Drug and environmentally induced manipulations of the opiate and serotonergic systems alter nociception in neonatal rat pups

Maternal presence or absence alters nociceptive responding and cortical anandamide levels in juvenile female rats

The influence of parental support on child pain experiences is well recognised. Accordingly, animal studies have revealed both short- and long-term effects of early life stress on nociceptive responding and neural substrates such as endocannabinoids. The endocannabinoid system plays an important role in mediating and modulating stress, social interaction, and nociception. This study examined the effects of maternal support or acute isolation on nociceptive responding of female rats to a range of stimuli during the juvenile pre-adolescent period and accompanying changes in the endocannabinoid system. The data revealed that juvenile female Sprague Dawley rats (PND21-24) isolated from the dam for 1 h prior to nociceptive testing exhibited increased latency to withdraw in the hot plate test and increased mechanical withdrawal threshold in the Von Frey test, compared to rats tested in the presence of the dam. Furthermore, isolated rats exhibited reduced latency to respond in the acetone drop test and enhanced nociceptive responding in the formalin test when compared to dam-paired counterparts. Anandamide, but not 2-AG, levels were reduced in the prefrontal cortex of dam-paired, but not isolated, juvenile rats following nociceptive testing. There was no change in the expression of CB1, FAAH or MAGL; however, CB2 receptor expression was reduced in both dam-paired and isolated rats following nociceptive testing. Taken together the data demonstrate that brief social isolation or the presence of the dam modulates nociceptive responding of juvenile rat pups in a modality specific manner, and suggest a possible role for the endocannabinoid system in the prefrontal cortex in sociobehavioural pain responses during early life.

The role of social isolation in ethanol effects on the preweanling rat

The present experiments investigated the effects of acute ethanol exposure on voluntary intake of 0.1% saccharin or water as well as behavioral and nociceptive reactivity in 12-day-old (P12) rats exposed to differing levels of isolation. The effects of ethanol emerged only during short-term social isolation (STSI) with different patterns observed in males and females and in pups exposed to saccharin or water. The 0.5g/kg ethanol dose selectively increased saccharin intake in females, decreased rearing activity in males and attenuated isolation-induced analgesia (IIA) in all water-exposed pups. Ingestion of saccharin decreased IIA, and the 0.5g/kg ethanol dose further reduced IIA. The 1.0g/kg ethanol dose, administered either intragastrically or intraparentionally, also decreased IIA in P12 females, but not in P9 pups. A significant correlation between voluntary saccharin intake and baseline nociceptive reactivity was revealed in saline injected animals, saccharin intake was inversely correlated with behavioral activation and latency of reaction to noxious heat after 0.5g/kg ethanol in females. The 0.5g/kg ethanol dose did not affect plasma corticosterone (CORT) measured 5h after maternal separation or 20min after ethanol injection. Female pups CORT level was inversely correlated with magnitude of IIA that accompanied the first episode of STSI (pretest isolation) 1.5-2h before CORT measurement. The present findings suggest that the anxiolytic properties of ethanol are responsible for enhancement of saccharin intake during STSI. Furthermore, differential reactivity of P12 males and females to STSI plays an important role in ethanol effects observed at this age.

Sensory feedback modulates quipazine-induced stepping behavior in the newborn rat

Research has shown that sensory feedback modulates locomotor behavior in intact as well as spinal adult animals. Here we examined if locomotor activity ("stepping") in newborn rats is influenced by cutaneous and proprioceptive feedback. One-day-old rats were treated with the serotonergic receptor agonist quipazine (3.0mg/kg) to induce air-stepping behavior or with saline (vehicle control). During stepping, a substrate/floor (elastic, stiff, or none) was placed beneath their limbs so that the feet could make plantar surface contact with a substrate. Pups treated with quipazine showed significantly more alternated fore- and hindlimb steps and plantar paw contact with the substrate, compared to pups treated with saline. Pups also made proportionately less contact with the stiff substrate versus the elastic substrate during stepping. Different types of movements made on the substrate (paw pushes, taps, swipes, and stances) were also characterized. These results indicate that sensory feedback modulates locomotor mechanisms and behavior in perinatal rats.

Alone but feeling no pain: Effects of social exclusion on physical pain tolerance and pain threshold, affective forecasting, and interpersonal empathy

Prior findings of emotional numbness (rather than distress) among socially excluded persons led the authors to investigate whether exclusion causes a far-reaching insensitivity to both physical and emotional pain. Experiments 1-4 showed that receiving an ostensibly diagnostic forecast of a lonesome future life reduced sensitivity to physical pain, as indicated by both (higher) thresholds and tolerance. Exclusion also caused emotional insensitivity, as indicated by reductions in affective forecasting of joy or woe over a future football outcome (Experiment 3), as well as lesser empathizing with another person's suffering from either romantic breakup (Experiment 4) or a broken leg (Experiment 5). The insensitivities to pain and emotion were highly intercorrelated.

