Morphine augments calcium-dependent potassium conductance in guinea-pig myenteric neurones

Morphine decreases enteric neuron excitability via inhibition of sodium channels

Gastrointestinal peristalsis is significantly dependent on the enteric nervous system. Constipation due to reduced peristalsis is a major side-effect of morphine, which limits the chronic usefulness of this excellent pain reliever in man. The ionic basis for the inhibition of enteric neuron excitability by morphine is not well characterized as previous studies have mainly utilized microelectrode recordings from whole mount myenteric plexus preparations in guinea pigs. Here we have developed a Swiss-Webster mouse myenteric neuron culture and examined their electrophysiological properties by patch-clamp techniques and determined the mechanism for morphine-induced decrease in neuronal excitability. Isolated neurons in culture were confirmed by immunostaining with pan-neuronal marker, β-III tubulin and two populations were identified by calbindin and calretinin staining. Distinct neuronal populations were further identified based on the presence and absence of an afterhyperpolarization (AHP). Cells with AHP expressed greater density of sodium currents. Morphine (3 µM) significantly reduced the amplitude of the action potential, increased the threshold for spike generation but did not alter the resting membrane potential. The decrease in excitability resulted from inhibition of sodium currents. In the presence of morphine, the steady-state voltage dependence of Na channels was shifted to the left with almost 50% of channels unavailable for activation from hyperpolarized potentials. During prolonged exposure to morphine (two hours), action potentials recovered, indicative of the development of tolerance in single enteric neurons. These results demonstrate the feasibility of isolating mouse myenteric neurons and establish sodium channel inhibition as a mechanism for morphine-induced decrease in neuronal excitability.

SK channels and the varieties of slow after-hyperpolarizations in neurons

Action potentials and associated Ca2+ influx can be followed by slow after-hyperpolarizations (sAHPs) caused by a voltage-insensitive, Ca2+-dependent K+ current. Slow AHPs are a widespread phenomenon in mammalian (including human) neurons and are present in both peripheral and central nervous systems. Although, the molecular identity of ion channels responsible for common membrane potential mechanisms has been largely determined, the nature of the channels that underlie the sAHPs in neurons, both in the brain and in the periphery, remains unresolved. This short review discusses why there is no clear molecular candidate for sAHPs.

Possible role of potassium channels in mu-receptor-mediated inhibition and muscarinic autoinhibition in acetylcholine release from myenteric plexus of guinea pig ileum

It is known that mu-agonists inhibit electrical field stimulation (EFS)-evoked ACh release from longitudinal muscle myenteric plexus (LMMP) preparation of guinea pig ileum when muscarinic autoinhibition does not fully work. In the present study, the possible role of K+ channels in the mechanisms of mu-agonists-induced inhibition and autoinhibition of ACh release was studied. In the presence of atropine, which blocks the autoinhibition, non-selective K+ channel blockers, tetraethylammonium (TEA) and 4-aminopyridine (4-AP), reversed the inhibitory effect of mu-agonists, morphine and [D-Ala2, N-Me-Phe4, Gly5-ol] enkephalin, on EFS-evoked ACh release, but not that of kappa-agonist U-50,488. Apamin, iberiotoxin or glibenclamide did not affect the inhibition of ACh release by morphine. On the other hand, in the absence of atropine (under the autoinhibition working condition), 4-AP increased EFS-evoked ACh release, but atropine did not further increase ACh release in the presence of 4-AP. In contrast, although TEA did not affect EFS-evoked ACh release, atropine increased ACh release in the presence of TEA. These results suggest that the inhibitory effects of mu-agonists and muscarinic autoinhibition on the ACh release are associated with activation of different types of K+ channels in the guinea pig LMMP preparations: the former is associated with 4-AP- and TEA-sensitive K+ channels and the latter is associated with 4-AP- but not TEA-sensitive K+ channels.

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.

The electrophysiological effects of [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin on guinea-pig myenteric neurons

The electrophysiological effects of a highly selective mu opioid agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAGO) were investigated on S and AH myenteric neurons in the guinea-pig ileum. Administration of DAGO (500 nM) caused a mean membrane hyperpolarization (+/- S.E.M.) of 7.8 +/- 0.5 mV in 58 of 138 S neurons, associated with a decrease in input resistance from 158 +/- 18 to 132 +/- 13 m omega. DAGO also produced a significant decrease in the mean amplitude of the cholinergic fast excitatory postsynaptic potentials (EPSPs), from 12.0 +/- 1.1 to 6.6 +/- 1.4 mV in 13 of 27 S neurons. On the other hand, in AH neurons, DAGO did not significantly affect the membrane potential, input resistance, action potential configuration, slow after-hyperpolarization or the antidromic action potential. The experiments indicate that the acute effects of mu opioids in the guinea-pig ileum are confined to a subpopulation of a single electrophysiological class of myenteric neurons.

