Helical structure of the COOH terminus of S3 and its contribution to the gating modifier toxin receptor in voltage-gated ion channels

The voltage-sensing domains in voltage-gated K(+) channels each contain four transmembrane (TM) segments, termed S1 to S4. Previous scanning mutagenesis studies suggest that S1 and S2 are amphipathic membrane spanning alpha-helices that interface directly with the lipid membrane. In contrast, the secondary structure of and/or the environments surrounding S3 and S4 are more complex. For S3, although the NH(2)-terminal part displays significant helical character in both tryptophan- and alanine-scanning mutagenesis studies, the structure of the COOH-terminal portion of this TM is less clear. The COOH terminus of S3 is particularly interesting because this is where gating modifier toxins like Hanatoxin interact with different voltage-gated ion channels. To further examine the secondary structure of the COOH terminus of S3, we lysine-scanned this region in the drk1 K(+) channel and examined the mutation-induced changes in channel gating and Hanatoxin binding affinity, looking for periodicity characteristic of an alpha-helix. Both the mutation-induced perturbation in the toxin-channel interaction and in gating support the presence of an alpha-helix of at least 10 residues in length in the COOH terminus of S3. Together with previous scanning mutagenesis studies, these results suggest that, in voltage-gated K(+) channels, the entire S3 segment is helical, but that it can be divided into two parts. The NH(2)-terminal part of S3 interfaces with both lipid and protein, whereas the COOH-terminal part interfaces with water (where Hanatoxin binds) and possibly protein. A conserved proline residue is located near the boundary between the two parts of S3, arguing for the presence of a kink in this region. Several lines of evidence suggest that these structural features of S3 probably exist in all voltage-gated ion channels.

Solution structure of hanatoxin1, a gating modifier of voltage-dependent K(+) channels: common surface features of gating modifier toxins

The three-dimensional structure of hanatoxin1 (HaTx1) was determined by using NMR spectroscopy. HaTx1 is a 35 amino acid residue peptide toxin that inhibits the drk1 voltage-gated K(+) channel not by blocking the pore, but by altering the energetics of gating. Both the amino acid sequence of HaTx1 and its unique mechanism of action distinguish this toxin from the previously described K(+) channel inhibitors. Unlike most other K(+) channel-blocking toxins, HaTx1 adopts an "inhibitor cystine knot" motif and is composed of two beta-strands, strand I for residues 19-21 and strand II for residues 28-30, connected by four chain reversals. A comparison of the surface features of HaTx1 with those of other gating modifier toxins of voltage-gated Ca(2+) and Na(+) channels suggests that the combination of a hydrophobic patch and surrounding charged residues is principally responsible for the binding of gating modifier toxins to voltage-gated ion channels.

A localized interaction surface for voltage-sensing domains on the pore domain of a K+ channel

Voltage-gated K+ channels contain a central pore domain and four surrounding voltage-sensing domains. How and where changes in the structure of the voltage-sensing domains couple to the pore domain so as to gate ion conduction is not understood. The crystal structure of KcsA, a bacterial K+ channel homologous to the pore domain of voltage-gated K+ channels, provides a starting point for addressing this question. Guided by this structure, we used tryptophan-scanning mutagenesis on the transmembrane shell of the pore domain in the Shaker voltage-gated K+ channel to localize potential protein-protein and protein-lipid interfaces. Some mutants cause only minor changes in gating and when mapped onto the KcsA structure cluster away from the interface between pore domain subunits. In contrast, mutants producing large changes in gating tend to cluster near this interface. These results imply that voltage-sensing domains interact with localized regions near the interface between adjacent pore domain subunits.

Inhibition of T-type voltage-gated calcium channels by a new scorpion toxin

The biophysical properties of T-type voltage-gated calcium channels are well suited to pacemaking and to supporting calcium flux near the resting membrane potential in both excitable and non-excitable cells. We have identified a new scorpion toxin (kurtoxin) that binds to the alpha 1G T-type calcium channel with high affinity and inhibits the channel by modifying voltage-dependent gating. This toxin distinguishes between alpha 1G T-type calcium channels and other types of voltage-gated calcium channels, including alpha 1A, alpha 1B, alpha 1C and alpha 1E. Like the other alpha-scorpion toxins to which it is related, kurtoxin also interacts with voltage-gated sodium channels and slows their inactivation. Kurtoxin will facilitate characterization of the subunit composition of T-type calcium channels and help determine their involvement in electrical and biochemical signaling.

Gating modifier toxins reveal a conserved structural motif in voltage-gated Ca2+ and K+ channels

Protein toxins from venomous animals exhibit remarkably specific and selective interactions with a wide variety of ion channels. Hanatoxin and grammotoxin are two related protein toxins found in the venom of the Chilean Rose Tarantula, Phrixotrichus spatulata. Hanatoxin inhibits voltage-gated K+ channels and grammotoxin inhibits voltage-gated Ca2+ channels. Both toxins inhibit their respective channels by interfering with normal operation of the voltage-dependent gating mechanism. The sequence homology of hanatoxin and grammotoxin, as well as their similar mechanism of action, raises the possibility that they interact with the same region of voltage-gated Ca2+ and K+ channels. Here, we show that each toxin can interact with both voltage-gated Ca2+ and K+ channels and modify channel gating. Moreover, mutagenesis of voltage-gated K+ channels suggests that hanatoxin and grammotoxin recognize the same structural motif. We propose that these toxins recognize a voltage-sensing domain or module present in voltage-gated ion channels and that this domain has a highly conserved three-dimensional structure.

Hanatoxin modifies the gating of a voltage-dependent K+ channel through multiple binding sites

We studied the mechanism by which Hanatoxin (HaTx) inhibits the drk1 voltage-gated K+ channel. HaTx inhibits the K+ channel by shifting channel opening to more depolarized voltages. Channels opened by strong depolarization in the presence of HaTx deactivate much faster upon repolarization, indicating that toxin bound channels can open. Thus, HaTx inhibits the drk1 K+ channel, not by physically occluding the ion conduction pore, but by modifying channel gating. Occupancy of the channel by HaTx was studied using various strength depolarizations. The concentration dependence for equilibrium occupancy as well as the kinetics of onset and recovery from inhibition indicate that multiple HaTx molecules can simultaneously bind to a single K+ channel. These results are consistent with a simple model in which HaTx binds to the surface of the drk1 K+ channel at four equivalent sites and alters the energetics of channel gating.

