Cellular and developmental patterns of expression of Ret and glial cell line-derived neurotrophic factor receptor alpha mRNAs

 Glial cell line-derived neurotrophic factor (GDNF) has recently been shown to signal by binding to GDNF receptor-alpha (GDNFR-α), after which the GDNF-GDNFR-α associates with and activates the tyrosine kinase receptor Ret. We have localized Ret messenger RNA (mRNA) in the developing and adult rodent and compared with to the expression of GDNF and GDNFR-α mRNA. Ret mRNA is strongly expressed in dopamine neurons and α-motorneurons as well as in thalamus, ruber and occlumotor nuclei, the habenular complex, septum, cerebellum, and brain stem nuclei. Ret mRNA was also found in several sensory systems, in ganglia, and in nonneuronal tissues such as teeth and vibrissae. Very strong Ret mRNA signals are present in kidney and the gastrointestinal tract, where Ret and GDNF mRNA expression patterns are precisely complementary. The presence of Ret protein was confirmed in adult dopamine neurons using immunohistochemistry. GDNFR-α mRNA was strongly expressed in the developing and adult dopamine neurons. It was also found in neurons in deep layers of cortex cerebri, in hippocampus, septum, the dentate gyrus, tectum, and the developing spinal cord. In the kidney and the gastrointestinal tract, GDNFR-α mRNA and Ret mRNA distribution overlapped. Dorsal root ganglia, cranial ganglia, and developing peripheral nerves were also positive. GDNFR-α was additionally found in sensory areas and in developing teeth. Sensory areas included inner ear, eye, olfactory epithelium, and the vomeronasal organ, as well as developing tongue papillae. The temporospatial pattern of expression of GDNFR-α mRNA did not always match that of Ret mRNA. For instance, GDNFR-α mRNA was also found in the developing ventral striatum, including the olfactory tubercle, and in hippocampus. These areas seemed devoid of Ret mRNA, suggesting that GDNFR-α might also have functions unrelated to Ret. Received: 2 January 1997 / Accepted: 26 February 1997     

Conditional ablation of mature olfactory sensory neurons mediated by diphtheria toxin receptor

The vertebrate olfactory epithelium provides an excellent model system to study the regulatory mechanisms of neurogenesis and neuronal differentiation due to its unique ability to generate new sensory neurons throughout life. The replacement of olfactory sensory neurons is stimulated when damage occurs in the olfactory epithelium. In this study, transgenic mice, with a transgene containing human diphtheria toxin receptor under the control of the olfactory marker protein promoter (OMP-DTR), were generated in which the mature olfactory sensory neurons could be specifically ablated when exposed to diphtheria toxin. Following diphtheria toxin induced neuronal ablation, we observed increased numbers of newly generated growth associated protein 43 (GAP43)-positive immature olfactory sensory neurons. OMP-positive neurons were continuously produced from the newly generated GAP43-positive cells. The expression of the signal transduction components adenylyl cyclase type III and the G-protein α subunit G_{α olf} was sensitive to diphtheria toxin exposure and their levels decreased dramatically preceding the disappearance of the OMP-positive sensory neurons. These data validate the hypothesis that OMP-DTR mice can be used as a tool to ablate the mature olfactory sensory neurons in a controlled fashion and to study the regulatory mechanisms of the neuronal replacement.     

Olfactory experience accelerates glomerular refinement in the mammalian olfactory bulb

Primary olfactory sensory neurons (OSNs) impart both molecular and functional organization to the olfactory bulb. Because OSNs can be selectively activated by odorants in vivo, we sought to determine whether odorant experience alters the cellular dynamics of specific OSNs or their axonal projections. Using mice, we found that odorant stimulation associated with behavioral conditioning influenced OSN wiring by accelerating glomerular refinement independent of OSN number; furthermore, this wiring was strongly associated with olfactory learning.     

