Eric P. Wiertelak

Characterization of cytokine-induced hyperalgesia

Agents which induce symptoms of illness, such as lipopolysaccharide (LPS), cause diverse effects including hyperalgesia. While previous studies have examined central pathways mediating LPS hyperalgesia, the initial steps in activating this system remain unknown. Since LPS induces the release of various cytokines and eicosinoids from immune cells, the present series of experiments examined the potential involvement of these substances in LPS hyperalgesia. This work demonstrates that: (a) Interleukin-1 beta (IL-1 beta) can produce hyperalgesia following either intraperitoneal or intracerebroventricular injection. In contrast, IL-1 beta delivered intrathecally did not affect pain responsivity. (b) Liver macrophages (Kupffer cells) appear to be critically involved, and relay signals to the brain via hepatic vagal afferents. (c) Both IL-1 beta and tumor necrosis factor appear to be critical mediators of LPS hyperalgesia. In contrast, prostaglandins do not appear to be involved. Taken together, these studies suggest that substances classically thought of as products of the immune system may dynamically enhance pain responsivity via actions either on the hepatic vagus or at central sites.

Neurocircuitry of illness-induced hyperalgesia

We have previously demonstrated that illness-inducing agents such as lithium chloride (LiCl) and the bacterial cell wall endotoxin lipopolysaccharide (LPS) produce hyperalgesia on diverse pain measures. The present series of studies attempted to identify the neurocircuitry mediating these effects. These studies have demonstrated that illness-inducing agents produce hyperalgesia by activating: (a) peripheral nerves rather than by generating a blood-borne mediator (Expt. 1); (b) vagal afferents, specifically afferents within the hepatic branch of the vagus (Expt. 2); (c) as yet unidentified brain site(s) rostral to the mid-mesencephalon (Expt. 6); (d) a centrifugal pathway that arises from the nucleus raphe magnus, and not from the adjacent nucleus reticularis paragigantocellularis pars alpha (Expts. 4 and 5); (e) a centrifugal pathway in the dorsolateral funiculus of the spinal cord (Expt. 3); and (f) the same centrifugal pathways for diverse illness inducing agents (Expts. 3, 7 and 8). These data call for the re-evaluation of a number of assumptions inherent in previous studies of hyperalgesia.

Interleukin-1 mediates the behavioral hyperalgesia produced by lithium chloride and endotoxin

The sickness-inducing agents lithium chloride (LiCl) and lipopolysaccharide (LPS) produce a long-lasting facilitation of the nociceptive tailflick reflex. Many of the behavioral and physiological changes produced by illness are mediated by interleukin-1 (IL-1) released from monocytes stimulated by the pathogenic substance. Monocytes also produce an IL-1 receptor antagonist (IL-1ra) which has been sequenced and cloned. The present experiments report that IL-1 can itself produce hyperalgesia as assessed by tailflick to radiant heat, and that recombinant IL-1ra blocks the hyperalgesia produced by LiCl and LPS.

The role of the amygdala and dorsal raphe nucleus in mediating the behavioral consequences of inescapable shock

It has been argued that exposure to inescapable shock produces later behavioral changes such as poor shuttle box escape learning because it leads to the conditioning of intense fear, which later transfers to the shuttle box test situation and interferes with escape. Both fear, as assessed by freezing, and escape were measured in Sprague-Dawley rats 24 hr after exposure to inescapable shock. Lesions of the basolateral region and central nucleus of the amygdala eliminated the fear that transfers to the shuttle box after inescapable shock, as well as the fear conditioned in the shuttle box by the shuttle box shocks. However, the amygdala lesions did not reduce the escape learning deficit produced by inescapable shock. In contrast, dorsal raphe nucleus lesions did not reduce the fear that transfers to the shuttle box after inescapable shock, but eliminated the enhanced fear conditioning in the shuttle box as well as the escape deficit. The implications of these results for the role of fear and anxiety in mediating inescapable shock effects are discussed.

Subcutaneous formalin produces centrifugal hyperalgesia at a non-injected site via the NMDA-nitric oxide cascade

Previous work has demonstrated that pain facilitation can occur following injection of subcutaneous irritants, such as formalin. Such studies have focused on apparent pain facilitation induced at the site of irritant injection. Changes in processing of incoming pain information have typically been assumed to result from activation of neurocircuitry intrinsic to the spinal cord. The present series of studies have examined hyperalgesia exhibited at a site distant from the site of irritant injection and have begun to define the neurocircuitry and neuropharmacology underlying this pain enhancement. This work demonstrates that s.c. formalin injected into the dorsum of one hindpaw in rats produces prolonged hyperalgesia as measured by the tailflick test. Hyperalgesia is not mediated solely by circuitry intrinsic to the spinal cord, but rather involves activation of centrifugal pathways originating within the brain and descending to the spinal cord via pathway(s) outside of the dorsolateral funiculus. At the level of the spinal cord, this hyperalgesic state is mediated by an NMDA-nitric oxide cascade, since hyperalgesia can be abolished by administration of either an NMDA antagonist (APV) or a nitric oxide synthesis inhibitor (L-NAME).

