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 anesthetize d rats. Few cells (3%) were classified as low-threshold-mechanorecepti ve ( LTM) neurons. The majority (97%) of somatosensory CPu and GP neur ons responded differentially or exclusively to noxious mechanical stim ulation of the skin. Nociceptive neurons were classified into the foll owing three groups on the basis of their response properties to noxiou s mechanical stimulation: wide-dynamic-range (WDR) neurons (21%); noci ceptive-specific (NS) neurons (67%); and inhibited (INH) neurons (13%) . 2. No differences in the response properties or in the proportions o f 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 microelectro de track. 3. The receptive fields of nociceptive CPu and GP neurons we re often large and bilateral; some receptive fields encompassed the en tire body. The trigeminal region, especially the perioral area, was in cluded in the receptive fields of nociceptive neurons more often (62 o f 63 cells) than any other part of the body. However, no preference fo r any particular division of the trigeminal nerve was observed in the receptive fields. Some neurons had receptive fields that were disconti nuous. 4. Noxious pinching of the skin significantly increased the spo ntaneous 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 decrease d their resting activity levels by an average of 43% after a noxious p inch. 5. The von Frey stimulus threshold of WDR neurons (11.0 g/mm2) w as significantly lower than that of NS neurons (33.6 g/mm2) and INH ne urons (32.6 g/mm2). Mean stimulus thresholds of WDR, NS, and INH neuro ns determined by using calibrated forceps were 1.6, 4.8, and 2.2 g/mm2 , respectively. 6. Individual stimulus-response functions of nocicepti ve neurons were best fit by a negatively accelerating (logarithmic) cu rves. However, WDR neurons had significantly steeper slopes than NS ne urons. 7. The results demonstrate that a large proportion of somatosen sory neurons within the neostriatum and globus pallidus (especially al ong the CPu-GP border) receive nociceptive information. These data are discussed in relation to several putative afferent nociceptive pathwa ys projecting to the CPu and GP. Because some WDR neurons have the abi lity to encode stimulus intensity, it is possible that these cells may play a role in grading motor responses to noxious stimulation. The in ability of NS and INH neurons to encode stimulus magnitude and the lar ge receptive fields of most nociceptive neurons in the CPu and GP sugg est that these neurons are involved with signaling the occurrence of n oxious stimuli and coordinating gross motor responses to noxious event s (e.g., withdrawal, orientation, and avoidance of painful stimuli).