Delaying reinforcement in an autoshaping task generates adjunctive and superstitious behaviors

Rats were autoshaped to touch a lever upon its insertion into an operant chamber on a 45 s random time schedule. Occurrence of a reinforced touch on each of 12 lever insertions per session and nose-pokes at the retracted lever were monitored, as was exploratory rearing activity. Delays of 2, 4 or 8 s interposed between the retraction of the lever, which occurred either after 15 s or after a touch response, and delivery of the food pellet reward, resulted in progressively slower acquisition of the extended lever touch response. However, if rats had already acquired the response under immediate reinforcement conditions, the delays subsequently introduced did not cause a decline in autoshaped touch responding. Nose-pokes at the retracted lever occurred during both intertrial and reinforcement delay intervals over the course of autoshaping. The appearance and frequency of these (adjunctive or superstitious) behaviors depended upon the reinforcement delay and behavioral history. These interval behaviors offer measurements of learning not based on arbitrary criteria; they thus provide information about effects on endpoints, or a more global approach to learning.

Embryonic "binge" cocaine exposure alters neural-immune and neural-endocrine interactions in young chickens: involvement of serotonin(2) receptors

As part of our characterization of the developmental consequences of prenatal cocaine exposure, cocaine was injected into eggs containing viable chicken embryos on embryonic day (E) 18 and the fever response to the endotoxin lipopolysaccharide (LPS) and a delayed-type hypersensitivity response to phytohemagglutinin (PHA) were assessed postnatally. E18 cocaine exposure did not affect basal body temperature. LPS induced a fever in the chicks at 4 h post-injection on post-hatch day (D) 4 and 2 h post-injection on D24. E18 cocaine exposure suppressed the peak LPS-induced fever by 50% at both ages. E18 cocaine exposure also suppressed the hypersensitivity reaction to an intradermal injection of PHA on D17, while having no effect on the response to a saline injection. To determine the importance of serotonin(2) (5-HT(2)) receptors in the developmental toxicity of cocaine, varying doses of the 5-HT(2) antagonist ritanserin were injected on E17 followed by cocaine on E18. Ritanserin, like cocaine, did not alter basal temperature, but it dose-relatedly attenuated or blocked cocaine's effect on LPS-induced fever on both D4 and D24. Ritanserin pretreatment was also able to block the blunted isolation stress response seen in D16 chicks following E18 cocaine exposure. Thus, late prenatal cocaine exposure significantly alters adaptive fever and hypersensitivity responses, and embryonic 5-HT(2) receptors played a mediating role in the fever effect.

Cocaine and salicylate: documentation of hydroxyl radical formation in hearts and brains of 18-day-old chick embryos and unexpected interactive toxicity

RATIONALE

Multiple low doses of cocaine (COC) may cause intermittent vasoconstriction and reperfusion, leading to elevations in damaging reactive oxygen species, such as hydroxyl free radicals (*OH). Salicylate may offer protection because it reacts with *OH and/or because of its anti-inflammatory actions.

OBJECTIVE

To measure *OH concentrations in hearts and brains of chicken embryos exposed to multiple, small doses of COC, and to determine if otherwise non-toxic doses of sodium salicylate (NaSal) protected against the marginal but significant reduction in hatchability caused by a model of "binge" COC exposure.

METHODS

Three experiments were carried out. In the first, 67.5 mg COC/kg egg was administered as five doses of 13.5 mg/kg egg or 0.675 mg/egg every 1.5 h, injected just beneath the shell, on day 18 of development (E18), 1 h after NaSal (25 or 100 mg/kg egg) was injected as a bolus. Hearts and brains taken shortly afterward were analyzed for *OH. In experiment 2, the dose of COC was reduced to 56.5 mg/kg egg so as to achieve a small but significant reduction in hatchability in order to determine if NaSal protected against or enhanced COC's toxicity, manifest as an increase or decrease in hatchability. The doses of NaSal for this experiment were 50, 100 or 200 mg/kg egg, all devoid of effects upon hatchability when injected alone. Experiment 3 was done to confirm the presence of vascular disruptions/hemorrhages observed on COC-exposed embryos while harvesting hearts and brains for chemical analyses and to quantify what appeared to be enhanced COC-related vascular accidents associated with NaSal pretreatment. The dose of NaSal used in experiment 3 was 200 mg/kg egg and COC was injected again at 5x13.5 mg/kg egg.

