Effects of polyamines on NMDA-induced currents in rat hippocampal neurons: a whole-cell and single-channel study

Endogenous antagonists of N-methyl-d-aspartate receptor in schizophrenia

Schizophrenia is a chronic neuropsychiatric brain disorder that has devastating personal impact and rising healthcare costs. Dysregulation of glutamatergic neurotransmission has been implicated in the pathobiology of the disease, attributed largely to the hypofunction of the N-methyl-d-aspartate (NMDA) receptor. Currently, there is a major gap in mechanistic analysis as to how endogenous modulators of the NMDA receptors contribute to the onset and progression of the disease. We present a systematic review of the neurobiology and the role of endogenous NMDA receptor antagonists in animal models of schizophrenia, and in patients. We discuss their neurochemical origin, release from neurons and glia with action mechanisms, and functional effects, which might contribute toward the impairment of neuronal processes underlying this complex pathological state. We consider clinical evidence suggesting dysregulations of endogenous NMDA receptor in schizophrenia, and highlight the pressing need in future studies and emerging directions, to restore the NMDA receptor functions for therapeutic benefits.

Polyamines as Snake Toxins and Their Probable Pharmacological Functions in Envenomation

While decades of research have focused on snake venom proteins, far less attention has been paid to small organic venom constituents. Using mostly pooled samples, we surveyed 31 venoms (six elapid, six viperid, and 19 crotalid) for spermine, spermidine, putrescine, and cadaverine. Most venoms contained all four polyamines, although some in essentially trace quantities. Spermine is a potentially significant component of many viperid and crotalid venoms (≤0.16% by mass, or 7.9 µmol/g); however, it is almost completely absent from elapid venoms assayed. All elapid venoms contained larger molar quantities of putrescine and cadaverine than spermine, but still at levels that are likely to be biologically insignificant. As with venom purines, polyamines impact numerous physiological targets in ways that are consistent with the objectives of prey envenomation, prey immobilization via hypotension and paralysis. Most venoms probably do not contain sufficient quantities of polyamines to induce systemic effects in prey; however, local effects seem probable. A review of the pharmacological literature suggests that spermine could contribute to prey hypotension and paralysis by interacting with N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, nicotinic and muscarinic acetylcholine receptors, γ-Aminobutyric acid (GABA) receptors, blood platelets, ryanodine receptors, and Ca²⁺-ATPase. It also blocks many types of cation-permeable channels by interacting with negatively charged amino acid residues in the channel mouths. The site of envenomation probably determines which physiological targets assume the greatest importance; however, venom-induced liberation of endogenous, intracellular stores of polyamines could potentially have systemic implications and may contribute significantly to envenomation sequelae.

Selective loss of NMDA receptor NR1 subunit isoforms in Alzheimer's disease

Previous work had shown that the ratio of NMDA receptor NR1 subunit mRNA transcripts containing an N-terminal splice cassette to those that do not is markedly lower in regions of the Alzheimer's disease (AD) brain that are susceptible to pathological damage, compared with spared regions in the same cases or homotropic regions in controls. To elucidate the origins of this difference in proportionate expression, we measured the absolute levels of each of the eight NR1 transcripts by quantitative internally standardized RT-PCR assay. Expression of transcripts with the cassette was strongly attenuated in susceptible regions of Alzheimer's brain, whereas expression of non-cassette transcripts differed little from that in controls. The expression of other NR1 splice variants was not associated with pathology relevant to disease status, although some combinations of splice cassettes were well maintained in AD cases. The population profile of NR1 transcripts in occipital cortex differed from the profiles in other brain regions studied. Western analysis confirmed that the expression of protein isoforms containing the N-terminal peptide was very low in susceptible areas of the Alzheimer's brain. Cells that express NR1 subunits with the N-terminal cassette may be selectively vulnerable to toxicity in AD.

Involvement of voltage- and ligand-gated Ca2+ channels in the neuroexcitatory and synergistic effects of putative uremic neurotoxins

BACKGROUND

Renal failure has been viewed as a state of cellular calcium toxicity due to the retention of small fast-acting molecules. We have tested this hypothesis and identified potentially neuroexcitatory compounds among a number of putative uremic neurotoxins by examining the acute in vitro effects of these compounds on cultured central neurons. The in vitro neuroexcitatory and synergistic effects of guanidinosuccinate and spermine were also examined in vivo.

METHODS

The acute effects of 17 candidate uremic neurotoxins on murine spinal cord neurons in primary dissociated cell culture were investigated using the tight-seal whole-cell recording technique. The compounds studied comprised low-molecular-weight solutes like urea, indoles, guanidino compounds, polyamines, purines and phenoles, homocysteine, orotate, and myoinositol. Currents evoked by these compounds were further examined using various ligand- and voltage-gated ion channel blockers. The acute in vivo effects of guanidinosuccinate and spermine were behaviorally assessed following their injection in mice.