Mapping the receptor site for hanatoxin, a gating modifier of voltage-dependent K+ channels

Hanatoxin (HaTx) binds to multiple sites on the surface of the drk1 voltage-gated K+ channel and modifies channel gating. We set out to identify channel residues that contribute to form these HaTx binding sites. Chimeras constructed using the drk1 and shaker K+ channels suggest that the S3-S4 linker may contain influential residues. Alanine scanning mutagenesis of the region extending from the C terminal end of S3 through S4 identified a number of residues that likely contribute to form the HaTx binding sites. The pore blocker Agitoxin2 and the gating modifier HaTx can simultaneously bind to individual K+ channels. These results suggest that residues near the outer edges of S3 and S4 form the HaTx binding sites and are eccentrically located at least 15 A from the central pore axis on the surface of voltage-gated K+ channels.

Inhibition of calcium channels in rat central and peripheral neurons by omega-conotoxin MVIIC

Inhibition of voltage-dependent calcium channels by omega-conotoxin MVIIC (omega-CTx-MVIIC) was studied in various types of rat neurons. When studied with 5 mM Ba2+ as charge carrier, omega-CTx-MVIIC block of N-type calcium channels in sympathetic neurons was potent, with half-block at 18 nM. Block of N-type channels had a rapid onset (tau approximately 1 sec at 1 microM omega-CTx-MVIIC) and quick reversibility (tau approximately 30 sec). The rate of block was proportional to toxin concentration, consistent with 1:1 binding of toxin to channels, with a rate constant (k on) of approximately 1 X 10(6) M-1. sec-1. Both potency and rate of block were reduced dramatically with increasing concentrations of extracellular Ba2+ omega-CTx-MVIIC also blocked P-type calcium channels in cerebellar Purkinje neurons, but both development and reversal of block were far slower than for N-type channels. The rate of block was proportional to toxin concentration, with k on -1.5 x 10(3) M-1. sec-1 at 5 mM Ba2+. From this value and an unblocking time constant of approximately 200 min, a dissociation constant of approximately 50 nM was estimated. Thus, block of P-type channels is potent but very slow. In hippocampal CA3 pyramidal neurons, omega-CTx-MVIIC blocked approximately 50% of the high-threshold calcium channel current; one component (approximately 20%) was blocked with the rapid kinetics expected for N-type channels, whereas the other component was blocked slowly. The component blocked slowly was reduced but not eliminated by preexposure to 200 nM or 1 microM omega-Aga-IVA.

An inhibitor of the Kv2.1 potassium channel isolated from the venom of a Chilean tarantula

The Kv2.1 voltage-activated K+ channel, a Shab-related K+ channel isolated from rat brain, is insensitive to previously identified peptide inhibitors. We have isolated two peptides from the venom of a Chilean tarantula, G. spatulata, that inhibit the Kv2.1 K+ channel. The two peptides, hanatoxin1 (HaTx1) and hanatoxin2 (HaTx2) are unrelated in primary sequence to other K+ channel inhibitors. The activity of HaTx was verified by synthesizing it in a bacterial expression system. The concentration dependence for both the degree of inhibition at equilibrium (Kd = 42 nM) and the kinetics of inhibition (kon = 3.7 x 10(4) M-1s-1; koff = 1.3 x 10(-3) s-1), are consistent with a bimolecular reaction between HaTx and the Kv2.1 K+ channel. Shaker-related, Shaw-related, and eag K+ channels were relatively insensitive to HaTx, whereas a Shal-related K+ channel was sensitive. Regions outside the scorpion toxin binding site (S5-S6 linker) determine sensitivity to HaTx. HaTx introduces a new class of K+ channel inhibitors that will be useful probes for studying K+ channel structure and function.

Intrastriatal injections of quinolinic acid or kainic acid: differential patterns of cell survival and the effects of data analysis on outcome

There is controversy about the extent to which lesions of the rat striatum with excitatory amino acids mimic the cellular pathology seen in Huntington's Disease (HD). We sought to resolve this debate by determining with cell counts in adjacent sections the patterns of survival of medium spiny and aspiny striatal neurons using enkephalin immunohistochemistry and NADPH-diaphorase histochemistry as markers of these cell populations, respectively. Results showed that 2 weeks after quinolinic acid lesions, cell loss was qualitatively similar for the two cell groups. However, by varying the size of the sampling area for quantitative analyses and its distance from the lesion zone, the outcome of the statistical analyses varied enormously. Thus, a relative sparing of NADPH-diaphorase-labeled cells compared to enkephalin-labeled cells could be detected quantitatively in transition areas bordering the lesion under some but not all analytical conditions. Kainic acid lesions depleted both cell populations similarly, except in regions of transition farthest from the lesion, where enkephalin-containing neurons were more resistant than NADPH-diaphorase-containing cells. The size of the transition area around the lesion also differed depending upon excitotoxin and cell population. These results help to reconcile the controversy and suggest that with highly specified quantitative conditions quinolinic acid-induced injury of the striatum can resemble the histopathology of HD.

Modulation of Ca2+ channels by protein kinase C in rat central and peripheral neurons: disruption of G protein-mediated inhibition

Activation of protein kinase C (PKC) reduced G protein-dependent inhibition of Ca2+ channels by glutamate, GA-BAB, adenosine, muscarinic, alpha-adrenergic, and LHRH receptors in a variety of central and peripheral neurons. PKC stimulation also relieved the inhibitory effect of internal GTP gamma S and reduced tonic G protein-mediated inhibition observed with internal GTP in the absence of transmitter receptor agonist. Basal Ca2+ channel currents were enhanced by PKC stimulation in most neurons studied. The PKC-induced enhancement of basal current was voltage dependent, and enhanced currents displayed altered kinetics. Inhibition of G proteins with GDP beta S attenuated the PKC-induced enhancement of basal Ca2+ channel current. These results show that PKC regulates the inhibitory effects of G proteins, possibly by disrupting the coupling of G proteins to Ca2+ channels. The PKC-induced enhancement of Ca2+ channel current results, at least in part, from the removal of tonic G protein-mediated inhibition.

