Objective Recently,there has been growing interest in the interaction between depressive disorders and pain.The purpose of this study was to examine whether depression would lead to a decreased sensitivity to noxious stimuli in rats with spontaneous pain.Methods The olfactory bulbectomized rats were used as a model of depression.The depression-like behaviors were assessed by open field test and changes in body weight.Formalin solution was injected into the rat hindpaw to produce ongoing pain.Noxious thermal stimuli were applied onto the hindpaw contralateral to formalin injection,and the withdrawal thresholds were measured.Results In non-depressive rats,the formalin-treated paw developed hypoalgesia to noxious stimuli while the contralateral paw was not affected.The depressive rats,however,showed a significantly lower sensitivity to noxious thermal stimulus,represented as higher withdrawal thresholds of the contralateral paw,when compared to the non-depressive rats.Conclusion These results demonstrate that depression can alleviate the stimulus-evoked pain even in the context of formalin inflammatory pain,consistent with the previous clinical observations that patients suffering from both depression and persistent pain have decreased sensitivities to noxious experimental stimuli.
Objective To investigate the effects of pentobarbital anesthesia on nociceptive processing in the medial and lateral pain pathways.Methods Laser stimulation was employed to evoke nociceptive responses in rats under awake or anesthetic conditions.Pain-related neuronal activities were simultaneously recorded from the primary somatosensory cortex （SI） ,ventral posterolateral thalamus （VPL） ,anterior cingulate cortex （ACC） ,and medial dorsal thalamus （MD） with 4 eight-wire microelec-trode arrays.Results Compared with the awake state,pentobarbital anesthesia significantly suppressed the neural activities induced by noxious laser stimulation.Meanwhile,the pain-evoked changes in the neuronal correlations between cortex and thalamus were suppressed in both medial and lateral pain pathways.In addition,the spontaneous firing rates in all the 4 areas were altered （including inhibition and excitation） under the condition of anesthesia.Conclusion The nociceptive processing in the brain can be dramatically changed by anesthesia,which indicates that there are considerable differences in the brain activities between awake and anesthetized states.It is better to employ awake animals for recording neural activity when investigating the sensory coding mechanisms,especially pain coding,in order to obtain data that precisely reflect the physiological state.