RESULTS

It was shown that 3-indoxyl sulfate, guanidinosuccinate, spermine, and phenol evoked significant whole-cell currents. Inward whole-cell current evoked by 3-indoxyl sulfate was not blocked by any of the applied ligand- or voltage-gated ion channel blockers, and the compound appeared to influence miscellaneous membrane ionic conductances, probably involving voltage-gated Ca2+ channels as well. Phenol-evoked outward whole-cell currents were at least partly due to the activation of voltage-gated K+ channels, but may also involve a variety of other ionic conductances. On the other hand, inward whole-cell currents evoked by guanidinosuccinate and spermine were shown to be due to specific interaction with voltage- and ligand-gated Ca2+ channels. Guanidinosuccinate-evoked current was caused by activation of N-methyl-d-aspartate (NMDA) receptor-associated ion channels. Low (micromol/L) concentrations of spermine potentiated guanidinosuccinate-evoked current through the action of spermine on the polyamine binding site of the NMDA receptor complex, whereas current evoked by high (mmol/L) concentrations of spermine alone involved direct activation of voltage-gated Ca2+ channels. Finally, intracerebroventricular administration of 0.25 micromol/L spermine potentiated clonic convulsions induced by guanidinosuccinate. These neuroexcitatory and synergistic effects of guanidinosuccinate and spermine could take place at pathophysiologic concentrations.

CONCLUSION

The observed in vitro and in vivo effects of uremic retention solutes suggest that the identified compounds could play a significant role in uremic pathophysiology. Some of the compounds tested displayed in vitro and in vivo neuroexcitatory effects that were mediated by ligand- and voltage-gated Ca2+ channels. The findings suggest a mechanism for the involvement of calcium toxicity in the central nervous system complications in renal failure with particular reference to guanidinosuccinate and spermine.

Glutamate(NMDA) receptor NR1 subunit mRNA expression in Alzheimer's disease

We analyzed the expression profile of two NMDAR1 mRNA isoform subsets, NR1(0XX) and NR1(1XX), in discrete regions of human cerebral cortex. The subsets are characterized by the absence or presence of a 21-amino acid N-terminal cassette. Reverse transcription polymerase chain reaction for NR1 isoforms was performed on total RNA preparations from spared and susceptible regions from 10 pathologically confirmed Alzheimer's disease (AD) cases and 10 matched controls. Primers spanning the splice insert yielded two bands, 342 bp (NR1(0XX)) and 405 bp (NR1(1XX)), on agarose gel electrophoresis. The bands were visualized with ethidium and quantified by densitometry. NR1(1XX) transcript expression was calculated as a proportion of the NR1(1XX) + NR1(0XX) total. Values were significantly lower in AD cases than in controls in mid-cingulate cortex, p < 0.01, superior temporal cortex, p < 0.01 and hippocampus, p approximately 0.05. Cortical proportionate NR1(1XX) transcript expression was invariant over the range of ages and areas of controls tested, at approximately 50%. This was also true for AD motor and occipital cortex. Proportionate NR1(1XX) expression in AD cingulate and temporal cortex was lower at younger ages and increased with age: this regression was significantly different from that in the homotropic areas of controls. Variations in NR1 N-terminal cassette expression may underlie the local vulnerability to excitotoxic damage of some areas in the AD brain. Alternatively, changes in NR1 mRNA expression may arise as a consequence of the AD disease process.

Mechanism of cGMP-gated channel block by intracellular polyamines

Polyamines block the retinal cyclic nucleotide-gated channel from both the intracellular and extracellular sides. The voltage-dependent mechanism by which intracellular polyamines inhibit the channel current is complex: as membrane voltage is increased in the presence of polyamines, current inhibition is not monotonic, but exhibits a pronounced damped undulation. To understand the blocking mechanism of intracellular polyamines, we systematically studied the endogenous polyamines as well as a series of derivatives. The complex channel-blocking behavior of polyamines can be accounted for by a minimal model whereby a given polyamine species (e.g., spermine) causes multiple blocked channel states. Each blocked state represents a channel occupied by a polyamine molecule with characteristic affinity and probability of traversing the pore, and exhibits a characteristic dependence on membrane voltage and cGMP concentration.

Mutation of structural determinants lining the N-methyl-D-aspartate receptor channel differentially affects phencyclidine block and spermine potentiation and block

Spermine and other endogenous polyamines potentiate, block and permeate the N-methyl-D-aspartate receptor channel. To identify structural determinants of the N-methyl-D-aspartate channel that mediate spermine's actions, we generated mutant receptors with asparagine (N) to glutamine (Q) or arginine (R) substitutions in the selectivity filter of the channel. We demonstrate that mutation of the three critical asparagines in this domain differentially affects block by phencyclidine and both potentiation and block by spermine. N-to-Q and N-to-R mutations in the N site of the NR1 subunit (N598 in NR1(011), N619 in NR1(100)) and N-to-Q mutations in the N and N + 1 sites (N595 and N596 in NR2A, respectively) of the NR2 subunit (Q/NN, R/NN, N/QN, N/NQ, Q/QN and Q/NQ receptors) reduced affinity for phencyclidine. The Q/NN receptor showed markedly reduced potentiation by spermine, with little or no change in spermine block. The R/NN receptor showed markedly reduced spermine potentiation and affinity for spermine at its block site. The N/QN, N/NQ and Q/QN mutant receptors showed somewhat enhanced spermine block, while the Q/ NQ double mutant exhibited significantly more enhanced spermine block. Thus, the asparagine residues critical to Ca2+ permeability and Mg2+ block of N-methyl-D-aspartate channels are also critical to block by spermine and phencyclidine. To examine the interaction of spermine and phencyclidine within the channel, we performed competition studies. Spermine appeared to compete with phencyclidine for binding to the receptor; however, blocks by phencyclidine and by spermine were not additive. The findings suggest that spermine can bind to a site in the external vestibule of the channel to impede phencyclidine binding, but allow Na+ influx.

