Nicotinic receptors in sensory ganglia

Presence of alpha7 nicotinic acetylcholine receptors on dorsal root ganglion neurons proved using knockout mice and selective alpha-neurotoxins in histochemistry

In complex tissues where multiple subtypes of nicotinic acetylcholine receptors (nAChRs) are expressed, immunohistochemistry has been the most popular tool for investigation of nAChR subunit distribution. However, recent studies with nAChR subunit knockout mice demonstrated that a large panel of antibodies is unsuitable. Thus, we aimed to develop a histochemical method for selective labeling of alpha7 nAChR with neurotoxins, utilizing alpha7 nAChR-transfected cells, dorsal root ganglia (DRG) and spinal cord from wild-type and knockout mouse. The specificity of Alexa Fluor 488-conjugated alpha-bungarotoxin (Alexa-alphaBgt) was demonstrated in binding to alpha7-transfected cells inhibited by long-chain alpha-cobratoxin (CTX), but not short-chain alpha-neurotoxin II (NTII). In contrast, binding to Torpedo muscle-type nAChRs and to motor end plates in mouse tongue sections was prevented by both CTX and NTII. In tissue sections of DRG, expressing all neuronal nAChR subunits, only CTX precluded Alexa-alphaBgt labeling of neurons, with no staining for alpha7 nAChR knockout tissue. It proved that alpha7 nAChRs are the major alphaBgt-binding sites in mouse DRG. Corresponding results were obtained for terminals in the spinal cord. Thus, we present a protocol utilizing Alexa-alphaBgt and non-labeled CTX/NTII that allows specific histochemical detection of alpha7 nAChR with a spatial resolution at the level of single axon terminals.

Are there interactions between acetylcholine- and ATP-induced responses at the level of a visceral sensory ganglion?

We investigate possible interactions between acetylcholine (ACh)- and adenosine 5'-triphosphate (ATP)-induced responses of petrosal ganglion, where the perikarya of most sensory neurons of the glossopharyngeal nerve are located. Experiments were performed on petrosal ganglia excised from pentobarbitone-anesthetized cats, desheathed and perfused in vitro. Separate applications of ACh and ATP to the exposed surface of the ganglion induced bursts of antidromic potentials recorded from the carotid (sinus) nerve branch of the glossopharyngeal nerve, which frequencies were dependent on the dose of the applied agonists. The simultaneous application of previously determined ED50s of ACh and ATP provoked responses corresponding closely to the simple addition of the responses elicited by the separate application of each agent. Responses usually subsided within 1 min of stimuli application but were followed by periods of refractoriness to subsequent application of the same agent. After determining the timing for recovering from desensitization to the ED50s of ACh and ATP applied separately, ACh was applied while the preparation had been desensitized to ATP and then ATP was applied during desensitization to ACh, but responses obtained were similar to control responses induced by each agent separately. In summary, ACh- and ATP-induced responses of petrosal ganglion neurons are simply additive, followed by a few minute lasting desensitization, but cross-desensitization was not observed. Thus, ACh and ATP seem to operate through independent receptors, activating separate ionic channels, whose coincident currents do not interfere each other.

Nicotinic acetylcholine receptor subtypes in nociceptive dorsal root ganglion neurons of the adult rat

Stimulation of nicotinic acetylcholine receptors (nAChR) excites peripheral sensory nerve fibres, but also exert antinociceptive effects. The differences in these nAChR-mediated effects could be related to the expression of different nAChR subtypes located on nociceptive neurons. In the present study, we focused on the recently described alpha 10-nAChR subunit, and on alpha 4 and alpha 7 subunits, which are the most abundant subunits in the central nervous system. In nociceptive neurons from thoracic and lumbar dorsal root ganglia (DRG), nAChR subunits were found at transcriptional (RT-PCR), translational (immunohistochemistry) and functional levels. Cultured DRG neurons express mRNA for the subunits alpha 2-7 and alpha 10. The alpha-subunit proteins 4, 7 and 10 were colocalised in virtually all nociceptive neurons that were identified by immunoreactivity for the vanilloid receptor TRPV-1. These findings were corroborated by current recordings and calcium measurements, which revealed excitatory inward currents and calcium responses in capsaicin sensitive neurons.

