BK-Type K(Ca) channels in two parasympathetic cell types: differences in kinetic properties and developmental expression

Structural mapping of patient-associated KCNMA1 gene variants

KCNMA1-linked channelopathy is a neurological disorder characterized by seizures, motor abnormalities, and neurodevelopmental disabilities. The disease mechanisms are predicted to result from alterations in KCNMA1-encoded BK K+ channel activity; however, only a subset of the patient-associated variants have been functionally studied. The localization of these variants within the tertiary structure or evaluation by pathogenicity algorithms has not been systematically assessed. In this study, 82 nonsynonymous patient-associated KCNMA1 variants were mapped within the BK channel protein. Fifty-three variants localized within cryoelectron microscopy-resolved structures, including 21 classified as either gain of function (GOF) or loss of function (LOF) in BK channel activity. Clusters of LOF variants were identified in the pore, the AC region (RCK1), and near the Ca2+ bowl (RCK2), overlapping with sites of pharmacological or endogenous modulation. However, no clustering was found for GOF variants. To further understand variants of uncertain significance (VUSs), assessments by multiple standard pathogenicity algorithms were compared, and new thresholds for sensitivity and specificity were established from confirmed GOF and LOF variants. An ensemble algorithm was constructed (KCNMA1 meta score (KMS)), consisting of a weighted summation of this trained dataset combined with a structural component derived from the Ca2+-bound and unbound BK channels. KMS assessment differed from the highest-performing individual algorithm (REVEL) at 10 VUS residues, and a subset were studied further by electrophysiology in HEK293 cells. M578T, E656A, and D965V (KMS+;REVEL-) were confirmed to alter BK channel properties in voltage-clamp recordings, and D800Y (KMS-;REVEL+) was assessed as benign under the test conditions. However, KMS failed to accurately assess K457E. These combined results reveal the distribution of potentially disease-causing KCNMA1 variants within BK channel functional domains and pathogenicity evaluation for VUSs, suggesting strategies for improving channel-level predictions in future studies by building on ensemble algorithms such as KMS.

Structural mapping of patient-associated KCNMA1 gene variants

KCNMA1-linked channelopathy is a neurological disorder characterized by seizures, motor abnormalities, and neurodevelopmental disabilities. The disease mechanisms are predicted to result from alterations in KCNMA1-encoded BK K+ channel activity; however, only a subset of the patient-associated variants have been functionally studied. The localization of these variants within the tertiary structure or evaluation by pathogenicity algorithms has not been systematically assessed. In this study, 82 nonsynonymous patient-associated KCNMA1 variants were mapped within the BK channel protein. Fifty-three variants localized within cryo-EM resolved structures, including 21 classified as either gain-of-function (GOF) or loss-of-function (LOF) in BK channel activity. Clusters of LOF variants were identified in the pore, the AC region (RCK1), and near the Ca 2+ bowl (RCK2), overlapping with sites of pharmacological or endogenous modulation. However, no clustering was found for GOF variants. To further understand variants of uncertain significance (VUS), assessments by multiple standard pathogenicity algorithms were compared, and new thresholds for sensitivity and specificity were established from confirmed GOF and LOF variants. An ensemble algorithm was constructed (KCNMA1 Meta Score), consisting of a weighted summation of this trained dataset combined with a structural component derived from the Ca 2+ bound and unbound BK channels. KMS assessment differed from the highest performing individual algorithm (REVEL) at 10 VUS residues, and a subset were studied further by electrophysiology in HEK293 cells. M578T, E656A, and D965V (KMS+;REVEL-) were confirmed to alter BK channel properties in voltage-clamp recordings, and D800Y (KMS-;REVEL+) was assessed as benign under the test conditions. However, KMS failed to accurately assess K457E. These combined results reveal the distribution of potentially disease-causing KCNMA1 variants within BK channel functional domains and pathogenicity evaluation for VUS, suggesting strategies for improving channel-level predictions in future studies by building on ensemble algorithms such as KMS.