RESULTS

COC increased *OH in hearts and brains of chicken embryos on E18, and non-toxic doses of NaSal (i.e. 100 or 200 mg/kg egg) enhanced COC's toxicity in a dose-related manner. The lowest NaSal dose (50 mg/kg egg) may have offered some protection against the effects of COC, as the reduction in hatchability caused by 56.5 mg COC/kg egg was no longer significant. Vascular disruptions/hemorrhages were associated with and most likely responsible for the interactive toxicity.

CONCLUSIONS

Our unexpected findings may be of clinical relevance because of the use of aspirin for treatment of misdiagnosed "preeclamptic" COC-abusing pregnant women and its possible use for COC abusers at risk for reduced cerebral blood flow and stroke.

Embryonic cocaine exposure and corticosterone: serotonin(2) receptor mediation

Cocaine activates the mature hypothalamic-pituitary-adrenal (HPA) axis, increasing corticosterone concentrations in animals and humans and serotonin(2) receptors (5-HT(2)) are involved in this effect. Although prenatal cocaine exposure is associated with altered responsiveness of the HPA axis to "stress" and serotonergic compounds postnatally, it is unknown whether cocaine directly activates the embryonic HPA axis or if 5-HT(2) receptors are involved. Domestic chicken eggs with viable embryos were exposed to either the 5-HT(2) receptor agonist dimethoxyiodophenylaminopropane (DOI: 0.4, 0.8, or 1.2 mg/kg egg) or saline on embryonic day 18 (E18). In a second study, the 5-HT(2) antagonist ritanserin (0.3 mg/kg egg, a dose found effective against other effects of DOI or cocaine) or vehicle was administered on E17, prior to treatment on E18 with either saline or cocaine (5 injections of 12 mg/kg egg, equivalent to a total dose of 3.5 mg/egg). Radioimmunoassay was used to measure serum corticosterone from blood samples taken approximately 1-2 h after drug injections. DOI significantly raised corticosterone in a dose-related fashion. Cocaine-induced corticosterone elevations were blocked by pretreatment with ritanserin, whereas ritanserin by itself did not affect corticosterone concentrations. These data indicate that 5-HT(2) receptors are involved in cocaine's effect on the HPA axis during late chicken embryogenesis.

Salicylate and cocaine: interactive toxicity during chicken mid-embryogenesis

Increased free radical production, due to ischemia and reperfusion, has been postulated as a cause of cocaine's (COC) developmental toxicity. Salicylate reacts with hydroxyl free radicals (*OH) to form stable, quantifiable reaction products, which can be measured with high-pressure liquid chromatography (HPLC). To determine if chicken embryos' brains and hearts were exposed to increased *OH concentrations after injection of COC, an injection of a nontoxic dose of sodium salicylate (NaSAL, 100 mg/kg egg, or 5 mg/egg), followed by 5 injections of COC (13.5 mg/kg or 0.675 mg/egg, every 1.5 h), was administered to eggs containing embryos on the 12th day of embryogenesis (E12). In addition to finding increased *OH concentrations in E12 embryonic hearts and brains, we observed that the developmental toxicity of COC, manifest as vascular disruption (hemorrhage) and lethality, was enhanced by NaSAL injection. These results confirm and extend results of similar experiments performed upon older embryos (E18), and indicate that increased &z.rad;OH concentration in embryonic tissues after COC exposure and toxic interactions of COC and NaSAL can also occur at an earlier stage of development. The results are discussed in light of possible exposure of human fetuses to both COC and salicylates, since COC-abusing pregnant women can be misdiagnosed with pre-eclampsia and aspirin is used to treat this syndrome.

Late embryonic ritanserin exposure fails to alter normal responses to immune system stimulation in young chicks

