NOCICEPTIVE RESPONSES IN THE NEOSTRIATUM AND GLOBUS-PALLIDUS OF THE ANESTHETIZED RAT

1. Extracellular recordings were made from neurons in the neostriatum (caudate nucleus-putamen, CPu) and globus pallidus (GP) of anesthetized rats. Few cells (3%) were classified as low-threshold-mechanoreceptive ( LTM) neurons. The majority (97%) of somatosensory CPu and GP neurons responded differentially or exclusively to noxious mechanical stimulation of the skin. Nociceptive neurons were classified into the following three groups on the basis of their response properties to noxious mechanical stimulation: wide-dynamic-range (WDR) neurons (21%); nociceptive-specific (NS) neurons (67%); and inhibited (INH) neurons (13%). 2. No differences in the response properties or in the proportions of WDR, NS, and INH neurons were found in the CPu compared with the GP. Nociceptive neurons were located most often along the CPu-GP border. Additionally, neurons of similar functional classification were often clustered within 200-400 mum of each other along a single microelectrode track. 3. The receptive fields of nociceptive CPu and GP neurons were often large and bilateral; some receptive fields encompassed the entire body. The trigeminal region, especially the perioral area, was included in the receptive fields of nociceptive neurons more often (62 of 63 cells) than any other part of the body. However, no preference for any particular division of the trigeminal nerve was observed in the receptive fields. Some neurons had receptive fields that were discontinuous. 4. Noxious pinching of the skin significantly increased the spontaneous neuronal discharge of WDR and NS neurons by an average of 482 and 221%, respectively. There were no significant differences between the discharge adaptation rates of WDR and NS neurons. Afterdischarge activity was observed in some WDR and NS neurons. INH neurons decreased their resting activity levels by an average of 43% after a noxious pinch. 5. The von Frey stimulus threshold of WDR neurons (11.0 g/mm2) was significantly lower than that of NS neurons (33.6 g/mm2) and INH neurons (32.6 g/mm2). Mean stimulus thresholds of WDR, NS, and INH neurons determined by using calibrated forceps were 1.6, 4.8, and 2.2 g/mm2, respectively. 6. Individual stimulus-response functions of nociceptive neurons were best fit by a negatively accelerating (logarithmic) curves. However, WDR neurons had significantly steeper slopes than NS neurons. 7. The results demonstrate that a large proportion of somatosensory neurons within the neostriatum and globus pallidus (especially along the CPu-GP border) receive nociceptive information. These data are discussed in relation to several putative afferent nociceptive pathways projecting to the CPu and GP. Because some WDR neurons have the ability to encode stimulus intensity, it is possible that these cells may play a role in grading motor responses to noxious stimulation. The inability of NS and INH neurons to encode stimulus magnitude and the large receptive fields of most nociceptive neurons in the CPu and GP suggest that these neurons are involved with signaling the occurrence of noxious stimuli and coordinating gross motor responses to noxious events (e.g., withdrawal, orientation, and avoidance of painful stimuli).