State-dependent sculpting of olfactory sensory neurons is attributed to sensory enrichment,odor deprivation,and aging

Gene-targeted deletion of the predominant Shaker potassium channel, Kv1.3, in the mitral cells of the olfactory bulb, decreases the number of presynaptic, odorant receptor (OR)-identified olfactory sensory neurons (OSNs) in the main olfactory epithelium (MOE) and alters the nature of their postsynaptic connections to mitral cell targets. The current study examined whether OSN density was state-dependent by examining the impact of (1) odor enrichment, (2) sensory deprivation, and (3) aging upon the number of P2- or M72-expressing neurons. Histological approaches were used to quantify the number of OSNs across entire epithelia for wildtype (WT) vs. Kv1.3-null (KO) mice bred onto an ORtauLacZ reporter background. Following either odor enrichment or early unilateral naris-occlusion, the number of M72-expressing OSNs was significantly decreased in WT mice, but was unchanged in KO animals. Following naris-occlusion, the number of P2-expressing OSNs was decreased regardless of genotype. Animals that were reared to 2 years of age demonstrated loss of both P2- and M72-expressing OSNs in WT mice and a concomitant loss of only M72-expressing neurons in KO mice. These findings suggest that voltage-gated activity of the mitral cells is important for OSN plasticity, and can prevent neuronal loss via sensory- and OR-dependent mechanisms.     

GDNF family ligand immunoreactivity in the gut of teleostean fish

Glial-derived neurotrophic factor (GDNF), neurturin (NRTN), persephin (PSPN), and artemin (ARTN) are a group of proteins belonging to the GDNF family ligands (GFLs). GDNF, NRTN, and ARTN support the survival of central, peripheral, and autonomic neuron populations, while PSPN supports the survival of only several central neuron populations. A common receptor, RET, modulates the action of this family and a co-receptor, GFRα, determines RET ligand specificity. GDNF and NRTN appear to be essential for enteric nervous system (ENS) development in mammals, zebrafish, and other teleostean species. GFLs are also essential for the maintenance and plasticity of adult mammalian ENS. In this study, the distribution pattern of GFLs in the intestine of five adult fish (bass, gilt-head, scorpionfish, trout, and zebrafish) was evaluated by immunochemical and immunocytochemical analysis. The results demonstrated the presence of GDNF, NRTN, and ARTN in the gut of all species studied. They appeared to be spread in the ENS and/or endocrine cells of the intestine. These findings suggest that the presence of GFLs in fish gut is not only limited to developmental period, but could be also involved in the enteric physiology of adult species.     

Diversity in the olfactory epithelium of bony fishes: development, lamellar arrangement, sensory neuron cell types and transduction components

In this study we use a taxon-based approach to examine previous, as well as new findings on several topics pertaining to the peripheral olfactory components in teleost fishes. These topics comprise (1) the gross anatomy of the peripheral olfactory organ, including olfactory sensory neuron subtypes and their functional parameters, (2) the ultrastructure of the olfactory epithelium, and (3) recent findings regarding the development of the nasal cavity and the olfactory epithelium. The teleosts are living ray-finned fish, and include descendants of early-diverging orders (e.g., salmon), specialized descendants (e.g., goldfish and zebrafish), as well as the Acanthopterygii, numerous species with sharp bony rays, including perch, stickleback, bass and tuna. Our survey reveals that the olfactory epithelium lines a multi-lamellar olfactory rosette in many teleosts. In Acanthopterygii, there are also examples of flat, single, double or triple folded olfactory epithelia. Diverse species ventilate the olfactory chamber with a single accessory nasal sac, whereas the presence of two sacs is confined to species within the Acanthopterygii. Recent studies in salmonids and cyprinids have shown that both ciliated olfactory sensory neurons (OSNs) and microvillous OSNs respond to amino acid odorants. Bile acids stimulate ciliated OSNs, and nucleotides activate microvillous OSNs. G-protein coupled odorant receptor molecules (OR-, V1R-, and V2R-types) have been identified in several teleost species. Ciliated OSNs express the G-protein subunit G_αolf/s, which activates cyclic AMP during transduction. Localization of G protein subunits G_α0 and G_αq/11 to microvillous or crypt OSNs, varies among different species. All teleost species appear to have microvillous and ciliated OSNs. The recently discovered crypt OSN is likewise found broadly. There is surprising diversity during ontogeny. In some species, OSNs and supporting cells derive from placodal cells; in others, supporting cells develop from epithelial (skin) cells. In some, epithelial cells covering the developing olfactory epithelium degenerate, in others, these retract. Likewise, there are different mechanisms for nostril formation. We conclude that there is considerable diversity in gross anatomy and development of the peripheral olfactory organ in teleosts, yet conservation of olfactory sensory neuron morphology. There is not sufficient information to draw conclusions regarding the diversity of teleost olfactory receptors or transduction cascades. This review focused on a topic that benefited enormously from the leadership of Dr. A. I. (Al) Farbman: the integration of morphology, development, and function in the study of the olfactory epithelium. Al has been an inspiration to us; we are indebted to you, thank you, Al.     

