Grueneberg Glomeruli in the Olfactory Bulb are Activated by Odorants and Cool Temperature

Neurons of the Grueneberg ganglion respond to cool temperatures as well as to distinct odorants and extend axonal processes to the olfactory bulb of the brain. Analyses of transgenic mice, in which Grueneberg ganglion neurons and their axons are labeled, revealed that these axons innervated nine distinct glomeruli distributed in a characteristic topographical pattern in dorsal, lateral, ventral, and medial regions of rather posterior areas in the bulb. To assess activation of these glomeruli (hereinafter designated as Grueneberg glomeruli) upon stimulation of Grueneberg ganglion neurons, mice were exposed to the odorant 2,3-dimethylpyrazine (2,3-DMP) and the expression of the activity-dependent marker c-Fos in juxtaglomerular cells of the relevant glomeruli was monitored. It was found that all of these glomeruli were activated, irrespective of their localization in the bulb. To verify that the activation of juxtaglomerular cells in Grueneberg glomeruli was indeed based on stimulation of Grueneberg ganglion neurons, the 2,3-DMP-induced responses in these glomeruli were investigated in mice lacking the cyclic nucleotide-gated channel CNGA3 which is critical for chemo- and thermosensory signal transduction in Grueneberg ganglion neurons. This approach revealed that elimination of CNGA3 led to a reduction of the odorant-induced activity in Grueneberg glomeruli, indicating that the activation of these glomeruli is based on a preceding stimulation of the Grueneberg ganglion. Analyzing whether Grueneberg glomeruli in the bulb might also process thermosensory information, it was found that upon exposure to coolness, Grueneberg glomeruli were activated. Investigating mice lacking CNGA3, the activation of these glomeruli by cool temperatures was attenuated.     

The Thermosensitive Potassium Channel TREK-1 Contributes to Coolness-Evoked Responses of Grueneberg Ganglion Neurons

Neurons of the Grueneberg ganglion (GG) residing in the vestibule of the murine nose are activated by cool ambient temperatures. Activation of thermosensory neurons is usually mediated by thermosensitive ion channels of the transient receptor potential (TRP) family. However, there is no evidence for the expression of thermo-TRPs in the GG, suggesting that GG neurons utilize distinct mechanisms for their responsiveness to cool temperatures. In search for proteins that render GG neurons responsive to coolness, we have investigated whether TREK/TRAAK channels may play a role; in heterologous expression systems, these potassium channels have been previously found to close upon exposure to coolness, leading to a membrane depolarization. The results of the present study indicate that the thermosensitive potassium channel TREK-1 is expressed in those GG neurons that are responsive to cool temperatures. Studies analyzing TREK-deficient mice revealed that coolness-evoked responses of GG neurons were clearly attenuated in these animals compared with wild-type conspecifics. These data suggest that TREK-1 channels significantly contribute to the responsiveness of GG neurons to cool temperatures, further supporting the concept that TREK channels serve as thermoreceptors in sensory cells. Moreover, the present findings provide the first evidence of how thermosensory GG neurons are activated by given temperature stimuli in the absence of thermo-TRPs.     

Grueneberg ganglion neurons are activated by a defined set of odorants

Based on a variety of recent findings, the Grueneberg ganglion (GG) in the vestibule of the nasal cavity is considered as an olfactory compartment. However, defined chemical substances that activate GG neurons have not been identified. In this study, the responsiveness of murine GG cells to odorants was examined by monitoring the expression of the activity-dependent gene c-Fos. Testing a number of odorous compounds, cells in the GG were found to respond to dimethylpyrazine (DMP) and a few related substances. These responses were dose-dependent and restricted to early postnatal stages. The DMP-responsive GG cells belonged to the subset of GG neurons that coexpress the signaling elements V2r83, GC-G, and CNGA3. These cells have been previously reported to respond to cool ambient temperatures as well. In fact, cool temperatures enhanced DMP-evoked responses of GG cells. These findings support the concept that the GG of neonatal mice operates as a dual sensory organ that is stimulated by both the odorous compound DMP and cool ambient temperatures.     

