Modulation by angiotensin III of nociception-related and arterial pressure-related neuronal responsiveness in the nucleus reticularis gigantocellularis of the rat

Angiotensin III modulates the nociceptive control mediated by the periaqueductal gray matter

Endogenous angiotensin (Ang) II and/or an Ang II-derived peptide, acting on Ang type 1 (AT(1)) and Ang type 2 (AT(2)) receptors, can carry out part of the nociceptive control modulated by periaqueductal gray matter (PAG). However, neither the identity of this putative Ang-peptide, nor its relationship to Ang II antinociceptive activity was clarified. Therefore, we have used tail-flick and incision allodynia models combined with an HPLC time course of Ang metabolism, to study the Ang III antinociceptive effect in the rat ventrolateral (vl) PAG using peptidase inhibitors and receptor antagonists. Ang III injection into the vlPAG increased tail-flick latency, which was fully blocked by Losartan and CGP 42,112A, but not by divalinal-Ang IV, indicating that Ang III effect was mediated by AT(1) and AT(2) receptors, but not by the AT(4) receptor. Ang III injected into the vlPAG reduced incision allodynia. Incubation of Ang II with punches of vlPAG homogenate formed Ang III, Ang (1-7) and Ang IV. Amastatin (AM) inhibited the formation of Ang III from Ang II by homogenate, and blocked the antinociceptive activity of Ang II injection into vlPAG, suggesting that aminopeptidase A (APA) formed Ang III from Ang II. Ang III can also be formed from Ang I by a vlPAG alternative pathway. Therefore, the present work shows, for the first time, that: (i) Ang III, acting on AT(1) and AT(2) receptors, can elicit vlPAG-mediated antinociception, (ii) the conversion of Ang II to Ang III in the vlPAG is required to elicit antinociception, and (iii) the antinociceptive activity of endogenous Ang II in vlPAG can be ascribed preponderantly to Ang III.

A new role for the renin-angiotensin system in the rat periaqueductal gray matter: angiotensin receptor-mediated modulation of nociception

Renin-angiotensin (Ang) system (RAS) peptides injected into the periaqueductal gray matter (PAG) elicit antinociception. Saralasin blocks Ang II-elicited antinociception. Thus, it is possible that endogenous RAS peptides could participate on the modulation of nociception in the PAG. This possibility was tested here injecting, in the PAG, the specific Ang type 1 and type 2 receptor (AT1 receptor and AT(2 receptor) antagonists losartan and CGP42,112A, respectively, either alone or before Ang II. The effects of Ang II, losartan and CGP42,112A on nociception were measured using the tail flick test and the model of incision allodynia. Ang II increased tail-flick latency, an effect inhibited by both losartan and CGP42,112A. Ang II reduced incisional allodynia. Either losartan or CGP42,112A alone increased incision allodynia, suggesting that endogenous Ang II and/or an Ang-peptide participates in the control of allodynia by the PAG. AT1 and AT2 receptors were immunolocalized in neuronal cell bodies and processes in the ventrolateral PAG. Taken together, the antinociceptive effect of Ang II injection into the ventrolateral PAG, the increase of allodynia elicited by injecting either losartan or CGP42,112A alone in the PAG, and the presence of AT1 and AT2 receptors in neurons and neuronal processes in the same region, represent the first evidence that part of the tonic nociceptive control mediated by the PAG is carried out locally by endogenous Ang II and/or an Ang-peptide acting on AT1 and AT2 receptors.

Effects of naloxone on rewarding and aversive brain sites

The effects of naloxone on the rewarding and aversive properties of brain stimulation derived from the ventral tegmental area and the nucleus reticularis gigantocellularis, respectively, were assessed in rats, based on the following measures-the current threshold for latency to escape aversive nucleus reticularis gigantocellularis stimulation, the frequency threshold for rewarding ventral tegmental area stimulation, and the frequency threshold for self-stimulation obtained from delivery of concurrent ventral tegmental area and nucleus reticularis gigantocellularis stimulation, before and after three systemic doses of naloxone (0, 10, and 20mg/kg); in the latter case, the stimulation trains were interdigitated with an interpulse interval of 2 ms. Initially, thresholds for concurrent stimulation were elevated relative to the values obtained for ventral tegmental area stimulation alone, returning to baseline values only when the nucleus reticularis gigantocellularis stimulation no longer induced escape. After each pairing of the two sites, the current threshold for escape gradually increased until the maximum value administered, 700 microA, at which point aversive responses were no longer observed. This required very few pairings, between one and five trials across animals. Drug tests were then begun and produced a significant dose-response threshold increase across animals, without reinstating the latency to escape nucleus reticularis gigantocellularis stimulation. These findings are discussed in terms of a dissociation between the analgesic and rewarding properties of ventral tegmental area stimulation.

Factors that influence the persistence of stimulation-induced aversion

Brain stimulation reward in certain regions has been shown to produce analgesia to externally applied painful stimuli. In the present experiments, we studied how electrical self-stimulation of the dorsal raphe (DR) nucleus modifies the aversive effects of electrical stimulation of the nucleus reticularis gigantocellularis (Gi) or of the dorsal tegmentum (DTg). In the first study, the threshold for latency to escape aversive Gi stimulation was tracked before and after exposure to rewarding DR stimulation. Only a few sessions of DR self-stimulation were required to produce a complete and long-lasting inhibition of Gi aversion. In the second study, the aversion induced by DTg stimulation rapidly disappeared following a few test sessions at that site. Unlike our previous experience with Gi aversion that required either pairing with rewarding lateral hypothalamic (LH) or ventral tegmental area (VTA) pulses in order to increase the threshold for latency to escape Gi aversion, in this study, simply brief experience with rewarding DR stimulation in unpaired trials was sufficient to entirely suppress Gi-induced aversion. Even more surprising was the finding that unlike the Gi, aversion obtained from activation of the DTg does not persist, its threshold for escape quickly increases, and within a few sessions is no longer evident. One interpretation of these findings is that the aversion mechanisms associated with the Gi and DTg are differentially susceptible to analgesic processes.

