Biophysical properties and responses to neurotransmitters of petrosal and geniculate ganglion neurons innervating the tongue

Is there a role for GABA in peripheral taste processing

In the peripheral neurons and circuits for hearing, balance, touch and pain, GABA plays diverse and important roles. In some cases, GABA is an essential player in the maintenance of sensory receptors and afferent neurons. In other instances, GABA modulates the sensory signal before it reaches CNS neurons. And in yet other instances, tonic GABA-mediated signals set the resting tone and excitability of afferent neurons. GABA_A receptors are present on gustatory afferent neurons that carry taste signals from taste buds to central circuits in the brainstem. Yet, the functional significance of these receptors is unexplored. Here, I outline some of the roles of GABA in other peripheral sensory systems. I then consider whether similar functions may be ascribed to GABA signaling in the taste periphery.

Bioelectronic tongue: Current status and perspectives

In the course of evolution, nature has endowed humans with systems for the recognition of a wide range of tastes with a sensitivity and selectivity which are indispensable for the evaluation of edibility and flavour attributes. Inspiration by a biological sense of taste has become a basis for the design of instruments, operation principles and parameters enabling to mimic the unique properties of their biological precursors. In response to the demand for fast, sensitive and selective techniques of flavouring analysis, devices belonging to the group of bioelectronic tongues (B-ETs) have been designed. They combine achievements of chemometric analysis employed for many years in electronic tongues (ETs), with unique properties of bio-inspired materials, such as natural taste receptors (TRs) regarding receptor/ligand affinity. Investigations of the efficiency of the prototype devices create new application possibilities and suggest successful implementation in real applications. With advances in the field of biotechnology, microfluidics and nanotechnologies, many exciting developments have been made in the design of B-ETs in the last five years or so. The presented characteristics of the recent design solutions, application possibilities, critical evaluation of potentialities and limitations as well as the outline of further development prospects related to B-ETs should contribute to the systematisation and expansion of our knowledge.

Transcriptomes and neurotransmitter profiles of classes of gustatory and somatosensory neurons in the geniculate ganglion

Taste buds are innervated by neurons whose cell bodies reside in cranial sensory ganglia. Studies on the functional properties and connectivity of these neurons are hindered by the lack of markers to define their molecular identities and classes. The mouse geniculate ganglion contains chemosensory neurons innervating lingual and palatal taste buds and somatosensory neurons innervating the pinna. Here, we report single cell RNA sequencing of geniculate ganglion neurons. Using unbiased transcriptome analyses, we show a pronounced separation between two major clusters which, by anterograde labeling, correspond to gustatory and somatosensory neurons. Among the gustatory neurons, three subclusters are present, each with its own complement of transcription factors and neurotransmitter response profiles. The smallest subcluster expresses both gustatory- and mechanosensory-related genes, suggesting a novel type of sensory neuron. We identify several markers to help dissect the functional distinctions among gustatory neurons and address questions regarding target interactions and taste coding.Characterization of gustatory neural pathways has suffered due to a lack of molecular markers. Here, the authors report single cell RNA sequencing and unbiased transcriptome analyses to reveal major distinctions between gustatory and somatosensory neurons and subclusters of gustatory neurons with unique molecular and functional profiles.

Transcriptomes and neurotransmitter profiles of classes of gustatory and somatosensory neurons in the geniculate ganglion

Taste buds are innervated by neurons whose cell bodies reside in cranial sensory ganglia. Studies on the functional properties and connectivity of these neurons are hindered by the lack of markers to define their molecular identities and classes. The mouse geniculate ganglion contains chemosensory neurons innervating lingual and palatal taste buds and somatosensory neurons innervating the pinna. Here, we report single cell RNA sequencing of geniculate ganglion neurons. Using unbiased transcriptome analyses, we show a pronounced separation between two major clusters which, by anterograde labeling, correspond to gustatory and somatosensory neurons. Among the gustatory neurons, three subclusters are present, each with its own complement of transcription factors and neurotransmitter response profiles. The smallest subcluster expresses both gustatory- and mechanosensory-related genes, suggesting a novel type of sensory neuron. We identify several markers to help dissect the functional distinctions among gustatory neurons and address questions regarding target interactions and taste coding.

Characterization of gustatory neural pathways has suffered due to a lack of molecular markers. Here, the authors report single cell RNA sequencing and unbiased transcriptome analyses to reveal major distinctions between gustatory and somatosensory neurons and subclusters of gustatory neurons with unique molecular and functional profiles.

Taste buds: cells, signals and synapses

The past decade has witnessed a consolidation and refinement of the extraordinary progress made in taste research. This Review describes recent advances in our understanding of taste receptors, taste buds, and the connections between taste buds and sensory afferent fibres. The article discusses new findings regarding the cellular mechanisms for detecting tastes, new data on the transmitters involved in taste processing and new studies that address longstanding arguments about taste coding.