4-Aminopyridine interrupts the modulation of acetylcholine release mediated by muscarinic and opiate receptors

The effect of 4-aminopyridine, a potassium channel blocker on the muscarinic and opiate modulation of acetylcholine release, was investigated. Rat frontal cortical slices were loaded with [3H]choline, superfused continuously, and stimulated electrically. 4-Aminopyridine enhanced the stimulation-evoked release of tritium without affecting basal release. The electrically evoked release of radioactivity was reduced by the muscarinic agonist oxotremorine and the delta selective opiate receptor agonist Metenkephalin, and was enhanced in the presence of the cholinesterase inhibitor physostigmine by the muscarinic antagonist atropine. These effects were completely abolished by 4-aminopyridine. Since 4-aminopyridine blocks potassium permeability of the neuron, it is suggested that the changes in potassium permeability and the consequent alteration of membrane polarization are involved in the presynaptic modulation of acetylcholine release.

Adenosine 5'-triphosphate modulates membrane potassium conductance in guinea-pig myenteric neurones

1. Intracellular recordings were made from myenteric neurones isolated from the guinea-pig small intestine to study actions of adenosine 5'-triphosphate (ATP). ATP was applied by superfusion (10 nM-100 microM) or pressure ejection from ATP-containing glass pipettes. 2. Myenteric neurones have been classified into two groups: type I/S neurones and type II/AH neurones. ATP produced a membrane hyperpolarization in 80% of AH neurones and a membrane depolarization in 90% of S neurones in a dose-dependent manner. Adenosine caused responses similar to those induced by ATP in both AH and S neurones, but was less effective than ATP. 3. The ATP-induced hyperpolarization was associated with a fall in input resistance, but the ATP-induced depolarization was accompanied by an increase in input resistance. Both responses reversed in polarity near the potassium equilibrium potential (-84 to -87 mV) and the reversal potential varied with extracellular potassium concentration, as predicted by the Nernst equation. These results indicate that the hyperpolarization is due to an increase, while the depolarization is due to a decrease in potassium conductance. 4. Both the hyperpolarization and the depolarization induced by ATP persisted in calcium-free solution containing 1.2 mM-magnesium, but were markedly reduced or abolished in calcium-free solutions containing 3.7-10 mM-magnesium and by 1 mM-nickel or cobalt. Both responses to ATP persisted in tetraethylammonium (1-10 mM) or tetrodotoxin (1-3 microM)-containing solutions. 5. Quinine and quinidine (1-100 microM) reversibly depressed both the ATP-induced responses. Caffeine (100 microM), theophylline (100 microM) and 3-isobutyl-1-methylxanthine (1-10 microM) did not significantly affect the ATP-induced depolarization but did reversibly depress the ATP-induced hyperpolarization. 6. These results suggest that the ATP-induced hyperpolarization may be due to activation, and the ATP-induced depolarization to inactivation, of a calcium-sensitive potassium conductance.

Specificities of afferents reinnervating cat muscle spindles after nerve section

1. We have made quantitative assessments of the sensory reinnervation and recovery of peroneus brevis muscle spindles following section and epineurial repair of the common peroneal nerve. After 6-50 weeks recovery, single-unit, dorsal-root recordings were made of the responses to ramp-and-hold or sinusoidal stretch of the reinnervated spindles, which were subsequently examined in teased, silver preparations. 2. Assessments of recovery used data obtained from cross-union experiments in which foreign afferents (including Ib) were given the opportunity of reinnervating spindles in the absence of their native (Ia, spindle II) afferents; and from an examination of tenuissimus spindles reinnervated by Ia and spindle II afferents in the absence of Ib afferents. These studies revealed: (i) that regenerating Ib afferents can terminate in sites originally occupied by the endings of Ia or spindle II afferents, and respond to stretch like normal Ia and spindle II afferents; (ii) that Ib and spindle II afferents reinnervating spindles are histologically identical apart from diameter range; and (iii) that some cutaneous afferents can reinnervate spindles and give highly abnormal, phasic stretch responses. 3. Recovery of afferents reinnervating spindles was marked by increases in conduction velocity and proportions firing tonically, but their firing rates at the three phases of ramp-and-hold stretch were considerably lower than normal and showed no tendency to increase. 4. Some relatively fast afferents that gave spindle II-type responses were identified as Ib afferents reinnervating secondary-ending sites; conversely, some relatively slow afferents that gave Ia-type responses were identified as spindle II afferents reinnervating primary-ending sites. 5. The estimated loss of spindle afferents from tenuissimus after nerve section (52% Ia, 49% spindle II) was considerably less than the estimated loss of these afferents from peroneus brevis after section of the common peroneal nerve (79% Ia, 86% spindle II). The proportion of spindles in tenuissimus reinnervated by free-ending afferents was also much lower (22%) than in peroneus brevis (73%). These differences are partly attributed to the greater size and degree of afferent complexity of the common peroneal nerve. 6. Similar proportions of spindles in peroneus brevis were reinnervated by Ia and Ib afferents after both partial (27% Ia, 20% Ib) and complete (21% Ia, 20% Ib) section of the common peroneal nerve.(ABSTRACT TRUNCATED AT 400 WORDS)

