mRNAS for one, two or three members of trk receptor family are expressed in single rat trigeminal ganglion neurons

Functional Diversity of Neurotrophin Actions on the Oculomotor System

Neurotrophins play a principal role in neuronal survival and differentiation during development, but also in the maintenance of appropriate adult neuronal circuits and phenotypes. In the oculomotor system, we have demonstrated that neurotrophins are key regulators of developing and adult neuronal properties, but with peculiarities depending on each neurotrophin. For instance, the administration of NGF (nerve growth factor), BDNF (brain-derived neurotrophic factor) or NT-3 (neurotrophin-3) protects neonatal extraocular motoneurons from cell death after axotomy, but only NGF and BDNF prevent the downregulation in ChAT (choline acetyltransferase). In the adult, in vivo recordings of axotomized extraocular motoneurons have demonstrated that the delivery of NGF, BDNF or NT-3 recovers different components of the firing discharge activity of these cells, with some particularities in the case of NGF. All neurotrophins have also synaptotrophic activity, although to different degrees. Accordingly, neurotrophins can restore the axotomy-induced alterations acting selectively on different properties of the motoneuron. In this review, we summarize these evidences and discuss them in the context of other motor systems.

Chronic exposure to 50Hz magnetic fields causes a significant weakening of antioxidant defence systems in aged rat brain

Several studies suggest that extremely low-frequency magnetic fields (ELF-MFs) may enhance the free radical endogenous production. It is also well known that one of the unavoidable consequences of ageing is an overall oxidative stress-based decline in several physiological functions and in the general resistance to stressors. On the basis of these assumptions, the aim of this study was to establish whether the ageing process can increase susceptibility towards widely present ELF-MF-mediated pro-oxidative challenges. To this end, female Sprague-Dawley rats were continuously exposed to a sinusoidal 50 Hz, 0.1 mT magnetic field for 10 days. Treatment-induced changes in the major antioxidant protection systems and in the neurotrophic support were investigated, as a function of the age of the subjects. All analyses were performed in brain cortices, due to the high susceptibility of neuronal cells to oxidative injury. Our results indicated that ELF-MF exposure significantly affects anti-oxidative capability, both in young and aged animals, although in opposite ways. Indeed, exposed young individuals enhanced their neurotrophic signalling and anti-oxidative enzymatic defence against a possible ELF-MF-mediated increase in oxygen radical species. In contrast, aged subjects were not capable of increasing their defences in response to ELF-MF treatment but, on the contrary, they underwent a significant decrease in the major antioxidant enzymatic activities. In conclusion, our data seem to suggest that the exposure to ELF-MFs may act as a risk factor for the occurrence of oxidative stress-based nervous system pathologies associated with ageing.

The TrkC receptor induces apoptosis when the dependence receptor notion meets the neurotrophin paradigm

The TrkC/NT-3 receptor/ligand pair is believed to be part of the classic neurotrophic theory claiming that neuronal death occurs by default when neurotrophic factors become limited, through loss of survival signals. Here, we show that TrkC is a dependence receptor and, as such, induces caspase-dependent apoptotic death in the absence of NT-3 in immortalized cells, a proapoptotic activity inhibited by the presence of NT-3. This proapoptotic activity of TrkC relies on the caspase-mediated cleavage of the intracellular domain of TrkC, which permits the release of a proapoptotic fragment. This fragment induces apoptosis through a caspase-9-dependent mechanism. Finally, we show that the death of dorsal root ganglion (DRG) neurons provoked by NT-3 withdrawal is inhibited when TrkC-proapoptotic activity is antagonized. Thus, the death of neurons upon disappearance of NT-3 is not only due to a loss of survival signals but also to the active proapoptotic activity of the unbound TrkC dependence receptor.

Differential Trk expression in explant and dissociated trigeminal ganglion cell cultures

During embryonic development, expression of neurotrophin receptor tyrosine kinases (Trks) by sensory ganglia is continuously and dynamically regulated. Neurotrophin signaling promotes selective survival and axonal differentiation of sensory neurons. In embryonic day (E) 15 rat trigeminal ganglion (TG), NGF receptor TrkA is expressed by small diameter neurons, NT-3 receptor TrkC and BDNF receptor TrkB are expressed by large diameter neurons. Organotypic explant and dissociated cell cultures of the TG (and dorsal root ganglia) are commonly used to assay neurotrophin effects on developing sensory neurons. In this study, we compared Trk expression in E15 rat TG explant and dissociated cell cultures with or without neurotrophin treatment. Only a subset of TG cells express each of the three Trk receptors in wholemount explant cultures as in vivo conditions. In contrast, all TG neurons co-express all three Trk receptors upon dissociation, regardless of neurotrophin treatment. Neurons cultured in low concentrations of one neurotrophin first, and switched to higher concentrations of another after 1 day, survive and display morphological characteristics of neurons cultured in a mixture of both neurotrophins for 3 days. Our results indicate that wholemount explant cultures of sensory ganglia represent in vivo conditions in terms of Trk expression patterns; whereas dissociation dramatically alters Trk expression by primary sensory neurons.