Reinforcing properties of ethanol in neonatal rats: involvement of the opioid system

Toward understanding why infant rats ingest high levels of ethanol without initiation procedures, the authors tested effects of mu and kappa receptor antagonists on ethanol reinforcement in neonatal rats. After an intracisternal injection of CTOP (micro antagonist), nor-Binaltorphimine (kappa antagonist), or saline, newborn (3-hr-old) rats were given conditioning pairings of an odor with intraorally infused ethanol or a surrogate nipple with ethanol administered intraperitoneally (to minimize ethanol's gustatory attributes). In each case, these opioid antagonists reduced or eliminated ethanol's reinforcement effect. The same effects occurred with saccharin as the reinforcer in olfactory conditioning. The results imply that activation of mu and kappa receptors, apparently acting jointly, is necessary for reinforcement or that antagonists of this activity impair basic conditioning.

Why Does Social Exclusion Hurt? The Relationship Between Social and Physical Pain

The authors forward the hypothesis that social exclusion is experienced as painful because reactions to rejection are mediated by aspects of the physical pain system. The authors begin by presenting the theory that overlap between social and physical pain was an evolutionary development to aid social animals in responding to threats to inclusion. The authors then review evidence showing that humans demonstrate convergence between the 2 types of pain in thought, emotion, and behavior, and demonstrate, primarily through nonhuman animal research, that social and physical pain share common physiological mechanisms. Finally, the authors explore the implications of social pain theory for rejection-elicited aggression and physical pain disorders.

Effect of neonatal beta-endorphin imprinting on sexual behavior and brain serotonin level in adult rats

A single dose (3 microg) beta-endorphin was administered to newborn female and male rats (hormonal imprinting). In adult age (at 5 months) sexual behavior, steroid hormone binding capacity and brain serotonin content was studied. Females' sexual activity (lordosis quotient) significantly decreased and more animals protested against mounting (ratio of kicking and crying 21/24 vs. 8/24; p < 0.001). Males' sexual activity did not change, however more males were aggressive (4/10 vs. 1/10). Uterine estrogen receptor density significantly increased and affinity decreased. There was no change in the binding capacity of thymic glucocorticoid receptors. In the brain, five regions were studied for serotonin content. There was a gender difference in serotonin level and the intragroup differences were also high. In the endorphin treated males the serotonin level was significantly lower than in the controls. In the endorphin treated females the intragroup scattering has been significantly reduced. Nociceptin content of the cerebrospinal fluid was not changed. The experiments call attention to the possibility of adjustment of sexual and behavioral sphere by the individually different endorphin surge during labor.

The first suckling episode in the rat: the role of endogenous activity at mu and kappa opioid receptors

The present study examined the role of endogenous activity at mu and kappa opioid receptors in attachment to and ingestion of milk from a surrogate nipple in cesarean-delivered newborn rats prior to regular suckling experience. Selective opioid antagonist drugs were injected into the cisterna magna (IC administration) or lateral ventricles (ICV administration). Blockade of endogenous activity at mu opioid receptors by IC administration of the selective antagonist CTOP reduced attachment time and markedly increased disengagements from the nipple. CTOP also increased the intensity of suckling measured as milk intake per min attached to the nipple, when milk was available from the nipple in a free-access regime, and enhanced intake when milk was infused through an intraoral cannula aside from the suckling context. The ICV administration of the selective kappa antagonist nor-BNI considerably increased latency to grasp the surrogate nipple, while time on the nipple and milk intake were decreased. The presented data suggest that populations of mu and kappa receptor-containing neurons, differentiable by the route of antagonist administration, play an important role in initiation and maintenance of suckling behavior in the newborn rat during its first encounter with the nipple and milk. The kappa opioid system is predominantly involved in the initiation of the newborn's behavior directed toward the nipple providing milk. The role of the mu opioid system seems more complicated: it transforms initial oral grasp responses into sustained attachment to the nipple and maintains the intake of milk at a certain physiological level.

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.