Protein kinase C modulates glutamate receptor inhibition of Ca2+ channels and synaptic transmission

Fast synaptic transmission in the central nervous system can be modulated by neurotransmitters and second-messenger pathways. For example, transmission at glutamatergic synapses can be depressed by the metabotropic glutamate receptor, providing autoreceptor-mediated negative feedback. Metabotropic glutamate receptor inhibition of Ca2+ channels may contribute to this pathway. In contrast, stimulation of protein kinase C can enhance excitatory synaptic transmission, whereas both depression and enhancement of Ca2+ current have been reported. Here we show that in hippocampal CA3 and cortical pyramidal neurons, activation of protein kinase C enhances current through N-type Ca2+ channels and, in addition, dramatically reduces G protein-dependent inhibition of these same channels by the metabotropic glutamate receptor. In parallel experiments on fast excitatory transmission at corticostriatal synapses, kinase C activators were similarly found to reduce the inhibitory effect produced by stimulation of the metabotropic glutamate receptor. The results show that second-to-second control of Ca2+ channels by the metabotropic glutamate receptor can itself be modulated on a slower timescale by protein kinase C. These mechanisms may be used in the control of fast excitatory synaptic transmission.

Inhibition of calcium channels in rat CA3 pyramidal neurons by a metabotropic glutamate receptor

L-Glutamate rapidly and reversibly suppressed Ca channel current in freshly dissociated pyramidal neurons from the CA3 region of the rat hippocampus. L-Glutamate inhibition of Ca channel current could be distinguished from activation of background conductance by appropriate ionic conditions and by distinct pharmacological profiles. Ca channel inhibition by glutamate was mimicked by quisqualate, ibotenate, racemïct-ACPD and 1S,3R-ACPD but not by kainate, AMPA, L-aspartate, NMDA, L-2-amino-4-phosphonobutyric acid, or 1R,3S-ACPD; 6-cyano-7-nitroquinoxaline-2,3-dione did not inhibit the response. All agonists inhibited a similar fraction of high-voltage-activated Ca channel current, typically approximately 30%. Concentration-response relations for the agonists were consistent with mediation by a metabotropic glutamate receptor. The stereospecific agonist 1S,3R-ACPD was especially useful since it did not activate background conductances. The fraction of Ca channel current sensitive to 1S,3R-ACPD was partially blocked by omega-conotoxin GVIA but was not sensitive to dihydropyridine antagonists or agonists. The suppression of Ca channels by 1S,3R-ACPD became irreversible when cells were dialyzed with GTP-gamma-S. 1S,3R-ACPD suppressed Ca channel currents in outside-out membrane patches but not in cell-attached patches when applied outside the patch. These results suggest that metabotropic glutamate receptors suppress the activity of N-type Ca channels in CA3 neurons by a mechanism involving G-proteins but not readily diffusible second messengers.

Developmental changes in brain kynurenic acid concentrations

The cerebral distribution and regulation of excitatory amino acid levels may play a crucial role in neuronal development. In the present study we examined concentrations of the endogenous excitatory amino acid antagonist kynurenic acid and related substances during development in fetal and neonatal rat brain and fetal non-human primate cerebral cortex. Kynurenic acid concentrations in rat fetal whole brain were significantly increased 4-5 fold prenatally, then declined rapidly at 1 day after birth, and reached adult concentrations at 7 days after birth. L-Kynurenine concentrations were also markedly increased prior to birth and then declined to adult concentrations at 1 day after birth. L-Tryptophan was increased 3 fold before birth, and decreased to adult concentrations 1 day after birth. In contrast concentrations of dopamine, norepinephrine, 3,4-dihydroxyphenylacetic acid and homovanillic acid increased 1 day prior to birth and continued to increase following birth. Fetal baboon cerebral cortex showed significant increases in kynurenic acid concentrations both pre-term and near-term as compared with adult concentrations. These results show that marked changes in kynurenic acid concentrations occur prior to and following birth. It is possible that high levels of kynurenic acid prior to birth inhibit neurite branching and development of excitatory synapses, which then develop rapidly in parallel with the decrease in kynurenic acid levels.

Competitive antagonism of glutamate receptor channels by substituted benzazepines in cultured cortical neurons

Whole-cell recordings from rat cortical neurons in dissociated cell culture were used to study the antagonism of glutamate receptors by several lipophilic benzazepine analogues of 2,5-dihydro-2,5-dioxo-3-hydroxy-1H-benzazepine (DDHB). DDHB and three substituted derivatives, 4-bromo-, 7-methyl-, and 8-methyl-DDHB, inhibited the activation of N-methyl-D-aspartate (NMDA) receptors at both the NMDA recognition site and the glycine allosteric site. In addition, all four compounds blocked the activation of non-NMDA receptors by kainate and L-glutamate. Antagonism by the four benzazepines was equivalent at holding potentials from -80 mV to +50 mV. Both the onset of and recovery from block of the agonist-gated currents were complete within seconds. Antagonist affinity was calculated from the displacement of steady state concentration-response curves for kainate, L-glutamate, glycine, and NMDA, based on the Gaddum-Schild relationship (dose ratio = 1 + [antagonist]/KB). The most potent blocker, 8-Me-DDHB, had an apparent dissociation constant (KB) of 470 nM at the glycine allosteric site and 27 microM at the NMDA recognition site. The apparent dissociation constant of 8-Me-DDHB for non-NMDA receptors was 6.4 microM when kainate was the agonist and 9.6 microM when L-glutamate was the agonist. Unsubstituted DDHB showed slightly higher affinity for the NMDA recognition site (KB = 16 microM) but was less potent than 8-Me-DDHB at the glycine allosteric site and at non-NMDA receptors (KB = 3 and 65 microM, respectively). At all three sites, the inhibitory actions of these benzazepine derivatives were consistent with a simple competitive mechanism of antagonism. In addition, the antagonist potency of the parent compound, DDHB, against kainate, NMDA, and glycine was equal to or greater than that of other bicyclic antagonists, including kynurenic acid, indole-2-carboxylic acid, and quinoxaline-2,3-dione. Substituted benzazepines represent a new class of glutamate receptor antagonists that show competitive action, significant potency at multiple sites, and a high degree of lipophilicity.