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.

Each sensory nerve arising from the geniculate ganglion expresses a unique fingerprint of neurotrophin and neurotrophin receptor genes

Neurons in the geniculate ganglion, like those in other sensory ganglia, are dependent on neurotrophins for survival. Most geniculate ganglion neurons innervate taste buds in two regions of the tongue and two regions of the palate; the rest are cutaneous nerves to the skin of the ear. We investigated the expression of four neurotrophins, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and NT-4, and five neurotrophin receptors, trkA, trkB, trkC, p75, and truncated trkB (Trn-B) in single sensory neurons of the adult rat geniculate ganglion associated with the five innervation fields. For fungiform papillae, a glass pipette containing biotinylated dextran was placed over the target papilla and the tracer was iontophoresed into the target papilla. For the other target fields, Fluoro-Gold was microinjected. After 3 days, geniculate ganglia were harvested, sectioned, and treated histochemically (for biotinylated dextran) or immunohistochemically (for Fluoro-Gold) to reveal the neurons containing the tracer. Single labeled neurons were harvested from the slides and subjected to RNA amplification and RT-PCR to reveal the neurotrophin or neurotrophin receptor genes that were expressed. Neurons projecting from the geniculate ganglion to each of the five target fields had a unique expression profile of neurotrophin and neurotrophic receptor genes. Several individual neurons expressed more than one neurotrophin receptor or more than one neurotrophin gene. Although BDNF is significantly expressed in taste buds, its primary high affinity receptor, trkB, was not prominently expressed in the neurons. The results are consistent with the interpretation that at least some, perhaps most, of the trophic influence on the sensory neurons is derived from the neuronal somata, and the trophic effect is paracrine or autocrine, rather than target derived. The BDNF in the taste bud may also act in a paracrine or autocrine manner on the trkB expressed in taste buds, as shown by others.

Expression of Trk receptors in the oculomotor system of the adult cat

We examined the expression of the three Trk receptors for neurotrophins (TrkA, TrkB, and TrkC) in the extraocular motor nuclei of the adult cat by using antibodies directed against the full-Trk proteins in combination with horseradish peroxidase retrograde tracing. The three receptors were present in all neuronal populations investigated, including abducens motoneurons and internuclear neurons, medial rectus motoneurons of the oculomotor nucleus, and trochlear motoneurons. They were also present in the vestibular and prepositus hypoglossi nuclei. TrkA, TrkB, and TrkC immunopositive cells were found in similar percentages in the oculomotor and in the trochlear nuclei. In the abducens nucleus, however, a significantly higher percentage of cells expressed TrkB than the other two receptors, among both motoneurons (81.8%) and internuclear neurons (88.4%). The percentages obtained for the three Trk receptors in identified neuronal populations pointed to the colocalization of two or three receptors in a large number of cells. We used confocal microscopy to elucidate the subcellular location of Trk receptors. In this case, abducens motoneurons and internuclear neurons were identified with antibodies against choline acetyltransferase and calretinin, respectively. We found a different pattern of staining for each neurotrophin receptor, suggesting the possibility that each receptor and its cognate ligand may use a different route for cellular signaling. Therefore, the expression of Trk receptors in oculomotor, trochlear, and abducens motoneurons, as well as abducens internuclear neurons, suggests that their associated neurotrophins may exert an influence on the normal operation of the oculomotor circuitry. The presence of multiple Trk receptors on individual cells indicates that they likely act in concert with each other to regulate distinct functions.

Nerve growth factor survival signaling in cultured hippocampal neurons is mediated through TrkA and requires the common neurotrophin receptor P75