Dorsal root ganglion neurons express multiple nicotinic acetylcholine receptor subtypes

Although nicotinic agonists can modulate sensory transmission, particularly nociceptive signaling, remarkably little is known about the functional expression of nicotinic acetylcholine receptors (nAChRs) on primary sensory neurons. We have utilized molecular and electrophysiological techniques to characterize the functional diversity of nAChR expression on mammalian dorsal root ganglion (DRG) neurons. RT-PCR analysis of subunit mRNA in DRG tissue revealed the presence of nAChR subunits alpha2-7 and beta2-beta4. Using whole cell patch-clamp recording and rapid application of nicotinic agonists, four pharmacologically distinct categories of nicotinic responses were identified in cultured DRG neurons. Capacitance measurements were used to divide neurons into populations of large and small cells, and the prevalence of nicotinic responses was compared between groups. Category I (alpha7-like) responses were seen in 77% of large neurons and 32% of small neurons and were antagonized by 10 nM methyllycaconitine citrate (MLA) or or 50 nM alpha-bungarotoxin (alpha-BTX). Category II (alpha3beta4-like) responses were seen in 16% of large neurons and 9% of small neurons and were antagonized by 20 microM mecamylamine but not 10 nM MLA or 1 microM DHbetaE. Category II responses had a higher sensitivity to cytisine than nicotine. Two other types of responses were identified in a much smaller percentage of neurons and were classified as either category III (alpha4beta2-like) or category IV (subtype unknown) responses. Both the alpha7-like and alpha3beta4-like responses could be desensitized by prolonged applications of the analgesic epibatidine.

Antihyperalgesic activity of epibatidine in the formalin model of facial pain

A growing body of evidence supports a nicotinic cholinergic approach to pain management, as neuronal nicotinic receptor agonists have shown efficacy across animal models of both inflammatory and neuropathic pain. However, most of these investigations have focused on the spinal system, and there is to date no report of nicotinic receptor-mediated antinociception in any pain model involving the trigeminal field of innervation. Thus, the purpose of the present studies was to evaluate whether the neuronal nicotinic receptor agonist epibatidine possesses antihyperalgesic activity in the formalin model of facial pain. Adult, male, Sprague--Dawley rats received a 50-microl, subcutaneous injection of 5% formalin into one vibrissal pad and the consequent, facial grooming behavior was monitored. Consistent with previous investigations using the formalin model, animals exhibited two periods of nocifensive grooming: (1) an acute phase that began immediately, peaked at 3 min and almost completely abated by 6 min, and (2) a tonic phase that began between 6 and 9 min, peaked at 21 min and slowly diminished over the ensuing 24 min. The subcutaneous administration of epibatidine (1--5 microg/kg) 5 min prior to the formalin injection led to a significant, dose-dependent reduction of both the acute and tonic phases of hyperalgesia. Separate groups of animals receiving epibatidine either 15, 30 or 60 min prior to the formalin injection exhibited a progressively diminishing antihyperalgesic response that was no longer significant in either phase by 30 min. Finally, pretreatment with the selective neuronal nicotinic receptor antagonist mecamylamine completely abolished the antihyperalgesic effect of epibatidine in both phases. Taken together, these studies demonstrate that in both the acute and tonic phases of the formalin model of facial pain, epibatidine produces a neuronal nicotinic receptor-mediated antihyperalgesia that is both dose- and time-dependent. These results support the rationale for exploring the clinical efficacy of nicotinic agonists as analgesics to treat certain types of trigeminal pain in humans.

Individual variation in the expression profiles of nicotinic receptors in the olfactory bulb and trigeminal ganglion and identification of alpha2, alpha6, alpha9, and beta3 transcripts

Nicotine evokes dose-dependent and often variable chemosensory responses in animals and humans. Earlier observations that nicotine binds to some nicotinic acetylcholine receptor (nAChR) subtypes in the olfactory bulb (OB) and trigeminal ganglion (TG) led us to investigate the complete nAChR expression profile in each tissue and to determine whether inter-individual differences exist in male and female rats. Total RNA was extracted from individual samples of dissected OB and TG and analyzed by a sensitive reverse transcription-polymerase chain reaction (RT-PCR) assay to determine the messenger RNA profiles of ten transcripts encoded by the alpha2, alpha3, alpha4, alpha5, alpha6, alpha7, alpha9, beta2, beta3, and beta4 nAChR genes. We found that (a) in the OB, all animals expressed alpha2, alpha3, alpha4, alpha5, alpha7, beta2, and beta4 subunit mRNAs, whereas alpha6, beta3, and alpha9 transcripts were expressed in only 17, 28, and 33% of the animals, respectively, and (b) in the TG, all animals expressed alpha2, alpha3, alpha6, alpha7, beta2, and beta4 subunit mRNAs, whereas alpha9, beta3, alpha4, and alpha5 transcripts were expressed in 4, 38, 88, and 92% of the animals, respectively. These results also identified new subunits that are expressed in each tissue (alpha2, alpha6, alpha9, and beta3) and demonstrated that individual rats may have different tissue-specific expression profiles for alpha4, alpha5, alpha6, alpha9, and beta3 transcripts. Such variations are likely to be reflected in the composition of functional receptor subtypes in the rat OB and TG that have different activation and desensitization characteristics to acetylcholine and nicotine.

