Activation of enkephalin receptors reduces calcium conductance in neuroblastoma cells

Developmental switch in phenotypic expression of preproenkephalin mRNA and 45Ca2+ accumulation following kainate-induced status epilepticus

Kainic acid-induced status epilepticus results in delayed degeneration of CA3 hippocampal neurons in the mature but not immature rat hippocampus. In adult rats, the putative opioid precursor, preproenkephalin (PPE) mRNA increases in the dentate gyrus (DG), a region resistant to damage, following continuous limbic seizures. To explore why the immature brain is resistant to seizure-induced damage, the regional distribution of PPE mRNA expression and 45Ca2+ accumulation were compared in postnatal day 14 (P14) pup, and adult hippocampus at 5-6 h after kainate-induced status epilepticus. Inverted patterns of PPE expression and Ca2+ uptake were observed at the two ages. In P14 pups, PPE mRNA expression increased in DG and escalated in CA3, regions where 45Ca2+ accumulations were absent. In adult rats, PPE mRNA expression increased only in DG; 45Ca2+ labeling was predominant in CA3a,c and absent in DG. Pronounced increases in enkephalin neuropeptide synthesis in immature CA3 neurons may reduce glutamate release presynaptically and also prevent voltage-gated Ca2+ uptake into these neurons despite recurrent seizure activity. Opioid-mediated inhibition may provide an explanation for the resistance of the immature CA3 region to seizure-induced damage.

Opioids decrease high-voltage activated calcium currents in acutely dissociated neostriatal neurons

Although the distribution of opioid receptors is central to the patch-matrix model of neostriatal organization, it has been unclear whether these receptors are located post-synaptically. Moreover, it has not yet been clarified whether opioid receptor activation in neostriatum results in the modulation of calcium and/or potassium conductances. To test this, neostriatal neurons were acutely isolated and their sensitivity to opioid receptor agonists determined. At nanomolar concentrations, both the mu-agonist [D-Ala2, MePhe4,Gly-ol5]-enkephalin (DAMGO) and the delta-agonist [D-Pen2, D-Pen5]-enkephalin (DPEPE) reversibly decreased whole-cell calcium currents in medium-sized neurons. These effects were blocked by the opiate antagonist naloxone. These findings argue that activation of post-synaptic, opioid receptors is capable of modulating the excitability of neostriatal neurons.

Functional expression of Ca(2+)-mobilizing opioid receptors in Xenopus oocytes injected with rat brain mRNA

Functional expression of opioid receptors was detected in the Xenopus oocyte translation system by a voltage-clamp recording. After injection of poly(A)+ RNA isolated from 3-week-old rat striatum or whole brain, the oocytes often demonstrated intracellular Ca(2+)-mediated oscillatory responsiveness to [D-Ala2, N-methyl-Phe4, Gly5-ol]enkephalin (DAMGO), [D-Pen2, D-Pen5]enkephalin (DPDPE) and U50488H at a concentration of 1 microM. These responses were very transiently expressed after injection of the mRNA, however, water-injected oocytes never responded to any of these opioid agonists. After fractionation by a sucrose-density gradient, an RNA size of about 3-4 kb encoded these opioid receptors. In the oocytes injected with size-selected striatal mRNA, DPDPE evoked the fluctuating current with higher probability and larger amplitude than other agonists, whereas oocytes injected with size-selected whole brain mRNA produced DAMGO and U50488H responses predominantly. The DPDPE response of striatal mRNA-injected oocytes was antagonized by naloxone as well as the delta-specific antagonist ICI 174864. The DAMGO and U50488H responses have not been characterized yet because of a strong desensitizing property making repeated recordings impossible. These observations suggest that putative mu, delta and kappa subtypes of opioid receptors mobilizing intracellular Ca2+ are expressed in Xenopus oocytes by rat brain mRNA.

Presynaptic inhibitory action of enkephalin on excitatory transmission in superficial dorsal horn of rat spinal cord

1. Tight-seal whole-cell recordings were made from marginal neurones visually identified in thin slices of 1- to 2-week-old rat lumbar spinal cord. Excitatory postsynaptic currents (EPSCs), either evoked by extracellular stimulation or those arising spontaneously in tetrodotoxin, i.e. miniature EPSCs (mEPSCs), were recorded after blocking inhibitory synaptic inputs with strychnine and bicuculline. 2. The EPSCs were abolished reversibly by kynurenic acid or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) but not affected by (+)-2-amino-5-phosphonovalerate (APV), suggesting that they were mediated by non-NMDA (N-methyl-D-aspartate) glutamate receptors. Micromolar concentrations of methionine [Met5]enkephalin reversibly reduced the magnitude of evoked EPSCs and the frequency of mEPSCs. 3. The enkephalin action on the mEPSC frequency was blocked by naloxone. A specific agonist of mu-opiate receptor, [D-Ala2,N-Me-Phe4, Gly5]enkephalin-ol (DAGO) suppressed the mEPSC frequency. In contrast, neither a delta-opiate receptor agonist, [D-Pen2, L-Pen5]enkephalin (DPLPE) nor a kappa-opiate receptor agonist, (5 alpha, 7 alpha, 8 beta)-(-)-N-methyl-N-[7-(1-pyrrolidinyl)-1- oxaspiro(4,5)-dec-8-yl]benzeneacetamide (U-69,593) significantly affected the mEPSC frequency. 4. The amplitude of mEPSCs or of currents induced by exogenous L-glutamate, was not affected by [Met5]enkephalin. It is suggested that [Met5]enkephalin presynaptically inhibits glutamatergic EPSCs by activating the mu-opiate receptor. 5. The frequency of mEPSC was reduced by about 50% by replacement of external Ca2+ with Mg2+ or by addition of Cd2+. In Ca(2+)-free-Mg2+ solution, [Met5]enkephalin did not reduce the remaining mEPSCs' frequency any further. 6. It is concluded that the opiates may suppress presynaptic Ca2+ entry, thereby inhibiting synaptic transmission.