Induced and constitutive heat shock protein expression in the olfactory system--a review, new findings, and some perspectives

Heat shock, or stress, proteins (HSPs) are cellular proteins induced in response to conditions that cause protein denaturation, and their induction is essential for survival of such conditions. In the olfactory system we have found intense HSP expression occurs during normal processing of environmental odorants/inhalants as well as following hyperthermia and drug exposure. The HSPs involved include ubiquitin, HSP70, HSC70, and HSP25. Responses are both cell type- and stress-specific, occurring primarily in olfactory supporting cells and to some extent in Bowman’s gland acinar cells. Responses to these stresses are not seen in olfactory sensory neurons. This article reviews those studies and the significance of their findings. It also discusses a distinct subpopulation of rat olfactory sensory neurons (OSNs), the 2A4(+)OSNs, found to be constitutively reactive with HSP70, the predominantly stress-inducible isoform of the 70 kD HSP family. Their high HSP70 expression appears to confer on the 2A4(+)OSNs an enhanced ability to survive damage-induced OSN turnover. New findings are also presented on HSP25-specific changes following olfactory bulbectomy. All data are discussed in the context of the overall olfactory and bioprotective functions of the olfactory mucosa.     

Glial cell line–derived neurotrophic factor family ligands enhance capsaicin-stimulated release of calcitonin gene-related peptide from sensory neurons

The glial cell line–derived neurotrophic factor (GDNF) family ligands (GFLs) are a group of peptides that have been implicated as important factors in inflammation, since they are released in increased amounts during inflammation and induce thermal hyperalgesia upon injection. Mouse isolated sensory neurons in culture and freshly dissociated spinal cord slices were used to examine the enhancement in stimulated-release of the neuropeptide, calcitonin gene-related peptide (CGRP), as a measure of sensitization. Exposure of isolated sensory neurons in culture to GDNF, neurturin, and artemin enhanced the capsaicin-stimulated release of immunoreactive calcitonin gene-related peptide (iCGRP) two- to threefold, but did not increase potassium-stimulated release of iCGRP. A similar profile of sensitization was observed in freshly dissociated spinal cord slices. Persephin, another member of the GFL family thought to be important in development, was unable to induce an enhancement in the release of iCGRP. These results demonstrate that specific GFLs are important mediators affecting sensory neuronal sensitivity, likely through modulation of the capsaicin receptor. The sensitization of sensory neurons during inflammation, and the pain and neurogenic inflammation resulting from this sensitization, may be due in part to the effects of these selected GFLs.     