Receptor localization in the mammalian dorsal horn and primary afferent neurons

The dorsal horn of the spinal cord is a primary receiving area for somatosensory input and contains high concentrations of a large variety of receptors. These receptors tend to congregate in lamina II, which is a major receiving center for fine, presumably nociceptive, somatosensory input. There are rapid reorganizations of many of these receptors in response to various stimuli or pathological situations. These receptor localizations in the normal and their changes after various pertubations modify present concepts about the wiring diagram of the nervous system. Accordingly, the present work reviews the receptor localizations and relates them to classic organizational patterns in the mammalian dorsal horn.

Spinal cholinergic and monoaminergic receptors mediate descending inhibition from the nuclei reticularis gigantocellularis and gigantocellularis pars alpha in the rat

Focal electrical stimulation and glutamate microinjection in the nuclei reticularis gigantocellularis (NGC) and gigantocellularis pars alpha (NGC alpha) both inhibit the nociceptive tail-flick (TF) reflex in rats. The present experiments were undertaken to determine the transmitter(s) at the level of the lumbar spinal cord mediating descending inhibition of the TF reflex produced by activation of the NGC/NGC alpha. Intrathecal administration of atropine (7.2-57.6 nmol) produced a dose-dependent increase in the electrical stimulation threshold required for inhibition of the TF reflex. Phentolamine (47.2 or 94.4 nmol) and methysergide (32 or 64 nmol) also increased the stimulation threshold significantly, but only at the greater doses. Neither naloxone (27.5 or 55 nmol) nor mecamylamine (49.1 or 98.2 nmol) affected stimulation thresholds for inhibition of the TF reflex. Stimulation at threshold intensities for inhibition did not change the blood pressure significantly at most sites of stimulation in the NGC/NGC alpha (25/39). Intrathecal administration of atropine, phentolamine or methysergide did not affect resting blood pressure or changes associated with stimulation in most cases, although inhibition of the TF reflex by stimulation in the NGC/NGC alpha was affected consistently by these pretreatments. Similarly, glutamate (100 nmol, 0.5 microliter/1.5 min) microinjection produced a short-lasting inhibition (4.63 +/- 0.70 min, n = 19) of the TF reflex. Glutamate microinjection produced both pressor and depressor effects which were not affected by intrathecal pretreatment. Inhibition of the TF reflex by glutamate was attenuated significantly by intrathecal pretreatment with atropine, scopolamine, phentolamine and methysergide, but not naloxone or mecamylamine. These findings suggest that either a descending or local spinal cholinergic system, together with descending serotonergic and noradrenergic systems, are involved in the centrifugal inhibition of spinal nociceptive transmission from the NGC/NGC alpha.

Ultrastructural distribution of alpha-bungarotoxin binding sites in the suprachiasmatic nucleus of the rat hypothalamus

The distribution of (125I) alpha bungarotoxin (alpha-BTX) binding sites in the suprachiasmatic nucleus (SCN) of the adult female rat was examined by electron-microscopic autoradiography. The ultrastructural distribution of silver grains was analysed by line source, direct point count, and 50% probability circle methods. Real grain distribution was significantly different from that of randomly generated hypothetical grains. Line source analysis demonstrated two populations of sources: one associated with membranes, and one inside neuronal structures. Probability circle analysis of shared grains indicated that membrane-bound-radioactive sources were mainly associated with axo-dendritic appositions. Only a small proportion of labeled neuronal interfaces exhibited synaptic differentiations in the plane of section. However, the compartment containing synaptic terminals was the most enriched when comparing real to hypothetical grains. Probability circle analysis of exclusive grains demonstrated that sources that were not associated with neuronal plasma membranes were likely to be within nerve cell bodies and dendrites. It is concluded that the majority of specifically labelled alpha-BTX binding sites in the SCN is membrane bound, and may be associated with axodendritic synaptic transmission. The presence of a significant proportion of the label in the soma and dendrites of suprachiasmatic neurons 24 h after ventricular infusion suggests that some of the labeled binding sites (junctional or nonjunctional) may be internalized within these two compartments.

