Different localization of receptors for omega-conotoxin and nitrendipine in rat brain

Stable respiratory activity requires both P/Q-type and N-type voltage-gated calcium channels

P/Q-type voltage-gated calcium channels (Ca(v)2.1) play critical presynaptic and postsynaptic roles throughout the nervous system and have been implicated in a variety of neurological disorders. Here we report that mice with a genetic ablation of the Ca(v)2.1 pore-forming α(1A) subunit (α(1A)⁻/⁻) encoded by CACNA1a (Jun et al., 1999) suffer during postnatal development from increasing breathing disturbances that lead ultimately to death. Breathing abnormalities include decreased minute ventilation and a specific loss of sighs, which was associated with lung atelectasis. Similar respiratory alterations were preserved in the isolated in vitro brainstem slice preparation containing the pre-Bötzinger complex. The loss of Ca(v)2.1 was associated with an alteration in the functional dependency on N-type calcium channels (Ca(v)2.2). Blocking N-type calcium channels with conotoxin GVIA had only minor effects on respiratory activity in slices from control (CT) littermates, but abolished respiratory activity in all slices from α(1A)⁻/⁻ mice. The amplitude of evoked EPSPs was smaller in inspiratory neurons from α(1A)⁻/⁻ mice compared with CTs. Conotoxin GVIA abolished all EPSPs in inspiratory neurons from α(1A)⁻/⁻ mice, while the EPSP amplitude was reduced by only 30% in CT mice. Moreover, neuromodulation was significantly altered as muscarine abolished respiratory network activity in α(1A)⁻/⁻ mice but not in CT mice. We conclude that excitatory synaptic transmission dependent on N-type and P/Q-type calcium channels is required for stable breathing and sighing. In the absence of P/Q-type calcium channels, breathing, sighing, and neuromodulation are severely compromised, leading to early mortality.

Distribution of omega-Conotoxin GVIA binding sites in teleost cerebellar and electrosensory neurons

The distribution of omega-Conotoxin GVIA (CgTx) binding sites was used to localize putative N-type Ca2+ channels in an electrosensory cerebellar lobule, the eminentia granularis pars posterior, and in the electrosensory lateral line lobe of a gymnotiform teleost (Apteronotus leptorhynchus). The binding sites for CgTx revealed by an anti-CgTx antibody had a consistent distribution on somatic and dendritic membranes of specific cell types in both structures. The distribution of CgTx binding was unaffected by co-incubation with nifedipine or AgaToxin IVA, blocking agents for L- and P-type Ca2+ channels, respectively. Incubation with CgTx in the presence of varying levels of extracellular Ca2+ altered the number but not the cell types exhibiting immunolabel. A punctate immunolabel was detected on somatic membranes of granule and stellate cell interneurons in both the eminentia granularis pars posterior and the electrosensory lateral line lobe. Punctate CgTx binding sites were also present on spherical cell somata and on the large presynaptic terminals of primary afferents that terminate on spherical cells in the electrosensory lateral line lobe. No label was detected in association with distal dendritic membranes of any cell class, or with parallel fibers in the respective molecular layers. Binding sites for CgTx in the eminentia granularis are consistent with the established role for N-type Ca2+ channels in cell migrations, an activity which is known to persist in this layer in adult Apteronotus. The distribution of labeled stellate cells with respect to topographic maps in the electrosensory lateral line lobe further suggest that N-type Ca2+ channels are expressed in relation to functional activity across these sensory maps.

Purified omega-conotoxin GVIA receptor of rat brain resembles a dihydropyridine-sensitive L-type calcium channel

The omega-conotoxin GVIA (CTX) receptor has been purified 1900-fold to apparent homogeneity by monitoring both reversible binding of 125I-labeled CTX (125I-CTX) and photoincorporation of N-hydroxysuccinimidyl-4-azidobenzoate-125I-CTX (HSA-125I-CTX). Photoincorporation of HSA-125I-CTX into a 230-kDa protein exhibits a pharmacologic and chromatographic profile indicating that the 230-kDa protein is the CTX-binding subunit of the receptor. The pharmacologic specificity of 125I-CTX binding to the purified CTX receptor closely resembles that of the native membrane-bound form with respect to sensitivity towards CTX (Kd = 32 pM) and other peptide toxin antagonists. The purified CTX receptor comprises the 230-kDa protein (alpha 1) and four additional proteins with apparent molecular masses of 140 (alpha 2), 110, 70 (beta 2), and 60 (beta 1) kDa. This subunit structure closely resembles that of the 1,4-dihydropyridine-sensitive L-type calcium channel.