Neurotrophins and target interactions in the development and regulation of sympathetic neuron electrical and synaptic properties

The electrical and synaptic properties of neurons are essential for determining the function of the nervous system. Thus, understanding the mechanisms that control the appropriate developmental acquisition and maintenance of these properties is a critical problem in neuroscience. A great deal of our understanding of these developmental mechanisms comes from studies of soluble growth factor signaling between cells in the peripheral nervous system. The sympathetic nervous system has provided a model for studying the role of these factors both in early development and in the establishment of mature properties. In particular, neurotrophins produced by the targets of sympathetic innervation regulate the synaptic and electrophysiological properties of postnatal sympathetic neurons. In this review we examine the role of neurotrophin signaling in the regulation of synaptic strength, neurotransmitter phenotype, voltage-gated currents and repetitive firing properties of sympathetic neurons. Together, these properties determine the level of sympathetic drive to target organs such as the heart. Changes in this sympathetic drive, which may be linked to dysfunctions in neurotrophin signaling, are associated with devastating diseases such as high blood pressure, arrhythmias and heart attack. Neurotrophins appear to play similar roles in modulating the synaptic and electrical properties of other peripheral and central neuronal systems, suggesting that information provided from studies in the sympathetic nervous system will be widely applicable for understanding the neurotrophic regulation of neuronal function in other systems.

Nephrin binds to the COOH terminus of a large-conductance Ca2+-activated K+ channel isoform and regulates its expression on the cell surface

We carried out a yeast two-hybrid screen to identify proteins that interact with large-conductance Ca2+-activated K+ (BKCa) channels encoded by the Slo1 gene. Nephrin, an essential adhesion and scaffolding molecule expressed in podocytes, emerged in this screen. The Slo1-nephrin interaction was confirmed by coimmunoprecipitation from the brain and kidney, from HEK-293T cells expressing both proteins, and by glutathione S-transferase pull-down assays. We detected nephrin binding to the Slo1 VEDEC splice variant, which is typically retained in intracellular stores, and to the beta4-subunit. However, we did not detect significant binding of nephrin to the Slo1 QEERL or Slo1 EMVYR splice variants. Coexpression of nephrin with Slo1 VEDEC increased expression of functional BKCa channels on the surface of HEK-293T cells but did not affect steady-state surface expression of the other COOH-terminal Slo1 variants. Nephrin did not affect the kinetics or voltage dependence of channel activation in HEK-293T cells expressing Slo1. Stimulation of Slo1 VEDEC surface expression in HEK-293T cells was also observed by coexpressing a small construct encoding only the distal COOH-terminal domains of nephrin that interact with Slo1. Reduction of endogenous nephrin expression by application of small interfering RNA to differentiated cells of an immortalized podocyte cell line markedly reduced the steady-state surface expression of Slo1 as assessed by electrophysiology and cell-surface biotinylation assays. Nephrin therefore plays a role in organizing the surface expression of ion channel proteins in podocytes and may play a role in outside-in signaling to allow podocytes to adapt to mechanical or neurohumoral stimuli originating in neighboring cells.

Beta1-subunits increase surface expression of a large-conductance Ca2+-activated K+ channel isoform

Auxiliary (beta) subunits of large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels regulate the gating properties of the functional channel complex. Here we show that an avian beta1-subunit also stimulates the trafficking of BK(Ca) channels to the plasma membrane in HEK293T cells and in a native population of developing vertebrate neurons. One C-terminal variant of BK(Ca) alpha-subunits, called the VEDEC isoform after its five last residues, is largely retained in intracellular compartments when it is heterologously expressed in HEK293T cells. A closely related splice variant, called QEERL, shows high levels of constitutive trafficking to the plasma membrane. Co-expression of beta1-subunits with the VEDEC isoform resulted in a large increase in surface BK(Ca) channels as assessed by cell-surface biotinylation assays, whole cell recordings of membrane current, and confocal microscopy in HEK293T cells. Co-expression of beta1-subunits slowed the gating kinetics of BK(Ca) channels, as reported previously. Consistent with this, overexpression of beta1-subunits in a native cell type that expresses intracellular VEDEC channels, embryonic day 9 chick ciliary ganglion neurons, resulted in a significant increase in macroscopic Ca(2+)-activated K(+) current. Both the cytoplasmic N- and C-terminal domains of avian beta1 are able to bind directly to VEDEC and QEERL channels. However, overexpression of the N-terminal domain by itself is sufficient to stimulate trafficking of VEDEC channels to the plasma membrane, whereas overexpression of either the cytoplasmic C-terminal domain or the extracellular loop domain did not affect surface expression of VEDEC.

