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.     

Rats with decreased brain cholecystokinin levels show increased responsiveness to peripheral electrical stimulation-induced analgesia

Using the P77PMC strain of rat, which is genetically prone to audiogenic seizures, and also has decreased levels of cholecystokinin (CCK), we examined the analgesic response to peripheral electrical stimulation, which is, in part, opiate-mediated. A number of studies have suggested that CCK may function as an antagonist to endogenous opiate effects. Therefore, we hypothesized that the P77PMC animals would show an enhanced analgesic response based on their decreased CCK levels producing a diminished endogenous opiate antagonism. We found that the analgesic effect on tail flick latency produced by 100 Hz peripheral electrical stimulation was more potent and longer lasting in P77PMC rats than in control rats. Moreover, the potency of the stimulation-produced analgesia correlated with the vulnerability to audiogenic seizures in these rats. We were able to block the peripheral electrical stimulation-induced analgesia (PSIA) using a cholecystokinin octapeptide (CCK-8) administered parenterally. Radioimmunoassay showed that the content of CCK-8 in cerebral cortex, hippocampus and periaqueductal gray was much lower in P77PMC rat than in controls. These results suggest that low CCK-8 content in the central nervous system of the P77PMC rats may be related to the high analgesic response to peripheral electrical stimulation, and further support the notion that CCK may be an endogenous opiate antagonist.     

Endogenous analgesia in the pregnant rat: An artifact of weight-dependent measures?

It has been reported that pregnancy produces an opioid-mediated, endogenous analgesia in the rat. In an attempt to confirm this finding, we used 5 different analgesic measures to compare the responsiveness of pregnant and non-pregnant female rats to painful stimuli. Pregnant and non-pregnant rats differed only when assessed by measures that were highly correlated with body weight. Furthermore, the reduced pain responsiveness of pregnant rats was not prevented by administration of the opioid antagonists, naloxone or naltrexone. We can find no evidence for an endogenous analgesia of pregnancy; instead, our results suggest that findings of a diminished response to painful stimuli in pregnant rats may be an artifact related to the greater body mass/weight of the pregnant animals.     

Paralemniscal reticular formation: Response of cells to a noxious stimulus

Extracellular single unit recordings were made in the paralemniscal reticular formation in adult male rats. A majority of the cells studied were characterized as nociceptive because a noxious stimulus evoked a change (either an increase or decrease) in their spontaneous firing rates. Norepinephrine (NE) administered microiontophoretically usually mimicked the response to the noxious stimulus (foot pinch). After a neuron had been characterized with respect to its response to NE and the noxious stimulus, horseradish peroxidase (HRP) was iontophoretically ejected from the micropipette. Following iontophoresis of HRP into the paralemniscal reticular formation, retrograde and orthograde labeling was observed in the periaqueductal gray and the nucleus raphe magnus. These data support a possible role of the paralemniscal reticular formation in an endogenous analgesic system.     

Stress induced analgesia: its opioid nature depends on the strain of rat but not on the mode of induction

Reports by several investigators have shown that both opioid and non-opioid analgesia can be induced by non-pharmacological manipulations such as the administration of electric shock, and that such analgesia depends on shock parameters, the affective state of the animal and the region of the body shocked. We tested several manipulations which have been reported to induce opioid analgesia using a local strain of rats (CR). Such manipulations included the used of 30 min of intermittent footshock (3 mA, 1 s on, 5 s off), brief shock to the forepaws, transpinal electroconvulsive shock (ECS) and tail shock induced helplessness. Administration of either naloxone or naltrexone to rats of the CR strain failed to attenuate the analgesic effect of these manipulations and in some cases even enhanced analgesia. The existence of functional opioid analgesia systems in CR rats was evident from the fact that electrical stimulation of the periaqueductal gray area produced naloxone sensitive analgesia. In additional experiments we compared the analgesic effect of brief continuous (3 min) footshock, prolonged intermittent footshock (30 min) and ECS in young (less than 75 days of age) and old (greater than 75 days of age) rats of the Sabra strain. Young Sabra rats showed naloxone sensitive analgesia following all 3 manipulations while adult rats displayed analgesia which was naloxone insensitive. Furthermore, no decrement in learning, indicative of helplessness, could be demonstrated in young Sabras following 3 min of shock which induced naloxone sensitive analgesia.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Organismic variables and pain inhibition: Roles of gender and aging

