Comment

Pain control: breaking the circuit

Background & Aims: The endogenous opioid system is involved in modulating the experience of pain, the response to stress, and the action of analgesic therapies. Recent human imaging studies have shown a significant tonic modulation of visceral pain, raising the question of whether endogenous opioids tonically modulate the pain of visceral cancer. Methods: Transgenic mice expressing the first 127 amino acids of simian virus 40 large T antigen, under the control of the rat elastase-1 promoter, that spontaneously develop pancreatic cancer were used to investigate the role of endogenous opioids in the modulation of pancreatic cancer pain. Visceral pain behaviors were assessed as degree of hunching and vocalization. Results Although mice with late-stage pancreatic cancer displayed spontaneous, morphine-reversible, visceral pain-related behaviors such as hunching and vocalization, these behaviors were absent in mice with early-stage pancreatic cancer. After systemic administration of the central nervous system (CNS)-penetrant opioid receptor antagonists naloxone or naltrexone, mice with early-stage pancreatic cancer displayed significant visceral pain-related behaviors, whereas systemic administration of the CNS-nonpenetrant opioid antagonist naloxone-methiodide did not induce an increase in visceral pain behaviors. Conclusions Our findings suggest that a CNS opioid-dependent mechanism tonically modulates early and late-stage pancreatic cancer pain. Understanding the mechanisms that mask this pain in early stage disease and drive this pain in late-stage disease may allow improved diagnosis, treatment, and care of patients with pancreatic cancer.

The nociceptive nature of spinal dorsal horn neurons expressing NK1 and γ-aminobutyric acid (GABA)_B receptors was evaluated in the rat. Immunodetection of the Fos protein, induced by noxious mechanical stimulation of the skin, was combined with immunocytochemistry for NK1 or GABA_B receptors (double-immunostaining study) or both receptors (triple-immunostaining study). Neurons double-labeled for Fos and for each receptor largely prevailed in lamina I. The proportions of Fos-positive cells immunostained for NK1 or GABA_B receptors were higher in lamina I than in the remaining spinal laminae. More Fos-positive cells were immunoreactive (IR) for GABA_B receptors than for NK1 in all dorsal horn laminae. In the triple-immunostaing study, co-localization of NK1 and GABA_B receptors occurred only in lamina I and was higher in neurons expressing Fos. As to the morphological lamina I cell class, NK1-positive cells belonged mainly to the fusiform type while similar proportions of fusiform, pyramidal and flattened NK1 neurons expressed GABA_B receptors. No differences were found between those cell types as to the degree of nociceptive activation. The present results suggest that the co-localization of NK1 and GABA_B receptors is a common feature of fusiform, pyramidal and flattened neurons in lamina I. Considering the participation of the three cell classes in various ascending systems, it is concluded that a simultaneous action of substance P (SP) and GABA may play an important role in the modulation of nociceptive input supraspinally transmitted from lamina I.

We have entered a new era in understanding CNS circuitry involved in acute and chronic pain. The ability to objectively measure a pain or analgesic state of the brain using non-invasive methods that define neural activation provides the possibility for top-down approaches to drug discovery. These brain maps represent the specific brain state. In the future, correlations with such states and behavioral, genetic, epigenetic or other chemical markers may help define specific diagnostic tools and novel approaches to drug discovery.

Aversion to capsaicin (0.1–10 ppm) was assessed using a two-bottle paired preference paradigm, before and after intracisternal injection of substance P conjugated to saporin (SP-SAP) to ablate neurons in superficial medullary and cervical dorsal horn that express NK-1 receptors. Before SP-SAP, there was a concentration-dependent decrease in consumption of capsaicin with a threshold of 0.1–0.3 ppm. Following SP-SAP, significantly more capsaicin solution was consumed at 1- and 10-ppm concentrations. These results support a role for substance P in the mediation of high, but not low, levels of capsaicin-induced oral irritation.

In the normal urinary bladder, tachykinins (TKs) are expressed in a population of bladder nociceptors that is sensitive to the excitatory and desensitizing effects of capsaicin (i.e., capsaicin-sensitive primary afferent neurons (CSPANs)). Several endobiotics or xenobiotics excite CSPANs and release TKs and other mediators at both the peripheral and spinal cord level. The peripheral release of TKs determines a set of responses (known as neurogenic inflammation) that includes vasodilatation, plasma protein extravasation, smooth muscle contraction and stimulation of afferent nerves. Following chronic inflammation, both immune cells and capsaicin-resistant sensory neurons can de novo express TKs: whether these pools of TKs are releasable and contribute to inflammatory processes is presently unsettled. At the spinal cord level, the release of TKs contributes in determining an altered pattern of vesicourethral reflexes in response to nociceptive stimulation of the bladder by conveying: (a) the afferent transmission to supraspinal sites, and (b) descending or sensory inputs to the sacral parasympathetic nucleus (SPN). Recent evidence also attribute a synergetic role of TKs in the supraspinal modulation of the sensory arm of the micturition reflex.

The overall available information suggests that TK receptor antagonists may affect bladder motility/reflexes which occur during different pathological states, while having little influence on the normal motor bladder function.

Diffuse noxious inhibitory control (DNIC) is a phenomenon whereby wide dynamic range neurons are selectively and powerfully inhibited through the central nervous system by noxious stimuli heterotopically applied to a body area distant from their excitatory receptive fields. Previous work has shown that systemic administration of an α1-adrenoceptor agonist, phenylephrine (PE), blocked the DNIC. We hypothesized that descending inhibitory pathways mediate the DNIC mechanism and that the neural network of the DNIC loop exists in the middle brainstem, likely in a more rostral part than formerly assumed, possibly the nucleus raphe magnus (RMg). The aim of this study was to determine whether DNIC is directly modulated by PE when administered close to the RMg.

The experiments were performed on anesthetized male Sprague-Dawley rats. For administration of different drugs close to the RMg, the tip of a 33-gauge cannula was placed into an area close to the RMg as determined using the atlas of Paxinos and Watson. Single square-wave electrical stimuli were applied to the digits of the left hindpaw. The C-fiber reflex response elicited by electrical stimulation within the receptive field of the ipsilateral sural nerve was recorded from the biceps femoris muscle in the absence and presence of noxious tail immersion in warm water at 50°C. The DNIC effect was calculated from a recorded electromyogram as the “inhibition rate.” Saline (0.05 μL) or PE (0.05 μg/0.05 μL) was microinjected close to the RMg through the cannula. The C-fiber reflex evoked by electromyographic activity was recorded the same way. The inhibition rate of the C-fiber reflex was compared before and after administration of drugs. A paired t test was used for statistical comparison between same drug administration groups, and 1-way analysis of variance and Bonferroni multiple comparison were used for statistical analysis between different drugs. At the end of all experiments, the tissue-contacting end of the cannula tip was cauterized with an electric current to localize the drug administration site. The brain was removed, sliced in coronal sections, and stained with hematoxylin and eosin.

The C-fiber reflex inhibited by noxious thermal stimuli (DNIC) was significantly blocked after the injection of PE close to the RMg.

Direct administration of PE close to the RMg inhibited DNIC, thereby affecting and modulating the intrinsic pain inhibition system. These findings suggest that the RMg may be involved in the regulation of DNIC.