Neurotrophin modulation of NMDA receptors in cultured murine and isolated rat neurons

BDNF release from single cells elicits local dendritic growth in nearby neurons

In cultured neurons, the exogenous application of neurotrophins (in homogenous concentrations) alters many features of axonal and dendritic arbors. In vivo, however, release of endogenous neurotrophins from neuronal processes creates spatially heterogeneous neurotrophin distributions. To probe the consequences of such endogenous neurotrophin distribution, we produced 'donor neurons' in ferret cortex brain slices that co-expressed brain-derived neurotrophic factor (BDNF) and red fluorescent protein (RFP). Using two-photon microscopy, we analyzed their effects on 'recipient neurons' that expressed green fluorescent protein (GFP) alone. BDNF released from dendrites and cell bodies acted directly on nearby recipient neurons to increase dendritic branching in a distance-dependent manner. Three-dimensional analysis of donor and recipient dendrites indicated that the BDNF source had to be within 4.5 microm to induce dendritic growth in the recipient neuron. Thus, BDNF released from an individual cell alters the structure of nearby dendrites on an exquisitely local scale.

Spinal brain-derived neurotrophic factor (BDNF) produces hyperalgesia in normal mice while antisense directed against either BDNF or trkB, prevent inflammation-induced hyperalgesia

Although known primarily for its role in neuronal development, brain-derived neurotrophic factor (BDNF) has also recently been implicated in processes mediated by the adult nervous system, such as spinal nociception. Peripheral inflammation increases expression of BDNF preferentially in dorsal root ganglion cells that contain substance P and/or calcitonin gene-related peptide, known nociceptive transmitters for which synthesis is also increased during inflammatory states. Expression of the tyrosine kinase receptor that selectively binds BDNF, trkB, is increased in the spinal dorsal horn during inflammation as well. Additionally, intrathecal (i.t.) administration of the BDNF-scavenging protein trkB-IgG attenuates inflammation-induced behavioral responses. Collectively, this evidence implicates BDNF in spinal nociceptive processes. Here we show that, in normal mice, i.t. BDNF produces an acute, dose-dependent thermal hyperalgesic response. Selective inhibition of BDNF expression by i.t. antisense oligodeoxynucleotide treatment produces antinociception in normal mice and attenuates carrageenan-induced hyperalgesia. Further, we demonstrate that i.t. antisense treatment directed against the full-length trkB receptor (trkB.FL) attenuates carrageenan-induced hyperalgesia. Consistent with a trkB.FL-mediated mechanism, the i.t. administration of another trkB ligand, neurotrophin-4/5, also produces hyperalgesia while the trkC agonist neurotrophin-3, which weakly cross-reacts with trkB, has little effect. Finally, with the accumulating evidence linking BDNF to synaptic plasticity, we investigated whether BDNF-induced hyperalgesia in normal mice involves the N-methyl-D-aspartate (NMDA) receptor. Interestingly, i.t. co-administration of the NMDA receptor antagonist D(-)-2-amino-5-phosphonovaleric acid (D-APV) with BDNF dose-dependently inhibits BDNF-induced hyperalgesia, suggesting that BDNF induces acute hyperalgesic responses and affects central sensitization in a process dependent on NMDA receptor activation.

Modulation of synaptic plasticity by stress and antidepressants

Recent preclinical and clinical studies have shown that mechanisms underlying neuronal plasticity and survival are involved in both the outcome of stressful experiences and the action of antidepressants. Whereas most antidepressants predominantly affect the brain levels of monoamine neurotransmitters, it is increasingly appreciated that they also modulate neurotransmission at synapses using the neurotransmitter glutamate (the most abundant in the brain). In the hippocampus, a main area of the limbic system involved in cognitive functions as well as attention and affect, specific molecules enriched at glutamatergic synapses mediate major changes in synaptic plasticity induced by stress paradigms or antidepressant treatments. We analyze here the modifications induced by stress or antidepressants in the strength of synaptic transmission in hippocampus, and the molecular modifications induced by antidepressants in two main mediators of synaptic plasticity: the N-methyl-D-aspartate (NMDA) receptor complex for glutamate and the Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). Both stress and antidepressants induce alterations in long-term potentiation of hippocampal glutamatergic synapses, which may be partly accounted for by the influence of environmental or drug-induced stimulation of monoaminergic pathways projecting to the hippocampus. In the course of antidepressant treatments significant changes have been described in both the NMDA receptor and CaM kinase II, which may account for the physiological changes observed. A central role in these synaptic changes is exerted by brain-derived neurotrophic factor (BDNF), which modulates both synaptic plasticity and its molecular mediators, as well as inducing morphological synaptic changes. The role of these molecular effectors in synaptic plasticity is discussed in relation to the action of antidepressants and the search for new molecular targets of drug action in the therapy of mood disorders.

Proton-gated channels in PC12 cells

Acid-sensing ion channels (ASICs) are expressed in various sensory and central neurons. The functional role of these channels remains elusive. Complex subunit combinations and lack of specific blockers for native receptors are likely to contribute to the difficulty of resolving the function of ASICs. Finding a neuronal cell line, which expresses a single population of ASICs, should prove to be useful in delineating the function of individual ASICs. Using patch-clamp, Ca(2+)-imaging, and RT-PCR techniques, we have explored the existence of ASICs in PC12 cells, a clonal neuronal cell line. Fast drops of extracellular pH activated transient inward currents in PC12 cells with pH(0.5) at 6.0-6.2. The ASICs in PC12 cells were selective for Na(+) with significant Ca(2+) permeability. Currents in PC12 cells were blocked by the nonselective ASIC blocker amiloride. PcTX1, a specific homomeric ASIC1a blocker, also blocked the ASIC currents with an IC(50) of approximately 1.5 nM. RT-PCR demonstrated the existence of ASIC1a transcript in both undifferentiated and nerve growth factor-differentiated PC12 cells. Our data suggest that PC12 cells likely contain a single population of functional proton-gated channel-homomeric ASIC1a. It might be an ideal neuronal cell line for the study of physiological and potential pathological roles of this key subunit of ASICs.

Spontaneous limbic seizures after intrahippocampal infusion of brain-derived neurotrophic factor

The results of several studies have contributed to the hypothesis that BDNF promotes seizure activity, particularly in adult hippocampus. To test this hypothesis, BDNF, vehicle (phosphate-buffered saline, PBS), or albumin was infused directly into the hippocampus for 2 weeks using osmotic minipumps. Rats were examined behaviorally, electrophysiologically, and anatomically. An additional group was tested for sensitivity to the convulsant pilocarpine. Spontaneous behavioral seizures were observed in BDNF-infused rats (8/32; 25%) but not in controls (0/20; 0%). In a subset of six animals (three BDNF, three albumin), blind electrophysiological analysis of scalp recordings contralateral to the infused hippocampus demonstrated abnormalities in all BDNF rats; but not controls. Neuronal loss in BDNF-treated rats was not detected relative to PBS- or albumin-treated animals, but immunocytochemical markers showed a pattern of expression in BDNF-treated rats that was similar to rats with experimentally induced seizures. Thus, BDNF-infused rats had increased expression of NPY in hilar neurons of the dentate gyrus relative to control rats. NPY and BDNF expression was increased in the mossy fiber axons of dentate gyrus granule cells relative to controls. The increase in NPY and BDNF expression in BDNF-treated rats was bilateral and occurred throughout the septotemporal axis of the hippocampus. Mossy fiber sprouting occurred in five BDNF-treated rats but no controls. In another group of infused rats that was tested for seizure sensitivity to the convulsant pilocarpine, BDNF-infused rats had a shorter latency to status epilepticus than PBS-infused rats. In addition, the progression from normal behavior to severe seizures was faster in BDNF-treated rats. These data support the hypothesis that intrahippocampal BDNF infusion can facilitate, and potentially initiate, seizure activity in adult hippocampus.