Expression of cGMP signaling elements in the Grueneberg ganglion

The Grueneberg ganglion (GG) is a cluster of neurons localized to the vestibule of the anterior nasal cavity. Based on axonal projections to the olfactory bulb of the brain, as well as expression of olfactory receptors and the olfactory marker protein, it is considered a chemosensory subsystem. Recently, it was observed that in mice, GG neurons respond to cool ambient temperatures. In mammals, coolness-induced responses in highly specialized neuronal cells are supposed to rely on the ion channel TRPM8, whereas in thermosensory neurons of the nematode worm Caenorhabditis elegans, detection of environmental temperature is mainly mediated by cyclic guanosine monophosphate (cGMP) pathways, in which cGMP is generated by transmembrane guanylyl cyclases. To unravel the molecular mechanisms underlying coolness-induced responses in GG neurons, potential expression of TRPM8 in the murine GG was investigated; however, no evidence was found that this ion channel is present in the GG. By contrast, a substantial number of GG neurons was observed to express the transmembrane guanylyl cyclase subtype GC-G. In the nose, GC-G expression appears to be confined to the GG since it was not detectable in other nasal compartments. In the GG, coolness-stimulated responses are only observed in neurons characterized by the expression of the olfactory receptor V2r83. Interestingly, expression of GC-G in the GG was found in this V2r83-positive subpopulation but not in other GG neurons. In addition to GC-G, V2r83-positive GG cells also co-express the phosphodiesterase PDE2A. Thus, in summary, coolness-sensitive V2r83-expressing GG neurons are endowed with a cGMP cascade which might underlie thermosensitivity of these cells, similar to the cGMP pathway mediating thermosensation in neurons of C. elegans. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. J. Fleischer and K. Mamasuew contributed equally to this work.     

Chemo- and thermosensory responsiveness of Grueneberg ganglion neurons relies on cyclic guanosine monophosphate signaling elements

Neurons of the Grueneberg ganglion (GG) in the anterior nasal region of mouse pups respond to cool temperatures and to a small set of odorants. While the thermosensory reactivity appears to be mediated by elements of a cyclic guanosine monophosphate (cGMP) cascade, the molecular mechanisms underlying the odor-induced responses are unclear. Since odor-responsive GG cells are endowed with elements of a cGMP pathway, specifically the transmembrane guanylyl cyclase subtype GC-G and the cyclic nucleotide-gated ion channel CNGA3, the possibility was explored whether these cGMP signaling elements may also be involved in chemosensory GG responses. Experiments with transgenic mice deficient for GC-G or CNGA3 revealed that GG responsiveness to given odorants was significantly diminished in these knockout animals. These findings suggest that a cGMP cascade may be important for both olfactory and thermosensory signaling in the GG. However, in contrast to the thermosensory reactivity, which did not decline over time, the chemosensory response underwent adaptation upon extended stimulation, suggesting that the two transduction processes only partially overlap.     

Receptor guanylyl cyclases in mammalian olfactory function

The contributions of guanylyl cyclases to sensory signaling in the olfactory system have been unclear. Recently, studies of a specialized subpopulation of olfactory sensory neurons (OSNs) located in the main olfactory epithelium have provided important insights into the neuronal function of one receptor guanylyl cyclase, GC-D. Mice expressing reporters such as β-galactosidase and green fluorescent protein in OSNs that normally express GC-D have allowed investigators to identify these neurons in situ, facilitating anatomical and physiological studies of this sparse neuronal population. The specific perturbation of GC-D function in vivo has helped to resolve the role of this guanylyl cyclase in the transduction of olfactory stimuli. Similar approaches could be useful for the study of the orphan receptor GC-G, which is expressed in another distinct subpopulation of sensory neurons located in the Grueneberg ganglion. In this review, we discuss key findings that have reinvigorated the study of guanylyl cyclase function in the olfactory system.     