Spermine and arcaine block and permeate N-methyl-D-aspartate receptor channels

Polyamines such as spermine are thought to be endogenous regulators of NMDA (N-methyl-D-aspartate)-type glutamate receptors. Polyamine block of NMDA receptors was studied in excised outside-out patches from rat hippocampal neurons and Xenopus oocytes expressing recombinant receptors. Extracellular spermine and arcaine reduced NMDA single-channel conductance in a voltage-dependent manner, with partial relief of block evident at large inside negative membrane potentials. Reducing extracellular Na+ concentration increased the apparent affinities for spermine and arcaine, indicating strong interaction between spermine and permeant ions. Internal spermine also blocked NMDA channels in a voltage-dependent manner, with relief of block evident at large inside positive potentials. The Woodhull model of channel block by an impermeant ion adequately described the actions of external spermine from -60 to +60 mV, but failed for more negative potentials. Eyring rate theory for a permeable blocker with two barriers and one binding site adequately described the voltage-dependent block and relief from block by both external and internal spermine over the range of -120 to +60 mV. These findings indicate that polyamines block and permeate neuronal NMDA receptor channels from the extracellular and intracellular sides, although sensitivity to internal spermine is probably too low to be physiologically relevant.

Blockade of a retinal cGMP-gated channel by polyamines

The cyclic nucleotide-gated (CNG) channel in retinal rods converts the light-regulated intracellular cGMP concentration to various levels of membrane potential. Blockade of the channel by cations such as Ca2+ and Mg2+ lowers its effective conductance. Consequently, the membrane potential has very low noise, which enables rods to detect light with extremely high sensitivity. Here, we report that three polyamines (putrescine, spermidine, and spermine), which exist in both the intracellular and extracellular media, also effectively block the CNG channel from both sides of the membrane. Among them, spermine has the greatest potency. Extracellular spermine blocks the channel as a permeant blocker, whereas intracellular spermine appears to block the channel in two conformations-one permeant, and the other non- (or much less) permeant. The membrane potential in rods is typically depolarized to approximately -40 mV in the dark. At this voltage, K1/2 of the CNG channel for extracellular spermine is 3 microM, which is 100-1,000-fold higher affinity than that of the NMDA receptor-channel for extracellular spermine. Blockade of the CNG channel by polyamines may play an important role in suppressing noise in the signal transduction system in rods.

Spermine/spermidine is expressed by retinal glial (Müller) cells and controls distinct K+ channels of their membrane

There is recent evidence that polyamines such as spermine (spm) and spermidine (spd) may act as endogenous modulators of the activity of inwardly rectifying K+ channels. This type of K+ channels is abundantly expressed by retinal glial (Müller) cells where they are involved in important glial cell functions such as the clearance of excess extracellular K+ ions. This prompted us to study the following questions, i) do mammalian Müller cells contain endogenous spm/spd?; ii) do Müller cells possess the enzymes (e.g., ornithine decarboxylase, ODC) necessary to produce spm/spd?; and iii) does application of exogenous spm/spd exert specific effects onto inwardly rectifying K+ channels of Müller cells? Immunocytochemical studies were performed on histological sections of guinea-pig, rabbit, porcine, and human retinae, and on enzymatically dissociated Müller cells. Whole-cell and patch-clamp recordings were performed on enzymatically dissociated porcine and guinea-pig Müller cells. All above-mentioned questions could be answered with "yes." Specifically, the majority of Müller cells were labeled with antibodies directed to spm/spd, both within retinal sections and enzymatically isolated from retinal tissue. Müller cells in normal retinae express low levels of ODC but increase this expression markedly in cases of retinal pathology such as experimental epiretinal melanoma. Externally applied polyamines (1 mM) reduce (predominantly inward) whole-cell K+ currents, with the efficacies being spm > spd > put. If applied at the inside of membrane patches, spm (1 mM) blocks completely the outward currents through inwardly rectifying K+ channels but fails to affect the activity of large conductance, Ca2+-activated K+ channels. It is concluded that Müller cells contain endogenous channel-active polyamines, the synthesis of which may be up-regulated in pathological situations, and which may be involved in the control of both glial function and cell proliferation.

Two blocking sites of amino-adamantane derivatives in open N-methyl-D-aspartate channels

Using whole-cell patch-clamp techniques, we studied the blockade of open N-methyl-D-aspartate (NMDA) channels by amino-adamantane derivatives (AADs) in rat hippocampal neurons acutely isolated by the vibrodissociation method. The rapid concentration-jump technique was used to replace superfusion solutions. A kinetic analysis of the interaction of AAD with open NMDA channels revealed fast and slow components of their blockade and recovery. Mathematical modeling showed that these kinetic components are evidence for two distinct blocking sites of AADs in open NMDA channels. A comparative analysis of different simplest models led us to conclude that these AAD blocking sites can be simultaneously occupied by two blocker molecules. The voltage dependence of the AAD block suggested that both sites were located deep in the channel pore.

