Pharmacology Biochemistry and Behavior, Vol. 63, No. 4, pp. 581–588, 1999 © 1999 Elsevier Science Inc. Printed in the USA. All rights reserved 0091-3057/99/$–see front matter PII S0091-3057(99)00032-5 581 Extracellular Glutamate in the Dorsal Horn of the Lumbar Spinal Cord in the Freely Moving Rat During Hindlimb Stepping W. M. WALWYN,* J. TA-HUANG,* L. ACKERSON,† N. T. MAIDMENT† AND V. R. EDGERTON* *Department of Physiological Science, UCLA, 621 Circle Drive South, Los Angeles, CA 90095, and †Department of Psychiatry and Biobehavioral Sciences, Neuropsychiatric Institute, UCLA School of Medicine, Los Angeles, CA 90024-1759 Received 10 July 1998; Revised 17 December 1998; Accepted 15 January 1999 WALWYN, W. M., J. TA-HUANG, L. ACKERSON, N. T. MAIDMENT AND V. R. EDGERTON. Extracellular glutamate in the dorsal horn of the lumbar spinal cord in the freely moving rat during hindlimb stepping. PHAMACOL BIO- CHEM BEHAV 63(4) 581–588, 1999.—The capacity to reestablish locomotor function after complete spinal cord transection in the adult mammal is now well documented. Further studies have shown different neurotransmitters to be involved in the initiation and maintenance of these locomotor patterns. However, there has been no in vivo evidence of the changes in glutamate or any other neurotransmitter in the extracellular space of the dorsal horn during an alternating motor pattern such as hindlimb stepping. This study describes an in vivo microdialysis technique to measure extracellular glutamate in the dorsal horn of the spinal cord in the fully awake intact rat. A concentric microdialysis probe was placed in the dorsal horn at L5, and 18 h later dialysate samples were collected at 20-min intervals before, during, and after 20 min of hindlimb stepping. During stepping, extracellular glutamate rose 150% above resting levels and returned to resting levels 40 min later. This in- crease may have occurred either as a result of primary afferent depolarization or modulation by the descending and ascend- ing supraspinal pathways. In another series of experiments extracellular glutamate was, therefore, measured in the dorsal horn of the chronic spinally transected rat during 20 min of hindlimb stepping. Although the spinal group did not take as many steps as the intact group, those taking more than 40 steps showed a significant rise in extracellular glutamate, and the number of steps taken by the individual spinal rats correlated positively with the individual values of extracellular glutamate (r 2 = 0.63). These results are consistent with glutamate being an important neurotransmitter in the spinal cord in normal locomotion. © 1999 Elsevier Science Inc. Glutamate Locomotion Microdialysis Spinal cord Spinal transection THE capacity to reestablish locomotor function after com- plete spinal cord transection in the adult mammal is now well- documented (3,35). Further ability of the spinal cord to inte- grate and execute complex motor patterns in the absence of supraspinal influence in acute and chronic spinally transected animals may be altered by the application of various pharma- cological agents targeting different receptors (4). These stud- ies, as well as many of fictive locomotor patterns, suggest that hindlimb locomotion results from a complex interplay of dif- ferent neurotransmitters and neuromodulators within the spi- nal cord. Although there has been no in vivo information re- garding glutamatergic transmission during alternating locomotor patterns of the left and right hindlimbs, or hindlimb stepping, in vitro studies indicate that glutamate and the glutamatergic receptors are involved in the rhythmic properties of hindlimb locomotion. When applied to an isolated neonatal spinal cord in situ, L-glutamate and agonists of the glutamatergic recep- tors induce fictive locomotion in the neonatal rat (1,10,14,32), chick (2), and lamprey (13,31). Glutamate, an excitatory amino acid, is found extensively throughout the mammalian spinal cord. It is found in neurons of the dorsal horn and dorsal root ganglia (5,66), stored in no- ciceptive and nonnociceptive primary afferent terminals (8), interneurons (42), and motoneurons (9,41). A number of de- Requests for reprints should be addressed to W. Walwyn, PhD., Department of Psychiatry and Biobehavioral Sciences, Neuropsychiatric In- stitute, UCLA School of Medicine, Los Angeles, CA 90024-1759.