The ionic mechanisms underlying opioid actions

Recent experiments using intracellular recording techniques in vitro have revealed that common ionic mechanisms may explain the actions of opioid drugs. Evidence is now available from studies on guinea pig gut myenteric and submucous plexi, from preparations of spinal cord and dorsal root ganglia, from brain slices including the locus coeruleus and from neuroblastoma/glioma hybrid cells. The concensus is that mu opioid receptors activate an outward potassium conductance, possibly by way of adenylate cyclase. Activation of the receptor increases the membrane permeability to potassium ions and thus produces a membrane hyperpolarisation and conductance increase, plus an indirect inhibition of calcium entry during the action potential. Kappa opioids appear to inhibit directly the entry of calcium through voltage-dependent calcium channels, although to date there is no conclusive evidence that this mechanism of action can be extended to neurones of the central nervous system. The mechanism of action of delta opioids has only recently been investigated and initial evidence suggests they increase a potassium conductance similar to that increased by mu opioids. However, work in neuroblastoma x glioma hybrid cells has suggested that in these cells at least, receptor activation depress a component of voltage-dependent calcium current. The link between the receptor and the calcium channel involves a G-protein, Go.

Opioid inhibition of synaptic transmission in the guinea-pig myenteric plexus

Intracellular recordings were made from neurones in the myenteric plexus of the guinea-pig ileum. Presynaptic nerves were excited by a focal stimulating electrode on an interganglionic strand. Fast excitatory postsynaptic potentials (e.p.s.ps) were depressed in amplitude by morphine and [Met5]enkephalin in the concentration range of 1 nM-1 microM. Nicotinic depolarizations evoked by exogenously applied acetylcholine (ACh) were not affected by these opioids. Hyperpolarization of the presynaptic fibres probably contributed to the depression of the fast e.p.s.p. because fast e.p.s.ps evoked by low stimulus voltages were more depressed than those evoked by high stimulus voltages and fast e.p.s.ps resulting from activation of a single presynaptic fibre were blocked in a non-graded manner. Opioids depressed the slow e.p.s.p. in those neurones in which they did not change the resting membrane potential. The slow e.p.s.p. was increased in amplitude in those neurones hyperpolarized by opioids. Depolarizations resulting from application of barium, substance P or ACh were also enhanced by opioids. Equivalent circuit models in which opioids increase, and substance P or ACh decrease, the same potassium conductance could account for this enhancement. The actions of opioids were prevented or reversed by naloxone (1 nM-1 microM). It is concluded that morphine and enkephalin inhibit the release of ACh and a non-cholinergic transmitter from fibres of the myenteric plexus, and that this may involve a hyperpolarization of presynaptic fibres. Additionally, opioids can interact postsynaptically with other substances which affect membrane potassium conductances.

Mu and kappa opioids inhibit transmitter release by different mechanisms

The actions of various opioids were examined on calcium action potentials in the cell somata of guinea pig myenteric neurones and on the release of acetylcholine at synapses onto these cells. The opioids morphine, normorphine, and [D-Ala2, MePhe4, Met5(O)]enkephalin-ol caused membrane hyperpolarizations resulting from an increase in potassium conductance; opioids that are more selective agonists for the kappa receptor subtype (dynorphin, tifluadom, U50488H) did not. Conversely, calcium action potentials were depressed or abolished by the kappa opioids but were not affected by morphine and [D-Ala2, MePhe4, Met(O)5]enkephalin-ol. Both groups of opioids caused presynaptic inhibition of acetylcholine release in the myenteric plexus, depressing the amplitude of the fast excitatory postsynaptic potential. The presynaptic inhibition caused by [D-Ala2, MePhe4, Met(O)5]enkephalin-ol, morphine, and normorphine, but not that caused by the kappa opioids, was prevented by pretreatment with the selective mu site-directed irreversible antagonist beta-funaltrexamine. Furthermore, the presynaptic inhibitory action of morphine and [D-Ala2, MePhe4, Met(O)5]enkephalin-ol, but not that of the kappa-receptor agonists, was reversibly blocked by barium. The results suggest that presynaptic inhibition caused by mu receptor activation probably results from an increase in potassium conductance, whereas kappa-receptor agonists may depress the release of acetylcholine by directly reducing calcium entry into the nerve terminals.