Localization of P-type calcium channels in the central nervous system

The distribution of the P-type calcium channel in the mammalian central nervous system has been demonstrated immunohistochemically by using a polyclonal specific antibody. This antibody was generated after P-channel isolation via a fraction from funnel-web spider toxin (FTX) that blocks the voltage-gated P channels in cerebellar Purkinje cells. In the cerebellar cortex, immunolabeling to the antibody appeared throughout the molecular layer, while all the other regions were negative. Intensely labeled patches of reactivity were seen on Purkinje cell dendrites, especially at bifurcations; much weaker reactivity was present in the soma and stem segment. Electron microscopic localization revealed labeled patches of plasma membrane on the soma, main dendrites, spiny branchlets, and spines; portions of the smooth endoplasmic reticulum were also labeled. Strong labeling was present in the periglomerular cells of the olfactory bulb and scattered neurons in the deep layer of the entorhinal and pyriform cortices. Neurons in the brainstem, habenula, nucleus of the trapezoid body and inferior olive and along the floor of the fourth ventricle were also labeled intensely. Medium-intensity reactions were observed in layer II pyramidal cells of the frontal cortex, the CA1 cells of the hippocampus, the lateral nucleus of the substantia nigra, lateral reticular nucleus, and spinal fifth nucleus. Light labeling was seen in the neocortex, striatum, and in some brainstem neurons.

The inhibition of [125I]omega-conotoxin GVIA binding to neuronal membranes by neomycin may be mediated by a GTP-binding protein

omega-Conotoxin GVIA (omega-CT) has been reported to block calcium currents at the L- and N-type calcium channels. In neuronal membranes omega-CT, and the aminoglycoside antibiotic neomycin, have been shown to inhibit [125I]omega-CT binding, presumably acting at the N-type calcium channel. We demonstrate here that the concentration curve for neomycin sulfate inhibition of [125I]omega-CT binding is shifted to the right by GTP analogues or fluoride, increasing the IC50 for neomycin. [125I]omega-CT binding is unaffected by these agents and in competition studies the potency of omega-CT, Ca2+, or La3+ is not modulated by GTP analogues or fluoride. These results indicate that the inhibition of [125I]omega-CT binding by neomycin may be mediated by a GTP binding protein.

A monoclonal antibody to conotoxin reveals the distribution of a subset of calcium channels in the rat cerebellar cortex

Voltage-sensitive calcium channels (VSCC) are a family of ionophores having different electrical and pharmacological properties. The omega-conotoxin GVIA (omega-CgTX) is a specific blocker of one subset of VSCCs. Because of the specificity of this toxin, a monoclonal anti-omega-CgTX antibody was generated against a omega-CgTX-key hole limpet hemocyanin conjugate and used as a specific marker to study VSCC distributions. This mab was shown to recognize omega-CgTX on Western blots and to display omega-CgTX-dependent immunoperoxidase staining of rat cerebellum. Incubation of fresh, unfixed sections of adult rat cerebellum in omega-CgTX followed by light fixation and peroxidase immunocytochemistry with mab anti-omega-CgTX revealed a specific pattern of labelling. All principal classes of cerebellar neurons were immunoreactive, but in general glial cells were not stained. Most interestingly, strong focal immunoreactivity was encountered at branching points of Purkinje cell dendrites. This characteristic staining pattern implies that a subset of VSCC is specifically concentrated in these regions and suggests that these channels may play a role in the functional integration of dendritic signals.

Synaptic and extra-synaptic distribution of histamine H1-receptors in rat and guinea pig brains

Localization of histamine H1-receptors in subcellular fractions from rat and guinea pig brains was examined in a [3H]mepyramine binding study. Major [3H]mepyramine binding sites with increased specific activities [( 3H]mepyramine binding vs. protein amount) were recovered from P2 fractions from both rat and guinea pig brains by differential centrifugation. Further subfractionation of both rat and guinea pig P2 fractions by a discontinuous sucrose density gradient centrifugation showed the highest recovery of [3H]mepyramine binding with further increased specific activities found in synaptic plasma membrane (SPM) fractions. Minor [3H]mepyramine binding sites with increased specific activities were detected in both rat and guinea pig P3 fractions. [3H]Mepyramine binding sites in SPM and P3 fractions showed identical Kd values in each species. These results indicate that histamine H1-receptors are located not only in synaptic but also in extra-synaptic membranes of both rat and guinea pig brains.