Pharmacological characterization of capsaicin-induced body movement of neonatal rat

In neonatal rats, nociceptive responses induced by capsaicin were characterized pharmacologically. Capsaicin, injected subcutaneously (s.c.), induced body movement including scratching and struggling responses, and the responses were quantified by using a device composed of an audio speaker as a detector. The capsaicin-induced body movement was inhibited by a tachykinin NK1-receptor antagonist RP-67580 with an ID50 value of 3.5 mg/kg, s.c. Opioid analgesics, morphine, buprenorphine and pentazocine, also inhibited the body movement with ID50 values of 0.085, 0.0079 and 0.92 (mg/kg, s.c.), respectively. Non-steroidal anti-inflammatory drugs, indomethacin, ibuprofen and acetaminophen, did not exert any effect on the capsaicin-induced body movement. Neither the sedative diazepam nor the sedative chlorpromazine inhibited the body movement. It is concluded that the capsaicin-induced body movement in neonatal rats, which is considered to be nociceptive responses mediated by substance P, is sensitive to centrally acting analgesics with micro-opioid receptor agonist activity.

A novel method for detecting the analgesic effect of drugs by quantifying body movement after noxious stimulation in neonatal rat

We developed a method for quantifying nociceptive struggling in neonatal rats induced by bolus, subcutaneous injection of capsaicin (3-3000 ng). The response was quantified by using an audio speaker and electrical instruments such as an amplifier, a rectifier, and a monostable multivibrator. Using this method, we were able to quantify the nociceptive response which appeared immediately after injection of capsaicin. The response peaked at 0-1 min, and then decayed during next 1-3 min. Furthermore, this method also detected that the magnitude of the response increased dose-dependently up to the maximum dose of 3000 ng. In an experiment testing the suitability of this method for screening analgesics, it was shown that morphine (0.03-0.3 mg/kg, s.c.) inhibited the response dose-dependently with an ID50 value of 0.089 mg/kg and that the analgesic effect of morphine (1 mg/kg, s.c.) was reversed by naloxone (0.03-0.1 mg/kg, s.c.). Thus it can be concluded that this method is useful to quantify the capsaicin-induced nociceptive response and is suitable for screening analgesics.

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.

Evidence for opioid and nonopioid processes mediating adaptive responses of infant rats that are repeatedly isolated

Previous research examining the ability of neonatal rats to adapt to repeated isolation demonstrated that an opioid-dependent decline in ultrasonic vocalizations occurred across a series of isolations (Goodwin, Molina, & Spear, 1994). These findings were expanded in the present study. In the first experiment, the decline in vocalization rates was found to result from the release of endogenous opioids throughout the series of isolations. Although naltrexone attenuated the decline in calling rates relative to vehicle-treated subjects, there was still a significant decline in calling rates following opioid receptor blockade. In the second experiment, two injections of naltrexone did not attenuate the decline in calling rates any more than a single injection did, suggesting that there must also be some nonopioid process that modulates this decline. In both experiments, activity levels and, in the first experiment, the amount of body heat lost in the repeatedly isolated subjects declined in a nonnaltrexone reversible manner. In a final study, after calling rates had been suppressed by a series of isolations, a brief exposure to the mother was found to restore baseline calling rates, suggesting the decline is not the consequence of fatigue. The attenuation of vocalization rates, activity, and loss of body heat are adaptive responses of infant rats to isolation; however, of these three, only the attenuation of vocalization rates is consistently modulated by the release of endogenous opioids.

Neonatal isolation alters LTP in freely moving juvenile rats: sex differences

We have previously reported that neonatal isolation significantly enhanced the magnitude of hippocampal long-term potentiation (LTP) recorded from freely moving male rats tested at 30 days of age. The present study extends this work to examine the effects of neonatal isolation on hippocampal LTP in male and female juvenile rats. Changes in dentate granule cell population measures, i.e., EPSP slope and population spike amplitude (PSA), evoked by tetanization of the medial perforant pathway were used to assess the effects of neonatal isolation on LTP over a period of 96 hrs. Prior to tetanization, significant sex differences were obtained for input/output (I/O) response measures of EPSP slope and PSA, with males showing consistently higher values than females. No significant effect of treatment was obtained within either sex for baseline measures. Following tetanization significant sex differences were also obtained for both measures, with males showing significantly greater enhancement than females. Comparisons made at 1 hr post-tetanization (establishment of LTP) indicated that isolated males showed significantly greater enhancement than any other group. On the other hand, treatment differences were not obtained from females. At 96 hrs (maintenance of LTP), however, both neonatally isolated males and females showed significantly greater enhancement than either non-isolated siblings or unhandled controls. These results indicate that males and females exhibit different enhancement profiles with respect to both the magnitude and duration of LTP, and that neonatal isolation alters these profiles in a sex-specific manner.

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.