Recent Advances in Molecular Mechanisms of Taste Signaling and Modifying

The sense of taste conveys crucial information about the quality and nutritional value of foods before it is ingested. Taste signaling begins with taste cells via taste receptors in oral cavity. Activation of these receptors drives the transduction systems in taste receptor cells. Then particular transmitters are released from the taste cells and activate corresponding afferent gustatory nerve fibers. Recent studies have revealed that taste sensitivities are defined by distinct taste receptors and modulated by endogenous humoral factors in a specific group of taste cells. Such peripheral taste generations and modifications would directly influence intake of nutritive substances. This review will highlight current understanding of molecular mechanisms for taste reception, signal transduction in taste bud cells, transmission between taste cells and nerves, regeneration from taste stem cells, and modification by humoral factors at peripheral taste organs.

Pragmatically on the sense of taste - a short treatise based on culinary art

The sense of taste is essential for proper functioning of the organism. The authors describe, in an accessible way, the complex mechanisms of taste perception. The structure of particular taste receptors, variants of their activation, as well as physical and chemical factors modifying the sensation of taste, are presented. Exquisite culinary examples are given in order to facilitate the reader with the understanding of why, at the level of the cerebral cortex, a virtually infinite number of combinations of taste sensations can be perceived. The discourse is spiced up by reflections of the eminent philosopher of taste, J.A. Brillat-Savarin, who convinces us that food intake should be not only a physiological act, but also a refined pleasure.

Tachykinins stimulate a subset of mouse taste cells

The tachykinins substance P (SP) and neurokinin A (NKA) are present in nociceptive sensory fibers expressing transient receptor potential cation channel, subfamily V, member 1 (TRPV1). These fibers are found extensively in and around the taste buds of several species. Tachykinins are released from nociceptive fibers by irritants such as capsaicin, the active compound found in chili peppers commonly associated with the sensation of spiciness. Using real-time Ca²⁺-imaging on isolated taste cells, it was observed that SP induces Ca²⁺ -responses in a subset of taste cells at concentrations in the low nanomolar range. These responses were reversibly inhibited by blocking the SP receptor NK-1R. NKA also induced Ca²⁺-responses in a subset of taste cells, but only at concentrations in the high nanomolar range. These responses were only partially inhibited by blocking the NKA receptor NK-2R, and were also inhibited by blocking NK-1R indicating that NKA is only active in taste cells at concentrations that activate both receptors. In addition, it was determined that tachykinin signaling in taste cells requires Ca²⁺-release from endoplasmic reticulum stores. RT-PCR analysis further confirmed that mouse taste buds express NK-1R and NK-2R. Using Ca²⁺-imaging and single cell RT-PCR, it was determined that the majority of tachykinin-responsive taste cells were Type I (Glial-like) and umami-responsive Type II (Receptor) cells. Importantly, stimulating NK-1R had an additive effect on Ca²⁺ responses evoked by umami stimuli in Type II (Receptor) cells. This data indicates that tachykinin release from nociceptive sensory fibers in and around taste buds may enhance umami and other taste modalities, providing a possible mechanism for the increased palatability of spicy foods.

Characteristics of sodium currents in rat geniculate ganglion neurons

Geniculate ganglion (GG) cell bodies of chorda tympani (CT), greater superficial petrosal (GSP), and posterior auricular (PA) nerves transmit orofacial sensory information to the rostral nucleus of the solitary tract. We have used whole cell recording to investigate the characteristics of the Na(+) channels in isolated Fluorogold-labeled GG neurons that innervate different peripheral receptive fields. GG neurons expressed two classes of Na(+) channels, TTX sensitive (TTX-S) and TTX resistant (TTX-R). The majority of GG neurons expressed TTX-R currents of different amplitudes. TTX-R currents were relatively small in 60% of the neurons but were large in 12% of the sampled population. In a further 28% of the neurons, TTX completely abolished all Na(+) currents. Application of TTX completely inhibited action potential generation in all CT and PA neurons but had little effect on the generation of action potentials in 40% of GSP neurons. Most CT, GSP, and PA neurons stained positively with IB(4), and 27% of the GSP neurons were capsaicin sensitive. The majority of IB(4)-positive GSP neurons with large TTX-R Na(+) currents responded to capsaicin, whereas IB(4)-positive GSP neurons with small TTX-R Na(+) currents were capsaicin insensitive. These data demonstrate the heterogeneity of GG neurons and indicate the existence of a subset of GSP neurons sensitive to capsaicin, usually associated with nociceptors. Since there are no reports of nociceptors in the GSP receptive field, the role of these capsaicin-sensitive neurons is not clear.

