Search

Peripheral nerve injury promotes the release of brain-derived neurotrophic factor (BDNF) from spinal microglial cells and primary afferent terminals. This induces an increase in dorsal horn excitability that contributes to "central sensitization" and to the onset of neuropathic pain. Although it is accepted that impairment of GABAergic and/or glycinergic inhibition contributes to this process, certain lines of evidence suggest that GABA release in the dorsal horn may increase after nerve injury. To resolve these contradictory findings, we exposed rat spinal cord neurons in defined-medium organotypic culture to 200 ng/ml BDNF for 6 days to mimic the change in spinal BDNF levels that accompanies peripheral nerve injury. Morphological and electrophysiological criteria and glutamic acid decarboxylase (GAD) immunohistochemistry were used to distinguish putative inhibitory tonic-islet-central neurons from putative excitatory delay-radial neurons. Whole cell recording in the presence of 1 μM tetrodotoxin showed that BDNF increased the amplitude of GABAergic and glycinergic miniature inhibitory postsynaptic currents (mIPSCs) in both cell types. It also increased the amplitude and frequency of spontaneous, action potential-dependent IPSCs (sIPSCs) in putative excitatory neurons. By contrast, BDNF reduced sIPSC amplitude in inhibitory neurons but frequency was unchanged. This increase in inhibitory drive to excitatory neurons and decreased inhibitory drive to inhibitory neurons seems inconsistent with the observation that BDNF increases overall dorsal horn excitability. One of several explanations for this discrepancy is that the action of BDNF in the substantia gelatinosa is dominated by previously documented increases in excitatory synaptic transmission rather than by impediment of inhibitory transmission.

Adult neuronal phenotype is maintained, at least in part, by the sensitivity of individual neurons to a specific selection of neurotrophic factors and the availability of such factors in the neurons' environment. Nerve growth factor (NGF) increases the functional expression of Na(+) channel currents (I(Na)) and both N- and L-type Ca(2+) currents (I(Ca,N) and I(Ca,L)) in adult bullfrog sympathetic ganglion (BFSG) B-neurons. The effects of NGF on I(Ca) involve the mitogen-activated protein kinase (MAPK) pathway. Prolonged exposure to the ganglionic neurotransmitter luteinizing hormone releasing hormone (LHRH) also increases I(Ca,N) but the transduction mechanism remains to be elucidated as does the transduction mechanism for NGF regulation of Na(+) channels. We therefore exposed cultured BFSG B-neurons to chicken II LHRH (0.45 microM; 6-9 days) or to NGF (200 ng/ml; 9-10 days) and used whole cell recording, immunoblot analysis, and ras or rap-1 pulldown assays to study effects of various inhibitors and activators of transduction pathways. We found that 1) LHRH signals via ras-MAPK to increase I(Ca,N), 2) this effect is mediated via protein kinase C-beta (PKC-beta-IotaIota), 3) protein kinase A (PKA) is necessary but not sufficient to effect transduction, 4) NGF signals via phosphatidylinositol 3-kinase (PI3K) to increase I(Na), and 5) long-term exposure to LHRH fails to affect I(Na). Thus downstream signaling from LHRH has access to the ras-MAPK pathway but not to the PI3K pathway. This allows for differential retrograde and anterograde neurotrophic regulation of sodium and calcium channels in an adult sympathetic neuron.