A comparison of the antinociceptive effects of opioid agonists in neonatal and adult rats in phasic and tonic nociceptive tests

Changes in the attitudes about neonatal pain and pain management have recently resulted in increases in the administration of opioids to neonates. Little is known, however, about the relative potencies of the various opioid agonists employed, especially in comparison to adult responses. The first objective in the present study was to compare the antinociceptive potency of four clinically relevant opioids in neonatal and adult rats. The second objective was to compare and contrast these agents in two different types of nociceptive tests: tonic (formalin-induced inflammation) and phasic (tail flick and hot plate). Our results indicate that the opioid agonists morphine, meperidine, and fentanyl, and the mixed agonist buprenorphine were all effective antinociceptive agents in both neonates and adults in each of the three tests employed, and that the relative potencies of these agents appeared to be similar in neonates and adults. In general, the pups were more sensitive to the antinociceptive agents when tested in the phasic nociceptive tests, and the drugs were more potent in the tonic test than either of the phasic tests.

Repeated exposure of rat pups to isolation attenuates isolation-induced ultrasonic vocalization rates: reversal with naltrexone

Young rat pups are dependent on the dam for their survival, thus isolation of the neonatal rat pup from the dam presents the young organism with a variety of stressors. The question examined in this study concerns the ability of the young rat pup to modify its response to isolation following repeated exposure to that isolation as well as the role played by endogenous opiates in this process. Following repeated isolations, pups were seen to decrease vocalization rates. Altering the context in an attempt to dishabituate animals failed to reverse the decreased vocalization rate. However, opiate receptor blockade attenuated this decrease when administered subsequent to the first isolation period but not prior to the last isolation period. These results suggest that the development of this attenuated response to isolation stress is opiate-mediated but that once established, its expression is not dependent on endogenous opiate release.

Prenatal cocaine exposure increases the behavioral sensitivity of neonatal rat pups to ligands active at opiate receptors

Offspring of dams given 40 mg/kg cocaine HCl (C40) from gestational day 8-20 (E8-E20), pair-fed dams injected daily with saline (PF), nutritional control dams placed on a 40% cellulose based diet and injected with saline daily (NC), and untreated dams (LC) were examined. Offspring were given morphine (0.0, 0.1, or 0.5 mg/kg SC) on postnatal day 10-11 (P10-11) in Experiment 1, and isolation-induced ultrasonic vocalizations were measured. Planned comparisons indicated that both C40 and NC offspring exhibited a greater sensitivity to the morphine-related decrease in isolation-induced ultrasounds than LC controls. However, the presence of an anesthetized littermate suppressed isolation-induced ultrasounds equally across all groups, with all groups of offspring spending equal amounts of time in physical contact with the littermate. A tail-flick measure of analgesia indicated that PF animals were hyperalgesic relative to the other prenatal treatment groups; however, no differences in sensitivity to morphine were seen across the prenatal groups. In Experiment 2, animals were given the selective delta, [D-Pen2,D-Pen5]-enkephalin (DPDPE), and mu, [D-Ala2-NMe-Phe4Gly ol]-enkephalin (DAMGO) agonists ICV and ultrasonic vocalizations were recorded. Results indicated that both C40 and NC offspring were more sensitive to the low dose of DAMGO; however, because of the profound suppression of vocalizations seen at both doses of DPDPE, potential differences among the prenatal treatment groups in responsiveness to the delta agonist were difficult to detect.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

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.

5-HT3 receptor-active drugs alter development of spinal serotonergic innervation: lack of effect of other serotonergic agents

Our work has focused on identifying the type of serotonin receptor through which serotonin acts as a developmental signal in the central nervous system. Previously, we have found that the regulation of development of ascending serotonergic neurons is through the balance of two serotonin receptors. One, the 5-HT1a receptor, releases a growth factor from astroglial cells. The other receptor is related to a release-regulating autoreceptor and can be stimulated indirectly by serotonin releasers such as fenfluramine. In the present study, we examined the receptors which regulate development of the descending neurons by treating pregnant rats with selective serotonergic drugs, from gestation day 12 until birth. Pups were subsequently tested for alterations in development by nociceptive testing (tail-flick latency) and by determining the binding of 3H-paroxetine, an indicator of serotonin terminal density, in spinal cord. Our results show that agents stimulating the 5-HT1a receptor (8-OH-DPAT) or the 5-HT1b receptor (TFMPP) or substances which release serotonin (fenfluramine) had no effect on the development of spinal serotonergic pathways. However, agents acting on the 5-HT3 receptor did--the agonist phenylbiguanide (PG) increased latency on tail-flick testing (postnatal days 10 and 30), while the antagonist, MDL 72222, decreased latency (postnatal days 10 and 18). Interestingly, both the agonist and the antagonist significantly increased 3H-paroxetine binding on postnatal day 18. Our results are discussed in terms of a possible mechanism by which 5-HT3 receptors may influence development.