The wiring of Grueneberg ganglion axons is dependent on neuropilin 1

The Grueneberg ganglion is a specialized olfactory sensor. In mice, its activation induces freezing behavior. The topographical map corresponding to the central projections of its sensory axons is poorly defined, as well as the guidance molecules involved in its establishment. We took a transgenic approach to label exclusively Grueneberg sensory neurons and their axonal projections. We observed that a stereotyped convergence map in a series of coalescent neuropil-rich structures is already present at birth. These structures are part of a peculiar and complex neuronal circuit, composed of a chain of glomeruli organized in a necklace pattern that entirely surrounds the trunk of the olfactory bulb. We found that the necklace chain is composed of two different sets of glomeruli: one exclusively innervated by Grueneberg ganglion neurons, the other by axonal inputs from the main olfactory neuroepithelium. Combining the transgenic Grueneberg reporter mouse with a conditional null genetic approach, we then show that the axonal wiring of Grueneberg neurons is dependent on neuropilin 1 expression. Neuropilin 1-deficient Grueneberg axonal projections lose their strict and characteristic avoidance of vomeronasal glomeruli, glomeruli that are innervated by secondary neurons expressing the repulsive guidance cue and main neuropilin 1 ligand Sema3a. Taken together, our observations represent a first step in the understanding of the circuitry and the coding strategy used by the Grueneberg system.     

Expression patterns of anoctamin 1 and anoctamin 2 chloride channels in the mammalian nose

Calcium-activated chloride channels are expressed in chemosensory neurons of the nose and contribute to secretory processes and sensory signal transduction. These channels are thought to be members of the family of anoctamins (alternative name: TMEM16 proteins), which are opened by micromolar concentrations of intracellular Ca²⁺. Two family members, ANO 1 (TMEM16A) and ANO 2 (TMEM16B), are expressed in the various sensory and respiratory tissues of the nose. We have examined the tissue specificity and sub-cellular localization of these channels in the nasal respiratory epithelium and in the five chemosensory organs of the nose: the main olfactory epithelium, the septal organ of Masera, the vomeronasal organ, the Grueneberg ganglion and the trigeminal system. We have found that the two channels show mutually exclusive expression patterns. ANO 1 is present in the apical membranes of various secretory epithelia in which it is co-localized with the water channel aquaporin 5. It has also been detected in acinar cells and duct cells of subepithelial glands and in the supporting cells of sensory epithelia. In contrast, ANO 2 expression is restricted to chemosensory neurons in which it has been detected in microvillar and ciliary surface structures. The different expression patterns of ANO 1 and ANO 2 have been observed in the olfactory, vomeronasal and respiratory epithelia. No expression has been detected in the Grueneberg ganglion or trigeminal sensory fibers. On the basis of this differential expression, we derive the main functional features of ANO 1 and ANO 2 chloride channels in the nose and suggest their significance for nasal physiology.     

Guanylyl cyclase‐G is an alarm pheromone receptor in mice

Many animals respond to threats by releasing alarm pheromones (APs) that warn conspecifics. In mice, detection of the AP 2‐sec‐butyl‐4,5‐dihydrothiazole (SBT) is mediated by chemosensory neurons residing in the Grueneberg ganglion (GG) of the anterior nasal region. Although the molecular mechanisms underlying activation of GG neurons by SBT and other substances are still unclear, recent studies have reported an involvement of the transmembrane guanylyl cyclase (GC) subtype GC‐G in chemosensory signaling in the GG. Here, we show that SBT directly binds with high affinity to the extracellular domain of GC‐G and elicits an enhanced enzymatic activity of this protein. In line with this finding, heterologous expression of GC‐G renders cells responsive to SBT while activation by SBT was strongly attenuated in GG neurons from GC‐G‐deficient mice. Consistently, SBT‐induced fear‐associated behaviors, SBT‐evoked elevated blood pressure, and increased serum levels of the stress hormone corticosterone were clearly reduced in GC‐G‐knockout animals compared to wild‐type mice. These observations suggest that GC‐G serves as an unusual receptor in GG neurons mediating the detection of the volatile AP substance SBT.     