The role of the common neurotrophin receptor p75 (p75NTR) in neuronal survival and cell death remains controversial. On the one hand, p75NTR provides a positive modulatory influence on nerve growth factor (NGF) signaling through the high affinity neurotrophin receptor TrkA, and hence increases NGF survival signaling. However, p75NTR may also signal independently of TrkA, causing cell death or cell survival, depending on the cell type and stage of development. Here we demonstrate that TrkA is expressed in primary cultures of hippocampal neurons and is activated by NGF within 10 min of exposure. In primary hippocampal cultures neuroprotection by NGF against glutamate toxicity was mediated by NF-kappaB and accompanied by an increased expression of neuroprotective NF-kappaB target genes Bcl-2 and Bcl-xl. In mouse hippocampal cells lacking p75NTR (p75NTR-/-) activation of TrkA by NGF was not detectable. Moreover, neuroprotection by NGF against glutamate toxicity was abolished in p75NTR-/- neurons, and the expression of bcl-2 and bcl-xl was markedly reduced as compared to wildtype cells. NGF increased TrkA phosphorylation in hippocampal neurons and provided protection that required phosphoinositol-3-phosphate (PI3)-kinase activity and Akt phosphorylation, whereas the mitogen-activated protein kinases (MAPK), extracellular-regulated kinases (Erk) 1/2, were not involved. P75NTR signaling independent of TrkA, such as increased neutral sphingomyelinase (NSMase) activity causing enhanced levels of ceramide, were not detected after exposure of hippocampal neurons to NGF. Interestingly, inhibition of sphingosine-kinase blocked the neuroprotective effect of NGF, suggesting that sphingosine-1-phosphate was also involved in NGF-mediated survival in our cultured hippocampal neurons. Overall, our results indicate an essential role for p75NTR in supporting NGF-triggered TrkA signaling pathways mediating neuronal survival in hippocampal neurons.

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.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Insulin-like growth factor 1 and a cytosolic tyrosine kinase activate chloride outward transport during maturation of hippocampal neurons

The development of hyperpolarizing inhibition is an important step in the maturation of neuronal networks. Hyperpolarizing inhibition requires Cl(-) outward transport that is accomplished by KCC2, a K(+)/Cl(-) cotransporter. We show that cultured hippocampal neurons initially contain an inactive form of the KCC2 protein, which becomes activated during subsequent maturation of the neurons. We also show that this process is accelerated by transient stimulation of IGF-1 receptors. Because the transporter can be rapidly activated by coapplication of IGF-1 and an Src kinase and can be deactivated by membrane-permeable protein tyrosine kinase inhibitors, we suggest that activation of K(+)/Cl(-) cotransporter function by endogenous protein tyrosine kinases mediates the developmental switch of GABAergic responses to hyperpolarizing inhibition.

Neuron-specific Bcl-2 homology 3 domain-only splice variant of Bak is anti-apoptotic in neurons, but pro-apoptotic in non-neuronal cells

We have identified and characterized N-Bak, a neuron-specific isoform of the pro-apoptotic Bcl-2 family member Bak. N-Bak is generated by neuron-specific splicing of a novel 20-base pair exon, which changes the previously described Bak, containing Bcl-2 homology (BH) domains BH1, BH2, and BH3, into a shorter BH3-only protein. As demonstrated by reverse transcription-polymerase chain reaction and RNase protection assay, N-Bak transcripts are expressed only in central and peripheral neurons, but not in other cells, whereas the previously described Bak is expressed ubiquitously, but not in neurons. Neonatal sympathetic neurons microinjected with N-Bak resisted apoptotic death caused by nerve growth factor (NGF) removal, whereas microinjected Bak accelerated NGF deprivation-induced death. Overexpressed Bak killed sympathetic neurons in the presence of NGF, whereas N-Bak did not. N-Bak was, however, still death-promoting when overexpressed in non-neuronal cells. Thus, N-Bak is an anti-apoptotic BH3-only protein, but only in the appropriate cellular environment. This is the first example of a neuron-specific Bcl-2 family member.

Neurotrophin-receptor immunoreactive neurons in mesopontine regions involved in the control of behavioral states

The microinjection of nerve growth factor (NGF) and neurotrophin-3 (NT-3) into the rostral pontine tegmentum of adult cats rapidly induces long-lasting episodes of rapid eye movement (REM) sleep [J. Yamuy, F.R. Morales, M.H. Chase, Induction of rapid eye movement sleep by the microinjection of nerve growth factor into the pontine reticular formation of the cat, Neuroscience 66 (1995) 9-13]. Because this effect may be mediated by neurotrophin receptors, we sought to determine the distribution of neurons that contain low- and high-affinity neurotrophin receptors in regions of the feline pons and mesencephalon which are involved in the generation of REM sleep as well as neuronal groups that are involved in the control of REM sleep-related patterns of physiological activity. Using antibodies directed against p75, trkA, trkB and trkC, immunolabeled neurons were present in the latero-dorsal and pedunculo-pontine tegmental nuclei, the peribrachial nuclei, medial and lateral pontine reticular formation, the raphe nuclei, and the locus coeruleus. Giant reticular cells and large neurons in the mesencephalic trigeminal nucleus were immunoreactive for p75 and all trk receptors. Neurons that were devoid of neurotrophin-receptor immunoreactivity were intermingled with immunostained neurons in all explored structures. Thus, both low- and high-affinity neurotrophin receptors are conspicuously present in neurons located in mesopontine regions of adult cats. These data underscore the importance of neurotrophin-induced trophic actions on mesopontine neurons. Furthermore, the results support the hypothesis that NGF and NT-3 may modulate the electrical activity of neurons in the rostral pontine tegmentum that are responsible for the generation of REM sleep by acting on one or more of the neurotrophin receptors.