Modulation of neuronal mitochondrial membrane potential by the NMDA receptor: role of arachidonic acid

Activation of NMDA receptors in dissociated cerebellar granule cells reduced mitochondrial membrane potential (MMP), as measured by rhodamine 123 fluorescence in a flow cytometer. This effect was inhibited by several NMDA-receptor antagonists with the following rank order of potency: MK-801 > PCP > TCP > dextrorphan > dichlorokynurenic acid > D-AP5 > dextromethorphan. Neither spermine nor arcaine modified the NMDA-induced reduction in MMP, whereas ifenprodil and eliprodil inhibited this response in the micromolar range. The mechanism responsible for the alteration of MMP mediated by NMDA was studied. Mepacrine and dibucaine prevented the MMP reduction induced by NMDA, as did W13 (calmodulin antagonist). In contrast, this effect was not blocked by cyclooxygenase or lipooxygenase inhibitors, H7 (a protein kinase C inhibitor) or nitroarginine (nitric oxide synthase inhibitor). These data suggest a direct interaction between NMDA-receptor activation and arachidonic acid formation, and indicate that NMDA receptor-mediated effect on MMP could involve arachidonic acid.

Spermine does not compete with omega-conotoxin GVIA in the striatum radiatum of the hippocampal slice

The effect of spermine (Spm) and of omega-conotoxin GVIA (CTX) on the population excitatory postsynaptic potentials (pEPSP) in stratum radiatum of the CA1 area were compared. CTX decreased irreversibly the initial slope of pEPSP by 57%. Spm produced a maximum inhibition of 85% with an apparent dissociation constant of 0.85 mM and a maximum Hill coefficient larger than 3. The effect of Spm was mostly reversible. Preincubation with Spm did not protect the slice from the irreversible effect of CTX suggesting that they interact with different sites. Since CTX and Spm inhibited pEPSPs with very different affinities and reversibilities a kinetic model was developed to compare their effects. This model relates the inhibitors' binding to presynaptic voltage-activated Ca2+ channels (VACC) with inhibition of pEPSP. The model suggest that: all CTX and Spm effects can be explained by inhibition of VACC. Spm and CTX do not compete for the same site. CTX inhibits 20% (N-type) and Spm 40% of channels (probably the Q-type). More than three Spm molecules bind per one channel molecule, while one CTX is sufficient to inhibit channel function. The model also illustrates that the inhibitor concentration-pEPSP inhibition curves display a Hill coefficient similar to that for inhibitor binding.

Interactions of polyamines with ion channels

Endogenous polyamines, in particular spermine, have been found to cause block and modulation of a number of types of ion channel. Intracellular spermine is responsible for intrinsic gating and rectification of strong inward rectifier K+ channels by directly plugging the ion channel pore. These K+ channels control the resting membrane potential in both excitable and non-excitable cells, and control the excitability threshold in neurons and muscle cells. Intracellular spermine causes inward rectification at some subtypes of Ca2+-permeable glutamate receptors in the central nervous system, again by plugging the receptor channel pore, and spermine can even permeate the ion channel of these receptors. Extracellular spermine has multiple effects at the N-methyl-d-aspartate (NMDA) subtype of glutamate receptor, including stimulation that increases the size of NMDA receptor currents, and voltage-dependent block. A number of polyamine-conjugated arthropod toxins and synthetic polyamine analogues are potent antagonists of glutamate receptors, and represent new tools with which to study these receptors. Interactions of polyamines with other types of cation channels have been reported. This area of research represents a new biology and a new pharmacology of polyamines.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

N1-dansyl-spermine and N1-(n-octanesulfonyl)-spermine, novel glutamate receptor antagonists: block and permeation of N-methyl-D-aspartate receptors

The effects of several N-sulfonyl-polyamines, including N1-dansyl-spermine (N1-DnsSpm) and N1-(n-octanesulfonyl)-spermine (N1-OsSpm), were studied at recombinant N-methyl-D-aspartate (NMDA) receptors expressed in Xenopus laevis oocytes. N1-DnsSpm and N1-OsSpm inhibited NMDA receptors and were approximately 1000-fold more potent than spermine in oocytes voltage-clamped at -70 mV. Block by N1-DnsSpm and N1-OsSpm was strongly voltage dependent, being more pronounced at hyperpolarized membrane potentials. With the Woodhull model of voltage-dependent channel block, the values of Kd(0) were 779 microM, 882 microM, and 7.4 mM and those of z delta were 2.58, 2.57, and 1.07 for N1-DnsSpm, N1-OsSpm, and spermine, respectively. This suggests that an increase in the voltage dependence of block together with an increase in affinity contributes to the increased potencies of N1-DnsSpm and N1-OsSpm compared with spermine. Sensitivity to N1-DnsSpm was reduced by mutation NR1(N616Q) and was increased by mutations NR1(N616G) and NR2A(N615G). The NR1(N616G) and NR2A(N615G) mutations decreased the Kd(0) value of N1-DnsSpm without affecting z delta, whereas the NR1(N616Q) mutation reduced z delta. These mutations may alter the accessibility of part of the polyamine binding site within the channel pore or directly alter the properties of that site. Block by N1-DnsSpm (0.3 microM) was almost complete at -100 mV, and there was no relief of block at extreme negative membrane potentials (-100 to -200 mV) at wild-type NR1/NR2A channels. In contrast, block by N1-DnsSpm was partially relieved at extreme negative potentials at receptors containing NR1(N616G) or NR2A(N615G), suggesting that N1-DnsSpm can permeate these mutant channels but not wild-type NR1/NR2A channels. This is hypothesized to be due to an increase in the pore size of channels containing NR1(N616G) or NR2A(N615G), which allows passage of the bulky head group of N1-DnsSpm. In contrast to N1-DnsSpm, N1-OsSpm could easily permeate wild-type NR1/NR2A channels, presumably because the head group of N1-OsSpm can pass through the narrowest part of the channel pore. N-Sulfonyl-polyamines such as N1-DnsSpm and N1-OsSpm represent a new class of polyamine antagonists with which to study glutamate receptor ion channels.