Delta/Notch signaling promotes formation of zebrafish neural crest by repressing Neurogenin 1 function

In zebrafish, cells at the lateral edge of the neural plate become Rohon-Beard primary sensory neurons or neural crest. Delta/Notch signaling is required for neural crest formation. ngn1 is expressed in primary neurons; inhibiting Ngn1 activity prevents Rohon-Beard cell formation but not formation of other primary neurons. Reducing Ngn1 activity in embryos lacking Delta/Notch signaling restores neural crest formation, indicating Delta/Notch signaling inhibits neurogenesis without actively promoting neural crest. Ngn1 activity is also required for later development of dorsal root ganglion sensory neurons; however, Rohon-Beard neurons and dorsal root ganglion neurons are not necessarily derived from the same precursor cell. We propose that temporally distinct episodes of Ngn1 activity in the same precursor population specify these two different types of sensory neurons.     

Expression of trace amine-associated receptors in the Grueneberg ganglion

The Grueneberg ganglion (GG) in the vestibule of the anterior nasal cavity is considered as an olfactory subcompartment based on expression of the olfactory marker protein (OMP) and axonal projection to the olfactory bulb. Searching for olfactory receptors present in the GG, it has been observed recently that V2r83, a member of the V2R class of olfactory receptors, is expressed in numerous cells in the GG of mice. However, no other olfactory receptors have been found to be present in a considerable number of GG neurons so far. Here, we report that GG neurons express trace amine-associated receptors (TAARs) that have most recently been described as a novel class of olfactory receptors. It was observed that several TAAR subtypes are expressed by defined subpopulations of GG neurons distinct from the V2r83-positive cells. Analyzing the time course of TAAR expression during pre- and postnatal development revealed that TAARs are expressed by a substantial portion of GG neurons in late embryonic and neonatal stages, whereas in juveniles and adults, the number of TAAR-positive cells in the GG was significantly decreased.     

Apoptosis of Central and Peripheral Neurons Can Be Prevented with Cyclin-Dependent Kinase/Mitogen-Activated Protein Kinase Inhibitors

Abstract: Previous studies have indicated that certain members of the cyclin-dependent kinase/mitogen-activated protein kinase superfamily are involved in apoptosis of neuronal cells. Here, we have examined programmed cell death induced by withdrawal of neurotrophic support from CNS (rat retinal) and PNS (chick sympathetic, sensory, and ciliary) neurons. All four neuron types were equally rescued by the purine analogues olomoucine and roscovitine. Olomoucine inhibits multiple cyclin-dependent and mitogen-activated protein kinases with similar potency. Roscovitine is a more selective cyclin-dependent kinase inhibitor; but, so is butyrolactone I, which did not prevent retinal ganglion cell death. The specific p38^MAPK inhibitor SB-203580 did not prevent apoptosis in retinal ganglion cells. Death of these cells in the absence of neurotrophic factors was accompanied by morphological changes indicative of apoptosis, including nuclear condensation and fragmentation. Treatment with olomoucine or roscovitine not only prevented these apoptotic changes in retinal ganglion cells but also blocked neurite outgrowth. The survival-promoting activity of olomoucine correlated with its in vitro IC₅₀ for c-Jun N-terminal kinase-1 and its potency to repress c- jun induction in live PC12 cells. Roscovitine was more potent in rescuing neurons than in inhibiting Jun kinase. Thus, the antiapoptotic action of roscovitine might be due to inhibition of additional kinases.     

