responses in P3-P5 neurons compared with glycine-evoked responses. In contrast, MNTB-evoked IPSCs in lateral limb neurons were more sensitive to SN throughout development. Consistent with the electrophysiological observations, there was a reduction in staining for the beta2,3-GABAA receptor subunit from P4 to P14, whereas staining for the glycine receptor-associated protein gephyrin increased. Brief exposure to baclofen depressed transmission at excitatory and inhibitory synapses for approximately 15 min, suggesting a GABAB-mediated metabotropic signal. Collectively, these data demonstrate a striking switch from GABAergic to glycinergic transmission during postnatal development. Although GABA and glycine elicit similar postsynaptic ionotropic responses, our results raise the possibility that GABAergic transmission in neonates may play a developmental role distinct from that of glycine.

A developmental shift from GABAergic to glycinergic transmission in the central auditory system

GABAergic and glycinergic circuits are found throughout the auditory brainstem, and it is generally assumed that transmitter phenotype is established early in development. The present study documents a profound transition from GABAergic to glycinergic transmission in the gerbil lateral superior olive (LSO) during the first 2 postnatal weeks. Whole-cell voltage-clamp recordings were obtained from LSO neurons in a brain slice preparation, and IPSCs were evoked by electrical stimulation of the medial nucleus of the trapezoid body (MNTB), a known glycinergic projection in adult animals. GABAergic and glycinergic components were identified by blocking transmission with bicuculline and strychnine (SN), respectively. In the medial limb of LSO, there was a dramatic change in the GABAergic IPSC component, decreasing from 78% at postnatal day 3 (P3)-P5 to 12% at P12-P16. There was an equal and opposite increase in the glycinergic component during this same period. Direct application of GABA also elicited significantly larger amplitude and longer duration responses in P3-P5 neurons compared with glycine-evoked responses. In contrast, MNTB-evoked IPSCs in lateral limb neurons were more sensitive to SN throughout development. Consistent with the electrophysiological observations, there was a reduction in staining for the beta2,3-GABAA receptor subunit from P4 to P14, whereas staining for the glycine receptor-associated protein gephyrin increased. Brief exposure to baclofen depressed transmission at excitatory and inhibitory synapses for approximately 15 min, suggesting a GABAB-mediated metabotropic signal. Collectively, these data demonstrate a striking switch from GABAergic to glycinergic transmission during postnatal development. Although GABA and glycine elicit similar postsynaptic ionotropic responses, our results raise the possibility that GABAergic transmission in neonates may play a developmental role distinct from that of glycine.

Leukemia inhibitory factor and ciliary neurotrophic factor regulate dendritic growth in cultures of rat sympathetic neurons

Cytokines such as leukemia inhibitory factor (LIF) and ciliary neurotrophic factor (CNTF) have previously been shown to regulate neurotransmitter and neuropeptide synthesis in sympathetic neurons [P.H. Patterson, Leukemia inhibitory factor, a cytokine at the interface between neurobiology and immunology, Proc. Natl. Acad. Sci. USA 91 (1994) 7833-7835]. We considered the possibility that these agents may also affect the development of neuronal cell shape. Intracellular dye injection and immunocytochemistry were used to assess dendritic growth in cultures of perinatal rat sympathetic neurons and the effects of LIF and CNTF were compared to those of osteogenic protein-1 (OP-1), a growth factor that induces profuse dendritic growth in these neurons [P. Lein, M. Johnson, X. Guo, D. Rueger, D. Higgins, Osteogenic protein-1 induces dendritic growth in rat sympathetic neurons, Neuron 15 (1995) 597-605]. Under control conditions, sympathetic neurons formed only axons. Exposure to either LIF or OP-1 stimulated dendritic growth, but the magnitude of the response to LIF was much less than that obtained with OP-1 with respect to both dendritic number and length. Simultaneous exposure to LIF and OP-1 resulted in dendritic growth equivalent to that observed in the presence of LIF alone, suggesting that LIF inhibits the response of neurons to OP-1. Both the stimulatory and inhibitory effects of LIF were mimicked by CNTF, but not by other growth factors. These data suggest that LIF and CNTF regulate dendritic development in a complex manner that is dependent on both the morphological state of the neuron and the presence of other growth factors. However, the net effect of exposure to these cytokines appears to be the production of a population of neurons with rudimentary arbors consisting of only one or two short dendrites.

