Pronociceptive and Antinociceptive Effects of Buprenorphine in the Spinal Cord Dorsal Horn Cover a Dose Range of Four Orders of Magnitude.

Due to its distinct pharmacological profile and lower incidence of adverse events compared with other opioids, buprenorphine is considered a safe option for pain and substitution therapy. However, despite its wide clinical use, little is known about the synaptic effects of buprenorphine in nociceptive pathways. Here, we demonstrate dose-dependent, bimodal effects of buprenorphine on transmission at C-fiber synapses in rat spinal cord dorsal horn in vivo. At an analgesically active dose of 1500 µg·kg(-1), buprenorphine reduced the strength of spinal C-fiber synapses. This depression required activation of spinal opioid receptors, putatively µ1-opioid receptors, as indicated by its sensitivity to spinal naloxone and to the selective µ1-opioid receptor antagonist naloxonazine. In contrast, a 15,000-fold lower dose of buprenorphine (0.1 µg·kg(-1)), which caused thermal and mechanical hyperalgesia in behaving animals, induced an enhancement of transmission at spinal C-fiber synapses. The ultra-low-dose buprenorphine-induced synaptic facilitation was mediated by supraspinal naloxonazine-insensitive, but CTOP-sensitive µ-opioid receptors, descending serotonergic pathways, and activation of spinal glial cells. Selective inhibition of spinal 5-hydroxytryptamine-2 receptors (5-HT2Rs), putatively located on spinal astrocytes, abolished both the induction of synaptic facilitation and the hyperalgesia elicited by ultra-low-dose buprenorphine. Our study revealed that buprenorphine mediates its modulatory effects on transmission at spinal C-fiber synapses by dose dependently acting on distinct µ-opioid receptor subtypes located at different levels of the neuraxis.

Presynaptic inhibition of optogenetically identified VGluT3+ sensory fibres by opioids and baclofen.

Distinct subsets of sensory nerve fibres are involved in mediating mechanical and thermal pain hypersensitivity. They may also differentially respond to analgesics. Heat-sensitive C-fibres, for example, are thought to respond to µ-opioid receptor (MOR) activation while mechanoreceptive fibres are supposedly sensitive to δ-opioid receptor (DOR) or GABAB receptor (GABABR) activation. The suggested differential distribution of inhibitory neurotransmitter receptors on different subsets of sensory fibres is, however, heavily debated. In this study, we quantitatively compared the degree of presynaptic inhibition exerted by opioids and the GABABR agonist baclofen on (1) vesicular glutamate transporter subtype 3-positive (VGluT3) non-nociceptive primary afferent fibres and (2) putative nociceptive C-fibres. To investigate VGluT3 sensory fibres, we evoked excitatory postsynaptic currents with blue light at the level of the dorsal root ganglion (DRG) in spinal cord slices of mice, expressing channelrhodopsin-2. Putative nociceptive C-fibres were explored in VGluT3-knockout mice through electrical stimulation. The MOR agonist DAMGO strongly inhibited both VGluT3 and VGluT3 C-fibres innervating lamina I neurons but generally had less influence on fibres innervating lamina II neurons. The DOR agonist SNC80 did not have any pronounced effect on synaptic transmission in any fibre type tested. Baclofen, in striking contrast, powerfully inhibited all fibre populations investigated. In summary, we report optogenetic stimulation of DRG neurons in spinal slices as a capable approach for the subtype-selective investigation of primary afferent nerve fibres. Overall, pharmacological accessibility of different subtypes of sensory fibres considerably overlaps, indicating that MOR, DOR, and GABABR expressions are not substantially segregated between heat and mechanosensitive fibres.

Astrocyte calcium signals at Schaffer collateral to CA1 pyramidal cell synapses correlate with the number of activated synapses but not with synaptic strength.

