Developmental aspects of nociception

Hyperthermic effects on behavior

This review focuses upon the past 8 years of research on hyperthermic effects on behavior. Heat stress and heat stoke become severe conditions when body temperatures exceed 40°C as this can lead to delirium, convulsions, coma, and death. The animal literature indicates that hyperthermia can increase glutamatergic and decrease GABAergic neurotransmission. Interestingly, µ-opiate receptor antagonists can attenuate the morphological and biochemical changes in brain, as well as, ameliorate some behavioral deficits induced by heart stress. In humans, heat stress can produce detrimental effects on motor and cognitive performance. Since most cognitive tasks require a motor response, some cognitive deficiencies may be attributed to decreased motor performance. Although hyperthermia may exert more deleterious effects on complex than simple cognitive tasks, systematic studies are needed to examine the effects of different levels and durations of hyperthermia (irrespective of dehydration) on cognition. Additionally, body temperatures should be carefully monitored where controls are run for baseline or brief exposures to a hyperthermic environment. Acute radiofrequency exposure can disrupt behavior when body temperatures increase >1°C with whole body SAR between 3.2-8.4 W/kg and time-averaged power densities at 8-140 mW/cm(2). Effects of lower levels of radiation are conflicting and some experiments fail to replicate even with the original investigators. This suggests either that brief exposure to the radiation is at a threshold where some individuals are affected while others are not, or that these levels are innocuous. Nevertheless, thermal changes appear to account for almost all of the behavioral effects reported.

Periadolescent male but not female rats have higher motor activity in response to morphine than do adult rats

Little has been done to investigate the effects of opioid exposure during adolescence. First we determined behavioral differences in response to acutely administered morphine between periadolescent and adult male and female rats. Second, we determined the impact of age of morphine exposure on sensitivity to morphine-induced locomotion later in life. For the acute morphine studies, antinociceptive responses were assessed using cumulative morphine dosing (0.5-12 mg/kg) followed by a time course after the last morphine injection (<or= 4 hr), and dose-response curves for motor activity (2-h test) were determined following saline and morphine (0.1-3.0 mg/kg) administration. For the long-term study, periadolescent and adult rats were given saline or 1, 3, or 5 days of morphine (5.0 mg/kg, 2x/day). Changes in locomotor activity in response to saline or morphine (0.1-3.0 mg/kg) were determined 1 month later. A number of age- and sex-related behavioral differences were observed: basal differences in behavior were assay-dependent; however, male periadolescent rats were generally more sensitive to acute morphine-induced motor stimulation, while both male and female periadolescent rats tended to be less sensitive to morphine-induced antinociception. Lastly, following morphine exposure, activity was dependent on age of treatment and treatment regimen, with the greatest effects in 5-day periadolescent-treated animals.

Postnatal development and gender-dependent expression of TIP39 in the rat brain

Tuberoinfundibular peptide of 39 residues (TIP39) is a selective agonist of the parathyroid hormone 2 (PTH2) receptor. The topographical distributions of TIP39 and the PTH2 receptor in the brain, described for young male rats, suggested that TIP39 has limbic and endocrine functions. In the present study, we investigated the expression of TIP39 and the PTH2 receptor in male and female rat brain during postnatal development by means of in situ hybridization histochemistry, quantitative RT-PCR, and immunocytochemistry. TIP39's distribution and expression levels are similar in young female and male brains. TIP39 mRNA levels peak at postnatal day 14 and subsequently decline both in the subparafascicular area and in the medial paralemniscal nucleus, the two major sites where TIP39 is synthesized. A greater developmental decrease in TIP39 expression in males leads to greater levels in older females than older males. The decrease is partially reversed by prepubertal but not postpubertal gonadectomy. TIP39 peptide levels in cell bodies change in parallel with mRNA levels, whereas TIP39 appears and disappears somewhat later in nerve fibers. In addition, TIP39 peptide levels are also sexually dimorphic in older rats. In contrast, PTH2 receptor expression in the brain does not decrease during puberty and is not sexually dimorphic even in old animals. The appearance of TIP39 during early, and decline during late, postnatal development together with the gender-dependent levels in mature animals suggests that TIP39 may play a role in sexual maturation or gender-specific functions.

