Fentanyl decreases catecholamine metabolism measured by in vivo voltammetry in the rat locus coeruleus

The objective of this study was to investigate under controlled conditions the effects of fentanyl on the rat locus coeruleus catechol oxidation current. Using differential normal pulse voltammetry combined with electrochemically treated carbon fiber electrodes to measure the catechol oxidation current, catecholamine metabolism can be reliably monitored. Male Sprague-Dawley rats weighing 500-600 g had carbon fiber electrodes implanted into the locus coeruleus under halothane - O2 - air anesthesia with controlled ventilation and muscle relaxation. Experiments consisted of four groups of rats given the following treatments: (A) saline (n = 6); (B) fentanyl, 10 micrograms.kg-1 i.v. (n = 6); (C) naloxone, 800 micrograms.kg-1 i.v. followed 2 min later by fentanyl, 10 micrograms.kg-1 (n = 5); (D) clonidine, 200 micrograms.kg-1 i.p. (n = 6). There was no significant change in the catechol oxidation current following saline. Fentanyl produced a significant (ANOVA, p less than 0.05) decrease in the catechol oxidation current (maximum 32 min postinjection was 75.8 +/- 4.6% of baseline). This decrease was prevented by a prior injection of naloxone. Clonidine produced a significant decrease in catechol oxidation current (maximum 40 min postinjection was 54.1 +/- 7.0% of baseline). Systolic blood pressure was significantly decreased following clonidine and there were no significant changes in arterial blood gases throughout the experiments. The alpha 2-adrenergic agonist clonidine and the opioid fentanyl produced a decrease in locus coeruleus catechol oxidation current measured by in vivo voltammetry, which monitors catecholamine turnover.     

Benzodiazepines attenuate single unit activity in the locus coeruleus

Several classes of anxiolytic compounds have the common effect of decreasing the firing of noradrenergic neurons or attenuating the post- synaptic effects of noradrenergic activity. In order to determine whether the benzodiazepines, the most widely used anxiolytics, also decrease noradrenergic activity, the effect of acute intravenous injections of diazepam (0.1–2.0 mg/kg) and chlordiazepoxide (0.5–4.0 mg/kg) were administered to anesthetized rats while spontaneous activity of single neurons in the principal noradrenergic nucleus, the locus coeruleus, was recorded. Diazepam and chlordiazepoxide decreased spontaneous single unit activity in the locus coeruleus at relatively low doses. This net effect on noradrenergic systems is consistent with the actions of several classes of nonbenzodiazepine anxiolytics, and with the involvement of noradrenergic systems in the neural mechanisms of anxiety.     

Methyl parathion increases neuronal activities in the rat locus coeruleus

Exposure to organophosphate insecticides induces undesirable behavioral changes in humans, including anxiety and irritability, depression, cognitive disturbances and sleep disorders. Little information currently exists concerning the neural mechanisms underlying such behavioral changes. The brain stem locus coeruleus (LC) could be a mediator of organophosphate insecticide-induced behavioral toxicities since it contains high levels of acetylcholinesterase and is involved in the regulation of the sleep-wake cycle, attention, arousal, memory, and pathological processes, including anxiety and depression. In the present study, using a multi-wire recording technique, we examined the effects of methyl parathion, a commonly used organophosphate insecticide, on the firing patterns of LC neurons in rats. Systemic administration of a single dose of methyl parathion (1 mg/kg, i.v.) increased the spontaneous firing rates of LC neurons by 240% but did not change the temporal relationships among the activities of multiple LC neurons. This dose of methyl parathion induced a 50% decrease in blood acetylcholinesterase activity and a 48% decrease in LC acetylcholinesterase activity. The methyl parathion-induced excitation of LC neurons was reversed by administration of atropine sulfate, a muscarinic receptor antagonist, indicating an involvement of muscarinic receptors. The methyl parathion-induced increase in LC neuronal activity returned to normal within 30 min while the blood acetylcholinesterase activity remained inhibited for over 1 h. These data indicate that methyl parathion treatment can elicit excitation of LC neurons. Such excitation could contribute to the neuronal basis of organophosphate insecticide-induced behavioral changes in human.     