Cultured rat hippocampal neural progenitors generate spontaneously active neural networks

We previously demonstrated that the neural cell adhesion molecule (N-CAM) inhibited the proliferation of cultured rat hippocampal progenitor cells and increased the number of neurons generated. We demonstrate here that the continued presence of fibroblast growth factor 2 along with N-CAM or brain-derived neurotrophic factor over 12 days of culture greatly increased the number of both progenitors and neurons. These progenitor-derived neurons expressed neurotransmitters, neurotransmitter receptors, and synaptic proteins in vitro consistent with those expressed in the mature hippocampus. Progenitor cells cultured on microelectrode plates formed elaborate neural networks that exhibited spontaneously generated action potentials after 21 days. This activity was observed only in cultures grown in the presence of fibroblast growth factor 2 and either N-CAM or brain-derived neurotrophic factor. Analysis of neuronal activity after various pharmacological treatments indicated that the networks formed functional GABAergic and glutamatergic synapses. We conclude that mitogenic growth factors can synergize with N-CAM or neurotrophins to generate spontaneously active neural networks from neural progenitors.

From acquisition to consolidation: on the role of brain-derived neurotrophic factor signaling in hippocampal-dependent learning

One of the most rigorously investigated problems in modern neuroscience is to decipher the mechanisms by which experience-induced changes in the central nervous system are translated into behavioral acquisition, consolidation, retention, and subsequent recall of information. Brain-derived neurotrophic factor (BDNF) has recently emerged as one of the most potent molecular mediators of not only central synaptic plasticity, but also behavioral interactions between an organism and its environment. Recent experimental evidence indicates that BDNF modulates synaptic transmission and plasticity by acting across different spatial and temporal domains. BDNF signaling evokes both short- and long-term periods of enhanced synaptic physiology in both pre- and postsynaptic compartments of central synapses. Specifically, BDNF/TrkB signaling converges on the MAP kinase pathway to enhance excitatory synaptic transmission in vivo, as well as hippocampal-dependent learning in behaving animals. Emerging concepts of the intracellular signaling cascades involved in synaptic plasticity induced through environmental interactions resulting in behavioral learning further support the contention that BDNF/TrkB signaling plays a fundamental role in mediating enduring changes in central synaptic structure and function. Here we review recent literature showing the involvement of BDNF/TrkB signaling in hippocampal-dependent learning paradigms, as well as in the types of cellular plasticity proposed to underlie learning and memory.

Effect of lamotrigine on the Ca(2+)-sensing cation current in cultured hippocampal neurons

Concentrations of extracellular calcium ([Ca(2+)](e)) in the CNS decrease substantially during seizure activity. We have demonstrated previously that decreases in [Ca(2+)](e) activate a novel calcium-sensing nonselective cation (csNSC) channel in hippocampal neurons. Activation of csNSC channels is responsible for a sustained membrane depolarization and increased neuronal excitability. Our study has suggested that the csNSC channel is likely involved in generating and maintaining seizure activities. In the present study, the effects of anti-epileptic agent lamotrigine (LTG) on csNSC channels were studied in cultured mouse hippocampal neurons using patch-clamp techniques. At a holding potential of -60 mV, a slow inward current through csNSC channels was activated by a step reduction of [Ca(2+)](e) from 1.5 to 0.2 mM. LTG decreased the amplitude of csNSC currents dose dependently with an IC(50) of 171 +/- 25.8 (SE) microM. The effect of LTG was independent of membrane potential. In the presence of 300 microM LTG, the amplitude of csNSC current was decreased by 31 +/- 3% at -60 mV and 29 +/- 2.9% at +40 mV (P > 0.05). LTG depressed csNSC current without affecting the potency of Ca(2+) block of the current (IC(50) for Ca(2+) block of csNSC currents in the absence of LTG: 145 +/- 18 microM; in the presence of 300 microM LTG: 136 +/- 10 microM. n = 5, P > 0.05). In current-clamp recordings, activation of csNSC channel by reducing the [Ca(2+)](e) caused a sustained membrane depolarization and an increase in the frequency of spontaneous firing of action potentials. LTG (300 microM) significantly inhibited csNSC channel-mediated membrane depolarization and the excitation of neurons. Fura-2 ratiometric Ca(2+) imaging experiment showed that LTG also inhibited the increase in intracellular Ca(2+) concentration induced by csNSC channel activation. The effect of LTG on csNSC channels may partially contribute to its broad spectrum of anti-epileptic actions.

Restoration of decreased N-methyl-d-asparate receptor activity by brain-derived neurotrophic factor in the cultured hippocampal neurons: involvement of cAMP

Brain-derived neurotrophic factor (BDNF) may play an important role in the modulation of N-methyl-d-asparate (NMDA) receptor function. To elucidate the underlying mechanisms, whole-cell patch-clamp recording was used to assess the effect of BDNF on the responses of cultured hippocampal neurons to the glutamate receptor agonist NMDA. We found that peak amplitude of NMDA-evoked currents in cultured hippocampal pyramidal neurons at Day 18 in vitro decreased significantly compared to that of NMDA currents at Day 10 or 14. Interestingly, NMDA-evoked currents were greatly enhanced by BDNF (50 ng/ml) in cultured neurons at Day 18, but not at Day 10 or 14. Treatment with Rp-cAMP abolished the potentiating effects of BDNF on NMDA current. Elevating the amount of cytosolic cAMP by preincubation with forskolin or Sp-cAMP also enhanced NMDA currents as effectively as BDNF in 18-day-old hippocampal neurons. Measurement of the cellular content of cAMP by RIA indicated that cultured hippocampal neurons showed decreased basal cAMP levels at the time NMDA currents were decreased and BDNF increased the decreased cAMP levels. Taken together, these results suggest that BDNF may restore decreased NMDA receptor activity in cultured hippocampal neurons by the cAMP pathway.

Some forms of cAMP-mediated long-lasting potentiation are associated with release of BDNF and nuclear translocation of phospho-MAP kinase

Long-lasting forms of synaptic plasticity like the late phase of LTP (L-LTP) typically require an elevation of cAMP, the recruitment of the cAMP-dependent protein kinase (PKA), and ultimately the activation of transcription and translation; some forms also require brain-derived neurotrophic factor (BDNF). Both cAMP and BDNF can activate mitogen-activated protein kinase (MAPK/ERK), which also plays a role in LTP. However, little is known about the mechanisms whereby cAMP, BDNF, and MAPK interact. We find that increases in cAMP can rapidly activate the BDNF receptor TrkB and induce BDNF-dependent long-lasting potentiation at the Schaffer collateral-CA1 synapse in hippocampus. Surprisingly, in these BDNF-dependent forms of potentiation, which are also MAPK dependent, TrkB activation is not critical for the activation of MAPK but instead appears to modulate the subcellular distribution and nuclear translocation of the activated MAPK.

