Hippocampal synaptic plasticity induced by excitatory amino acids includes changes in sensitivity to the calcium channel blocker, omega-conotoxin

Molecular characterization and embryonic expression of the family of N-methyl-D-aspartate receptor subunit genes in the zebrafish

We present the cloning of 10 N-methyl-D-aspartate (NMDA) receptor subunits from the zebrafish. These subunits fall into five subtypes, each containing two paralogous genes. Thus, we report two NMDAR1 genes (NR1.1 and NR1.2), and eight NMDAR2 genes, designated NR2A.1 and NR2A.2, NR2B.1 and NR2B.2, NR2C.1 and NR2C.2, and NR2D.1 and NR2D.2. The predicted sequences of the NR1 paralogs display 90% identity to the human protein. The NR2 subunits show less identity, differing most at the N- and C-termini. The NR1 genes are both expressed embryonically, although in a nonidentical manner. NR1.1 is found in brain, retina, and spinal cord at 24 hours postfertilization (hpf). NR1.2 is expressed in the brain at 48 hpf but not in the spinal cord. NR2 developmental gene expression varies: both paralogs of the NR2A are expressed at 48 hpf in the retina, only one paralog of the NR2B is expressed at low levels in the heart at 48 hpf. Neither of the NR2C is expressed embryonically. Both paralogs of the NR2D are expressed: 2D.1 is in the forebrain, retina, and spinal cord at 24 hpf, whereas the 2D.2 is only found in the retina. Our findings demonstrate that the zebrafish can serve as a useful model system for investigating the role of NMDA receptors in the development of the nervous system.

Pharmacological characterization of morphine-induced in vivo release of cholecystokinin in rat dorsal horn: effects of ion channel blockers

Previous studies indicate that an increased release of cholecystokinin (CCK) in response to morphine administration may counteract opioid-induced analgesia at the spinal level. In the present study we used in vivo microdialysis to demonstrate that systemic administration of antinociceptive doses of morphine (1-5 mg/kg, s.c.) induces a dose-dependent and naloxone-reversible release of CCK-like immunoreactivity (CCK-LI) in the dorsal horn of the spinal cord. A similar response could also be observed following perfusion of the dialysis probe for 60 min with 100 microM but not with 1 microM morphine. The CCK-LI release induced by morphine (5 mg/kg, s.c.) was found to be calcium-dependent and tetrodotoxin-sensitive (1 microM in the perfusion medium). Topical application of either the L-type calcium channel blocker verapamil (50 microg) or the N-type calcium channel blocker omega-conotoxin GVIA (0.4 microg) onto the dorsal spinal cord completely prevented the CCK-LI release induced by morphine (5 mg/kg, s.c.). Our data indicate that activation of L- and N-type calcium channels is of importance for morphine-induced CCK release, even though the precise site of action of morphine in the dorsal horn remains unclear. The present findings also suggest a mechanism for the potentiation of opioid analgesia by L- and N-type calcium channel blocking agents.

Toxins affecting calcium channels in neurons

Calcium enters the cytoplasm mainly via voltage-activated calcium channels (VACC), and this represents a key step in the regulation of a variety of cellular processes. Advances in the fields of molecular biology, pharmacology and electrophysiology have led to the identification of several types of VACC (referred to as T-, N-, L-, P/Q- and R-types). In addition to possessing distinctive structural and functional characteristics, many of these types of calcium channels exhibit differential sensitivities to pharmacological agents. In recent years a large number of toxins, mainly small peptides, have been purified from the venom of predatory marine cone snails and spiders. Many of these toxins have specific actions on ion channels and neurotransmitter receptors, and the toxins have been used as powerful tools in neuroscience research. Some of them (omega-conotoxins, omega-agatoxins) specifically recognize and block certain types of VACC. They have common structural backbones and some been synthesized with identical potency as the natural ones. Natural, synthetic and labeled calcium channel toxins have contributed to the understanding of the diversity of the neuronal calcium channels and their function. In particular, the toxins have been useful in the study of the role of different types of calcium channels on the process of neurotransmitter release. Neuronal calcium channel toxins may develop into powerful tools for diagnosis and treatment of neurological diseases.