Functional recuperation of the serotoninergic innervation in the rat locus coeruleus

1. 5,6-dihydroxytryptamine (5,6-DHT) or a lesion of the raphe centralis superior (RCS) cause significant decreases in the serotonin (5-HT) content and significant increases in the tyrosine hydroxylase activity in the locus coeruleus (LC) of the rat. This suggests that noradrenaline (NA) synthesis is controlled by serotonin-containing neurons in the raphe system via their terminals in the LC. 2. Radioautography after intraventricular infusion of tritiated serotonin (3H-5-HT) and biochemical determinations of endogenous 5-HT content showed an almost complete disappearance of serotoninergic axonal varicosities and content in the LC region 10-15 days after intraventricular administration of 75 micrograms of 5,6-DHT. Two to 4 months after neurotoxin administration, 5-HT fibers had regrown in the LC but, contrary to the normal innervation pattern, the majority of them invaded the medial most portion of the nucleus and the adjacent subependymal region. The LC region regained almost all of its endogenous 5-HT content in the same time period. 3. Functional recuperation of these 5-HT fibers was demonstrated by the fact that the RCS had, after regeneration, the same functional control on NA synthesis as in the normal animal.     

Noradrenaline overflow in mouse dentate gyrus following locus coeruleus and natural stimulation: real-time monitoring by in vivo voltammetry

The pattern of catecholaminergic innervation of the dentate gyrus (DG) of the hippocampus, particularly the relatively dense and selective noradrenergic input, creates favourable conditions for real-time monitoring of noradrenaline (NA) release following stimulation of the locus coeruleus (LC) by in vivo voltammetry. Two electrochemically active species with different temporal characteristics were registered in the DG following electrical stimulation of the LC. Several approaches, including testing of anatomical and pharmacological specificity, coating of microelectrodes with Nafion and use of fast cyclic voltammetry, were used to verify the characteristics of electrochemical responses. The first sharp peak that appeared immediately during stimulation was definitely associated with NA overflow. The second late peak was possibly attributable to ascorbic acid. We examined the characteristics of alpha-2 adrenoceptor regulation of NA release in the DG, and showed for the first time that noradrenergic terminals resemble dopaminergic terminals in their mechanisms of increasing the refilling rate of the readily releasable pool following stimulation repeated at short intervals. Amperometric registration of NA in the DG was complicated by interference with electrical activity of hippocampus. This interference could be used, after appropriate filtration, for simultaneous recording from the same microelectrode of NA release and electrical activity of the hippocampus.     

Generators of synchronous activity of the locus coeruleus during development

The locus coeruleus is thought to play an important role in the development of the central nervous system through a coordinated release of noradrenaline. The influence of the locus coeruleus on its diverse targets is strongly synchronized by the existence of electrical and chemical coupling between neurones, afferent supply of the nucleus from restricted sources and diffuse innervation of target areas. Extensive coupling between neonatal locus coeruleus neurones produces rhythmic synchronized electrical activity and distributes afferent synaptic activity throughout the entire nucleus. The strength of electrical coupling declines with age but appears to persist to a limited extent in the adult.     

Open-field behavior of rats following bilateral coagulation of the locus coeruleus

A modified open-field test was used in the experiments on rats. It was shown that animals 6 days after bilateral coagulation of locus coeruleus displayed weight changes and some behavioural parameters distinct from those of sham-operated and control animals. The most pronounced differences observed in animals after locus coeruleus coagulation, as compared to the control, were inversion and adaptive responses to a sudden brief stimulus, locomotor dysfunction and absence of response to handling.     

Activation of locus coeruleus neurons by peripheral stimuli: modulation by a collateral inhibitory mechanism.

Noradrenergic neurons of the rat locus coeruleus (LC) respond to noxious stimuli or peripheral nerve stimulation with a burst of spikes followed by a period of suppressed activity. During this period of post-activation suppression, responses to additional stimuli were attenuated. After antidromic activation of the LC there was also a period of reduced responsivity, presumably mediated by inhibitory recurrent LC collaterals. The suppression of LC unit firing which follows nerve stimulation was reduced by piperoxane, an α-adrenergic antagonist which is known to block the norepinephrine-mediated autoinhibitory action of recurrent LC axon collaterals. The specificity of piperoxane in blocking norepinephrine was shown by the fact that it did not antagonize several other putative transmitters in the LC (i.e., GABA, glycine, and met-enkephalin). It is concluded that the post-activation reduction of LC neuronal responsivity may be mediated in part through noradrenergic autoinhibitory mechanisms within the LC.     

Role of the locus coeruleus in the mechanism of different types of analgesia

The activity of antinociceptive mechanisms during exposure to cold swim stress (CSS), inescapable foot shock (FS) and morphine administration (10 mg/kg) was studied in rats subjected to bilateral lesions of locus coeruleus (LC). It has been shown that under all types of stimulation the latent periods (LP) of nociceptive reactions of paw licking and tail flick were significantly increased, as compared to baseline level, thus suggesting suppression of the pain sensitivity. In rats subjected to CSS and morphine administration significantly shorter LP were revealed in the experimental group, compared to the control, which suggested partial inhibition of antinociceptive mechanisms activity after LC lesion. FS did not cause any differences in the experimental and control groups. It is concluded that LC lesions cannot completely inhibit the antinociceptive mechanisms activated by CSS and morphine injection. In FS monoamine mechanisms of LC do not seem to play an important role in the functioning of antinociceptive mechanisms.     