Behavioural effects in adulthood of neonatal manipulation of brain serotonin levels in normal and androgenized females

5HT concentrations in the hypothalamus are higher in females than males over the second week of life and this differentiation is testosterone-dependent. We have investigated the possible influence of 5HT over this period on the development of systems that control adult behaviour, in particular those influenced by neonatal testosterone. Neonatal androgenization (250 micrograms/pup testosterone propionate; TP; on day 1 postpartum) induced a masculine pattern of behaviour in females ovariectomised in adulthood and bearing a TP implant. The neonatal treatment reduced exploration, motor activity and female sexual behaviour and increased anxiety, orientation toward the incentive female and male sexual behaviour. Depletion of 5HT by pCPA (100 mg/kg days 8-16 postpartum) enhanced the TP-induced increment in locomotion and female sexual behaviour and increased sexual orientation toward the incentive female, while 5HTP (20 mg/kg days 8-16 postpartum) antagonised the reduction in exploration by TP. Thus 5HT may normally exert an inhibitory control on the action of neonatal testosterone on exploration, motor activity and sexual behaviour. Neonatal PCPA treatment also had a marked anxiolytic effect which was independent of the presence of T as it was noted in normal and androgenized females and previously has been observed in intact males. This might indicate a primary control by a serotonergic system on the development of the systems controlling anxiety.

Development of stress-induced responses in preweanling rats

This study examined in postnatal Days 7, 14, and 21 male rats the effects of social isolation and social isolation with administration of brief foot shocks on the development of stress-induced behavioral and pituitary-adrenal hormone responses. Day 21 rats appeared similar to adult rats in their repsonses to the two test conditions. That is, exposure to either isolation or to shock increased both pituitary-adrenal hormone secretion and tail-flick latencies but only administration of shock potentiated freezing and ultrasonic vocalizations. Younger rats differed from Day 21 rats in their responses to the two test conditions. In both tests, Day 7 rats produced the highest number of ultrasounds, which may be due to a significant decrease in body temperature. In contrast, in Day 14 pups, exposure to shock significantly reduced isolation-induced ultrasonic vocalizations. Additional age-dependent differences were found in the analgesic responses of Days 7 and 14 rats. Day 7 rats exposed to stress became consistently hyperalgesic whereas Day 14 rats showed only a short-lasting analgesic response. Although in Days 7 and 14 rats exposure to the two stress conditions produced significant elevations in pituitary-adrenal hormone concentrations, plasma levels were lower than those measured in Day 21 rats. To summarize, preweanling rats exhibit varied age-dependent responses when exposed to different stress environments.

Opioid-dependent behaviors in infant rats: effects of prenatal exposure to ethanol

Pregnant rats were given diets containing either 5% ethanol, an isocaloric (pair-fed) diet, or casein pellets. Offspring were tested at postnatal day 10 for isolation-induced ultrasonic vocalizations and subsequent stress-induced analgesia. Rats prenatally exposed to ethanol vocalized significantly less in the five minutes during isolation. The opiate, morphine, caused a greater suppression of vocalizations in alcohol-exposed pups compared to controls, while the increased calling normally seen with the opiate antagonist, naltrexone, was attenuated. In a test in which the pup withdraws a paw from a hot plate (48 degrees C), prenatal alcohol offspring demonstrated baseline latencies (no isolation) similar to controls but had greatly attenuated responses in their isolation-induced analgesia. Since both vocalization and analgesia responses have been determined to be modulated by the endogenous opioid system, the aberrant responses of the prenatal-ethanol-exposed offspring can be interpreted as failures to respond by opioid release/secretion to appropriate stimuli.

The effect of prenatal methadone exposure on development and nociception during the early postnatal period of the rat

The effects of prenatal exposure to methadone via Alzet osmotic minipump on early postnatal development and on nociceptive behavioral endpoints were assessed in Sprague-Dawley rat pups during the first three postnatal weeks. This treatment regimen appeared to produce no maternal toxicity, with dams developing and maintaining dependence upon methadone through parturition. Methadone-exposed dams exhibited a withdrawal syndrome consisting of wet-dog shakes, diarrhea, vocalizations and irritability when challenged with naloxone 24 h postpartum. Pups exhibited a similar withdrawal syndrome following naloxone challenge consisting of mouthing and licking, hyperactive response to touch and vocalizations 24 h postpartum. Although no significant difference in litter size was evident in methadone-treated litters, a 16% pup mortality rate was observed in these litters. Prenatal methadone-exposed pups exhibited a significant body weight reduction at birth that resolved by postnatal day 2 (P2) in males and P4 in females. Methadone-exposed pups exhibited significant developmental delay in the expression of the negative geotaxic response to a morphine challenge while, conversely, 21-day-old pups exhibited a significantly reduced analgesic response to this challenge. These studies indicate that this method of prenatal exposure to methadone can produce dependence in the dam and offspring without substantial mortality, induce developmental delay and alter analgesic responses to opiate challenge in exposed pups during the preweanling period.

