Distribution of mu-opioid receptors in rat visceral premotor neurons

Parasympathetic preganglionic cardiac motoneurons labeled after voluntary diving

A dramatic bradycardia is induced by underwater submersion in vertebrates. The location of parasympathetic preganglionic cardiac motor neurons driving this aspect of the diving response was investigated using cFos immunohistochemistry combined with retrograde transport of cholera toxin subunit B (CTB) to double-label neurons. After pericardial injections of CTB, trained rats voluntarily dove underwater, and their heart rates (HR) dropped immediately to 95 ± 2 bpm, an 80% reduction. After immunohistochemical processing, the vast majority of CTB labeled neurons were located in the reticular formation from the rostral cervical spinal cord to the facial motor nucleus, confirming previous studies. Labeled neurons caudal to the rostral ventrolateral medulla were usually spindle-shaped aligned along an oblique line running from the dorsal vagal nucleus to the ventrolateral reticular formation, while those more rostrally were multipolar with extended dendrites. Nine percent of retrogradely-labeled neurons were positive for both cFos and CTB after diving and 74% of these were found rostral to the obex. CTB also was transported transganglionically in primary afferent fibers, resulting in large granular deposits in dorsolateral, ventrolateral, and commissural subnuclei of the nucleus tractus solitarii (NTS) and finer deposits in lamina I and IV-V of the trigeminocervical complex. The overlap of parasympathetic preganglionic cardiac motor neurons activated by diving with those activated by baro- and chemoreceptors in the rostral ventrolateral medulla is discussed. Thus, the profound bradycardia seen with underwater submersion reinforces the notion that the mammalian diving response is the most powerful autonomic reflex known.

Effects of fentanyl on gastric myoelectrical activity: a possible association with polymorphisms of the mu-opioid receptor gene?

BACKGROUND

Opioids have inhibitory effects on gastric motility, but the mechanism is far from clear. Electrical slow waves in the stomach determine the frequency and the peristaltic nature of gastric contractions. The primary aim of this study was to investigate the effects of the opioid fentanyl on gastric myoelectric activity. As there were large variations between the subjects, we investigated whether the variation was correlated to single nucleotide polymorphisms (SNP) of the mu-opioid receptor (MOR) gene.

METHODS

We used cutaneous multichannel electrogastrography (EGG) to study myoelectrical activity in 20 patients scheduled for elective surgery. Fasting EGG was recorded for 30 min, followed by intravenous administration of fentanyl 1 microg/kg and subsequent EGG recording for 30 min. Spectral analysis of the two recording periods was performed and the variables assessed were dominant frequency (DF) of the EGG and its power (DP). Genetic analysis of the SNP A118G and G691C of the MOR gene was performed with the polymerase chain reaction technique.

RESULTS

There was a significant reduction in DF and DP after intravenous fentanyl. However, there was a large variation between the patients. In eight subjects EGG was unaffected, five subjects had a slower DF (bradygastria) and in six subjects the slow waves disappeared. We found no correlation between the EGG outcome and the presence of A118G or G691C in the MOR gene.

CONCLUSIONS

Fentanyl inhibited gastric myoelectrical activity in about half of the subjects. The variation could not be explained by SNP in the MOR gene. Because of small sample size, the results must be regarded as preliminary observations.

Misidentification of cardiac vagal pre-ganglionic neurons after injections of retrograde tracer into the pericardial space in the rat

We evaluated whether pericardial injections of the retrograde tracers cholera toxin subunit B (CTb) or Fast Blue (FB) reliably labelled cardiac vagal pre-ganglionic neurons. Injections of CTb into the pericardial space of the rat labelled neurons in both the external and compact formations of the nucleus ambiguus. Most labelled neurons were found in the compact formation of the nucleus ambiguus, and the majority of these, and only these, expressed immunoreactivity for calcitonin gene-related peptide. This distribution of labelled neurons and their immunohistochemical properties is characteristic of oesophageal motoneurons. Examination of the oesophagus following intra-pericardial CTb applications revealed strong labelling of motor end plates within the skeletal muscle of the thoracic but not the abdominal oesophagus. When a second retrograde tracer, FB, was injected into the abdominal oesophagus, labelled somata were found adjacent to CTb-labelled neurons in the compact formation of the nucleus ambiguus. No co-localisation of tracers was found, but identical proportions of calcitonin gene-related peptide (CGRP) immunoreactivity were observed in both groups of neurons. FB injected into the pericardial space labelled intra-cardiac neurons but not brainstem neurons. We conclude that intra-pericardial, and perhaps sub-epicardial, injections of some retrograde tracers are likely to label a subset of oesophageal, as well as cardiac, vagal motor neurons in the brainstem.

