Peripheral target-specific influences on embryonic neurite growth vigor and patterns

The relevance of the superior cervical ganglion for cardiac autonomic innervation in health and disease: a systematic review

PURPOSE

The heart receives cervical and thoracic sympathetic contributions. Although the stellate ganglion is considered the main contributor to cardiac sympathetic innervation, the superior cervical ganglia (SCG) is used in many experimental studies. The clinical relevance of the SCG to cardiac innervation is controversial. We investigated current morphological and functional evidence as well as controversies on the contribution of the SCG to cardiac innervation.

METHODS

A systematic literature review was conducted in PubMed, Embase, Web of Science, and COCHRANE Library. Included studies received a full/text review and quality appraisal.

RESULTS

Seventy-six eligible studies performed between 1976 and 2023 were identified. In all species studied, morphological evidence of direct or indirect SCG contribution to cardiac innervation was found, but its contribution was limited. Morphologically, SCG sidedness may be relevant. There is indirect functional evidence that the SCG contributes to cardiac innervation as shown by its involvement in sympathetic overdrive reactions in cardiac disease states. A direct functional contribution was not found. Functional data on SCG sidedness was largely unavailable. Information about sex differences and pre- and postnatal differences was lacking.

CONCLUSION

Current literature mainly supports an indirect involvement of the SCG in cardiac innervation, via other structures and plexuses or via sympathetic overdrive in response to cardiac diseases. Morphological evidence of a direct involvement was found, but its contribution seems limited. The relevance of SCG sidedness, sex, and developmental stage in health and disease remains unclear and warrants further exploration.

Evaluation of Cisplatin Neurotoxicity in Cultured Rat Dorsal Root Ganglia via Cytosolic Calcium Accumulation

BACKGROUND

Calcium homeostasis is considered to be important in antineoplastic as well as in neurotoxic adverse effects of cisplatin.

AIMS

This study aimed to investigate the role of Ca(2+) in cisplatin neurotoxicity in cultured rat dorsal root ganglia (DRG) cells.

STUDY DESIGN

Cell culture study.

METHODS

DRG cells prepared from 1-day old Sprague-Dawley rats were used to determine the role of Ca(2+) in the cisplatin (10-600 μM) neurotoxicity. The cells were incubated with cisplatin plus nimodipine (1-3 μM), dizocilpine (MK-801) (1-3 μM) or thapsigargin (100-300 nM). Toxicity of cisplatinon DRG cells was determined by the MTT assay.

RESULTS

The neurotoxicity of cisplatin was significant when used in high concentrations (100-600 μM). Nimodipine (1 μM) but not MK-801 or thapsigargin prevented the neurotoxic effects of 200 μM of cisplatin.

CONCLUSION

Voltage-dependent calcium channels may play a role in cisplatin neurotoxicity.

Molecular determinants of the face map development in the trigeminal brainstem

The perception of external sensory information by the brain requires highly ordered synaptic connectivity between peripheral sensory neurons and their targets in the central nervous system. Since the discovery of the whisker-related barrel patterns in the mouse cortex, the trigeminal system has become a favorite model for study of how its connectivity and somatotopic maps are established during development. The trigeminal brainstem nuclei are the first CNS regions where whisker-specific neural patterns are set up by the trigeminal afferents that innervate the whiskers. In particular, barrelette patterns in the principal sensory nucleus of the trigeminal nerve provide the template for similar patterns in the face representation areas of the thalamus and subsequently in the primary somatosensory cortex. Here, we describe and review studies of neurotrophins, multiple axon guidance molecules, transcription factors, and glutamate receptors during early development of trigeminal connections between the whiskers and the brainstem that lead to emergence of patterned face maps. Studies from our laboratories and others' showed that developing trigeminal ganglion cells and their axons depend on a variety of molecular signals that cooperatively direct them to proper peripheral and central targets and sculpt their synaptic terminal fields into patterns that replicate the organization of the whiskers on the muzzle. Similar mechanisms may also be used by trigeminothalamic and thalamocortical projections in establishing patterned neural modules upstream from the trigeminal brainstem.

Dose and age-dependent axonal responses of embryonic trigeminal neurons to localized NGF via p75NTR receptor

Nerve growth factor (NGF) and related neurotrophins are target-derived survival factors for sensory neurons. In addition, these peptides modulate neuronal differentiation, axon guidance, and synaptic plasticity. We tested axonal behavior of embryonic trigeminal neurons towards localized sources of NGF in collagen gel assays. Trigeminal axons preferentially grow towards lower doses of localized NGF and grow away from higher concentrations at earlier stages of development, but do not show this response later. Dorsal root ganglion axons also show similar responses to NGF, but NGF-dependent superior cervical ganglion axons do not. Such axonal responses to localized NGF sources were also observed in Bax-/- mice, suggesting that the axonal effects are largely independent of cell survival. Immunocytochemical studies indicated that axons, which grow towards or away from localized NGF are TrkA-positive, and TrkA-/- TG axons do not respond to any dose of NGF. We further show that axonal responses to NGF are absent in TG derived from mice that lack the p75 neurotrophin receptor (p75NTR). Collectively, our results suggest that localized sources of NGF can direct axon outgrowth from trigeminal ganglion in a dose- and age-dependent fashion, mediated by p75NTR signaling through TrkA expressing axons.