Interactions between rewarding lateral hypothalamic and aversive nucleus reticularis gigantocellularis stimulation

The interaction between rewarding and aversive consequences of brain stimulation were assessed in two studies. In the first, the frequency threshold for 300 ms trains of combined lateral hypothalamic (LH) and nucleus reticularis gigantocellularis (Gi) stimulation, in which each LH pulse was followed 2 ms later by the Gi one, was determined for one month. Compared to the threshold for trains of single LH pulses, combined LH-Gi stimulation initially increased the frequency threshold; however, this effect reversed within one session and was subsequently maintained for the duration of the study. The aversion produced by Gi stimulation, as measured by latency to escape, was abolished following a single session of LH-Gi pairs. In the second study, a subset of animals received both presentations of combined pulses, LH followed by Gi, and the reverse; the interval between pulses was varied from 0.2 to 6.4 ms. The effectiveness of combined stimulation, determined by the ratio of LH frequency thresholds to that of the LH-Gi ranged from 0 to 50% across animals but the individual effectiveness functions within animals did not vary with different intervals. In addition, the order of presentation of pulses was of no consequence. Thus, not only did exposure to LH stimulation appear to obliterate Gi aversion, but the combination of LH and Gi pulses added to the rewarding effect produced by LH stimulation alone.

Hypertension-associated hypalgesia. Evidence in experimental animals and humans, pathophysiological mechanisms, and potential clinical consequences

A behavioral hypalgesia (increased response threshold to noxious stimuli) has been consistently, although not invariably, reported in spontaneous and experimental acute and chronic hypertension in the rat. Studies in human hypertension have also demonstrated a diminished perception of pain, assessed as pain thresholds or ratings. The sensitivity to painful stimuli correlated inversely with blood pressure levels, and this relationship extended into the normotensive range. Evidence in humans and rats points to a role of the baroreflex system in modulating nociception. In the rat, blood pressure-related antinociception may be due to attenuated transmission of noxious stimuli at the spinal level secondary to descending inhibitory influences that are projected from brain stem sites involved in cardiovascular regulation and that may depend on baroreceptor activation and/ or on a central "drive." Both endorphinergic and noradrenergic central neurons (the latter acting through postsynaptic alpha 2-receptors) have been shown to be involved, and other mediators probably also play a role. Functionally, blood pressure-related antinociception may represent an aspect of a more-complex coordinated adaptive response of the body to "stressful" situations. It is still uncertain whether in human essential hypertension hypalgesia is secondary to elevated blood pressure or whether both depend on some common mechanism. Studies on the effect of hypotensive treatment are too few to allow conclusions. According to one hypothesis, the reduction in pain perception caused by baroreceptor activation secondary to blood pressure elevation may represent a rewarding mechanism that may be reinforced with repeated stress and may be involved in the development of hypertension in some individuals. Hypertension-associated hypalgesia may have clinically relevant consequences, especially in silent myocardial ischemia and unrecognized myocardial infarction, both of which are more prevalent in hypertensive individuals.

Further Characterization of Nociception-Related and Arterial Pressure-Related Neuronal Responses in the Nucleus Reticularis Gigantocellularis of the Rat

The present study was undertaken to further characterize the nucleus reticularis gigantocellularis (NRGC) of the medulla oblongata in the central processing of nociceptive and cardiovascular signals, and its modulation by met-enkephalin. In Sprague-Dawley rats anesthetized with pentobarbital sodium, we found that all 125 spontaneously active NRGC neurons that responded to noxious stimuli (tail clamp) also exhibited arterial pressure-relatedness. Forty neurons additionally manifested cardiac periodicity that persisted even during nociceptive responses. While maintaining their cardiovascular responsive characteristics, the nociception-related NRGC neuronal activity was blocked, naloxone-reversibly (0.5 mg/kg, i.v.), by morphine (5 mg/kg, i.v.). Microiontophoretically applied met-enkephalin suppressed the responsiveness of NRGC neurons to individually delivered tail clamp or transient hypertension induced by phenylephrine (5 &mgr;g/kg, i.v.). Interestingly, in NRGC neurons that manifested both nociception and arterial pressure relatedness, the preferential reduction in the response to noxious stimuli upon simultaneous elevation in systemic arterial pressure was reversed to one that favored nociception in the presence of met-enkephalin. All actions of met-enkephalin were discernibly blocked by the opioid receptor antagonist, naloxone. Our results suggest that individual NRGC neurons may participate in the processing of both nociceptive and cardiovascular information, or in the coordination of the necessary circulatory supports during nociception. In addition, neuropeptides such as met-enkephalin may exert differential modulation on neuronal responsiveness according to the prevailing physiologic status of the animal. They also showed that NRGC may be a central integrator for pain and cardiovascular-related functions. Copyright 1996 S. Karger AG, Basel