Modulation of vertebrate neuronal calcium channels by transmitters

A large number of neurotransmitters have now been shown to reduce the amplitude and slow the activation kinetics of whole cell HVA ICa in a great diversity of neurons. These transmitters include L-glutamate (AMPA/kainate, metabotropic and NMDA receptors), GABA (via GABAB receptors, NA (via alpha 2 receptors), 5-HT, NA (via alpha 2 receptors), DA and several peptides. Both whole-cell and single-channel studies have demonstrated that the N-channel is the most common channel type to be blocked by transmitters, although an inhibition of the L-type channel has also occasionally been reported. The suppression of the N-type Ca current was commonly shown to be voltage-dependent, with a relief at large positive voltages. Strong evidence has been put forward showing that the transmitter action is mediated by a G-protein, with GDP-beta-S blocking transmitter action, and GTP-gamma-S directly inhibiting the Ca channel. Moreover, pertussis toxin blocked the transmitter action in most neurons, and following such block, injection of the G-protein Go restored transmitter action. A direct link between the G-protein and the Ca channel has been widely theorized to mediate the action of transmitters on certain neurons. There is also some evidence that certain transmitters in specific neurons mediate calcium channel inhibition through a 2nd messenger, perhaps protein kinase C. Transmitters have also been found, although uncommonly, to inhibit HVA L-type and LVA T-type channels. In addition, an enhancement of both HVA and LVA Ca currents by transmitters has been demonstrated, and substantial evidence exists for mediation of this action by cAMP.

Activation of mu opioid receptors inhibits transient high- and low-threshold Ca2+ currents, but spares a sustained current

Opioids and opiates decrease the duration of action potentials and the amount of neurotransmitter released from sensory neurons. The mu-type opioid receptor, the binding site for morphine, is thought to act exclusively on K+ channels. Here, we show that activation of the mu receptor inhibits Ca2+ channels in rat sensory neurons; the effect is blocked by a mu antagonist and is not mimicked by kappa or delta receptor agonists. Both low-threshold (T-type) and high-threshold Ca2+ currents are partially suppressed. omega-Conotoxin-sensitive and omega-conotoxin-insensitive, high-threshold Ca2+ currents are inhibited. The kinetic effect on high-threshold current is like that caused by diminished rest potential: the transient component is selectively lost, whereas the sustained component is spared.

Antibodies to the GTP binding protein, Go, antagonize noradrenaline-induced calcium current inhibition in NG108-15 hybrid cells

The voltage-dependent calcium current in chemically differentiated NG108-15 cells is depressed by noradrenaline acting on alpha-adrenoreceptors. The response is absent in cells pretreated with pertussis toxin, implicating the involvement of a G-protein. To identify this G-protein, we have studied the response to noradrenaline in cells preinjected with antibodies specific for two G-proteins, Gi and Go. Cells injected with the Gi antibody responded normally to noradrenaline. In contrast, the response to noradrenaline in cells injected with the Go antibody was markedly attenuated. We conclude that Go is employed in coupling alpha-adrenoreceptors to the calcium channels in NG108-15 cells.

Noradrenaline- and Enkephalin-Induced Inhibition of Voltage-Sensitive Calcium Currents in NG108-15 Hybrid Cells

Voltage-sensitive calcium currents were recorded from chemically differentiated neuroblastoma x glioma hybrid (NG108-15) cells using the whole-cell clamp technique. Both noradrenaline and [D-Ala2, D-Leu5] enkephalin (DADLE) reversibly depressed the amplitude of the calcium current by up to 30%. The response to noradrenaline occluded that to DADLE suggesting that both agonists depress the same fraction of current. The response to DADLE but not that to noradrenaline desensitized rapidly. Cells responded normally to noradrenaline when desensitized to the opioid. Responses to either agonist were absent in cells pre-incubated with pertussis toxin. In addition the response to noradrenaline became irreversible in cells dialysed internally with a non-hydrolysable analogue of GTP. The response to noradrenaline was not affected by treatment of the cells with either membrane-permeable analogues of cAMP or a combination of forskolin and isobutylmethylxanthine. It is concluded that both noradrenaline and DADLE depress the same fraction of voltage-dependent calcium current in NG108-15 cells; that the responses are mediated by a pertussis-sensitive GTP-binding protein but are not secondary to a reduction in the intracellular concentration of cAMP; and that desensitization of the opioid response occurs at a site linked intimately to the opioid receptor rather than at a common site in the transduction pathway between receptor activation and reduction in the calcium channel current.