Pain in the Brain

Science's STKE  11 Dec 2007:
Vol. 2007, Issue 416, pp. tw449
DOI: 10.1126/stke.4162007tw449

Delaney et al. provide new insights into signaling mechanisms that may help explain the integration of pain with stress reactions and with cognitive and emotional processing of pain signals in the brain. The discoveries come from an analysis of neurons in the central amygdala, a brain region where emotional responses to sensory stimuli are derived. Neurons carrying nociceptive information from a region known as the parabranchial nucleus make large synapses on cells of the central amygdala. Neurotransmission at these sites in rat brain slices was inhibited by noradrenaline, which is released by noradrenergic afferent neurons in the region. Electrophysiological experiments showed that noradrenaline acted presynaptically and appeared not to affect the probability of vesicle release but rather to limit the number of active release sites within the synapse. Experiments with inhibitors indicated that the noradrenaline acted at α2-adrenergic receptors coupled to Gi- or Go-type heterotrimeric guanine nucleotide-binding proteins (G proteins). Loading of presynaptic terminals with a membrane-permeable peptide that binds to G protein βγ subunits inhibited the effects of noradrenaline, implicating Gβγ subunits in the process. Imaging of intracellular calcium concentrations with fluorescent dye showed that noradrenaline’s effects were independent of calcium concentrations. Thus, the Gβγ subunits appeared not to act by modulating channel activity but possibly by interacting with the vesicle release machinery. The actions of exogenous noradrenaline were also reproduced in experiments where release of noradrenaline from endogenous neurons was electrically stimulated. Tully et al. point out in commentary on the paper that this calcium-independent regulation might be particularly appropriate to reduce painful sensations during stressful situations that cause release of noradrenaline. Repetitive neuronal firing might gradually increase intracellular calcium concentrations sufficiently to overcome calcium-mediated inhibitory mechanisms, whereas the noradrenaline effect should be maintained even in the presence of rapid firing of presynaptic neurons.

A. J. Delaney, J. W. Crane, P. Sah, Noradrenaline modulates transmission at a central synapse by a presynaptic mechanism. Neuron 56, 880-892 (2007). [PubMed]

K. Tully, Y. Li, V. Y. Bolshakov, Keeping in check painful synapses in central amygdala. Neuron 56, 757-759 (2007). [PubMed]