Olfactory epithelia differentially express neuronal markers

All three olfactory epithelia, the olfactory epithelium proper (OE), the septal organ of Masera (SO), and the vomeronasal organ of Jacobson (VNO) originate from the olfactory placode. Here, their diverse neurochemical phenotypes were analyzed using the immunohistochemical expression pattern of different neuronal markers. The olfactory bulb (OB) served as neuronal control. Neuronal Nuclei Marker (NeuN) is neither expressed in sensory neurons in any of the three olfactory epithelia, nor in relay neurons (mitral/tufted cells) of the OB. However, OB interneurons (periglomerular/granule cells) labeled, as did supranuclear structures of VNO supporting cells and VNO glands. Protein Gene Product 9.5 (PGP9.5 = C-terminal ubiquitin hydrolase L1 = UCHL1) expression is exactly the opposite: all olfactory sensory neurons express PGP9.5 as do OB mitral/tufted cells but not interneurons. Neuron Specific Enolase (NSE) expression is highest in the most apically located OE and SO sensory neurons and patchy in VNO. In contrast, the cytoplasm of the most basally located neurons of OE and SO immunoreacted for Growth Associated Protein 43 (GAP-43/B50). In VNO neurons GAP-43 labeling is also nuclear. In the cytoplasm, Olfactory Marker Protein (OMP) is most intensely expressed in SO, followed by OE and least in VNO neurons; further, OMP is also expressed in the nucleus of basally located VNO neurons. OB mitral/tufted cells express OMP at low levels. Neurons closer to respiratory epithelium often expressed a higher level of neuronal markers, suggesting a role of those markers for neuronal protection against take-over. Within the VNO the neurons show clear apical–basal expression diversity, as they do for factors of the signal transduction cascade. Overall, expression patterns of the investigated neuronal markers suggest that OE and SO are more similar to each other than to VNO.     

Direct visualization of cell movement in the embryonic olfactory bulb using green fluorescent protein transgenic mice: evidence for rapid tangential migration of neural cell precursors

We analyzed motile behavior of neuronal precursor cells in the intact olfactory bulbs (OBs) using transgenic mice expressing GFP under the control of T alpha 1 tubulin promoter. In the olfactory bulbs at the embryonic days 12.5-14.5, a large number of immature neurons expressed GFP in this transgenic line. Embryonic OBs were maintained in an organ culture system and the migratory behavior of GFP-positive cells was analyzed by time-lapse confocal microscopy. We observed rapid tangential movement of GFP-positive cells in the ventral olfactory bulb. In contrast to the typical bipolar morphology of translocating immature neurons within the developing cortex, the motile cells had neither leading nor trailing processes and changed their overall shape frequently. Comparison of the behavior of cells expressing GFP under the control of T alpha 1 tubulin or nestin promoter revealed that rapid motility was specific to cells in the neuronal lineage. The rapid movement was sensitive to an actin perturbing reagent and also dependent on the calcium influx through L-type calcium channels. These results indicate the presence of a specific form of precursor cell migration in the embryonic olfactory bulb.     

GDNF、GFAP和NGFRp75在成年大鼠和金黄地鼠嗅球成鞘细胞的免疫组织化学研究

目的　研究GDNF在成年大鼠和金黄地鼠嗅球成鞘细胞的表达 ,探索成鞘细胞在中枢神经再生中的作用。　方法　用免疫组织化学ABC法 ,显示GDNF在成年大鼠和金黄地鼠嗅球成鞘细胞的表达和分布 ,同时用NGFRp75和GFAP染色作为阳性对照。　结果　在成年大鼠和金黄地鼠嗅球的纤维层和小球层内均可见深棕色的GDNF免疫组织化学反应的成鞘细胞。在小球层与纤维层分界处和小球层与分子层分界处及嗅小球之间密集分布 ,在嗅小球之内较稀疏。同时在同一嗅球组织的另两组切片的相同部位 ,分别出现GFAP和NGFRp75免疫反应性细胞 ,间接说明GDNF免疫反应的结构是嗅球成鞘细胞。　结论　嗅球成鞘细胞含有胶质细胞源性神经营养因子     