Prior studies in our laboratory have demonstrated that prenatal treatment with the serotonin2 (5-HT2) antagonist ritanserin is effective in blocking some of the lethal, dysmorphic, cardiovascular, and behavioral consequences of excessive direct or indirect stimulation of 5-HT2 receptors in the developing chicken. The efficacious dose range for ritanserin in these studies had very little or no effect on the above measures of toxicity when administered alone. In the present study, we extend our characterization of ritanserin's potential toxicity, or lack thereof, to include the normal behavioral and endocrine responses to immune system stimulation by the endotoxin lipopolysaccharide (LPS). LPS administration induces a syndrome collectively known as sickness behavior, manifest as altered thermoregulatory processes leading to fever, and increased serum concentrations of neuroendocrine hormones, including corticosterone. These survival-promoting responses to LPS were assessed in young chickens that had been treated with doses of ritanserin ranging from 0 to 2.7 mg/kg on embryonic day 17 (E17). When sickness behavior was assessed in 5-7-day-old chicks 1 h post-LPS injection, E17 ritanserin-treated subjects did not differ from controls. At 4-6 h post-LPS, 4-day-old chicks displayed a robust fever, and E17 ritanserin did not affect the magnitude of this response. Similarly, E17 ritanserin treatment failed to affect corticosterone concentrations 2 h post-LPS in 14-day-old chicks. Thus, ritanserin treatment during late embryogenesis, a time when it is effective against direct and indirect acting 5-HT2 agonists, failed to modify the survival promoting and beneficial interactions between the nervous, endocrine, and immune systems that are elicited following immunostimulation.

OCD-Like behaviors caused by a neuropotentiating transgene targeted to cortical and limbic D1+ neurons

To study the behavioral role of neurons containing the D1 dopamine receptor (D1+), we have used a genetic neurostimulatory approach. We generated transgenic mice that express an intracellular form of cholera toxin (CT), a neuropotentiating enzyme that chronically activates stimulatory G-protein (Gs) signal transduction and cAMP synthesis, under the control of the D1 promoter. Because the D1 promoter, like other CNS-expressed promoters, confers transgene expression that is regionally restricted to different D1+ CNS subsets in different transgenic lines, we observed distinct but related psychomotor disorders in different D1CT-expressing founders. In a D1CT line in which transgene expression was restricted to the following D1+ CNS regions-the piriform cortex layer II, layers II-III of somatosensory cortical areas, and the intercalated nucleus of the amygdala-D1CT mice showed normal CNS and D1+ neural architecture but increased cAMP content in whole extracts of the piriform and somatosensory cortex. These mice also exhibited a constellation of compulsive behavioral abnormalities that strongly resembled human cortical-limbic-induced compulsive disorders such as obsessive-compulsive disorder (OCD). These compulsive behaviors included episodes of perseverance or repetition of any and all normal behaviors, repetitive nonaggressive biting of siblings during grooming, and repetitive leaping. These results suggest that chronic potentiation of cortical and limbic D1+ neurons thought to induce glutamatergic output to the striatum causes behaviors reminiscent of those in human cortical-limbic-induced compulsive disorders.

Voltage associated with spontaneous embryonic motility in the developing chicken: an automated characterization during mid-late embryogenesis

Movement of developing chicken embryos and their associated membranes generates voltage detectable with electrodes inserted just beneath the eggshell. Use of such voltages as a motility indicator offers an embryonic behavioral assessment method less subjective and invasive than observational methods using windows that disrupt substantial portions of the eggshell. We used a computerized signal recording and processing procedure to compare voltages from embryonic Day 12 (E12), E15, and E18 chicken eggs with embryos, assessed on the same day. Larger voltages were recorded from E18 subjects than from E12 or E15 subjects. Because this could have been due to embryonic size (mass) and/or proximity to the electrodes, making age comparisons uninterpretable, we used standard deviation-normalized and Z-score-based data transformations, comparing groups for relative deviations from basal voltages. E18 subjects still appeared more active than E12 subjects, with E15 a transitional age, in contrast to results from earlier window-based studies. The automated assessment method we used could enhance behavioral teratology studies of avian species.

Open-field and LPS-induced sickness behavior in young chickens: effects of embryonic cocaine and/or ritanserin