Multiple pain-inhibitory systems dependent upon both opioid and nonopioid mechanisms of action have been identified, particularly in the rodent. The experimental subject has typically been the young, adult male rat, and generalizations concerning these systems have been made from this subject pool. This review focuses upon the roles of two organismic factors, aging and gender, in the modulation of analgesic processes. Using an array of age cohorts (4, 9, 14, 19, 24 months), these data illustrate that aging produces differential decrements in the analgesic responses following morphine, different parameters of footshock, continuous cold-water swims (CCWS: a nonopioid stressor), intermittent cold-water swims (ICWS: an opioid stressor) and 2-deoxy-D-glucose (a mixed opioid/nonopioid stressor). In contrast, neither beta-endorphin nor food deprivation analgesia is affected by aging. This review identifies that CCWS and ICWS analgesia are sensitive to gender differences, gonadectomy differences and steroid replacement differences such that females display less analgesia than males, gonadectomy reduces both analgesic responses, and that testosterone is most effective in reinstating gonadectomy-induced analgesic deficits. These data are considered in terms of therapeutic implications for the organismic variables under study as well as for the conceptual and methodological modifications that must be made in studying intrinsic pain inhibition.     

Effects of thyroid deficiency at birth on deoxyribonucleic acid synthesis and deoxythymidine kinase activity in the developing rat brain

In thyroid deficiency at birth, the endogenous pool sizes of cellular dTTP did not change much in the cerebellum, cerebrum and brain stem at the whole ages studied, but the specific radioactivities of dTTP, at 2 hours after the subcutaneous injection of [3H]-thymidine, apparently increased only in the cerebellum on the 14th and 21st days as compared with the normal controls. The highest rate of DNA synthesis in vivo, expressed in terms of the specific radioactivity ratio of DNA to dTTP, was observed at the four-day-old normal rat cerebellum, and in the thyroid deficiency it appeared to shift between seven and 14 days of age. On the other hand, no apparent effects of thyroid deprivation on the rates of both the cerebrum and brain stem were found. The results suggest that a temporal alteration of DNA synthesis as well as thymidine metabolism occurred by thyroid deficiency was confined to the cerebellum in the early postnatal development of rats. Indeed, cerebellar thymidine kinase activity, related to DNA synthesis, also displayed a concomitant delay in thyroid deficiency to the characteristic age-dependence of DNA synthesis. No significant difference between normal and thyroidectomized rats was revealed in this respect on both the cerebrum and brain stem throughout the experimental periods.     

Differential effects of dPTyr(Me)AVP, a vasopressin antagonist, upon foot shock analgesia

Acute exposure to either prolonged intermittent foot shock (PIFS) or brief continuous foot shock, (BCFS) decreases the sensitivity of rats to noxious stimuli, but differ in their mechanisms of actions. Since the peptide vasopressin (VP) has been implicated in analgesic and stress-related processes, the present study examined whether antagonism of central VP receptors with dPTyr(Me)AVP would alter the analgesic responses following PIFS or BCFS. While intracerebroventricular administration of dPTyr(Me)AVP, a V1 receptor antagonist, significantly attenuated the analgesic response to PIFS, it potentiated the analgesic response to BCFS. It should be noted that the form of PIFS employed in the present study was not blocked by naloxone. These results are discussed in terms of multiple forms of pain-inhibitory systems that may utilize collateral inhibition as a means of providing selective activation.     

Biological sex influences learning strategy preference and muscarinic receptor binding in specific brain regions of prepubertal rats