Endogenous opioids and oligodendroglial function: possible autocrine/paracrine effects on cell survival and development

Previous work has shown that oligodendrocytes (OLs) express both micro- and kappa-opioid receptors. In developing OLs, micro receptor activation increases OL proliferation, while the kappa-antagonist nor-binaltorphimine (NorBNI) affects OL differentiation. Because exogenous opioids were not present in our defined culture medium, we hypothesized that NorBNI blocked endogenous opioids produced by the OLs themselves. To test this, intact and partially processed proenkephalin and prodynorphin-derived peptides were assessed in OLs using immunocytochemistry or Western blot analysis, or both. Immature OLs possessed large amounts of intact and partially processed proenkephalin precursors, as well as posttranslational products of prodynorphin including dynorphin A (1-17). With maturation, however, intact or partially processed proenkephalin was expressed by only about 50% of OLs, while dynorphin A (1-17) was undetectable. To assess the function of OL-derived opioids, the effect of kappa-agonists/antagonists on OL differentiation and death was explored. kappa-Agonists alone had no effect. In contrast, NorBNI significantly increased OL death. Additive OL losses were evident when NorBNI was paired with toxic levels of glutamate, suggesting that kappa-receptor blockade alone is sufficient to induce OL death. Thus, the results indicate that OLs express proenkephalin and prodynorphin peptides in a developmentally regulated manner, and further suggest that opioids produced by OLs modulate OL maturation and survival through local (i.e., autocrine and/or paracrine) mechanisms.

BDNF but not NT-4 is required for normal flexion reflex plasticity and function

Neurotrophins can directly modulate the function of diverse types of central nervous system synapses. Brain-derived neurotrophic factor (BDNF) might be released by nociceptors onto spinal neurons and mediate central sensitization associated with chronic pain. We have studied the role of BDNF and neurotrophin-4 (NT-4), both ligands of the trkB tyrosine kinase receptor, in synaptic transmission and reflex plasticity in the mouse spinal cord. We used an in vitro spinal cord preparation to measure monosynaptic and polysynaptic reflexes evoked by primary afferents in BDNF- and NT-4-deficient mice. In situ hybridization studies show that both these neurotrophins are synthesized by sensory neurons, and NT-4, but not BDNF, also is expressed by spinal neurons. BDNF null mutants display selective deficits in the ventral root potential (VRP) evoked by stimulating nociceptive primary afferents whereas the non-nociceptive portion of the VRP remained unaltered. In addition, activity-dependent plasticity of the VRP evoked by repetitive (1 Hz) stimulation of nociceptive primary afferents (termed wind-up) was substantially reduced in BDNF-deficient mice. This plasticity also was reduced in a reversible manner by the protein kinase inhibitor K252a. Although the trkB ligand NT-4 is normally present, reflex properties in NT-4 null mutant mice were normal. Pharmacological studies also indicated that spinal N-methyl-d-aspartate receptor function was unaltered in BDNF-deficient mice. Using immunocytochemistry for markers of nociceptive neurons we found no evidence that their number or connectivity was substantially altered in BDNF-deficient mice. Our data therefore are consistent with a direct role for presynaptic BDNF release from sensory neurons in the modulation of pain-related neurotransmission.

Removal of NMDA receptor Mg(2+) block extends the action of NT-3 on synaptic transmission in neonatal rat motoneurons

NT-3 has previously been reported to enhance AMPA/kainate receptor-mediated synaptic responses in motoneurons via an effect on the N-methyl-D-aspartate (NMDA) receptor. To investigate neurotrophin-3 (NT-3) action further, we measured the NMDA receptor (NMDAR)-mediated synaptic response directly by intracellular recording in motoneurons after blocking AMPA/kainate, GABA(A), GABA(B) and glycine receptor-mediated responses pharmacologically. Two pathways were stimulated, the segmental dorsal root (DR) and the descending ventrolateral fasciculus (VLF). The DR-evoked NMDAR-mediated response in motoneurons of rats younger than 1 wk has two components, the initial one of which is generated monosynaptically. NT-3 strongly potentiated both NMDA components in a rapidly reversible manner. No NMDAR-mediated responses were present at VLF connections and at DR connections in older (1- to 2-wk-old) neonates. Bath-applied NT-3-induced potentiation of the AMPA/kainate receptor-mediated response occurred only at connections that exhibit a synaptic NMDA receptor-mediated response. Reducing Mg(2+) concentration in the bathing solution restored the NMDAR-mediated response elicited by DR stimulation in older neonates and by VLF throughout the neonatal period (0-2 wk). In low-Mg(2+), NT-3 enhanced AMPA/kainate receptor-mediated responses elicited by inputs normally not influenced by NT-3. Thus a major reason for the loss of NT-3 action on AMPA/kainate synaptic responses is the reduced activity of the NMDA receptor due to developing Mg(2+) block of NMDA receptor-channel complex as the animal matures, and both can be re-established by reducing Mg(2+) concentration in fluid bathing the spinal cord.

Neurotrophins and synaptic plasticity in the mammalian spinal cord

The pathway mediating the monosynaptic stretch reflex has served as an important model system for studies of plasticity in the spinal cord. Its usefulness is extended by evidence that neurotrophins, particularly neurotrophin-3 (NT-3), which has been shown to promote spinal axon elongation, can modulate the efficacy of the muscle spindle-motoneurone connection both after peripheral nerve injury and during development. The findings summarized here emphasize the potential for neurotrophins to modify function of both damaged and undamaged neurones. It is important to recognize that these effects may be functionally detrimental as well as beneficial.

Growth factors regulate phototransduction in retinal rods by modulating cyclic nucleotide-gated channels through dephosphorylation of a specific tyrosine residue

Illumination of vertebrate rod photoreceptors leads to a decrease in the cytoplasmic cGMP concentration and closure of cyclic nucleotide-gated (CNG) channels. Except for Ca(2+), which plays a negative feedback role in adaptation, and 11-cis-retinal, supplied by the retinal pigment epithelium, all of the biochemical machinery of phototransduction is thought to be contained within rod outer segments without involvement of extrinsic regulatory molecules. Here we show that insulin-like growth factor-I (IGF-I), a paracrine factor released from the retinal pigment epithelium, alters phototransduction by rapidly increasing the cGMP sensitivity of CNG channels. The IGF-I-signaling pathway ultimately involves a protein tyrosine phosphatase that catalyzes dephosphorylation of a specific residue in the alpha-subunit of the rod CNG channel protein. IGF-I conjointly accelerates the kinetics and increases the amplitude of the light response, distinct from events that accompany adaptation. These effects of IGF-I could result from the enhancement of the cGMP sensitivity of CNG channels. Hence, in addition to long-term control of development and survival of rods, growth factors regulate phototransduction in the short term by modulating CNG channels.

Insulin-like growth factor I potentiates kainate receptors through a phosphatidylinositol 3-kinase dependent pathway

Neurotrophic factors modulate synaptic plasticity through mechanisms that include regulation of membrane ion channels and neurotransmitter receptors. Recently, it was shown that insulin-like growth factor I (IGF-I) induces depression of AMPA-mediated currents without affecting NMDA-receptor function in neurons. We now report that IGF-I markedly potentiates the kainate-preferring ionotropic glutamate receptor in young cerebellar granule neurons expressing functional kainate-, but not AMPA-mediated currents. Potentiation of kainate responses by IGF-I is blocked by wortmannin, a phosphatidylinositol 3-kinase (P13K) inhibitor, indicating a role for this kinase in the effect of IGF-I. These results reinforce the notion that modulation of ionotropic glutamate receptors are involved in the regulatory actions of IGF-I on neuronal plasticity.

Release of immunoreactive brain-derived neurotrophic factor in the spinal cord of the rat following sciatic nerve transection

Using the antibody microprobe method, the sites of spinal release of immunoreactive brain-derived neurotrophic factor (BDNF) was studied in normal rats, and rats with prior sciatic nerve transection. In normal rats, a significant basal release of immunoreactive BDNF was found in the superficial dorsal horn. Following sciatic nerve transection (performed 14 days previously), release of BDNF was found throughout the whole of the dorsal horn, extending into deeper laminae. Electrical stimulation of the ipsilateral sciatic nerve at a strength adequate to excite either A fibres (20 Hz at 2x threshold voltage) or A and C fibres (2 Hz at 20x threshold voltage) did not alter the basal release of immunoreactive BDNF in normal or in nerve-injured rats. The results suggest that BDNF is released from the central terminals of primary afferent fibres, but such release is not solely dependent upon action potential invasion of these terminals. The increased extent of release following nerve transection is consistent with the hypothesis that BDNF plays a role in the central response to peripheral nerve injury.