Exposure to drugs of abuse during embryogenesis may adversely affect nervous, immune, and endocrine systems development. We compared exposure on embryonic day 18 (E18) by single or multiple cocaine (COC) injections (56.25 mg/kg total dose for both) or saline on hatching and activity measures. In saline-exposed controls, repeated testing, age, and gender affected activity levels. A single or multiple COC injections increased the median latency to explore and multiple COC injections decreased the median number of lines crossed by female chicks in the open field. We also determined if pretreatment with the serotonin2 (5-HT2) receptor antagonist ritanserin could attenuate COC's effects on open-field behavior as well as behaviors sensitive to immune system stimulation (lipopolysaccharide (LPS)-induced sickness behavior). Eggs containing embryos were pretreated on E17 with 0.4 mg ritanserin/kg or its vehicle followed by multiple COC injections or saline on E18. E18 COC treatment decreased the median number of lines crossed and distress vocalizations in females. Ritanserin pretreatment mitigated the COC induced effects. E18 COC exposure also suppressed LPS-induced sickness behaviors in both males and females, increasing food consumption and the time spent awake and active, as well as decreasing the time spent sleeping. Ritanserin alone had no effect on the food consumed or time spent active, nor did this dose affect COC-induced alterations in sickness behavior. Ritanserin alone decreased time spent sleeping and also failed to affect the COC-induced suppression. Thus, embryonic COC exposure can suppress open field and LPS-induced sickness behavior in the young chick, and ritanserin pretreatment can block the former, but not the latter effects at the dose chosen for these experiments.

Relationships between midembryonic 5-HT2 agonist and/or antagonist exposure and detour learning by chickens

The importance of serotonin (5-HT) as both a transmitter and a regulatory signal during development of many species is well established. The availability of 5-HT receptor subtype agonists and antagonists will enable pharmacological dissection of the importance of one or more of the 5-HT receptors for their involvement in the mediation of developmental insults by drugs that are less selective but include actions upon serotonergic function. Such insults include exposure to cocaine or opiate withdrawal, both of which are blocked or attenuated by 5-HT2 antagonists. The 5-HT2 receptor agonist dimethoxyiodophenylaminopropane (DOI), like cocaine, causes vasoconstriction during embryogenesis, herniated umbilici in hatchlings, and altered detour learning by young chickens after injection into eggs at late stages of embryogenesis. The 5-HT2 antagonist ritanserin (RIT) blocks or significantly attenuates these effects. This study describes an effect of DOI on posthatch detour learning when injected earlier during embryogenesis (i.e., on embryonic day 12, E12) which is opposite its effect when injected later (i.e., on E15). Both effects are blocked by an inactive dose of RIT (0.3 mg/kg egg) and by a higher dose of RIT (0.9 mg/kg egg), which itself retards posthatch detour learning following E12 injection. Thus, excessive stimulation or blockade of 5-HT2 receptors around midembryogenesis can cause a similar behavioral teratogenic outcome. The data are discussed in relation to the likelihood that potential use of 5-HT2 antagonists for treating pregnant women and their fetuses who are not at risk is nil.

Behavioral and neuroendocrine assessment of ritanserin exposure in the developing chicken: lack of toxicity at effective doses

The 5-HT2 antagonist ritanserin (RIT) is undergoing Phase III clinical trials for the treatment of substance abuse disorders. RIT has also shown preclinical therapeutic potential for attenuating or blocking lethal and/or toxic effects of exposure to cocaine or the selective 5-HT2 agonist dimethoxyiodophenyl-aminopropane (DOI) in the developing chicken. To assess the potential toxicity ("side effects") of RIT itself during development, we exposed chicken embryos to 0, 0.1, 0.3, 0.9, or 2.7 mg RIT/kg egg by injecting the drug into eggs with 14-day-old embryos (E14). Voltage generated by spontaneous embryonic activity (motility) was measured on E15 to assess short-term effects of RIT; none were observed. There was no overall effect of these RIT doses on hatchability, though sample sizes were small (n = 13-15 per group). One to 2 weeks after hatching, chicks' acquisition of a detour learning response was tested. There were no observable effects of any RIT dose on detour learning. To assess potential effects of RIT on responsiveness to stress, some chicks were exposed to isolation stress approximately 3 weeks after hatching and killed 15 min later. Blood was assayed for serum corticosterone. There was no effect of any embryonic RIT dose on corticosterone concentrations in nonstressed subjects. Although corticosterone was elevated in all stressed groups, the group exposed to the highest embryonic RIT dose (2.7 mg/kg egg) showed a stress-induced elevation greater than other groups. Thus, except for the highest RIT dose (six to seven times greater than a therapeutically effective dose used in earlier work), embryonic RIT exposure on E14 had no effect on embryonic behavior, hatchability, posthatch learned behavior, and basal serum corticosterone concentrations. At a supraefficacious dose it appears to have modified the responsiveness of the neuroendocrine axis to mild stress.

