Does the medial thalamus play a role in the negative affective component of visceral pain in rats?

Microcircuit Mechanisms through which Mediodorsal Thalamic Input to Anterior Cingulate Cortex Exacerbates Pain-Related Aversion

Hyperexcitability of the anterior cingulate cortex (ACC) is thought to drive aversion associated with chronic neuropathic pain. Here, we studied the contribution of input from the mediodorsal thalamus (MD) to ACC, using sciatic nerve injury and chemotherapy-induced mouse models of neuropathic pain. Activating MD inputs elicited pain-related aversion in both models. Unexpectedly, excitatory responses of layer V ACC neurons to MD inputs were significantly weaker in pain models compared to controls. This caused the ratio between excitation and feedforward inhibition elicited by MD input to shift toward inhibition, specifically for subcortically projecting (SC) layer V neurons. Furthermore, direct inhibition of SC neurons reproduced the pain-related aversion elicited by activating MD inputs. Finally, both the ability to elicit pain-related aversion and the decrease in excitation were specific to MD inputs; activating basolateral amygdala inputs produced opposite effects. Thus, chronic pain-related aversion may reflect activity changes in specific pathways, rather than generalized ACC hyperactivity.

Core Outcome Measures in Preclinical Assessment of Candidate Analgesics

All preclinical procedures for analgesic drug discovery involve two components: 1) a "pain stimulus" (the principal independent variable), which is delivered to an experimental subject with the intention of producing a pain state; and 2) a "pain behavior" (the principal dependent variable), which is measured as evidence of that pain state. Candidate analgesics are then evaluated for their effectiveness to reduce the pain behavior, and results are used to prioritize drugs for advancement to clinical testing. This review describes a taxonomy of preclinical procedures organized into an "antinociception matrix" by reference to their types of pain stimulus (noxious, inflammatory, neuropathic, disease related) and pain behavior (unconditioned, classically conditioned, operant conditioned). Particular emphasis is devoted to pain behaviors and the behavioral principals that govern their expression, pharmacological modulation, and preclinical-to-clinical translation. Strengths and weaknesses are compared and contrasted for procedures using each type of behavioral outcome measure, and the following four recommendations are offered to promote strategic use of these procedures for preclinical-to-clinical analgesic drug testing. First, attend to the degree of homology between preclinical and clinical outcome measures, and use preclinical procedures with behavioral outcome measures homologous to clinically relevant outcomes in humans. Second, use combinations of preclinical procedures with complementary strengths and weaknesses to optimize both sensitivity and selectivity of preclinical testing. Third, take advantage of failed clinical translation to identify drugs that can be back-translated preclinically as active negative controls. Finally, increase precision of procedure labels by indicating both the pain stimulus and the pain behavior in naming preclinical procedures.

Distributions of different types of nociceptive neurons in thalamic mediodorsal nuclei of anesthetized rats

Mediodorsal thalamic nucleus (MD) is a critical relay of nociception. This study recorded responses of MD neurons to noxious mechanical and thermal stimuli in isoflurane anesthetized rats. We found the threshold of noxious mechanical stimulation was 141 gw and that of noxious heat stimulation was 46 °C. A significantly higher percentage of noxious inhibitory neurons were found in the medial and central part of the MD, whereas a higher percentage of noxious excitatory neurons were found in the lateral part of the MD and adjacent intralaminar nuclei. The differential distribution of excitatory and inhibitory neurons implies functional differentiation between the medial and lateral part of the MD in nociception processing. Furthermore, by an analysis of the stimulus-response function (SRF), we found 80% of these excitatory neurons had a step-function or hat-shape-like SRF. This suggests that most of the MD neurons may serve as a system to distinguish innocuous versus noxious stimuli.