The polyamine spermine affects omega-conotoxin binding and function at N-type voltage-sensitive calcium channels

1. The effects of the polyamines, spermine and spermidine on neuronal N-type voltage-sensitive calcium channels were investigated using the binding and function of the ligand omega-conotoxin GVIA (omega-CT). 2. Spermine and spermidine enhanced (EC50 approximately 0.16 and 0.45 microM) and, at higher concentrations, inhibited (IC50 of 9 and 240 microM) the binding of [125I]omega-CT to rat hippocampal synaptosomes. 3. Spermine and, less potently, spermidine inhibited the neurotransmitter-mediated, omega-CT-sensitive, electrical-field-stimulated contractile responses of the rat vas deferens. 4. The polyamines also inhibited the phenylephrine-evoked contractile responses of the vas deferens with the same rank order, consistent with a postsynaptic mechanism of inhibition. 5. However, pre-exposure to spermine prevented the irreversible inhibition of vas deferens twitch responses by omega-CT (previously found to be presynaptic). The prevention of inhibition by omega-CT demonstrates that the neuronal binding of spermine and omega-CT is mutually exclusive. Thus spermine (and presumably spermidine at higher concentrations) appears to modulate the actions of omega-CT at N-type voltage-sensitive calcium channels.

Neomycin and omega-conotoxin GVIA interact at a common neuronal site in peripheral tissues

1. The present study examined the interaction of omega-conotoxin GVIA (omega-CT) and aminoglycoside antibiotics on electrically evoked, nerve-mediated contractile responses in the rat vas deferens, guinea-pig ileum and guinea-pig left atria. 2. omega-CT caused a time- and concentration-dependent inhibition of the electrically evoked twitch responses of the rat vas deferens and guinea-pig ileum. Aminoglycoside antibiotics inhibited the twitch responses of these preparations with a rank order of potency: neomycin greater than gentamycin greater than kanamycin. omega-CT had no effect on the postjunctional contractile responses of either noradrenaline (vas deferens) or carbachol (ileum). However, at high concentrations neomycin and gentamycin caused significant postjunctional inhibition. The results suggest that omega-CT and aminoglycosides cause prejunctional inhibition in these preparations, with the aminoglycoside antibiotics exhibiting postjunctional inhibitory effects as well at high concentrations. 3. omega-CT caused a concentration- and frequency-dependent inhibition of the neuronally mediated field stimulation enhancement of electrically paced guinea-pig left atria. omega-CT had no effect on either the electrically paced contractile response that was elicited by direct muscle stimulation or the enhancement of the paced response caused by beta-adrenoceptor agonist stimulation. Neomycin caused a concentration-dependent inhibition of the electrically paced contractile response and inhibited the field stimulation response only at concentrations which caused pronounced inhibition of the paced response. Neomycin also caused insurmountable inhibition of responses elicited by beta-adrenoceptor agonist stimulation. Thus, omega-CT caused an exclusive prejunctional inhibition in guinea-pig left atria, whereas the substantial postjunctional effects of neomycin made it difficult to discern any prejunctional activity of neomycin in these experiments. 4. In the vas deferens, ileum and atria the inhibitory effects of omega-CT were long-lasting, whereas the effects of neomycin could be reversed upon wash-out. The disparate kinetics of omega-CT and neomycin allowed for the design of receptor protection studies to determine whether neomycin acts at a prejunctional site in common with omega-CT. The pre-equilibration of a competitive antagonist (neomycin) should prevent the irreversible antagonist (omega-CT) from gaining access to receptors. Pre-exposure of tissues with neomycin prevented the irreversible inhibition of omega-CT. These receptor protection studies suggest that omega-CT and neomycin interact at common neuronal sites in the rat vas deferens, guinea-pig ileum and guinea-pig atria. Neomycin, however, exhibits activity at postjunctional sites as well.

[125I]-omega conotoxin binding to human frontal cortex from normal, Alzheimer's and non-Alzheimer's dementia patients

We studied the binding of the calcium antagonist neurotoxin [125I]-omega conotoxin (GVIA) in age-matched human brains from normal, Alzheimer's disease and non-Alzheimer's dementia patients. Crude preparations of plasmalemmal membranes from frontal cortex were utilized. Saturation isotherms were subjected to Scatchard analysis to determine maximal binding capacity (Bmax) and binding affinity (Kd). In all brain samples tested, [125I]-GVIA binding was homogenous to a single class of high affinity binding sites. Scatchard analysis of saturation isotherms gave the following estimates for normal brains (mean +/- S.D., n = 7): Bmax = .630 +/- .200 pmol/mg and Kd = .177 +/- .054 nM. No significant change was observed in the Kd or Bmax estimates for [125I]-omega conotoxin binding in Alzheimer's disease or non-Alzheimer's dementia brains when compared to normal brains. Although these findings do not rule out the existence of localized changes in calcium channel receptor binding in the frontal cortex of Alzheimer's disease patients, the results do suggest that the neuronal voltage sensitive calcium channel may be unaltered in Alzheimer's disease.

