Analgesic effects of intraventricular and intrathecal injection of morphine and ketocyclazocine in the infant rat

Effects of plant-derived analgesic compounds sinomenine and salvinorin A in infant rats

OBJECTIVE

Premature and ill neonates undergo painful but medically necessary procedures while hospitalized. Although opiate drugs are administered as analgesics, problems associated with their side effects, tolerance, and potential dependence necessitate research into alternative pain-relieving medications. Here we test two plant-derived compounds in infant rats: sinomenine, which targets the Mas-related G-protein-coupled receptor member X2 opioid receptor; and salvinorin A, which is a κ opioid receptor agonist. In adult animals both sinomenine and salvinorin A are analgesic, but neither has been tested in infants.

METHODS

We used the formalin and thermal plantar tests in rats 7 and 21 days of age (PN7 and PN21) for behavioral signs of pain. In addition, brain sections were stained using Fos immunohistochemistry to examine patterns of brain activation in the midbrain periaqueductal gray and the paraventricular nucleus of the hypothalamus.

RESULTS

Sinomenine was analgesic in both the formalin and thermal tests on animals 21 days of age. At PN7 only the highest dose elevated response latencies in the thermal test and there were no effects of sinomenine in the formalin test. Analysis of Fos expression in the sinomenine-treated animals showed no drug effect, in contrast to the behavioral results. Salvinorin A was analgesic in the formalin test only at the highest dose at 21 days of age but not in the thermal test at either age.

CONCLUSION

The increased modest effectiveness of sinomenine in older animals and the minimum salvinorin A drug effect suggest that the compounds act on sites that develop during the preweaning period (sinomenine) or after weaning (salvinorin A).

Lamina specific postnatal development of PKCγ interneurons within the rat medullary dorsal horn

Protein kinase C gamma (PKCγ) interneurons, located in the superficial spinal (SDH) and medullary dorsal horns (MDH), have been shown to play a critical role in cutaneous mechanical hypersensitivity. However, a thorough characterization of their development in the MDH is lacking. Here, it is shown that the number of PKCγ-ir interneurons changes from postnatal day 3 (P3) to P60 (adult) and such developmental changes differ according to laminae. PKCγ-ir interneurons are already present at P3-5 in laminae I, IIo, and III. In lamina III, they then decrease from P11-P15 to P60. Interestingly, PKCγ-ir interneurons appear only at P6 in lamina IIi, and they conversely increase to reach adult levels at P11-15. Analysis of neurogenesis using bromodeoxyuridine (BrdU) does not detect any PKCγ-BrdU double-labeling in lamina IIi. Quantification of the neuronal marker, NeuN, reveals a sharp neuronal decline (∼50%) within all superficial MDH laminae during early development (P3-15), suggesting that developmental changes in PKCγ-ir interneurons are independent from those of other neurons. Finally, neonatal capsaicin treatment, which produces a permanent loss of most unmyelinated afferent fibers, has no effect on the development of PKCγ-ir interneurons. Together, the results show that: (i) the expression of PKCγ-ir interneurons in MDH is developmentally regulated with a critical period at P11-P15, (ii) PKCγ-ir interneurons are developmentally heterogeneous, (iii) lamina IIi PKCγ-ir interneurons appear less vulnerable to cell death, and (iv) postnatal maturation of PKCγ-ir interneurons is due to neither neurogenesis, nor neuronal migration, and is independent of C-fiber development. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 102-119, 2017.

Altered formalin-induced pain and Fos induction in the periaqueductal grey of preadolescent rats following neonatal LPS exposure

Animal and human studies have demonstrated that early pain experiences can produce alterations in the nociceptive systems later in life including increased sensitivity to mechanical, thermal, and chemical stimuli. However, less is known about the impact of neonatal immune challenge on future responses to noxious stimuli and the reactivity of neural substrates involved in analgesia. Here we demonstrate that rats exposed to Lipopolysaccharide (LPS; 0.05 mg/kg IP, Salmonella enteritidis) during postnatal day (PND) 3 and 5 displayed enhanced formalin-induced flinching but not licking following formalin injection at PND 22. This LPS-induced hyperalgesia was accompanied by distinct recruitment of supra-spinal regions involved in analgesia as indicated by significantly attenuated Fos-protein induction in the rostral dorsal periaqueductal grey (DPAG) as well as rostral and caudal axes of the ventrolateral PAG (VLPAG). Formalin injections were associated with increased Fos-protein labelling in lateral habenula (LHb) as compared to medial habenula (MHb), however the intensity of this labelling did not differ as a result of neonatal immune challenge. These data highlight the importance of neonatal immune priming in programming inflammatory pain sensitivity later in development and highlight the PAG as a possible mediator of this process.