Modulation and block of ion channels: a new biology of polyamines

The endogenous polyamines, spermine, spermidine, and putrescine have effects on several types of cation channels. Intracellular polyamines, in particular spermine, contribute to intrinsic gating and rectification of strong inward rectifier K+ channels. Intracellular spermine is also responsible for inward rectification of some types of Ca(2+)-permeable AMPA and kainate receptors. Spermine has a number of effects on the activity of the NMDA subtype of glutamate receptor, involving two or more extracellular polyamine binding sites on the NMDA receptor. In K+ channels and glutamate receptors, some of the amino acids in the receptor/channel structure that influence to polyamines have been identified, leading to a partial understanding of the effects of polyamines at a molecular level. Block of K+ channels by intracellular polyamines is likely to be an important receptors by intracellular spermine and modulation by extracellular spermine may affect excitability and the influx of Ca2+ in neurons and glial cells of the nervous system.

Differential effects of histamine on the N-methyl-D-aspartate channel in hippocampal slices and cultures

The effect of histamine on N-methyl-D-aspartate currents was investigated in pyramidal neurons in the CA1 region of acute hippocampal slices from juvenile rats. The objective was to compare histamine effects in the slice with those previously reported in acutely dissociated and cultured hippocampal neurons. Micromolar concentrations of histamine had no effect on N-methyl-D-aspartate mediated excitatory postsynaptic currents in the slice, in contrast to the large enhancement seen in culture under identical conditions. However, millimolar concentrations of histamine blocked these currents both in the slice and in culture. Possible reasons for the lack of enhancement in the slice were explored as follows. (1) Histamine could not penetrate the slice or was already present at high concentrations inside the slice. This was tested by recording N-methyl-D-aspartate currents elicited in outside-out patches pulled from the somas of CA1 slice neurons. Histamine still had no effect in patches, whereas the corresponding experiment for cultured neurons showed robust enhancement. (2) Slices release an endogenous ligand that binds with high affinity to the histamine site on the N-methyl-D-aspartate receptor, blocking its activation. This was tested by superfusing cultures with supernatant from homogenized slice tissue. Histamine enhancement was maintained in these cultures. (3) CA1 slices and cultures express different N-methyl-D-aspartate receptor subtypes. The reverse transcription-polymerase chain reaction technique was used to examine the expression of messenger RNA encoding N-methyl-D-aspartate receptor subunits in the two systems. No difference was found in the whole-tissue expression of messenger RNA for the NR2A, 2B or 2C subunits or for the eight known splice variants of the NR1 subunit. It is hypothesized that the differential enhancing effect of histamine in slices and culture involves posttranslational modifications or other factors that modulate the N-methyl-D-aspartate receptor/ion channel according to its environment.

Excitatory amino acid-induced currents in rat septal cholinergic neurons in culture

Whole-cell voltage-clamp recordings were used to study excitatory amino acid-induced currents in neurons isolated from the septum of fetal rat brains. The neurons were cultured for more than four weeks on a feeder layer composed of glial cells obtained from the septal region. Septal neurons were either fusiform, triangular or multipolar and 83% of cells showed acetylcholinesterase activity. L-Glutamate, kainate, quisqualate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) applied by local perfusion produced inward currents (Iglu, Ikai, Iquis and IAMPA, respectively) at -44mV which increased in amplitude with increasing concentration of agonist; they desensitized when induced at higher concentrations except for the Ikai. The EC50s for the peak Ikai and sustained Iglu, Iquis and IAMPA were 55, 13, 0.39 and 3.5 microM, respectively. 6-Cyano-7-nitroquinoxaline-2,3-dione (CNQX) depressed Ikai and IAMPA evoked at a concentration of 10 microM (IC50s: 0.58 and 0.84 microM, respectively). Schild analysis for the CNQX action on Ikai gave a dissociation constant of 0.27 microM for CNQX. n-Methyl-D-aspartate (NMDA) (with glycine, 3 microM) produced an inward current (INMDA) at -44 mV whose peak amplitude enhanced with increased concentrations (EC50 = 32 microM). INMDA was potentiated by glycine (EC50 = 0.15 microM) and inhibited by D-2-amino-5-phosphovalerate (IC50 = 9.9 microM for INMDA evoked at a concentration of 50 microM). MK-801 (0.1-10 microM) inhibited INMDA in a dose- and use-dependent manner. INMDA was (0.1-10 microM) inhibited INMDA in a dose- and use-dependent manner. INMDA was potentiated by spermine (EC50 = 247 microM; 91% increase at 1mM) in a manner independent of holding potential (VH). INMDA was inhibited by Mg2+ and Zn2+ (IC50 = 673 and 39 microM, respectively, at -44 mV) in a manner dependent on VH; the magnitudes of a depolarization required for an e-fold increase in their IC50s in a range of -64 to -24 mV were 16 and 22 mV, respectively. The action of Zn2+ was independent of VH > -24 mV. Current-voltage relations for Ikai, Iquis and IAMPA exhibited outward rectification, while that of INMDA showed a region of negative conductance at VH < -30 mV, which disappeared in a Mg(2+)-free solution. Reversal potentials for Ikai, Iquis, IAMPA and INMDA were close to 0 mV, indicating the involvement of non-specific cation channels. Increasing extracellular Ca2+ concentration from 2.4 to 30 mM did not affect the Ikai and Iquis, reversal potential showing negligible Ca2+ component, but shifted INMDA reversal potential to a more positive potential, yielding a ratio of Ca2+ permeability to that of monovalent cation to be 13. Cholinergic septal neurons in culture express non-NMDA-(AMPA/kainate-) and NMDA-type of glutamate receptor channels. Their properties were quantitatively similar to those of glutamate receptor channels on other types of neurons in the brain except for the actions of endogenous neuromodulators (Mg2+, Zn2+ and spermine) on NMDA receptor channels. It is suggested that NMDA receptor channels on different types of neurons may play a distinct role depending on a difference in the actions of these neuromodulators.

