The NK1 receptor is essential for the full expression of noxious inhibitory controls in the mouse.

Behavioral analysis of the NK1 receptor gene knock-out (NK1-/-) mouse indicated that substance P was closely involved in orchestrating the physiological and behavioral response of the animal to major environmental stressors. In particular, endogenous pain control mechanisms, such as stress-induced analgesia were substantially impaired in mutant mice, suggesting a reduction in descending inhibitory controls to the spinal cord from the brainstem. To directly test the integrity of descending controls in NK1-/- mice, we have analyzed c-Fos expression in laminae I-II of the lumbar and cervical cord and in the rostral ventromedial medulla in an experimental paradigm known to require recruitment of descending inhibitory controls. Anesthetized mice were stimulated with water at 50 degrees C either on their forepaw, hindpaw, or on both the hindpaw plus forepaw concurrently. Wild-type mice, naive or treated with an NK1 antagonist (RP67580) or its inactive isomer (RP68651), were compared with NK1-/- mice. C-Fos expression at the lumbar laminae I-II level was significantly reduced, whereas it was significantly greater in the raphe magnus and pallidus nuclei in the double stimulation situation in wild-type compared with NK1-/- mice. Blocking the NK1 receptor pharmacologically reproduced, in an enantiomere-selective manner, the data from NK1-/- mice, with no evidence for recruitment of descending inhibition at the lumbar cord level after forepaw stimulation. The present study demonstrates that the NK1 receptor is essential for the full development of noxiously evoked descending inhibition.

Changes in tactile stimuli-induced behavior and c-Fos expression in the superficial dorsal horn and in parabrachial nuclei after sciatic nerve crush.

Neurons in the superficial laminae of the dorsal horn are dominated by input from peripheral nociceptors. Following peripheral nerve injury, low threshold mechanoreceptive Abeta-fibers sprout from their normal termination site in laminae III/IV into laminae I-II and this structural reorganization may contribute to neuropathic tactile pain hypersensitivity. We have now investigated whether a sciatic nerve crush injury alters the behavioral response in rats to tactile stimuli and whether this is associated with a change in the pattern of c-Fos expression in the dorsal horn and the parabrachial area of the brainstem. Sciatic nerve crush resulted in a patchy but marked tactile allodynia manifesting first at 3 weeks and persisting for up to 52 weeks. C-Fos expression in the dorsal horn and parabrachial region was never observed on brushing the skin of the sciatic nerve territory in animals with intact nerves, but was found after sciatic nerve crush with peripheral regeneration. We conclude that after nerve injury, low threshold mechanoreceptor fibers may play a major role in producing pain-related behavior by activating normally nociceptive-specific regions of the central nervous system such as the superficial laminae of the dorsal horn and the parabrachial area.

Physiological properties of the lamina I spinoparabrachial neurons in the rat.

Single-unit extracellular recordings of spino-parabrachial (spino-PB) neurons (n = 53) antidromically driven from the contralateral parabrachial (PB) area were performed in the lumbar cord in anesthetized rats. All the spino-PB neurons were located in the lamina I of the dorsal horn. Their axons exhibited conduction velocities between 2.8 and 27.8 m/s, in the thin myelinated fibers range. They had an extremely low spontaneous activity (median = 0. 064 Hz) and a small excitatory receptive field (

Differential projections to the intralaminar and gustatory thalamus from the parabrachial area: a PHA-L study in the rat.

The organization of projections from the parabrachial (PB) area to the ventral posterior parvicellular (VPpc) "gustatory" and intralaminar nuclei of the thalamus was studied in the rat by using microinjections of Phaseolus vulgaris leucoagglutinin (PHA-L), into subregions of the PB area. The present study is a follow-up of three former studies (Bernard et al. [1993] J. Comp. Neurol. 329:201-229; Aldén et al. [1994] J. Comp. Neurol. 341:289-314; Bester et al. [1997a] J. Comp. Neurol. 383:245-281) that examined PB projections onto the amygdala, the bed nucleus of the stria terminalis, and the hypothalamus. Our data showed that (1) the region centered in the internal lateral PB subnucleus projects densely with a bilateral and symmetric pattern to the caudal portion of the paracentral and, to a lesser extent, to the adjacent portion of the central and parafascicular medial thalamic nuclei; (2) the mesencephalic PB region centered in the ventral lateral subnucleus and scattered neurons in the subjacent brachium conjunctivum project primarily, although diffusely, to the central medial thalamic nucleus. The third region includes two subgroups: (3a) the medial subgroup, including the medial, the waist area, and the ventral lateral subnuclei of the pontine PB area, projects bilaterally but with a weak ipsilateral predominance to the VPpc, terminals bearing large varicosities. Additionally, a diffuse projection with small varicosities spreads in the area between the two VPpc nuclei and the central medial nucleus. (3b) The lateral subgroup, centered in the external medial subnucleus, projects with a contralateral predominance in the periphery of the VPpc nuclei, most terminals being located around the dorsomedial tip. It is suggested that the PB projections to the intralaminar nucleus could be involved in cortical limbic arousal processing in relation with nociceptive, (somatic, visceral, and intraoral) and gustatory aversive stimuli. The projection with large varicosities inside the VPpc could process gustatory discrimination.