Evidence for a serotonin-mediated effect of cocaine causing vasoconstriction and herniated umbilici in chicken embryos

Some of cocaine (COC)'s pathophysiological effects on exposed embryos likely result from its vasoconstrictive action, and serotonin2 (5-HT2) agonists such as dimethoxyiodophenylaminopropane (DOI) can mimic these effects. Infusions of COC (5 mg/kg/min) or DOI (0.5 mg/kg/min) for 15 min into chicken eggs with embryos on E15 caused a significant reduction in blood vessel diameters (14 and 30%, respectively). Pretreatment with the 5-HT2 antagonist ritanserin (RIT, 0.9 mg/kg) 18-22 h earlier blocked the effect of COC and blocked or attenuated the effect of DOI. In separate groups of chicken embryos exposed to multiple injections of low doses of COC on E18, herniated umbilici were prominent in hatchlings. A single bolus injection of the same absolute amount of COC did not cause herniated umbilici. An additional experiment replicated the induction of herniated umbilici by multiple injections of COC and demonstrated the probable involvement of 5-HT2 receptors because RIT blocked COC's ability to induce this anomaly. These data suggest that COC's vasoconstrictive effect, via 5-HT2 receptors, may play a mechanistic role in some adverse outcomes in embryos exposed to COC.

Ontogeny of glucocorticoid receptors in the hyperstriatum-hippocampus-parahippocampal area and optic tectum of the embryonic chicken (Gallus domesticus) brain

The importance of glucocorticoids and their perturbation during development is an active research area. Developmental insults, including direct and indirect consequences of exposure to drugs of abuse or withdrawal from them, may act upon or via the neuroendocrine axis of the pregnant experimental subject (e.g. rat) and/or directly upon the neuroendocrine axis of the embryo or fetus. The use of the domestic chicken embryo may constitute a good experimental subject for studying these effects in the absence of maternal influences. Thus, the pattern of brain glucocorticoid cytosolic receptors were characterized in an early developing brain region, the optic tectum (OT) and a later developing region with a different function, the hyperstriatum-hippocampus-parahippocampal (HHP) area, on embryonic days (E) 11, 15, 18 and on the day of hatching (HD). The influence of the glucocorticoid synthesis inhibitor metyrapone, injected into eggs on E14 and on E17, upon glucocorticoid receptors (on E15 and E18) was also studied to determine effects of a 'chemical adrenalectomy'. Receptors for this steroid are high on E11 and E15, decreasing as they approach the time of hatching, with the HHP generally showing greater numbers of specific binding sites for [3H]-corticosterone (CORT). Although metyrapone treatment did not alter the apparent number of receptors on E15, on E18 it unmasked receptors otherwise occupied by endogenous ligand(s) and/or induced their synthesis, resulting in significantly more receptors identified with [3H]-CORT. Nevertheless, the HHP continued to display more of these receptors than the OT on E15 and E18 after injection of metyrapone. These observations are consistent with the hypotheses that the HHP of embryos of this species contains a higher density of glucocorticoid receptors than does the OT; that glucocorticoid receptor quantification is related to steroid synthesis inhibition in late embryonic development; and that neuroendocrine feedback control of serum glucocorticoids may become functional between E15 and E18. The results also suggest the use of this experimental approach for assessing the effects of developmental insults with drugs, other than metyrapone, as a marker for altered neuroendocrine development and/or function.

Excessive stimulation of serotonin2 (5-HT2) receptors during late development of chicken embryos causes decreased embryonic motility, interferes with hatching, and induces herniated umbilici

The existence and functional significance of 5-HT2 receptors in chicken embryos was studied by injecting the selective agonist dimethoxyiodophenylaminopropane (DOI), alone or in conjunction with the selective 5-HT2 antagonist ritanserin (RIT), into domestic chicken eggs with embryos of varying ages. DOI caused dose-dependent reductions in hatchability and herniated umbilici in hatchlings. These effects were observed after injection early, mid, or late during embryonic development, with evidence of the toxic effects of DOI being greater in older embryos, probably due to 5-HT2 receptor activation late in development, even after injecting DOI as early as on day 3 of embryogenesis. This is based upon the fact that embryos in eggs injected with DOI early continued to develop apparently normally, failing to hatch, often after pipping their shells. Additionally, those that hatched often did so with herniated umbilici, as did late-exposed embryos, indicating that DOI's effects upon this organ were most likely mediated during the prehatching period (i.e., days 18-20). The agonist's selectivity was confirmed by the capacity of RIT to dose dependently block both of these toxic effects of DOI. Reduced embryonic motility monitored on day 19, after injection of DOI on the evening of day 18, suggests that excessive activation of 5-HT2 receptors late during development of this species interferes with some normal embryonic behaviors and physiological changes necessary for inducing and/or maintaining the hatching process.