Glycine-independent and subunit-specific potentiation of NMDA responses by extracellular Mg2+

Extracellular Mg2+, which blocks NMDA channels in a voltage-dependent manner and increases the receptor's affinity for glycine, is shown here to potentiate NMDA responses at saturating glycine concentrations. This potentiation, induced by millimolar concentrations of Mg2+, is not mimicked by Ca2+ and Ba2+ and is voltage independent. The potentiation is variable in native receptors of cultured mouse central neurons; in recombinant receptors, it is "permitted" by the NR2B subunit and prevented by the NR1 splice variant containing an N-terminal insert. Mg2+ also induces a shift of the pH sensitivity of NMDA receptors. The similarity and nonadditivity of the effects of Mg2+ and spermine suggest that Mg2+ may be the physiological agonist acting at the subunit-specific spermine site.

Intracellular polyamines mediate inward rectification of Ca(2+)-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors that lack the glutamate receptor GluR2 subunit are Ca(2+)-permeable and exhibit inwardly rectifying current responses to kainate and AMPA. A proportion of cultured rat hippocampal neurons show similar Ca(2+)-permeable inwardly rectifying AMPA receptor currents. Inward rectification in these neurons was lost with intracellular dialysis and was not present in excised outside-out patches but was maintained in perforated-patch whole-cell recordings, suggesting that a diffusible cytoplasmic factor may be responsible for rectification. Inclusion of the naturally occurring polyamines spermine and spermidine in the recording pipette prevented loss of rectification in both whole-cell and excised-patch recordings; Mg2+ and putrescine were without effect. Inward rectification of Ca(2+)-permeable AMPA receptors may reflect voltage-dependent channel block by intracellular polyamines.

A photoaffinity probe for the polyamine site regulating the NMDA receptor

Polyamines produce on the NMDA-receptor channel activity a regulatory effect subsequent to their binding to specific sites on the receptor-complex. The photoactivatable polyamine derivative L-azidophenylspermine shows properties which suggest its potential as a photoaffinity probe to investigate the nature and topology of these sites. In the dark, its effect on the binding of tritiated N-(1-[thienyl]cyclohexyl)piperidine ([3H]TCP) to synaptosomal plasma membranes is similar to that of diaminodecane. Arcaine antagonizes both the effects of L-azidophenylspermine and diaminodecane on [3H]TCP binding. L-Azidophenylspermine competes in post-synaptic densities with tritiated spermidine for a unique binding site with an EC50 similar to that of spermine. Upon irradiation, L-azidophenyl-spermine incorporates into this material with a high efficiency to a level consistent with both the Bmax for tritiated spermidine and the estimated density of NMDA receptors in this fraction.

Control of proton sensitivity of the NMDA receptor by RNA splicing and polyamines

The function of the N-methyl-D-aspartate (NMDA)-preferring glutamate receptor can be regulated by extracellular pH, a process that may be important during ischemia in the brain or during seizures. Protons inhibit NMDA receptor function by 50 percent at pH 7.3 through interactions with the NR1 subunit, and both polyamines and NR1 exon 5 potentiate receptor function through relief of the tonic proton inhibition present at physiological pH. A single amino acid (lysine 211) was identified that mediates the effects of exon 5 in the rat brain. Electroneutral substitutions at this position restored pH sensitivity and, consequently, polyamine relief of tonic inhibition. This effect, together with the structural similarities between polyamines and the surface loop encoded by exon 5, suggest that exon 5 may act as a tethered pH-sensitive constitutive modulator of NMDA receptor function.

Polyamines antagonize N-methyl-D-aspartate-evoked depolarizations, but reduce Mg2+ block

This study utilized a grease-gap preparation to investigate the effects of polyamines on responses of CA1 hippocampal pyramidal cells to N-methyl-D-aspartate (NMDA) and on the block of the NMDA channel by Mg2+. In the absence of added Mg2+, 1,10-diaminodecane (0.1-1 mM) non-competitively antagonized NMDA-evoked depolarizations. Its antagonism slowly progressed to a stable value, was not use-dependent and did not reverse completely upon washout. Similar results were obtained with 100 microM spermine and 1 mM diethylenetriamine. Addition of 1 mM Mg2+ to the superfusion medium greatly reduced these effects. Conversely, the polyamines attenuated the blocking action of Mg2+. Postnatal treatment with alpha-difluoromethylornithine reduced the total polyamine content of area CA1 in 10- to 15-day-old rats almost to the adult level (although spermine content was unaffected). Mg2+ less potently antagonized NMDA-evoked depolarizations in slices from 10- to 15-day-old rats than in slices from adult rats, and this difference was unaffected by the alpha-difluoromethylornithine treatment. These results suggest (1) that there are rapid and slow components to the antagonism of NMDA-evoked depolarizations by polyamines, both of which may involve permeation of the polyamine into or through the NMDA channel: (2) that polyamine release in brain could modulate the Mg2+ sensitivity of responses to NMDA; and (3) that changes in the total content of endogenous polyamine do not explain developmental differences in the sensitivity of NMDA-evoked depolarizations to Mg2+.