Non-neuronal cell conditioned medium stimulates peptidergic expression in sympathetic and sensory neurons in vitro

Regulation of peptide neurotransmitter metabolism was examined in dissociated cell cultures of neonatal rat sympathetic and sensory ganglia. Previous studies have shown that pineal gland conditioned medium (PCM) influences substance P (SP) and somatostatin (SS) metabolism in sympathetic neurons in vitro. The present study examines mechanisms mediating these effects, and compares the actions of PCM on sympathetic and sensory neurons. PCM treatment increased SP levels in a dose-dependent manner without altering SS content of sympathetic neurons cultured in the presence of ganglion non-neuronal cells. Conversely, treatment of pure sympathetic neuron cultures resulted in a dose-dependent increase in SS, while SP was virtually undetectable at all doses. By contrast, dorsal root ganglion, trigeminal ganglion, and nondose ganglion sensory neurons contained SP both in the presence and absence of ganglion non-neuronal cells. Moreover, in each of these neuronal populations treatment with PCM increased SP levels both in the presence and in the absence of ganglion non-neuronal cells. These observations suggest that ganglion non-neuronal cells are necessary for sympathetic but not sensory neuron expression of SP. Moreover, PCM apparently stimulates SP in neurons which already contain the peptide, but the factor cannot foster de novo expression of the phenotype. PCM also influenced other transmitter traits in sympathetic neurons, suggesting linkage between mechanisms regulating peptides and other transmitters. In cultures containing both sympathetic neurons and non-neuronal cells, PCM treatment increased cholineacetyltransferase (CHAC) activity as well as SP, and decreased tyrosine hydroxylase (TOH) activity. By contrast, PCM treatment of pure sympathetic neuron cultures led to parallel increases in SS and TOH activity with negligible levels of SP and CHAC. These observations suggest that in sympathetic neurons, SS may be linked with noradrenergic expression, while SP is associated with cholinergic development, although more data are required to confirm this relationship. Moreover, there may be a reciprocal relationship between SP and SS expression by sympathetic neurons analogous to previous observations regarding cholinergic-noradrenergic expression (P. H. Patterson and L. L. Y. Chun, Proc. Natl. Acad. Sci. USA 71, 3607-3610, 1974; Dev. Biol. 56, 263-280, 1977). Consequently, neurotransmitter phenotypic expression is a complex process in which the environment regulates a balance among multiple transmitters.     

Adult rat otic placode-derived neurons and sensory epithelium express all four erbB receptors: a role in regulating vestibular ganglion neuron viability.

The erbB receptor family consists of erbB1/epidermal growth factor receptor, erbB2/neu, erbB3, and erbB4, all of which have been implicated in cell proliferation, differentiation, and survival in several tissues. In the nervous system, these family members can function in a trophic capacity for certain subpopulations of neurons and some types of non-neuronal cells. Vestibular sensory epithelial cells and vestibular ganglion neurons are derived from ectodermal otic placode and are essential components of the peripheral vestibular system, the sensory system for balance. Recent studies in mammals suggest that certain ligands of the epidermal growth factor receptor can induce proliferation of vestibular sensory epithelial cells. We now show that vestibular ganglion neurons and vestibular sensory epithelial cells express all four erbB receptors in adult rats. Cultured vestibular ganglion neurons also expressed all four erbB family members and were therefore used to analyze the effects of modulating erbB signaling on differentiated vestibular ganglion neurons. Transforming growth factor-alpha (a ligand for epidermal growth factor receptor) and sensory and motor neuron-derived factor (a ligand for erbB3 and erbB4) promoted vestibular ganglion neuron viability, whereas epidermal growth factor (another ligand for epidermal growth factor receptor) did not. Glial growth factor 2 (another ligand for erbB3 and erbB4) and an antibody that blocks erbB2/neu-mediated signaling inhibited vestibular ganglion neuron viability. Collectively, these observations indicate that erbB signaling regulates the viability of differentiated otic placode-derived cells in mammals and suggest that exogenous modulation of erbB signaling in peripheral vestibular tissues may prove therapeutically useful in peripheral vestibular disorders.     