Multiple Ca2+ channel types coexist to regulate synaptosomal neurotransmitter release

The regulation of excitation-secretion coupling by Ca2+ channels is a fundamental property of the nerve terminal. Peptide toxins that block specific Ca2+ channel types have been used to identify which channels participate in neurotransmitter release. Subsecond measurements of [3H]-glutamate and [3H]dopamine release from rat striatal synaptosomes showed that P-type channels, which are sensitive to the Agelenopsis aperta venom peptide omega-Aga-IVA, trigger the release of both transmitters. Dopamine (but not glutamate) release was also controlled by N-type, omega-conotoxin-sensitive channels. With strong depolarizations, where neither toxin was very effective alone, a combination of omega-Aga-IVA and omega-conotoxin produced a synergistic inhibition of 60-80% of Ca(2+)-dependent dopamine release. The results suggest that multiple Ca2+ channel types coexist to regulate neurosecretion under normal physiological conditions in the majority of nerve terminals. P- and N-type channels coexist in dopaminergic terminals, while P-type and a omega-conotoxin- and omega-Aga-IVA-resistant channel coexist in glutamatergic terminals. Such an arrangement could lend a high degree of flexibility in the regulation of transmitter release under diverse conditions of stimulation and modulation.

Glutamate induces long-term increase in the frequency of single N-methyl-D-aspartate channel openings in hippocampal CA1 neurons examined in situ

Long-term potentiation is currently a leading candidate for a physiological memory mechanism in CNS. The interpretation of this phenomenon is contradictory in many respects. However, there is clear evidence that long-term potentiation is critically dependent on activation of N-methyl-D-aspartate receptors. Recently it has been shown that extracellularly applied glutamate also induces long-lasting changes in the properties of synaptic transmission in the hippocampus that can be attributed to long-term potentiation. The involvement of presynaptic mechanisms has been reported. Here we demonstrate a definite increase in both the open time and open-state probability of N-methyl-D-aspartate-operated channels, induced by prolonged application of glutamate to hippocampal slices.

Glutamate and theta-rhythm stimulation selectively enhance NMDA component of EPSC in CA1 neurons of young rats

Using in situ whole-cell patch clamp of hippocampal CA1 pyramidal neurons we demonstrate that glutamate initiates processes resulting in an increase in the amplitude of the excitatory post-synaptic current (EPSC). In adult animals both, NMDA and non-NMDA components of the EPSC increase in parallel. In young animals only the NMDA component is increased. A similar enhancement of the EPSC can be achieved by the stimulation of excitatory synaptic inputs to CA1 neurons with the frequency of the theta-rhythm. EPSCs remain enhanced for more than 60 min. The selective enhancement of the NMDA component in young animals is inhibited by preincubation of slices with the NO-synthase blocker, N omega-nitro-L-arginine (NA) or by the NO-scavenger, hemoglobin.

Inositol phosphate regulation of voltage-dependent calcium channels in cerebellar granule neurons

The effects of intracellularly applied inositol phosphates on voltage-dependent calcium channel currents were assessed in rat cerebellar neurons using the whole-cell recording configuration of the patch-clamp technique. Intraneuronal perfusion of 10 microM inositol 1,4,5-trisphosphate (IP3) increased the amplitude of currents elicited by depolarization from a holding potential of -40 mV. IP3 did not modify current activation, but shifted the steady-state inactivation curve toward more positive values. The dose-response curve indicated an EC50 of 0.5 microM for IP3. Inositol 1,3,4,5-tetrakisphosphate (IP4), but not inositol 4,5,-bisphosphate, mimicked the effect of IP3. The effect of IP3 persisted in the presence of 100 micrograms/ml heparin and did not depend on intracellular calcium mobilization, as similar responses were not produced by 10 mM caffeine or by intrapipette calcium buffering at pCa 6 instead of pCa 7.7. Preincubation with omega-conotoxin led to a 55% inhibition of barium current; however, inhibition was reversed by IP3, which reestablished the control current amplitude. These results imply that IP3 and IP4 can elicit calcium entry by modifying both the gating characteristics and the pharmacological properties of voltage-dependent calcium channels.

Properties of vertebrate glutamate receptors: calcium mobilization and desensitization

