Analgesic effects of intrathecally applied noradrenergic compounds in the developing rat: differences due to thermal vs mechanical nociception

Evaluation of neurotoxicity and long-term function and behavior following intrathecal 1 % 2-chloroprocaine in juvenile rats

Spinally-administered local anesthetics provide effective perioperative anesthesia and/or analgesia for children of all ages. New preparations and drugs require preclinical safety testing in developmental models. We evaluated age-dependent efficacy and safety following 1 % preservative-free 2-chloroprocaine (2-CP) in juvenile Sprague-Dawley rats. Percutaneous lumbar intrathecal 2-CP was administered at postnatal day (P)7, 14 or 21. Mechanical withdrawal threshold pre- and post-injection evaluated the degree and duration of sensory block, compared to intrathecal saline and naive controls. Tissue analyses one- or seven-days following injection included histopathology of spinal cord, cauda equina and brain sections, and quantification of neuronal apoptosis and glial reactivity in lumbar spinal cord. Following intrathecal 2-CP or saline at P7, outcomes assessed between P30 and P72 included: spinal reflex sensitivity (hindlimb thermal latency, mechanical threshold); social approach (novel rat versus object); locomotor activity and anxiety (open field with brightly-lit center); exploratory behavior (rearings, holepoking); sensorimotor gating (acoustic startle, prepulse inhibition); and learning (Morris Water Maze). Maximum tolerated doses of intrathecal 2-CP varied with age (1.0 μL/g at P7, 0.75 μL/g at P14, 0.5 μL/g at P21) and produced motor and sensory block for 10-15 min. Tissue analyses found no significant differences across intrathecal 2-CP, saline or naïve groups. Adult behavioral measures showed expected sex-dependent differences, that did not differ between 2-CP and saline groups. Single maximum tolerated in vivo doses of intrathecal 2-CP produced reversible spinal anesthesia in juvenile rodents without detectable evidence of developmental neurotoxicity. Current results cannot be extrapolated to repeated dosing or prolonged infusion.

Contribution of CD137L to Sensory Hypersensitivity in a Murine Model of Neuropathic Pain

CD137L (4-1BBL) is a costimulatory molecule whose signaling can promote monocyte/macrophage functions; however, CD137L-mediated microglial response and its role in neuropathic pain remain unknown. We investigated CD137L following peripheral nerve injury-induced neuropathic pain using a spinal nerve L5 transection (L5Tx) murine model in both sexes. First, C57BL/6_CD137L knock-out (KO) mice displayed decreased mechanical and diminished heat hypersensitivity compared to wild-type (WT) controls, beginning on day 3 to up to day 35 post-L5Tx. Purified anti-mouse CD137L neutralizing monoclonal antibody (0.1 or 0.5 µg) was also used to identify CD137L’s window of action in BALB/c mice. Anti-CD137L antibody was intrathecally administered either from day 0 (before surgery) to day 7 (early treatment), or from day 6 to 13 post-L5Tx (late treatment), and nociceptive thresholds were assessed before surgery to up to day 35 post-surgery. Early treatment with anti-CD137L reduced L5Tx-induced mechanical but not heat hypersensitivity, while later treatment did not alter either sensitivity. Pro- versus anti-inflammatory responses within the lumbar spinal cord following L5Tx were further evaluated via quantitative real-time PCR (qRT-PCR) and immunohistochemistry (IHC) in time-course studies. Following L5Tx, female CD137L KO mice did not show increased iNOS mRNA and had reduced numbers of IL-1β⁺ cells compared to WT. At 21 d post-surgery, CD137L KO mice had higher total numbers of arginase (Arg)-1⁺ cells and Arg-1⁺ microglia. Altogether, results indicate that spinal cord CD137L contributes to the development of peripheral nerve injury-induced neuropathic pain, which may be in part mediated through CD137L’s modulation of the pro- and anti-inflammatory balance within the spinal cord.

