The concept of pain has remained a topic of long debate since its emergence in ancient times. The initial ideas of pain were formulated in both the East and the West before 1800. Since 1800, due to the development of experimental sciences, different theories of pain have emerged and become central topics of debate. However, the existing theories of pain may be appropriate for the interpretation of some aspects of pain, but are not yet comprehensive. The history of pain problems is as long as that of human beings;however, the understanding of pain mechanisms is still far from sufficient. Thus, intensive research is required. This historical review mainly focuses on the development of pain theories and the fundamental discoveries in this field. Other historical events associated with pain therapies and remedies are beyond the scope of this review.
Simultaneous multisite recording using multi-electrode arrays（MEAs） in cultured and acutely-dissociated brain slices and other tissues is an emerging technique in the field of network electrophysiology.Over the past 40 years,great efforts have been made by both scientists and commercial concerns,to advance this technique.The MEA technique has been widely applied to many regions of the brain,retina,heart and smooth muscle in various studies at the network level.The present review starts from the development of MEA techniques and their uses in brain preparations,and then specifically concentrates on the use of MEA recordings in studies of synaptic plasticity at the network level in both the temporal and spatial domains.Because the MEA technique helps bridge the gap between single-cell recordings and behavioral assays,its wide application will undoubtedly shed light on the mechanisms underlying brain functions and dysfunctions at the network level that remained largely unknown due to the technical difficulties before it matured.
Objective Melittin is the main peptide in bee venom and causes both persistent spontaneous nociception and pain hypersensitivity. Our recent studies indicated that both transient receptor potential （TRP） vanilloid receptor 1 （TRPV 1） and canonical TRPs （TRPCs） are involved in mediating the melittin-induced activation of different subpopulations of pri- mary nociceptive cells. Here, we further determined whether TRPC channels are involved in melittin-induced inflamma- tory nociceptive responses in behavioral assays. Methods The anti-nociceptive and anti-hyperalgesic effects of localized peripheral administration of three doses of the non-selective TRPC antagonist, SKF-96365 （1-{[3-[3-（4-methoxyphenyl） propoxy]-4-methoxyphenyl}-lH-imidazole hydrochloride）, were evaluated in melittin tests. Pain-related behaviors were rated by counting the number of paw flinches, and measuring paw withdrawal thermal latency （s） and paw withdrawl me- chanical threshold （g）, over a l-h time-course. Results Localized peripheral SKF-96365 given before melittin prevented, and given after melittin significantly suppressed, the melittin-evoked persistent spontaneous nociception. Pre-blockade and post-suppression of activation of primary nociceptive activity resulted in decreased hypersensitivity to both thermal and mechanical stimuli applied to the primary injury site of the ipsilateral hindpaw, despite dose-effect differences between thermal and mechanical hyperalgesia. However, local administration of SKF-96365 into the contralateral hindpaw had no significant effect on any pain-associated behaviors. In addition, SKF-96365 had no effect on baseline threshold for either thermal or mechanical sensitivity under normal conditions. Conclusion Besides TRPV1, SKF-96365-sensitive TRPC channels might also be involved in the pathophysiological processing of melittin-induced inflammatory pain and hyper- sensitivity. Therapeutically, SKF-96365 is equally effective in preventing primary thermal and mechanical hyperalgesia