GABA, its receptors, and GABAergic inhibition in mouse taste buds

Taste buds consist of at least three principal cell types that have different functions in processing gustatory signals: glial-like (type I) cells, receptor (type II) cells, and presynaptic (type III) cells. Using a combination of Ca2+ imaging, single-cell reverse transcriptase-PCR and immunostaining, we show that GABA is an inhibitory transmitter in mouse taste buds, acting on GABA(A) and GABA(B) receptors to suppress transmitter (ATP) secretion from receptor cells during taste stimulation. Specifically, receptor cells express GABA(A) receptor subunits β2, δ, and π, as well as GABA(B) receptors. In contrast, presynaptic cells express the GABA(A) β3 subunit and only occasionally GABA(B) receptors. In keeping with the distinct expression pattern of GABA receptors in presynaptic cells, we detected no GABAergic suppression of transmitter release from presynaptic cells. We suggest that GABA may serve function(s) in taste buds in addition to synaptic inhibition. Finally, we also defined the source of GABA in taste buds: GABA is synthesized by GAD65 in type I taste cells as well as by GAD67 in presynaptic (type III) taste cells and is stored in both those two cell types. We conclude that GABA is an inhibitory transmitter released during taste stimulation and possibly also during growth and differentiation of taste buds.

Characteristics of calcium currents in rat geniculate ganglion neurons

Geniculate ganglion (GG) cell bodies of chorda tympani (CT), greater superficial petrosal (GSP), and posterior auricular (PA) nerves transmit orofacial sensory information to the rostral nucleus of the solitary tract (rNST). We used whole cell recording to study the characteristics of the Ca(2+) channels in isolated Fluorogold-labeled GG neurons that innervate different peripheral receptive fields. PA neurons were significantly larger than CT and GSP neurons, and CT neurons could be further subdivided based on soma diameter. Although all GG neurons possess both low voltage-activated (LVA) "T-type" and high voltage-activated (HVA) Ca(2+) currents, CT, GSP, and PA neurons have distinctly different Ca(2+) current expression patterns. Of GG neurons that express T-type currents, the CT and GSP neurons had moderate and PA neurons had larger amplitude T-type currents. HVA Ca(2+) currents in the GG neurons were separated into several groups using specific Ca(2+) channel blockers. Sequential applications of L, N, and P/Q-type channel antagonists inhibited portions of Ca(2+) current in all CT, GSP, and PA neurons to a different extent in each neuron group. No difference was observed in the percentage of L- and N-type Ca(2+) currents reduced by the antagonists in CT, GSP, and PA neurons. Action potentials in GG neurons are followed by a Ca(2+) current initiated after depolarization (ADP) that may influence intrinsic firing patterns. These results show that based on Ca(2+) channel expression the GG contains a heterogeneous population of sensory neurons possibly related to the type of sensory information they relay to the rNST.

Responses induced by acetylcholine and ATP in the rabbit petrosal ganglion

Acetylcholine and ATP appear to mediate excitatory transmission between receptor (glomus) cells and the petrosal ganglion (PG) neuron terminals in the carotid body. In most species these putative transmitters are excitatory, while inhibitory effects had been reported in the rabbit. We studied the effects of the application of acetylcholine and ATP to the PG on the carotid nerve activity in vitro. Acetylcholine and ATP applied to the PG increased the carotid nerve activity in a dose-dependent manner. Acetylcholine-induced responses were mimicked by nicotine, antagonized by hexamethonium, and enhanced by atropine. Bethanechol had no effect on basal activity, but reduced acetylcholine-induced responses. Suramin antagonized ATP-induced responses, and AMP had little effect on the carotid nerve activity. Our results suggest that rabbit PG neurons projecting through the carotid nerve are endowed with nicotinic acetylcholine and purinergic P2 receptors that increase the carotid nerve activity, while simultaneous activation of muscarinic cholinergic receptors reduce the maximal response evoked by nicotinic cholinergic receptor activation.

Distinct expression of cold receptors (TRPM8 and TRPA1) in the rat nodose-petrosal ganglion complex

TRPM8 and TRPA1 are cold-activated transient receptor potential (TRP) cation channels. TRPM8 is activated by moderate cooling, while TRPA1 is activated by extreme, noxious cold temperatures. These cold receptors are expressed in different subpopulations of primary afferent neurons. TRPA1 is co-expressed in a subpopulation of somatosensory neurons expressing TRPV1, which is activated by heat. However, the distribution and co-expression of these channels in the nodose-petrosal ganglion complex, which contains the jugular (JG), petrosal (PG), and nodose ganglia (NG) (mainly involved in putative somatic, chemo- and somato-sensation, and somato and visceral sensation, respectively), remain unknown. Here, we conducted in situ hybridization analysis of the rat nodose-petrosal ganglion complex using specific riboprobes for TRPM8, TRPA1, and TRPV1 to compare the features of the cranial sensory ganglia. Hybridization signals for TRPA1 were diffusely observed throughout these ganglia, whereas TRPM8 transcripts were seen in the JG and PG but not in the NG. We retrogradely labeled cranial nerve X with Fast Blue (fluorescent dye) and found TRPM8 transcripts in the jugular-vagal ganglion but not the NG neurons. TRPA1 transcripts were not detected in TRPM8-expressing neurons but were present in the subpopulation of TRPV1-expressing visceral sensory neurons. Taken together, these findings support that in the vagal system the expression of cold-activated TRP channels differs between nodose- and jugular-ganglion neurons suggesting different mechanisms of cold-transduction and that the TRPA1 distribution is consistent with its proposed function as a cold-sensing receptor in the visceral system.