Frequency encoding of cholinergic- and purinergic-mediated signaling to mouse urinary bladder smooth muscle: modulation by BK channels

In the urinary bladder, contractions of the detrusor muscle and urine voiding are induced by the neurotransmitters ACh and ATP, released from parasympathetic nerves. Activation of K(+) channels, in particular the large-conductance Ca(2+)-activated K(+) (BK) channels, opposes increases in excitability and contractility of urinary bladder smooth muscle (UBSM). We have shown that deleting the gene mSlo1 in mice (Slo(-/-)), encoding the BK channel, leads to enhanced nerve-mediated and neurotransmitter-dependent contractility of UBSM (38). Here, we examine the location of the BK channel in urinary bladder strips from mouse. Immunohistochemical analysis revealed that the channel is expressed in UBSM but not in nerves that innervate the smooth muscle. The relationship between electrical field stimulation and force generation of the cholinergic and purinergic pathways was examined by applying blockers of the respective receptors in UBSM strips from wild-type and from Slo(-/-) (knockout) mice. In wild-type strips, the stimulation frequency required to obtain a half-maximal force was significantly lower for the purinergic (7.2 +/- 0.3 Hz) than the cholinergic pathway (19.1 +/- 1.5 Hz), whereas the maximum force was similar. Blocking BK channels with iberiotoxin or ablation of the Slo gene increased cholinergic- and purinergic-mediated force at low frequencies, i.e., significantly decreased the frequency for a half-maximal force. Our results indicate that the BK channel has a very significant role in reducing both cholinergic- and purinergic-induced contractility and suggest that alterations in BK channel expression or function could contribute to pathologies such as overactive detrusor.

Temporal dynamics of neurite outgrowth promoted by basic fibroblast growth factor in chick ciliary ganglia

Basic fibroblast growth factor (bFGF) is a potent and multifunctional neurotrophic factor that can influence neuronal survival and differentiation. It has been shown to modulate growth and orientation of neuritic processes both in intact organs and in neuronal cultures, with a wide spectrum of effects on different preparations. Here we report that it promotes neurite growth in developing parasympathetic neurons from the chick ciliary ganglion. We have used both organotypic cultures and dissociated neurons, and we have combined assessment of global neurite growth by immunocytochemical techniques with evaluation of dynamic parameters of single neurites via time-lapse microscopy. We show that laminin, a molecule of the extracellular matrix that has been associated with stimulation of neurite extension, has only a limited and short-lived effect on neurite outgrowth. In contrast, bFGF can promote global growth of the neuritic network both in whole ganglia and in dissociated cultures for times up to 48 hr, and this effect is related to an increase in the growth rate of single neurites. Moreover, the effect can be observed even in enriched neuronal cultures, pointing to a direct action of bFGF on neurons.

Increased large conductance calcium-activated potassium (BK) channel expression accompanied by STREX variant downregulation in the developing mouse CNS

Background

Large conductance calcium- and voltage activated potassium (BK) channels are important determinants of neuronal excitability through effects on action potential duration, frequency and synaptic efficacy. The pore- forming subunits are encoded by a single gene, KCNMA1, which undergoes extensive alternative pre mRNA splicing. Different splice variants can confer distinct properties on BK channels. For example, insertion of the 58 amino acid stress-regulated exon (STREX) insert, that is conserved throughout vertebrate evolution, encodes channels with distinct calcium sensitivity and regulation by diverse signalling pathways compared to the insertless (ZERO) variant. Thus, expression of distinct splice variants may allow cells to differentially shape their electrical properties during development. However, whether differential splicing of BK channel variants occurs during development of the mammalian CNS has not been examined.