Mu opioid receptors in the ventrolateral periaqueductal gray mediate stress-induced analgesia but not immobility in rat pups

Rat pups become immobile and analgesic when exposed to an adult male rat. The aim of this study was to determine whether these reactions are under the control of endogenous opioids and to determine the role of the midbrain periaqueductal gray (PAG), which mediates stress-induced immobility and analgesia in adult animals. In Experiment 1, 14-day-old rats were injected systemically with the general opioid receptor antagonist naltrexone (1 mg/kg), which blocked male-induced analgesia to thermal stimulation but did not affect immobility. In Experiment 2, the selective mu opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP; 50 or 100 ng/200 nl) was microinjected into the ventrolateral and lateral PAG. CTOP suppressed male-induced analgesia when injected into the ventrolateral PAG. Male-induced immobility was not affected by CTOP. Male proximity therefore seems to induce analgesia in rat pups by releasing endogenous opioids that bind to mu opioid receptors in the ventrolateral PAG.

Ontogeny of defensive behavior and analgesia in rat pups exposed to an adult male rat

Aversive situations may reduce nociception. The mechanism underlying such analgesia has been suggested to involve the interaction between the two separate but interconnected motivational systems "defense" and "pain." To determine the developmental course of defense and nociception, these processes were analyzed during early ontogeny in rats. To elicit a defensive reaction, a huddle of preweanling rat pups was exposed to an unfamiliar, unrelated adult male, or, for comparison, to the mother. On postnatal Day 7 the pups did not show a behavioral reaction to the presence of the mother or the male, and no reduction in nociceptive threshold in a thermal paw withdrawal test. On Day 14, pups in the presence of the male stopped ongoing behaviors and became immobile, and showed reduced paw withdrawal after the exposure. At Day 21, 22 pups of 32 became immobile when exposed to the male, whereas 10 pups explored the partition separating them from the male. Neither group showed reduced paw withdrawal. Immobility was considered a defensive reaction because it reduces auditory and visual cues and therefore the probability of being detected. The developmental course of immobility seems to reflect both the changes in threat imposed on the pups by a potentially infanticidal male and the ability of pups to react to that threat. The reduction in paw withdrawal that followed male exposure indicates an inhibitory mechanism. It is discussed whether the activation of the defense system results in an inhibition of nociception.

Sex and genotype determine the selective activation of neurochemically-distinct mechanisms of swim stress-induced analgesia

A growing literature documents the important influence of organismic factors such as sex and genotype on pain sensitivity and pain modulation. We recently determined that 3-min forced swims in 15 degrees C water produce non-opioid (i.e., naloxone-insensitive) analgesia in outbred Swiss-Webster mice of both sexes; this form of stress-induced analgesia (SIA) is significantly attenuated by the N-methyl-D-aspartate (NMDA) antagonist, dizocilpine (MK-801) in males, but not females. A pilot study designed to confirm the non-opioid and (in male mice) NMDAergic nature of 15 degrees C swim SIA in the C57BL/6J and DBA/2J inbred strains used widely in gene mapping was conducted, using the hot-plate (54 degrees C) assay of nociception. In female mice of both strains, 15 degrees C swim SIA was insensitive to antagonism by either naloxone (10 mg/kg, i.p.) or dizocilpine (0.1 mg/kg. i.p.). In male C57BL/ 6J mice, the observed SIA was naloxone-insensitive, but was attenuated by dizocilpine. This pattern of results is virtually identical to that obtained using Swiss-Webster mice in this and previous studies. However, male DBA/2J mice displayed SIA that was significantly attenuated by naloxone, but insensitive to dizocilpine antagonism. These findings support the hypothesis that genetic factors and sex, in addition to stressor parameters, can determine the selective recruitment of alternative central mechanisms of pain inhibition.