Preferred tastes and opioid-modulated behaviors in neonatal rats

Ten-day-old rats were given various tastants to ingest, independent of mother and suckling. Relative to water, the pups ingested significantly more saccharin (0.5%) and NaCl (2.5%) and less quinine (0.4%). Pretreatment with the opiate antagonist, naltrexone, had no effect on water or quinine intake but significantly reduced that of saccharin and NaCl. Furthermore, the preferred solutions of saccharin and NaCl given intraorally caused a decrease in ultrasonic vocalizations of isolated pups and in a separate experiment caused an analgesic response to heat. Both phenomena were reversible with naltrexone administration, suggesting that preferred tastes elicit an endogenous opioid response significantly affecting behaviors seen during isolation.

Neurobehavioral assessment during the early postnatal period

Few laboratories investigating the neurobehavioral consequences of developmental toxicants assess offspring early in ontogeny other than examining physical maturation, reflex development and perhaps locomotor activity, measures which tap only a limited portion of the neurobehavioral capacities of young organisms. The importance of including a wider range of neurobehavioral assessments during the early postnatal period in developmental toxicology test batteries is discussed. Special considerations for the design of testing early in life are enumerated, and examples are given of suckling, cognitive and psychopharmacological tests that have been shown to be sensitive indicators early in life of the effects of gestational drug exposure.

Injection of 5,7-dihydroxytryptamine into the B3 raphe region of neonatal rat pups induces hyperalgesia but only slight alterations in ingestion-related behaviors

The effects of intrabrainstem injections of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into the B3 raphe region (nucleus raphe magnus and nucleus reticularis paragigantocellularis) on early ingestive behavior and nociception were assessed in Sprague-Dawley rat pups during the first postnatal week. Lesions resulted in a marked depletion of serotonin (5HT) in hindbrain without influencing 5HT levels in forebrain. Pretreatment with desipramine (DMI) resulted in a sparing of noradrenergic neurons from neurotoxic effects. The B3 lesion resulted in significant hyperalgesia as reflected by decreased latencies in tail flick testing. Although nipple attachment latencies in suckling tests were slightly increased by the lesion, no notable effects on mouthing or other ingestive-related behaviors were observed in testing conducted in an independent ingestion paradigm. These results suggest that whereas B3 serotonergic neurons may be functioning in an adult-typical manner to regulate analgesia during the early postnatal period, this raphe region may play only a slight role in the modulation of ingestion-related behaviors early in life.

Ontogeny of behavioral and hormonal responses to stress in prenatally stressed male rat pups

Effects of prenatal stress on stress-induced behavioral and hormonal responses were investigated in preweanling rats at two ages. Prenatal stress treatments involved the application of uncontrollable electric shocks to pregnant rats every other day throughout gestation. Offspring of undisturbed rats in home cages served as controls. When male pups were 14 and 21 days old, ultrasonic vocalizations and freezing were recorded in 10-min tests involving isolation, and isolation with the application of electric foot shocks at either 0.5- or 2.0-mA intensity. Immediately before and after each test, tail-flick latencies were measured in order to assess alterations in stress-induced analgesia. Stress-induced secretion of ACTH was measured in plasma obtained after the second tail-flick test. Results indicated that 14-day-old prenatally stressed pups emitted significantly fewer ultrasonic vocalizations and exhibited significantly lower percent increases in tail-flick latencies than control pups. Plasma ACTH, however, was significantly elevated in prenatally stressed rats, suggesting that exposure to different tests was a stress-inducing event. At 21 days of age, prenatally stressed rats no longer differed significantly from control males in the exhibition of ultrasonic vocalizations, defensive freezing, and tail-flick latencies. Plasma ACTH content, however, was significantly lower in prenatally stressed than control males after exposure to the isolation with 2.0-mA shock test. The involvement of motivational, maturational, and mediational factors is examined in order to account for these age-dependent and stressor-dependent differences in behavioral and hormonal responses occurring between prenatally stressed and control pups.