Aminooxyacetic acid results in excitotoxin lesions by a novel indirect mechanism

Aminooxyacetic acid (AOAA) is an inhibitor of several pyridoxal phosphate-depedent enzymes in the brain. In the present experiments intrastriatal injections of AOAA produced dose-dependent excitotoxic lesions. The lesions were dependent on a pyridoxal phosphate mechanisms because pyridoxine blocked them. The lesions were blocked by the noncompetitive N-methyl-D-aspartate (NMDA) antagonist MK-801 and by coinjection of kynurenate, a result indicating an NMDA receptor-mediated excitotoxic process. Electrophysiologic studies showed that AOAA does not directly activate ligand-gated ion channels in cultured cortical or striatal neurons. Pentobarbital anesthesia attenuated the lesions. AOAA injections resulted in significant increases in lactate content and depletions of ATP levels. AOAA striatal lesions closely resemble Huntington's disease both neurochemically and histologically because they show striking sparing of NADPH-diaphorase and large neurons within the lesioned area. AOAA produces excitotoxic lesions by a novel indirect mechanism, which appears to be due to impairment of intracellular energy metabolism, secondary to its ability to block the mitochondrial malate-aspartate shunt. These results raise the possibility that a regional impairment of intracellular energy metabolism may secondarily result in excitotoxic neuronal death in chronic neurodegenerative illnesses, such as Huntington's disease.

Chronic quinolinic acid lesions in rats closely resemble Huntington's disease

We previously found a relative sparing of somatostatin and neuropeptide Y neurons 1 week after producing striatal lesions with NMDA receptor agonists. These results are similar to postmortem findings in Huntington's disease (HD), though in this illness there are two- to threefold increases in striatal somatostatin and neuropeptide Y concentrations, which may be due to striatal atrophy. In the present study, we examined the effects of striatal excitotoxin lesions at 6 months and 1 yr, because these lesions exhibit striatal shrinkage and atrophy similar to that occurring in HD striatum. At 6 months and 1 yr, lesions with the NMDA receptor agonist quinolinic acid (QA) resulted in significant increases (up to twofold) in concentrations of somatostatin and neuropeptide Y immunoreactivity, while concentrations of GABA, substance P immunoreactivity, and ChAT activity were significantly reduced. In contrast, somatostatin and neuropeptide Y concentrations did not increase 6 months after kainic acid (KA) or alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) lesions. At both 6 months and 1 yr, QA lesions showed striking sparing of NADPH-diaphorase neurons as compared with both AMPA and KA lesions, neither of which showed preferential sparing of these neurons. Long-term QA lesions also resulted in significant increases in concentrations of both 5-HT and 5-hydroxyindoleacetic acid (HIAA), similar to findings in HD. Chronic QA lesions therefore closely resemble the neurochemical features of HD, because they result in increases in somatostatin and neuropeptide Y and in 5-HT and HIAA. These findings strengthen the possibility that an NMDA receptor-mediated excitotoxic process could play a role in the pathogenesis of HD.

2-Chloroadenosine attenuates NMDA, kainate, and quisqualate toxicity

Excitatory amino acid (EAA)-induced cell death in the striatum is dependent upon intact glutamatergic afferents arising from the cerebral cortex. Through a mechanism possibly related to inhibition of glutamate release, adenosine receptor agonists attenuate EAA induced toxicity in the rat striatum. In the present study, we examined whether 2-chloroadenosine (2CLA), a stable adenosine analog, protects against toxicity induced by kainate (KA), quisqualate (QUIS), N-methyl-D-aspartate (NMDA), and ibotenate (IBO). In vivo intrastriatal injections of 2CLA (50 nmol) with each EAA tested provided a partial but significant protective effect versus injection of the EAA alone, as measured by striatal concentrations of gamma-aminobutyric acid (GABA) and substance P-like immunoreactivity (SP-LI). These results show that 2CLA attenuates both NMDA- and non-NMDA-mediated neuronal cell death.

Regional brain and cerebrospinal fluid quinolinic acid concentrations in Huntington's disease

Many of the characteristics neuroanatomical and neurochemical features of Huntington's disease (HD) are produced in experimental animals by an intrastriatal injection of the endogenous N-methyl-D-aspartate receptor agonist quinolinic acid (QUIN). Conceivably, a chronic over-production of QUIN in brain could be involved in the pathogenesis of HD. To investigate this hypothesis, concentrations of QUIN were measured both in cerebrospinal fluid (CSF) and postmortem tissue from patients with HD and neurologically normal age-matched controls. CSF QUIN concentrations were slightly lower in patients with HD, however the changes were not significant. Mean concentrations of QUIN tended to be lower in HD putamen, dentate nucleus and several cortical regions, although significant reductions were found only in Brodmann areas 17, 20 and 28. The mechanisms responsible for these small reductions in brain QUIN concentrations remain to be determined. These results do not support the hypothesis that a chronic increase of QUIN production is responsible for neurodengeneration in HD.