According to the theory of multiple memory systems, specific brain regions interact to determine how the locations of goals are learned when rodents navigate a spatial environment. A number of factors influence the type of strategy used by rodents to remember the location of a given goal in space, including the biological sex of the learner. We recently found that prior to puberty male rats preferred a striatum‐dependent stimulus‐response strategy over a hippocampus‐dependent place strategy when solving a dual‐solution task, while age‐matched females showed no strategy preference. Because the cholinergic system has been implicated in learning strategy and is known to be sexually dimorphic prior to puberty, we explored the relationship between learning strategy and muscarinic receptor binding in specific brain regions of prepubertal males and female rats. We confirmed our previous finding that at 28 days of age a significantly higher proportion of prepubertal males preferred a stimulus‐response learning strategy than a place strategy to solve a dual‐solution visible platform water maze task. Equal proportions of prepubertal females preferred stimulus‐response or place strategies. Profiles of muscarinic receptor binding as assessed by autoradiography varied according to strategy preference. Regardless of biological sex, prepubertal rats that preferred stimulus‐response strategy exhibited lower ratios of muscarinic receptor binding in the hippocampus relative to the dorsolateral striatum compared to rats that preferred place strategy. Importantly, much of the variance in this ratio was related to differences in the ventral hippocampus to a greater extent than the dorsal hippocampus. The ratios of muscarinic receptors in the hippocampus relative to the basolateral amygdala also were lower in rats that preferred stimulus‐response strategy over place strategy. Results confirm that learning strategy preference varies with biological sex in prepubertal rats with males biased toward a stimulus‐response strategy, and that stimulus‐response strategy is associated with lower ratios of muscarinic binding in the hippocampus relative to either the striatum or amygdala. © 2012 Wiley Periodicals, Inc.     

Ontogeny of adrenomedullary responses to hypoxia and hypoglycemia: Role of splanchnic innervation

Adrenals of neonatal rats were denervated at 3 days of age (just before functional neuronal connections are ordinarily made). At 14 days of age, rats with denervated adrenals did not secrete catecholamines in response to a neurogenic reflex stimulus (insulin-induced hypoglycemia), confirming that innervation had failed to develop; sham-operated littermates were fully responsive to insulin. In contrast, hypoxia was still able to cause depletion of adrenal catecholamines in the denervated group, indicating the persistence of a non-neurogenic secretory mechanism well past the age at which it should have disappeared (8 days). Prolonged deprivation of neural input in adult rats also led to the re-emergence of non-neurogenic capabilities. Twenty-one days after surgery in adulthood, adrenal responses to insulin were still not present but those to hypoxia were. Thus, the development of neural stimulation itself is responsible for the ontogenetic loss of the immature type of adrenal catecholamine release mechanism and does so by suppressing the non-neurogenic component; consequently the mechanism can reappear when nerve traffic is interrupted for extended periods.     

Immature rat convulsions and long-term effects on hippocampal cholinergic neurotransmission

Generalized tonic-clonic convulsions were induced on 2 consecutive days by pentylenetetrazol (PTZ) in immature rats (postnatal days 10 and 20), and hippocampal slices were prepared at different intervals post-injection. The anticholinesterase eserine provoked interictal-like discharges in the CA3 area of PTZ-injected rats (19/33), but not in controls (0/15), an effect mimicked by carbachol and reversed by atropine. This enhanced response to eserine was recorded in slices from 25-100% of the PTZ-injected rats, the percentage varying with the age at injection and post-injection interval. These results suggest that seizures in immature brain may have long-term consequences in cholinergic neurotransmission, converting a rise in endogenous ACh into an epileptogenic stimulus, which in turn would presumably facilitate the recurrence of seizures.     

Involvement of spinal opioid systems in footshock-induced analgesia: Antagonism by naloxone is possible only before induction of analgesia

Footshock reliably produces analgesia in rats which is mediated either by opiate or non-opiate systems. It has recently been demonstrated that a critical factor determining the involvement of endogenous opioids is the body region shocked; front paw shock produces a naloxone-reversible analgesia and hind paw shock produces an analgesia which fails to be attenuated by this opiate antagonist. The present study demonstrated that a crucial opiate site for the production of front paw footshock-induced analgesia (FSIA) exists within the spinal cord. One microgram of naloxone delivered directly to the lumbosacral cord immediately prior to shock significantly attenuated this analgesia. However, the efficacy of naloxone antagonism was order-dependent in that naloxone failed to antagonize fron paw FSIA if delivered immediately after shock; naloxone could prevent but could not reverse the analgesic state. The body region shocked was again observed to be a critical factor determining the involvement of endogenous opioids since 1 microgram of spinal naloxone failed to antagonize hind paw FSIA. These results were discussed in light of recent evidence proposing a neuromodulatory role of opioids within the spinal cord.     