Analgesia induced by localized injection of opiate peptides into the brain of infant rats

BACKGROUND

Stimulation of a variety of brain sites electrically or by opiates activates descending inhibitory pathways to attenuate noxious input to the spinal cord dorsal horn and produce analgesia. Analgesia induced by electrical stimulation of the periaqueductal grey (PAG) of the midbrain or medial rostral ventral medulla (RVM) matures late, towards the end or past the pre-weaning period. Descending facilitation takes precedence over inhibition. Yet opiates injected intracerebroventricularly or directly into the PAG induce analgesia relatively early in development. Our goal was to re-examine the role of opiates specific to individual receptor types in analgesia at several supraspinal sites.

METHODS

Antinociception was tested following microinjection of DAMGO (μ-opiate agonist), DPDPE (∂-opiate agonist) or U50,488 (κ-opiate agonist) into the PAG, RVM or dorsal lateral pons (DLP) in 3-, 10- and 14-day-old rats.

RESULTS

DAMGO produced analgesia at 3 days of age at each brain area; the RVM was the most effective and the dorsal PAG was the least effective site. DPDPE produced modest analgesia at 10 and 14 days of age at the ventral PAG, RVM or DLP, but not the dorsal PAG. U50,488H was ineffective at all sites and all ages.

CONCLUSIONS

Antinociception could be elicited at all three sites by DAMGO as early as 3 days of age and DPDPE at 10 and 14 days of age. The degree of analgesia increased gradually during the first 2 weeks of life, and likely reflects the maturation of connections within the brain and of descending inhibitory paths from these sites.

Periaqueductal gray neuroplasticity following chronic morphine varies with age: role of oxidative stress

The development of tolerance to the antinociceptive effects of morphine has been associated with networks within ventrolateral periaqueductal gray (vlPAG) and separately, nitric oxide signaling. Furthermore, it is known that the mechanisms that underlie tolerance differ with age. In this study, we used a rat model of antinociceptive tolerance to morphine at two ages, postnatal day (PD) 7 and adult, to determine if changes in the vlPAG related to nitric oxide signaling produced by chronic morphine exposure were age-dependent. Three pharmacological groups were analyzed: control, acute morphine, and chronic morphine group. Either morphine (10mg/kg) or equal volume of normal saline was given subcutaneously twice daily for 6½ days. Animals were analyzed for morphine dose-response using Hot Plate test. The expression of several genes associated with nitric oxide metabolism was evaluated using rtPCR. In addition, the effect of morphine exposure on immunohistochemistry for Fos, and nNOS as well as nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) reaction at the vlPAG were measured. In both age groups acute morphine activated Fos in the vlPAG, and this effect was attenuated by chronic morphine, specifically in the vlPAG at the level of the laterodorsal tegmental nucleus (LDTg). In adults, but not PD7 rats, chronic morphine administration was associated with activation of nitric oxide function. In contrast, changes in the gene expression of PD7 rats suggested superoxide and peroxide metabolisms may be engaged. These data indicate that there is supraspinal neuroplasticity following morphine administration as early as PD7. Furthermore, oxidative stress pathways associated with chronic morphine exposure appear age-specific.

Neuraxial analgesia in neonates and infants: a review of clinical and preclinical strategies for the development of safety and efficacy data

Neuraxial drugs provide robust pain control, have the potential to improve outcomes, and are an important component of the perioperative care of children. Opioids or clonidine improves analgesia when added to perioperative epidural infusions; analgesia is significantly prolonged by the addition of clonidine, ketamine, neostigmine, or tramadol to single-shot caudal injections of local anesthetic; and neonatal intrathecal anesthesia/analgesia is increasing in some centers. However, it is difficult to determine the relative risk-benefit of different techniques and drugs without detailed and sensitive data related to analgesia requirements, side effects, and follow-up. Current data related to benefits and complications in neonates and infants are summarized, but variability in current neuraxial drug use reflects the relative lack of high-quality evidence. Recent preclinical reports of adverse effects of general anesthetics on the developing brain have increased awareness of the potential benefit of neuraxial anesthesia/analgesia to avoid or reduce general anesthetic dose requirements. However, the developing spinal cord is also vulnerable to drug-related toxicity, and although there are well-established preclinical models and criteria for assessing spinal cord toxicity in adult animals, until recently there had been no systematic evaluation during early life. Therefore, in the second half of this review, we present preclinical data evaluating age-dependent changes in the pharmacodynamic response to different spinal analgesics, and recent studies evaluating spinal toxicity in specific developmental models. Finally, we advocate use of neuraxial drugs with the widest demonstrable safety margin and suggest minimum standards for preclinical evaluation before adoption of new analgesics or preparations into routine clinical practice.