Extremely low frequency magnetic fields promote neurite varicosity formation and cell excitability in cultured rat chromaffin cells

We report the use of an extremely low frequency magnetic field (ELF-MF, 60 Hz, 10 Grms) to stimulate cultured chromaffin cells to determine the possible changes that may occur in their electrical properties. The results showed that ELF-MF not only facilitated neurite outgrowth, but also formation of neurite varicosity with high concentration of catecholamines. This report also shows for the first time that a greater number of cultured chromaffin cells differentiated by ELF-MF have spontaneous extracellular electrical activity and that their firing frequency is higher than that seen in non-stimulated cells.

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.

Leukemia inhibitory factor induces neurotransmitter switching in transgenic mice

Leukemia inhibitory factor (LIF) is a cytokine growth factor that induces rat sympathetic neurons to switch their neurotransmitter phenotype from noradrenergic to cholinergic in vitro. To test whether LIF can influence neuronal differentiation in vivo, we generated transgenic mice that expressed LIF in pancreatic islets under the control of the insulin promoter and evaluated the neurotransmitter phenotype of the pancreatic sympathetic innervation. We also used the insulin promoter to coexpress nerve growth factor in the islets, which greatly increased the density of sympathetic innervation and facilitated analysis of the effects of LIF. Our data demonstrate that tyrosine hydroxylase and catecholamines declined and choline acetyltransferase increased in response to LIF. We conclude that LIF can induce neurotransmitter switching of sympathetic neurons in vivo.

Target determination of neurotransmitter phenotype in sympathetic neurons

While the majority of sympathetic neurons are noradrenergic, a minority population are cholinergic. At least one population of cholinergic sympathetic neurons arises during development by a target-dependent conversion from an initial noradrenergic phenotype. Evidence for retrograde specification has been obtained from transplantation studies in which sympathetic neurons that normally express a noradrenergic phenotype throughout life were induced to innervate sweat glands, a target normally innervated by cholinergic sympathetic neurons. This was accomplished by transplanting footpad skin containing sweat gland primordia from early postnatal donor rats to the hairy skin region of host rats. The sympathetic neurons innervating the novel target decreased their expression of noradrenergic traits and developed choline acetyltransferase (ChAT) activity. In addition, many sweat gland-associated fibers acquired acetylcholinesterase (AChE) staining and VIP immunoreactivity. These studies indicate that sympathetic neurons in vivo alter their neurotransmitter phenotype in response to novel environmental signals and that sweat glands play a critical role in the cholinergic and peptidergic differentiation of the sympathetic neurons that innervate them. The sweat gland-derived cholinergic differentiation factor is distinct from leukemia inhibitory factor and ciliary neurotrophic factor, two well-characterized cytokines that alter the neurotransmitter properties of cultured sympathetic neurons in a similar fashion. Recent studies indicate that anterograde signalling is also important for the establishment of functional synapses in this system. We have found that the production of cholinergic differentiation activity by sweat glands requires sympathetic innervation, and the acquisition and maintenance of secretory competence by sweat glands depends upon functional cholinergic innervation.

Reduced cortical inhibitory synaptogenesis in organotypic cerebellar cultures developing in the absence of neuronal activity

Organotypic cerebellar cultures derived from newborn mice were continuously exposed to medium containing tetrodotoxin and elevated levels of magnesium to block all electrical activity. After 2 weeks in vitro, no activity was evident during the first 15-20 minutes following transfer to a recording medium without blocking agents. Thereafter, cortical discharge rates increased until a state of sustained hyperactivity was reached. Ultrastructural examination of such cultures revealed a reduction of inhibitory Purkinje cell somatic synapses to half the control value along with an even greater reduction of axodendritic synapses (largely inhibitory) in the cortical neuropil. No loss of axospinous synapses (excitatory) was evident. These results support the concept that spontaneous neuronal activity is necessary for the full development of inhibitory circuitry.