Neurochemical characterization of excitotoxin lesions in the cerebral cortex

Neuronal degeneration that occurs in both ischemia and degenerative neurologic illnesses may involve excitotoxic mechanisms. In the present study, we examined whether cortical lesions with agonists acting at subtypes of glutamate receptors result in selective patterns of neuronal death. Injections of quinolinic acid, NMDA, homocysteic acid, kainic acid (KA), and alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) were made at 2 sites in the dorsolateral frontoparietal cortex in rats. After 1 week, the cerebral cortex was either dissected for neurochemical studies, or animals were perfused for histologic evaluation. Concentrations of somatostatin (SS), neuropeptide Y (NPY), substance P (SP), cholecystokinin (CCK), and vasoactive intestinal polypeptide (VIP) were measured by radioimmunoassay, while amino acids and catecholamines were measured by high-performance liquid chromatography (HPLC) with electrochemical detection. NMDA agonists (quinolinic acid, homocysteic acid, and NMDA itself) resulted in dose-dependent reductions in glutamate and GABA, while SS, NPY, SP, CCK, and VIP were either unchanged or significantly increased in concentration. KA and AMPA at doses that resulted in comparable GABA depletions caused significant reductions in SS concentrations. Markers of cortical afferents were spared. All excitotoxins resulted in dose-dependent marked increases in uric acid concentrations. Histologic examination verified that lesions with NMDA agonists produced relative sparing of NADPH-diaphorase, SS, VIP, and CCK neurons. These results show that NMDA excitotoxin lesions result in a pattern of selective neuronal damage in the cerebral cortex that is similar to that which occurs in both ischemia and Huntington's disease.

Kynurenine pathway measurements in Huntington's disease striatum: evidence for reduced formation of kynurenic acid

Recent evidence suggests that there may be overactivation of the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptors in Huntington's disease (HD). Tryptophan metabolism by the kynurenine pathway produces both quinolinic acid, an NMDA receptor agonist, and kynurenic acid, an NMDA receptor antagonist. In the present study, multiple components of the tyrosine and tryptophan metabolic pathways were quantified in postmortem putamen of 35 control and 30 HD patients, using HPLC with 16-sensor electrochemical detection. Consistent with previous reports in HD putamen, there were significant increases in 5-hydroxyindoleacetic acid, 5-hydroxytryptophan, and serotonin concentrations. Within the kynurenine pathway, the ratio of kynurenine to kynurenic acid was significantly (p less than 0.01) increased twofold in HD patients as compared with controls, consistent with reduced formation of kynurenic acid in HD. CSF concentrations of kynurenic acid were significantly reduced in HD patients as compared with controls and patients with other neurologic diseases. Because kynurenic acid is an endogenous inhibitor of excitatory neurotransmission and can block excitotoxic degeneration in vivo, a relative deficiency of this compound could directly contribute to neuronal degeneration in HD.

Cerebral synthesis and release of kynurenic acid: an endogenous antagonist of excitatory amino acid receptors

Excitatory amino acid (EAA)-mediated synaptic transmission is the most prevalent excitatory system within the mammalian brain. Activation of EAA receptors has been postulated to contribute to neuronal cell death in stroke, epilepsy, hypoglycemia, and Huntington's disease. Kynurenic acid is an endogenous substance that inhibits EAA receptors and may therefore influence important physiologic and pathologic processes. The release of intracerebrally synthesized kynurenic acid into the extracellular fluid (ECF), where it may act at EAA receptors, has not been established in vivo. We studied the synthesis and release of kynurenic acid in the rat striatum using intracerebral microdialysis coupled with high performance liquid chromatography and fluorescence detection. The basal ECF concentration of kynurenic acid in the rat corpus striatum was 17.1 +/- 1.1 nM. Peripheral administration of the immediate biosynthetic precursor of kynurenic acid, L-kynurenine, resulted in marked dose-dependent increases in striatal ECF concentrations of kynurenic acid, peaking at 2-2.5 hr. The highest dose of L-kynurenine (100 mg/kg), administered peripherally, resulted in a 108-fold increase in plasma kynurenic acid levels and a 37-fold increase in cerebral ECF levels. Peripheral administration of kynurenic acid, at a dose that caused plasma levels to increase 430-fold, resulted in only 4-fold increases in striatal ECF concentrations. The precursor responsiveness of striatal ECF kynurenic acid to peripherally infused L-kynurenine was blocked by the central application (via the dialysis probe) of aminooxyacetic acid, an inhibitor of the immediate synthetic enzyme for kynurenic acid, kynurenine aminotransferase. Administration of L-tryptophan was less effective than L-kynurenine in increasing ECF kynurenic acid concentrations and did so at a considerably later time interval (6 hr).(ABSTRACT TRUNCATED AT 250 WORDS)

Galanin immunoreactivity is increased in the nucleus basalis of Meynert in Alzheimer's disease

A depletion of large cholinergic neurons in the nucleus basalis of Meynert is a consistent finding in Alzheimer's disease (AD). The nucleus basalis of Meynert also contains interneurons and afferents that may modulate its functioning. In the present study we examined neurochemical markers for neuropeptides, amino acid neurotransmitters, and monoaminergic neurotransmitters in postmortem samples of the nucleus basalis in 16 control subjects and 30 patients with AD. There were no significant changes in glutamate, aspartate, taurine, gamma-aminobutyric acid (GABA), and catecholamines; however, concentrations of serotonin, 5-hydroxyindoleacetic acid, and 5-hydroxytryptophol were significantly reduced. Choline acetyltransferase activity was significantly reduced, consistent with previous reports. Galanin immunoreactivity was significantly increased twofold in the patients with AD, but there were no significant changes in substance P, somatostatin, or neuropeptide Y immunoreactivity. Since galanin inhibits acetylcholine release, and produces cognitive deficits in animals, increased galanin immunoreactivity in the nucleus basalis of Meynert in AD may contribute to the cognitive deficits that characterize the illness.

Kynurenine metabolites of tryptophan: implications for neurologic diseases

Over the past 2 decades, a number of studies have demonstrated that amino acids act as precursors for the biosynthesis of a variety of neuroactive compounds, including catecholamines and indoleamines. For example, the aromatic amino acid L-tryptophan is a precursor for serotonin biosynthesis. Based on this observed precursor relationship, dietary tryptophan supplementation is used to treat a number of neurologic disorders attributed to alterations in serotoninergic neurotransmission. Recent studies have revealed that, in addition to serotonin, a number of neuroactive compounds, the kynurenines, are metabolities of tryptophan. Of these, perhaps the most important is quinolinic acid, a neurotoxin that acts at the N-methyl-D-aspartate (NMDA) receptor and whose precursor responsiveness to tryptophan far exceeds that of serotonin. In the central nervous system, kynurenines, and in particular quinolinic acid, may modulate excitatory amino acid transmission, and may act as neurotoxic agents implicated in the pathogenesis of several neurologic diseases.