Action of κ and Δ opioid agonists on premotor cardiac vagal neurons in the nucleus ambiguus

Both enkephalin and dynorphin containing fibers are in close proximity to neurons in the nucleus ambiguus, including cardiac vagal neurons. Microinjection of Delta and kappa agonists into the nucleus ambiguus have been shown to evoke decreases in heart rate. Yet little is known about the mechanisms by which Delta and kappa opioid receptors alter the activity of cardiac vagal neurons. This study tests whether kappa and Delta opioid agonists can alter the activity of cardiac vagal neurons by modulating likely opioid targets including voltage gated calcium currents, and both glycinergic and GABA) neurotransmission to cardiac vagal neurons. Cardiac vagal neurons were identified in vitro by a fluorescent tracer and studied using patch clamp techniques. Neither the kappa agonist spiradoline or the Delta agonist [D-Pen(2), D-Pen(5)]enkephalin (DPDPE) modulated the voltage gated calcium currents in cardiac vagal neurons. DPDPE also did not alter either glycinergic or GABAergic synaptic neurotransmission. Spiradoline did not change GABAergic synaptic inputs, but did significantly inhibit glycinergic synaptic inputs to cardiac vagal neurons. At a concentration of 1 microM, spiradoline inhibited the amplitude of glycinergic events, and at a concentration of 5 microM, spiradoline inhibited both glycinergic amplitude and frequency. Spiradoline also inhibited both the amplitude and frequency of glycinergic miniature inhibitory post-synaptic currents, indicating kappa agonists likely act at both presynaptic and postsynaptic sites to inhibit glycinergic neurotransmission to cardiac vagal neurons.

Ionotropic glutamate receptor subunit immunoreactivity of vagal preganglionic neurones projecting to the rat heart

The ionotropic glutamate receptor subunits expressed by vagal preganglionic neurones in the rat medulla oblongata were examined by using fluorescence immunolabelling combined with retrograde neuronal tracing. The general population of these neurones in the medulla was identified by intraperitoneal injections of Fluorogold and also with choline acetyltransferase antibodies. Cardiac projecting neurones were specifically identified by applying the fluorescent tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine (DiI) to the heart or by injecting cholera toxin B-subunit into the pericardium. Both tracers labelled populations of neurones lying in the dorsal vagal nucleus, intermediate reticular formation and nucleus ambiguus, and when both tracers were applied simultaneously, approximately 50% of cells were dual-labelled. Control experiments established that the labelling was specific for neurones projecting to the heart. Most vagal preganglionic neurones, including those projecting to the heart, irrespective of their location in the medulla, had a similar profile of glutamate receptor immunoreactivity. Labelling of somata for the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA) subunit GluR1 was weak or absent, while labelling with antibodies directed to GluR2, a common sequence of GluR2 and GluR3, and GluR4 was moderate or intense. All neurones studied appeared to express the N-methyl-D-aspartate (NMDA) receptor subunit NR1, and while antibodies recognising the NR2A and NR2B splice variants gave strong labelling, immunoreactivity with a NR2B specific antibody was weaker. Weak to moderate labelling was seen in some neurones using antibodies to the kainate receptor subunits KA2 and GluR5-7. These results are consistent with neurophysiological data indicating the presence of AMPA, NMDA and kainate responses in cardiac vagal preganglionic neurones, and suggest that these neurones are similar to other vagal parasympathetic preganglionic neurones in expressing mainly AMPA receptor subunits GluR2-4.

Mu-opioid receptors are located postsynaptically and endomorphin-1 inhibits voltage-gated calcium currents in premotor cardiac parasympathetic neurons in the rat nucleus ambiguus

Activation of opioid receptors in the CNS evokes a dramatic decrease in heart rate which is mediated by increases in inhibitory parasympathetic activity to the heart. Injection of opiates into the nucleus ambiguus, where premotor cardiac parasympathetic nucleus ambiguus neurons are located elicits an increase in parasympathetic cardiac activity and bradycardia. However, the mechanisms responsible for altering the activity of premotor cardiac parasympathetic nucleus ambiguus neurons is unknown. This study examined at the electron microscopic level whether premotor cardiac parasympathetic nucleus ambiguus neurons possess postsynaptic opioid receptors and whether mu-opioid receptor agonists alter voltage-gated calcium currents in these neurons. Premotor cardiac parasympathetic nucleus ambiguus neurons were identified in the rat using retrograde fluorescent tracers. One series of experiments utilized dual-labeling immunocytochemical methods combined with electron microscopic analysis to determine if premotor cardiac parasympathetic nucleus ambiguus neurons contain mu-opioid receptors. In a second series of experiments whole cell patch clamp methodologies were used to determine whether activation of postsynaptic opioid receptors altered voltage-gated calcium currents in premotor cardiac parasympathetic nucleus ambiguus neurons in brainstem slices. The perikarya and 78% of the dendrites of premotor cardiac parasympathetic nucleus ambiguus neurons contain mu-opioid receptors. Voltage-gated calcium currents in premotor cardiac parasympathetic nucleus ambiguus neurons were comprised nearly entirely of omega-agatoxin-sensitive P/Q-type voltage-gated calcium currents. Activation of mu-opioid receptors inhibited these voltage-gated calcium currents and this inhibition was blocked by pretreatment with pertusis toxin. The mu-opioid receptor agonist endomorphin-1, but not the mu-opioid receptor agonist endomorphin-2, inhibited the calcium currents. In summary, mu-opioid receptors are located postsynaptically on premotor cardiac parasympathetic nucleus ambiguus neurons. The mu-opioid receptor agonist endomorphin1 inhibited the omega-agatoxin-sensitive P/Q-type voltage-gated calcium currents in premotor cardiac vagal nucleus ambiguus neurons. This inhibition is mediated via a G-protein mediated pathway which was blocked by pretreatment with pertusis toxin. It is possible that the inhibition of calcium currents may act to indirectly facilitate the activity of premotor cardiac parasympathetic nucleus ambiguus neurons by disinhibition, such as by a reduction in inhibitory calcium activated potassium currents.