The Necklace Olfactory System in Mammals

In the mammalian olfactory system, there exist several parallel specialized subsystems, one of which is the necklace olfactory system. This subsystem has several interesting features in its anatomical organization and physiological responses. Its olfactory sensory neurons (OSNs) in the olfactory epithelium project their axons to a set of glomeruli in the caudal olfactory bulb, forming the shape of “beads-on-a-string” and thus being named as “necklace glomeruli.” Physiologically, necklace OSNs lack components suggesting cAMP as the second messenger in the signal transduction cascade as those observed in the OSNs of the canonical olfactory system. In contrast, necklace OSNs possess several signaling components suggesting cGMP as the second messenger. Our recent studies demonstrate that one of the major functions of the necklace olfactory system is to detect atmospheric carbon dioxide (CO₂) and mediate avoidance behavior, suggesting novel molecular and cellular mechanisms of CO₂ sensing. Here, I will review recent progresses on our understanding of the organization and function of the necklace olfactory subsystem. These recent studies suggest the exciting potentials of using the necklace olfactory system as an advantageous model system for studying neural circuits underlying innate avoidance behavior.     

Dual functions of mammalian olfactory sensory neurons as odor detectors and mechanical sensors

Most sensory systems are primarily specialized to detect one sensory modality. Here we report that olfactory sensory neurons (OSNs) in the mammalian nose can detect two distinct modalities transmitted by chemical and mechanical stimuli. As revealed by patch-clamp recordings, many OSNs respond not only to odorants, but also to mechanical stimuli delivered by pressure ejections of odor-free Ringer solution. The mechanical responses correlate directly with the pressure intensity and show several properties similar to those induced by odorants, including onset latency, reversal potential and adaptation to repeated stimulation. Blocking adenylyl cyclase or knocking out the cyclic nucleotide-gated channel CNGA2 eliminates the odorant and the mechanical responses, suggesting that both are mediated by a shared cAMP cascade. We further show that this mechanosensitivity enhances the firing frequency of individual neurons when they are weakly stimulated by odorants and most likely drives the rhythmic activity (theta oscillation) in the olfactory bulb to synchronize with respiration.     

Occurrence of neuropeptides and tyrosine hydroxylase in the olfactory epithelium of the lesser-spotted catshark (Scyliorhinus canicula Linnaeus, 1758)

Immunohistochemical studies using antisera against various neuropeptides (Substance P, vasoactive intestinal polypeptide, and cholecystokinin octapeptide) and tyrosine hydroxylase revealed both olfactory sensory neuron (OSN) polymorphisms and transepithelial-subepithelial nerves in the olfactory epithelium of the cartilaginous fish, Scyliorhinus canicula. This study provides the first evidence of three morphological types of OSNs within the olfactory epithelium of cartilaginous fish that are similar to those found in the teleosts. In fishes there is evidence that OSNs differ functionally, including their differential olfactory bulb projections and molecular properties. The Substance P positive olfactory neurons in S. canicula may have a separate bulbar projection site that is not known, but may indicate a characteristic found in olfactory neuron subtypes in both lampreys and teleost fish. Numerous Substance P immunopositive nerves are found at the base of and in the olfactory epithelium. Some of them were observed to extend outwards almost reaching the epithelial surface. Their presumptive origin from the trigeminal nerve and their interrelationship with chemosensory cells in the nasal passages of vertebrates are discussed.     