Developmental Changes in Pain and Spinal Immune Gene Expression after Radicular Trauma in the Rat

Neuropathic pain is chronic pain that develops after nerve injury and is less frequent in infants and children than in adults. Likewise, in animal models of neuropathic pain, allodynia and hyperalgesia are non-existent or attenuated in the infant, with a “switch” during development by which acute nerve injury transitions to chronic pain. Concomitant with the delay in neuropathic pain, there is a parallel delay in the ability of nerve injury to activate the immune system. Models of neuropathic pain in the infant have used various ligation methods and find that neuropathic pain does not occur under after postnatal days 21–28 (PN21–PN28), linked to activation of immune processes and developmental regulation of anti-inflammatory cytokines. We applied a model of neuropathic pain in the adult using a transient compression of the cervical nerve or nerve root in infant rats (injured at 10, 14, 21, or 28 days of age) to define transition periods during which injury results in no change in thermal and mechanical pain sensitivity or in short-term changes in pain. There was little to no hyperalgesia when the injury was imposed at PN10, but significant thermal hyperalgesia and mechanical allodynia 1 day after compression injury when performed at PN14, 21, or 28. Thermal withdrawal latencies returned to near baseline by 7 days postsurgery when the injuries were at PN14, and lasted up to 14 days when the injury was imposed at PN28. There was mechanical allodynia following injury at 1 day postinjury and at 14 days after injury at PN14. Measurements of mRNA from spinal cord at 1, 7, and 14 days postinjury at PN14, 21, and 28 showed that both the magnitude and duration of elevated immune markers and chemokines/cytokines were greater in the older animals, corresponding to the development of hyperalgesia. Thus, we confirm the late onset of neuropathic pain but found no evidence of emergent hyperalgesia if the injury was before PN21. This may be due to the use of a transient, and not sustained, compression ligation model.

Interactions between glia, the immune system and pain processes during early development

Pain is a serious problem for infants and children and treatment options are limited. Moreover, infants born prematurely or hospitalized for illness likely have concurrent infection that activates the immune system. It is now recognized that the immune system in general and glia in particular influence neurotransmission and that the neural bases of pain are intimately connected to immune function. We know that injuries that induce pain activate immune function and suppressing the immune system alleviates pain. Despite this advance in our understanding, virtually nothing is known of the role that the immune system plays in pain processing in infants and children, even though pain is a serious clinical issue in pediatric medicine. This brief review summarizes the existing data on immune-neural interactions in infants, providing evidence for the immaturity of these interactions.

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.

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.

Intrathecal clonidine in the neonatal rat: dose-dependent analgesia and evaluation of spinal apoptosis and toxicity

BACKGROUND

Neuraxial clonidine is used for perioperative analgesia in children of all ages. Preclinical studies in the postnatal rat allow comparison of the relative toxicity and safety of spinal analgesics throughout postnatal development.

METHODS

Rat pups aged 3, 7, or 21 postnatal (P) days were briefly anesthetized for intrathecal injections of saline or clonidine. At each age, the maximum tolerated, antinociceptive (increased hindlimb mechanical withdrawal threshold) and antihyperalgesic (hindpaw carrageenan inflammation) doses were determined. Lumbar spinal cord sections were assessed for apoptosis and cell death (histology, activated caspase-3 immunohistochemistry, Fluoro-Jade C staining), histopathology (hematoxylin and eosin staining), and increased glial reactivity (microglial and astrocytic markers). P3 intrathecal ketamine sections served as positive controls. In additional groups, thermal latency and mechanical withdrawal threshold were measured at P35.

RESULTS

Intrathecal clonidine produces age- and dose-dependent analgesia in rat pups. Maximal doses of clonidine did not alter the degree or distribution of apoptosis or increase glial reactivity in the neonatal spinal cord. No spinal histopathology was seen 1 or 7 days after injection at any age. Intrathecal clonidine did not produce persistent changes in reflex sensitivity to mechanical or thermal stimuli at P35.

CONCLUSIONS

Intrathecal clonidine in the postnatal rat did not produce signs of spinal cord toxicity, even at doses much larger than required for analgesia. The therapeutic ratio (maximum tolerated dose/antihyperalgesic dose) was >300 at P3, >30 at P7, and >10 at P21. These data provide additional information to inform the clinical choice of spinal analgesic drug in early life.

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.

The development of nociceptive circuits

The study of pain development has come into its own. Reaping the rewards of years of developmental and molecular biology, it has now become possible to translate fundamental knowledge of signalling pathways and synaptic physiology into a better understanding of infant pain. Research has cast new light on the physiological and pharmacological processes that shape the newborn pain response, which will help us to understand early pain behaviour and to design better treatments. Furthermore, it has shown how developing pain circuitry depends on non-noxious sensory activity in the healthy newborn, and how early injury can permanently alter pain processing.