Investigatory behavior of a novel conspecific by Wistar Kyoto, Wistar and Sprague-Dawley rats

The assumption was made that investigatory behaviors (i.e., ano-genital and general body sniffing) of a female conspecific by a mature male rat, has positive hedonic characteristics. Because reduced interest in pleasurable events (i.e., anhedonia) is diagnostically related to depressive behavior, the hypothesis was advanced that less investigatory behavior would be observed in an animal model of depression, namely the Wistar Kyoto (WKY) rat strain. In Experiment 1, WKY, Wistar and Sprague-Dawley male rats were subjected, in the first test series, to three consecutive 2-min exposures to one intruder stimulus female, followed later by another three consecutive 2-min exposures to a second stimulus intruder female. On the second test series, 24 h later, the male rats were exposed to one female for 2 min, followed 6 min later to another 2-min exposure to another stimulus female. Half the male subjects were subjected to tail shock stress 2 h before the first test series. All males demonstrated a habituation of the investigatory response to the same stimulus female, but a dishabituation when subsequently exposed to a new stimulus female. Only WKY rats, exposed to prior stress, revealed a significant reduction in investigatory behavior. In Experiment 2, using only WKY and Wistar rats, a factorial design was used to observe any differences between two stressors, namely tail shock and water restraint, and also to observe possible differences in investigatory behavior towards male vs. female intruder rats. Restraint stress and shock stress elicited significant reductions in investigatory behavior for WKY rats, but not Wistar rats, when confronted with female intruder rats. Male intruders elicited more freezing behavior, as well as aggressive defensive fighting behavior from resident male rats. The results are interpreted to suggest that the significant decrease in investigatory behavior towards a female intruder, which was observed primarily in stressed WKY males, reflects the presence of anhedonia in stressed WKY rats, and reinforces our assertion that the WKY rat strain represents a useful animal model of depressive behavior.     

Changes in mu opiate receptors following inescapable shock

Rats exposed to inescapable shock exhibit profound hypoalgesia. Pharmacological evidence has suggested that changes in endogenous opiate activity may be responsible for the hypoalgesic response. We measured the binding of [3H]DAGO, a selective mu-opiate receptor agonist, in brains of rats exposed to no shock, inescapable shock, or escapable shock. Binding of [3H]DAGO in the midbrains of rats in the inescapable shock group was decreased relative to the other two groups. The decrease in binding appeared to result from a decrease in number of mu-receptors and not a change in affinity. These results support the hypothesis that inescapable shock produces long-term changes in endogenous opiate systems.     

Vitamin E deficiency does not induce compensatory antioxidant increases in central nervous system tissue of apolipoprotein E-deficient mice

Compensatory upregulation in endogenous antioxidants has been shown to accompany certain genetic and dietary deficiencies that promote oxidative stress, including that related to Alzheimer's disease. We compared antioxidant levels in brain tissue of normal and transgenic mice lacking apolipoprotein E following dietary deprivation of vitamin E or folate. As described previously, ApoE-deficient mice displayed increased levels of the endogenous antioxidant glutathione as compared to normal mice, and increased these levels further following folate deprivation. By contrast, glutathione was depleted following vitamin E deprivation in brain tissue of normal and ApoE-deficient mice. TBAR analyses confirmed increased oxidative damage following vitamin E deprivation. However, combined deprivation of folate and vitamin E resulted in levels of glutathione intermediate between those observed following deprivation of either agent, indicating that the lack of compensatory increase in glutathione following vitamin E deprivation was not due to overt neurotoxicity. Similar results were observed for total antioxidant levels in brain tissue. The differential response to vitamin E and folate deprivation is consistent with the possibility that specific differences in oxidative damage may result from deficiencies in either of these agents. The lack of a compensatory response to vitamin E deprivation highlights the importance of dietary vitamin E in prevention of chronic neurodegeneration.     

Increased basal activity of the HPA axis and renin-angiotensin system in congenital learned helpless rats exposed to stress early in development