Human Skeletal Muscle Cells Synthesise a Neuronotrophic Factor Reactive with Spinal Neurons

Retrograde trophic influences originating in the skeletal musculature have been postulated to be involved in regulating survival and differentiation of embryonic motor neurons and reactive terminal sprouting of mature motor fibres. We have previously described the use of a quantitative immunoassay for neurofilament protein to bioassay in vitro the cell-type-specific neuronotrophic activity of nerve growth factor (NGF) on sensory ganglion neurons. In the present study, the effect of media conditioned by adult human muscle cells (MCM) on the in vitro development of chicken spinal neurons has been studied using a similar approach. Significant increases in neurofilament protein levels in 7-day chicken embryonic spinal cord cultures were found with doses of MCM protein as low as 0.4 microgram/ml, with a dose-response relationship yielding maximal and half-maximal effects at 4 and 1 microgram/ml, respectively. Maximal increases in neurofilament protein levels were associated with an approximate two-fold increase in neuronal cell survival. MCM also induced increases in choline acetyltransferase activity in chick spinal cord cultures. In both the absence and presence of NGF, MCM did not increase neurofilament protein expression in primary cultures of sensory neurons.     

Differential expression of the capsaicin receptor TRPV1 and related novel receptors TRPV3, TRPV4 and TRPM8 in normal human tissues and changes in traumatic and diabetic neuropathy

Background  

Transient receptor potential (TRP) receptors expressed by primary sensory neurons mediate thermosensitivity, and may play a role in sensory pathophysiology. We previously reported that human dorsal root ganglion (DRG) sensory neurons co-expressed TRPV1 and TRPV3, and that these were increased in injured human DRG. Related receptors TRPV4, activated by warmth and eicosanoids, and TRPM8, activated by cool and menthol, have been characterised in pre-clinical models. However, the role of TRPs in common clinical sensory neuropathies needs to be established.     

Transient expression of somatostatin peptide is a widespread feature of developing sensory and sympathetic neurons in the embryonic rat.

Previous studies from this and other laboratories demonstrated that many embryonic sensory ganglion cells in the rat transiently express the catecholamine synthesizing enzyme tyrosine hydroxylase (TH), a trait not expressed by most mature sensory neurons. We, therefore, sought to determine whether transient expression was uniquely associated with catecholaminergic traits, or, alternatively, whether embryonic ganglion cells transiently expressed peptidergic properties as well. Of the four peptides examined (somatostatin [somatotropin release inhibiting factor] (SRIF), galanin (Gal), calcitonin gene-related peptide (CGRP), and substance P (SP)), only SRIF was found to be transiently expressed during early stages of sensory gangliogenesis. Surprisingly, SRIF immunoreactivity was observed in virtually all cranial and spinal sensory ganglion cells on embryonic day (E) 12.5. In addition to perikaryal labeling, intense SRIF immunoreactivity was also observed in the central and peripheral processes of E12.5 sensory neurons, suggesting the peptide may be released from nerve endings. The time course of SRIF appearance in cranial ganglion cells paralleled that previously described for TH, and double-labeling studies revealed extensive co-localization of these two phenotypes. By E16.5, however, the number of neurons expressing SRIF had diminished markedly, indicating that SRIF is only transiently expressed by most sensory neurons during early stages of ganglion development. An unexpected finding was that transient expression of SRIF is also a prominent feature of sympathetic ganglion cells; however, the temporal pattern of staining in the sympathetic and sensory ganglia differed substantially. Whereas virtually no SRIF staining was observed in E12.5 sympathetics, the vast majority of cells in the E16.5 superior cervical ganglion (SCG) were labeled. This contrasted sharply with the adult SCG, in which only low levels of SRIF expression were found. These findings demonstrate that SRIF peptide is transiently expressed at high levels in peripheral sensory and sympathetic neurons during embryogenesis. The time course and widespread distribution of SRIF expression indicates that the peptide may play a role in early stages of ganglion cell growth and development. Moreover, these data, in conjunction with previous studies demonstrating SRIF immunoreactivity in developing central neurons, suggest that transient expression of this peptide is a common property of diverse neuronal cell types.     