Functional interaction between medial thalamus and rostral anterior cingulate cortex in the suppression of pain affect

The medial thalamic parafascicular nucleus (PF) and the rostral anterior cingulate cortex (rACC) are implicated in the processing and suppression of the affective dimension of pain. The present study evaluated the functional interaction between PF and rACC in mediating the suppression of pain affect in rats following administration of morphine or carbachol (acetylcholine agonist) into PF. Vocalizations that occur following a brief noxious tailshock (vocalization afterdischarges) are a validated rodent model of pain affect, and were preferentially suppressed by injection of morphine or carbachol into PF. Vocalizations that occur during tailshock were suppressed to a lesser degree, whereas, spinal motor reflexes (tail flick and hindlimb movements) were only slightly suppressed by injection of carbachol into PF and unaffected by injection of morphine into PF. Blocking glutamate receptors in rACC (NMDA and non-NMDA) by injecting D-2-amino-5-phosphonovalerate (AP-5) or 6-cyano-7-nitroquinoxaline-2,3-dione disodium (CNQX) produced dose-dependent antagonism of morphine-induced increases in vocalization thresholds. Carbachol-induced increases in vocalization thresholds were not affected by injection of either glutamate receptor antagonist into rACC. The results demonstrate that glutamate receptors in the rACC contribute to the suppression of pain affect produced by injection of morphine into PF, but not to the suppression of pain affect generated by intra-PF injection of carbachol.

Neuronal mechanisms for pain-induced aversion behavioral studies using a conditioned place aversion test

Pain consists of sensory discriminative and negative affective components. Although the neural systems responsible for the sensory component of pain have been studied extensively, the neural basis of the affective component is not well understood. Recently, behavioral studies using conditioned place aversion (CPA) tests have successfully elucidated the neural circuits and mechanisms underlying the negative affective component of pain. Excitotoxic lesions of the anterior cingulate cortex (ACC), central amygdaloid nucleus, basolateral amygdaloid nucleus (BLA), or bed nucleus of the stria terminalis (BNST) suppressed intraplantar formalin-induced aversive responses. Glutamatergic transmission within the ACC and BLA via NMDA receptors was shown to play a critical role in the affective component of pain. In the BNST, especially its ventral part, noradrenergic transmission via beta-adrenergic receptors was demonstrated as important for pain-induced aversion. Because persistent pain is frequently associated with psychological and emotional dysfunction, studies of the neural circuits and the molecular mechanisms involved in the affective component of pain may have considerable clinical importance in the treatment of chronic pain.

Roles of corticosterone in formalin-induced conditioned place aversion in rats

Glucocorticoid hormones have been shown to contribute to many cognitive functions, such as depressions, learning and memory, and abnormal glucocorticoid secretion results in functional changes in prefrontal cortex and amygdala. In the present study, we used the conditioned place aversion (CPA) paradigm to investigate the role of corticosterone (CORT) in the negative affective component of chemical somatic pain induced by intraplantar injection of formalin into male adult Long-Evan rats. Five percent of formalin produced acute biphasic nociceptive behaviors, including flinching and licking of hindpaw, and CPA. Intraplantar formalin induced CPA was abolished by bilateral adrenalectomy and the impairment of CPA can be restored by the CORT treatment. However, the adrenalectomy failed to affect the formalin-produced acute nociceptive behaviors. Therefore, data from the present study suggest that CORT secretion by the adrenal cortex may play a role in chemical somatic noxious stimuli-induced avoidance learning and aversive memory, but not sensory discrimination of noxious stimulation.

Examining the role of the medial thalamus in modulating the affective dimension of pain

The purpose of this project was to explore the role of the medial thalamus (MT), including the medial dorsal thalamus (MD) and associated midline nuclei in pain processing. Experiment 1 explored the role of electrolytic lesions to the MT in the formalin test. It was hypothesized that animals with electrolytic lesions to the MT would have attenuated paw licking behavior during the second phase of the formalin tests as compared to sham lesion controls. This hypothesis was based on evidence of projections from the MD to the ACC, and previous research demonstrating attenuation of paw licking behavior in the second phase of the formalin test in animals with ACC lesions. Experiment 2 tested the effects of electrolytic MT lesions on mechanical paw withdrawal thresholds in the L5 nerve ligation model. It was hypothesized that lesions of the MT would not alter mechanical paw withdrawal thresholds. Experiment 3 tested the effects of electrolytic MT lesions on escape/avoidance behavior in the place escape avoidance paradigm. For experiment 1, animals with MT lesions were found to have slightly elevated paw licking behavior, but only across two time points. No differences in mechanical paw withdrawal thresholds and in escape/avoidance behavior were detected as compared to the sham lesion group. These results indicate a limited role for the medial thalamic nuclei in coding for pain intensity and the affective dimension of pain. Additional research is needed to explore the role of individual medial nuclei in pain processing.