Effects of undernutrition during suckling on novelty-induced analgesia in young and adult rats

The effects of undernutrition on novelty-induced analgesia were investigated in young and adult rats. Rats were undernourished by feeding their dams an 8% casein diet from birth until weaning (21 days of age). Rats were exposed to an open field (novelty) for 2 min and the nociception was measured by the tail-flick method. At adult age, only well-nourished rats presented novelty-induced analgesia, suggesting that early undernutrition abolishes this response. At 21 days of age, the exposure to the open field had no effect on nociception of both nutritional groups, suggesting that some developmental factor is necessary for the emergence of novelty-induced analgesia.

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).

Developmental aspects of nociception

Research has documented the existence of multiple, endogenous systems that modulate nociception. Based on the effects of opioid antagonists and endocrine lesions, endogenous analgesia systems have been organized into four classes: neural-opioid, neural-nonopioid; hormonal-opioid; hormonal-nonopioid. Developmental research on the ontogeny of endogenous analgesic function has revealed differential rates of maturation. Front-paw shock, a stimulus that activates a neural-opioid analgesic response, has been shown to be functionally mature by 28 days of age in the rat. Similarly, hind-paw shock, a stimulus that elicits a neural-nonopioid analgesic response, reaches maturity after two months of age. However, the hormonal-opioid analgesic system activated by cold-water immersion reaches adult levels by 10 days of age. Food deprivation produces a hormonal-opioid analgesic response in adult rats, and food deprivation/isolation of rat pups has been found to elicit an analgesic response in 6-day-old rats. From these data it seems that the rate of development of the different endogenous analgesic systems is related to the activation of neural or hormonal components. Whether the differential rates of development and the neural-hormonal distinction are related to the ecological validity of the activating stimulus remains to be determined.

Serotonergic and opiate interactions in the modulation of drug- and environmental-induced analgesia in the neonatal rat pup

Serotonergic and opiate interactions in the modulation of drug- and environmental-induced analgesia were assessed in 6-day-old Sprague-Dawley-derived rat pups using tail-flick testing procedures. In these experiments the serotonergic antagonist metergoline was observed to attenuate both the analgesia induced by the opiate agonist morphine and the analgesia induced by isolation from siblings and the dam, an environmental manipulation which has previously been shown to be associated with increases in opiate activity. In contrast, the opiate antagonist naloxone was observed to be ineffective in blocking not only analgesia induced by the serotonergic agonist quipazine, but also analgesia induced by long-term deprivation from the dam and food, a manipulation that has been previously reported to induce increases in serotonergic utilization. These results suggest that in the neonate, as in the adult, the serotonergic modulation of nociception appears to occur "downstream" from the opiate systems serving to regulate nociception following both drug- and environmental-induced alterations in pain sensitivity. Analgesia induced by long-term deprivation from food and the dam appears to be strongly related to increases in serotonergic activity and relatively unaffected by opiate antagonism, whereas analgesia induced by isolation from siblings and the dam may be related to increases in opiate activity, but modulated by serotonergic systems serving to regulate pain responsivity. Thus alterations in the environment, mediated at least in part by alterations in opiate and serotonergic activity, appear to play an important role in influencing the sensitivity of the neonate to pain stimuli.

Effects of undernutrition during suckling and of training on the hypothalamic beta-endorphin of young and adult rats

The involvement of the hypothalamic beta-endorphinergic system in behavioral processes has previously been studied in adult rats. In the present report, we studied the effects of undernutrition and of inhibitory avoidance training on the hypothalamic beta-endorphin-like immunoreactivity of 21-day-old and adult rats. Rats were undernourished by feeding their dams an 8% protein diet from the day of delivery until weaning (21 days of age). The beta-endorphin-like immunoreactivity was measured by radioimmunoassay. In adult rats, undernutrition decreased the basal level hypothalamic beta-endorphin. Avoidance training decreased the content of beta-endorphin in the hypothalamus of well-nourished adults, but had no effect on the levels of previously undernourished rats. In 21-day-old rats, neither undernutrition nor avoidance training altered the levels of beta-endorphin. These results suggest that the hypothalamic beta-endorphinergic system of weaning rats is not yet functional in relation to the parameters analyzed. Probably, other developmental factors are necessary for the emergence of the effects of undernutrition found in adult rats and for the emergence of the response of this system to training (novelty).