The Salted Food Addiction Hypothesis may explain overeating and the obesity epidemic

One plausible explanation for the controversy that surrounds the causes and clinical management of obesity is the notion that overeating and obesity may only be a couple of "symptoms" associated with a yet to be discovered medical disorder.

OBJECTIVES

To introduce the Salted Food Addition Hypothesis. This theory proposes that salted food acts in the brain like an opiate agonist, producing a hedonic reward which has been perceived as being only peripherally "flavorful", "tasty" or "delicious". The Salted Food Addition Hypothesis also proposes that opiate receptor withdrawal has been perceived as "preference," "urges," "craving" or "hunger" for salted food.

METHODS

The Salted Food Addiction Hypothesis is made manifest by individually presenting a basic review of its primary coexisting components; the Neurological Component and the Psychosocial Component. We also designed a prospective study in order to test our hypothesis that opiate dependent subjects increase their consumption of salted food during opiate withdrawal.

RESULTS

The neuropsychiatric evidence integrated here suggests that salted food acts like an, albeit mild, opiate agonist which drives overeating and weight gain. The opiate dependent group studied (N=27) developed a 6.6% increase in weight during opiate withdrawal.

CONCLUSIONS

Salted Food may be an addictive substance that stimulates opiate and dopamine receptors in the brain's reward and pleasure center more than it is "tasty", while salted food preference, urge, craving and hunger may be manifestations of opiate withdrawal. Salted food and opiate withdrawal stimulate appetite, increases calorie consumption, augments the incidence of overeating, overweight, obesity and related illnesses. Obesity and related illnesses may be symptoms of Salted Food Addiction.

Postsynaptic action of GABA in modulating sensory transmission in co-cultures of rat carotid body via GABA(A) receptors

GABA is expressed in carotid body (CB) chemoreceptor type I cells and has previously been reported to modulate sensory transmission via presynaptic GABA(B) receptors. Because low doses of clinically important GABA(A) receptor (GABA(A)R) agonists, e.g. benzodiazepines, have been reported to depress afferent CB responses to hypoxia, we investigated the potential contribution of GABA(A)R in co-cultures of rat type I cells and sensory petrosal neurones (PNs). During gramicidin perforated-patch recordings (to preserve intracellular Cl-), GABA and/or the GABA(A) agonist muscimol (50 microm) induced a bicuculline-sensitive membrane depolarization in isolated PNs. GABA-induced whole-cell currents reversed at approximately -38 mV and had an EC50 of approximately 10 microm (Hill coefficient = approximately 1) at -60 mV. During simultaneous PN and type I cell recordings at functional chemosensory units in co-culture, bicuculline reversibly potentiated the PN, but not type I cell, depolarizing response to hypoxia. Application of the CB excitatory neurotransmitter ATP (1 microm) over the soma of functional PN induced a spike discharge that was markedly suppressed during co-application with GABA (2 microm), even though GABA alone was excitatory. RT-PCR analysis detected expression of GABAergic markers including mRNA for alpha1, alpha2, beta2, gamma2S, gamma2L and gamma3 GABA(A)R subunits in petrosal ganglia extracts. Also, CB extracts contained mRNAs for GABA biosynthetic markers, i.e. glutamate decarboxylase (GAD) isoforms GAD 67A,E, and GABA transporter isoforms GAT 2,3 and BGT-1. In CB sections, sensory nerve endings apposed to type I cells were immunopositive for the GABA(A)R beta subunit. These data suggest that GABA, released from the CB during hypoxia, inhibits sensory discharge postsynaptically via a shunting mechanism involving GABA(A) receptors.