Structure-activity analysis of dynorphin A toxicity in spinal cord neurons: intrinsic neurotoxicity of dynorphin A and its carboxyl-terminal, nonopioid metabolites

Dynorphin A [dynorphin A (1-17)] is an endogenous opioid peptide that is antinociceptive at physiological concentrations. Levels of dynorphin A increase markedly following spinal cord trauma and may contribute to secondary neurodegeneration. Both kappa opioid and N-methyl-d-aspartate (NMDA) receptor antagonists can modulate the effects of dynorphin, suggesting that dynorphin is acting through kappa opioid and/or NMDA receptor types. Despite these findings, few studies have critically examined the mechanisms of dynorphin A neurotoxicity at the cellular level. To better understand how dynorphin affects cell viability, structure-activity studies were performed examining the effects of dynorphin A and dynorphin A-derived peptide fragments on the survival of mouse spinal cord neurons coexpressing kappa opioid and NMDA receptors in vitro. Time-lapse photography was used to repeatedly follow the same neurons before and during experimental treatments. Dynorphin A caused significant neuronal losses that were dependent on concentration (> or = 1 microM) and duration of exposure. Moreover, exposure to an equimolar concentration of dynorphin A fragments (100 microM) also caused a significant loss of neurons. The rank order of toxicity was dynorphin A (1-17) > dynorphin A (1-13) congruent with dynorphin A (2-13) congruent with dynorphin A (13-17) (least toxic) > dynorphin A (1-5) ([Leu(5)]-enkephalin) or dynorphin A (1-11). Dynorphin A (1-5) or dynorphin A (1-11) did not cause neuronal losses even following 96 h of continuous exposure, while dynorphin A (3-13), dynorphin A (6-17), and dynorphin A (13-17) were neurotoxic. The NMDA receptor antagonist MK-801 (dizocilpine) (10 microM) significantly attenuated the neurotoxic effects of dynorphin A and/or dynorphin-derived fragments except dynorphin A (13-17), suggesting that the neurotoxic effects of dynorphin were largely mediated by NMDA receptors. Thus, toxicity resides in the carboxyl-terminal portion of dynorphin A and this minimally includes dynorphin A (3-13) and (13-17). Our findings suggest that dynorphin A and/or its metabolites may contribute significantly to neurodegeneration during spinal cord injury and that alterations in dynorphin A biosynthesis, metabolism, and/or degradation may be important in determining injury outcome.

Brain-derived neurotrophic factor in the control human brain, and in Alzheimer's disease and Parkinson's disease

Brain-derived neurotrophic factor (BDNF) is a small dimeric protein, structurally related to nerve growth factor, which is abundantly and widely expressed in the adult mammalian brain. BDNF has been found to promote survival of all major neuronal types affected in Alzheimer's disease and Parkinson's disease, like hippocampal and neocortical neurons, cholinergic septal and basal forebrain neurons, and nigral dopaminergic neurons. In this article, we summarize recent work on the molecular and cellular biology of BDNF, including current ideas about its intracellular trafficking, regulated synthesis and release, and actions at the synaptic level, which have considerably expanded our conception of BDNF actions in the central nervous system. But our primary aim is to review the literature regarding BDNF distribution in the human brain, and the modifications of BDNF expression which occur in the brain of individuals with Alzheimer's disease and Parkinson's disease. Our knowledge concerning BDNF actions on the neuronal populations affected in these pathological states is also reviewed, with an aim at understanding its pathogenic and pathophysiological relevance.

Brain-derived neurotrophic factor increases activity of NR2B-containing N-methyl-D-aspartate receptors in excised patches from hippocampal neurons

Growth factors, including members of the neurotrophin gene family, play a central role in the regulation of neuronal survival and differentiation during development. In addition to these relatively long-term actions of neurotrophins, recent studies have shown that these factors also rapidly modulate synaptic transmission. Brain-derived neurotrophic factor (BDNF), in particular, regulates both pre- and postsynaptic aspects of hippocampal synaptic transmission. The postsynaptic effects include an increase in glutamate responsiveness, mediated by the N-methyl-D-aspartate (NMDA) glutamate receptor subtype. It is not clear, however, where BDNF-trkB signal transduction is initiated, because trkB receptors are located in both pre- and postsynaptic membranes. In the present study, we used excised membrane patches from cultured hippocampal neurons to determine whether BDNF directly modulates postsynaptic NMDA receptor activity. The results indicate that acute exposure to BDNF increases NMDA single channel open probability via postsynaptic trkB receptors and that this effect is dependent on the presence of the NR2B subunit of the NMDA receptor.

The role of brain-derived neurotrophic factor receptors in the mature hippocampus: modulation of long-term potentiation through a presynaptic mechanism involving TrkB

The neurotrophin BDNF has been shown to modulate long-term potentiation (LTP) at Schaffer collateral-CA1 hippocampal synapses. Mutants in the BDNF receptor gene trkB and antibodies to its second receptor p75NTR have been used to determine the receptors and cells involved in this response. Inhibition of p75NTR does not detectably reduce LTP or affect presynaptic function, but analyses of newly generated trkB mutants implicate TrkB. One mutant has reduced expression in a normal pattern of TrkB throughout the brain. The second mutant was created by cre-loxP-mediated removal of TrkB in CA1 pyramidal neurons of this mouse. Neither mutant detectably impacts survival or morphology of hippocampal neurons. TrkB reduction, however, affects presynaptic function and reduces the ability of tetanic stimulation to induce LTP. Postsynaptic glutamate receptors are not affected by TrkB reduction, indicating that BDNF does not modulate plasticity through postsynaptic TrkB. Consistent with this, elimination of TrkB in postsynaptic neurons does not affect LTP. Moreover, normal LTP is generated in the mutant with reduced TrkB by a depolarization-low-frequency stimulation pairing protocol that puts minimal demands on presynaptic terminal function. Thus, BDNF appears to act through TrkB presynaptically, but not postsynaptically, to modulate LTP.

NT-3 evokes an LTP-like facilitation of AMPA/kainate receptor-mediated synaptic transmission in the neonatal rat spinal cord

Neurotrophin-3 (NT-3) is a neurotrophic factor required for survival of muscle spindle afferents during prenatal development. It also acts postsynaptically to enhance the monosynaptic excitatory postsynaptic potential (EPSP) produced by these fibers in motoneurons when applied over a period of weeks to the axotomized muscle nerve in adult cats. Similar increases in the amplitude of the monosynaptic EPSP in motoneurons are observed after periodic systemic treatment of neonatal rats with NT-3. Here we show an acute action of NT-3 in enhancing the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA/kainate) receptor-mediated fast monosynaptic EPSP elicited in motoneurons by dorsal root (DR) stimulation in the in vitro hemisected neonatal rat spinal cord. The receptor tyrosine kinase inhibitor K252a blocks this action of NT-3 as does the calcium chelator bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) injected into the motoneuron. The effect of NT-3 resembles long-term potentiation (LTP) in that transient bath application of NT-3 to the isolated spinal cord produces a long-lasting increase in the amplitude of the monosynaptic EPSP. An additional similarity is that activation of N-methyl-D-aspartate (NMDA) receptors is required to initiate this increase but not to maintain it. The NMDA receptor blocker MK-801, introduced into the motoneuron through the recording microelectrode, blocks the effect of NT-3, indicating that NMDA receptors in the motoneuron membrane are crucial. The effect of NT-3 on motoneuron NMDA receptors is demonstrated by its enhancement of the depolarizing response of the motoneuron to bath-applied NMDA in the presence of tetrodotoxin (TTX). The potentiating effects of NT-3 do not persist beyond the first postnatal week. In addition, EPSPs with similar properties evoked in the same motoneurons by stimulation of descending fibers in the ventrolateral funiculus (VLF) are not modifiable by NT-3 even in the initial postnatal week. Thus, NT-3 produces synapse-specific and age-dependent LTP-like enhancement of AMPA/kainate receptor-mediated synaptic transmission in the spinal cord, and this action requires the availability of functional NMDA receptors in the motoneuron.