Melittin is a basic 26-amino-acid polypeptide that constitutes 40-60% of dry honeybee（Apis mellifera）venom.Although much is known about its strong surface activity on lipid membranes,less is known about its painproducing effects in the nervous system.In this review,we provide lines of accumulating evidence to support the hypothesis that melittin is the major pain-producing substance of bee venom.At the psychophysical and behavioral levels,subcutaneous injection of melittin causes tonic pain sensation and pain-related behaviors in both humans and animals.At the cellular level,melittin activates primary nociceptor cells through direct and indirect effects.On one hand,melittin can selectively open thermal nociceptor transient receptor potential vanilloid receptor channels via phospholipase A2-lipoxygenase/cyclooxygenase metabolites,leading to depolarization of primary nociceptor cells.On the other hand,algogens and inflammatory/proinflammatory mediators released from the tissue matrix by melittin＇s pore-forming effects can activate primary nociceptor cells through both ligand-gated receptor channels and the G-protein-coupled receptor-mediated opening of transient receptor potential canonical channels.Moreover,subcutaneous melittin up-regulates Nav1.8 and Nav1.9subunits,resulting in the enhancement of tetrodotoxinresistant Na~＋currents and the generation of long-term action potential firing.These nociceptive responses in the periphery finally activate and sensitize the spinal dorsal horn pain-signaling neurons,resulting in spontaneous nociceptive paw flinches and pain hypersensitivity to thermal and mechanical stimuli.Taken together,it is concluded that melittin is the major pain-producing substance of bee venom,by which peripheral persistent pain and hyperalgesia（or allodynia）,primary nociceptive neuronal sensitization,and CNS synaptic plasticity（or metaplasticity） can be readily induced and the molecular and cellular mechanisms underlying naturally-occurring venomous biotoxins can be experimentally
Intractable central post-stroke pain（CPSP） is one of the most common sequelae of stroke, but has been inadequately studied to date. In this study, we first determined the relationship between the lesion site and changes in mechanical or thermal pain sensitivity in a rat CPSP model with experimental thalamic hemorrhage produced by unilateral intra-thalamic collagenase IV（ITC） injection. Then, we evaluated the efficacy of gabapentin（GBP）, an anticonvulsant that binds the voltage-gated Ca2＋ channel α2δ and a commonly used anti-neuropathic pain medication. Histological case-by-case analysis showed that only lesions confined to the medial lemniscus and the ventroposterior lateral/medial nuclei of the thalamus and/or the posterior thalamic nucleus resulted in bilateral mechanical pain hypersensitivity. All of the animals displaying CPSP also had impaired motor coordination, while control rats with intra-thalamic saline developed no central pain or motor deficits. GBP had a dose-related anti-allodynic effect after a single administration（1, 10, or 100 mg/kg） on day 7 post-ITC, with significant effects lasting at least 5 hfor the higher doses. However, repeated treatment, once a day for two weeks, resulted in complete loss of effectiveness（drug tolerance） at 10 mg/kg, while effectiveness remained at 100 mg/kg, although the time period of efficacious analgesia was reduced. In addition, GBP did not change the basal pain sensitivity and the motor impairment caused by the ITC lesion, suggesting selective action of GBP on the somatosensory system.
The α2δ-1 subunit of the voltage-gated Ca2＋ channel （VGCC） is a molecular target of gabapentin （GBP）, which has been used as a first-line drug for the relief of neuropathic pain. GBP exerts its anti-nociceptive effects by disrupting trafficking of the α2δ-1 subunit to the presynaptic membrane, resulting in decreased neurotrans- mitter release. We previously showed that GBP has an anti- allodynic effect in the first two weeks; but this is followed by insensitivity in the later stage after repeated adminis- tration in a rat model of central post-stroke pain （CPSP） hypersensitivity induced by intra-thalamic hemorrhage. To explore the mechanisms underlying GBP insensitivity, the cellular localization and time-course of expression of the α2δ-1 subunit in both the thalamus and spinal dorsal horn were studied in the same model. We found that the α2δ-1 subunit was mostly localized in neurons, but not astrocytes and microglia. The level of α2δ-1 protein increased in the first two weeks after injury but then decreased in the third week, when GBP insensitivity occurred. Furthermore, the c^2g-1 down-regulation was likely caused by later neuronal loss in the injured thalamus through a mechanism other than apoptosis. In summary, the present results suggest that the GBP receptor ~2^-1 is mainly expressed in thalamic neurons in which it is up-regulated in the early stage of CPSP but this is followed by dramatic down-regulation, which is likely associated with GBP insensitivity after long-term use.
