Targeting analgesic drugs for spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn and this output informs the brain as to the peripheral environment. This encoding process is subject to strong upregulation resulting in hyperesthetic states and downregulation reducing the ongoing processing of nociceptive stimuli reversing the hyperesthesia and pain processing. The present review addresses the biology of spinal nociceptive processing as relevant to the effects of intrathecally-delivered drugs in altering pain processing following acute stimulation, tissue inflammation/injury and nerve injury. The review covers i) the major classes of spinal agents currently employed as intrathecal analgesics (opioid agonists, alpha 2 agonists; sodium channel blockers; calcium channel blockers; NMDA blockers; GABA A/B agonists; COX inhibitors; ii) ongoing developments in the pharmacology of spinal therapeutics focusing on less studied agents/targets (cholinesterase inhibition; Adenosine agonists; iii) novel intrathecal targeting methodologies including gene-based approaches (viral vectors, plasmids, interfering RNAs); antisense, and toxins (botulinum toxins; resniferatoxin, substance P Saporin); and iv) issues relevant to intrathecal drug delivery (neuraxial drug distribution), infusate delivery profile, drug dosing, formulation and principals involved in the preclinical evaluation of intrathecal drug safety.Keywords: Adenovirus transfection, dorsal horn, neurotoxins, pain pathways, spinal drug delivery, spinal analgesics, toxicity.
RATIONALETargeting analgesic drugs for direct spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn. Thus, high intensity (e.g., nociceptive) stimulation activates populations of small primary afferents, generating an intensity-dependent increase in activity of second order dorsal horn projection neurons. This excitation is transmitted through spinofugal projection pathways to higher centers, such as the somatosensory thalamus -somatosensory cortex, and to the medial thalamus -limbic cortices that are respectively believed to underlie the sensory-discriminative and affectivemotivational components of the pain experience [1-3]. The dorsal horn encoding process reflects a remarkable plasticity wherein the input-output function of the spinal dorsal horn is subject to pronounced regulation by local neuronal and nonneuronal circuits as well as supraspinal (bulbospinal) input. Thus, following tissue or nerve injury there is an enhanced neuronal response to moderate or low intensity stimuli, e.g., *Address correspondence to this author at the University of California, San Diego, Anesthesia Research Lab 0818, 9500 Gilman Dr. LaJolla, CA 92093, USA; ...
