Targeting acute ischemic stroke with a calcium-sensitive opener of maxi-K potassium channels

During ischemic stroke, neurons at risk are exposed to pathologically high levels of intracellular calcium (Ca++), initiating a fatal biochemical cascade. To protect these neurons, we have developed openers of large-conductance, Ca++-activated (maxi-K or BK) potassium channels, thereby augmenting an endogenous mechanism for regulating Ca++ entry and membrane potential. The novel fluoro-oxindoles BMS-204352 and racemic compound 1 are potent, effective and uniquely Ca++-sensitive openers of maxi-K channels. In rat models of permanent large-vessel stroke, BMS-204352 provided significant levels of cortical neuroprotection when administered two hours after the onset of occlusion, but had no effects on blood pressure or cerebral blood flow. This novel approach may restrict Ca++ entry in neurons at risk while having minimal side effects.

Maxi-K potassium channels: form, function, and modulation of a class of endogenous regulators of intracellular calcium

Large-conductance calcium-activated (maxi-K, BK) potassium channels are widely distributed in the brain. Maxi-K channels function as neuronal calcium sensors and contribute to the control of cellular excitability and the regulation of neurotransmitter release. Little is currently known of any significant role of maxi-K channels in the genesis of neurological disease. Recent advances in the molecular biology and pharmacology of these channels have revealed sources of phenotypic variability and demonstrated that they can be successfully modulated by pharmacological agents. A potential role is suggested in the treatment of conditions such as ischemic stroke and cognitive disorders.

Differential effects of coadministration of fluoxetine and WAY-100635 on serotonergic neurotransmission in vivo: sensitivity to sequence of injections

Serotonin 5-HT(1A) receptor antagonists potentiate the effects of serotonin reuptake inhibitors on extracellular serotonin levels in a variety of brain regions. These effects are quite variable, however, with reports indicating potentiations of anywhere from 100-1900%. One factor that might impact the magnitude of such potentiations is the timing of administration of the two agents; reports in which the reuptake inhibitor is given prior to the serotonin receptor antagonist consistently report larger potentiations than reports in which the antagonist is given first. To test this relationship directly, microdialysis and electrophysiology studies were performed to assess the magnitude of increase in extracellular serotonin and changes in cellular activity produced by the serotonin reuptake inhibitor fluoxetine and the 5-HT(1A) receptor antagonist WAY-100635 under various dosing regimens. In microdialysis studies, when WAY-100635 (0.5 mg/kg s.c.) was administered 80 min after fluoxetine (10 mg/kg i.p.) the increase in serotonin was more than twice that observed when the compounds were coadministered. In electrophysiology studies in vivo, WAY-100635 reversed the depression of cell firing produced by fluoxetine when administered 30 min after fluoxetine, but when the two compounds were coadministered, a depression in firing rate was observed comparable to that produced by fluoxetine alone. In contrast, slice recording studies showed that WAY-100635 blocked the effects of fluoxetine regardless of the order of administration. These results indicate that fluoxetine and WAY-100635 can interact in a fashion not predicted by the currently accepted model. It is likely that neuronal circuitry outside of the raphe nuclei underlies this relationship.

The augmentation hypothesis for improvement of antidepressant therapy: is pindolol a suitable candidate for testing the ability of 5HT1A receptor antagonists to enhance SSRI efficacy and onset latency?

The development of selective serotonin reuptake inhibitors (SSRIs) provided a major advancement in the treatment of depression. However, these drugs suffer from a variety of drawbacks, most notably a delay in the onset of efficacy. One hypothesis suggests that this delay in efficacy is due to a paradoxical decrease in serotonergic (5-HT) neuronal impulse flow and release, following activation of inhibitory presynaptic 5-HT1A autoreceptors, following acute administration of SSRIs. According to the hypothesis, efficacy is seen only when this impulse flow is restored following desensitization of 5-HT1A autoreceptors and coincident increases in postsynaptic 5-HT levels are achieved. Clinical proof of this principal has been suggested in studies that found a significant augmenting effect when the beta-adrenergic/5-HT1A receptor antagonist, pindolol, was coadministered with SSRI treatment. In this article, we review preclinical electrophysiological and microdialysis studies that have examined this desensitization hypothesis. We further discuss clinical studies that utilized pindolol as a test of this hypothesis in depressed patients and examine preclinical studies that challenge the notion that the beneficial effect of pindolol is due to functional antagonism of the 5-HT1A autoreceptors.

An assessment of the present and future roles of non-ligand gated ion channel modulators as CNS therapeutics

Few approved drugs have, as their primary known mechanism of action, modulation of non-ligand gated ion channels. However, these proteins are important regulators of neuronal function through their control of sodium, potassium, calcium and chloride flux, and are ideal candidates as drug discovery targets. Recent progress in the molecular biology and pharmacology of ion channels suggests that many will be associated with specific pharmacological profiles that will include both activators and inhibitors. Ion channels, through their regulation by G-proteins, are a major component of the final common pathway of many drugs acting at classical neuronal receptors. Thus, targeting of the ion channels themselves may confer different profiles of efficacy and specificity to drug action in the brain and spinal cord. Three areas for drug discovery are profiled that the authors consider prime targets for ion channel based therapies, anticonvulsant drugs, cognition enhancing drugs and drugs for improving neurone survival following ischaemia.