Inhibition of histamine release from rat hypothalamic slices by omega-conotoxin GVIA, but not by nilvadipine, a dihydropyridine derivative

Histamine release in response to 40 mM high K+-stimulation from the rat hypothalamic slice preparations perifused in vitro was significantly inhibited by 1.0 nM-1.0 microM omega-conotoxin GVIA, a peptide modulator of N- and L-type voltage-sensitive calcium channels, but not by similar concentrations of nilvadipine, a dihydropyridine derivative of L-type calcium channel antagonist. These results indicate that the voltage-sensitive calcium channel controlling histamine release from hypothalamic slices is omega-conotoxin-sensitive but dihydropyridine-insensitive.

Distribution of the omega-conotoxin receptor in rat brain. An autoradiographic mapping

The distribution of [125I]omega-conotoxin GVIA binding sites, the putative voltage-sensitive calcium channels, was studied by an autoradiographic method in the rat brain. The toxin binding sites were distributed throughout the brain in a highly heterogeneous manner. The highest density of the binding sites was observed in the cerebral cortex, hippocampus, amygdaloid complex, substantia nigra, caudate putamen, superior colliculus, nucleus of the solitary tract, and the dorsal horn of the cervical spine. The glomerular layer of the olfactory bulb, molecular layer of the cerebellar cortex, and posterior lobe of the hypophysis showed intermediate density but the density was higher than in the surrounding areas. The globus pallidus, thalamic areas, inferior olive, and pontine nuclei showed low density, while no binding sites were observed in the white matter tract regions such as the internal and external capsule, corpus callosum, fimbria of the hippocampus, fornix, stria medullaris of the thalamus, and fasciculus retroflexus. This distribution of omega-conotoxin binding sites indicates that the toxin binding sites are localized in those areas of the brain enriched in synaptic connections. This distribution pattern resembles that reported for voltage-sensitive sodium channels but it differs from that of the binding sites of dihydropyridines and verapamil. These results suggest that omega-conotoxin recognizes different molecules from organic calcium channel antagonist binding sites and that omega-conotoxin-sensitive voltage-sensitive calcium channels are concentrated in the synaptic zones and play a key role in the excitation-secretion coupling of neurotransmitters.

Distribution of [125I]omega-conotoxin GVIA and [3H]isradipine binding sites in the central nervous system of rats of different ages

Potential age-related changes in L- and N-type voltage-sensitive calcium channels (L- and N-VSCCs) were assessed by the in vitro binding of [3H]isradipine ([3H]ISR, 150 pM) and [125]omega-conotoxin GVIA ([125I]omega-CT, 4 pM) to membranes prepared from discrete central nervous system regions of 0.5-, 2-, and 18-month-old rats. The rank orders of [3H]ISR and [125I]omega-CT binding, although differing, indicated that the highest binding was in neocortex, corpus striatum, and hippocampus; radioligand binding was generally not affected by the variable of age. These results suggest that the nonidentical [3H]ISR and [125I]omega-CT binding sites are concentrated in those regions characterized by high densities of synaptic connections, and that these sites, as presumed components of L- and N-VSCCs, are relatively stable during the aging process.

Autoradiographic visualization in rat brain of receptors for omega-conotoxin GVIA, a newly discovered calcium antagonist

Putative N-type voltage-sensitive calcium channels were localized autoradiographically in thaw-mounted rat brain slices using [125I]omega-conotoxin GVIA as a ligand. Density of the toxin binding sites were highly heterogeneous throughout the brain. The highest density of the binding sites was observed in the glomerular layer of the olfactory bulb, cerebral cortex, molecular layer of the hippocampus, amygdaloid complex, reticular part of the substantia nigra, molecular layer of the cerebellar cortex, and nucleus of the solitary tract. White matter tract regions such as the internal capsule, corpus callosum, fimbria of the hippocampus, fornix, and fasciculus retroflexus showed an extremely low density.