Results

Using quantitative real-time polymerase chain reaction (RT-PCR) Taqman™ assays, we demonstrate that total BK channel transcripts are up regulated throughout the murine CNS during embryonic and postnatal development with regional variation in transcript levels. This upregulation is associated with a decrease in STREX variant mRNA expression and an upregulation in ZERO variant expression.

Conclusion

As BK channel splice variants encode channels with distinct functional properties the switch in splicing from the STREX phenotype to ZERO phenotype during embryonic and postnatal CNS development may provide a mechanism to allow BK channels to control distinct functions at different times of mammalian brain development.

Developmental Changes in Electrophysiological Properties and Synaptic Transmission in Rat Intracardiac Ganglion Neurons

We charted postnatal changes in the intrinsic electrophysiological properties and synaptic responses of rat intrinsic cardiac ganglion (ICG) neurons. We developed a whole-mount ganglion preparation of the excised right atrial ganglion plexus. Using intracellular recordings and nerve stimulation we tested the hypothesis that substantial transformations in the intrinsic electrical characteristics and synaptic transmission accompany postnatal development. Membrane potential ( Em) did not change but time constant (τ) and cell capacitance increased with postnatal development. Accordingly, input resistance ( Rin) decreased but specific membrane resistance ( Rm) increased postnatally. Comparison of the somatic active membrane properties revealed significant changes in electrical phenotype. All neonatal neurons had somatic action potentials (APs) with small overshoots and small afterhyperpolarizations (AHPs). Adult neurons had somatic APs with large overshoots and large AHP amplitudes. The range of AHP duration was larger in adults than in neonates. The AP characteristics of juvenile neurons resembled those of adults, with the exception of AHP duration, which fell midway between neonate and adult values. Phasic, multiply adapting, and tonic evoked discharge activities were recorded from ICG neurons. Most neurons displayed phasic discharge at each developmental stage. All neurons received excitatory synaptic inputs from the vagus or interganglionic nerve trunk(s), the strength of which did not change significantly with postnatal age. The changes in the electrophysiological properties of the postganglionic neuron suggest that increased complexity of parasympathetic regulation of cardiac function accompanies postnatal development.

The p38 mitogen-activated protein kinase pathway negatively regulates Ca2+-activated K+ channel trafficking in developing parasympathetic neurons

The trafficking of large-conductance Ca2+-activated K+ channels (K(Ca)) in chick ciliary ganglion neurons is regulated by growth factors. Here we show that a canonical p38 cascade inhibits K(Ca) trafficking in ciliary ganglion neurons. Two different p38 inhibitors (SB202190 or SB203580) or over-expression of dominant-negative forms of several components of the p38 cascade increased K(Ca) in ciliary neurons. Inhibition of protein synthesis or Golgi processing had no effect on this phenomenon, suggesting that p38 is acting at a distal step of the trafficking pathway. Depolymerization of filamentous actin (F-actin) increased functional expression of K(Ca), whereas stabilization of F-actin inhibited the effect of SB202190 on K(Ca) trafficking. SB202190 also caused an immunochemically detectable increase in K(Ca) on the plasma membrane. Inhibition of p38 decreased the extent of cortical F-actin in ciliary neurons. Macroscopic K(Ca) is suppressed by transforming growth factor (TGF) beta3. Application of TGFbeta3 increased the phosphorylation of p38 in ciliary neurons and increased cortical F-actin. Thus, the p38 signaling cascade endogenously suppresses development of functional K(Ca), in part by stabilizing an F-actin barrier that prevents plasma membrane insertion of functional channel complexes. This cascade also appears to mediate inhibitory effects of TGFbeta3 on the expression of K(Ca).

