Neuropharmacology
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Antidepressants that inhibit the recapture of noradrenaline have variable effects in chronic pain which may be related to the complex role of noradrenaline in pain modulation. Whereas at the spinal cord noradrenaline blocks nociceptive transmission, both antinociception and pronociception were reported after noradrenaline release in the brain. To study the role of noradrenaline in pain modulatory areas of the brain, we elected the dorsal reticular nucleus (DRt), a key pain facilitatory area located at the medulla oblongata. ⋯ Finally, we show that reducing noradrenaline release into the DRt, using an HSV-1 vector which decreases the expression of tyrosine hydroxylase in noradrenergic DRt-projecting neurons, attenuates pain behavioral responses in both phases of the formalin test. The increased noradrenaline levels induced by the infusion of nomifensine at the DRt, along with the hyperalgesic effects of noradrenaline released at the DRt upon noxious stimulation, indicates that noradrenaline may enhance pain facilitation from the brain. It is important to evaluate if antidepressants that inhibit noradrenaline recapture enhance pain facilitation from the brain herein attenuating their analgesic effects.
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Considerable evidence indicates that adenosine A(2A) receptors (A(2A)Rs) modulate cholinergic neurotransmission, nicotinic acetylcholine receptor (nAChR) function, and nicotine-induced behavioural effects. To explore the interaction between A(2A) and nAChRs, we examined if the complete genetic deletion of adenosine A(2A)Rs in mice induces compensatory alterations in the binding of different nAChR subtypes, and whether the long-term effects of nicotine on nAChR regulation are altered in the absence of the A(2A)R gene. Quantitative autoradiography was used to measure cytisine-sensitive [¹²⁵I]epibatidine and [¹²⁵I]α-bungarotoxin binding to α4β2* and α7 nAChRs, respectively, in brain sections of drug-naïve (n = 6) or nicotine treated (n = 5-7), wild-type and adenosine A(2A)R knockout mice. ⋯ The genetic ablation of adenosine A(2A)Rs prevented nicotine-induced upregulation of α7 nAChRs, without affecting α4β2* receptor upregulation. This selective effect was observed at plasma levels of nicotine that were within the range reported for smokers (10-50 ng ml⁻¹). Our data highlight the involvement of adenosine A(2A)Rs in the mechanisms of nicotine-induced α7 nAChR upregulation, and identify A(2A)Rs as novel pharmacological targets for modulating the long-term effects of nicotine on α7 receptors.
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Comparative Study
Involvement of the nucleus accumbens shell dopaminergic system in prelimbic NMDA-induced anxiolytic-like behaviors.
Nucleus accumbens (NAc) and prefrontal cortex (PFC) dopaminergic and glutamatergic systems are involved in fear/anxiety-related behaviors; meanwhile NAc dopaminergic system activity is mediated by PFC via NAc glutamatergic projections. This study has investigated the involvement of NAc shell dopaminergic system in prelimbic NMDA-induced anxiolytic-like behaviors. ⋯ Our results suggested a modulatory effect of the NAc shell dopaminergic system on prelimbic NMDA-induced anxiolytic-like behaviors.
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Brain-derived neurotrophic factor (BDNF) signaling via TrkB crucially regulates synaptic plasticity in the brain. Although BDNF is abundant at hippocampal mossy fiber (MF) synapses, which critically contribute to hippocampus dependent memory, its role in MF synaptic plasticity (long-term potentiation, LTP) remained largely unclear. Using field potential recordings in CA3 of adult heterozygous BDNF knockout (ko, BDNF+/-) mice we observed impaired (∼50%) NMDAR-independent MF-LTP. ⋯ Basal synaptic transmission, short-term plasticity, and synaptic fatigue during LTP induction were not significantly altered by treatment with k252a or TrkB-Fc, or by chronic BDNF reduction in BDNF+/- mice. Since the acute interference with BDNF-signaling did not completely block MF-LTP, our results provide evidence that an additional mechanism besides BDNF induced TrkB signaling contributes to this type of LTP. Our results prove for the first time a mechanistic action of acute BDNF/TrkB signaling in presynaptic expression of MF-LTP in adult hippocampus.
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Comparative Study
The multitarget opioid ligand LP1's effects in persistent pain and in primary cell neuronal cultures.
Persistent pain states, such as those caused by nerve injury or inflammation, are associated with altered sensations, allodynia and hyperalgesia, that are resistant to traditional analgesics. A contribution to development and maintenance in altered pain perception comes from nociceptive processing and descending modulation from supraspinal sites. A multitarget ligand seems to be useful for pain relief with a decreased risk of adverse events and a considerable analgesic efficacy. ⋯ The EC₅₀ values in this functional screening seem to confirm LP1 as a potent opioid ligand (EC₅₀ = 0.35 fM and EC₅₀ = 44 pM in spinal cord and frontal cortex, respectively). Using a NeuroProof data-base of well characterised reference compounds, a similarity profile of LP1 to opioid and non-opioid drugs involved in pain modulation was detected. Our studies seem to support that multitarget ligand approach should be useful for persistent pain conditions in which mechanical allodynia and thermal hyperalgesia are significant components of the nociceptive response.