Differential release of neurotransmitters from superficial and deep layers of the dorsal horn in response to acute noxious stimulation and inflammation of the rat paw Natalia Dmitrieva a, * , Antonio J. Rodr  ıguez-Malaver b , Jackeline P erez c , Luis Hern andez c a Program in Neuroscience, Florida State University, Tallahassee, FL 32306-1270, USA b Laboratory of Biochemical Adaptation, Los Andes University, M erida 5101, Venezuela c Laboratory of Physiology of Behavior, Los Andes University, M erida 5101, Venezuela Received 7 January 2003; accepted 1 September 2003 Abstract Experimental evidence suggests that release of neurotransmitters in response to acute noxious stimulation and inflammation can differ in superficial and deeper dorsal horn (DH) laminae. Using two different microdialysis probes, we studied changes in levels of glutamate, aspartate, arginine and GABA in dialysates collected from the surface of the spinal cord and within the DH induced by pinching the paw or paw inflammation. In penthotal anaesthetized rats, a flexible microdialysis probe was placed on the dorsal surface of the L4–L5 or L6–S2 spinal segments. In other rats, a rigid microdialysis probe was implanted within the DH of the same segments. Samples were collected every minute before, during and after pinching the hind paw (acute pain), and every half an hour after injecting either carrageenan or saline into the same paw (inflammation-induced pain). Amino acids were measured by capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIFD). Pinching the paw induced a significant but short lasting increase in extracellular glutamate and aspartate in dialysates from the surface of the DH. Carrageenan, but not saline, injected into the paw significantly increased concentrations of glutamate, aspartate and arginine both on the surface and within the DH of L4–L5 and also within the DH of the L6–S2 segments. The GABA level was significantly increased following carrageenan only within the DH. The maximum increase on the surface was detected 60–120 min after the onset of inflammation whereas the response within the DH reached a maximum between 150 and 180 min after carrageenan. These results indicate that unlike acute mechanical noxious stimulation which enhances amino acid neu- rotransmitters in surface dialysate, inflammation induced neurotransmitter release in all layers of the DH suggesting sensiti- zation of the DH. Ó 2003 European Federation of Chapters of the International Association for the Study of Pain. Published by Elsevier Ltd. All rights reserved. Keywords: Dorsal horn; Glutamate; GABA; Inflammation; Sensitization 1. Introduction Noxious stimulation induces glutamate (Glu) and aspartate (Asp) release in the dorsal horn (DH) (Burit- ova et al., 1996a; Chapman et al., 1995; Chapman and Dickenson, 1995; Davies and Watkins, 1983; Malmberg and Yaksh, 1995; Okuda et al., 2001; Skilling et al., 1988; Sluka and Westlund, 1992; Yaksh et al., 1999). Spinal GABA administration prevents nociception-re- lated release of excitatory amino acids (Sluka et al., 1994) and c-fos expression (Buritova et al., 1996b). Moreover, excitatory amino acids activate nociceptive specific and wide dynamic range DH spinothalamic neurons (Dougherty et al., 1992b,a, 1993). Therefore, it is believed that these amino acids play an important role in nociceptive information transmission in the spinal cord (Dickenson et al., 1997; Millan, 1999). In spinal cord slices, DH neurons in laminae I–II but not in deeper laminae are rapidly excited by both application of Glu and Asp (Schneider and Perl, 1985, European Journal of Pain 8 (2004) 245–252 www.EuropeanJournalPain.com * Corresponding author. Tel.: +1-850-644-5884; fax: +1-850-644- 9874. E-mail address: dmitrieva@psy.fsu.edu (N. Dmitrieva). 1090-3801/$30 Ó 2003 European Federation of Chapters of the International Association for the Study of Pain. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ejpain.2003.09.001