The development of MK-801, kainate, AMPA, and muscimol binding sites in cat visual cortex

Previous work using homogenate binding has shown that the development of (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10imin e maleate (MK-801) binding in cat visual cortex increases from 21 days to 42 days, the height of the plastic period, and decreases in adulthood. We have studied the generality of this finding by examining the development of NMDA binding sites in several brain regions and by examining the development of other binding sites in the visual cortex. After confirming the original finding, we extended it by showing that the sensitivity of MK-801 binding sites to glutamate and glycine decreases when the cat becomes an adult. We then examined the regional specificity of MK-801 binding. Retinal binding did not change significantly with age. Binding in both visual cortex and hippocampus increased significantly from 7 days to 42 days regardless of whether binding was measured per milligram wet weight or per milligram protein. The decline from 42 days to adulthood was less dramatic in the hippocampus than in the visual cortex and was statistically significant only when binding was measured per milligram protein. Saturation analyses also showed a difference in the two structures. Bmax in the visual cortex, but not in the hippocampus, decreased from 42 days to adulthood. To determine whether these developmental changes were specific to MK-801 binding sites, we compared the age-dependent binding of MK-801, kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), and muscimol. Like MK-801, kainate binding increased from 7 days to 42 days and decreased from 42 days to adulthood. AMPA and muscimol binding showed a similar increase in binding from 7 days to 42 days but did not decrease significantly from 42 days to adulthood. Displacement experiments suggest that AMPA and kainate bind to separate sites. The 42-day peak in NMDA and kainate binding suggests that their associated receptors may have a role in determining the plastic period of visual cortex.

Spermine potentiation of recombinant N-methyl-D-aspartate receptors is affected by subunit composition

The present study shows that both the NR1 and NR2 subunits critically affect spermine potentiation of heteromeric recombinant N-methyl-D-aspartate receptors. NR1(011), the most prominent NR1 splice variant in rat forebrain, and NR1(100), prominent in midbrain, were expressed in Xenopus oocytes singly and in combination with NR2A, NR2B, and NR2C subunits. As for NR1(011) homomers, NR1(011)/NR2B receptors exhibited spermine potentiation by two mechanisms: by increasing glycine affinity and by increasing current through receptors with bound N-methyl-D-aspartate and glycine. NR1(011)/NR2A receptors exhibited only the increase in glycine affinity, and NR1(011)/NR2C receptors exhibited neither. As for NR1(100) homomers, NR1(100)/NR2B and NR1(100)/NR2A receptors exhibited spermine potentiation only by increasing the glycine affinity. Spermine produced no potentiation of NR1(100)/NR2C receptors. Thus, the NR2B subunit "permits" both forms of spermine potentiation, the NR2A subunit permits spermine potentiation only by increasing the glycine affinity, and th NR2C subunit permits neither form of potentiation. Spermine actions on NR1/NR2 showed little voltage dependence. These observations are of interest because the NR1 and NR2 subunits are differentially distributed and developmentally regulated. At early postnatal ages, NR2B subunit mRNA was more highly expressed than NR2A and NR2C mRNAs in hippocampus, neocortex, and caudate-putamen. These findings account for many of the observed differences among neurons in polyamine actions and suggest that these actions will vary in a cell-specific and age-related manner.

Blockade by ifenprodil of high voltage-activated Ca2+ channels in rat and mouse cultured hippocampal pyramidal neurones: comparison with N-methyl-D-aspartate receptor antagonist actions