Cognition-enhancing drugs increase stimulated hippocampal theta rhythm amplitude in the urethane-anesthetized rat

Synchronous hippocampal electroencephalographic activity occurring in a frequency range of 3 to12 Hz (i.e., hippocampal theta rhythm) has been associated with mnemonic processes in vivo. However, this link is tenuous and theta rhythm may be secondary to processes that underlie mnemonic function. If theta rhythm is associated with mnemonic or cognitive function, cognition-enhancing drugs should enhance theta rhythm regardless of their primary biological target. In the current study, we evaluated several drugs that were shown to have cognition-enhancing properties in preclinical behavioral models and that vary with respect to their primary biological target: 1) the nootropic piracetam (250 and 500 mg/kg); 2) the small-conductance calcium-activated potassium-channel blocker apamin (0.1 and 0.4 mg/kg); and 3) the acetylcholinesterase inhibitor donepezil (0.1-10.0 mg/kg). All of the cognition-enhancing drugs produced dose-dependent increases in hippocampal theta rhythm amplitude elicited by stimulation of the brainstem reticular formation at doses that did not affect peak theta frequency in the urethane-anesthetized rat. These increases were reversed by the muscarinic receptor antagonist scopolamine, suggesting a common final cholinergic action of these compounds. The use-dependent N-methyl-D-aspartate antagonist dizocilipine maleate and scopolamine reduced theta amplitude (both) and frequency (dizocilipine maleate only). These data demonstrate that hippocampal theta rhythm is sensitive to cognition-modulating compounds, suggesting that theta rhythm may be closely associated with cognitive function.

A reexamination of the role of GABA in the mammalian suprachiasmatic nucleus

Three independent electrophysiological approaches in hypothalamic slices were used to test the hypothesis that gamma-amino butyric acid (GABA)A receptor activation excites suprachiasmatic nucleus (SCN) neurons during the subjective day, consistent with a recent report. First, multiple-unit recordings during either the subjective day or night showed that GABA or muscimol inhibited firing activity of the SCN population in a dose-dependent manner. Second, cell-attached recordings during the subjective day demonstrated an inhibitory effect of bath- or microapplied GABA on action currents of single SCN neurons. Third, gramicidin perforated-patch recordings showed that bicuculline increased the spontaneous firing rate during the subjective day. Therefore, electrophysiological data obtained by three different experimental methods provide evidence that GABA is inhibitory rather than excitatory during the subjective day.

Formation of intermediate-conductance calcium-activated potassium channels by interaction of Slack and Slo subunits

Large-conductance calcium-activated potassium channels (maxi-K channels) have an essential role in the control of excitability and secretion. Only one gene Slo is known to encode maxi-K channels, which are sensitive to both membrane potential and intracellular calcium. We have isolated a potassium channel gene called Slack that is abundantly expressed in the nervous system. Slack channels rectify outwardly with a unitary conductance of about 25-65 pS and are inhibited by intracellular calcium. However, when Slack is co-expressed with Slo, channels with pharmacological properties and single-channel conductances that do not match either Slack or Slo are formed. The Slack/Slo channels have intermediate conductances of about 60-180 pS and are activated by cytoplasmic calcium. Our findings indicate that some intermediate-conductance channels in the nervous system may result from an interaction between Slack and Slo channel subunits.

Phase shifting of circadian rhythms and depression of neuronal activity in the rat suprachiasmatic nucleus by neuropeptide Y: mediation by different receptor subtypes

Neuropeptide Y (NPY) has been implicated in the phase shifting of circadian rhythms in the hypothalamic suprachiasmatic nucleus (SCN). Using long-term, multiple-neuron recordings, we examined the direct effects and phase-shifting properties of NPY application in rat SCN slices in vitro (n = 453). Application of NPY and peptide YY to SCN slices at circadian time (CT) 7.5-8.5 produced concentration-dependent, reversible inhibition of cell firing and a subsequent significant phase advance. Several lines of evidence indicated that these two effects of NPY were mediated by different receptors. NPY-induced inhibition and phase shifting had different concentration-response relationships and very different phase-response relationships. NPY-induced phase advances, but not inhibition, were blocked by the GABAA antagonist bicuculline, suggesting that NPY-mediated modulation of GABA may be an underlying mechanism whereby NPY phase shifts the circadian clock. Application of the Y2 receptor agonists NPY 13-36 and (Cys2,8-aminooctanoic acid5,24,D-Cys27)-NPY advanced the peak of the circadian rhythm but did not inhibit cell firing. The Y1 and Y5 agonist [Leu31,Pro34]-NPY evoked a substantial inhibition of discharge but did not generate a phase shift. NPY-induced inhibition was not blocked by the specific Y1 antagonist BIBP-3226; the antagonist also had no effect on the timing of the peak of the circadian rhythm. Application of the Y5 agonist [D-Trp32]-NPY produced only direct neuronal inhibition. These are the first data to indicate that at least two functional populations of NPY receptors exist in the SCN, distinguishable on the basis of pharmacology, each mediating a different physiological response to NPY application.

Molecular dissection of two distinct actions of melatonin on the suprachiasmatic circadian clock

The pineal hormone melatonin elicits two effects on the suprachiasmatic nuclei (SCN): acute neuronal inhibition and phase-shifting. Melatonin evokes its biological effects through G protein-coupled receptors. Since the Mel1a melatonin receptor may transduce the major neurobiological actions of melatonin in mammals, we examined whether it mediates both melatonin effects on SCN function by using mice with targeted disruption of the Mel1a receptor. The Mel1a receptor accounts for all detectable, high affinity melatonin binding in mouse brain. Functionally, this receptor is necessary for the acute inhibitory action of melatonin on the SCN. Melatonin-induced phase shifts, however, are only modestly altered in the receptor-deficient mice; pertussis toxin still blocks melatonin-induced phase shifts in Mel1a receptor-deficient mice. The other melatonin receptor subtype, the Mel1b receptor, is expressed in mouse SCN, implicating it in the phase-shifting response. The results provide a molecular basis for two distinct, mechanistically separable effects of melatonin on SCN physiology.