Differential efficacy of intrathecal NMDA receptor antagonists on inflammatory mechanical and thermal hyperalgesia in rats

Spinal cord dorsal horn N-methyl-D-aspartate (NMDA) receptors have been implicated in central sensitization, enhanced responsiveness to peripheral stimuli following peripheral injury. Since hyperalgesia is a behavioral consequence of central sensitization, it should be attenuated at the level of the dorsal horn with NMDA receptor antagonists. However, responsiveness to thermal and mechanical hyperalgesia may be distinct, and have thus far not been directly compared in chronic inflammatory pain models. In the present study, inflammation was induced with complete Freund's adjuvant (CFA) injected into the rat hind paw and NMDA receptor antagonists dizocilpine (MK-801) or 2-amino-5-phosphonovaleric acid (AP-5) were intrathecally injected in rats to determine the effects on both mechanical and thermal hyperalgesia. Locomotor tests and reflexes were also conducted to evaluate potential motor side effects. The NMDA receptor antagonists dose-dependently ameliorated mechanical hyperalgesia, but had marginal effects on thermal hyperalgesia. In ranges near antihyperalgesic doses, significant disruption of motor coordination was observed for both antagonists. These results suggest that, depending on the stimulus, NMDA receptors may have variable significance for central sensitization-mediated hyperalgesia, and that NMDA receptor antagonists may have therapeutic potential for some, but not all components in the clinical manifestation of inflammatory pain.

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 trigeminal nerve and augmentation of regional cerebral blood flow during experimental bacterial meningitis

We investigated whether trigeminal nerve fibers contribute to enhanced regional cerebral blood flow (rCBF) in a rat model of experimental bacterial meningitis. rCBF was measured continuously for 6 h by laser Doppler flowmetry through thinned bone over the frontal cortex. Meningitis was induced with pneumococcal cell wall components and confirmed by a significant increase of (a) leukocytes within the cerebrospinal fluid, (b) brain water content, (c) intracranial pressure and (d) rCBF. The increase of rCBF was significantly attenuated (p < 0.05) at 3, 4, 5, and 6 h in animals after a chronic (200 +/- 21% versus 138 +/- 13% at 6 h on the intact and denervated sides, respectively) but not after an acute section of the nasociliary branch of the trigeminal nerve. We conclude that elevations in blood flow during the early phase of bacterial meningitis are mediated in part by the trigeminal nerve, probably by local perivascular release of neuropeptides from afferent axons innervating the meninges.

Lumbar catheterization of the spinal subarachnoid space in the rat

The method commonly used for catheterization of the lumbar subarachnoid space in the rat implies inserting the catheter through the atlanto-occipital (A-O) membrane and moving the catheter caudally along the spinal cord. The method is associated with a considerable morbidity. A method for direct catheterization of the lumbar subarachnoid space was therefore developed. Major surgery was avoided by using a catheter-through-needle technique. Of 32 rats, none died. There were no signs of neurological disturbances, and all animals gained weight as normal the first week after implantation. Data from rats catheterized by the A-O method were used for comparison. Of 40 animals, 2 died, 11 showed signs of neurological disturbances, and the mean weight was reduced during the first week after catheterization. The two groups of animals showed different behavioural responses to intrathecal injection of N-methyl-D-aspartate (NMDA, 0.1-1.6 mM, 15 microliters) which is thought to stimulate afferent pathways mediating nociception. Animals with a lumbar catheter showed licking, biting and scratching behaviour in a dose-related manner for concentrations up to 1.6 mM. The animals with A-O catheters showed a maximum level of this behaviour already at 0.4 mM, while 0.5 mM induced convulsions. A possible explanation of this difference in response to NMDA could be a long-lasting pain state in the A-O group, caused by catheter-induced changes in the spinal cord and by the extensive surgery. It is concluded that the direct lumbar catheterization has several advantages compared to the A-O method, decreasing the suffering of the animals, the neurological disturbances and the interference with nociceptive functions of the spinal cord.

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.

Role of the spinal cord in intracisternal beta-endorphin-evoked suppression of liver DNA synthesis in 10-day-old rats