Immuno-localization of vesicular acetylcholine transporter in mouse taste cells and adjacent nerve fibers: indication of acetylcholine release

Acetylcholine (ACh) is well established as a neurotransmitter and/or neuromodulator in various organs. Previously, it has been shown by Ogura (J Neurophysiol 87:2643-2649, 2002) that in both physiological and immunohistochemical studies the muscarinic acetylcholine (ACh) receptor is present in taste receptor cells. However, it has not been determined if ACh is released locally from taste receptor cells and/or surrounding nerve fibers. In this study we investigated the sites of ACh release in mouse taste tissue using the antisera against vesicular ACh transporter (VAChT), a key element of ACh-containing vesicles. Our data show that VAChT-immunoreactivity is present in many taste receptor cells, including cells expressing the transient receptor potential channel M5 (TRPM5). In taste cells, VAChT-immunoreactivity was colocalized with the immunoreactivity to choline-acetyltransferase (ChAT), which synthesizes ACh. Additionally, enhanced green fluorescent protein (eGFP) was detected in the taste cells of BAC-transgenic mice, in which eGFP was placed under the control of endogenous ChAT transcriptional regulatory elements (ChAT(BAC)-eGFP mice). Furthermore, many ChAT-immunolabeled taste cells also reacted to an antibody against the vesicle-associated membrane protein synaptobrevin-2. These data suggest that ACh-containing vesicles are present in taste receptor cells and ACh release from taste cells may play a role in autocrine and/or paracrine cell-to-cell communication. In addition, certain nerve fibers surrounding or within taste buds were immunoreactive for the VAChT antibody. Some of these fibers were also immunolabeled with antibody against calcitonin gene-related peptide (CGRP), a marker for trigeminal peptidergic fibers. Thus, functions of taste receptor cells could be modulated by trigeminal fibers via ACh release as well.

Electrical and pharmacological properties of petrosal ganglion neurons that innervate the carotid body

The petrosal ganglion (PG) contains the somata of primary afferent neurons that innervate the chemoreceptor (glomus) cells in the carotid body (CB). The most accepted model of CB chemoreception states that natural stimuli trigger the release of one or more transmitters from glomus cells, which in turn acting on specific post-synaptic receptors increases the rate of discharge in the nerve endings of PG neurons. However, PG neurons that project to the CB represent only small fraction (roughly 20%) of the whole PG and their identification is not simple since their electrophysiological and pharmacological properties are not strikingly different as compared with other PG neurons, which project to the carotid sinus or the tongue. In addition, differences reported on the actions of putative transmitters on PG neurons may reflect true species differences. Nevertheless, some experimental strategies have contributed to identify and characterize the properties of PG neurons that innervate the CB. In this review, we examined the electrophysiological properties and pharmacological responses of PG neurons to putative CB excitatory transmitters, focusing on the methods of study and species differences. The evidences suggest that ACh and ATP play a major role in the fast excitatory transmission between glomus cells and chemosensory nerve endings in the cat, rat and rabbit. However, the role of other putative transmitters such as dopamine, 5-HT and GABA is less clear and depends on the specie studied.

ATP- and ACh-induced responses in isolated cat petrosal ganglion neurons

Chemoreceptor (glomus) cells of the carotid body are synaptically connected to the sensory nerve endings of petrosal ganglion (PG) neurons. In response to natural stimuli, the glomus cells release transmitters, which acting on the nerve terminals of petrosal neurons increases the chemosensory afferent discharge. Among several transmitter molecules present in glomus cells, acetylcholine (ACh) and adenosine 5'-triphosphate (ATP) are considered to act as excitatory transmitter in this synapse. To test if ACh and ATP play a role as excitatory transmitters in the cat CB, we recorded the electrophysiological responses from PG neurons cultured in vitro. Under voltage clamp, ATP induces a concentration-dependent inward current that partially desensitizes during 20-30 s application pulses. The ATP-induced current has a threshold near 100 nM and saturates between 20-50 muM. ACh induces a fast, inactivating inward current, with a threshold between 10-50 muM, and saturates around 1 mM. A large part of the population of PG neurons (60%) respond to both ATP and ACh. Present results support the hypothesis that ACh and ATP act as excitatory transmitters between cat glomus cells and PG neurons.

Electrophysiological characterization of nicotinic acetylcholine receptors in cat petrosal ganglion neurons in culture: Effects of cytisine and its bromo derivatives

Petrosal ganglion neurons are depolarized and fire action potentials in response to acetylcholine and nicotine. However, little is known about the subtype(s) of nicotinic acetylcholine receptors involved, although alpha4 and alpha7 subunits have been identified in petrosal ganglion neurons. Cytisine, an alkaloid unrelated to nicotine, and its bromo derivatives are agonists exhibiting different affinities, potencies and efficacies at nicotinic acetylcholine receptors containing alpha4 or alpha7 subunits. To characterize the receptors involved, we studied the effects of these agonists and the nicotinic acetylcholine receptor antagonists hexamethonium and alpha-bungarotoxin in isolated petrosal ganglion neurons. Petrosal ganglia were excised from anesthetized cats and cultured for up to 16 days. Using patch-clamp technique, we recorded whole-cell currents evoked by 5-10 s applications of acetylcholine, cytisine or its bromo derivatives. Agonists and antagonists were applied by gravity from a pipette near the neuron surface. Neurons responded to acetylcholine, cytisine, 3-bromocytisine and 5-bromocytisine with fast inward currents that desensitized during application of the stimuli and were reversibly blocked by 1 microM hexamethonium or 10 nM alpha-bungarotoxin. The order of potency of the agonists was 3-bromocytisine >> acetylcholine approximately = cytisine >> 5-bromocytisine, suggesting that homomeric alpha7 neuronal nicotinic receptors predominate in cat petrosal ganglion neurons in culture.