Use of subunit-specific antisense oligodeoxynucleotides to define developmental changes in the properties of N-methyl-D-aspartate receptors

Antisense oligodeoxynucleotides were used to determine whether alterations in the expression of N-methyl-D-aspartate (NMDA) receptor subunit mRNA are responsible for developmental changes in the sensitivity of receptors to agonists and antagonists. Xenopus laevis oocytes were injected with mRNA prepared from neonatal and adult rat cerebral cortex, and the effects of agonists and antagonists were determined under voltage-clamp conditions. Glycine-site antagonists like 7-chlorokynurenate and glutamate-site antagonists like CGP-39653 were more potent at NMDA receptors expressed from mRNA from adult rat cerebral cortex than those expressed from mRNA from 1-day-old rat. NMDA receptors from 1-day-old rat cerebral cortex were more sensitive to activation by glycine than were receptors from adult rat cerebral cortex. 7-Chlorokynurenate and CGP-39653 were more potent inhibitors of responses seen with heteromeric NR1/NR2A receptors than with NR1/ NR2B receptors. Conversely, heteromeric NR1/NR2B receptors were more sensitive to activation by glycine than were NR1/NR2A receptors. We previously described a delay in the expression of the NR2A subunit in developing rat brain. Anti-sense oligodeoxynucleotides were used to determine whether the delayed expression of the NR2A subunit underlies changes in pharmacological properties observed during development. The properties of receptors seen when adult brain mRNA was coinjected with antisense oligodeoxynucleotides against the NR2A subunit were similar to those found in receptors from 1-day-old rat brain. These data suggest that changes in the sensitivity of NMDA receptors to antagonists and to glycine seen during development are a result of alterations in the expression of different species of NR2 subunit mRNA.

Phenotypic alteration of a human BK (hSlo) channel by hSlobeta subunit coexpression: changes in blocker sensitivity, activation/relaxation and inactivation kinetics, and protein kinase A modulation

A human homolog of the large-conductance calcium-activated potassium (BK) channel beta subunit (hSlobeta) was cloned, and its effects on a human BK channel (hSlo) phenotype are reported. Coexpression of hSlo and hSlobeta, in both oocytes and human embryonic kidney 293 cells, resulted in increased Ca2+ sensitivity, marked slowing of BK channel activation and relaxation, and significant reduction in slow inactivation. In addition, coexpression changed the pharmacology of the BK channel phenotype: hSlo-mediated currents in oocytes were more sensitive to the peptide toxin iberiotoxin than were hSlo + hSlobeta currents, and the potency of blockade by the alkaloid BK blocker tetrandrine was much greater on hSlo + hSlobeta- mediated currents compared with hSlo currents alone. No significant differences in the response to charybdotoxin or the BK channel opener NS1619 were observed. Modulation of BK channel activity by phosphorylation was also affected by the presence of the hSlobeta subunit. Application of cAMP-dependent protein kinase increased P(OPEN) of hSlo channels, but decreased P(OPEN)of most hSlo + hSlobeta channels. Taken together, these altered characteristics may explain some of the wide diversity of BK channel phenotypes observed in native tissues.

Synthesis and biological evaluation of an iodinated iberiotoxin analogue, [mono-iodo-Tyr5, Phe36]-iberiotoxin

The synthesis and iodination of a structural analogue of the specific large-conductance calcium-activated potassium (BK) channel blocker, iberiotoxin (IbTX), a 37-amino acid scorpion neurotoxin, is reported. The synthesis of this analogue, [Tyr5, Phe36]-IbTX, was accomplished using standard solid-phase Fmoc (9-fluorenylmethoxycarbonyl) chemistry protocols. The linear peptide was cyclized via the formation of three intramolecular disulfide bridges and subsequently iodinated at the Tyr5 position. Upon purification, the iodinated analogue, [mono-iodo-Tyr5, Phe36]-IbTX, exhibited comparable biological activity to native IbTX in blocking BK-mediated currents. These findings suggest the synthesis and use of an 125I labelled IbTX analogue for BK channel localization in autoradiography experiments.

Effects of channel modulators on cloned large-conductance calcium-activated potassium channels

Through expression of the cloned mouse (mSlo) or human (hSlo) large-conductance (BK) Ca(2+)-activated K+ channel in Xenopus laevis oocytes and HEK 293 cells, we characterized the effects of reported blockers and openers of BK channels to initiate the study of the molecular determinants of BK channel modulation. In oocytes, iberiotoxin and charybdotoxin, peptidyl scorpion toxins, were both equally effective blockers of BK current, although iberiotoxin was significantly more potent than charybdotoxin. The structurally related peptide kaliotoxin was not a potent blocker of BK current. Paxilline, a fungal tremorgenic alkaloid, was an effective but complex blocker of BK current. Tetrandrine, a putative blocker of type II BK channels, and ketamine were relatively ineffective. The putative BK openers NS004 and NS1619, phloretin, niflumic acid, flufenamic acid, and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) increased BK current in oocytes at microM concentrations; many of these produced biphasic concentration-response relationships. Coapplication of representative blockers and openers revealed several patterns of interaction, including competitive and noncompetitive antagonism. NS1619, niflumic acid, and phloretin were tested by using excised inside-out membrane patches from HEK 293 cells and were found to increase the activity of hSlo BK channels and produce a leftward shift in the G/Gmax-versus-voltage relationship of these channels. These results represent the first comprehensive examination of the molecular pharmacology of BK channels.

Reduced electrical excitability of PC12 cells deficient in GAP-43: comparison with GAP-43-positive cells

The electrical excitability of 3 lines of rat pheochromocytoma (PC12) cells were determined under current-clamp recording conditions. In the presence of nerve growth factor (NGF), PC12(A) 'control' cells expressed high levels of GAP-43 protein, PC12(B) cells were highly deficient in GAP-43, and PC12(AB) cells, created by transfection of PC12(B) cells with a rat GAP-43 gene construct, expressed high levels of GAP-43. All 3 lines had similar resting membrane potentials, but significantly greater proportions of GAP-43-containing PC12(A) and PC12(AB) cells exhibited spiking in response to depolarizing current pulses. These spikes were resistant to TTX, were greatly enhanced in TEA and TTX, and were substantially reduced by L-type Ca(2+)-channel antagonists. GAP-43 expression may regulate PC12 cell excitability following NGF treatment, as reflected in a lower proportion of cells capable of discharging with Ca(2+)-spikes in a GAP-43-deficient cell line.