Negative feedback regulation of nerve-mediated contractions by KCa channels in mouse urinary bladder smooth muscle

When the urinary bladder is full, activation of parasympathetic nerves causes release of neurotransmitters that induce forceful contraction of the detrusor muscle, leading to urine voiding. The roles of ion channels that regulate contractility of urinary bladder smooth muscle (UBSM) in response to activation of parasympathetic nerves are not well known. The present study was designed to characterize the role of large (BK)- and small-conductance (SK) Ca(2+)-activated K(+) (K(Ca)) channels in regulating UBSM contractility in response to physiological levels of nerve stimulation in UBSM strips from mice. Nerve-evoked contractions were induced by electric field stimulation (0.5-50 Hz) in isolated strips of UBSM. BK and SK channel inhibition substantially increased the amplitude of nerve-evoked contractions up to 2.45 +/- 0.12- and 2.99 +/- 0.25-fold, respectively. When both SK and BK channels were inhibited, the combined response was additive. Inhibition of L-type voltage-dependent Ca(2+) channels (VDCCs) in UBSM inhibited nerve-evoked contractions by 92.3 +/- 2.0%. These results suggest that SK and BK channels are part of two distinct negative feedback pathways that limit UBSM contractility in response to nerve stimulation by modulating the activity of VDCCs. Dysfunctional regulation of UBSM contractility by alterations in BK/SK channel expression or function may underlie pathologies such as overactive bladder.

Urodynamic properties and neurotransmitter dependence of urinary bladder contractility in the BK channel deletion model of overactive bladder

Overactive bladder and incontinence are major medical issues, which lack effective therapy. Previously, we showed (Meredith AL, Thornloe KS, Werner ME, Nelson MT, and Aldrich RW. J Biol Chem 279: 36746-36752, 2004) that the gene mSlo1 encodes large-conductance Ca2+-activated K+ (BK) channels of urinary bladder smooth muscle (UBSM) and that ablation of mSlo1 leads to enhanced myogenic and nerve-mediated contractility and increased urination frequency. Here, we examine the in vivo urodynamic consequences and neurotransmitter dependence in the absence of the BK channel. The sensitivity of contractility to nerve stimulation was greatly enhanced in UBSM strips from Slo-/- mice. The stimulation frequency required to obtain a 50% maximal contraction was 8.3 +/- 0.9 and 19.1 +/- 1.8 Hz in Slo-/- and Slo+/+ mice, respectively. This enhancement is at least partially due to alterations in UBSM excitability, as muscarinic-induced Slo-/- contractility is elevated in the absence of neuronal activity. Muscarinic-induced Slo-/- contractility was mimicked by blocking BK channels with iberiotoxin (IBTX) in Slo+/+ strips, whereas IBTX had no effect on Slo-/- strips. IBTX also enhanced purinergic contractions of Slo+/+ UBSM but was without effect on purinergic contractions of Slo-/- strips. In vivo bladder pressure and urine output measurements (cystometry) were performed on conscious, freely moving mice. Slo-/- mice exhibited increased bladder pressures, pronounced pressure oscillations, and urine dripping. Our results indicate that the BK channel in UBSM has a very significant role in urinary function and dysfunction and as such likely represents an important therapeutic target.

Acute modulation of adrenal chromaffin cell BK channel gating and cell excitability by glucocorticoids

Although adrenal glucocorticoids cortisol and corticosterone (CORT) have numerous "genomic" effects on adrenomedullary chromaffin cells, acute modulatory actions remain largely unknown, despite rapid stress-related changes in secretion. We report that 1 microM glucocorticoids rapidly modulate gating of chromaffin cell BK channels and action potential firing. In general, CORT, or the analog dexamethasone (DEX), increased channel activity in inside-out bovine patches, an effect not blocked by the glucocorticoid receptor (GR) antagonist RU38486. By contrast, these steroids could profoundly inhibit BK activation in many rat patches, while facilitating activation in others. We show that BK inhibition arises from a negative shift in the voltage dependence of BK inactivation paralleling that for activation. We report that rat cells characteristically exhibit greater repetitive firing ability than bovine cells in the absence of glucocorticoids. In both species, steroid application typically increased firing responses to smaller current injections, attributable to BK-enhanced repolarization and Na+ channel deinactivation. However, in rat cells, where BK inactivation is generally faster and more complete, glucocorticoids tended to dampen responses to stronger stimuli. Thus, in the context of natural variation in BK gating, glucocorticoids can either promote or limit firing responses. We suggest that steroids exploit BK gating variety to tailor catecholamine output in a species- and context-specific fashion.