Olfactory sensory neurons in the sea lamprey display polymorphisms

The sea lamprey (Petromyzon marinus) is an ancient jawless fish phyletically removed from modern (teleost) fishes. It is an excellent organism in the study of olfaction due to its accessible olfactory pathway, which is susceptible to manipulation, and its important location in the evolution of vertebrates. There are many similarities in the olfactory systems of all fishes, and they also share characteristics with the olfactory system of mammals. Teleost fishes lack the distinctive vomeronasal organ of mammals; rather all odours are processed initially by olfactory sensory neurons (OSNs) of three morphotypes within the olfactory epithelium. We sought to identify olfactory sensory neuron polymorphisms in the sea lamprey. Using retrograde tracing with dyes injected into the olfactory bulb, we identified three morphotypes which are highly similar to those found in teleosts. This study provides the first evidence of morphotypes in the sea lamprey peripheral olfactory organ, and indicates that olfactory sensory neuron polymorphism may be a trait highly conserved throughout vertebrate evolution.     

Ciliary neurotrophic factor (CNTF) in the olfactory system of rats and mice.

The rodent olfactory system is a regarded model for the relationship between neurotrophic factors, their receptors, and their compound influence on the notable lifelong neuroplasticity occurring in this sensory system. It was known that high amounts of ciliary neurotrophic factor (CNTF), a hematopoietic cytokine, can be found in the olfactory bulb. In the awarded work, a detailed cellular characterization of CNTF-localization in the olfactory system was obtained. The results demonstrated CNTF-immunoreactivity in olfactory ensheathing cells, newborn interneurons in the olfactory bulb, and in a subpopulation of mature olfactory sensory neurons in the olfactory epithelium. Three-dimensional reconstructions of CNTF-immunoreactive axonal bulbar projections of these neurons revealed an ordered bilaterally symmetric pattern. This finding implies a potential connection between neuronal CNTF-expression in the olfactory epithelium and olfactory information processing.     

Developmental changes in neurite outgrowth responses of dorsal root and sympathetic ganglia to GDNF, neurturin, and artemin.

The ability of glial cell line-derived neurotrophic factor (GDNF), neurturin, and artemin to induce neurite outgrowth from dorsal root, superior cervical, and lumbar sympathetic ganglia from mice at a variety of development stages between embryonic day (E) 11.5 and postnatal day (P) 7 was examined by explanting ganglia onto collagen gels and growing them in the presence of agarose beads impregnated with the different GDNF family ligands. Artemin, GDNF, and neurturin were all capable of influencing neurite outgrowth from dorsal root and sympathetic ganglia, but the responses of each neuron type to the different ligands varied during development. Neurites from dorsal root ganglia responded to artemin at P0 and P7, to GDNF at E15.5 and P0, and to neurturin at E15.5, P0, and P6/7; thus, artemin, GDNF, and neurturin are all capable of influencing neurite outgrowth from dorsal root ganglion neurons. Neurites from superior cervical sympathetic ganglia responded significantly to artemin at E15.5, to GDNF at E15.5 and P0, and to neurturin at E15.5. Neurites from lumbar sympathetic ganglia responded to artemin at all stages from E11.5 to P7, to GDNF at P0 and P7 and to neurturin at E11.5 to P6/7. Combined with the data from previous studies that have examined the expression of GDNF family members, our data suggest that artemin plays a role in inducing neurite outgrowth from young sympathetic neurons in the early stages of sympathetic axon pathfinding, whereas GDNF and neurturin are likely to be important at later stages of sympathetic neuron development in inducing axons to enter particular target tissues once they are in the vicinity or to induce branching within target tissues. Superior cervical and lumbar sympathetic ganglia showed temporal differences in their responsiveness to artemin, GDNF, and neurturin, which probably partly reflects the rostrocaudal development of sympathetic ganglia and the tissues they innervate.     