Brain-derived neurotrophic factor restores long-term potentiation in polysialic acid-neural cell adhesion molecule-deficient hippocampus

The neural cell adhesion molecule (NCAM) and its polysialylated form (PSA-NCAM) contribute to long-term potentiation (LTP) in the CA1 hippocampus. Here we report that the deficient LTP found in slices prepared from NCAM knockout mice and in organotypic slice cultures treated with Endo-N, an enzyme that cleaves the PSA moiety of NCAM, can be rescued by brain-derived neurotrophic factor (BDNF). This effect is not reproduced by nerve growth factor, but can be obtained with high concentrations of NT4/5. The effect of BDNF cannot be accounted for by modifications of N-methyl-D-aspartate receptor-dependent responses or of high-frequency bursts. PSA-NCAM, however, could directly interact with BDNF. Exogenous application of PSA residues or recombinant PSA-NCAM also prevents LTP. Furthermore trkB phosphorylation, and thus BDNF signaling, is reduced in both NCAM knockout mice and Endo-N-treated slice cultures. These results suggest that one action of PSA-NCAM could be to sensitize pyramidal neurons to BDNF, thereby modulating activity-dependent synaptic plasticity.

Differential effects of BDNF, ADNF9, and TNFalpha on levels of NMDA receptor subunits, calcium homeostasis, and neuronal vulnerability to excitotoxicity

Calcium influx through N-methyl-d-aspartate (NMDA) receptors can result in neuronal apoptosis or necrosis and may play a pivotal role in neuronal death in many different neurodegenerative diseases. In the present study we employed primary neuronal cultures and three different excitoprotective factors, brain-derived neurotrophic factor (BDNF), activity-dependent neurotrophic factor (ADNF9), and tumor necrosis factor alpha (TNFalpha), to elucidate the mechanisms whereby trophic factors modify the excitotoxic process. Neurons pretreated with BDNF exhibited increased levels of the NMDA receptor subunits NR1 and NR2A, which was associated with increased calcium responses to NMDA and vulnerability to excitotoxic necrosis and reduced vulnerability to apoptosis. ADNF9 and TNFalpha suppressed calcium responses to glutamate and protected neurons against both excitotoxic necrosis and apoptosis, but had no effect on levels of NMDA receptor subunits. Inhibition of phosphorylation and DNA binding of NF-kappaB, by H7 and kappaB decoy DNA, respectively, suggest that the excitotoxic-modulating actions of BDNF are mediated by kinases, while those of ADNF9 and TNFalpha are mediated by both kinases and the transcription factor NF-kappaB. Our data show that, whereas BDNF increases neuronal responses to glutamate while ADNF9 and TNFalpha decrease the same, all three protect against excitotoxic apoptosis.

Dynorphin A (1-13) neurotoxicity in vitro: opioid and non-opioid mechanisms in mouse spinal cord neurons

Dynorphin A is an endogenous opioid peptide that preferentially activates kappa-opioid receptors and is antinociceptive at physiological concentrations. Levels of dynorphin A and a major metabolite, dynorphin A (1-13), increase significantly following spinal cord trauma and reportedly contribute to neurodegeneration associated with secondary injury. Interestingly, both kappa-opioid and N-methyl-D-aspartate (NMDA) receptor antagonists can modulate dynorphin toxicity, suggesting that dynorphin is acting (directly or indirectly) through kappa-opioid and/or NMDA receptor types. Despite these findings, few studies have systematically explored dynorphin toxicity at the cellular level in defined populations of neurons coexpressing kappa-opioid and NMDA receptors. To address this question, we isolated populations of neurons enriched in both kappa-opioid and NMDA receptors from embryonic mouse spinal cord and examined the effects of dynorphin A (1-13) on intracellular calcium concentration ([Ca2+]i) and neuronal survival in vitro. Time-lapse photography was used to repeatedly follow the same neurons before and during experimental treatments. At micromolar concentrations, dynorphin A (1-13) elevated [Ca2+]i and caused a significant loss of neurons. The excitotoxic effects were prevented by MK-801 (Dizocilpine) (10 microM), 2-amino-5-phosphopentanoic acid (100 microM), or 7-chlorokynurenic acid (100 microM)--suggesting that dynorphin A (1-13) was acting (directly or indirectly) through NMDA receptors. In contrast, cotreatment with (-)-naloxone (3 microM), or the more selective kappa-opioid receptor antagonist nor-binaltorphimine (3 microM), exacerbated dynorphin A (1-13)-induced neuronal loss; however, cell losses were not enhanced by the inactive stereoisomer (+)-naloxone (3 microM). Neuronal losses were not seen with exposure to the opioid antagonists alone (10 microM). Thus, opioid receptor blockade significantly increased toxicity, but only in the presence of excitotoxic levels of dynorphin. This provided indirect evidence that dynorphin also stimulates kappa-opioid receptors and suggests that kappa receptor activation may be moderately neuroprotective in the presence of an excitotoxic insult. Our findings suggest that dynorphin A (1-13) can have paradoxical effects on neuronal viability through both opioid and non-opioid (glutamatergic) receptor-mediated actions. Therefore, dynorphin A potentially modulates secondary neurodegeneration in the spinal cord through complex interactions involving multiple receptors and signaling pathways.

Neurotrophic factors in the auditory periphery

Many of the neurotrophic factors promote the survival of developing peripheral sensory neurons, and they might be useful as therapeutic agents in the adult neuronal systems. During development, neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) mRNAs are expressed in the auditory sensory epithelium, which composes the peripheral target field of the cochlear (spiral) neurons. NT-3 mRNA is also expressed in the inner hair cells of the mature organ of Corti. mRNAs encoding their signal-transducing receptors, TrkC and TrkB, respectively, are expressed in the cochlear neurons. In addition to neurotrophins, which seem to have an important role during development, another neurotrophic factor, glial cell-line-derived neurotrophic factor (GDNF), seems to be involved in the maintenance of postnatal auditory neurons. In the present work, the cellular distribution of neurotrophins and GDNF in the developing and adult rat inner ear are compared. The effects of recombinant neurotrophins and GDNF on dissociated cochlear neurons in vitro are also compared. Recently, NT-3 and BDNF were used in vivo as therapeutic agents to protect guinea pig cochlear neurons from aminoglycoside-induced degeneration and GDNF from noise-induced degeneration. These data demonstrate that NT-3, BDNF, and GDNF might be potential candidates for prevention of degeneration of the auditory nerve in man.