Validation of a preclinical spinal safety model: effects of intrathecal morphine in the neonatal rat

BACKGROUND

Preclinical studies demonstrate increased neuroapoptosis after general anesthesia in early life. Neuraxial techniques may minimize potential risks, but there has been no systematic evaluation of spinal analgesic safety in developmental models. We aimed to validate a preclinical model for evaluating dose-dependent efficacy, spinal cord toxicity, and long-term function after intrathecal morphine in the neonatal rat.

METHODS

Lumbar intrathecal injections were performed in anesthetized rats aged postnatal day (P) 3, 10, and 21. The relationship between injectate volume and segmental spread was assessed postmortem and by in vivo imaging. To determine the antinociceptive dose, mechanical withdrawal thresholds were measured at baseline and 30 min after intrathecal morphine. To evaluate toxicity, doses up to the maximum tolerated were administered, and spinal cord histopathology, apoptosis, and glial response were evaluated 1 and 7 days after P3 or P21 injection. Sensory thresholds and gait analysis were evaluated at P35.

RESULTS

Intrathecal injection can be reliably performed at all postnatal ages and injectate volume influences segmental spread. Intrathecal morphine produced spinally mediated analgesia at all ages with lower dose requirements in younger pups. High-dose intrathecal morphine did not produce signs of spinal cord toxicity or alter long-term function.

CONCLUSIONS

The therapeutic ratio for intrathecal morphine (toxic dose/antinociceptive dose) was at least 300 at P3 and at least 20 at P21 (latter doses limited by side effects). These data provide relative efficacy and safety for comparison with other analgesic preparations and contribute supporting evidence for the validity of this preclinical neonatal safety model.

Spinal cord ionotropic glutamate receptors function in formalin-induced nociception in preweaning rats

RATIONALE

Neonates respond to noxious stimuli at or before birth, but the organization of nociceptive systems changes well into postnatal life. It is unknown how nociceptive information is processed in the immature animal and, specifically, whether noxious stimulation is transmitted by glutamatergic circuits, known to play an important role in nociception in the adult. Both N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are found within the neonatal spinal cord, but in immature form, and when they become involved in pain processing in vivo is not known.

OBJECTIVES

The objective was to determine the age-related changes in the involvement of spinal NMDA and AMPA receptors in formalin-induced nociception during early life. Because the formalin test provides a measure of immediate nociceptive responding (first phase) and of peripheral and central sensitization (second phase), a second aim was to determine if there is specificity of the effects to either phase.

MATERIALS AND METHODS

NMDA antagonists (MK801, AP5) or an AMPA antagonist (YM872) was administered intrathecally, and pups were assessed in the formalin test behaviorally and by Fos expression within the spinal cords of 3-, 10-, and 21-day-old rats.

RESULTS

The NMDA antagonists attenuated formalin-induced behavioral responses at the youngest age tested with some selectivity for the second phase of responding. MK-801 did not induce motor impairment at any age. YM872 also attenuated formalin-induced nociceptive responses at all ages throughout the test session, although there was some motor impairment in the 3-day-old subjects. Spinal administration of either YM872 or MK-801 reduced Fos expression in the spinal cord at all ages.

CONCLUSIONS

These data suggest that spinal NMDA and AMPA receptor are functional and involved in formalin-induced nociception throughout development.

Maturation of NK1 receptor involvement in the nociceptive response to formalin

Administration of NK1 antagonists in adult animals attenuates the nociceptive response in the formalin test, indicating that the neurokinins and the NK1 receptor play a role in mediating this pain response. The number and distribution of NK1 receptors change dramatically during development, and the age at which they become involved in pain processing is not known. We examined the role of NK1 receptors in the formalin model in rats ranging in age between 3- and 21-days old. An NK1 antagonist, CP99,994, and its less active enantiomer CP100,263 were administered to the spinal cord (intrathecal), systemically (subcutaneous), or locally (intraplantar). Intrathecal administration of CP99,994, but not CP100,263, attenuated pain behaviors in the second phase of the formalin response in 14-day and 21-day old rats, but did not alter the pain response in 3-day or 10-day old rats. CP99,994 also reduced the expression of the c-fos protein in the superficial dorsal horn of 21-day old rats. Systemic and intraplantar injection of either CP99,994 or CP100,263 reduced the pain response to formalin in 3-day and 21-day old rats, suggesting a non-NK1 mediated mechanism of action. These results indicate that, within the spinal cord, NK1 receptors start to play a role in the pain response to formalin between 10 and 21 days. Moreover, analgesia induced by systemic or local injection of NK1 antagonists involves mechanisms other than, or in addition to, the NK1 receptor.