Regulation of A-currents by cell-cell interactions and neurotrophic factors in developing chick parasympathetic neurones

1. The developmental regulation of ion channel expression was studied in parasympathetic neurones isolated from the chick ciliary ganglion. Whole-cell patch clamp recordings were made from ciliary ganglion neurones that were removed from the embryo on the ninth embryonic day (E9) and maintained in dissociated cell culture for an additional 4 days. Previous studies have shown that the expression of a transient voltage-activated K+ current (IA) is regulated by unidentified environmental stimuli during these developmental stages. 2. The effect of interactions between neurones and target tissue on the expression of IA was tested by co-culturing ciliary ganglion neurones with chick striated muscle cells. Neurones from the nerve-muscle co-cultures expressed normal amplitudes of IA, but the neurones did not express normal levels of IA when they were plated onto lysed muscle fibres. 3. The effect of interactions between ganglionic neurones and non-neuronal ganglionic cells was tested by culturing ganglia as explants rather than as dissociated cells. Neurones isolated from the explant cultures did not express normal levels of IA. Similarly, when dissociated ganglionic neurones were co-cultured with fibroblasts isolated from embryonic chick skin, they did not express normal amplitudes of IA. 4. Chronic depolarization caused by growing ciliary ganglion neurones in the presence of elevated K+ concentrations did not allow for the normal expression of IA, although it did promote the survival of these neurones in vitro. 5. Addition of 40 ng ml-1 of recombinant human ciliary neurotrophic factor (CNTF) or basic fibroblast growth factor (bFGF) to the cell culture medium had no effect on IA expression in developing chick ciliary ganglion neurones. However, 40 ng ml-1 of acidic fibroblast growth factor (aFGF) stimulated the expression of IA. All trophic factors promoted the growth and survival of ciliary ganglion neurones in vitro. 6. Dissociated ciliary ganglion neurones were maintained in a culture medium containing an aqueous extract of the whole brain. Neurones developing under these conditions expressed normal levels of IA. The stimulatory activity of the brain extract was destroyed by heating. 7. The expression of IA in chick ciliary ganglion neurones developing in vitro can be regulated by soluble growth factors and by interactions with certain other cell types. Similar interactions may regulate the expression of IA in ciliary ganglion neurones developing in situ.

Synapse elimination from the mouse neuromuscular junction in vitro: a non-Hebbian activity-dependent process

The effect of action potentials on elimination of mouse neuromuscular junctions (NMJ) was studied in a three-compartment cell culture preparation. Axons from superior cervical ganglion or ventral spinal cord neurons in two lateral compartments formed multiple neuromuscular junctions with muscle cells in a central compartment. The loss of synapses over a 2-7-day period was determined by serial electrophysiological recording and a functional assay. Electrical stimulation of axons from one side compartment during this period, using 30-Hz bursts of 2-s duration, repeated at 10-s intervals, caused a significant increase in synapse elimination compared to unstimulated cultures (p < 0.001). The extent of homosynaptic and heterosynaptic elimination was comparable, i.e., of the 226 functional synapses of each type studied, 111 (49%) of the synapses that had been stimulated were eliminated, and 87 (39%) of unstimulated synapses on the same muscle cells were eliminated. Also, simultaneous bilateral stimulation caused significantly greater elimination of synapses than unilateral stimulation (p < 0.005). These observations are contrary to the Hebbian hypothesis of synaptic plasticity. A spatial effect of stimulus-induced synapse elimination was also evident following simultaneous bilateral stimulation. Prior to stimulation, most muscle cells were innervated by axons from both side compartments, but after bilateral stimulation, muscle cells were predominantly unilaterally innervated by axons from the closer compartment. These experiments suggest that synapse elimination at the NMJ is an activity-dependent process, but it does not follow Hebbian or anti-Hebbian rules of synaptic plasticity. Rather, elimination is a consequence of postsynaptic activation and a function of location of the muscle cell relative to the neuron. An interaction between spatial and activity-dependent effects on synapse elimination could help produce optimal refinement of synaptic connections during postnatal development.

Accommodation of mouse DRG growth cones to electrically induced collapse: kinetic analysis of calcium transients and set-point theory