Ras is a mediator of TGFbeta1 signaling in developing chick ciliary ganglion neurons

Large-conductance Ca(2+)-activated K(+) channels (K(Ca)) in chick ciliary ganglion neurons are regulated by target-derived TGFbeta1. Here we show that TGFbeta1 stimulation of K(Ca) expression was blocked by the structurally dissimilar Ras protein farnesyl transferase inhibitors manumycin-A and FTI-277. A similar effect was produced in ciliary neurons overexpressing RasN17, a widely used dominant-negative form of Ras. Moreover, TGFbeta1-evoked increases in phosphorylation of SMAD2 were reduced by manumycin-A, suggesting that Ras-dependent transduction cascades activated by TGFbeta1 feed back onto SMAD signaling. Thus, Ras is a mediator of pleiotropic TGFbeta1 signaling in developing neurons.

Target-mediated control of neural differentiation

The development of the nervous system entails the coordination of the spatial and chemical development of both pre- and postsynaptic elements. This coordination is accomplished by signals passing between neurons and the target cells that they innervate. This review focuses on well-characterized examples of target-mediated neuronal differentiation in the central and peripheral nervous systems. These include control of neurogenesis in the leech by male genitalia, presynaptic differentiation induced by postsynaptic molecules expressed by skeletal muscle, postsynaptic adhesion molecules that induce presynaptic differentiation in the central nervous system (CNS), target-mediated control of neurotransmitter phenotype in peripheral neurons, and target-regulated control of neuronal nicotinic acetylcholine receptors (nAChRs) and large conductance calcium-activated potassium channels (BK). The detailed understanding of these processes will uncover signals critical for the directed differentiation of stem cells as well as identify future targets for therapies in neural regeneration that promote the reestablishment of functional connections.

Glial cell line-derived neurotrophic factor and target-dependent regulation of large-conductance KCa channels in developing chick lumbar motoneurons

The functional expression of large-conductance Ca2+-activated K+ (K(Ca)) channels in lumbar motoneurons (LMNs) of the developing chick embryo is regulated in part by interactions with striated muscle target tissues. Here we show that the functional expression of K(Ca) channels in LMNs developing in vitro can be stimulated by application of a skeletal muscle extract (MEX) or by coculture with hindlimb myotubes. A similar stimulation of K(Ca) channels in vitro can be produced by the trophic factors glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor but not by neurotrophin (NT)-3 or NT-4. The actions of MEX and hindlimb myotubes are blocked by a GDNF-neutralizing antiserum. Moreover, injection of this same antiserum into the embryonic hindlimb reduced the functional expression of K(Ca) channels in vivo to levels seen in LMNs deprived of interactions with the hindlimb. The effects of GDNF on K(Ca) channel expression in LMNs require 24 hr of continuous exposure to reach maximum and are blocked by the translation inhibitor anisomycin, indicating the need for synthesis of new proteins. GDNF actions are also blocked by the farnesyl transferase inhibitor manumycin, suggesting a role for Ras in the actions of GDNF. Finally, the actions of GDNF are inhibited by PP2, an inhibitor of Src family tyrosine kinases, and by LY29003, an inhibitor of phosphatidylinositol 3 kinases, but not by PD98059, an inhibitor of the Erk signaling cascade. None of these treatments alter expression of voltage-activated Ca2+ channels. Thus, the actions of GDNF on LMN K(Ca) channel expression appear to use a transduction pathway similar to that used for regulation of apoptosis.