The effect of periaqueductal gray lesions on responses to age-specific threats in infant rats

During early ontogeny infant rats show specific responses to a variety of age-dependent threatening situations. When isolated from nest and dam, they emit ultrasonic vocalizations and show decreased reactivity to noxious stimulation, or analgesia. When exposed to an unfamiliar adult male, they become immobile and analgesic. The midbrain periaqueductal gray (PAG) is an important area within the circuitry that controls responses to threatening stimuli in the adult. Little is known about the functions of the PAG in early life. It was hypothesized that the PAG mediates the responses to the age-specific threats social isolation and male exposure in the infant rat. Rat pups were lesioned electrolytically either in the lateral or the ventrolateral PAG on postnatal day 7, tested in social isolation on day 10, and exposed to a male on day 14. On day 10 during isolation, ultrasonic vocalizations and isolation-induced analgesia were decreased in both lesion groups. On day 14, male-induced immobility and analgesia were decreased in ventrally lesioned animals. In conclusion, the PAG seems to play a developmentally continuous role in age-specific responses to threat such as ultrasonic vocalization, analgesia, and immobility.

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.

Maturation of the biphasic behavioral and heart rate response in the formalin test

The biological processes that mediate and modulate the perception of pain in the infant animal are not well studied and thus nociception during early development is poorly understood. In the adult animal, injection of formalin into the hind paw produces distinct phases of behavioral and autonomic responses: an early nociceptive response followed by a period of quiescence and a later second phase that matches or exceeds the initial response. The delayed reaction of the second phase has been suggested to be a model of inflammation-induced changes in neuronal sensitivity. Studies in the infant rat have demonstrated that the first phase is present in the fetus and neonate but the onset of the second phase is later maturing. We report here that the first phase occurs in 7- to 35-day-old pups in the formalin test when measured behaviorally and in 14- to 35-day-old pups when assessed by increased heart rate. However, the behavioral response in second phase is greatly attenuated or absent in 7- or 14-day-old pups, a finding consistent with that of others, appearing first at 21 days of age. The biphasic tachycardic response was not noted until even later, at 35 days of age. These data confirm that the neural mechanisms that mediate the secondary behavioral phase in the formalin test are late maturing, that the biphasic cardiovascular response does not occur until substantially later, after weaning, and that the behavioral and cardiovascular responses are dissociated developmentally.

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.

The suppression of formalin-induced fos expression by different anesthetic agents in the infant rat

Despite the importance of pediatric anesthesiology, the sites and mechanisms of anesthetic action in the neonate are not well described in either human or nonhuman species. This experiment investigated suppression produced by different anesthetic agents of neuronal activity in the lumbar spinal cord of the 3-day-old rat. The expression of the c-fos immediate early gene following formalin injection into the hindpaw was used as a marker for neuronal activity. Pups were anesthetized by one of the following often-used agents: methoxyflurane, acepromazine, a mixture of ketamine and xylazine, and hypothermia. All treatments induced behavioral anesthesia. Despite the behavioral anesthesia, the ketamine-xylazine mixture was completely ineffective in suppressing formalin-induced-Fos expression. In contrast, methoxyflurane and hypothermia blocked the appearance of the Fos protein. Similarly, acepromazine was effective in eliminating some of the Fos-labeled nuclei. These data suggest that, in the infant rat, both hypothermia and methoxyflurane act in part at the spinal level by depressing either primary afferents or dorsal horn neuronal activity whereas the site of action for ketamine-xylazine may be located supraspinally.