Electrical stimulation causes growth cones of mouse dorsal root ganglion neurons to collapse. During chronic stimulation, however, growth cones resume motility. In addition, these growth cones are now resistant to the collapsing effects of subsequent stimulation, a process we term accommodation. We compared the kinetics of electrically induced Ca2+ transients in naive and accommodated growth cones in order to determine whether the accommodation process results from a change in the Ca2+ transient, or a change in the Ca2+ sensitivity of the growth cones. Three kinetics were determined: (1) the initial increase to peak Ca2+ levels produced by 10 Hz stimulation; (2) recovery from peak Ca2+ levels during stimulus trains lasting 15 min; and (3) clearing of Ca2+ from growth cones after terminating the stimulus. These kinetics were analyzed using single exponential fits to changes in fura-2 fluorescence ratios. The electrically evoked increase in Ca2+ was significantly slower in accommodated growth cones (tau = 6.0 s) compared to naive growth cones (tau = 1.4 s). Despite the slower increase of [Ca2+]i in accommodated growth cones, peak [Ca2+]i was similar to that reached in naive growth cones, and the steady-state Ca2+ level was significantly elevated after chronic stimulation. Thus, accommodated growth cones maintained outgrowth at [Ca2+]i that caused collapse initially. Time course experiments show that accommodation is a slow process (t 1/2 = about 3 h). Accommodation did not induce measurable changes in the rates of Ca2+ homeostasis during or after stimulus trains. The kinetics of Ca2+ recovery during (tau = 90 s) and after 15 min of stimulation (tau = 8.5 s) was not significantly different in accommodated versus naive growth cones. Rates of 45Ca2+ efflux were also similar in both types of growth cones. These results suggest two regulatory processes contributing to growth cone motility during chronic stimulation: (1) recovery of [Ca2+]i to levels permissive to neurite outgrowth, and (2) an increase in the range of optimal [Ca2+]i for growth cone motility. These adaptive responses of mammalian growth cones to chronic stimulation could be involved in the modulation of CNS development by electrical activity of neurons.

Selective modulation of cholinergic properties in cultures of avian embryonic sympathetic ganglia

We have studied the expression of catecholaminergic and cholinergic phenotypes in sympathetic ganglia removed from 7- to 10-day-old quail embryos and grown in vitro under different conditions. Quantitative data were obtained by measuring the conversion of (3H) tyrosine and (3H) choline to catecholamines (CA) and acetylcholine (ACh), respectively. In explant cultures, large amounts of both neurotransmitters were synthesized from the onset, but CA generally predominated, the molar ratios of CA:ACh being, on average, of the order of 2:1. If the ganglia were dissociated before plating, there was a selective increase in ACh synthesis (three- to fivefold) such that the CA:ACh ratio fell strikingly. The early expression of the cholinergic phenotype appears to be species-specific in that, under identical conditions, dissociated cell cultures of newborn mouse superior cervical ganglia were overwhelmingly catecholaminergic (CA:ACh ratio of approximately 40:1) and ACh synthesis was only just detectable. Addition of veratridine (1.5 microM) either to explant or to dissociated cell cultures of embryonic quail sympathetic ganglia barely altered CA-synthesizing ability; in contrast, ACh synthesis and accumulation were stimulated about threefold. This effect, which we found to correspond to a quantitatively similar increase in the activity of choline acetyltransferase (ChAT), was completely blocked by tetrodotoxin, indicating that it was due to Na(+)-dependent depolarization. A preferential stimulation of ACh production was also observed when the concentration of K+ was raised to 20 mM. Veratridine treatment of cultures of presumptive sympathoblasts, in the form of sclerotome-associated neural crest cells, had identical effects. Our results reveal the quantitative importance of ACh-related properties in avian sympathetic ganglia from the earliest stages of their development and suggest that depolarization may be one of the factors selectively enhancing expression of the cholinergic phenotype during ontogeny. In these respects, the neurochemical differentiation of sympathetic neurons unfolds according to dissimilar scenarios in birds and mammals.

Cell interactions that determine sympathetic neuron transmitter phenotype and the neurokines that mediate them

The transmitter properties of both developing and mature sympathetic neurons are plastic and can be modulated by a number of environmental cues. Cell culture studies demonstrate that noradrenergic neurons can be induced to become cholinergic and that the expression of neuropeptides can be altered. Similar changes in transmitter phenotype occur in vivo. During development, noradrenergic neurons that innervate eccrine sweat glands acquire cholinergic and peptidergic function. This change is dependent upon interactions with the target tissue. Following injury of sympathetic neurons in developing and adult animals, striking alterations take place in peptide expression. Ciliary neurotrophic factor and cholinergic differentiation factor/leukemia inhibitory factor, members of a family that includes several hematopoietic cytokines, induce cholinergic function and modulate neuropeptide expression in cultured sympathetic neurons. Studies in progress provide evidence that members of this new cytokine family influence the transmitter phenotype of sympathetic neurons not only in vitro but also in vivo.