Cloning and expression of a human large-conductance calcium-activated potassium channel

A putative BK channel gene was cloned from a human brain (substantia nigra) cDNA library by hybridization screening. The sequence of the full length clone shows high homology with the mSlo gene, suggesting that this cDNA is the human homologue (hSlo). The hSlo clone does not contain either alternative exon A or B at its splice sites; and similar to mSlo, it has a long string of serines at its 5' end. Reverse transcription coupled with the PCR technique demonstrated the differences in expression of the isoforms among the CNS and the periphery. Expression of hSlo in Xenopus oocytes showed a family of outward currents, induced by step depolarizations, that were blocked by iberiotoxin and activated by the compound NS004, a known opener of native and cloned maxi-K channels. Single channel recordings of hSlo channels showed a high degree of voltage- and Ca(2+)-dependence, and an average single channel conductance of 285.9 pS.

Reduced bicuculline response and GABAA agonist binding in aged rat hippocampus

Extracellular field recordings from CA1 pyramidal cells in the rat hippocampal slice preparation were used to examine the effects of age on gamma-aminobutyric acid (GABA)-mediated recurrent inhibition. The actions of bicuculline (1-100 microM), a GABAA antagonist, were assessed in slices from young (1-3 months) and aged (26 months) Fischer 344 rats. Pre-drug population spike amplitudes were smaller in slices from aged rats. Bicuculline increased population spike amplitudes in slices from both age groups, but slices from young rats were more sensitive to the antagonist. Bicuculline also produced multiple population spikes in slices from both age groups, however the increase in population spike burst durations was much greater in slices from young rats than in slices from aged rats. Agonist radiolabeled GABAA binding site density was significantly decreased in hippocampal tissue from aged rats. Our results suggest there is a reduction in GABAergic inhibition in hippocampal slices from aged rats, possibly mediated by a decrease in GABAA receptors.

Opening of large-conductance calcium-activated potassium channels by the substituted benzimidazolone NS004

1. We used electrophysiological techniques to examine the effects of 5-trifluoromethyl-1-(5-chloro-2-hydroxyphenyl)-1,3-dihydro-2H-benzimidaz ole- 2-one (NS004) on large-conductance calcium-activated potassium (BK) channels. 2. We used recordings from excised membrane patches (cell-attached and inside-out single-channel configurations) and whole-cell patch-clamp recordings to examine the effects of NS004 on single BK channels and whole-cell outward currents, respectively, in rat GH3 clonal pituitary tumor cells. We also tested NS004 on voltage-clamped BK channels isolated from rat brain plasma membrane preparations and reconstituted into planar lipid bilayers. Finally, we used two-electrode voltage-clamp techniques to study the effects of NS004 on currents expressed in Xenopus laevis oocytes by the recently described Slo BK clone from Drosophila. 3. In GH3 cells and in Xenopus oocytes expressing the Slo gene product NS004 produced an increase in an iberiotoxin- or tetraethylammonium-sensitive whole-cell outward current, respectively. NS004 produced a significant increase in the activity of single GH3 cell BK channels and rat brain BK channels reconstituted into planar lipid bilayers. In both systems this was characterized by an increase in channel mean open time, a decrease in interburst interval, and an apparent increase in channel voltage/calcium sensitivity. 4. These data indicate that NS004 could be useful for investigating the biophysical and molecular properties of BK channels and for determining the functional consequences of the opening of BK channels.

The substituted benzimidazolone NS004 is an opener of the cystic fibrosis chloride channel

Cystic fibrosis is a major inherited disorder involving abnormalities of fluid and electrolyte transport in a number of different organs. Epithelial cells of cystic fibrosis patients have a decreased capacity to secrete chloride in response to cAMP-mobilizing agents because of the mutation of a single gene. The gene product, the cystic fibrosis transmembrane conductance regulator or CFTR, is a chloride channel. The most frequent mutation is a deletion of phenylalanine in position 508 (delta F508-CFTR) that reduces both the expression of the CFTR protein at the cell surface, and the activity of the Cl- channel. This work presents the properties of NS004, a substituted benzimidazolone, which is the first activator of normal and mutant CFTR-associated chloride channels to be described. NS004 activated CFTR and delta F508-CFTR Cl- channels expressed in Xenopus oocytes, and increased 125I efflux (via the Cl- channel) from Vero cells expressing CFTR and delta F508-CFTR. Application of NS004 to the external side of outside-out patches excised from these CFTR- and delta F508-CFTR-expressing cells induced a marked and reversible increase in channel activity.

The sensitivity of hippocampal long-term potentiation to nitric oxide synthase inhibitors is dependent upon the pattern of conditioning stimulation

Inhibition of nitric oxide synthase prior to conditioning has been previously found to reduce levels of hippocampal long-term potentiation. In the present experiments in the rat, the reduction of long-term potentiation by nitric oxide synthase inhibitors was highly conditioning-dependent. We have characterized the relative importance of the number of conditioning stimulus trains, pulse number, intensity, and pattern in the reduction of long-term potentiation by nitric oxide synthase inhibitors. Long-term potentiation was reduced relative to control values only when multiple conditioning stimulus trains were presented at maximal stimulus intensity; potentiation produced by an equivalent number and intensity of stimuli presented in a single conditioning train was not reduced by nitric oxide synthase inhibitors, and multiple-train-induced potentiation was sensitive to nitric oxide synthase inhibitors only when maximal stimulation intensity was employed. Another form of synaptic potentiation, primed-burst potentiation, was reduced by nitric oxide synthase inhibition, while homosynaptic and heterosynaptic depression were unaffected. Our results support the hypothesis that conditioning-dependent release of nitric oxide can contribute to long-term potentiation, but also show that its blockade by nitric oxide synthase inhibitors is dependent on the nature of the conditioning stimulus, and that long-term potentiation can be generated that is apparently resistant to the effects of these drugs.