Interaction between mitogens upon intracellular Ca2+ pools in murine fibroblasts

A comparison of the effect of platelet-derived growth factor (PDGF) and bombesin on intracellular Ca2+ stores was carried out in Swiss 3T3 cells loaded with Fura-2. It was found that the tumor promoter thapsigargin (Tg) almost completely inhibited both the PDGF- and the bombesin-induced intracellular Ca2+ concentration ([Ca2+]i) rise, indicating that the two mitogens mobilize Ca2+ from intracellular pool(s) sensitive to the tumor promoter. It was also found that pre-treatment with PDGF almost totally and persistently (up to at least 30 min) inhibited the bombesin-, Tg- and ionomycin-induced rise in [Ca2+]i, whereas pre-treatment with bombesin had only a partial inhibitory effect on the PDGF, Tg and ionomycin [Ca2+]i response, both in the absence and in the presence of external Ca2+. On the other hand, vasopressin and bradykinin, which also stimulate hydrolysis of phosphoinositides in these cells, did not affect the [Ca2+]i response induced by the same agents. These results indicate that, despite the poor production of inositol 1,4,5-trisphosphate (InsP3), PDGF was capable of totally discharging and maintaining discharged the InsP3-sensitive stores of intracellular Ca2+, regardless of whether extracellular Ca2+ was present in the medium. Bombesin only partially caused this effect. On the contrary, bradykinin and vasopressin, after releasing intracellular Ca2+ allowed an almost total refilling of the pools. It is interesting to note that, at variance with PDGF and bombesin, neither bradykinin nor vasopressin are able to induce a mitogenic response in Swiss 3T3 cells.

Acute opiate withdrawal in rats undernourished during infancy: impact of the undernutrition method

Acute morphine withdrawal was assessed in adult rats following early postnatal undernutrition produced by two different methods (Large Litter procedure-20 pups/litter and Modified Slob procedure-rats cross-fostered on days 2, 4, and 6 to nonlactating dams for 24-hour periods). Response rates were first stabilized on a FR16 operant schedule. A single dose of morphine (20 mg/kg) was then administered, followed 4 h later by a single injection of naloxone (2.5 mg/kg). Males reared in large litters showed little behavioral disruption after morphine, suggesting either insensitivity to the opiate or the rapid development of tolerance. After naloxone. Modified Slob males displayed milder withdrawal than those in the well-nourished control or large litter groups. Thus the method of undernutrition influenced morphine's action and expression of withdrawal. A clear sex difference was also evident, females appearing to be generally less sensitive to the opiate- and naloxone-induced withdrawal than males. Body temperature underwent a characteristic elevation following morphine and a depression following naloxone across all groups, but undernutrition did not affect these responses. Hence, behavior proved to be the more sensitive measure for revealing differences in opiate dependence and withdrawal following early life undernutrition, under the test conditions employed.

Long-term effects of neonatal exposure to isobutylmethylxanthine. II. Attenuation of acute morphine withdrawal in mature rats

An acute model of morphine withdrawal was used to determine if neonatal exposure to 3-isobutyl-1-methylxanthine (IBMX) would cause alterations in the expression of withdrawal in the adult rat. IBMX induces a quasi-morphine withdrawal syndrome (QMWS), which is almost identical to true morphine withdrawal both behaviorally and neurochemically. Transient IBMX treatment during infancy (on days 7-10 of life) caused an attenuated suppression of fixed ratio (FR) responding during acute morphine withdrawal in adulthood; however, there appeared to be no attenuation of withdrawal-induced hypothermia. The attenuated behavioral response was not due to an altered ability to express withdrawal, as these rats did not react differently to various doses of IBMX plus naloxone (i.e., varying severities of quasi-morphine withdrawal) in adulthood. Coadministration of the serotonin (5-HT) antagonist mianserin with IBMX in the neonate prevented the effects of IBMX. Both the mianserin-treated and the IBMX plus mianserin-treated groups had increased levels of [3H]naloxone binding in brainstem, while IBMX treatment alone apparently had no significant effect. None of the neonatal drug treatments affected [3H]naloxone binding in frontal cortex. Thus, the long-term effects of IBMX on the opioid withdrawal response cannot be explained by changes in the number of opioid binding sites (labelled with [3H]naloxone) within the brain. The results indicate that exposure to a methylxanthine, and thus quasi-morphine withdrawal, during development results in long-lasting alterations of a system which is involved in opioid withdrawal. Because coadministration of mianserin prevented the effects of IBMX, 5-HT and 5-HT2 receptors are implicated in these effects.