Ion channel expression by white matter glia: the O-2A glial progenitor cell

We describe electrophysiological properties of the O-2A glial progenitor cell in a new serum-free culture system. O-2A progenitors have many properties characteristic of neurons: they have glutamate-activated ion channels, express the neuronal form of the sodium channel, fire single regenerative potentials, and synthesize the neurotransmitter GABA by an alternative synthetic pathway. Nearly identical properties were observed in acutely isolated O-2A progenitors, indicating that this phenotype is not an artifact of culture. The O-2A did not express a simple subset of channel types found in its descendant cells, the type-2 astrocyte and oligodendrocyte, studied in the same culture system. During development, these electrophysiological properties may contribute to O-2A function in vivo.

Measurement of kynurenic acid in mammalian brain extracts and cerebrospinal fluid by high-performance liquid chromatography with fluorometric and coulometric electrode array detection

Kynurenic acid is a broad-spectrum excitatory amino acid (EAA) receptor antagonist which is present in the mammalian central nervous system. We describe a method for the measurement of kynurenic acid using isocratic reverse-phase high-performance liquid chromatography (HPLC) with fluorometric detection enhanced by Zn2+ as a postcolumn reagent. The method requires no prior sample preparation procedures other than extraction with 0.1 M HClO4. The reliability of the primary fluorometric method was verified by comparing measurements of tissue concentrations of kynurenic acid in human cerebral cortex and putamen using three different methods of separation with fluorometric detection, as well as four methods utilizing HPLC with coulometric electrode array system (CEAS) detection. All seven methods produced comparable results. The concentration of kynurenic acid in human cerebral cortex was 2.07 +/- 0.61 pmol/mg protein, and in human putamen, 3.38 +/- 0.81 pmol/mg protein. Kynurenic acid was also found to be present in human cerebrospinal fluid (CSF) at a concentration of 5.09 +/- 1.04 nM. The regional distribution of kynurenic acid in the rat brain was examined. Kynurenic acid concentrations were highest in brainstem (149.6 fmol/mg protein) and olfactory bulb (103.9 fmol/mg protein) and lowest in thalamus (26.0 fmol/mg protein). There were no significant postmortem changes in kynurenic acid concentrations in cerebral cortex, hippocampus, and striatum at intervals ranging from 0 to 24 h. Perfusion of the cerebral vasculature with normal saline prior to sacrifice did not significantly alter kynurenic acid content in rat hippocampus, cerebral cortex, or striatum. The analytical methods described are the most sensitive (10-30 fmol injection-1) and specific (utilizing both excitation and emissions properties and electrochemical reaction potentials, respectively) methods for determining kynurenic acid in brain tissue extracts and CSF. These methods should prove useful in examining whether kynurenic acid modulates EAA-mediated neurotransmission under physiologic conditions, as well as in determining the role of kynurenic acid in excitotoxic neuronal death.

Homocysteic acid lesions in rat striatum spare somatostatin-neuropeptide Y (NADPH-diaphorase) neurons

L-Homocysteic acid (L-HCA) is a sulfated amino acid which is present in mammalian striatum and is a putative excitatory striatal neurotransmitter. In the present study we examined the histologic and neurochemical effects of L-HCA induced striatal lesions to determine how closely changes resemble those of Huntington's disease (HD). Increasing doses of L-HCA injected into the anterior striatum resulted in dose-dependent reductions in both substance P-like immunoreactivity (SP-LI) and gamma-aminobutyric acid (GABA) while there was a relative sparing of both somatostatin-like immunoreactivity (SS-LI) and neuropeptide Y-like immunoreactivity (NPY-LI). Immunocytochemical studies showed a relative sparing of NADPH-diaphorase neurons (which colocalize with SS and NPY) within regions in which there was a significant depletion of enkephalin stained neurons. The lesions were blocked by pretreatment with MK-801, a systemically effective non-competitive antagonist of N-methyl-D-aspartate (NMDA) receptors or coinjection of equimolar concentrations of 2-amino-5-phosphonovalerate (APV). These findings are similar to those produced with the NMDA agonist quinolinic acid, and suggest that other endogenous NMDA agonists, such as L-HCA, could be potential excitotoxins in HD.

Somatostatin, neuropeptide Y, GABA and cholinergic enzymes in brain of pentylenetetrazol-kindled rats

We studied the effect of pentylenetetrazol (PTZ)-induced kindling (35 mg/kg, i.p., daily) on somatostatin-like immunoreactivity (SOM) with special attention to the duration of changes (rats were sacrificed either 10 days or 4 months after the development of kindling) and to transmitters or modulators related to somatostatin (neuropeptide Y (NPY), GABA, choline acetyltransferase (ChAT), acetylcholinesterase (AchE]. In rats sacrificed 10 days after the last kindled seizure, SOM was elevated in frontal cortex and striatum (p less than 0.01); NPY was elevated in frontal cortex, striatum and hippocampus (p less than 0.05) of kindled or prekindled rats (i.e., rats which were treated daily with PTZ but did not express three consecutive generalized seizures). ChAT activity was slightly decreased (p less than 0.05) in cortex. GABA levels and AchE activity were unchanged in kindled cortex. In rats sacrificed 4 months after the development of kindling none of the parameters analyzed differed from controls. The present study suggests that the cortical and striatal neurons containing SOM/NPY are affected by PTZ-kindling. The cortical cholinergic system is affected to a much smaller extent. The neuropeptide changes are not persistent, as is the lowered seizure threshold, so they are probably not involved in the maintainance of the latter.