Previously, we have shown that intracisternal (i.c.) administration of beta-endorphin (an opioid peptide naturally occurring in the brain) to preweanling rats markedly decreases DNA synthesis (an index of cell proliferation) in both brain and liver. This observation is consistent with our hypothesis that endogenous CNS beta-endorphin plays an important role in controlling postnatal growth. The current research specifically undertook to investigate, in 10-day-old rats, whether or not i.c. beta-endorphin-evoked suppression of liver DNA synthesis is actually mediated by spinal opioid receptors and/or by descending endorphinergic pathways. In contrast to the i.c. route of administration, beta-endorphin given directly into the spinal subarachnoid space via intrathecal (i.t.) injection did not alter liver DNA synthesis, yet was able to evoke profound antinociceptive responses. This demonstrates that intracisternally applied beta-endorphin exerts its effect on liver DNA by acting at supraspinal sites, and not by directly stimulating spinal opioid receptors after diffusion from its intracerebral site of injection. As it is possible that beta-endorphin's supraspinal actions may activate a descending inhibitory endorphinergic pathway to reduce DNA synthesis, we conducted studies in rat pups administered naloxone intrathecally. Naloxone i.t. was completely ineffective in preventing beta-endorphin i.c. from inhibiting liver DNA synthesis. On the other hand, i.t. coinjection of naloxone plus beta-endorphin was able to block the analgesic response, while their i.c. coinjection reversed the DNA effect. The results from these studies indicate that opioid systems within the spinal cord do not play a major role in mediating CNS beta-endorphins regulation of DNA synthesis in peripheral tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of the novel analgesic, S 12813-4, on the spinal release of substance P- and calcitonin gene-related peptide-like materials in the rat

The possible inhibitory control by the novel analgesic S 12813-4 (3-(2-(4-phenylpiperazine-1-yl)-ethyl)-2-oxo-2,3- dihydrooxazolo(b)pyridine) of spinal neurones containing substance P (SP) and/or calcitonin gene-related peptide (CGRP) was assessed in vitro and in vivo in the rat. S 12813-4 (10 nM-0.1 mM) did not affect the spinal release of CGRP-like material (CGRPLM) but inhibited in a concentration dependent manner the K(+)-evoked overflow of SP-like material (SPLM) from slices of the dorsal half of the rat lumbar enlargement. The inhibitory effect of 10 microM S 12813-4 on SPLM release was not additive with that of Na (0.1 mM), and could be prevented by the alpha 2-adrenoceptor antagonist idazoxan (10 microM). Similarly, idazoxan (10 microM) suppressed the inhibition by intrathecally administered S 12813-4 (10 microM) of the spinal outflow of SPLM in halothane anaesthetized rats whose intrathecal space was perfused with an artificial cerebrospinal fluid. These data suggest that the analgesic effect of S 12813-4 might involve some alpha 2-adrenoreceptor-mediated control of SPLM release within the spinal cord. Whether this control concerns SP-containing primary afferent fibres (presynaptic inhibition) or SP-containing interneurones and/or bulbo-spinal SP-ergic pathways (postsynaptic inhibition) deserves further investigations.

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

Little is known of the neural bases of analgesia in immature animals. This experiment examined the effects of intracerebroventricular (i.c.v.) and intrathecal (i.t.) administration of morphine or ketocyclazocine in tests of antinociception in rats aged 3 to 14 days of age. Analgesia tests were conducted using both thermal and mechanical (pressure) noxious stimuli applied to the forepaw, hindpaw or tail. In the 3-day-old morphine-injected i.c.v. produced analgesia in the forepaws when either the mechanical or thermal noxious stimulus was used. There was no effect when the hindpaw or tail was tested. At 10 days of age, when the mechanical stimulus was used, morphine was analgesic in tests on all three appendages but was only effective in the forepaw when the thermal stimulus was used. Morphine was fully effective in all tests with both stimuli at 14 days of age. Ketocyclazocine had no consistent effect when given i.c.v. When injected i.t., morphine produced analgesia in the forepaws in the thermal test at 4 days of age and in all appendages by 10 days. When the mechanical test was used, morphine was effective in all appendages at all ages tested. Ketocyclazocine was analgesic at all appendages for the mechanical stimulus at all ages but was only transiently effective in the thermal test. The results demonstrate differential development of analgesia mediated at different levels of the neural axis and are consistent with the development of descending inhibitory that may mediate analgesia induced by i.c.v. injections of morphine. Neural mechanisms that are involved in the analgesic effects of these drugs against the two types of stimuli are also developmentally distinct.

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.

Tolerance and withdrawal to chronic morphine treatment in the week-old rat pup

Chronic treatment of adult animals with morphine results in tolerance but there are fewer reports on the effects of chronic opiates during ontogeny. The present experiments assessed the development of morphine-induced tolerance and withdrawal in infant rats. Pups were injected with morphine twice daily from ages 1-7 days and then tested on day 7 for morphine-induced analgesia in a hot-water immersion test, and separation-induced ultrasonic vocalizations in response to isolation from the dam and littermates at 7 and 10 days of age. Tolerance occurred to the analgesic effects of morphine but not to its suppression of ultrasonic vocalizations. Separation-induced vocalizations were greatly increased in chronic morphine-treated pups following naltrexone-precipitated withdrawal at 7 days of age. The increase in ultrasonic vocalizations following naltrexone treatment in morphine exposed pups may be a developmentally unique sign of opiate withdrawal.