Recovery of C-fiber-induced extravasation following peripheral nerve injury in the rat.

Peripheral nerve injury leads to substantial alterations in injured sensory neurons. These include cell death, phenotypic modifications, and regeneration. Primary sensory neurons have recently been shown not to die until a time beyond 4 months following a nerve crush or ligation and this loss is, moreover, limited to cells with unmyelinated axons, the C-fibers. The late loss of C-fibers may be due to a lack of target reinnervation during the regenerative phase. In order to investigate this, we have used a particular peripheral function, unique to C-fibers, as a measure of peripheral reinnervation: an increase in capillary permeability on antidromic activation of C-fibers, i.e., neurogenic extravasation. This was investigated in rats that had received a nerve crush injury 1 to 50 weeks earlier. Some recovery of the capacity of C-fibers to generate extravasation was detected at 8-10 weeks, which increased further at 12-14 weeks, and then plateaued at this level with no further recovery at 30 or 50 weeks. In intact and damaged sciatic nerves, A beta-fibers never induced extravasation. These findings are compatible with the hypothesis that those C-fibers which make it back to their peripheral targets do not subsequently die and those that do not, may die.

Further evidence for the involvement of the spinoparabrachial pathway in nociceptive processes: a c-Fos study in the rat.

We have analysed in briefly anaesthetised rats (1% halothane for 18 minutes) the effects of innocuous and noxious heat, applied to the hindpaw, on evoked c-Fos immunoreactivity at the levels of the parabrachial area (PB), spinal cord, and nucleus of the solitary tract (NTS). After anaesthesia recovery, animals were left to move freely for 2 hours. At the spinal level, c-Fos was expressed primarily in the ipsilateral superficial laminae, increasing with the applied temperatures in a dependent manner in the noxious range (correlation coefficient r = 0.954, n = 20). At the NTS level, no noxiously evoked c-Fos expression was observed. At the PB level, c-Fos was expressed preferentially contralaterally, increasing with the applied temperatures in a dependent manner in the noxious range (r = 0.971, n = 25). The maximum expression was observed in the outer portion of the external lateral, the lateral crescent, and the superior lateral subnuclei around the pontomesencephalic junction. This was congruent with the densest supraspinal projection of lamina I neurones of the dorsal horn. Labelling in the PB area was highly correlated (r = 0.936, n = 20) with labelling in the superficial laminae. We conclude that, under our experimental procedures, noxious heat-induced c-Fos expression at the PB level depends on the intensity of the noxious stimulation. These data further support the relevance of the recently described spino-PB pain pathway. Because of their location, the Fos-immunoreactive neurones observed in the pontine and the mesencephalic divisions of the PB area were likely PB-amygdaloid and PB-hypothalamic nociceptive neurones, respectively.

Organization of efferent projections from the parabrachial area to the hypothalamus: a Phaseolus vulgaris-leucoagglutinin study in the rat.

The organization of projections from the parabrachial (PB) area to the hypothalamus was studied in the rat by using microinjections of Phaseolus vulgaris-leucoagglutinin (PHA-L) into subregions of the PB area. The present study is a follow-up of two former studies (Bernard et al. [1993] J. Comp. Neurol. 329:201-229; Aldén et al. [1994] J. Comp. Neurol. 341:289-314) that examined PB projections onto the amygdala and the bed nucleus of the stria terminalis. The results demonstrate that 1) the mesencephalic PB region, centered in the lateral portion of the superior lateral subnucleus projects extremely densely to almost the entire dorsomedial subdivision of the ipsilateral ventromedial hypothalamic nucleus; 2) the mesencephalic PB region, located in the medial portion of the superior lateral subnucleus and weakly overflowing into the rostralmost dorsal lateral pontine subnucleus, projects densely to the retrochiasmatic area and, to a lesser extent, to the ipsilateral ventromedial nucleus of the hypothalamus; 3) the PB region, including the central lateral, a portion of the superior lateral, and the outer external lateral subnuclei, projects densely to the ipsilateral median, anteroventral, and periventricular preoptic hypothalamic nuclei and projects more weakly to the dorsal border of the paraventricular nucleus (PVN). No consistent projection was found in the magnocellular PVN. All of these PB regions also project diffusely to the dorsomedial area and to a small tuberal subfornical hypothalamic area. In addition, the medial half of the PB area projects consistently to the posterior lateral hypothalamus. It is suggested that these pathways may be involved in aversive-defensive behavior, in autonomic and neuroendocrine aspects of pain, and in feeding and energy metabolism regulation.