The effects of the adenosine reuptake inhibitor soluflazine on synaptic potentials and population hypoxic depolarizations in area CA1 of rat hippocampus in vitro

Adenosine has recently been shown to play a potentially important role in the regulation of synaptic excitability during experimental hypoxia in the hippocampus of the rat. Endogenous adenosine, rapidly released at the initiation of a hypoxic episode, produced synaptic depression, which could protect sensitive neurons. In the present experiments, an inhibitor of the reuptake of adenosine, soluflazine (R64719) was employed to increase the levels of endogenous adenosine under normoxic and hypoxic conditions in slices of the hippocampus of the rat. Soluflazine produced a slow-onset, concentration-dependent depression of population excitatory postsynaptic potentials, which was reversed by the specific A1 adenosine receptor antagonist, 8-cyclopentyltheophylline. During severe N2-induced hypoxia, soluflazine significantly delayed hypoxic depolarization. These results suggest that inhibition of the reuptake of adenosine may have therapeutic potential in the amelioration of hypoxic/ischemic neuronal damage, particularly in the hippocampus.

Endogenous adenosine contributes to hypoxic synaptic depression in hippocampus from young and aged rats

1. Extracellular field potentials were recorded to study the role of endogenous adenosine during hypoxia in area CA1 of rat hippocampal slices. 2. Hypoxic conditions, induced by 15 min exposure to 95% N2-5% CO2 at 32 degrees C and in high-glucose incubation medium, produced a rapid and reversible depression of evoked synaptic potentials. 3. In slices from young Sprague-Dawley rats, the hypoxia-induced synaptic depression was reduced in a concentration-dependent manner by the adenosine antagonist 8-cyclopentyltheophylline (8-CPT; 100 nM-2.0 microM). 4. Recovery of synaptic potentials after hypoxia was complete under each experimental condition. 5. Extended periods of hypoxia lasting 30 min likewise produced a rapid and near total suppression of the evoked synaptic potentials. In the presence of 8-CPT, both the population excitatory postsynaptic potential (EPSP) slope and population spike amplitude were significantly preserved throughout the hypoxic episode. 6. Neither the onset rate nor the degree of the hypoxia-induced synaptic depression were significantly different in slices from young, adult, or aged Fischer 344 rats. Reduction of the hypoxia-induced response depression by 8-CPT was also similar in all age groups. 7. These findings have further characterized the important involvement of endogenous adenosine in the potentially neuroprotective synaptic depression observed in hippocampal slices from young and aged rats during hypoxia.

Age-dependence of effects of A1 adenosine receptor antagonism in rat hippocampal slices

1. Population excitatory postsynaptic potentials (EPSPs) and population spikes evoked in area CA1 of hippocampal slices from aged Fischer 344 rats were significantly smaller in amplitude than responses obtained in slices from young Fischer 344 rats. 2. The A1 adenosine receptor antagonist 8-cyclopentyltheophylline (8-CPT) produced a concentration-dependent increase in synaptic potentials in slices from both young and aged rats. Low concentrations (1 nM) of 8-CPT were effective in producing increases in both population spike amplitudes and population EPSP slopes in young and aged rat slices. Response increases were maximized by 100 nM 8-CPT in slices from rats of both age groups. 3. Adenosine antagonism produced greater average increases in synaptic responses in hippocampal slices from aged rats at all concentrations tested (1.0 nM-1.0 microM). A qualitative age-related difference in the response to 8-CPT was also observed; 8-CPT produced a late component, consisting of multiple population spikes, in evoked responses in slices obtained from aged but not young rats. 4. Adenosine antagonism significantly increased the maximum evocable response (both spike amplitude and EPSP slope) in slices from aged rats, relative to increases observed in slices from young rats. This suggested that smaller synaptic potentials seen in slices from aged rats were in part due to greater levels of "tonic" adenosinergic inhibition. 5. Slices from young and aged rats were incubated in the adenosine reuptake inhibitor soluflazine (R64719; 1.0, 10, and 100 microM) and the inhibition of population EPSPs was observed for 60 min. No difference was observed in the rate of inhibition or the maximal level of inhibition produced by soluflazine, in slices from rats of either age group. 6. Application of (+)-5-methyl-10,11-dihydro-5H-dibenzo-[a,d]cyclo-hepten- 5,10-imine hydrogen maleate (MK-801) and 2-amino-5-phosphonopentanoic acid (2-AP5), antagonists of N-methyl-D-aspartate (NMDA) excitatory amino acid (EAA) receptors, reduced the late multiple population spike component in slices from aged rats incubated in 8-CPT. A smaller direct effect of the NMDA antagonists was observed in slices from aged rats in the absence of 8-CPT treatment at maximal response levels. No effect of NMDA receptor antagonism was observed in slices from young rats under either condition. 7. Hippocampal tissue, from young and old rats utilized in the electrophysiological experiments, was assayed for A1 adenosine binding site density with a saturating concentration of radiolabeled agonist and antagonist. Guanine nucleotide modulation of agonist binding was also measured.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for nitric oxide synthase inhibitor-sensitive and insensitive hippocampal synaptic potentiation