Both the neuronal and inducible isoforms contribute to upregulation of retinal nitric oxide synthase activity by brain-derived neurotrophic factor

Although neurotrophins are best known for their trophic functions, growing evidence suggests that neurotrophins can also be neurotoxic, for instance by enhancing excitotoxic insults. We have shown recently that brain-derived neurotrophic factor (BDNF) limits its neuroprotective action on axotomized rat retinal ganglion cells (RGCs) by upregulating nitric oxide synthase (NOS) activity (Klöcker et al., 1998). The aim of the present study was to investigate this interaction of BDNF and NOS in the lesioned adult rat retina in more detail. We used NOS immunohistochemistry and NADPH-diaphorase (NADPH-d) reaction to characterize morphologically retinal NOS expression and activity. Using reverse transcription-PCR and Western blot analysis, we were able to identify the NOS isoforms being regulated. Six days after optic nerve lesion, we observed an increase in neuronal NOS (NOS-I) mRNA and protein expression in the inner retina. This did not lead to a marked increase in overall retinal NOS activity. Only RGC axons displayed strong de novo NADPH-d reactivity. In contrast, intraocular injection of BDNF resulted in a marked upregulation of NOS activity in NOS-I-immunoreactive structures, leaving the level of NOS-I expression unchanged. In addition, an induction of inducible NOS (NOS-II) was found after BDNF treatment. We identified microglial cells increasing in number and being activated by BDNF, which could serve as the cellular source of NOS-II. In summary, our data suggest that BDNF upregulates retinal NOS activity by both a post-translational regulation of NOS-I activity and an induction of NOS-II. These findings might be useful for developing pharmacological strategies to improve BDNF-mediated neuroprotection.

Actions of brain-derived neurotrophic factor in slices from rats with spontaneous seizures and mossy fiber sprouting in the dentate gyrus

This study examined the acute actions of brain-derived neurotrophic factor (BDNF) in the rat dentate gyrus after seizures, because previous studies have shown that BDNF has acute effects on dentate granule cell synaptic transmission, and other studies have demonstrated that BDNF expression increases in granule cells after seizures. Pilocarpine-treated rats were studied because they not only have seizures and increased BDNF expression in granule cells, but they also have reorganization of granule cell "mossy fiber" axons. This reorganization, referred to as "sprouting," involves collaterals that grow into novel areas, i.e., the inner molecular layer, where granule cell and interneuron dendrites are located. Thus, this animal model allowed us to address the effects of BDNF in the dentate gyrus after seizures, as well as the actions of BDNF on mossy fiber transmission after reorganization. In slices with sprouting, BDNF bath application enhanced responses recorded in the inner molecular layer to mossy fiber stimulation. Spontaneous bursts of granule cells occurred, and these were apparently generated at the site of the sprouted axon plexus. These effects were not accompanied by major changes in perforant path-evoked responses or paired-pulse inhibition, occurred only after prolonged (30-60 min) exposure to BDNF, and were blocked by K252a. The results suggest a preferential action of BDNF at mossy fiber synapses, even after substantial changes in the dentate gyrus network. Moreover, the results suggest that activation of trkB receptors could contribute to the hyperexcitability observed in animals with sprouting. Because human granule cells also express increased BDNF mRNA after seizures, and sprouting can occur in temporal lobe epileptics, the results may have implications for understanding temporal lobe epilepsy.

Excess target-derived brain-derived neurotrophic factor preserves the transient uncrossed retinal projection to the superior colliculus

During early postnatal development, a widespread ipsilateral projection to the superior colliculus is secondarily restricted to a small topographically defined region by elimination of ipsilaterally projecting retinal ganglion cells. Brain-derived neurotrophic factor (BDNF) has been proposed as the target-derived neurotrophic factor for retinal ganglion cells in several studies. Here we investigated the long-term effects of excess BDNF in the retinal ganglion cell target on naturally occurring retinal ganglion cell (RGC) elimination and on the restriction of the ipsilateral projection. To this end, sustained overexpression of BDNF was achieved in the postnatal superior colliculus using an adenoviral vector. While the total number of retinal ganglion cells in the adenovirus-BDNF treated animals was unchanged, a much higher proportion of RGCs retained a projection to the ipsilateral superior colliculus. We conclude that an excess of target-derived BDNF does not reduce the net amount of naturally occurring cell death in the retino-collicular system, but prevents the negative selection of retinal ganglion cells making inappropriate topographic connections.

Neurotrophin-3 potentiates excitatory GABAergic synaptic transmission in cultured developing hypothalamic neurones of the rat

1. Neurotrophin-3 (NT-3) supports the survival and differentiation of neurones in the central and peripheral nervous systems through a number of mechanisms that occur in a matter of hours or days. NT-3 may also have a more rapid mode of action that influences synaptic activity in mature neurones. In the present study, the effect of NT-3 on developing GABAergic synapses was investigated in 3- to 7-day-old cultures of rat hypothalamic neurones with whole-cell patch-clamp recording. 2. NT-3 induced a substantial dose-dependent potentiation of the frequency of spontaneous postsynaptic currents (sPSCs; 160 %) in developing neurones during a period when GABA evoked inward (depolarizing) current, as determined with gramicidin-perforated patch recordings. The NT-3 effect was long lasting; continued enhancement was found > 30 min after NT-3 wash-out. NT-3 evoked a substantial 202 % increase in total GABA-mediated inward current, measured as the time-current integral. Action potential frequency was also increased by NT-3 (to 220 %). 3. The frequency of GABA-mediated miniature postsynaptic currents in developing neurones in the presence of tetrodotoxin was potentiated (to 140%) by NT-3 with no change in the mean amplitude, suggesting a presynaptic locus of the effect. 4. In striking contrast to immature neurones, when more mature neurones were studied, NT-3 did not enhance the frequency of GABA-mediated spontaneous postsynaptic currents (sPSCs), but instead evoked a slight (16%) decrease. The frequency of miniature post-synaptic currents was also slightly decreased (16%) by the NT-3, with no change in amplitude. These results were recorded during a later period of neuronal maturity when GABA would evoke outward (hyperpolarizing) currents. NT-3 had no effect on the mean amplitude of GABA-evoked postsynaptic currents in either developing or mature neurones. 5. Intracellular application of K252a, a non-selective tyrosine kinase inhibitor, did not block the NT-3 effect postsynaptically. In contrast, bath application of K252a prevented the enhancement of sPSCs by NT-3, consistent with NT-3 acting through presynaptic induction of tyrosine kinase. Decreasing extracellular calcium with BAPTA or inhibiting calcium channels with Cd2+ blocked the augmentation of sPSC frequency by NT-3, suggesting that an increase of calcium entry may be required for the facilitation of NT-3. 6. Together, our results suggest NT-3 enhances GABA release during the developmental period when GABA is depolarizing and calcium elevating, but not later when GABA is inhibitory, suggesting that one mechanism through which NT-3 may influence neuronal development is via presynaptic potentiation of GABA excitation.

Brain-derived neurotrophic factor modulates nociceptive sensory inputs and NMDA-evoked responses in the rat spinal cord

Central sensitization, the hyperexcitability of spinal processing that often accompanies peripheral injury, is a major component of many persistent pain states. Here we report that the neurotrophin, brain-derived neurotrophic factor (BDNF), is a modulator of excitability within the spinal cord and contributes to the mechanism of central sensitization. BDNF, localized in primary sensory neuron cell bodies and central terminals, potentiates nociceptive spinal reflex responses in an in vitro spinal cord preparation and induces c-fos expression in dorsal horn neurons. NMDA receptor-mediated responses, known as a major contributor to central sensitization, were significantly enhanced by exogenous BDNF. Systemic NGF treatment, a procedure that mimics peripheral inflammatory states, raises BDNF levels in sensory neurons and increases nociceptive spinal reflex excitability. This increased central excitability is reduced by trkB-IgG, a BDNF "antagonist." We also show directly that inflammatory pain-related behavior depends on BDNF release in vivo. Thus behavioral nociceptive responses induced by intraplantar formalin and by intraplantar carageenan are significantly attenuated by trkB-IgG. Hence BDNF is appropriately localized and regulated in inflammatory states and is sufficient and necessary for the expression of central sensitization in the spinal cord. We propose that BDNF may function as a modulator of central sensitization in pathological states, and our results suggest that pharmacological antagonism of BDNF may prove an effective and novel analgesic strategy for the treatment of persistent inflammatory pain states.