Learned helpless behavior has been successfully bred in rats and designated as a genetic animal model of human depression and/or anxiety. Since congenital learned helpless animals have an impaired stress response in adulthood, we examined the effects of early stressors (at postnatal day 7, 14 or 21) on the hypothalamic–pituitary–adrenal axis and the renin-angiotensin system. The functioning of the hypothalamic–pituitary–adrenal axis was monitored through changes in corticosterone plasma levels in the adult animals after acute exposure to cold stress and maternal deprivation early in development. Renin-angiotensin system functioning was assessed by plasma renin activity. Unstressed congenital learned helpless rats had corticosterone levels that were similar to control animals (congenital non-learned helpless rats not stressed during development), but unstressed plasma renin activity levels of congenital learned helpless rats were lower than congenital non-learned helpless rats. There was a step-wise increase in corticosterone plasma levels in the congenital learned helpless rats with age of acute presentation of either cold stress or maternal deprivation stress (day 7, 49%; day 14, 84%; and day 21, 543% for cold stress). However, these baseline corticosterone levels were significantly lower in congenital learned helpless rats compared to congenital non-learned helpless controls. Similarly, in response to early exposure to cold stress and maternal deprivation, there was an increase in plasma renin activity levels of congenital learned helpless rats with age of presentation to either stressors. However, this increase in plasma renin activity levels was not evident in congenital non-learned helpless controls. Taken together, these results suggest that exposure to stress early in development has long-term effects on both the hypothalamic–pituitary–adrenal axis and the renin-angiotensin system, two neuroendocrine indicators of stress responsivity.     

Emotional and footshock stimuli induce differential long-lasting behavioural effects in rats; involvement of opioids

Rats were exposed to either a footshock stimulus (FS) or emotional stimulus (ES, forced perception of another rat receiving footshocks) during a daily 10-min session for 5 consecutive days. The consequences of FS and ES on their behavioural responsiveness were assessed at different post-stress intervals using a small open-field. FS induced a decrease in ambulation, rearing and sniffing and an increased immobility in the small open field. These effects were present in rats tested immediately after the last session and remained present for at least 15 days. In contrast, ES induced a transient decrease in ambulation and rearing immediately after the last session, but in the period from half an hour until at least 15 days after the stimulus experience, an increase in ambulation, rearing and sniffing was observed. Exposure to one footshock per session for 5 consecutive days or to 10 footshocks in a single session also resulted in a long-lasting reduction in ambulation and sniffing and an increase in immobility. The former regime did not influence the behavioural response of ES rats, but the latter resulted in an increase in ambulation, rearing and sniffing in ES rats. Naloxone (1 mg/kg s.c.) pretreatment antagonized the increased behavioural activity of the ES rats whereas the activity of control and FS animals was not affected, suggesting an involvement of endogenous opioid systems in the behavioural responses observed in ES rats. It is suggested that the behavioural responses of the ES and FS animals are regulated by different mechanisms.     

Ontogeny of drinking behavior of preweanling rats with lateral preoptic damage

Male (N = 9) and female (N = 7) infant albino rats sustained bilateral lateral preoptic area (LPO) destruction on the tenth day of life. During development and as adults, these brain-damaged rats were subjected to repeated testing of drinking behaviors in response to cellular and extracellular dehydration. Immediately following LPO destruction the pups ceased suckling and most rats required tube-feeding to maintain life; however, all pups were maintaining body weight through voluntary suckling and/or feeding-drinking by 20–21 days of age. Daily water intakes were elevated from 30–42 days of age, following which intakes decreased to control volumes from 43–160 days of age. Brain damaged rats had attenuated water intakes in response to hypertonic saline injection, however these rats were hyperdipsic in response to water deprivation, polyethylene glycol injection and food deprivation treatments. These results are consistent with the suggestion that the LPO contains osmoreceptors for drinking behavior, and the present results suggest that such osmoreceptors may be functional by the tenth day of life for rats.     

The effects of early maternal deprivation on auditory information processing in adult Wistar rats.

BACKGROUND: There is now ample evidence that schizophrenia is due to an interaction between genetic and (early) environmental factors which disturbs normal development of the central nervous system and ultimately leads to the development of clinical symptoms. Recently, we showed that a single 24-hour period of maternal deprivation of rat pups at postnatal day 9 leads to a disturbance in prepulse inhibition, similar to what is seen in schizophrenia. The present set of experiments was designed to further characterize the information processing deficits of maternally deprived Wistar rats. METHODS: Wistar rats were deprived from their mother for 24 hours on postnatal day 9. At adult age, rats were tested in the acoustic startle paradigm for prepulse inhibition and startle habituation. Rats were also tested in the evoked potentials paradigm for auditory sensory gating. RESULTS: The results show that maternal deprivation led to a reduction in acoustic startle habituation and auditory sensory gating in adult rats. Moreover, maternal deprivation disrupted prepulse inhibition but only when the prepulses were given shortly (50-100 milliseconds) before the startle stimulus. At longer intervals (250-1000 milliseconds), no effect was seen. CONCLUSIONS: The implications for the model and the development of disturbances in information processes are discussed.