Neuroactive metabolites of L-tryptophan, serotonin and quinolinic acid, in striatal extracellular fluid. Effect of tryptophan loading

Extracellular fluid levels of the neurotoxin quinolinic acid in the corpus striatum of rats, measured by in vivo microdialysis, were increased in a dose-dependent manner following the intraperitoneal administration of tryptophan. The lowest dose of tryptophan (12.5 mg/kg), equivalent to about 5% of the normal daily intake, increased peak quinolinic acid levels nearly 3-fold. At higher doses of tryptophan (up to 250 mg/kg), concentrations of quinolinic acid increased over 200-fold and exceeded potentially neurotoxic levels (10 microM). In contrast, the increase in extracellular serotonin following even the highest tryptophan dose was small (less than 2-fold). These data indicate that quinolinic acid is present in the extracellular fluid where it may function as a neuromodulator and that it is very responsive to physiological changes in precursor availability.

Differential sparing of somatostatin-neuropeptide Y and cholinergic neurons following striatal excitotoxin lesions

We previously found that quinolinic acid striatal excitotoxin lesions result in a relative sparing of somatostatin and neuropeptide Y neurons. In the present study we examined dose-response effects of excitotoxins acting at the three subtypes of glutamate receptors: N-methyl-D-aspartate (AA1), quisqualate (AA2), and kainic acid (AA3). Concentrations of both somatostatin-like immunoreactivity (SLI) and neuropeptide a Y-like immunoreactivity (NPYLI) were compared with those of substance P-like immunoreactivity (SPLI) and GABA. Kainic acid (AA3), quisqualic acid (AA2), and AMPA (AA2) resulted in dose-dependent reductions in all four neurochemical markers examined, while N-methyl-D,L-aspartate (AA1) and quinolinic acid (AA1) resulted in relative sparing of SLI and NPYLI. At doses of each excitotoxin which resulted in comparable 50% reductions in both GABA and SPLI only N-methyl-D,L-aspartate and quinolinic acid had no significant effect on concentrations of SLI and NPYLI. The relative sparing of somatostatin-neuropeptide Y neurons was confirmed histologically by using histochemical staining for NADPH-diaphorase neurons combined with either Nissl stains, or immunohistochemical staining for enkephalin. Lesions with N-methyl-D-aspartate agonists resulted in preferential sparing of NADPH-diaphorase neurons while these neurons were more vulnerable than other neurons to kainic acid or AMPA. Choline acetyltransferase neurons were relatively spared, as compared with other neurons, by agents acting at all three glutamate receptor subtypes. N-methyl-D,L-aspartate lesions were blocked with MK-801, while there was no effect on quisqualic acid or kainic acid lesions. The relative sparing of somatostatin-neuropeptide Y neurons following striatal excitotoxin lesions with N-methyl-D-aspartate (AA1) agonists probably reflects a paucity of AA1 receptors on these neurons. Since these neurons are also spared in Huntington's disease, excitotoxins acting at the N-methyl-D-aspartate (AA1) site provide an improved neurochemical model of this illness.

Developmental changes of neuropeptides and amino acids in baboon cortex

The pattern of developmental changes in concentrations of substance P, somatostatin and neuropeptide Y immunoreactivity and amino acids was studied in baboon cortex. Samples of occipital or frontal neocortex were obtained from preterm (100-105 days gestation), near-term (170-176 days gestation), and young adult animals. Substance P concentrations were low at preterm, highest at near-term, and then declined to adult levels. Neuropeptide Y and somatostatin immunoreactivity increased steadily across the three age groups. Concentrations of aspartate and gamma-aminobutyric acid (GABA) also increased progressively from preterm to adulthood, whereas glutamate concentrations showed small increases that were not statistically significant. Concentrations of taurine and alanine were highest preterm and declined progressively to adulthood. Levels of neuropeptides and amino acids show distinct patterns of change during development of neocortex in the baboon.

Systemic approaches to modifying quinolinic acid striatal lesions in rats

Quinolinic acid (QA) is an endogenous excitotoxin present in mammalian brain that reproduces many of the histologic and neurochemical features of Huntington's disease (HD). In the present study we have examined the ability of a variety of systemically administered compounds to modify striatal QA neurotoxicity. Lesions were assessed by measurements of the intrinsic striatal neurotransmitters substance P, somatostatin, neuropeptide Y, and GABA. Histologic examination was performed with Nissl stains. The antioxidants ascorbic acid, beta-carotene, and alpha-tocopherol administered s.c. for 3 d prior to striatal QA lesions had no significant effect. Other drugs were administered i.p. 1/2 hr prior to QA striatal lesions. The following were ineffective in blocking QA excitotoxicity: allopurinol, 50 and 100 mg/kg; ketamine, 75 mg/kg; nimodipine, 2.4, and 10 mg/kg; baclofen, 10 mg/kg; 2-amino-5-phosphonovalerate, 50 mg/kg; and 2-amino-7-phosphonoheptanoate, 50 mg/kg. Oral taurine administration for 4 weeks resulted in significantly increased levels of brain taurine but had no significant effect in blocking QA neurotoxicity. Systemic administration of the noncompetitive N-methyl-D-aspartate (NMDA) antagonist MK-801 resulted in a dose-responsive protection against QA toxicity, with complete block at a dose of 4 mg/kg. If the pathogenesis of HD involves QA or another excitotoxin acting at the NMDA receptor, it is possible that MK-801 could retard the degenerative process.

Distribution of galanin-like immunoreactivity in baboon brain

Galanin-like immunoreactivity (GLI) was measured in baboon brains using a recently developed radioimmunoassay. Concentrations were measured in 10 cortical regions, hippocampus and 20 subcortical regions. The highest concentrations were in the median eminence, followed by hypothalamus, locus ceruleus, periaqueductal grey, bed nucleus of the stria terminalis, septum, amygdala and substantia innominata. Substantial amounts were also measurable in the inferior olive, basal ganglia and thalamus with very low levels in cerebellum. In cerebral cortex, concentrations were lowest in occipital cortex and highest in dorsolateral frontal cortex. Hippocampal concentrations were higher than those in cerebral cortex. Concentrations of GLI in cerebral cortex were significantly correlated with choline acetyltransferase activity and substance P immunoreactivity but not with concentrations of somatostatin or neuropeptide Y. Approximately half the GLI coeluted with porcine standards while half corresponded to a lower molecular weight species on gel permeation chromatography. With reverse phase high performance liquid chromatography (HPLC) the majority of the immunoreactivity eluted just in front of the porcine standard with a smaller amount coeluting with the porcine standard. These results show a widespread distribution of GLI in primate brain and are in accord with previous immunocytochemical studies.

