Prior work has shown that pain can be modulated by low-level analgesic systems in the brainstem. These systems can be activated by exposure to aversive stimuli. With mild aversive stimuli the activation of these systems depends on forebrain structures. One purpose of the present research program is to elucidate the forebrain structures involved. A second purpose is to investigate the manner by which opiate analgesics, such as morphine, alter the experience of pain. Presumably opiates induce analgesia by interacting with one of the neural systems rich in opioid receptors. It remains unclear, however, what systems are involved. The experiments we propose will evaluate the role of one forebrain structure, the central nucleus of the amygdala. This nucleus is of particular interest since it is rich in opioids and opioid receptors. In addition, it is connected with the systems in the brainstem which mediate analgesia. We will evaluate the role of the central nucleus in pain by assessing how manipulations of this region in rats alter pain reactivity as measured by the tail-flick test and shock induced vocalization. We will also assess the impact of manipulations of this nucleus on the activation of the analgesic systems in the brainstem. In order to assess the contribution of the central nucleus we will evaluate the impact of bilateral lesions of the nucleus. The role of opioids in this nucleus will be investigated by microinjecting opiate agonists and antagonists directly onto the central nucleus. The present experiments will clarify whether manipulations of the central nucleus, and opioids in this region, alter pain reactivity. This work will elucidate whether opiate analgesics alter pain, in part, by interacting with opioid receptors in the central nucleus. The experiments will also elucidate whether the central nucleus plays a role in activating the analgesic systems in the brainstem. In addition, to clarifying the manner by which opiate analgesics alter pain, this work will clarify the functional role of the central nucleus. This is of major importance since this nucleus has been implicated as critical to learning and memory. Much of the evidence for this conclusion has been obtained with aversively motivated tasks. The present experiments will reveal whether manipulations of the central nucleus alter learning and memory by changing the affective impact of the aversive reinforcer.
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