Activity- and target-dependent regulation of large-conductance Ca2+-activated K+ channels in developing chick lumbar motoneurons

The functional expression of large-conductance (BK-type) Ca2+-activated K+ (K(Ca)) channels was examined in developing chick lumbar motoneurons (LMNs) between embryonic day 6 (E6) and E13 using patch-clamp recording techniques. The macroscopic K(Ca) current of E13 LMNs is inhibited by iberiotoxin and resistant to apamin. The average macroscopic K(Ca) density was low before E8 and increased 3.3-fold by E11, with an additional 1.8-fold increase occurring by E13. BK-type K(Ca) channels could not be detected in inside-out patches from E8 LMNs but were readily detected at E11. The density of voltage-activated Ca2+ currents did not change between E8 and E11. Surgical ablation of target tissues at E5 caused a significant reduction in average K(Ca) density in LMNs measured at E11. Conversely, chronic in ovo administration of d-tubocurarine, which causes an increase in motoneuron branching on the surface of the muscle target tissue, evoked a 1.8-fold increase in average LMN K(Ca) density measured at E11. Electrical activity also contributed to developmental regulation of LMN K(Ca) density. A significant reduction in E11 K(Ca) density was found after chronic in ovo treatment with the neuronal nicotinic antagonist mecamylamine or the GABA receptor agonist muscimol, agents that reduce activation of LMNs in ovo. Moreover, 3 d exposure to depolarizing concentrations of external K+ to LMNs cultured at E8 caused an increase in K(Ca) expression. Conversely, tetrodotoxin caused a decrease in K(Ca) expression in cultured E8 LMNs developing for 3 d in the presence of neurotrophic factors that promote neuronal survival in the absence of target tissues.

Development of electrophysiological and morphological diversity in autonomic neurons

The generation of neuronal diversity requires the coordinated development of differential patterns of ion channel expression along with characteristic differences in dendritic geometry, but the relations between these phenotypic features are not well known. We have used a combination of intracellular recordings, morphological analysis of dye-filled neurons, and stereological analysis of immunohistochemically labeled sections to investigate the development of characteristic electrical and morphological properties of functionally distinct populations of sympathetic neurons that project from the celiac ganglion to the splanchnic vasculature or the gastrointestinal tract of guinea pigs. At early fetal stages, neurons were significantly more depolarized at rest compared with neurons at later stages, and they generally fired only a single action potential. By mid fetal stages, rapidly and slowly adapting neurons could be distinguished with a topographic distribution matching that found in adult ganglia. Most rapidly adapting neurons (phasic neurons) at this age had a long afterhyperpolarization (LAH) characteristic of mature vasomotor neurons and were preferentially located in the lateral poles of the ganglion, where most neurons contained neuropeptide Y. Most early and mid fetal neurons showed a weak M current, which was later expressed only by rapidly-adapting and LAH neurons. Two different A currents were present in a subset of early fetal neurons and may indicate neurons destined to develop a slowly adapting phenotype (tonic neurons). The size of neuronal cell bodies increased at a similar rate throughout development regardless of their electrical or neurochemical phenotype or their topographical location. In contrast, the rate of dendritic growth of neurons in medial regions of the ganglion was significantly higher than that of neurons in lateral regions. The apparent cell capacitance was highly correlated with the surface area of the soma but not the dendritic tree of the developing neurons. These results demonstrate that the well-defined functional populations of neurons in the celiac ganglion develop their characteristic electrophysiological and morphological properties during early fetal stages of development. This is after the neuronal populations can be recognized by their neurochemical and topographical characteristics but long before the neurons have finished growing. Our data provide strong circumstantial evidence that the development of the full phenotype of different functional classes of autonomic final motor neurons is a multi-step process likely to involve a regulated sequence of trophic interactions.

Circadian regulation of cGMP-gated cationic channels of chick retinal cones. Erk MAP Kinase and Ca2+/calmodulin-dependent protein kinase II

cGMP-gated channels are essential for phototransduction in the vertebrate retina. Here we show that the affinity of these channels for cGMP in chick cones is substantially higher during the subjective night than during the subjective day. This effect persists in constant environmental conditions after entrainment to 12:12 hr light-dark cycles in vitro or in ovo. Circadian modulation of ligand affinity is a posttranslational effect and is driven by rhythms in the activities of two protein kinases: Erk and Ca2+/calmodulin-dependent protein kinase II (CaMKII). Erk is maximally active during the subjective night, whereas CaMKII is maximally active during the subjective day. Acute inhibition of these signaling pathways causes phase-dependent changes in the affinity of the channels for cGMP.