Regulation of cannabinoid and mu opioid receptors in rat lumbar spinal cord following neonatal capsaicin treatment Andrea G. Hohmann*, Miles Herkenham Section on Functional Neuroanatomy, National Institute of Mental Health, Building 36, Room 2D15, Bethesda, MD 20892-4070, USA Received 15 May 1998; received in revised form 19 June 1998; accepted 19 June 1998 Abstract In vitro receptor binding and quantitative autoradiography were used to determine whether cannabinoid receptors in rat lumbar spinal cord are localized to the central terminals of nociceptive primary afferents. Rats were treated as neonates with capsaicin to destroy sensory C-fibers. The densities of cannabinoid and mu opioid receptors in the spinal cord of the adult rats were compared with age-matched vehicle controls. Neonatal capsaicin produced a moderate but reliable suppression (16%) of [ 3 H]CP55,940 binding to cannabinoid receptors. By contrast, the binding of [ 3 H][D-Ala 2 -MePhe 4 ,Gly-ol 5 ]enkephalin (DAMGO) to mu receptors was depleted by approximately 60% in near adjacent sections. These data suggest that only a subpopulation of cannabinoid receptors is situated on the central terminals of primary afferent C-fibers. The present data provide anatomical evidence for a dissociation between cannabinoid and mu opioid modulation of sensory transmission at the level of the primary afferent inputs to the spinal cord.  1998 Elsevier Science Ireland Ltd. All rights reserved Keywords: Cannabinoid; Mu opioid; C-fiber; Capsaicin; Autoradiography Several lines of evidence suggest that cannabinoids act at the spinal level to modulate pain. Intrathecal administration of cannabinoids produces potent analgesic effects in beha- vioral tests [21]. Cannabinoids also inhibit noxious stimu- lus-evoked activity in wide dynamic range neurons in the spinal dorsal horn after systemic [9] or spinal [10] admin- istration. Further support for a spinal site of analgesic action of cannabinoids is suggested by the ability of cannabinoids to suppress FOS protein-like immunoreactivity, a neuro- chemical marker for pain, in rat lumbar dorsal horn [20]. An anatomical substrate capable of mediating the beha- vioral effects of cannabinoids is also suggested by the pre- sence of cannabinoid receptors in the spinal dorsal horn [5,19]. The analgesic actions of cannabinoids may be attributed, at least in part, to the actions of these compounds to inhibit the release of neurotransmitters from the central and periph- eral terminals of primary afferents. For example, ananda- mide, a putative endogenous ligand for the central cannabinoid receptor, CB1, has been observed to inhibit capsaicin-evoked release of calcitonin gene-related peptide (CGRP) from rat isolated hind paw skin [16] and lumbar dorsal horn [15] in vitro. These data are consistent with the expression of CB1 mRNA in neuronal subpopulations of rat dorsal root ganglia that express nociceptive primary affer- ents markers, namely substance P and CGRP mRNAs [8]. These findings suggest that cannabinoid receptors may be situated on the central terminals of nociceptive primary afferents. The majority of opiate receptors in the superficial laminae are found presynaptically on the central terminals of pri- mary afferents. Unilateral dorsal rhizotomies produce marked ipsilateral decreases in the binding of mu, delta and kappa opioid receptors in the superficial laminae [2,18]. Moreover, destruction of sensory C-fibers with neo- natal capsaicin treatment depletes opioid receptor binding sites in the dorsal horn [4,13]. However, it is unclear whether cannabinoid receptors in the superficial dorsal horn are localized to presynaptic or postsynaptic sites. Con- sequently, identifying the locations of cannabinoid recep- tors in the spinal cord would provide an anatomical Neuroscience Letters 252 (1998) 13–16 0304-3940/98/$19.00  1998 Elsevier Science Ireland Ltd. All rights reserved PII S0304-3940(98)00534-5 * Corresponding author. E-mail: agh@codon.nih.gov