enkephalin--ala(2)-mephe(4)-gly(5)- and bicuculline-methiodide

The spontaneous, synchronous activity induced by 4-aminopyridine (4AP, 50 microM) in the adult rat entorhinal cortex was analyzed with simultaneous field potential and intracellular recordings in an in vitro slice preparation. Four-AP induced isolated negative-going field potentials (interval of occurrence = 27.6 +/- 9.9 (SD) s; n = 27 slices) that corresponded to intracellular long-lasting depolarizations (LLDs), and ictallike epileptiform discharges (interval of occurrence = 10.4 +/- 5.7 min; n = 27 slices) that were initiated by the negative field potentials. LLDs recorded with K-acetate-filled microelectrodes triggered few action potentials of variable amplitude and had a duration of 1.7 +/- 0.8 s (n = 26 neurons), a peak amplitude of 11.8 +/- 5.0 mV (n = 26 neurons) and a reversal potential of -66.2 +/- 3.9 mV (n = 17 neurons). The ictal discharges studied with K-acetate microelectrodes consisted of prolonged depolarizations (duration = 72.9 +/- 44.3 s; peak amplitude = 29.2 +/- 11.4 mV; n = 25 neurons) with action-potential firing during both the tonic and the clonic phase. These depolarizations had a reversal potential of -45.3 +/- 3.8 mV (n = 4 neurons). Intracellular Cl- diffusion from KCl-filled microelectrodes made both LLDs and ictal depolarizations increase in amplitude (30.5 +/- 8.2 mV, n = 8 and 41.8 +/- 9.8 mV, n = 6 neurons, respectively). LLDs recorded with KCl and 2-(trimethyl-amino)N-(2, 6-dimethylphenyl)-acetamide (QX-314) microelectrodesreached an amplitude of 36.3 +/- 5.2 mV, lasted 12.5 +/- 6.5 s, and had a reversal potential of -31.3 +/- 2.5 mV (n = 4 neurons); under these recording procedures the ictal discharge amplitude was 41.5 +/- 5.0 mV and the reversal potential -24.0 +/- 7.0 mV (n = 4 neurons). The N-methyl-D-aspartate (NMDA) receptor antagonist 3,3-(2-carboxy-piperazine-4-yl)-pro-pyl-l-phosphonate (10 microM, n = 5 neurons) alone or concomitant with the nonNMDA receptor antagonist 6-cyano-7-nitro-quinoxaline-2,3-dione (10 microM, n = 4 neurons) abolished ictal discharges, without influencing LLDs. LLDs were blocked by the gamma-aminobutyric acid-A (GABAA) receptor antagonist bicuculline methiodide (BMI, 10 microM, n = 6 neurons) or the mu-opioid receptor agonist (-Ala2-N-Me-Phe, Gly-ol) enkephalin (DAGO, 10 microM, n = 2 neurons). Application of BMI (n = 4 neurons) or DAGO (n = 2 neurons) to control the medium abolished LLDs and ictal discharges but disclosed a novel type of epileptiform depolarization that lasted 3.5 +/-

Topics: 4-Aminopyridine; 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Animals; Bicuculline; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; Excitatory Amino Acid Antagonists; GABA-A Receptor Antagonists; In Vitro Techniques; Lidocaine; Male; Membrane Potentials; Neurons; Potassium Acetate; Rats; Rats, Sprague-Dawley; Receptors, GABA-A