Glutamate is now recognized as a major excitatory neurotransmitter in the vertebrate CNS, participating in a number of physiological and pathological processes. The importance of glutamate in the mobilization of intracellular Ca2+ as well as the relationship between excitatory and toxic properties has made it important to understand factors that regulate the responsivity of glutamate receptors. In recent years considerable insight has been gained about regulatory sites on NMDA receptors, with the recognition that these receptors are modulated by multiple endogenous and exogenous agents. Less is known about the regulation of responses mediated by AMPA, kainate, ACPD or APB receptors. Desensitization represents a potentially powerful means by which glutamate responses may be regulated. Indeed, two agents closely linked to the physiology of NMDA receptors, glycine and Ca2+, appear to modulate different types of desensitization. In the case of glycine, alteration of a rapid form of desensitization may be important in the role of this amino acid as a necessary cofactor for NMDA receptor activation. Additionally, changes in the affinity of the receptor complex for glycine may underlie the use-dependent decline in NMDA responses under certain conditions. Likewise, Ca2+ is a crucial player in the synaptic and toxic effects mediated by NMDA receptors, and is involved in a slower form of desensitization, in effect helping to regulate its own influx into neurons. The site and mechanism of the Ca2+ regulatory effects remain uncertain with evidence supporting both intracellular and ion channel sites of action. A clear role for Ca(2+)-dependent desensitization in the function of NMDA receptors under physiological conditions has not yet been demonstrated. AMPA receptor desensitization has been an area of intense investigation in recent years. The rapidity and degree of this process, coupled with its apparent rapid recovery, has suggested that desensitization is a key mechanism for the short-term regulation of responses mediated by these receptors. Furthermore, rapid desensitization appears to be one factor determining the time course and efficacy of fast excitatory synaptic transmission mediated by AMPA receptors, highlighting the physiological relevance of the process. The molecular mechanisms underlying desensitization remain uncertain. Traditionally, desensitization, like inactivation of voltage-gated channels, has been thought to represent a conformational change in the ion channel complex (Ochoa et al., 1989). However, it is unknown to what extent desensitization, in particular rapid AMPA receptor desensitization, has mechanistic features in common with inactivation. In voltage-gated channels, conformational changes in the channel protein restrict ion flow through the channel (Stuhmer, 1991).(ABSTRACT TRUNCATED AT 400 WORDS)

Escape from inhibition of synaptic transmission during in vitro hypoxia and hypoglycemia in the hippocampus

Electrophysiological recordings were made from rat hippocampal slices exposed to in vitro ischemic conditions in which the superfused medium is hypoxic and lacking glucose. Under these conditions, the evoked population spike recorded in CA1 is initially depressed and then transiently returns prior to an anoxic depolarization. This transient return in synaptic function under ischemic-like conditions also occurs if the population spike is inhibited by pretreatment with adenosinergic agonists or with the gamma-aminobutyric acid (GABA)B agonist, baclofen.

Apparent disappearance of postseizure inhibitions and intensity of seizures during the development of rapid kindling in rabbits

The evolution of seizures and postseizure inhibitions in the course of 'rapid kindling' and after the termination of stimulation were studied in rabbits with chronically implanted electrodes (neocortex, dorsal hippocampus, amygdala, caudate nucleus). The amygdala (n = 4) or hippocampus (n = 7) was electrically stimulated every 5 min. Generalized convulsions and wide-spread electrographic epileptic changes together with a striking shortening of postictal refractory periods were produced by this procedure within 2-6 h. In most cases, these epileptogenic effects continued their progression after the termination of stimulation for more than 2-4 weeks. The degree of reduction of postseizure inhibition durations was significantly greater than the degree of increase of generalized motor seizure durations. These may be mediated by mechanisms which facilitate the onset of seizure but do not significantly influence seizure expression.

Adenosine-dependent enhancement by methylxanthines of excitatory synaptic transmission in hippocampus of rats

Whole-cell patch-clamp was used to investigate synaptic transmission in hippocampal slices. Excitatory post-synaptic currents (EPSCs) were facilitated by low (less than or equal to 1 microM) adenosine (Ado) concentrations, while high concentrations had well-known inhibitory effects on the EPSC. When added on the background of preapplied Ado, methylxanthines caused a large potentiation of EPSCs. At saturation, the enhanced EPSC could exceed the control almost by an order of magnitude. Pertussis toxin strongly impaired the ability of Ado to block EPSCs but did not augment the facilitatory effect. The two components of the EPSC mediated by N-methyl-D-aspartate (NMDA) and non-NMDA receptors were facilitated simultaneously and in equal proportions.

Agonists selective for phosphoinositide-coupled receptors sensitize neurons to depolarization by L-2-amino-4-phosphonobutanoic acid (L-AP4)

Exposure of hippocampal and cortical slices to quisqualate induces a 30- to 100-fold decrease in the half-maximal concentration of L-2-amino-4-phosphonobutanoic acid (L-AP4) required to depolarize neurons. This sensitization persists for hours and has previously been shown to be induced only by quisqualate, via the interaction of quisqualate with a 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)-insensitive site. Here we tested the hypothesis that quisqualate may act on phosphoinositide (PI) metabolism to enhance the response to L-AP4, and found that sensitization to L-AP4 was induced by trans-1-amino-1,3-cyclopentanedicarboxylic acid (trans-ACPD), an agonist selective for PI-coupled excitatory amino acid receptors, and by carbachol and norepinephrine, agonists at other PI-coupled receptors. However, these compounds produced only a 2- to 5-fold sensitization to L-AP4, that was of shorter duration than that induced by quisqualate. These results suggest that sensitization to L-AP4 may be induced, at least in part, via PI-coupled receptors, and that the sensitivity of neurons to an excitatory amino acid agonist may be modified by heteroreceptor activation of the PI second messenger system.