The K+/Cl- co-transporter KCC2 renders GABA hyperpolarizing during neuronal maturation

GABA (gamma-aminobutyric acid) is the main inhibitory transmitter in the adult brain, and it exerts its fast hyperpolarizing effect through activation of anion (predominantly Cl-)-permeant GABA(A) receptors. However, during early neuronal development, GABA(A)-receptor-mediated responses are often depolarizing, which may be a key factor in the control of several Ca2+-dependent developmental phenomena, including neuronal proliferation, migration and targeting. To date, however, the molecular mechanism underlying this shift in neuronal electrophysiological phenotype is unknown. Here we show that, in pyramidal neurons of the rat hippocampus, the ontogenetic change in GABA(A)-mediated responses from depolarizing to hyperpolarizing is coupled to a developmental induction of the expression of the neuronal (Cl-)-extruding K+/Cl- co-transporter, KCC2. Antisense oligonucleotide inhibition of KCC2 expression produces a marked positive shift in the reversal potential of GABAA responses in functionally mature hippocampal pyramidal neurons. These data support the conclusion that KCC2 is the main Cl- extruder to promote fast hyperpolarizing postsynaptic inhibition in the brain.

BDNF and NT-3 applied in the whisker pad reverse cortical changes after peripheral deafferentation in neonatal rats

It has been known for a long time that subcortical input drives the specification of cortical areas. Molecular signals mediating this instructive effect from the periphery are poorly understood. In foetal or neonatal rats, ablation of whisker follicles, transection of the infraorbital nerve, inhibition of axonal transport, but not impulse activity blockade, prevent formation of barrels in the primary somatosensory cortex (S1). These findings suggest that a chemical signal, possibly arising from the skin or the follicle, may be responsible for somatotopic pattern formation in S1. Neurotrophins promote survival and differentiation of primary sensory neurons, and are expressed in the whisker pad during development. Neonatal rats received gelfoam impregnated with NGF, BDNF or NT-3 under the whisker pad following surgical denervation of whisker rows D and E on P0. Barrel formation in S1 was assessed on P7 by acetylcholinesterase histochemistry and 5-HT-immunohistochemistry. BDNF and NT-3, but not NGF, promoted development of the cortical barrels corresponding to denervated whiskers. Furthermore, BDNF and NT-3 prevented the lesion-induced expansion of row C barrels, while NGF appeared to promote row C expansion. Our results suggest that BDNF and NT-3 arising from the whisker pad are involved in the formation and/or maintenance of the barrel pattern in S1. These findings are potentially relevant for the prevention of sensory disturbances possibly due to reorganization of central sensory circuits after peripheral nerve lesions in humans.

Distribution of GABA receptor rho subunit transcripts in the rat brain

The gamma-aminobutyric acid (GABA) receptor rho subunits recently cloned from rat and human retina are thought to form GABA receptor channels belonging to a pharmacologically distinct receptor class, termed GABA(C). In this work we have examined the distribution of rho1, rho2 and rho3 subunits, and found expression of all three transcripts in several regions of the rat nervous system. In situ hybridization revealed expression of rho2 in the adult rat retina and some other parts of the visual pathways. A high local rho2 expression was seen in the superficial grey layer of the superior colliculus, and in the dorsal lateral geniculate nucleus. Expression was also detected in the 6th layer of visual cortex and in the CA1 pyramidal cell layer of hippocampus. With reverse transcriptase-polymerase chain reaction, expression of rho1 was mainly seen in the adult rat retina and dorsal root ganglia, as well as, at a significantly lower level, in the superior colliculus, hippocampus, brain stem, thalamus, postnatal day 8 (P8) superior colliculus and P8 hippocampus. Expression pattern of rho3 mRNA was clearly different from that of rho1 and rho2, being strongest in the hippocampus, and significantly lower in the retina, dorsal root ganglia and cortex. No rho3 expression was observed in adult or P8 superior colliculus or in P8 hippocampus. The present results clearly demonstrate that expression of GABA receptor rho subunits is not restricted to the retina, but significant expression can also be detected in many other brain regions, especially in those belonging to the visual pathways. The expression pattern of the rho subunits may be helpful in solving the functional significance of the receptors formed from these subunits.