Changes in the responsiveness of parabrachial neurons in the arthritic rat: an electrophysiological study.

1. Rats rendered polyarthritic by injection of Mycobacterium butyricum into the tail were used as a model for the study of "chronic pain". In such rats, anesthetized with halothane in a nitrous oxide-oxygen mixture, spontaneous activity and responses of parabrachial (PB) neurons to somatic stimulations were studied in comparison with those in a control group of healthy animals processed under the same experimental conditions. 2. The size of the somatic receptive field of PB neurons was similar in both arthritic and control groups. In the control group 13%, 55%, and 32% of the receptive fields were small, medium, and large, respectively. Similarly, in the arthritic group, 10%, 60%, and 30% of the receptive fields were small, medium, and large, respectively. 3. The spontaneous activity was significantly (P < 0.001) increased in the arthritic rats (0.1 < 3 < 16 Hz, n = 31; 10th percentile < median < 90th percentile) in comparison with the healthy rats (0.03 < 0.3 < 5 Hz, n = 22). 4. The sensitivity to mechanical stimuli was markedly increased in arthritic compared with healthy rats: 1) although PB neurons in normal rats never responded to innocuous stimuli, several PB neurons in arthritic rats responded to touch and/or joint movement; 2) the mean mechanical threshold decreased from 15.8 N/cm2 in normal rats to 5.9 N/cm2 in arthritic rats; 3) the mean pressure evoking 50% of the maximum response decreased from 34 N/cm2 in normal rats to 21 N/cm2 in arthritic rats; and 4) the intensity of the maximum response increased from 15.7 Hz in normal rats to 26.3 Hz in arthritic rats. 5. The mechanical encoding properties were clearly modified in arthritic rats compared with healthy rats. In this latter group, the PB neurons exhibited a clear capacity to encode mechanical stimuli in the noxious range: 1) the stimulus-response curves were always positive and monotonic until 48 N/cm2; and 2) the slope of the mean curve increased progressively from 2 to 8 N/cm2 before reaching a roughly linear maximum for a wide range of pressure (8-64 N/cm2) and plateauing beyond. In the arthritic rat, the PB neurons also encoded mechanical stimuli, but clearly from a lower pressure range: the slope of the mean curve was maximum and remained steep from the lowest pressure tested (1 N/cm2) up to 16 N/cm2; afterward the slope decreased progressively from 16 to 64 N/cm2 before plateauing. 6. The sensitivity to heat stimuli was only weakly modified. The thermal threshold was weakly, but significantly, increased from 44 degrees C in the normal rat to 45.8 degrees C in the arthritic rat. Other parameters for thermal modality were not changed, with the mean stimulus-response curves being similar in both arthritic and normal groups. 7. In conclusion, these experiments demonstrate that the activity of PB neurons is clearly changed in arthritic rats. These changes are reminiscent of some behavioral and electrophysiological modifications observed during arthritis. Considering the current literature, it is hypothesized that the PB relay could be responsible, at least in part, for several affective-emotional, behavioral, autonomic, and energy metabolism changes observed in arthritic rats.

Parabrachial area: electrophysiological evidence for an involvement in cold nociception.