Sex differences in the expression and antagonism of swim stress-induced analgesia in deer mice vary with the breeding season

Swim stress-induced analgesia (SSIA) was examined in photoperiodically induced 'breeding' (reproductive) and 'non-breeding' (non-reproductive) adult male and female deer mice, Peromyscus maniculatus. Nociceptive responses (50 degrees C, hot-plate) of breeding and non-breeding deer mice were determined after either a 1- or 3-min swim in 20 degrees C water. The 1-min swim induced an immediate and relatively short-lasting naloxone (1.0 mg/kg) insensitive 'non-opioid' -mediated SSIA that was antagonized by the N-methyl-D-aspartate (NMDA) antagonist, MK-801 (0.10 mg/kg) in all of the groups of mice except the breeding (reproductive) females. Breeding females displayed a non-opioid analgesia that was insensitive to MK-801. The 3-min swim induced a relatively more prolonged mixed opioid and 'non-opioid' SSIA of which the initial portion was sensitive to antagonism by MK-801 in all groups of the mice except the breeding females, while the latter portion (15 min after swim) was induced by naloxone in all of the groups of mice. Overall, the breeding males displayed greater levels of SSIA than the breeding females, with no consistent sex differences in the non-breeding mice. Within sexes, the breeding males displayed greater levels of opioid and non-opioid SSIA than the non-breeding males, while the non-breeding females displayed greater levels of SSIA than the breeding females. These results show that both sex and reproductive status affect the expression and neurochemical mediation of non-opioid SSIA. These findings also suggest that reproductive females may have an unique or novel hormonally (estrogen) dependent mechanism associated with the expression of SSIA.

Predator-induced opioid and non-opioid mediated analgesia in young meadow voles: sex differences and developmental changes

The present study examined developmental changes in the nociceptive responses of male and female meadow voles, Microtus pennsylvanicus, exposed to a garter snake, a natural predator of young voles. After 15 min of exposure to the presence of a garter snake, neonatal-juvenile voles (5-20 days of age) displayed naloxone (1.0 mg/kg)-sensitive opioid mediated analgesic responses, while after a brief 30-s exposure to the snake, voles displayed a higher amplitude, non-opioid analgesia that was insensitive to naloxone and blocked by the serotonin-1A (5-HT1A) agonist, 8-hydroxy-2-(di-n-propylamino)tetralin. The levels of opioid and non-opioid mediated analgesia declined during development as the threat presented by the snake decreased. Young female voles also displayed a significantly greater non-opioid, 5-HT1A sensitive analgesia than males, with no significant sex differences in the lower amplitude opioid analgesia. These results indicate that young (neonatal) meadow voles that are exposed to a naturally threatening stimulus display sexually dimorphic analgesic responses. These findings also illustrate the need to consider the ecological context when examining environmentally-induced analgesia.

Sex differences in the antagonism of swim stress-induced analgesia: effects of gonadectomy and estrogen replacement

Sex differences in the neurochemical mediation of swim stress-induced analgesia (SSIA) were examined in Swiss-Webster mice. Intact and gonadectomized adult mice of both sexes were tested for their analgesic response (hot-plate test) to 3 min of forced swimming in 15 degrees C and 20 degrees C water. SSIA resulting from 15 degrees C swim was previously shown to be naloxone-insensitive (i.e., non-opioid) whereas SSIA resulting from 20 degrees C swim produced an analgesia that was partially reversible by naloxone (i.e., mixed opioid/non-opioid). The non-opioid components of these SSIA paradigms were attenuated by the N-methyl-D-aspartate (NMDA) receptor antagonist, dizocilpine (MK-801). We now report that in males, but not females, dizocilpine (0.075 mg/kg, i.p.) and naloxone (10 mg/kg, i.p.) antagonized the non-opioid and opioid components of SSIA, respectively. After ovariectomy, females displayed a pattern of antagonism similar to males such that dizocilpine attenuated non-opioid SSIA, although naloxone remained ineffective in antagonizing 20 degrees C SSIA. Thus, SSIA in intact females was neither opioid- nor NMDA-mediated, yet it was of similar magnitude to the SSIA displayed by intact males. In separate experiments, estrogen replacement (estrogen benzoate; 5.0 micrograms/day, i.p.) administered to ovariectomized mice over a 6-8 day period reinstated the dizocilpine-insensitivity of 15 degrees C SSIA characteristic of intact females. However, a similar estrogen regimen administered to both intact and castrated males did not compromise the sensitivity to dizocilpine previously noted in male mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Age, strain and anesthetic dependent differences in the nociceptive responses produced by i.v. 5-HT in the rat