Catecholaminergic primary sensory neurons: autonomic targets and mechanisms of transmitter regulation

Contrary to traditional teaching, mammalian primary sensory neurons may express catecholaminergic (CA) neurotransmitter characteristics in vivo. Sensory neurons in the nodose, petrosal, and dorsal root ganglia of rats express tyrosine hydroxylase, the rate-limiting enzyme in CA biosynthesis, and formaldehyde-induced CA fluorescence, in addition to other CA traits. These findings suggest that catecholamines may function as sensory as well as autonomic motor (e.g., sympathetic) neurotransmitters. Most CA cells in the petrosal ganglion project peripherally to the carotid body, which indicates a striking correlation between CA expression in sensory neurons and the pattern of sensory innervation. Inasmuch as petrosal ganglion afferents make synaptic contact with chemoreceptive glomus cells in the carotid body, it is likely that CA sensory neurons in the ganglion transmit chemoreceptor information to the brain stem. Comparison with sympathetic neurons indicates that some mechanisms of CA regulation, such as altered activity of tyrosine hydroxylase in response to depolarizing stimuli, are shared among sensory and traditional CA populations. Other mechanisms, including trophic regulation, appear to be distinct. Therefore, despite expression of common phenotypic traits, CA expression in diverse populations of peripheral neurons is not necessarily associated with a common repertoire of regulatory mechanisms.     

Transient expression of somatostatin peptide is a widespread feature of developing sensory and sympathetic neurons in the embryonic rat

Previous studies from this and other laboratories demonstrated that many embryonic sensory ganglion cells in the rat transiently express the catecholamine synthesizing enzyme tyrosine hydroxylase (TH), a trait not expressed by most mature sensory neurons. We, therefore, sought to determine whether transient expression was uniquely associated with catecholaminergic traits, or, alternatively, whether embryonic ganglion cells transiently expressed peptidergic properties as well. Of the four peptides examined {somatostatin [somatotropin release inhibiting factor] (SRIF), galanin (Gal), calcitonin gene-related peptide (CGRP), and substance P (SP)}, only SRIF was found to be transiently expressed during early stages of sensory gangliogenesis. Surprisingly, SRIF immunoreactivity was observed in virtually all cranial and spinal sensory ganglion cells on embryonic day (E) 12.5. In addition to perikaryal labeling, intense SRIF immunoreactivity was also observed in the central and peripheral processes of E12.5 sensory neurons, suggesting the peptide may be released from nerve endings. The time course of SRIF appearance in cranial ganglion cells paralleled that previously described for TH, and double labeling studies revealed extensive co-localization of these two phenotypes. By E16.5, however, the number of neurons expressing SRIF had diminished markedly, indicating that SRIF is only transiently expressed by most sensory neurons during early stages of ganglion development. An unexpected finding was that transient expression of SRIF is also a prominent feature of sympathetic ganglion cells; however, the temporal pattern of staining in the sympathetic and sensory ganglia differed substantially. Whereas virtually no SRIF staining was observed in E12.5 sympathetics, the vast majority of cells in the E16.5 superior cervical ganglion (SCG) were labeled. This contrasted sharply with the adult SCG, in which only low levels of SRIF expression were found. These findings demonstrate that SRIF peptide is transiently expressed at high levels in peripheral sensory and sympathetic neurons during embryogenesis. The time course and widespread distribution of SRIF expression indicates that the peptide may play a role in early stages of ganglion cell growth and development. Moreover, these data, in conjunction with previous studies demonstrating SRIF immunoreactivity in developing central neurons, suggest that transient expression of this peptide is a common property of diverse neuronal cell types. © 1992 John Wiley & Sons, Inc.