In the presence of 4-aminopyridine (4AP, 50 microM) two types of spontaneous field potentials can be recorded in the CA3 stratum radiatum of adult rat hippocampal slices. First, epileptiform interictal discharges (0.85 +/- 0.25 Hz) that are blocked by excitatory amino acid ionotropic receptor antagonists. Second, negative-going synchronous potentials (0.036 +/- 0.015 Hz) which are solely abolished by application of bicuculline methiodide (BMI). Bath application of the specific adenosine A1 receptor agonist, N6-(L-2-phenylisopropyl) adenosine (L-PIA), reduced the frequency of interictal discharges in a dose-dependent manner (IC50 = 8.75 microM; n = 9 slices) and this effect was reversed by the specific adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 100 microM; n = 3 slices). L-PIA did not affect the frequency of occurrence of the negative-going field potential during application of excitatory amino acid receptor antagonists. This BMI-sensitive event was depressed, however, by application of the mu-opioid receptor agonist [D-Ala2-N-Me-Phe4,Gly5(5)-ol]enkephalin (DAGO, 10 microM; 15.1 +/- 8.7% of rate in control; n = 6 slices), an effect that was antagonized by naloxone (20 microM). Our results indicate that L-PIA reduces the 4AP-induced epileptiform activity through the activation of adenosine A1 receptors. This procedure does not influence the BMI-sensitive field potential, which is abolished, however, by DAGO. Thus, our findings support the hypothesis that the BMI-sensitive potential is due to the presynaptic release of GABA from interneurons.

Topics: 4-Aminopyridine; Action Potentials; Animals; Bicuculline; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; Hippocampus; Male; Purinergic P1 Receptor Agonists; Rats; Rats, Sprague-Dawley; Receptors, Opioid, mu

Modulation of gamma-aminobutyric acid (GABA)-mediated inhibition, and glutamate-mediated excitation by highly selective mu and delta opioid agonists was studied using intracellular recordings of CA1 pyramidal neuron synaptic responses in superfused hippocampal slices. Equimolar concentrations of the selective mu agonist, [Tyr-(D-Ala)-Gly-(N-Me-Phe)-Gly-ol]-enkephalin (DAGO), or the delta selective agonist, [D-Pen2,D-Pen5]-enkephalin (DPDPE), reversibly increased the amplitudes of excitatory post-synaptic potentials (EPSPs), evoked by Schaffer collateral/commissural stimulation, without altering the input resistance or resting membrane potential of these CA1 pyramidal neurons. The increased EPSP amplitudes resulting from superfusion with the enkephalin analogs were qualitatively similar to those caused by the GABAA receptor antagonist, bicuculline methiodide (BMI). Specific stimulation/recording protocols and micro-lesions of the slices were used to evoke relatively pure forms of recurrent and feed-forward GABA-mediated inhibitory post-synaptic potentials (IPSPs). The mu opioid agonist DAGO reduced both recurrent and feed-forward IPSPs, while the delta agonist DPDPE had no effect upon these responses. To test the hypothesis that the enhancement of pyramidal neuron EPSPs by delta (and mu) opioids was due to the reduction of an inhibitory potential that was coincident with the EPSP, DPDPE or the mu agonist, DAGO, were applied while recording monosynaptic IPSPs following the elimination of EPSPs by the glutamate receptor antagonists, D,L-2-amino-5-phosphonovalerate (APV) and 6,7-dinitroquinoxaline-2,3-dione (DNQX). The mu agonist, DAGO, reversibly reduced these pharmacologically isolated IPSPs, while the delta agonist, DPDPE, had no effect upon these responses. Despite the fact that the delta agonist, DPDPE, had no effect on recurrent, feed-forward or monosynaptic evoked IPSPs, this enkephalin did reversibly reduce the frequency of spontaneously occurring IPSPs, measured using whole-cell recordings with pipettes containing 65 mM KCl. The mu agonist, DAGO, and the GABAA antagonist, BMI, similarly reduced spontaneous IPSP rates. We conclude from these data that mu and delta opioid receptor activation increases EPSPs via the reduction of a form of GABAergic inhibition that is difficult to characterize, and which may be distinct from conventional feed-forward and recurrent inhibition. Furthermore, delta opioids seem to reduce this form of GABAergic inhibition sel

Topics: 2-Amino-5-phosphonovalerate; Analgesics; Animals; Axons; Baclofen; Bicuculline; Electric Stimulation; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, D-Penicillamine (2,5)-; Enkephalins; Evoked Potentials; gamma-Aminobutyric Acid; Glutamates; Glutamic Acid; Hippocampus; In Vitro Techniques; Male; Neurons; Pyramidal Tracts; Quinoxalines; Rats; Rats, Sprague-Dawley; Receptors, Opioid, delta; Receptors, Opioid, mu; Synapses