1. Nitric oxide (NO) has been proposed as a retrograde messenger, mediating the postsynaptic to presynaptic transfer of the effects of conditioning stimulation, responsible for the initiation of hippocampal long-term potentiation (LTP). To further test this hypothesis, we inhibited nitric oxide synthase (NOS) to determine whether synaptic potentiation produced by different conditioning stimulus patterns and intensities was differentially affected by reduction of stimulation-dependent NO production. 2. Synaptic potentiation was produced in hippocampal slices from young F-344 rats by two different conditioning stimulation protocols. Conditioning stimuli were delivered to the Schaffer-collateral commissural system, and moderate levels of potentiation of the population excitatory postsynaptic potential (EPSP) in area CA1 were produced by a single 100 Hz, 1-s conditioning train delivered at half-maximal stimulus intensity. Higher levels of potentiation of the population EPSP were obtained by delivering two 100 Hz, 1-s conditioning stimulus trains, with a 60-s intertrain interval, at high stimulus currents. 3. Application of the nitric oxide synthase inhibitors NG-nitro-L-arginine (NOARG; 0.1-200 microM) and NG-monomethyl-L-arginine (NMMA; 100 microM) produced no significant direct effects on synaptic responses. 4. In slices that received a single conditioning stimulus train, both NOARG and NMMA were ineffective in blocking or reducing potentiation at concentrations between 0.1 and 200 microM. In slices receiving the more intense pair of conditioning stimulus trains, levels of potentiation in control slices were higher, and there was a very significant reduction by both NOARG (50 and 100 microM) and NMMA (100 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Hypoxia produces cell death in the rat hippocampus in the presence of an A1 adenosine receptor antagonist: an anatomical and behavioral study

Endogenous adenosine depresses synaptic transmission in rat hippocampal slices during periods of hypoxia, a potentially neuroprotective cellular response that is attenuated by the adenosine antagonist 8-cyclopentyltheophylline. In this study, rats were exposed to moderate hypoxic conditions (5% O2- 95% N2, 40 min x three days) in the presence or absence of 8-cyclopentyltheophylline, and the effects of reducing adenosinergic inhibition during hypoxia were assessed histologically and behaviorally. Half the rats exposed to low levels of oxygen in the presence of 8-cyclopentyltheophylline (10 mg/kg) suffered unilateral or bilateral hippocampal damage largely limited to the CA1 subfield. Animals which had suffered hippocampal damage were also impaired in their performance of a working memory version of the Morris Water Maze, but not a passive avoidance task (step-through). Hypoxia alone did not result in neuronal damage or behavioral impairment. These results provide further evidence that endogenous adenosine provides an important level of neuronal protection during even prolonged periods of hypoxia.

Effects of oral BMY 21502 on Morris water task performance in 16-18 month old F-344 rats

In the present study we have examined the effects of oral administration of BMY 21502, a potential cognition enhancing drug, on the impaired Morris water task performance of 16-18 month old F-344 rats. BMY 21502 did not affect swim speeds or performance on the first trial of each day, but it did increase the rate of acquisition and initial retention, resulting in decreased swim distances on the second trial of each day. This increased rate of acquisition was dose-dependent, increasing to a peak at 5.0 mg/kg; the effect was decreased at 10 mg/kg, but still above control values. These results suggest that BMY 21502 is orally active over a broad range of doses, and lend further support for its potential as a therapeutic agent for the treatment of dementia.

Relationship between performance in the Morris water task, visual acuity, and thermoregulatory function in aged F-344 rats

The present experiments were designed to determine whether the loss of visual acuity and thermoregulatory control in aged rats contributes significantly to age-related deficits in the Morris water task. Relative to 2.5 mo rats, 16-18 mo F-344 rats were found to perform poorly in this spatial learning task. Their performance was also impaired in a test of visual acuity, and they became hypothermic during testing in the Morris water task. Nevertheless, 23 mo F-344 rats still retained a fairly high degree of visual acuity, and reducing the degree of visual acuity required to perform spatial mapping by adding large visual cues in close proximity to the target platform did not improve their performance. However, preventing hypothermia by warming 23 mo rats between trials in the Morris water task did significantly improve performance. These results suggest that age-related deficits in the Morris water task are not due to the loss of visual acuity; but, as a specific measure of cognitive function, performance in the Morris water task may be confounded by the loss of thermoregulatory control.

Electrophysiological evidence for N-methyl-D-aspartate excitatory amino acid receptors in the rat supraoptic nucleus in vitro

The responses of neurons in slices of the rat supraoptic nucleus (SON) to afferent stimulation were recorded under current-clamp conditions. In magnesium (Mg2+)-free incubation medium, synaptic responses were prolonged and were partially antagonized by the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist (+)-5-methyl-10,11-dihydro-5H- dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (MK-801). During blockade of non-NMDA excitatory amino acid (EAA) receptors, the synaptic responses in Mg(2+)-free medium were blocked by the competitive NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (AP5). The results of these experiments provide electrophysiological evidence for the existence of NMDA receptors in the rat SON.

A 1-heteroaryl-4-piperidinyl-methyl pyrrolidinone, BMY 21502, delays the decay of hippocampal synaptic potentiation in vitro

The effects of the substituted pyrrolidinone, BMY 21502, on the properties of cell membranes, synaptic transmission and synaptic plasticity, were assessed in area CA1 of hippocampal slices from the rat. Application of the compound to the bath had no consistent direct effects on parameters of the cell membrane or evoked synaptic potentials, at concentrations of less than 30 microM. In a blind experimental design, BMY 21502 at 1.0 and 10 microM, but not 25 microM, significantly delayed the decay of long-term potentiation in slices obtained from young animals; in slices obtained from very old rats (2.5-3.2 yr), 10 microM BMY 21502 significantly delayed decay of long-term potentiation. Therefore BMY 21502 was active in a physiological model that may predict of cognitive enhancement.

The adenosine antagonist 8-cyclopentyltheophylline reduces the depression of hippocampal neuronal responses during hypoxia

Exposure of rat hippocampal slices to hypoxic conditions for 15 min produced a rapid, profound, but completely reversible depression of evoked synaptic potentials. The specific A1 adenosine receptor antagonist 8-cyclopentyltheophylline (8-CPT) significantly reduced hypoxia-induced synaptic depression in a concentration-dependent manner. It is concluded that adenosine, which is neuroprotective when exogenously applied during severe hypoxia because of its ability to depress synaptic transmission, may have an important and exploitable endogenous role in the protection of sensitive neurons.

