Noradrenaline-mediated synaptic inhibition in rat locus coeruleus neurones

Catecholamine inhibition of calcium action potentials in rat locus coeruleus neurones

Intracellular recordings were made from neurones in the nucleus locus coeruleus in a slice of tissue cut from the rat pons. Clonidine (100 nM-10 microM), noradrenaline (10 microM-1 mM) and adrenaline (10 microM-1 mM) all reduced the duration of the spontaneously occurring action potential of the neurones. This effect was also observed on the action potential in the presence of tetrodotoxin, which results from calcium entering the cell. These concentrations of clonidine, noradrenaline and adrenaline always hyperpolarized the membrane. This hyperpolarization was prevented by two procedures which block potassium currents--intracellular caesium and extracellular barium. In conditions of potassium current blockade, noradrenaline (100 microM-1 mM) and adrenaline (20 microM-1 mM) shortened the calcium action potential but clonidine was ineffective even at 10 microM. Adrenaline and noradrenaline also suppressed inward calcium and barium currents measured under voltage clamp. This action of noradrenaline and adrenaline was not prevented by yohimbine (10 microM), propranolol (20 microM) or prazosin (1 microM); it was reduced by a concentration of phentolamine about 100 times higher than its Ke for alpha 2-adrenoceptors on locus coeruleus neurones. It is concluded that noradrenaline and adrenaline can directly inhibit calcium action potentials in locus coeruleus neurones when applied in high concentrations, but that this does not involve an alpha 2-adrenoceptor.

Characterization of alpha 2-adrenoceptors which increase potassium conductance in rat locus coeruleus neurones

Intracellular recordings were made from locus coeruleus neurones in a slice of rat pons superfused in vitro. A single-electrode voltage-clamp amplifier was used to measure membrane currents. Superfusion of the slice with clonidine (3-100 nM) or noradrenaline (100 nM-100 microM), or brief application of noradrenaline from a pipette by a pressure pulse, caused dose-dependent membrane hyperpolarizations. Phenylephrine (10 microM) and isoprenaline (10 microM) were ineffective. The hyperpolarizations were accompanied by a decrease in neurone input resistance. The hyperpolarization evoked by pressure ejection of noradrenaline could be reversed by membrane polarization to -110 mV. Clonidine and noradrenaline caused a membrane current which was linearly related to membrane potential between -50 and -120 mV, being outward at resting levels and reversing at -110 mV. The concentration-response curves for clonidine and noradrenaline were shifted rightwards in a parallel manner by alpha 2-adrenoceptor antagonists. The antagonist KeS estimated from the degree of shift were: RX 781094 9 nM, yohimbine 14 nM, phentolamine 20 nM and piperoxane 49 nM. These experiments indicate that in locus coeruleus neurones an increase in potassium conductance results from activation of alpha 2-adrenoceptors similar to those characterized on peripheral neurones.

Mechanism of alpha 2-adrenergic inhibition of neuroeffector transmission in the mouse vas deferens

The process by which the activation of presynaptic alpha 2-adrenoceptors inhibits the release of noradrenaline from terminals of postganglionic sympathetic nerves was studied in the mouse isolated vas deferens. Clonidine was used as a prototypic agonist. Field stimulation-evoked excitatory junction potentials (e.j.p.s) were recorded from individual muscle cells. The e.j.p. amplitudes were taken as a measure of transmitter release. Changes in the external Ca2+ concentration from 2.5 to 1.25 or 5 mM caused corresponding changes in the size of e.j.p.s. When the normal Ca2+ concentration of the medium (2.5 mM) was substituted by equimolar quantities of Ba2+ or Sr2+, the e.j.p. amplitudes decreased considerably. Clonidine (0.3-30 nM) inhibited the nerve stimulation-evoked e.j.p. amplitudes in a concentration-dependent manner, without altering appreciably the frequency of spontaneous e.j.p.s. Procedures known to enhance Ca2+ entry into nerve terminals, like a high Ca2+ medium (Ca2+ 5 mM) or 4-aminopyridine 30 microM reduced the effect of clonidine. Repetitive nerve stimulation at 3 Hz, which is supposed to lead to an accumulation of free Ca2+ inside nerve terminals, similarly counteracted the effect of clonidine 10 nM. Whereas the alpha 2-adrenergic inhibition of the first e.j.p. in a train was unaffected, the inhibition of all successive e.j.p.s was gradually decreased. At 5 mM Ca2+ only the time-course of facilitation became faster, the decrease in alpha 2-adrenergic inhibition proceeded with the same pulse-dependent rate as at a normal external Ca2+ concentration, although from a lower initial level.(ABSTRACT TRUNCATED AT 250 WORDS)

Presynaptic opiate receptor- and alpha 2-adrenoceptor-mediated inhibition of noradrenaline release in the rat brain: role of hyperpolarization?

The inhibitory actions of exogenous noradrenaline (1 microM) and clonidine (1 microM) as well as well as of the opiate receptor agonists morphine (1 microM) and [D-Ala2,D-Leu5]enkephalin (DADLE, 1 microM) on the potassium-induced Ca2+-dependent release of [3H]noradrenaline from superfused rat brain cortex slices were independent of the degree of depolarization when release was effected by 15 or 56 mM K+ for 5 min. The non-depolarization-dependent release of [3H]noradrenaline induced by exposing the slices for 5 min to medium with NaCl replaced by LiCl was only partially Ca2+-dependent and was not inhibited by the Ca2+ antagonist Cd2+ (50 microM). This release was strongly inhibited by morphine, DADLE, exogenous noradrenaline and clonidine at 1 microM concentrations both in the presence and the absence of extracellular Ca2+. Together with other data in the literature these results strongly suggest that opiate receptors and alpha 2-adrenoceptors located on noradrenergic axonal varicosities, unlike those located on cell bodies, do not primarily mediate hyperpolarization of the neuronal membrane. Instead the activation of these presynaptic receptors causes a reduction of Ca2+ availability for, or the utilization of Ca2+ by the secretion process upon invasion of an action potential.