Local neurotrophin effects on central trigeminal axon growth patterns

In dissociated cell and wholemount explant cultures of the embryonic trigeminal pathway NGF promotes exuberant elongation of trigeminal ganglion (TG) axons, whereas NT-3 leads to precocious arborization [J. Comp. Neurol. 425 (2000) 202]. In the present study, we investigated the axonal effects of local applications of NGF and NT-3. We placed small sepharose beads loaded with either NGF or NT-3 along the lateral edge of the central trigeminal tract in TG-brainstem intact wholemount explant cultures prepared from embryonic day 15 rats. Labeling of the TG with carbocyanine dye, DiI, revealed that NGF induces local defasciculation and diversion of trigeminal axons. Numerous axons leave the tract, grow towards the bead and engulf it, while some axons grow away from the neurotrophin source. NT-3, on the other hand, induced localized interstitial branching and formation of neuritic tangles in the vicinity of the neurotrophin source. Double immunocytochemistry showed that axons responding to NGF were predominantly TrkA-positive, whereas both TrkA and TrkC-positive axons responded to NT-3. Our results indicate that localized neurotrophin sources along the routes of embryonic sensory axons in the central nervous system, far away from their parent cell bodies, can alter restricted axonal pathways and induce elongation, arborization responses.

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.

Slit2, a branching-arborization factor for sensory axons in the Mammalian CNS

Axons that carry information from the sensory periphery first elongate unbranched and form precisely ordered tracts within the CNS. Later, they begin collateralizing into their proper targets and form terminal arbors. Target-derived factors that govern sensory axon elongation and branching-arborization are not well understood. Here we report that Slit2 is a major player in branching-arborization of central trigeminal axons in the brainstem. Embryonic trigeminal axons initially develop unbranched as they form the trigeminal tract within the lateral brainstem; later, they emit collateral branches into the brainstem trigeminal nuclei and form terminal arbors therein. In whole-mount explant cultures of this pathway, embryonic day 15 (E15) rat central trigeminal axons retain their unbranched growth within the tract, whereas E17 trigeminal axons show branching and arborization in the brainstem trigeminal nuclei, much like that seen in vivo. Similar observations were made in E13 and E15 mouse embryos. We cocultured Slit2-expressing tissues or cells with the whole-mount explant cultures of the central trigeminal pathway derived from embryonic rats or mice. When central trigeminal axons are exposed to ectopic Slit2 during their elongation phase, they show robust and premature branching and arborization. Blocking available Slit2 reverses this effect on axon growth. Spatiotemporal expression of Slit2 and Robo receptor mRNAs within the brainstem trigeminal nuclei and the trigeminal ganglion during elongation and branching-arborization further corroborates our experimental results.

Target specific differentiation of peripheral trigeminal axons in rat-chick chimeric explant cocultures

Avian and rodent trigeminal ganglion (TG) neurons share common features in their neurotrophin requirements and axonal projections between the sensory periphery and the brainstem. In rodents, the whisker pad (WP) is a major peripheral target of the infraorbital (IO) nerve component of the TG. The chick IO nerve is much smaller and innervates the maxillary process (MP). In the embryonic WP, IO axons course in fascicles from a caudal to rostral direction and form terminal plexuses around follicles. In the chick, IO axons travel as a thin bundle to the MP and branch out with no specific patterning. We cocultured E15 rat TG with E5-6 chick MP or chick TG with rat WP explants to examine target influences on trigeminal axon growth patterns as visualized with DiI labeling or neurofilament immunohistochemistry. Chick TG axons showed robust growth into WP explants, and the ganglion increased in size. Thick bundles of axons traveled between rows of follicles and formed a distinct pattern as they developed terminal arbors around individual follicles. In contrast, rat TG axon growth was sparse in chick MP explants and the ganglion size reduced over time. Furthermore, rat TG axons did not show any patterning in the chick MP. Similar target-specific growth patterns were observed when TG explants were given a choice between chick MP and rat WP explants. Collectively these results indicate that both the chick and rat TG cells respond to similar target-specific peripheral cues in the establishment of innervation density and patterning in peripheral orofacial targets.

Directional specificity and patterning of sensory axons in trigeminal ganglion-whisker pad cocultures

In the rodent trigeminal pathway, trigeminal axons invade the developing whisker pad from a caudal to rostral direction. We investigated directional specificity of embryonic day (E) 15 rat trigeminal axons within this peripheral target field using explant cocultures. E15 trigeminal axons readily grow into the same age whisker pad explants and form follicle-related patterns along a caudal to rostral direction. They also can grow into this target from its lateral aspects. In contrast, they are unable to invade the whisker pad from the rostral (nasal) pole. We did not find any correlation between the distribution of extracellular matrix molecules and trigeminal axon growth preferences. We also examined age-related changes in trigeminal axon responsiveness to directional cues. E19 trigeminal axons readily grew into E15 whisker pad explants from either the caudal or the rostral pole. These results suggest the presence of growth permissive and repulsive cues that guide sensory axons in the whisker pad. Furthermore, trigeminal axons lose their responsiveness to growth inhibitory cues at later stages of development.