The cholinergic stimulating effects of ciliary neurotrophic factor and leukemia inhibitory factor are mediated by protein kinase C

The intracellular mechanisms through which two trophic factors, ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF), regulate cholinergic development were examined in sympathetic neuron cultures. Treatment with CNTF or LIF increased levels of choline acetyltransferase (ChAT) activity by 375 and 350%, respectively. However, in neuronal cultures depleted of protein kinase C (PKC) activity by chronic phorbol ester treatment, neither CNTF nor LIF elevated ChAT activity. Further, the stimulation of ChAT due to increased cell density was not observed in PKC-depleted sympathetic neurons. The inhibition of CNTF-stimulated ChAT by phorbol ester occurred in a dose-dependent manner and chronic phorbol ester treatments did not alter the levels of the catecholamine biosynthetic enzyme tyrosine hydroxylase. Moreover, increased levels of diacylglycerol, an endogenous activator of PKC, were observed in sympathetic neurons treated with CNTF. However, neither CNTF nor LIF stimulated the hydrolysis of phosphatidylinositol 4,5-bisphosphate. These observations suggest that a common PKC-dependent pathway, which is independent of phosphatidylinositol 4,5-bisphosphate hydrolysis, mediates the cholinergic stimulating effects of CNTF, LIF, and cell-cell contact in cultured sympathetic neurons.

Phenotypic plasticity of avian embryonic sympathetic neurons grown in a chemically defined medium: direct evidence for noradrenergic and cholinergic properties in the same neurons

Avian embryonic sympathetic ganglia possess both catecholaminergic and cholinergic features and can synthesize noradrenaline (NAd) and acetylcholine (ACh) simultaneously. In the present study we sought to determine (1) whether or not this coproduction of NAd and ACh corresponds to the existence of two non-overlapping populations, and (2) to what extent the levels of synthesis are influenced by non-neuronal ganglion cells. We have focused on the correlation between the immunocytochemically demonstrable presence of the noradrenergic and cholinergic enzymes tyrosine hydroxylase (TH) and choline acetyltransferase (ChAT), respectively, and the synthesis of the corresponding neurotransmitters in embryonic quail sympathetic neuronal and non-neuronal cells purified by fluorescence-activated cell sorting. We show that (1) freshly sorted neurons synthesize both NAd and ACh, whereas non-neuronal cells produce neither; (2) the overwhelming majority of the sympathetic neurons display TH immunoreactivity; (3) about half of the TH-positive neurons are recognized by an anti-ChAT antibody in an artificial medium that selectively enhances synthesis and/or accumulation of ACh; (4) the non-neuronal cells are important for survival of the neurons and potentiate their synthesis of ACh in this medium, and (5) finally, we present evidence that expression of TH in noradrenergic neurons and in small intensely fluorescent cells of sympathetic ganglia is differentially regulated.

Segment-specific modifications of a neuropeptide phenotype in embryonic neurons of the moth, Manduca sexta

We have studied differences in the development of segmentally homologous neurons to identify factors that may regulate a neuropeptide phenotype. Bilaterally paired homologs of the peripheral neuron L1 were identified in the thoracic and abdominal segments in embryos of the moth Manduca: each bipolar neuron arises at a stereotyped location and, at 40% of embryogenesis, projects its major process within the transverse nerve of its own segment. Shortly after the initiation of axonogenesis (approximately 41%), L1 homologs in all but the prothoracic segment (T1) were labelled specifically by an antiserum to the molluscan neuropeptide Phe-Met-Arg-Phe-NH2 (authentic FMRFamide). Levels of peptide-immunoreactivity (IR) were comparable in all such segmental homologs up to the approximately 60% stage of embryogenesis, whereupon two distinct levels of peptide IR were displayed: homologs in the three most rostral segments (T2, T3, and A1; [abdominal segment 1]) showed high levels and were called Type I L1 neurons; homologs in the more caudal segments (A2-A8) typically showed low levels of IR and were called Type II L1 neurons. This segment-specific difference represented mature differentiated states and was retained in postembryonic stages. Intracellular dye fills of embryonic L1 neurons revealed that the morphogenesis of the Type I and II L1 neuron homologs was similar until approximately 48% of embryogenesis; thereafter it differed in two salient ways: (1) the cell bodies of Type II L1 neurons migrated approximately 150 microns laterally from their point of origin, and (2) the distal processes of the Type II L1 neurons contacted the heart, whereas those of Type I L1 neurons did not. Ultrastructural studies of both mature and developing L1 homologs showed that the FMRFamide-like antigen(s) localized specifically to secretory granules. Further, whereas the secretory granules in segmental homologs appeared similar initially (i.e., at approximately 50% of development), following the establishment of segment-specific differences, secretory granules found in mature Type I and II L1 neurons were cell type-specific.