1. The block by ifenprodil of voltage-activated Ca2+ channels was investigated in intracellular free calcium concentration ([Ca2+]i) evoked by 50 mM K+ (high-[K+]o) in Fura-2-loaded rat hippocampal pyramidal neurones in culture and on currents carried by Ba2+ ions (IBa) through Ca2+ channels in mouse cultured hippocampal neurones under whole-cell voltage-clamp. The effects of ifenprodil on voltage-activated Ca2+ channels were compared with its antagonist actions on N-methyl-D-aspartate- (NMDA) evoked responses in the same neuronal preparations. 2. Rises in [Ca2+]i evoked by transient exposure to high-[K+]o in our preparation of rat cultured hippocampal pyramidal neurones are mediated predominantly by Ca2+ flux through nifedipine-sensitive Ca2+ channels, with smaller contributions from nifedipine-resistant, omega-conotoxin GVIA-sensitive Ca2+ channels and Ca2+ channels sensitive to crude funnel-web spider venom (Church et al., 1994). Ifenprodil (0.1-200 microM) reversibly attenuated high-[K+]o-evoked rises in [Ca2+]i with an IC50 value of 17 +/- 3 microM, compared with an IC50 value of 0.7 +/- 0.1 microM for the reduction of rises in [Ca2+]i evoked by 20 microM NMDA. Tested in the presence of nifedipine 10 microM, ifenprodil (1-50 microM) produced a concentration-dependent reduction of the dihydropyridine-resistant high-[K+]o-evoked rise in [Ca2+]i with an IC50 value of 13 +/- 4 microM. The results suggest that ifenprodil blocks Ca2+ flux through multiple subtypes of high voltage-activated Ca2+ channels. 3. Application of the polyamine, spermine (0.25-5 mM), produced a concentration-dependent reduction of rises in [Ca2+]i evoked by high-[K+]o. The antagonist effects of ifenprodil 20 micro M on high-[K+]0-evoked rises in [Ca2+]. were attenuated by spermine 0.25 mM but not by putrescine 1 or 5 mM. In contrast,spermine 0.1 mM increased rises in [Ca2+]i evoked by NMDA and enhanced the ifenprodil (5 micro M) block of NMDA-evoked rises in [Ca2+]i.4. Similar results were obtained in mouse cultured hippocampal pyramidal neurones under whole-cell voltage-clamp. Ifenprodil attenuated both the peak and delayed whole-cell IB. with an IC% value of 18 +/- 2 micro M, whilst it attenuated steady-state NMDA-evoked currents with an IC50 of 0.8 +/- 0.2 micro M. Block of IBa by ifenprodil 10 JaM was rapid in onset, fully reversible and occurred without change in thecurrent-voltage characteristics of Ba. The ifenprodil block of IBa was enhanced on membrane depolarization and was weakly dependent on the frequency of current activation. Spermine 0.1 mM potentiated control NMDA-evoked currents but attenuated IB,. In agreement with the microspectrofluorimetric studies, co-application of spermine produced a small enhancement of the inhibitory effect of ifenprodil 10 micro M on NMDA-evoked responses whereas the reduction of I4 by ifenprodil 10 micro M in the presence of spermine was less than expected if the inhibitory effects of ifenprodil and spermine on IBa were simply additive.5. The results indicate that ifenprodil blocks high voltage-activated Ca2+ channels in rat and mouse cultured hippocampal pyramidal neurones. Although the Ca2+ channel blocking actions of ifenprodil are observed at higher concentrations than those associated with NMDA antagonist activity, Ca2+ channel blockade may contribute, at least in part, to the established neuroprotective and anticonvulsant properties of the compound.

Spermine depresses NMDA, K/AMPA and GABAA-mediated synaptic transmission in the rat hippocampal slice preparation

The effects of spermine, an endogenous polyamine, were examined in area CA1 of the rat hippocampal slice preparation. Spermine, at low millimolar concentrations, rapidly and potently depressed NMDA and K/AMPA-mediated population EPSPs, and GABA-mediated monosynaptic population IPSPs. These effects contrast with its well-known potentiation of NMDA currents at lower concentrations. Our results raise the possibility that the large intracellular stores of spermine that are released after various neural insults could act as an endogenous neuroprotective mechanism by limiting excessive calcium entry.

Mutagenesis rescues spermine and Zn2+ potentiation of recombinant NMDA receptors

Alternative splicing generates distinct forms of the NMDA receptor subunit NR1. NR1 subunits with an N-terminal insert (termed N1) form receptors in Xenopus oocytes with greatly reduced potentiation by spermine and Zn2+. Oocytes expressing NR1 receptors with N1 exhibited larger NMDA currents than oocytes expressing corresponding receptors without N1. In the present study, we used mutational analysis to investigate structural features of the N1 insert that control current amplitude and spermine and Zn2+ potentiation. Neutralization of positive charges in N1 rescued spermine and Zn2+ potentiation. Positive charges in N1 did not affect spermine or Zn2+ affinity. Neutralization of positive charges in N1 diminished the responses to the level of NR1 receptors lacking N1. The positively charged N1 may increase NMDA currents by causing a conformational change similar to that produced by spermine and Zn2+ in NR1 receptors lacking N1.

Splice variants of the N-methyl-D-aspartate receptor NR1 identify domains involved in regulation by polyamines and protein kinase C

The N-methyl-D-aspartate (NMDA) receptor NR1 gene encodes RNA that is alternatively spliced to generate at least seven variants. The variants arise from splicing in or out of three exons; one encodes a 21-amino acid insert in the N-terminal domain, and two encode adjacent sequences of 37 and 38 amino acids in the C-terminal domain. Splicing out of the second C-terminal exon deletes a stop codon and results in an additional open reading frame encoding an unrelated sequence of 22 amino acids before arriving at a second stop codon. We denote the NR1 variants by the presence or absence of the three alternatively spliced exons (from 5' to 3'); thus, NR1(111) has all three exons, NR1(000) has none, and NR1(100) has only the N-terminal exon. We report here electrophysiological characterization of six splice variants of the NR1 receptor expressed in Xenopus oocytes. NR1 receptors that lacked the N-terminal exon (NR1(000), NR1(010), and NR1(011)) exhibited a relatively high affinity for NMDA (EC50 approximately 13 microM) and marked potentiation by spermine. In contrast, those receptor variants with the N-terminal insert (NR1(100), NR1(101), and NR1(111)) showed a lower agonist affinity and little or no spermine potentiation at saturating glycine. All six variants showed spermine potentiation at low glycine and inhibition by spermine at more negative potentials. Variants differing only in the C-terminal domain differed little in agonist affinity and spermine potentiation. These findings indicate that the N-terminal insert either participates in agonist and polyamine binding domains or indirectly modifies their conformations. The splice variants differed in the extent to which they could be potentiated by activators of protein kinase C (PKC) from 3- to 20-fold. Presence of the N-terminal insert and absence of the C-terminal sequences increased potentiation by PKC. These findings identify the contributions of the separate polypeptide domains to modulation by polyamines and PKC and provide further support for the concept that subunit composition determines functional properties of NMDA receptors.