Comparison of the effects of nucleus tractus solitarius and ventral medial medulla lesions on illness-induced and subcutaneous formalin-induced hyperalgesias

We have previously demonstrated that illness-inducing agents (lipopolysaccharide (LPS)) and inflammatory agents (subcutaneous (s.c.) formalin) induce hyperalgesia by similar pathways. The present series of experiments compared the effects of medullary lesions on these phenomena. These experiments demonstrate that s.c. formalin-induced hyperalgesia, like illness-induced hyperalgesia, is dependent on the nucleus raphe magnus (NRM) but independent of the nucleus reticularis paragigantocellularis (NRPgc). However, these two forms of hyperalgesia differ with regards to their dependence on the nucleus tractus solitarius (NTS). Illness-induced hyperalgesia is abolished by unilateral (left) NTS lesions, whereas formalin-induced hyperalgesia remains unaffected by this procedure. These data provide further evidence that hyperalgesias induced by illness agents and by inflammatory agents are mediated by similar but not identical pathways. They also illustrate that neural structures have the capacity for opposed actions, in that both the NTS and NRM are documented to underlie hyperalgesia as well as analgesia. This capacity for opposed action may prove to be characteristic of structures involved in pain modulation.

Illness-induced hyperalgesia is mediated by a spinal NMDA-nitric oxide cascade

A variety of experimental manipulations produce enhanced pain responsivity. Recent work has demonstrated that activation of N-methyl-D-aspartate (NMDA) receptors in the spinal cord can produce persistent enhancement of pain via production of nitric oxide and/or prostaglandins. To date, the behavioral paradigms used to study NMDA mediated hyperalgesia have all involved direct excitation of spinal cord dorsal horn neurons via activation of primary nociceptive afferents. The present series of experiments examined whether the NMDA cascade would also be activated by events that do not produce direct pain input to the spinal cord dorsal horn. The hyperalgesia-inducing paradigm used was intraperitoneal lipopolysaccharide (LPS), which causes transient illness. Prior work has shown that LPS induces hyperalgesia via activation of hepatic vagal afferents to the brain, thereby activating a centrifugal pain facilitory circuit. The present study demonstrates that this centrifugal hyperalgesia is produced via activation of the NMDA-nitric oxide cascade at the level of the spinal cord.

The amygdala is necessary for the expression of conditioned but not unconditioned analgesia

Lesions of the basolateral region and central nucleus of the amygdala prevent conditioned analgesia (Helmstetter, 1992). In general, these regions of the amygdala are more critically involved in the expression of conditioned reactions to aversive events than in the mediation of unconditioned reactions. The impact of amygdala lesions on both conditioned and unconditioned analgesia was explored in Sprague-Dawley rats. The lesions completely prevented the expression of conditioned analgesia, but had no effect on unconditioned analgesia.

Peak shift revisited: A test of alternative interpretations

In Experiment 1, 2 groups of human subjects were trained to respond to 1 of 2 light intensity stimuli, S2 or S4, and then were tested for generalization with a randomized series of increasing values from S1 to S11. Both groups, including the group trained to respond to dimmer value, showed peak shifts to a brighter more centrally located test stimulus. In Experiment 2, which used line angle stimuli, both the size of the difference between S+ and S- and the range of test stimuli that extended beyond S+ were varied. The larger the S(+)-S- separation and the larger the range, the greater was the peak shift obtained. In Experiment 3, training involved an S- (line angle) surrounded by 2 S+ values with testing symmetrical about the training values and covering either a narrow or a wide range. The wide range produced greater peak shifts in both directions from S-. All 3 experiments support an adaptation-level interpretation of intradimensional discrimination learning and generalization test performance in human subjects. Related work with animals suggests the presence of similar processes.

Neurocircuitry of conditioned inhibition of analgesia: Effects of amygdala, dorsal raphe, ventral medullary, and spinal cord lesions on antianalgesia in the rat.

Pain inhibition (analgesia) is produced by learned danger signals and inhibited by learned safety signals (antianalgesia). Conditioned analgesia is mediated by brain-to-spinal pathways releasing spinal endogenous opiates. Spinal morphine mimics learned danger signals in producing analgesia, which is inhibited by antianalgesia. The circuitry mediating antianalgesia is unknown. These experiments demonstrate that raphe dorsalis, raphe magnus, and spinal dorsolateral funiculus lesions abolish antianalgesia. Other lesions had no effect on antianalgesia. More important, lesions that blocked development of conditioned analgesia did not block development of antianalgesia. Thus, neural circuitries mediating analgesia and antianalgesia were found to be distinct, and conditioned inhibition of analgesia was found to act by inhibiting the most distal part of the conditioned analgesia circuit, namely, the spinal cord.