1. Thirty-five percent of 120 neurons recorded extracellularly in the parabrachial (PB) area of anesthetized rats responded to a peripheral cold stimulus (0 degrees C). The cold-sensitive neurons were located in the lateral PB area, and most of those exhibiting a strong response to cold stimuli were inside or in close vicinity to the area receiving a high density of projections from superficial neurons of the dorsal horn. 2. The receptive fields for cold stimulation often were restricted to one or two parts of the body with a contralateral predominance for the limbs. No side predominance was observed for the face. 3. From a low spontaneous activity (10th percentile < median < 90th percentile: 0.1 < 1.5 < 5 Hz), the PB neurons responded to cold noxious stimuli (0 degree C water bath or waterjet, 20 s), without observable delay, with a sustained discharge. The mean maximal response to the stimulus was 16.1 +/- 1.2 Hz (mean +/- SE; n = 42). 4. About one-half (45%) of these cold-sensitive neurons were activated specifically by cold stimulation and did not respond or were inhibited by noxious heat and/or pinch. The remaining (55%) cold-sensitive neurons were also driven by heat and/or pinch. 5. The cold-sensitive neurons exhibited a clear capacity to encode cold stimuli in the noxious range: the stimulus-response function was always positive and monotonic from 30 to 0 degrees C; the mean curve was linear between 20 and 0 degrees C before plateauing between 0 to -10 degrees C; the mean threshold to cold stimulation was 17.1 +/- 1 degrees C (n = 21) and the mean t50 was 10.7 +/- 1.1 degrees C (n = 13). 6. The cold-sensitive neurons responded to intense transcutaneous electrical stimulation with an early and/or a late peak of activation, the latencies of which were in the 15-50 ms and 80-170 ms ranges (n = 8), respectively, i.e., compatible with the activation of A delta and C fibers. Interestingly, the cold-specific neurons predominantly responded with a late peak, suggesting these neurons were primarily driven by peripheral C fibers. 7. The intravenous injection of morphine depressed the responses of PB neurons to cold noxious stimuli in a dose-related (1, 3, and 9 mg/kg) and naloxone reversible fashion. The ED50 value was estimated approximately 2 mg/kg. Furthermore, two populations of neurons could be separated according to their morphine sensitivity. 8. It is concluded that PB cold-nonspecific neurons could be involved in affective-emotional, autonomic and neuroendocrine reactions in response to noxious cold events. The PB cold-specific neurons could be, in addition, involved in some thermoregulatory processes.

Spino (trigemino) parabrachiohypothalamic pathway: electrophysiological evidence for an involvement in pain processes.

1. Parabrachiohypothalamic (PB-H) neurons (n = 71) were recorded with extracellular micropipettes in the parabrachial (PB) area and were antidromically driven from the ventromedial nucleus (VMH) or the retrochiasmatic area (RCh) of the hypothalamus, in the anesthetized rat. The spontaneous activity of these neurons was very low, (10th percentile < median frequency < 90th percentile were 0.01 < 0.2 < 7 Hz). The axons of these neurons exhibited a very slow conduction velocity in the range of 0.2-1.4 m/s, i.e., corresponding to thin unmyelinated fibers. 2. Most PB-H neurons (89%) were located in the mesencephalic division of the PB area (mPB) mainly in the superior lateral (mPBsl) and external lateral (mPBel) subnuclei. 3. These units were separated in three groups: 1) a group of nociceptive-specific (NS) neurons (49%) activated by mechanical and/or thermal (heat) cutaneous stimuli only in noxious range; 2) a group of inhibited neurons (7%), not activated by any of the mechanical or thermal cutaneous stimuli but inhibited, by at least one of these stimuli, which had to be in noxious range; and 3) a group of nonresponsive neurons (44%). 4. The NS neurons responded exclusively to mechanical (pinch or squeeze) and/or thermal (waterbath or waterjet > 44 degrees C) noxious stimuli with a rapid onset, a marked and sustained activation, and generally no afterdischarge. The magnitude of the responses was between 2 and 30 Hz with a mean value of 14.8 +/- 1.4 Hz (mean +/- SE, n = 49). These neurons exhibited a clear capacity to encode thermal stimuli in the noxious range: 1) the stimulus-response function was always positive and monotonic; 2) the slope of the mean curve increased up to a maximum (between 46 and 50 degrees C) then beyond the slope decreased; and 3) the mean threshold was 44.3 +/- 2.2 degrees C. 5. The excitatory receptive fields of the NS neurons were often large including all (22% of the population) or several (67% of the population) parts of the body. In the few remaining cases (11%) they were restricted to one part of the body. In addition, in several cases, noxious stimuli applied outside the excitatory receptive field were found to strongly inhibit the discharge of NS neurons. 6. Most NS neurons responded to intense transcutaneous electrical stimulation with two peaks of activation.(ABSTRACT TRUNCATED AT 400 WORDS)

Prevention of post-tonsillectomy pain with analgesic doses of ketamine.

The prevention of postoperative pain in children who had undergone tonsillectomy was investigated in a double-blind trial. Ketamine (Ketalar; Parke-Davis) 0.5 mg/kg was given intravenously before the operation to 20 children and saline to a control group of 20 children. Premedication consisted of oral trimeprazine 4 mg/kg given 2 hours pre-operatively. The anaesthetic technique was standardised. There were no significant differences between the groups pre-or intra-operatively. Postoperatively there were significant differences in the measurement of pain but not in that of sedation. No hallucinations were encountered in those receiving ketamine. It is concluded that analgesic doses of ketamine are safe and effective.