Dual effect of the locus coeruleus on neuronal activity in the visual cortex

The effects exerted by electrical stimulation of the locus coeruleus (LC) on neuronal activity in the visual cortex have been studied in acute experiments on rabbits. The pattern of cortical afterprocesses affecting poststimulus histograms of neuronal activity has been found not always to correspond to the direction of change in the neuronal excitation level in the period of LC stimulation. An analysis of crossed interval histograms for a pair of neurons located in the same microvolume of the cortex has revealed the existence of two independent postsynaptic effects of LC stimulation: a fast (synaptic) effect and a slow (modulatory) effect. The findings are discussed with allowance for morphological features of the synaptic connections and interneuronal transmission in the noradrenergic system of the brain.Neirofiziologiya/Neurophysiology, Vol. 25, No. 4, pp. 243–246, July–August, 1993.     

Hyperphagia and hyperdipsia after locus coeruleus lesions in the stumptailed monkey.

Bilateral lesions of the nucleus locus coeruleus in 7 female stumptail monkeys were followed by long lasting hyperphagia and hyperdipsia. The percentage increase in weight at five weeks after lesioning correlated highly with 3-methoxy-4-hydroxy-phenethylene glycol (MHPG) concentration in the cerebral cortex. This relationship suggests that the effects are due to the locus coeruleus system and are not the result of variable destruction of the ventral noradrenergic or adjacent non-noradrenergic pathways.     

In vivo activation of renal phospholipase activity by bradykinin in the rat

Activation of a renal acylhydrolase by bradykinin (BK) with subsequent release of prostaglandins precursor arachidonic acid has been postulated but not yet demonstrated. BK was infused into the left artery of 27 rats which were subdivided into 9 groups according to BK concentration (10, 100 and 1000 ng/min) and time of infusion (20, 40 and 60 min). The rats were then sacrificed and the left to right ratio of renal phospholipase activity was determined. The data obtained were processed by a factorial analysis of variance which allowed the effect of BK and the time of infusion to be evaluated independently as well as interdependently. The results of the statistical analysis showed that phospholipase activity depends on both BK dosage and infusion time and that there is no interaction between dose and time. These findings offer evidence for the “in vivo” activation of the kidney phospholipase activity by BK.     

Monoaminergic interaction in the central nervous system: a morphological analysis in the locus coeruleus of the rat.

The locus coeruleus of the rat is richly innervated by many aminergic neurons varying in amine content and in site of origin. There are adrenergic and noradrenergic neurons originating in the medulla oblongata, dopaminergic from the hypothalamus, serotonergic from the mesencephalon and also intrinsic noradrenergic neurons in the locus coeruleus complex. Of these, adrenergic and dopaminergic inputs appear relatively specific and powerful.     

Morphine interference with noradrenaline-mediated inhibition from the locus coeruleus.

The effect of morphine on inhibition of relay neurons in the spinal trigeminal nucleus (STN) produced by conditioning stimulation of the locus coeruleus (LC) was studied in cats. Alterations in orthodromic and antidromic spike generations in STN were absent. However, the narcotic did selectively reduce the inhibitory effect of LC conditioning stimulation on STN neurons without affecting that of the sensory ortex. Nalorphine partially antagonized this effect of the narcotic. A reduction in orthodromic spike number of STN neurons induced by intraventricular noradrenaline in reserpinized animals was unaltered by morphine treatment. These data demonstrate that morphine selectively interferes with LC-induced inhibition of STN neurons and in addition may block noradrenaline release from terminals of LC neurons.     

Arecoline excites rat locus coeruleus neurons by activating the M2-muscarinic receptor

The action of arecoline on rat locus coeruleus neurons was studied by intracellular recording from the in vitro brain slice preparation. Superfusion of arecoline (0.1-100 microM) caused two dose-related effects, an increased firing rate and, in neurons previously hyperpolarized to a constant potential by passing a steady hyperpolarizing current across the membrane, depolarization. Both effects were associated with a reduction in membrane input resistance. Moreover, the arecoline-induced excitatory effects were antagonized by the muscarinic receptor antagonist, atropine, but not by the nicotinic receptor antagonist, hexamethonium. Methoctramine, a selective M2-muscarinic receptor antagonist, was also effective in reversing the arecoline-induced effects, with a dissociation equilibrium constant of 14.2+/-1.2 nM (n=6). These results therefore suggest that arecoline exerts its excitatory actions by binding to M2-muscarinic receptors on the cell membrane of neurons of the locus coeruleus.