Reassembling a system from the sensor to cerebral representation: The olfactory system in vitro

An odorant's code is represented by activity in a dispersed ensemble of olfactory sensory neurons in the nose, activation of a specific combination of groups of mitral cells in the olfactory bulb and is considered to be mapped at divergent locations in the olfactory cortex. We present here an in vitro model of the mammalian olfactory system developed to gain easy access to all stations of the olfactory pathway. Mouse olfactory epithelial explants are cocultured with a brain slice that includes the olfactory bulb and olfactory cortex areas and maintains the central olfactory pathway intact and functional. Organotypicity of bulb and cortex is preserved and mitral cell axons can be traced to their target areas. Calcium imaging shows propagation of mitral cell activity to the piriform cortex. Long term coculturing with postnatal olfactory epithelial explants restores the peripheral olfactory pathway. Olfactory receptor neurons renew and progressively acquire a mature phenotype. Axons of olfactory receptor neurons grow out of the explant and rewire into the olfactory bulb. The extent of reinnervation exhibits features of a postlesion recovery. Functional imaging confirms the recovery of part of the peripheral olfactory pathway and shows that activity elicited in olfactory receptor neurons or the olfactory nerves is synaptically propagated into olfactory cortex areas. This model is the first attempt to reassemble a sensory system in culture, from the peripheral sensor to the site of cortical representation. It will increase our knowledge on how neuronal circuits in the central olfactory areas integrate sensory input and counterbalance damage.     

Integration of CO2 and odorant signals in the mouse olfactory bulb

Carbon dioxide (CO₂) is an important environmental cue for many animal species. In both vertebrates and invertebrates, CO₂ is detected by a specialized subset of olfactory sensory neurons (OSNs) and mediates several stereotypical behaviors. It remains unknown how CO₂ cues are integrated with other olfactory signals in the mammalian olfactory bulb, the first stage of central olfactory processing. By recording from the mouse olfactory bulb in vivo, we found that CO₂-activating neurons also respond selectively to odorants, many of which are putative mouse pheromones and natural odorants. In addition, many odorant-responsive bulbar neurons are inhibited by CO₂. For a substantial number of CO₂-activating neurons, binary mixtures of CO₂ and a specific odorant produce responses that are distinct from those evoked by either CO₂ or the odorant alone. In addition, for a substantial number of CO₂-inhibiting neurons, CO₂ addition can completely block the action potential firing of the cells to the odorants. These results indicate strong interaction between CO₂ signals and odorant signals in the olfactory bulb, suggesting important roles for the integration of these two signals in CO₂-mediated behavioral responses.     

GFR α2/neurturin signalling regulates noxious heat transduction in isolectin B4-binding mouse sensory neurons

The GFR α2 receptor is the cognate co-receptor for the neurotrophic factor neurturin and GFR α2 is selectively expressed by isolectin B₄ (IB₄)-binding nociceptive sensory neurons. Here, we used two physiological approaches in combination with mice that have a targeted deletion of the GFR α2 gene (GFR α2 -/- mice) in order to determine whether GFR α2/neurturin signalling regulates the functional properties or the survival of IB₄-binding nociceptors. Because 50 % of IB₄-binding neurons respond to noxious heat and because patch clamp recordings of isolated dorsal root ganglion sensory neurons allow one to neurochemically identify subpopulations of neurons, we analysed the noxious heat responsiveness of IB₄-positive and -negative small-diameter neurons isolated from adult GFR α2 -/- and littermate control mice. The percentage of IB₄-positive neurons that had large (> 100 pA) heat-evoked inward currents was severely reduced in GFR α2 -/- mice (12 %) compared to wild-type littermates (47 %), and this loss in large-magnitude heat currents was accounted for by an increase in neurons with very small (< 100 pA) heat-evoked currents as well as an increase in neurons with no detectable heat current. Counts of IB₄-positive and -negative neurons, as well as counts of unmyelinated axons in the saphenous nerve, confirmed that the loss in neurons with large-amplitude heat currents was due to a deficit in heat transduction and not a decrease in cell survival. The effect was modality specific for heat because mechanical transduction of all fibre types, including IB₄-positive C fibres, was normal. Our data are the first to indicate a transduction-function role for GFR α2/neurturin signalling in a specific class of sensory neurons.