Galanin mRNA in the nucleus basalis of Meynert complex of baboons and humans

Galanin, a 29-amino acid peptide, has been shown by immunocytochemistry to occur in most large acetylcholinergic neurons of the complex that includes the nucleus basalis of Meynert and the nucleus of the diagonal band of Broca in nonhuman primates. In contrast, several studies have reported that most large neurons of the human nucleus basalis of Meynert complex appear to lack galanin immunoreactivity. We investigated this apparent species-difference by hybridization histochemistry for galanin messenger ribonucleic acid (mRNA) in humans and baboons. The results confirm previous immunocytochemical data; very few large neurons of the nucleus basalis of Meynert complex in humans contained detectable galanin messenger RNA, whereas most such cells in baboons were labeled by the oligodeoxynucleotide probe. The few labeled neurons in humans were primarily medial or ventral to the main body of the nucleus basalis of Meynert and corresponded in location to a minor population of relatively intensely labeled cells in baboons. These findings indicate that the indetectability of immunoreactive galanin in most cells of the nucleus basalis of Meynert complex in humans is due to a paucity or an absence of galanin messenger RNA and not to differences in posttranslational processing or transport of the peptide. Inasmuch as the probe labeled neurons in several other nuclei of both species, it is unlikely that differences in galanin messenger RNA sequences underlie the species-related disparity in hybridization in the nucleus basalis of Meynert complex. The indetectability of galanin messenger RNA in most cells of the human nucleus basalis of Meynert complex indicates that the expression of the galanin gene is regulated by as yet unidentified influences that differ in human and nonhuman primates. The varying phenotypes of galanin in primates suggest potentially important species-differences in the function of galanin in neurons of the nucleus basalis of Meynert complex.

Depolarization regulates expression of a synaptic vesicle protein in rat superior cervical ganglia in vitro

The role of membrane depolarization in the regulation of expression of a neuron specific protein was evaluated by culturing superior cervical ganglia from neonatal rats in defined medium and manipulating neuronal activity by depolarizing agents. P65 is an integral membrane protein of synaptic vesicles and can be used as a marker for general neuronal maturation. P65 antigen levels were quantified by indirect radioimmunoassay, using monoclonal antibodies. The expression of p65 in ganglion explants increased by 40-100% when the cultures were treated with the depolarizing agents, veratridine or high potassium. The veratridine effect could be blocked by simultaneous treatment with the sodium channel blocker, tetrodotoxin (TTX). The rise in p65 was not evident until 36 h after depolarizing treatment had begun and reached peak levels after 48 h, with no further increases observed with sustained treatment. After removal of the depolarizing treatment, p65 levels returned to control values after 24 h. P65 joins a growing number of molecules whose expression is regulated by membrane depolarization.

Eliminating afferent impulse activity does not alter the dendritic branching of the amphibian Mauthner cell

In the developing amphibian, the formation of extra vestibular contacts on the Mauthner cell (M-cell) enhances dendritic branching, while deprivation reduces it (Goodman and Model, 1988a). The mechanism underlying the interaction between afferent fibers and developing dendritic branches is not known; neural activity may be an essential component of the stimulating effect. We examined the role of afferent impulse activity in the regulation of M-cell dendritic branching in the axolotl (Ambystoma mexicanum) embryo. M-cells occur as a pair of large, uniquely identifiable neurons in the axolotl medulla. Synapses from the ipsilateral vestibular nerve (nVIII) are restricted to a highly branched region of the M-cell lateral dendrite. We varied the amount of nVIII innervation and eliminated neural activity. First, unilateral transplantation of a vestibular primordium deprived some M-cells of nVIII innervation and superinnervated others. Second, surgical fusion of axolotls to TTX-harboring California newt (Taricha torosa) embryos paralyzed the Ambystoma twin: voltage-sensitive Na+ channel blockade by TTX eliminated action potential propagation. Reconstruction of M-cells in 18 mm larvae revealed that dendritic growth was influenced by in-growing axons even in the absence of incoming impulses: impulse blockade had no effect on the stimulation of dendritic growth by the afferent fibers.