Cardiovascular and respiratory actions of U50,488H in the unanaesthetized ovine foetus

In an effort to evaluate the feasibility of kappa-opioid receptor agonists for use in pregnancy, we have investigated the actions of U50,488H (trans-3,4-dichloro-N-methyl-N-[2-(1- pyrrolidinyl)cyclohexyl]benzeneacetamide) on cardiovascular and respiratory control in the unanaesthetized ovine foetus. Intravenous administration of U50,488H (1.0 mg/kg) to the foetus resulted in an immediate increase in foetal blood pressure (P < 0.0001) and heart rate (P < 0.0001) which lasted 15 min, followed by a prolonged loss of heart rate variability for up to 3 h. There was also a significant suppression of foetal breathing movements for 2-3 h (P < 0.008). Pretreatment with naloxone (12 mg/h) completely blocked the hypertensive and tachycardiac response to U50,488H, but was unable to prevent the loss of variation in heart rate or respiratory depression. These data suggest that U50,488H can exert direct cardiovascular and respiratory actions in the ovine foetus via both opioid and non-opioid mechanisms. The naloxone-insensitive suppression of foetal breathing would severely limit the use of U50,488H as an obstetrical analgesic.

Effects of morphine, pentobarbital and amphetamine on formalin-induced behaviours in infant rats: sedation versus specific suppression of pain

The behavioural response of infant rats to intraplantar injection of formalin consists of specific directed behaviours (limb flexion, shaking and licking the injected paw) and non-specific behaviours that are also induced by non-nociceptive stimulation (squirming, hind limb kicks and whole body jerks), with specific indicators becoming more frequent as pups mature. The present study examined the effects of systemic morphine, pentobarbital and D-amphetamine on formalin-induced behaviours and behavioural state in rat pups from 1 to 20 days of age. Morphine (1 mg/kg) almost completely suppressed both specific and non-specific indicators of pain, and produced mild sedation relative to handled control pups. Pentobarbital (10 mg/kg) produced a similar degree of sedation and suppression of non-specific measures as morphine, but only had weak effects on specific measures in pups less than 1 week old, and no effects thereafter. Suppression of both specific and non-specific pain measures after amphetamine (2 mg/kg) emerged during the 2nd week of life and was not associated with sedation. Thus, morphine produced behavioural analgesia in infant rats in a model of injury-induced inflammatory pain from the 1st postnatal day, when their neurological maturity is similar to a 25-week human fetus, and 1 week before antinociception is observed in thermal and pressure tests. The effects of morphine were qualitatively different from a sedative dose of pentobarbital. The data support the contention that opioids have specific analgesic effects in premature human neonates and underline the need for pain measures that discriminate between sedation and analgesia.

Development of water-immersion produced analgesia

Nociceptive and antinociceptive systems change dramatically during the first 10 days of life in rats. The present studies sought to determine whether exposure to water stress can produce analgesia during this period, and when in the ontogeny changes in magnitude and duration of such stress-induced analgesia occur. It was found that exposure to water reliably induced analgesia as early as 3 days postnatally. This analgesia increased in magnitude and duration at the age of 17-20 days, when supra-spinal descending inhibition develops, and depended on the temperature of the water and duration of exposure.

Differential ontogenesis of thermal and mechanical antinociception induced by morphine and beta-endorphin

The antinociceptive effects induced by beta-endorphin and morphine given supraspinally have been previously demonstrated to be mediated by the activation of different neural mechanisms. The present experiments were to examine the effects of intraventricular administration of beta-endorphin and morphine in mechanical paw-withdrawal and thermal tail-flick nociceptive tests in rats of 2-28 days of age. 2-4-day-old neonates were not responsive to i.c.v. injection of beta-endorphin or morphine for the inhibition of the tail-flick response. The thermal antinociceptive responses induced by beta-endorphin and morphine started to develop in 7-14-day-old rats and continued to increase at 21-28 days. The inhibition of the mechanical paw-withdrawal response to beta-endorphin was already present in 2-day-old rats and morphine in 4-day-old rats. The mechanical antinociception progressively increased and reached a plateau at 7 days of age for beta-endorphin and 28 days of age for morphine. beta-Endorphin was found to be more efficacious than morphine in producing mechanical antinociception. The results demonstrate that beta-endorphin- and morphine-induced antinociception to mechanical and thermal stimuli develops differently and are consistent with the hypothesis that two descending pain inhibitory systems activated by beta-endorphin and morphine are differentially developed.