Amino acid and neuropeptide neurotransmitters in Huntington's disease cerebellum

Several neuropathologic studies have suggested that there may be pathologic involvement of the cerebellum in Huntington's disease (HD). To investigate this further, we measured concentrations of neurotransmitter amino acids and the neuropeptides, somatostatin, neuropeptide Y and substance P, in HD cerebellar cortex and dentate nucleus. Twenty-seven pathologically confirmed cases of HD were compared with 20 controls. There were no significant changes in concentrations were significantly increased by 21% in HD cerebellar cortex. In the dentate nucleus, there were small significant increases of neuropeptide Y-like immunoreactivity and substance P-like immunoreactivity. The meaning of the neurotransmitter changes found is unclear: however, the lack of change in GABA and glutamate concentrations argues against a substantial loss of intrinsic cerebellar neurons.

Somatostatin and neuropeptide Y concentrations in pathologically graded cases of Huntington's disease

Somatostatin and neuropeptide Y concentrations have previously been reported to be increased in the basal ganglia in Huntington's disease (HD). In the present study we have extended these findings by examining both somatostatin-like immunoreactivity (SLI) and neuropeptide Y-like immunoreactivity (NPYLI) in cases of HD, which were graded according to the severity of pathological degeneration in the striatum. In addition, we surveyed a large number of subcortical nuclei and cortical regions for alterations. Both SLI and NPYLI were significantly increased about threefold in the caudate, putamen, and the nucleus accumbens. Increases in mild and severe grades were similar, which is consistent with a relative but not absolute sparing of striatal aspiny neurons in which somatostatin and neuropeptide Y are colocalized. Significant increases of NPYLI were also found in the external pallidum, subthalamic nucleus, substantia nigra compacta, claustrum, anterior and dorsomedial thalamus, bed nucleus of the stria terminalis, and locus ceruleus. SLI was significantly increased in the external pallidum, red nucleus, and locus ceruleus. Measurements of both neuropeptides were made in 24 regions of the cerebral cortex. Significant increases in both NPYLI and SLI were found in the frontal cortex (Brodmann areas 6, 8, 9, 10, 11, and 45) and temporal cortex (Brodmann area 21), whereas NPYLI was also increased in Brodmann areas 12, 20-22, 25, and 42. Alterations in the cerebral cortex were as pronounced in cases with mild striatal pathological changes as in those with severe striatal pathological changes. These alterations may occur early in HD and could reflect a selective sparing of somatostatin-neuropeptide Y cortical neurons combined with cortical atrophy.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of galanin-like immunoreactivity in the rat brain: effects of neonatal glutamate treatment

Concentrations of the neuropeptide, galanin, were measured using a newly characterized radioimmunoassay in brain regions of adult male rats treated neonatally with monosodium glutamate. Galanin-like immunoreactivity (galanin-IR) was significantly reduced 57% in the median eminence, 15% in the medial basal hypothalamus, and 27% in the septal region when compared to untreated littermates. Concentrations of galanin-IR were reduced 22% in the preoptic region and unchanged in the parietal cortex. These studies suggest that glutamate-sensitive, galanin-containing neurons in the arcuate nucleus project to regions of the basal forebrain of the rat in addition to the median eminence. The galanin projection from the arcuate nucleus to the median eminence suggests that this peptide plays a role in the regulation of anterior pituitary hormone secretion.

A detailed examination of substance P in pathologically graded cases of Huntington's disease

Substance P concentrations have been found to be reduced in the basal ganglia in Huntington's disease (HD). In order to further examine this finding in the present study we measured substance P-like immunoreactivity (SPLI) in cases of HD which had been graded as to the severity of pathological changes in the striatum. Marked significant reductions of SPLI were found in all striatal nuclei which were significantly correlated with the percentage of neuronal loss in the varying pathologic grades. Similar changes were found in the projection sites of striatal substance P neurons, the globus pallidus interna and the substantia nigra. These changes are consistent with a loss of striatal substance P containing projection neurons in HD. Significant reductions in SPLI were also found in the external pallidum, bed nucleus of the stria terminalis and the subthalamic nucleus. Small significant increases in SPLI (20-30%) were found in 3 frontal cortical regions (Brodmann areas 6, 8 and 9). The finding of neurochemical changes in the subthalamic nucleus is of particular interest since lesions in this nucleus are known to result in chorea and therefore might contribute to the chorea which is a cardinal symptom of HD.

Replication of the neurochemical characteristics of Huntington's disease by quinolinic acid

Huntington's disease (HD) is an autosomal dominant neurological disorder characterized by progressive chorea, cognitive impairment and emotional disturbance. The disease usually occurs in midlife and symptoms progress inexorably to mental and physical incapacitation. It has been postulated that an excitotoxin is involved in the pathogenesis of HD. Schwarcz and colleagues have shown that quinolinic acid (QA) can produce axon-sparing lesions similar to those observed in HD. The lesions result in a depletion of neurotransmitters contained within striatal spiny neurones, for example gamma-aminobutyric acid (GABA), while dopamine is unaffected. Recently, we and others have demonstrated that in HD striatum there is a paradoxical 3-5-fold increase in both somatostatin and neuropeptide Y which is attributable to selective preservation of a subclass of striatal aspiny neurones in which these peptides are co-localized. In the present study we demonstrate that lesions due to quinolinic acid closely resemble those of HD as they result in marked depletions of both GABA and substance P, with selective sparing of somatostatin/neuropeptide Y neurones. Lesions produced by kainic acid (KA), ibotenic acid (IA) and N-methyl-D-aspartate (MeAsp) were unlike those produced by QA, as they affected all cell types without sparing somatostatin/neuropeptide Y neurones. These results suggest that QA or a similar compound could be responsible for neuronal degeneration in HD.