Neurotrophin-mediated potentiation of neuronal injury

The neurotrophins are a diverse family of peptides which activate specific tyrosine kinase-linked receptors. Over the past five decades, since the pioneering work of Levi-Montalcini and colleagues, the critical role that neurotrophins play in shaping the developing nervous system has become increasingly established. These molecules, which include the nerve growth factor (NGF)-related peptides, NGF, brain-derived neurotrophic factor (BDNF), NT-4/5 and NT-3, promote differentiation and survival in the developing nervous system, and to a lesser extent in the adult nervous system. As survival-promoting molecules, neurotrophins have been studied as potential neuroprotective agents, and have shown beneficial effects in many model systems. However, a surprising "dark side" to neurotrophin behavior has emerged from some of these studies implying that, under certain pathological conditions, neurotrophins may exacerbate, rather than alleviate, injury. How neurotrophins cause these deleterious consequences is a question which is only beginning to be answered, but initial work supports altered free radical handling or modification of glutamate receptor expression as possible mechanisms underlying these effects. This review will focus on evidence suggesting that neurotrophins may enhance injury under certain circumstances and on the mechanisms behind these injury-promoting aspects.

Activity-Dependent Activation of TrkB Neurotrophin Receptors in the Adult CNS

In this paper we have investigated the hypothesis that neural activity causes rapid activation of TrkB neurotrophin receptors in the adult mammalian CNS. These studies demonstrate that kainic acid-induced seizures led to a rapid and transient activation of TrkB receptors in the cortex. Subcellular fractionation demonstrated that these activated Trk receptors were preferentially enriched in the synaptosomal membrane fraction that also contained postsynaptic glutamate receptors. The fast activation of synaptic TrkB receptors could be duplicated in isolated cortical synaptosomes with KCl, presumably as a consequence of depolarization-induced BDNF release. Importantly, TrkB activation was also observed following pharmacological activation of brain-stem noradrenergic neurons, which synthesize and anterogradely transport BDNF; treatment with yohimbine led to activation of cortical TrkB receptors within 30 min. Pharmacological blockade of the postsynaptic α1-adrenergic receptors with prazosin only partially inhibited this effect, suggesting that the TrkB activation was partially due to a direct effect on postsynaptic cortical neurons. Together, these data support the hypothesis that activity causes release of BDNF from presynaptic terminals, resulting in a rapid activation of postsynaptic TrkB receptors. This activity-dependent TrkB activation could play a major role in morphological growth and remodelling in both the developing and mature nervous systems.

Regulation of Synaptic Function by Neurotrophic Factors in Vertebrates and Invertebrates: Implications for Development and Learning

Recent studies have demonstrated that neurotrophic factors contribute to the molecular events involved in synaptic plasticity, both during vertebrate development and in the mature nervous system. Although it is well established that many of the cellular and molecular mechanisms underlying synaptic plasticity are conserved between invertebrates and vertebrates, there are, as yet, very few neurotrophic factors identified in invertebrate species. Nonetheless, vertebrate neurotrophins can influence invertebrate neuronal growth and plasticity. In addition, homologs of neurotrophic factor receptors have been identified in several invertebrate species. These studies may indicate that the roles of neurotrophins in both developmental and adult plasticity are highly conserved across diverse phyla.

Neurotrophins and synaptic plasticity

Despite considerable evidence that neuronal activity influences the organization and function of circuits in the developing and adult brain, the molecular signals that translate activity into structural and functional changes in connections remain largely obscure. This review discusses the evidence implicating neurotrophins as molecular mediators of synaptic and morphological plasticity. Neurotrophins are attractive candidates for these roles because they and their receptors are expressed in areas of the brain that undergo plasticity, activity can regulate their levels and secretion, and they regulate both synaptic transmission and neuronal growth. Although numerous experiments show demonstrable effects of neurotrophins on synaptic plasticity, the rules and mechanisms by which they exert their effects remain intriguingly elusive.

Brain-derived neurotrophic factor and neurotrophin-3 enhance somatostatin gene expression through a likely direct effect on hypothalamic somatostatin neurons

Although neurotrophins (NTs) have been extensively studied as neuronal survival factors in some areas of the central nervous system, little is known about their function or cellular targets in the hypothalamus. To understand their functional significance and sites of action on hypothalamic neurons, we examined the effects of their cognate ligands on neuropeptide content and messenger RNA (mRNA) expression in somatostatin neurons present in fetal rat hypothalamic cultures. Treatments were performed in defined insulin-free medium between days 6 and 8 of culture, since the maximal effects of NTs on somatostatin content and mRNA expression were observed after 48-h incubations. Brain-derived neurotrophic factor and NT-3, but not nerve growth factor, induced a dose-dependent increase in somatostatin content, which was influenced by plating density. The same treatment increased somatostatin mRNA and immunostaining intensity of somatostatin neurons, but had no effect on the number of these labeled neurons. The increased levels of somatostatin (peptide and mRNA) induced by NTs were not blocked by tetrodotoxin or by glutamate receptor antagonists, suggesting that endogenous neurotransmitters (e.g. glutamate) were not involved in these effects. In contrast, the stimulatory effects were completely blocked by K-252a, an inhibitor of tyrosine kinase (Trk) receptors, whereas the less active analog K-252b was ineffective. Double-labeling studies demonstrated that both TrkB or TrkC receptors were located on somatostatin neurons. Our results show that, in rat hypothalamic cultures, brain-derived neurotrophic factor, and NT-3 have a potent stimulatory effect on peptide synthesis in somatostatinergic neurons, likely through direct activation of TrkB and TrkC receptors.

Evidence that brain-derived neurotrophic factor neuroprotection is linked to its ability to reverse the NMDA-induced inactivation of protein kinase C in cortical neurons

Several lines of evidence indicate that a rapid loss of neuronal protein kinase C (PKC) activity is a characteristic feature of cerebral ischemia and is a necessary step in the NMDA-induced death of cultured neurons. Exposing embryonic day 18 primary rat cortical neurons to 50 microM NMDA or 50 microM glutamate for 10 min caused approximately 80% cell death over the next 24 h, but excitotoxic death was largely averted, i.e., by 70-80%, in cells pretreated with brain-derived neurotrophic factor (BDNF). An 8-h preexposure to BDNF (50-100 ng/ml) maximally protected cortical cells from the effects of NMDA and glutamate, although the transient application of BDNF between 8 and 4 h before NMDA was equally protective. These effects of BDNF were abolished at supralethal, i.e., >100 microM, NMDA concentrations. It is significant that BDNF pretreatment prevented the inactivation of PKC in cortical cells normally seen 30 min to 2 h following lethal NMDA or glutamate exposure. This BDNF effect did not arise from changes in NMDA channel activity because neither whole-cell NMDA current amplitudes nor increases in intracellular free Ca2+ concentration were altered by the 8-h BDNF pretreatment. Furthermore, BDNF offered no neuroprotection to cells treated with the PKC inhibitors staurosporine (10-20 nM), calphostin C (1-2.5 microM), or GF-109203X (100 nM) at the time of NMDA addition. These results underscore the importance of PKC inactivation in glutamate-induced neuronal death. They also suggest that BDNF neuroprotection arises, at least in part, via its ability to block the mechanism by which pathophysiological Ca2+ influx through the NMDA receptor causes membrane PKC inactivation.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Neurotrophin-dependent modulation of glutamatergic synaptic transmission in the mammalian CNS