Vasoactive intestinal peptide regulates mitosis, differentiation and survival of cultured sympathetic neuroblasts

Although acute, millisecond-to-millisecond actions of neurotransmitters are well documented, diverse longer-term effects have been discovered only recently. Emerging evidence indicates that these signals regulate a variety of neuronal processes, from phenotypic expression to neurite outgrowth. Here we show that a single putative transmitter, vasoactive intestinal peptide, can exert multiple, long-term effects simultaneously: it stimulates mitosis, promotes neurite outgrowth and enhances survival of sympathetic neuron precursors in culture. As the peptide seems to be a normal presynaptic transmitter in the sympathetic system, synaptic transmission may exert hitherto unexpected effects.

Regulation of discrete sub-populations of transmitter-identified neurones after inhibition of electrical activity in cultures of mouse spinal cord

The effects of blockade of electrical activity by tetrodotoxin in cultures of mouse spinal cord and dorsal root ganglion on immunohistochemically-identified neuronal sub-populations have been investigated. Some spinal cord neuronal types, such as those storing methionine-enkephalin, substance P or calcitonin gene-related peptide were almost totally depleted after inhibition of electrical activity for 4 days. By contrast, putative substance P- and calcitonin gene-related peptide-immunoreactive dorsal root ganglion neurones were not significantly affected by such treatment. Several other neuronal types were reduced by about 30-40% after exposure to tetrodotoxin. The decrement in methionine-enkephalin-, substance P- and calcitonin gene-related peptide-immunoreactive neurones caused by tetrodotoxin was reversible, and, in the case of methionine-enkephalin, could not be elicited after day 30 in culture. Radioimmunoassay of levels of methionine-enkephalin in cultures confirmed the immunohistochemical data. It is concluded, therefore, that exposure to tetrodotoxin selectively reduces peptide immunoreactivity in specific neuronal sub-populations, but that the selectivity is not based on a single known neuronal characteristic such as transmitter phenotype, or a particular structural protein. The action of tetrodotoxin on those cells most severely attenuated is an alteration in transmitter expression rather than a lethal effect. The diminution with time of the ability of tetrodotoxin to attenuate methionine-enkephalin levels may reflect a reduction in the activity-dependent regulation of peptide expression relative to other competing trophic influences.

Neuronal membrane depolarization and the control of cholinergic muscarinic receptors: selective effect on different neuronal cell types

1. The possibility that a long-lasting neuronal activation regulates the expression of muscarinic cholinergic receptors was studied with three cultured neuronal cell lines. 2. Continuous depolarization of a subclone of the neuroblastoma-glioma NG108-15 hybrid cells with potassium chloride increased by 45-75% the number of cholinergic muscarinic receptors, monitored with 3H-QNB, whereas a short incubation with KCl for 10 min or 6 hr had no effect. 3. The calcium channel blocker verapamil increased the effect of KCl. 4. Two cell lines, named SC9 and WC5, that originate from the rat brain, also bind 3H-QNB. They were therefore used to test whether the effect of chronic depolarization is universal. Depolarized SC9 and WC5 cells, in the presence or absence of verapamil, did not show an increased 3H-QNB binding. 5. Muscarinic receptors of both SC9 and WC5 cells have a higher affinity to pirenzepine than the M-3 receptor subtype of the neuroblastoma-glioma cells, suggesting therefore that the two rat brain cell lines possess M-1 or M-2 receptors. 6. The physiological significance of this differential role of depolarization on the expression of different muscarinic receptors is discussed in the context of their postreceptor second messengers.

Neuronal aggregation and neurotransmitter regulation: partial purification and characterization of a membrane-derived factor

Cell membrane contact induces marked differential changes in neurotransmitter expression. In cultures of virtually pure dissociated sympathetic neurons, when such contact is provided by either high cell densities or addition of membranes derived from specific tissues, there is a marked increase in cell-specific content of substance P and de novo induction of choline acetyltransferase. To identify molecular mechanisms underlying regulation of transmitter expression by neuronal aggregation and membrane contact, we have begun to isolate and characterize a membrane-associated factor responsible for stimulation of choline acetyltransferase activity. The factor was found in substantial quantities in membranes from adult rat spinal cord as well as from sympathetic and sensory ganglia. Ionic mechanisms were employed to extract transmitter-inducing activity from spinal cord membranes in soluble form. The solubilized factor was then partially purified by ion exchange and gel filtration chromatography. It appears to be an extrinsic (non-integral) protein with an apparent molecular weight of 27. It is inactivated by trypsin and chymotrypsin, but is only moderately sensitive to heat inactivation, retaining activity at 60 degrees C but not at 90 degrees C. Neuronal perikaryal contact via aggregation represents a critical mechanism by which neurons themselves may influence phenotypic expression. Membrane localization of the factor provides a means by which cell contact may regulate transmitter expression.

