The role of the basal ganglia in nociception and pain.

The involvement of the basal ganglia in motor functions has been well studied. Recent neurophysiological, clinical and behavioral experiments indicate that the basal ganglia also process non-noxious and noxious somatosensory information. However, the functional significance of somatosensory information processing within the basal ganglia is not well understood. This review explores the role of the striatum, globus pallidus and substantia nigra in nociceptive sensorimotor integration and suggests several roles of these basal ganglia structures in nociception and pain. Electrophysiological experiments have detailed the non-nociceptive and nociceptive response properties of basal ganglia neurons. Most studies agree that some neurons within the basal ganglia encode stimulus intensity. However, these neurons do not appear to encode stimulus location since the receptive fields of these cells are large. Many basal ganglia neurons responsive to somatosensory stimulation are activated exclusively or differentially by noxious stimulation. Indirect techniques used to measure neuronal activity (i.e., positron emission tomography and 2-deoxyglucose methods) also indicate that the basal ganglia are activated differentially by noxious stimulation. Neuroanatomical experiments suggest several pathways by which nociceptive information may reach the basal ganglia. Neuroanatomical studies have also indicated that the basal ganglia are rich in many different neuroactive chemicals that may be involved in the modulation of nociceptive information. Microinjection of opiates, dopamine and gamma-aminobutyric acid (GABA) into the basal ganglia have varied effects on pain behavior. Administration of these neurochemicals into the basal ganglia affects supraspinal pain behaviors more consistently than spinal reflexive behaviors. The reduction of pain behavior following electrical stimulation of the substantia nigra and caudate nucleus provides additional evidence for a role of the basal ganglia in pain modulation. Some patients with basal ganglia disease (e.g., Parkinson's disease, Huntington's disease) have alterations in pain sensation in addition to motor abnormalities. Frequently, these patients have intermittent pain that is difficult to localize. Collectively, these data suggest that the basal ganglia may be involved in the (1) sensory-discriminative dimension of pain, (2) affective dimension of pain, (3) cognitive dimension of pain, (4) modulation of nociceptive information and (5) sensory gating of nociceptive information to higher motor areas. Further experiments that correlate neuronal discharge activity with stimulus intensity and escape behavior in operantly conditioned animals are necessary to fully understand how the basal ganglia are involved in nociceptive sensorimotor integration.