Long-term effects of neonatal exposure to isobutylmethylxanthine. I. Retardation of learning with antagonism by mianserin

Pregnant women regularly ingest the methylxanthines, caffeine and theophylline, during pregnancy and lactation. Also, theophylline is used to treat apnea in premature infants. In this study, rat pups were treated with 3-isobutyl-1-methylxanthine (IBMX), on days 7-10 of life. Transient IBMX treatment during infancy caused a retardation of acquisition of a delayed reinforced autoshaped lever touch response in adulthood. Treated rats required more trials to learn the task, but did not show altered exploratory activity in the operant chambers. Coadministration of the serotonin (5-HT) antagonist mianserin with IBMX was able to attenuate significantly the effects of IBMX in both males and females, even though mianserin treatment alone caused an apparent learning deficit in the males. The results indicate that 5-HT and 5-HT receptors are important during development for normal expression of a specific cognitive function later in life. Furthermore, a 5-HT system appears to play a role in the mechanism whereby perinatal methylxanthine exposure could lead to learning impairments or other undesirable behavioral consequences. The use of IBMX in developing rats may also offer a model for studying the long-term consequences of the expression of opioid withdrawal during the neonatal period, since this agent induces a quasi-morphine withdrawal syndrome (QMWS) in mature rats. It is of interest that mianserin can block or attenuate effects of both quasi- and true morphine withdrawal.

Hyperthermia sensitizes rats to cocaine's proconvulsive effects and unmasks EEG evidence of kindling after chronic cocaine

In phase I, 64 male and female Sprague-Dawley rat siblings from 8 litters were divided equally among 4 treatment groups; saline plus normothermia (S37), saline plus hyperthermia (S45), cocaine (30 mg/kg) plus normothermia (C37), and cocaine plus hyperthermia (C45) and treated daily from 45-60 days of age. Cocaine plus hyperthermia produced protracted, intense and often fatal convulsions, whereas animals from either treatment alone did not convulse. Subsequently, 12 males, representing all phase I treatment groups equally, were implanted with telemetric transmitters to monitor the EEG and core body temperature in phase II. Survivors of this second phase were exposed to one trial each of saline plus hyperthermia, cocaine plus normothermia, and cocaine plus hyperthermia, in that order. The data obtained suggests that 1) the telemetered EEG and temperature can be used to detect changes reflecting sensitization/kindling in the absence of behavioral expression (convulsions), 2) analysis of EEG power spectral bands and body temperature curves showed that a history of daily cocaine exposure seems to have contributed more than daily hyperthermia to subsequently observed seizure patterns and thermic responses, and, finally, 3) cocaine plus hyperthermia resulted in a shorter latency to convulse and a lower maximal EEG seizure voltage, while increasing the variety, severity and duration of its behavioral expression (convulsion).

Delayed effects of amphetamine or phencyclidine: interaction of food deprivation, stress and dose

Tritium-labelled phencyclidine (PCP) hydrochloride (12 mg/kg) was injected SC for six consecutive days into two groups of eight male rats maintained at 85% of their initial free-feeding weights. Eight days after the last injection, electric footshock raised fat levels of PCP 28% over nonshocked controls, and lowered blood levels 18%, but did not alter brain levels of the drug significantly. Thus, application of an acute stressor does result in redistribution of tissue stores of phencyclidine as predicted in the literature; however, the direction of the redistributions was to fat, rather than to brain. To explore the relation of a long-term stressor (one that eliminates adipose tissue as a sink for mobilized PCP), exploratory behavior was evaluated in male rats during six days of food deprivation commencing after six daily injections of PCP HCl (2 or 4 mg/kg, SC). Exploratory behavior of the 4 mg/kg dose group was abruptly altered, compared to saline controls, at six days of food deprivation, when the rats' body weights were about 70% of initial weights and when body fat would be severely reduced or depleted. To assess replicability and generalizability of this phenomenon, PCP HCl (4 or 8 mg/kg, SC) or dextroamphetamine sulfate (3.2 or 6.4 mg/kg, SC) was injected into male rats for six days and food deprivation followed afterward for nine consecutive days, or until similar body weight reductions as in the first experiment were achieved. Again, exploratory behavior was altered in comparison to saline controls in phencyclidine-treated rats (at the 4 mg/kg dose level) when rats reached about 70% of initial weights.(ABSTRACT TRUNCATED AT 250 WORDS)