The development of stimulation-produced analgesia (SPA) in the rat

The present study studied the development of stimulation produced analgesia (SPA) from the periaqueductal gray (PAG) in rats. A monopolar stimulating electrode was lowered into the dorsal or ventral PAG of animals aged 7, 14, 21, or 90-120 days. Constant current cathodal pulses (100 Hz, 100 microseconds) were delivered, starting 10 s before analgesia was tested by the tail-flick (TF) test and continuing throughout each TF trial or until cut-off (7 s). Current intensity was increased stepwise (3-200 microA). It was found that SPA can be elicited starting at 21 days, but not earlier. However, supraspinal modulation of nociception is still immature at 3 weeks after birth. First, stimulation intensities needed to produce SPA are higher in 21-day-old pups than in adult animals. Second, in 21-day-old pups, but not in adults effective current intensities in the dorsal PAG are higher than in the ventral PAG. Third, naltrexone decreases SPA from the ventral PAG in 21-day-old pups, but not in adult animals. These findings indicate that supraspinal modulation of nociception develops only 3 weeks after birth, with the ventral PAG maturing prior to the dorsal PAG, and that the contribution of endogenous opioids to SPA does not remain constant throughout the ontogeny of rats.

Effects of systemically administered monoamine reuptake blocking agents on patterns of buspirone-induced analgesia in rats

We have recently shown the serotonin 5-HT1A receptor agonist buspirone to produce analgesia in several pain tests in rats. As a 5-HT1A agonist, buspirone may change serotonergic (5-HT) tone to alter the balance of central monoaminergic (MA) systems that function in analgesia. MA-reuptake blocking drugs have been shown to produce analgesia, both when given alone and in combination with a variety of other agents, presumably via their action upon MA neurochemistry. The present study was undertaken to examine the effect of systemic administration of the 5-HT-reuptake blocker amitriptyline (AMI: 10 mg/kg), NE-reuptake blocker desipramine (DMI: 10 mg/kg) or DA-reuptake blocker GBR-12909 (7.5 mg/kg) on patterns of analgesia produced by buspirone (1-5 mg/kg) in thermal and mechanical pain tests in rats. Neither reuptake blocking agents or buspirone, when administered alone or in combination, produced overt changes in motor activity at the doses tested. AMI alone was not analgesic in either thermal or mechanical pain tests. In both assays, AMI reduced the analgesic action of buspirone, with greater effects seen in the thermal test. When administered alone, DMI produced significant analgesia against thermal and mechanical pain. DMI significantly attenuated the analgesic action of all doses of buspirone in both pain tests. Alone, GBR-12909 did not affect nociception in thermal or mechanical tests. GBR-12909 decreased buspirone-induced analgesia at all doses in the thermal test, while having no effect on buspirone-induced analgesia against mechanical pain. These results demonstrate that facilitation of 5-HT, NE and DA function with reuptake blocking drugs did not enhance the analgesic action of buspirone. These data indicate against the adjuvant use of reuptake blocking compounds and buspirone as analgesics. Furthermore, such findings may suggest other possible non-MA substrates of buspirone-induced analgesia.

Effects of neonatal spinal cord serotonin depletion on opiate-induced analgesia in tests of thermal and mechanical pain

There is considerable evidence that serotonin (5-HT) is involved in the analgesic actions of various opiates. However, it is less clear which opioid receptor types interact with these descending systems and whether the various monoaminergic pathways are specific for different types of nociceptive signals. In the present study we lesioned the spinal cord serotonin pathways by neonatal spinal injections of 5,7-dihydroxytryptamine (5,7-DHT) and tested the analgesic effects of morphine and ketocyclazocine one and two weeks later using both mechanical and thermal noxious stimuli. The treatment depleted spinal cord serotonin by more than 90% while not affecting norepinephrine levels. The effects of morphine were greatly attenuated in the depleted animals when the thermal noxious stimulus was applied. The analgesic actions of morphine were only slightly affected when the mechanical stimulus was applied. The effects of ketocyclazocine were not reduced by the treatment. The results further buttress the conclusion that at least part of morphine's analgesic effects are mediated by descending serotonin systems and that these systems are primarily effective against a thermal stimulus. The data suggest that non-5-HT brainstem system(s) are involved in morphine-induced analgesia to a mechanical noxious stimulus.