Glial cells respond to neuronal activity by transient increases in their intracellular calcium concentration. At hippocampal Schaffer collateral to CA1 pyramidal cell synapses, such activity-induced astrocyte calcium transients modulate neuronal excitability, synaptic activity, and LTP induction threshold by calcium-dependent release of gliotransmitters. Despite a significant role of astrocyte calcium signaling in plasticity of these synapses, little is known about activity-dependent changes of astrocyte calcium signaling itself. In this study, we analyzed calcium transients in identified astrocytes and NG2-cells located in the stratum radiatum in response to different intensities and patterns of Schaffer collateral stimulation. To this end, we employed multiphoton calcium imaging with the low-affinity indicator dye Fluo-5F in glial cells, combined with extracellular field potential recordings to monitor postsynaptic responses to the afferent stimulation. Our results confirm that somata and processes of astrocytes, but not of NG2-cells, exhibit intrinsic calcium signaling independent of evoked neuronal activity. Moderate stimulation of Schaffer collaterals (three pulses at 50 Hz) induced calcium transients in astrocytes and NG2-cells. Astrocyte calcium transients upon this three-pulse stimulation could be evoked repetitively, increased in amplitude with increasing stimulation intensity and were dependent on activation of metabotropic glutamate receptors. Activity-induced transients in NG2-cells, in contrast, showed a rapid run-down upon repeated three-pulse stimulation. Theta burst stimulation and stimulation for 5 min at 1 Hz induced synaptic potentiation and depression, respectively, as revealed by a lasting increase or decrease in population spike amplitudes upon three-pulse stimulation. Synaptic plasticity was, however, not accompanied by corresponding alterations in the amplitude of astrocyte calcium signals. Taken together, our results suggest that the amplitude of astrocyte calcium signals reflects the number of activated synapses but does not correlate with the degree of synaptic potentiation or depression at Schaffer collateral to CA1 pyramidal cell synapses.

A new role for interferon gamma in neural stem/precursor cell dysregulation.

BACKGROUND: The identification of factors that compromise neurogenesis is aimed at improving stem cell-based approaches in the field of regenerative medicine. Interferon gamma (IFNγ) is a main pro-inflammatory cytokine and up-regulated during several neurological diseases. IFNγ is generally thought to beneficially enhance neurogenesis from fetal or adult neural stem/precursor cells (NSPCs). RESULTS: We now provide direct evidence to the contrary that IFNγ induces a dysfunctional stage in a substantial portion of NSPC-derived progeny in vitro characterized by simultaneous expression of glial fibrillary acid protein (GFAP) and neuronal markers, an abnormal gene expression and a functional phenotype neither typical for neurons nor for mature astrocytes. Dysfunctional development of NSPCs under the influence of IFNγ was finally demonstrated by applying the microelectrode array technology. IFNγ exposure of NSPCs during an initial 7-day proliferation period prevented the subsequent adequate differentiation and formation of functional neuronal networks. CONCLUSIONS: Our results show that immunocytochemical analyses of NSPC-derived progeny are not necessarily indicating the correct cellular phenotype specifically under inflammatory conditions and that simultaneous expression of neuronal and glial markers rather point to cellular dysregulation. We hypothesize that inhibiting the impact of IFNγ on NSPCs during neurological diseases might contribute to effective neurogenesis and regeneration.

Neuron-glia communication via EphA4/ephrin-A3 modulates LTP through glial glutamate transport.

Astrocytes are critical participants in synapse development and function, but their role in synaptic plasticity is unclear. Eph receptors and their ephrin ligands have been suggested to regulate neuron-glia interactions, and EphA4-mediated ephrin reverse signaling is required for synaptic plasticity in the hippocampus. Here we show that long-term potentiation (LTP) at the CA3-CA1 synapse is modulated by EphA4 in the postsynaptic CA1 cell and by ephrin-A3, a ligand of EphA4 that is found in astrocytes. Lack of EphA4 increased the abundance of glial glutamate transporters, and ephrin-A3 modulated transporter currents in astrocytes. Pharmacological inhibition of glial glutamate transporters rescued the LTP defects in EphA4 (Epha4) and ephrin-A3 (Efna3) mutant mice. Transgenic overexpression of ephrin-A3 in astrocytes reduces glutamate transporter levels and produces focal dendritic swellings possibly caused by glutamate excitotoxicity. These results suggest that EphA4/ephrin-A3 signaling is a critical mechanism for astrocytes to regulate synaptic function and plasticity.