Selective gentamicin uptake by cytochemical subpopulations of guinea-pig geniculate ganglion cells

Cytochemical subpopulations of geniculate ganglion (GG) cells were identified in guinea-pigs using immunohistochemistry and selective gentamicin accumulation. Two subpopulations of GG cells were evident based upon their location and immunoreactivity for peptide 19 (PEP 19), for plasma membrane Ca2+-ATPase (PMCA-ATPase), and for neurofilament proteins. Cells within the posterior part of GG were positive for PEP 19 and PMCA-ATPase, but not for 68 kD or 160 kD neurofilament proteins. Cells within the anterior part showed complementary staining properties. Cells within these populations showed differences in accumulation of gentamicin, depending upon the administration route. Cells within the posterior part showed avid accumulation of gentamicin when animals received the drug systemically. When the drug was administered directly into the middle ear, cells within the anterior part showed avid gentamicin accumulation. Immunostaining for gentamicin in both cell populations was much more extreme and remained so for longer post-administration times when compared with spiral ganglion and vestibular ganglion cells. The results suggest that cells in the anterior part of GG have little exposure to gentamicin in the serum and that perhaps they innervate the middle ear mucosa or they absorb the drug through their axons within the middle ear. In contrast, cells in the posterior part of GG have greater access to systemically administered gentamicin either directly or via their axon terminals.

Neurotransmission in the carotid body: transmitters and modulators between glomus cells and petrosal ganglion nerve terminals

The carotid body (CB) is the main arterial chemoreceptor. The most accepted model of arterial chemoreception postulates that carotid body glomus (type I) cells are the primary receptors, which are synaptically connected to the nerve terminals of petrosal ganglion (PG) neurons. In response to natural stimuli, glomus cells are expected to release one (or more) transmitter(s) which, acting on the peripheral nerve terminals of processes from chemosensory petrosal neurons, increases the sensory discharge. Among several molecules present in glomus cells, acetylcholine and adenosine nucleotides and dopamine are considered as excitatory transmitter candidates. In this review, we will examine recent evidence supporting the notion that acetylcholine and adenosine 5'-triphosphate are the main excitatory transmitters in the cat and rat carotid bodies. On the other hand, dopamine may act as a modulator of the chemoreception process in the cat, but as an excitatory transmitter in the rabbit carotid body.

Neurocalcin-immunoreactive neurons in the petrosal ganglion innervate the taste bud

The distribution and origin of neurocalcin-immunoreactive (NC-ir) nerve fibers in the taste bud and carotid body were examined by an immunofluorescence method. In the circumvallate papilla of the tongue, NC-ir nerve fibers made subepithelial nerve plexuses and occasionally penetrated the taste bud. However, the carotid body was devoid of ir nerve fibers. In the petrosal ganglion, 32% of neurons were immunoreactive for NC. Such neurons were mostly medium-sized to large, and scattered throughout the ganglion. In the superior cervical and intralingual ganglia, numerous ir varicose fibers surrounded postsynaptic neurons. However, NC-ir could not be detected in cell bodies of these neurons. The retrograde tracing method indicated that NC-ir petrosal neurons innervated taste buds in the circumvallate papilla. NC-ir neurons may have a gustatory function in the petrosal ganglion.

ACh and ATP mediate excitatory transmission in cat carotid identified chemoreceptor units in vitro

Several molecules have been proposed as excitatory transmitters between glomus (type 1) cells and nerve terminals of petrosal ganglion (PG) neurons in the carotid body (CB). We tested whether ACh and ATP have a role to play as excitatory transmitters in the cat CB by recording intracellularly from identified PG neurons functionally connected to the CB in vitro. PG neurons projecting to the CB were classified according to their intracellular responses as: (a) neurons with humped action potentials (hAP neurons) responding phasically to long-lasting depolarizing pulses (53/67), and (b) neurons with smooth action potentials (non-hAP neurons) that fire tonically during long-lasting depolarizations (14/67). CB stimulation by stop flow and/or acidosis induced activity in 28 of 39 hAP-type neurons, being classified as chemosensory, but in none of the non-hAP neurons. Hexamethonium (10 microM) and suramin (100 microM) reversibly abolished the increased discharges evoked in chemosensory neurons (8/9) by stop flow or acidosis. Moreover, 24 of 27 chemosensory neurons responded to ganglionar application of ACh and ATP, while two neurons responded only to ACh and one to ATP. Mechanical deformation of the carotid sinus induced firing activity in 10 of 13 non-hAP neurons, but in none of the hAP neurons tested. Interestingly, 4/10 non-hAP neurons, which responded to carotid sinus mechanical stimulation also responded to ganglionar application of ATP, but were insensitive to ACh. Present results favor the hypothesis that ACh and ATP are excitatory transmitters in the cat CB, acting-at least-on the PG neuron terminals in the CB.