Biological mechanisms and perinatal exposure to abused drugs

Use of illicit and licit drugs during pregnancy is a major public health concern, as it can have adverse effects on the developing fetus. Infants born of women addicted to narcotics, cocaine, alcohol, or polydrugs often undergo a characteristic withdrawal syndrome and may have physical, behavioral, and/or neurological abnormalities. As it is not feasible to ascertain whether these functional changes in human infants are produced by abused substance(s) per se or by a combination of complex socioeconomic factors and polydrug use, researchers in recent years have developed and utilized various innovative animal models to assess drug-induced alterations and their biological mechanisms during the developmental period under a controlled environment. To promote interdisciplinary communications as well as to assess the progress and the future needs in this area, the National Institute on Drug Abuse (NIDA) held a technical review at which biomedical researchers discussed their current findings in various physiological systems. This report summarizes the major findings and the methodological and experimental issues discussed at the conference. The meeting was held in Washington, DC, on May 25 and 26, 1994.

Ontogeny of neuropeptide FF pharmacology and receptors in mouse brain

The postnatal ontogeny of neuropeptide FF (NPFF) receptors in mouse brain was mapped by quantitative autoradiography using [125I](1DME)Y8Famide as selective radioligand. By day 14, the adult distribution of NPFF sites was established in almost all brain areas. The density of NPFF receptors reached adult levels between 14 and 28 days after birth, earlier than did mu or delta opioid receptors. During ontogeny, morphine analgesia was measured in hindpaw, forepaw and tail immersion tests (57 degrees C). An antinociceptive effect of morphine in the forepaw and tail immersion test was evident in 14-day-old but not in 7-day-old mice. In 14 and 21 day-old mice, the influence of (1DME)Y8Famide on morphine analgesia depended on the dose and on the particular test. A low dose of (1DME)Y8Famide inhibited morphine analgesia in the tail or hindpaw test, whereas a higher dose had either no effect (tail test) or increased morphine activity (hindpaw test). In adult mice, only the inhibition of morphine antinociception by (1DME)Y8Famide was observed. These observations revealed the establishment of an equilibrium between NPFF and opioid functions during ontogeny.

Kappa opioid effects on fetal behavior: central administration of U50,488

The kappa opioid agonist U50,488 was administered to E21 rat fetuses via intracisternal (IC), intrahemispheric (IH), or intrathecal (IT) injection. The IC administration of U50,488 promoted a threefold increase in motor activity, which was predominated by movements of caudal regions of the body (rearlimbs, body trunk, and tail). The agonist effect was reversed by IC administration of the selective kappa opioid antagonist nor-binaltorphimine. The IH injection of U50,488 had little effect on fetal behavior, but IT injection resulted in pronounced increases in fetal activity, including rearlimb, trunk, and tail movements. The IT administration of nor-binaltorphimine blocked U50,488 effects, whereas IH injection of the antagonist had little influence on fetal behavior. These findings suggest that kappa opioid receptors located in the spinal cord may play a role in the regulation of fetal motor behavior.

Analgesia from the periaqueductal gray in the developing rat: focal injections of morphine or glutamate and effects of intrathecal injection of methysergide or phentolamine

The aim of these experiments was to examine the changes in antinociception elicited by morphine or glutamate stimulation of the periaqueductal gray of the midbrain (PAG) during the postnatal development of the rat. Pups, aged 3, 10, and 14 days, were implanted with cannulas aimed at either the dorsal or the ventral aspect of the PAG, and glutamate (vehicle, 60 mM or 180 mM) or morphine (vehicle, 2 micrograms or 6 micrograms) was microinjected into one of those two sites. Pups were tested for analgesia against noxious thermal and mechanical stimuli. Morphine produced analgesia at 3 and 10 days of age only when administered to the ventral part of the PAG and the thermal noxious stimulus was tested. Conversely, analgesia induced by glutamate was seen at 3 and 10 days of age only when glutamate was given to the dorsal aspect of the PAG and the mechanical stimulus was used. In 14-day-old pups, both drugs produced analgesia against both types of noxious stimuli regardless of their site of administration within the PAG. Systemically administered naloxone attenuated the analgesic effects of both drugs when they were administered to the ventral PAG, but did not consistently attenuate the analgesic effect of either compound given to the dorsal aspect of the PAG. When either morphine or glutamate was injected into the ventral PAG, intrathecal injections of methysergide attenuated analgesia against the thermal stimulus to a significantly greater degree than the mechanical stimulus and intraspinal injection of phentolamine attenuated analgesia against the mechanical stimulus more potently. When glutamate was given to the dorsal PAG, analgesia against both stimulus types was significantly attenuated. These results indicate that the morphine- and glutamate-induced analgesia mediated by the PAG are developmentally differentiated. These ontogenetic differences most likely reflect differences in the mechanism of action by which these drugs produce analgesia when administered to the PAG, as well as neuroanatomical differences within the dorsal and the ventral regions of the PAG.