Membrane properties of rat locus coeruleus neurones

Intracellular recordings were made from neurones in the locus coeruleus contained within a slice cut from rat pons and maintained in vitro. Most neurones fired action potentials spontaneously at frequencies of between 1 and 5 Hz; this did not arise from spontaneous synaptic input but appeared to result from endogenous properties of the membrane conductances. Under voltage clamp at potentials near threshold for action potential generation (-55 mV) there was a persistent inward calcium current. This current became less with membrane hyperpolarization and was abolished at about -70 mV. Two potassium currents were observed. The first had properties similar to that generally described as the "fast" potassium current (IK,A); it flowed transiently (for about 200 ms) when the membrane potential passed from about -65 to -45 mV, and was blocked by 4-aminopyridine. The second was a calcium-activated potassium current (IK,Ca); it flowed for several seconds following a burst of calcium action potentials. Spontaneous and evoked action potentials had both tetrodotoxin-sensitive and tetrodotoxin-resistant components. The latter was apparently due to calcium entry. The potential changes occurring during the spontaneous firing of locus coeruleus neurones could be reconstructed qualitatively from the ionic conductances observed. The membrane properties of the locus coeruleus neurones were remarkably uniform; however, about 5% of cells impaled within the region of the locus coeruleus were electrophysiologically distinct. These atypical cells had short duration action potentials, did not fire spontaneously and had large spontaneous depolarizing synaptic potentials.

Hyperpolarizing and age-dependent depolarizing responses of cultured locus coeruleus neurons to noradrenaline

The electrical activity and responses to noradrenaline (NA) of locus coeruleus (LC) neurons have been studied in organotypic cultures using intracellular recording. Most LC neurons were predominantly quiescent, though occasional bursts of activity were observed; a few cells were tonically active at rates of 0.5-5/s. In most cells tested, iontophoretic application of NA evoked responses which were initially hyperpolarizing, sometimes followed by a depolarizing phase and frequently followed by a period of increased excitatory synaptic activity. The enhanced synaptic activity appeared to be an indirect effect since it was blocked by bath application of tetrodotoxin (TTX). In the presence of TTX, responses to NA of all but one cell were simple hyperpolarizations or biphasic (hyperpolarization/depolarization) responses. The presence of the depolarizing component appeared to be age-dependent, since it was frequently observed in cultures grown in vitro for less than 26 days, while neurons in older cultures exhibited only hyperpolarizing responses. If such age-dependent depolarizing responses are present in vivo, they would represent a unique example of a transmitter response which is present only during a transient developmental phase.

The actions of noradrenaline on neurones of the rat substantia gelatinosa in vitro

Intracellular recordings were made from substantia gelatinosa (s.g.) neurones in slices cut from adult rat spinal cord and maintained in vitro. Noradrenaline applied by superfusion (1-50 microM), or by brief pressure ejection from a micropipette, reversibly hyperpolarized 80% of the s.g. cells. The noradrenaline induced hyperpolarization was associated with an increase in conductance and it reversed in polarity at -88 mV. The reversal potential changed when the external potassium concentration was changed, as predicted by the Nernst equation. The noradrenaline hyperpolarization was antagonized by phentolamine and yohimbine but not by propranolol and prazosin. The hyperpolarization was probably a direct action on the impaled cell and not due to release or block of release of other transmitters, because the effects persisted during a perfusion with a low calcium/high magnesium solution or in a solution containing cobalt and high magnesium. In 35 of 148 cells, noradrenaline caused a dose-related increase of spontaneous excitatory post-synaptic potentials (e.p.s.p.s). This effect was blocked by tetrodotoxin. The noradrenaline induced increase in e.p.s.p.s was blocked by phentolamine and prazosin but not by the alpha 2-blockers yohimbine and RX 781094. A few cells were depolarized by noradrenaline, and this was blocked by prazosin but not by yohimbine. It is suggested that noradrenaline may inhibit nociceptive input to the spinal cord by increasing the potassium conductance of s.g. neurones.

Noradrenergic actions of Purkinje and locus coeruleus neurons in culture

Explant cultures prepared from neonatal mice were used to study the actions of iontophoretically applied noradrenaline (NA) on Purkinje and locus coeruleus neurons, with extracellular and intracellular recording respectively. NA depressed spontaneous activity of Purkinje neurons and enhanced excitatory responses to glutamate in 14/16 cells. In cultures older than 26 days, NA hyperpolarized LC neurons but had no effect on or depressed depolarizing responses to glutamate. The hyperpolarizations were blocked by the selective alpha 2 antagonist yohimbine. Enhancement of glutamate responses is not a ubiquitous characteristic of NA action and appears not to be associated with alpha 2 adrenergic receptors.

Opiate activation of potassium conductance inhibits calcium action potentials in rat locus coeruleus neurones

Opiates act on mu-receptors to increase the potassium conductance of rat locus coeruleus neurones. Opiates also depress the rate of rise and peak amplitude of calcium action potentials in these cells. The action of opiates on calcium action potentials was prevented by two procedures which blocked the opiate-induced potassium current, intracellular caesium and extracellular barium. This indicates that the opiate reduction in calcium entry is secondary to an increased potassium current.