Catecholamine release from isolated sensory neurons of cat petrosal ganglia in tissue culture

The petrosal ganglion (PG) is entirely constituted by the perikarya of primary sensory neurons, part of which innervates the carotid body via the carotid sinus nerve (CSN). Application of acetylcholine (ACh) or nicotine (Nic) as well as adenosine 5'-triphosphate (ATP) to the PG in vitro increases the frequency of CSN discharges, an effect that is modified by the concomitant application of dopamine (DA). Since a population of PG neurons expresses tyrosine hydroxylase, and DA is released from the cat carotid body in response to electrical stimulation of C-fibers in the CSN, it is possible that DA may be released from the perikarya of PG neurons. Therefore, we studied whether ACh or Nic, ATP and high KCl could induce DA release from PG neurons in culture. Petrosal ganglia were excised from pentobarbitone-anesthetized adult cats, dissociated and their neurons maintained in culture for 7-21 days. Catecholamine release was measured by amperometry via carbon-fiber microelectrodes. In response to KCl, Nic, ACh or ATP application, about 25% of neurons exhibited electrochemical signals compatible with DA release. This percentage increased to 41% after loading the neurons with exogenous DA. The present results suggest that DA release may be induced from the perikarya of a population of PG neurons.

Embryonic geniculate ganglion neurons in culture have neurotrophin-specific electrophysiological properties

Geniculate ganglion neurons provide a major source of innervation to mammalian taste organs, including taste buds in the soft palate and in fungiform papillae on the anterior two thirds of the tongue. In and around the fungiform papillae, before taste buds form, neurotrophin mRNAs are expressed in selective spatial and temporal patterns. We hypothesized that neurotrophins would affect electrophysiological properties in embryonic geniculate neurons. Ganglia were explanted from rats at gestational day 16, when growing neurites have entered the papilla core, and maintained in culture with added brain-derived neurotrophic factor (BDNF), neurotrophin 4 (NT4), nerve growth factor (NGF) or neurotrophin 3 (NT3). Neuron survival with BDNF or NT4 was about 80%, whereas with NGF or NT3 less than 15% of neurons survived over 6 days in culture. Whole cell recordings from neurons in ganglion explants with each neurotrophin condition demonstrated distinctive neurophysiological properties related to specific neurotrophins. Geniculate neurons cultured with either BDNF or NT4 had similar passive-membrane and action potential properties, but these characteristics were significantly different from those of neurons cultured with NGF or NT3. NGF-maintained neurons had features of increased excitability including a higher resting membrane potential and a lower current threshold for the action potential. About 70% of neurons produced repetitive action potentials at threshold. Furthermore, compared with neurons cultured with other neurotrophins, a decreased proportion had an inflection on the falling phase of the action potential. NT3-maintained neurons had action potentials that were of relatively large amplitude and short duration, with steep rising and falling slopes. In addition, about 20% responded with a repetitive train of action potentials at threshold. In contrast, with BDNF or NT4 repetitive action potential trains were not observed. The data demonstrate different neurophysiological properties in developing geniculate ganglion neurons maintained with specific neurotrophins. Therefore, we suggest that neurotrophins might influence acquisition of distinctive neurophysiological properties in embryonic geniculate neurons that are fundamental to the formation of peripheral taste circuits and a functioning taste system.

Substance P mobilizes intracellular calcium and activates a nonselective cation conductance in rat spiral ganglion neurons

We demonstrate the expression of functional tachykinin receptors in rat spiral ganglion neurons (SGNs) using calcium signal measurement and whole-cell patch clamp recording. Substance P (SP; 10 microm, 1 s application) induced a transient increase in intracellular calcium. The SP dose-response study showed an EC50 of 18.8 microm and a Hill slope of 0.77. Comparison between specific agonists for the three tachykinin receptor (NKR) types showed the potency NKR3 > NKR1 > NKR2 at 10 microm. The Ca2+ response could be evoked in Ca2+-free medium and was blocked by N-ethylmaleimide and U-73122, indicating that Ca2+ was released from intracellular stores via a G-protein and phospholipase C pathway. Under whole-cell voltage clamp recording at a holding potential of -50 mV, SP (10 microm, 1 s) evoked a slowly developing transient inward current. The current reversed near to 0 mV and ionic permeability experiments revealed a cation nonselective conductance also permeable to large organic cations such as N-methyl-D-glucamine and tetraethylammonium. Neither removing extracellular calcium nor chelating intracellular calcium with 10 mm BAPTA could block the SP-evoked current. This conductance appeared coupled to G-protein activation as intracellular GDP-betaS blocked the SP-evoked current. Mutual desensitization and occlusion studies with acetylcholine and ATP showed that the SP-evoked conductance share effector channels and/or intracellular processes with the purinergic/cholinergic conductance. In SGNs, SP could have both a trophic action, via a calcium response, and a neuromodulatory role, by a depolarizing action through the activation of nonselective cation channels.