1. The protein family of the neurotrophins, consisting of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and Neurotrophin-3, -4/5, and -6 (NT-3; NT-4/5; NT-6) is well known to enhance the survival and to stabilize the phenotype of different populations of neurons in the central and the peripheral nervous system. These effects are mediated via binding to specific tyrosine kinase receptors (Trks) and to the low-affinity p75 neurotrophin receptor. 2. The neurotrophins NGF, BDNF, and NT-3 and the BDNF and NT-3 selective receptors (TrkB, TrkC) are expressed at high levels in neurons of the basal forebrain, the hippocampus, and the neocortex of the mammalian brain. The expression and secretion of NGF and BDNF in these brain areas is regulated by (physiological levels of) neuronal activity. 3. Exogenous application of the neurotrophins to hippocampal and neocortical neurons can enhance excitatory glutamatergic synaptic transmission via activation of Trk receptors. In addition, long-term potentiation (a potential cellular correlate for learning and memory formation in mammals) in the rodent hippocampus depends on endogenous supply of neurons with BDNF. 4. Judged by the analysis of electrophysiological data, the BDNF- and NT-3-induced enhancement of glutamatergic synapses is mediated by increasing the efficacy of glutamate release from the presynaptic neuron. However, neurotrophin-dependent postsynaptic enhancement of NMDA (but not AMPA) receptor responsiveness has also been shown. 5. On the molecular level, neither the pre- nor the postsynaptic modulation of glutamatergic synapses by neurotrophins is well understood. However, neurotrophins were shown to acutely affect intraneuronal Ca2+ levels and to influence molecular components of the transmitter release machinery, which could underly the presynaptic modifications, whereas BDNF-induced phosphorylation of NMDA-type glutamate receptors could account for the postsynaptic effects. 6. Taken together, these results suggest that the activity-dependent release of neurotrophins at frequently used synapses could modulate the synaptic efficacy at these junctions. Thus, neurotrophins might operate as locally released feedback modulators of synaptic transmission, and this could be a cellular correlate for certain aspects of information processing in the mammalian brain.

Multiple sites of action of neomycin, Mg2+ and spermine on the NMDA receptors of rat hippocampal CA1 pyramidal neurones

1. The effects of neomycin on NMDA-evoked currents in isolated CA1 hippocampal pyramidal neurones were investigated and single channel activity was examined in outside-out patches taken from cultured hippocampal neurones. The effects of neomycin on two combinations of NMDA receptor subunits (NR1a-NR2A and NR1a-NR2B) expressed in human embryonic kidney (HEK293) cells were also studied. 2. Neomycin (0. 01-1 mM) caused a potentiation of NMDA-activated currents in all neurones examined. No evidence of a voltage-dependent depression was observed in whole-cell recordings. 3. In outside-out patch recordings relatively low concentrations (30 and 100 microM) of neomycin caused a voltage-dependent reduction in single channel current amplitude as well as a large increase in the frequency of channel opening. 4. In saturating concentrations of glycine, neomycin enhanced NMDA-activated currents and this glycine-independent enhancement was confirmed using recombinant NR1a-NR2B receptors. Neomycin substantially increased the potency of glycine for the receptor by reducing the rate of dissociation of glycine from the receptor. Neomycin demonstrated a glycine-dependent enhancement of currents mediated by the NR1a-NR2A combination of subunits but a paradoxical depression was observed in saturating concentrations of glycine. 5. Neomycin increased the rate of deactivation of glutamate-activated currents consistent with neomycin causing a reduction in the affinity of the receptor for agonist. 6. These results indicate that neomycin has multiple and complex effects on NMDA receptors.

Presynaptic modulation of synaptic transmission and plasticity by brain-derived neurotrophic factor in the developing hippocampus

In addition to the regulation of neuronal survival and differentiation, neurotrophins may play a role in synapse development and plasticity. Application of brain-derived neurotrophic factor (BDNF) promotes long-term potentiation (LTP) in CA1 synapses of neonatal hippocampus, which otherwise exhibit only short-term potentiation. This is attributable, at least in part, to an attenuation of the synaptic fatigue induced by high-frequency stimulation (HFS). However, the prevention of synaptic fatigue by BDNF could be mediated by an attenuation of synaptic vesicle depletion from presynaptic terminals and/or a reduction of the desensitization of postsynaptic receptors. Here we provide evidence supporting a presynaptic effect of BDNF. The effect of BDNF on synaptic fatigue depended on the stimulation frequency, not on the stimulus duration nor on the number of stimulation pulses. BDNF was only effective when the synapses were stimulated at frequencies >50 Hz. Treatment with BDNF also potentiated paired-pulse facilitation (PPF), a parameter reflecting changes in the properties of presynaptic terminals. This effect of BDNF was restricted only to PPF elicited with interpulse intervals </=20 msec. Changes in the extracellular calcium concentration altered the magnitude of the BDNF effect on PPF and synaptic responses to HFS, suggesting that BDNF regulates neurotransmitter release. When the desensitization of glutamate receptors was blocked by cyclothiazide or aniracetam, the BDNF potentiation of the synaptic responses to HFS was unaltered. Taken together, these results suggest that BDNF acts presynaptically. When two pathways in the same slice were monitored simultaneously, BDNF treatment potentiated the tetanized pathway without affecting the synaptic efficacy of the untetanized pathway. The selective potentiation of high-frequency transmission by BDNF appears to contribute directly to the effect of BDNF on LTP rather than indirectly by inducing the release of additional diffusible factors. The preferential potentiation of highly active synapses by BDNF may have implications in the Hebbian mechanism of synaptic plasticity.

Brain-derived neurotrophic factor modulates hippocampal synaptic transmission by increasing N-methyl-D-aspartic acid receptor activity

Neurotrophins (NTs) have recently been found to regulate synaptic transmission in the hippocampus. Whole-cell and single-channel recordings from cultured hippocampal neurons revealed a mechanism responsible for enhanced synaptic strength. Specifically, brain-derived neurotrophic factor augmented glutamate-evoked, but not acetylcholine-evoked, currents 3-fold and increased N-methyl-D-aspartic acid (NMDA) receptor open probability. Activation of trkB NT receptors was critical, as glutamate currents were not affected by nerve growth factor or NT-3, and increased open probability was prevented by the tyrosine kinase inhibitor K-252a. In addition, the NMDA receptor antagonist MK-801 blocked brain-derived neurotrophic factor enhancement of synaptic transmission, further suggesting that NTs modulate synaptic efficacy via changes in NMDA receptor function.

Brain-derived neurotrophic factor alters the synaptic modification threshold in visual cortex

The effects of brain-derived neurotrophic factor (BDNF) were investigated on synaptic transmission and two forms of activity-dependent synaptic plasticity, long-term potentiation (LTP) and long-term depression (LTD), in visual cortex slices prepared from young (P21 -28) rats. The slices treated for 2-5 h in BDNF showed no difference from control slices when a 'strong' tetanus was used (theta-burst stimulation) to elicit a maximal level of LTP but displayed significantly greater synaptic potentiation in response to a 'weak' (20 Hz) tetanus. The BDNF-treated slices also showed significantly less LTD in response to a 1 Hz tetanus. Thus, BDNF treatment alters the relationship between stimulation frequency and synaptic plasticity in the visual cortex, shifting the modification threshold to the left. The effects of BDNF on LTP and LTD induction may be attributed to the significant enhancement of synaptic responses that was observed during conditioning stimulation. These data suggest that one role of BDNF during development of the visual cortex may be to modulate the properties of synaptic plasticity, enhancing synaptic strengthening and reducing synaptic weakening processes which contribute to the formation of specific synaptic connections.