Effects of excitatory amino acid antagonists on synaptic responses of supraoptic neurons in slices of rat hypothalamus

1. Intracellular recordings from magnocellular neurons in the supraoptic nucleus (SON) were obtained from rat hypothalamic slices to determine the effects of specific transmitter antagonists on evoked postsynaptic potentials (PSPs), action potential after-discharge, and spontaneously occurring PSPs. 2. Broad-spectrum excitatory amino acid (EAA) antagonists, kynurenic acid (KYN) and gamma-d-glutamylglycine (DGG), significantly diminished or eliminated electrically evoked depolarizing PSPs and spike discharges. These compounds also greatly reduced the amplitude and frequency of spontaneous PSPs. 3. The specific N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonopentanoic acid (AP5), did not significantly reduce these measures of synaptic activation under these experimental conditions. 4. The gamma-aminobutyric acid (GABA) antagonist, bicuculline methiodide (BIC), partially antagonized some PSPs when the cells were hyperpolarized (-75 to -80 mV) with steady injected currents; KYN antagonized BIC-resistant PSPs. 5. The involvement of a hypothetical cholinergic input to the SON in the responses to stimulation of the region dorsolateral to the SON was tested by bath application of nicotinic cholinergic antagonists, particularly d-tubocurarine (dTC). Nicotinic cholinergic antagonists, even after prolonged exposure to high concentrations, did not block the responses of SON cells to dorsolateral stimulation. 6. These findings strongly suggest that EAAs mediate fast excitatory synaptic responses of SON neurons to stimulation of cells and axons in the region dorsolateral to the SON. The blockade of almost all spontaneous EPSPs by broad-spectrum EAA antagonists likewise argues that EAAs are responsible for the majority of ongoing fast excitatory input. These responses appear to involve an interaction with kainate- and/or quisqualate-type EAA receptors.

Cholinergic excitation of supraoptic neurons in hypothalamic slices of rat

The actions of the cholinergic agonists acetylcholine (ACh), nicotine (NIC) and carbamylcholine (CARB) were studied in the supraoptic nucleus with extracellular recordings from coronal slices of the hypothalamus of the rat. Agonists were either applied to the bath or pressure-ejected from micropipettes. Acetylcholine, NIC and CARB produced short-latency increases in discharge rate of about one-half of the cells of the supraoptic nucleus, particularly if the cell was spontaneously active. Excitations produced by NIC were typically brief and the firing rate often declined during prolonged exposure to the agonist. A vigorous excitation frequently occurred in normal medium immediately after cessation of prolonged application of NIC. Local application of glutamate (GLUT) reliably excited cells of the supraoptic nucleus. In some of the cells that were unresponsive to cholinergic agonists, ACh and NIC enhanced responses to electrical stimulation in the dorsolateral area. The nicotinic cholinergic antagonists, hexamethonium (HEX) and d-tubocurarine (dTC), reduced or blocked excitations induced by NIC, but not those from GLUT. This study provides further evidence that nicotinic receptors can mediate cholinergic influences on cells of the supraoptic nucleus. However, these effects were not observed in all cells in the region of the supraoptic nucleus and the excitations by NIC showed densensitization. Acetylcholine may subserve a modulatory role to other transmitters in the supraoptic nucleus.

The effects of the excitatory amino acid antagonist kynurenic acid on synaptic transmission to supraoptic neuroendocrine cells

The magnocellular neuroendocrine system of the mammalian hypothalamus regulates numerous physiological functions by releasing the peptide hormones oxytocin and vasopressin from the neurohypophysis. Although several substances are known to promote hormone release, relatively little research has been aimed at testing the hypothesis that excitatory amino acids (EAAs) mediate synaptic transmission in this neuroendocrine system. In the present experiments, intracellular recordings in slices of rat hypothalamus revealed that kynurenic acid, a specific antagonist of EAA receptors, strongly and reversibly blocked excitatory postsynaptic potentials (EPSPs) in supraoptic neurons. The profound antagonism of EPSPs by kynurenic acid strongly suggests that EAAs may be an important class of fast excitatory neurotransmitter within this central regulatory nucleus.

Reduction of dye coupling in glial cultures by microinjection of antibodies against the liver gap junction polypeptide

Intracellular injection of antibodies to the 27-kDa liver gap junction polypeptide have been shown previously to uncouple pairs of cultured mammalian hepatocytes, cardiac myocytes, and ganglionic neurons (Proc. Natl. Acad. Sci. U.S.A., 82 (1985) 2412-2416). In confluent primary cultures of astrocytes, similar injections significantly reduced dye coupling for cells closer than 80 micron to the injected glial cell. Western blots identified a 27-kDa protein in extracts of the astrocyte cultures that cross-react with the gap junction-specific antibodies. These results suggest that homologous gap junction polypeptides exist in liver and glial cells.

Synaptic activation of slow depolarization in rat supraoptic nucleus neurones in vitro