Distinctive neurophysiological properties of embryonic trigeminal and geniculate neurons in culture

Neurons in trigeminal and geniculate ganglia extend neurites that share contiguous target tissue fields in the fungiform papillae and taste buds of the mammalian tongue and thereby have principal roles in lingual somatosensation and gustation. Although functional differentiation of these neurons is central to formation of lingual sensory circuits, there is little known about electrophysiological properties of developing trigeminal and geniculate ganglia or the extrinsic factors that might regulate neural development. We used whole cell recordings from embryonic day 16 rat ganglia, maintained in culture as explants for 3-10 days with neurotrophin support to characterize basic properties of trigeminal and geniculate neurons over time in vitro and in comparison to each other. Each ganglion was cultured with the neurotrophin that supports maximal neuron survival and that would be encountered by growing neurites at highest concentration in target fields. Resting membrane potential and time constant did not alter over days in culture, whereas membrane resistance decreased and capacitance increased in association with small increases in trigeminal and geniculate soma size. Small gradual differences in action potential properties were observed for both ganglion types, including an increase in threshold current to elicit an action potential and a decrease in duration and increase in rise and fall slopes so that action potentials became shorter and sharper with time in culture. Using a period of 5-8 days in culture when neural properties are generally stable, we compared trigeminal and geniculate ganglia and revealed major differences between these embryonic ganglia in passive membrane and action potential characteristics. Geniculate neurons had lower resting membrane potential and higher input resistance and smaller, shorter, and sharper action potentials with lower thresholds than trigeminal neurons. Whereas all trigeminal neurons produced a single action potential at threshold depolarization, 35% of geniculate neurons fired repetitively. Furthermore, all trigeminal neurons produced TTX-resistant action potentials, but geniculate action potentials were abolished in the presence of low concentrations of TTX. Both trigeminal and geniculate neurons had inflections on the falling phase of the action potential that were reduced in the presence of various pharmacological blockers of calcium channel activation. Use of nifedipine, omega-conotoxin-MVIIA and GVIA, and omega-agatoxin-TK indicated that currents through L-, N-, and P/Q- type calcium channels participate in the action potential inflection in embryonic trigeminal and geniculate neurons. The data on passive membrane, action potential, and ion channel characteristics demonstrate clear differences between trigeminal and geniculate ganglion neurons at an embryonic stage when target tissues are innervated but receptor organs have not developed or are still immature. Therefore these electrophysiological distinctions between embryonic ganglia are present before neural activity from differentiated receptive fields can influence functional phenotype.

A comparative study of three cranial sensory ganglia projecting into the oral cavity: in situ hybridization analyses of neurotrophin receptors and thermosensitive cation channels

Peripheral cranial sensory nerves projecting into the oral cavity receive food intake stimuli and transmit sensory signals to the central nervous system. To describe and compare the features of the cranial sensory ganglia that innervate the oral cavity, i.e., the trigeminal, petrosal, and geniculate ganglia (TG, PG, and GG, respectively), in situ hybridization was conducted using riboprobes for neurotrophin receptors (TrkA, TrkB, and TrkC), a neurotransmitter (substance P), and ion channels important for thermosensation (VR1 and TREK-1). In TG, all in six probes yielded positive signals to various extent in intensity and frequency. In addition, a strong correlation between the expression of VR1 and those of TrkA and substance P was observed as in the case of the dorsal root ganglia. In PG, positive signals to all six probes were also detected, and the correlation of expression was similar to that shown by TG. On the other hand, most cells in GG were positive to the TrkB probe, and a small number of cells were positive to the TrkC probe, but no significant signal was observed for the other four probes. These results indicate that TG and PG consist of cells that are heterogeneous in terms of neurotrophin requirement and somatosensory functions, and that GG seems to consist mainly of a homogeneous cell type, gustatory neurons. In conclusion, TG, PG, and GG, show gene expression characteristics intrinsic to the three ganglia. It is also concluded that TG and a portion of PG project several types of somatosensory nerves. This is consistent with the finding that GG and a portion of PG project gustatory nerves.