The effect of omega-conotoxin GVIA on synaptic transmission within the nucleus accumbens and hippocampus of the rat in vitro

1. The actions of two calcium channel antagonists, the N-channel blocker omega-conotoxin GVIA (omega-CgTx) and the L-channel antagonist nisoldipine, on synaptic transmission were investigated in the hippocampus and nucleus accumbens of the rat in vitro. 2. omega-CgTx (100 nM for 10 min) produced a marked and irreversible reduction of focally evoked population spikes and intracellularly recorded excitatory postsynaptic potentials (e.p.s.ps) in the nucleus accumbens, which could not be overcome by increasing the stimulus strength. 3. Nisoldipine (10 microM for 10 min) had no effect on population spikes in the nucleus accumbens or the CA1 of the hippocampus. 4. In the hippocampus, population spikes were not irreversibly reduced by omega-CgTx (100 nM for 10 min) but rather, multiple population spikes were produced along with spontaneous synchronous discharges. This indicated that inhibitory synaptic transmission was being preferentially reduced. 5. Intracellular recordings demonstrated that omega-CgTx powerfully reduced inhibitory synaptic transmission in an irreversible manner and that excitatory transmission was also reduced but to a lesser extent. Unlike excitatory transmission in the nucleus accumbens and inhibitory transmission in the hippocampus, increasing the stimulus strength overcame the reduction of hippocampal excitatory transmission. 6. It is concluded that omega-CgTx-sensitive calcium channels are involved in the calcium entry that precedes the synaptic transmission in all these synapses. The apparent lower sensitivity of the hippocampal excitatory fibres to omega-CgTx may indicate that calcium entry that promotes transmitter release at central synapses may be mediated by pharmacologically distinct calcium channels.

NMDA receptor agonists selectively block N-type calcium channels in hippocampal neurons

The modulation of voltage-dependent calcium channels by various neurotransmitters has been demonstrated in many neurons. Because of the critical role of Ca2+ in transmitter release and, more generally, in transmembrane signalling, this modulation has important functional implications. Hippocampal neurons possess low-threshold (T-type) Ca2+ channels and both L- and N-type high voltage-activated Ca2+ channels. N-type Ca2+ channels are blocked selectively by omega-conotoxin and adenosine. These substances both block excitatory synaptic transmission in the hippocampus, whereas dihydropyridines, which selectively block L-type channels, are ineffective. Excitatory synaptic transmission in the hippocampus displays a number of plasticity phenomena that are initiated by Ca2+ entry through ionic channels operated by N-methyl-D-aspartate (NMDA) receptors. Here we report that NMDA receptor agonists selectively and effectively depress N-type Ca2+ channels which are involved in neurotransmitter release from presynaptic sites. The inhibitory effect is eliminated by the competitive NMDA antagonist D-2-amino-5-phosphonovalerate, does not require Ca2+ entry into the cell, and is probably receptor-mediated. This phenomenon may provide a negative feedback between the liberation of excitatory transmitter and entry of Ca2+ into the cell, and could be important in presynaptic inhibition and in the regulation of synaptic plasticity.

Adenosine antagonists alter the synaptic response to in vitro ischemia in the rat hippocampus

Exposure of the submerged hippocampal slice to in vitro ischemic conditions (superfusion with hypoxic medium lacking glucose) resulted in a progression of changes in the orthodromically evoked response recorded from the CA1 pyramidal region. There was an early depression of the population spike with no change in the presynaptic fiber volley, followed by a transient return of the population spike and, finally, a complete loss of both the population spike and fiber volley. The adenosine A1 subtype-selective antagonists, 8-phenyltheophylline (8-PT) and 8-cyclopentyltheophylline (8-CPT), greatly attenuated the early depression of the population spike such that the initial loss of the population spike was associated with the loss of the fiber volley. This result suggests that the initial loss of synaptic function in the hippocampal slice during exposure to in vitro ischemic conditions is due to increased levels of the inhibitory neuromodulator, adenosine.