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

Peak noradrenergic receptor development in rat spinal cord has been shown to occur around 12 days of postnatal life. The intent of the present study was to examine the development of analgesia produced by spinally applied noradrenergic agonists. The extent to which these drugs modulate pain information evoked by a thermal vs mechanical stimulus in the infant rat was also addressed. Intrathecal norepinephrine resulted in analgesia that was more pronounced against a mechanical than thermal stimulus and more pronounced in 10-day-olds than 3-day-olds. The alpha 2 receptor agonist clonidine produced a dose-dependent analgesia that first appeared at 7 days of age when tested with a thermal stimulus and 3 days of age when tested with a mechanical stimulus. The analgesic effect of clonidine was also greatest at 10 days of age. The alpha 1 agonist phenylephrine was without analgesic effects. The developmental profile of behavioral analgesia correlates with the ontogeny of noradrenergic receptor activity in the spinal cord. The finding that intrathecal norepinephrine produced a more pronounced analgesia against a mechanical rather than thermal stimulus in the adult is supported by our investigation in the infant rat.

Stressor controllability during pregnancy influences pituitary-adrenal hormone concentrations and analgesic responsiveness in offspring

Repeated escapable shock, yoked-inescapable shock, or no-shock treatments were administered to female rats before parturition to investigate the effects of stressor controllability on offspring pituitary-adrenal hormone concentrations and stress-induced analgesic reactions. Female rats exposed to escapable shock treatments received tail-shock in boxes containing a wheel that allowed shocks to be terminated after rotation. Rats in the yoked-inescapable shock group received an identical amount and pattern of tail-shock. However, shock was terminated only after wheel rotation by the rat undergoing escapable shock treatments. Female rats in the no-shock group were simply placed in wheel-turn boxes. Fourteen-day-old offspring were exposed for 10-min to either a separation-stress or shock-induced stress test. The former test consisted of separating and isolating the pup from the mother and siblings, whereas the latter involved the administration of five brief, 1.0 sec, low intensity, 0.5 mA, foot-shocks. Immediately after exposure to foot-shocks, pups were given a tail-flick test to assess their analgesic response. Plasma was obtained from pups immediately after separation and tail-flick tests and ACTH and corticosterone concentrations were assayed by radioimmunoassay. Results indicated that prenatal inescapable shock treatments resulted in offspring with significantly higher plasma ACTH and corticosterone concentrations than offspring exposed to prenatal escapable shock or no-shock treatments. Offspring of females exposed to inescapable shock also exhibited greater increases from basal concentrations in ACTH and corticosterone after stress. Furthermore, prenatal escapable and inescapable shock treatments significantly altered shock-induced analgesic thresholds.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous opiates: 1985

This paper is the eighth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1985. The specific topics this year include stress, tolerance and dependence, eating, drinking and alcohol consumption, gastrointestinal and renal activity, mental illness, learning and memory, cardiovascular responses, respiration and thermoregulation, seizures and neurological disorders, activity, and some other selected topics.

Visceral nociception in developing rats

Maturational changes in visceral nociception were measured in developing or adult rats challenged with hypertonic saline or acetylcholine. Chemically induced abdominal constrictions were absent in rats younger than 7 days of age, regardless of the dose of hypertonic saline or acetylcholine used. Age-related increases in the percent of animals responding, the number of abdominal constrictions emitted per responder, and total response duration occurred in animals 10-20 days of age, until adult-like patterns of responding were attained at the time of weaning. Additional changes in the percent animals responding, as well as in the frequency and total duration of abdominal constriction responses, were also seen in postweanling, but not in preweanling, animals. Five-day-old animals did emit audible vocalizations to the intraperitoneal insertion of a hypodermic needle, however, at a time when these animals failed to show observable responses to the noxious visceral stimuli. Hence, mechanisms mediating pain associated with intraperitoneal needle insertion may be functional during the first postnatal week, at a time when mechanisms mediating visceral pain appear to be immature. These differences may be caused by the differential maturation of sensory, neural, or motoric mechanisms important for hypodermic needle insertion versus visceral nociception.

Opioid-mediation of separation distress in 10-day-old rats: reversal of stress with maternal stimuli

A relationship between distress vocalizations, response to nociception and their opioid mediation in 10-day-old maternally isolated rat pups was established. The comforting effects of several classes of biological stimuli were examined. Short-term (5 min) isolation from mother, siblings and nest caused a significant analgesic response to heat (48 degrees C) relative to nonisolated siblings. Morphine administration markedly increased heat escape latencies and decreased distress vocalizations during isolation. Naltrexone, an opioid antagonist, had the opposite effect; escape latencies were halved and distress vocalizations doubled. Contact with an anesthetized female, dam or virgin, immediately reduced both analgesia and vocalizations. Home-bedding was only effective after 5 min exposure, whereas clean bedding did not reduce isolation-induced behaviors. These results are discussed in terms of infant learning and motivation under natural circumstances.