Axonal transport of alpha-bungarotoxin binding sites in rat sciatic nerve

[125I]alpha-Bungarotoxin (alpha-BuTX) binding sites accumulate both proximal and distal to a ligature positioned around the sciatic nerve of rats. [125I]alpha-BuTX binding sites, localized using quantitative receptor autoradiography, were found to accumulate at nerve ligatures at a relatively constant rate which suggests that they undergo both anterograde and retrograde axonal transport. [125I]alpha-BuTX binding to sections of ligated sciatic nerve was saturable with apparent dissociation constants of 0.97 nM proximal and 0.53 nM distal to the ligature. D-Tubocurarine, nicotine, decamethonium and atropine displaced [125]alpha-BuTX from sciatic nerve sections with affinities comparable to those previously reported for the toxin binding component of rat brain. These data indicate that [125I]alpha-BuTX binding sites pharmacologically similar to those of rat brain are transported in sciatic nerve. Axonally transported toxin binding sites may correspond to those previously localized to the plasma membrane of peripheral nerve axons and on the terminals of motor neurons.

Alpha-bungarotoxin binding sites on sensory neurones and their axonal transport in sensory afferents

The autoradiographic localization of [125I]alpha-bungarotoxin binding sites on primary sensory fibres was investigated. Nicotinic alpha-bungarotoxin binding sites were localized to a small sub-population of large dorsal root ganglion cells in the rat, monkey, cat and human dorsal root ganglia. Ligation of the sciatic nerve or dorsal root in the rat resulted in an anterograde accumulation of binding sites proximal to the dorsal root ganglion, and a small retrograde accumulation. Unilateral dorsal root section in the rat produced a loss of toxin binding sites mainly within lamina III of the dorsal horn. These results suggest that nicotinic alpha-bungarotoxin binding sites manufactured in large dorsal root ganglion cell bodies are transported both centrally to the spinal cord and also peripherally.

Localization of opiate and histamine H1-receptors in the primate sensory ganglia and spinal cord

The autoradiographic localizations of opiate and histamine H1-receptors were studied quantitatively and compared following dorsal root section in the primate spinal cord. High densities of opiate and H1-receptors were found in the superficial layers of the dorsal horn. Dorsal root section produced a 40-50% loss of both receptors types throughout layers I and II with no loss in deeper layers of the dorsal horn. "Bursts' of opiate and H1-receptor labelling was observed over a population of small diameter cells (35-45 micron or micrometer) in the primate dorsal root ganglion. These cells represented approximately 8% of the total cells present. Serial consecutive sections demonstrated a population of cells bearing both opiate and histamine H1-receptors.

Atropine-sensitive gastric excitation by stimulation of thoracic dorsal roots--antidromic activation of afferents?

In chloralosed cats, usually with ligated adrenals and paralysed with gallamine, the thoracic dorsal roots were electrically stimulated and gastric motor responses were recorded. Thoracic dorsal root stimulation regularly elicited gastric contractions after pretreatment with guanethidine and/or hexamethonium which were readily blocked by atropine. It was established that these gastric motor responses were due to peripheral stimulation of the dorsal root and not to spread to the ventral roots. Together with other results (Delbro & Lisander 1980) the findings suggest an antidromic activation of thin afferent fibres with excitatory collaterals to intramural cholinergic neurons which convey the gastric contractions.

Localization of synaptic and nonsynaptic nicotinic-acetylcholine receptors in the goldfish retina

The localization of nicotinic-cholinergic receptors in the inner plexiform layer (IPL) of goldfish retina was studied by electron microscopic analysis of the binding pattern of a conjugate or horseradish peroxidase and alpha bungarotoxin (HRP-alpha BTx). Specific HRP reaction product (blockade by 1mM curare) was found at both synaptic and nonsynaptic sites. Synaptic binding sites for HRP-alpha BTx, which accounted for only 16% of the total specific reaction product sites, always involved an amacrine process as the presynaptic element, whereas amacrine, ganglion, and bipolar cells could be post-synaptic elements at labeled synapses. Only 17.5% of the total number of amacrine synapses were labeled by HRP-alpha BTx. Labeled synapses showed the same distribution in the IPL as unlabeled synapses: bimodal for amacrine-to-bipolar synapses with peak concentrations at the 20% and 80% layers and unimodal for amacrine-to-nonbipolar synapses with a peak concentration at the 60% layer. Nonsynaptic binding sites for HRP-alpha BTx (84% of total) were seen on the dendrites of ganglion, amacrine, and bipolar cells. The distribution of the nonsynaptic sites in the IPL largely accounts for the trilaminar binding pattern of 125I-alpha BTx as observed in light microscopic autoradiographs. If, as appears likely, the distribution of synapses is the relevant variable in determining the sites of neuronal interaction for a given transmitter system, then this study further illustrates the importance of distinguishing synaptic from nonsynaptic binding when using receptor-ligand probes to localize sites of chemical synaptic transmission.