Coordinate hormonal and synaptic regulation of vasopressin messenger RNA

Previous studies have shown that adrenalectomy augments arginine vasopressin (AVP) messenger RNA levels in the adult paraventricular nucleus. It is now demonstrated that unilateral lesions in the lateral septal nucleus enhance the adrenalectomy-induced expression of AVP mRNA. This effect was entirely ipsilateral to the lesion and most prominent in the rostral paraventricular nucleus and related nuclei. Moreover, AVP and AVP mRNA were found to be colocalized with oxytocin in a few neurons. These results indicate that mRNA expression is modulated by synaptic influences and raise the possibility that synaptically mediated selection of neuronal phenotypes is a dynamic feature of the mature central nervous system.

Monoclonal antibodies to and immunoaffinity purification of choline acetyltransferase from bovine brain

Three hybridomas producing monoclonal antibodies to bovine brain choline acetyltransferase (ChAT) have been established by fusion of the spleen cells from a mouse immunized with purified enzyme with myeloma NS-1 cells. All three clones produced IgGl antibodies that reacted with enzyme protein denatured with sodium dodecyl sulfate. By using one of the monoclonal antibodies, a rapid and efficient immunoaffinity purification procedure of bovine ChAT has been established. Immunoblot analysis and immunoaffinity purification indicated that bovine ChAT is a single 68-kilodalton protein. The monoclonal antibodies will offer us a good tool to isolate the cDNA clones encoding ChAT.

The cAMP cascade in the nervous system: molecular sites of action and possible relevance to neuronal plasticity

Many intercellular messages regulate the activity of their target cells by altering the intracellular level of cAMP and, as a consequence, the phosphorylation state of proteins which serve as substrates for cAMP-dependent protein kinase. Such regulation plays a crucial role in neuronal development, neuronal function, and neuronal plasticity (e.g., elementary learning mechanisms). Ample information has been accumulated in recent years on the enzymes that regulate the level of cAMP or respond to it, on the regulation of cAMP synthesis by neurohormones, neurotransmitters, ions, and toxins, on neuronal-specific substrate proteins that are phosphorylated by the cAMP-dependent kinase, and on the interaction of the cAMP-cascade with other second-messenger systems within neurons. Such data, obtained by a combination of molecular-biological, biochemical, and cellular approaches, shed light on the detailed mechanisms by which modulation of a ubiquitous molecular cascade leads to a great variety of short-term as well as long-term specific neuronal responses and alterations.

Effects of pineal factors on the action potentials of sympathetic neurons

Neurons from rat superior cervical ganglia were grown in coculture with pineal cells. Action potentials of neurons in cocultures were 25% longer and were 25% greater in amplitude than those recorded from neurons grown in the presence of ganglionic nonneuronal cells alone. Neurons showed an increase in action potential duration with increasing time in culture. This may have been related to the recovery of nonneuronal cell populations after an initial exposure to the antimitotic agent Ara-C. In cultures not initially exposed to Ara-C, a subsequent exposure after 7 days in culture resulted in a shortening of the action-potential duration. Neuronal cultures were exposed to gel slabs containing the pineal indolamines, serotonin, N-acetylserotonin, and melatonin. Serotonin and N-acetylserotonin showed no effect on the neuronal action potentials at the concentrations tested. Melatonin caused an increase in action-potential duration that was associated not with an increase in action-potential amplitude, but with a decrease in action-potential rise rates. The effects of long-term exposure in melatonin appeared to be reversible in some cells but not in all. Short-term effects of melatonin were observed in older cultures and in younger cultures after the cells were stimulated repeatedly. Older cultures also had higher levels of spontaneous activity. The dependence of the short-term effects of melatonin on electrical activity may suggest a role for melatonin as a neuromodulator.

Suppression of neurite elongation and growth cone motility by electrical activity

Electrical activity may regulate a number of neuronal functions in addition to its role in transmitting signals along nerve cells. The hypothesis that electrical activity affects neurite elongation in sprouting neurons was tested by stimulating individual snail neurons isolated in cell culture. The findings demonstrated that growth cone advance, and thus neurite elongation, is reversibly stopped during periods when action potentials are experimentally evoked. A decrease in filopodial number and growth cone area was also observed. Thus, action potentials can mediate the cessation of neurite outgrowth and thereby may influence structure and connectivity within the nervous system.