The intravenous (i.v.) administration of serotonin (5-HT) to rats is a noxious visceral stimulus which produces distinct vagal afferent-mediated pseudaffective responses, a passive avoidance behavior, a vagal afferent-mediated inhibition of the tail-flick (TF) reflex and a complex cardiovascular response. In the present study, we examined the effects of age (10 or 16 weeks), strain (Sprague-Dawley, SD; Wistar-Kyoto, WKY; spontaneously hypertensive rats, SHR) and anesthetic (conscious or lightly pentobarbital-anesthetized) on nociceptive (TF reflex and pseudaffective responses) and cardiovascular responses produced by 5-HT (3-288 micrograms/kg, i.v.). There were no age-related differences in baseline TF latencies in the 3 strains. Further, latencies were generally not significantly different whether rats were tested conscious or lightly anesthetized. There were, however, strain differences. Both conscious or lightly pentobarbital-anesthetized SHR and WKY rats at 10 weeks of age had significantly faster response latencies than 10 week old SD rats. At 16 weeks of age, only the lightly pentobarbital-anesthetized WKY and SHR showed faster response latencies than SD rats. The WKY and SHR, but not the SD rats, were more sensitive to the nociceptive effect of i.v. 5-HT at 16 weeks of age compared to 10 weeks of age. At both ages, WKY and SHR, but not SD rats, showed an anesthetic-dependent increase in nociceptive sensitivity to i.v. 5-HT. In addition, at both ages, regardless of the presence of anesthetic, the order of sensitivity to the nociceptive effects of i.v. 5-HT was SD greater than WKY much much greater than SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental changes in opiate-induced analgesia in deer mice: sex and population differences

We examined developmental changes in nociception and mu (morphine) and kappa (U-50,488) opiate-induced analgesia in male and female deer mice of two different populations; Peromyscus maniculatus artemisiae from a mainland region and P. m. angustus from a small island. Both populations displayed significant developmental changes in nociception and morphine (10 mg/kg) and U-50,488 (10 mg/kg)-induced analgesia. Basal thermal response latencies (nociceptive responses) and the levels of mu and kappa opiate-induced analgesia increased over 14-35 days of age, with maximum analgesic responses in adults (35+ days of age). In both of the populations, young (neonatal-weaning) male mice displayed significantly higher thermal response latencies and greater levels of naloxone (1.0 mg/kg) antagonized opiate-induced analgesia than young females. There were also population differences in the levels of analgesia, the insular mice displaying greater mu and lower kappa opiate-induced analgesic responses than the mainland animals. The population differences in mu and kappa opiate-induced analgesia were evident in young and adult mice of both sexes. These results show that there are significant sex and population differences in nociception and opiate-induced analgesia in young (neonatal-weaning) and adult deer mice.

Opioids and the developing organism: a comprehensive bibliography, 1984-1988

A comprehensive bibliography of the literature concerned with opioids and the developing organism for 1984-1988 is presented. Utilized with companion papers (Neurosci. Biobehav. Rev. 6:439-479; 1982; 8:387-403; 1984), these articles cover the clinical and laboratory references beginning in 1875. For the years 1984, 1985, 1986, 1987, and 1988, a total of 877 citations were recorded. A series of indexes accompanies the citations in order to make the literature more accessible. These indexes are divided into clinical and laboratory topics, and subdivided into such topics as the type of opioid explored and the general area of biological interest (e.g., physiology).

Evolutionary and comparative aspects of nociception

The ability to detect and respond to aversive environmental stimuli is a basic feature of animals that is expressed in the term 'nociception.' Nociception and nociceptive responses provide an index of the sensitivity of individuals to aversive physical stimuli. Measurements of alterations in nociceptive responses (antinociception and analgesia) are commonly used to monitor the behavioral and physiological status of animals following experimental manipulation that usually, but not always, involve exposure to either noxious, stressful or potentially stressful physical and/or biological stimuli. This review briefly considers: i) the phylogenetic development of nociceptive responses and behaviors ii) evolutionary and comparative patterns of the neuromodulation of nociceptive behaviors by opioid peptides and other nonopioid peptidal regulatory mechanisms; iii) the effects of various biological variables, including; age, development, sex, and temporal factors (biological rhythms) on nociception in rodents.