1. The effects of synaptic activation on rat supraoptic nucleus (s.o.n.) neurones were studied in the hypothalamic slice preparation. Intracellular recordings were obtained from forty-one probable magnocellular neuroendocrine cells using microelectrodes filled with 3 M-potassium acetate. Responses to single and repetitive stimulation of the area dorsolateral to the s.o.n., which would be expected to activate a cholinergic pathway (Hatton, Ho & Mason, 1983), were analysed. 2. In forty of forty-one cells, responses to single stimuli consisted of a short-latency excitatory post-synaptic potential (e.p.s.p.), which was often followed by a brief burst of fast depolarizing events which resembled spontaneous e.p.s.p.s. When the membrane was depolarized, single stimuli could consistently produce a burst of action potentials. 3. Brief trains of orthodromic stimuli produced three effects in most cells. Spontaneous fast depolarizing events, which appeared to be primarily e.p.s.p.s. significantly increased in frequency after the train. A slow membrane depolarization, which lasted up to 1-2 min, was observed in twenty-eight of forty-one cells. In several cells the slow depolarization was accompanied by an increase in input resistance (Ri). In some cells an after-discharge occurred during the slow depolarization. Slow depolarizations were observed in each of eleven phasic neurones, and in a smaller percentage of non-phasic and silent cells. 4. All components of the response to dorsolateral stimulation could be reduced or blocked in low-Ca2+, high-Mg2+ bathing medium. 5. Slow depolarizations were observed when action potentials were not elicited by the stimulus train. The slow depolarization was still present after manipulations that blocked discharge during the stimulus train, including injection of hyperpolarizing current and diffusion of the quaternary ammonium compound QX314. These data argue that the slow depolarization can occur independent of spike depolarizing after-potentials (d.a.p.s). 6. In some cells antidromic stimulation at an intensity just suprathreshold for the recorded cell did not produce comparable bursts of fast depolarizing events or slow depolarizations; similar periods of depolarizing current injection, which produced repetitive discharge, also did not mimic the effects of orthodromic stimulation. 7. The fast depolarizing events appear to reflect spontaneous e.p.s.p.s; increases in the frequency of these events may reflect the after-discharge of nearby neurones that are presynaptic to the recorded neurone.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of opossum and rat hippocampal CA1 cells to paired stimulus volleys

Short-term synaptic plasticity was studied in the in vitro hippocampus of the North American opossum (Didelphis virginiana) and rat (Rattus norvegicus). Conditioning and test stimulus pulses were delivered to fibers in stratum radiatum, and intracellular and extracellular recordings were obtained from area CA1 pyramidal cells. In rat, the amplitude of the population spike in response to the second (test) of two stimulus impulses is suppressed at short inter-pulse-intervals (IPI's). In opossum, the amplitude of the test population spike is facilitated at comparable IPI's. Facilitation of the test population spike in rat occurs only when the test stimulus is separated from the first stimulus (conditioning) by a longer IPI. Peak values of facilitation do not significantly differ between species. Intracellular responses, elicited by stimulus pulses that were subthreshold for spike production, indicate that the amplitude of test EPSP's recorded from opossum pyramidal cells are facilitated at IPI's that result in suppression of test EPSP's in rat pyramidal cells.

Modulation by dopamine of population spikes in area CA1 hippocampal neurons elicited by paired stimulus pulses

Extracellular recording techniques were used to study the effects of dopamine on postactivation excitability of rat area CA1 hippocampal neurons maintained in vitro. Population spikes were elicited by delivery of conditioning and test stimulus pulses to afferent fibers. The interval between the conditioning and test volley was set to separate delivery of stimuli by 10 to 80 msec. The effect of superfusion or microtopical application of dopamine (DA) on population responses to test stimulus pulses was studied. When paired stimulus volleys, separated by brief intervals (up to 40 msec), were delivered to afferent fibers, paired-pulse suppression (PPS) was indicated by the amplitude of the population spike elicited by the test volley being smaller than that elicited by the conditioning volley. When paired volleys were separated by longer intervals (40 to 80 msec), the response elicited by the test volley was larger in amplitude than that elicited by the conditioning volley, indicating paired-pulse facilitation (PPF). Following exposure to DA, the amplitude of the population response elicited by the conditioning volley was larger than the amplitude before exposure to DA. This effect was long-lasting, enduring for tens of minutes. However, when the amplitude of the conditioning population response was held constant, the PPS was decreased, indicating disinhibition. It is suggested that dopamine produces a long-lasting attenuation of an intervening inhibitory influence onto CA1 pyramidal neurons.

Modulation by dopamine of population responses and cell membrane properties of hippocampal CA1 neurons in vitro

Dopamine (DA) was applied to rat hippocampal slices maintained in vitro. Extracellular and intracellular recording techniques were used to study the effect of DA on population responses, membrane potentials, and membrane responses to hyperpolarizing current pulses in CA1 pyramidal cells. Temporary exposure of hippocampal slices to DA has a dual effect. The initial action of DA is to produce a suppression of the extra-cellularly recorded population responses. In individual neurons, this initial effect is seen as a membrane hyperpolarization accompanied by a decrease in the amplitude of responses to hyperpolarizing current pulses. The frequency of occurrence of spontaneous depolarizations and spikes is reduced. The early action of DA is followed by a profound potentiation of the population responses that can last for hours. This long-lasting potentiation of the population response, induced by DA, is depressed by spiroperidol, a DA antagonist. In individual neurons, the late effect of DA is a long-lasting membrane depolarization associated with an increase in the amplitude of responses to hyperpolarizing current pulses. During this late phase, spontaneous activity is increased, as are single cell responses to stimulation of afferents. The evidence presented here indicates that DA is able to induce a long-lasting modification of the excitability of CA1 hippocampal neurons. This modulation of excitability by DA may be similar in nature to previously described DA-modulatory actions in the peripheral nervous system.

GDEE antagonism of iontophoretic amino acid excitations in the intact hippocampus and in the hippocampal slice preparation

Glutamic acid diethylester (GDEE) reversibly antagonized excitations produced by glutamate and aspartate but not those produced by acetylcholine when applied iontophoretically to rat CA1 hippocampal neurons in penthrane (methoxyfluorane) anesthetized rats and to CA1 neurons in in vitro slice preparations. GDEE did not appear to differentiate between the excitations produced by glutamate aspartate and appeared to be a more potent antagonist than has previously been reported. CA1 cells were remarkably sensitive to acetylcholine; 5-50 nA being sufficient to produce marked amino acid-like excitations, which were unrelated to the pH of the acetylcholine. The nature of the responses to applied substances was virtually identical between the intact animal and the in vitro slice preparation. A description of the in vitro technique is given as an Appendix.