Food deprivation alters behavioral and plasma corticosterone responses to phencyclidine in rats

Phencyclidine (PCP) sensitivity of rats, whose body weights were maintained at 70% of free-feeding controls, was compared to drug sensitivity of the controls in terms of unconditioned (exploratory) behavior and plasma corticosterone levels. Low doses of PCP HCl [0 (saline vehicle), 0.3 or 0.9 mg/kg, SC] were given to food-deprived rats and to free-feeding controls 15 minutes before measuring unconditioned behavior for 90 minutes; then PCP in brain and corticosterone in plasma were assayed. An additional group (0.43 mg/kg) was established from the reduced-weight rats in order to compare with free-feeding rats given 0.3 mg/kg, the same absolute dose-a circumstance reflecting "street" usage in which doses are not adjusted for body weight differences among users. These low doses of PCP altered exploratory behaviors, but there did not appear to be an interaction between food-deprivational status and drug, with the possible exception of an altered effect of PCP upon habituation in the lighter animals. PCP elevated plasma corticosterone levels over saline controls only in the reduced-weight rats. The drug, possibly reflecting a tranquilizing action of the lowest dose, reduced corticosterone levels in free-feeding controls. Brain levels of drug were directly related to dose, and were elevated in the food-deprived animals 26-30% over those at the same per-weight dose levels in the free-feeding rats, in spite of being given lower absolute amounts of drug. In the 0.43 mg/kg reduced-weight dose group, given the same absolute dose as the 0.3 mg/kg free-feeding group, brain levels were doubled over the latter group, and exploratory behavior was correspondingly different from the free-feeding group.(ABSTRACT TRUNCATED AT 250 WORDS)

The serotonin2 antagonist ritanserin blocks quasi-morphine withdrawal at a time when mianserin is no longer effective

A quasi-morphine withdrawal syndrome (QMWS), produced in opiate-naive rats with an injection of isobutylmethylxanthine (IBMX) and the opioid antagonist naloxone, allows one to study the expression of opiate withdrawal in the absence of the acute or chronic effects of opiates and the adaptive processes termed dependence. The allegedly selective and long-acting serotonin2 (5-HT2) antagonist ritanserin attenuated the QMWS-induced suppression of fixed ratio (FR) operant responding, which is a sensitive measure of the expression of a QMWS. When administered 30 min prior to precipitation of the QMWS, the lowest dose of ritanserin tested (0.158 mg/kg) was the most effective in blocking the expression of withdrawal; however, there was not complete reversal of the behavioral suppression. Acutely, the two higher doses of ritanserin tested (2.5 and 10 mg/kg) suppressed responding when given alone. This may have masked their ability to attenuate a QMWS. At a dose of 2.5 mg/kg, ritanserin completely blocked the QMWS-induced suppression of responding 24 h post-administration, at a time when its actions at other receptors (e.g., alpha 2) have dissipated. At an equivalent dose, the shorter-acting 5-HT2 antagonist mianserin was unable to attenuate the QMWS-induced suppression of FR operant responding 24 h post-administration. The 5-HT2 antagonists reportedly produce a paradoxical down-regulation of 5-HT2 binding sites upon chronic treatment, rather than the expected supersensitivity. Chronic treatment with ritanserin (2.5 mg/kg/day for 7 days), but not mianserin (same regimen), attenuated a QMWS 24 h after the final injection, thus supporting with a functional measure, the down-regulation of such binding sites by ritanserin.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence of single dose opioid dependence in 12- to 14-day-old chicken embryos

We have previously reported that chicken embryos injected with a single dose of methadone (Meth) on day 3, 7 or 11 of embryogenesis fail to show dependence on day 14, measured as a significant overshoot in motility above baseline after challenge with the opioid antagonist naloxone (Nx). Constant infusion of Meth from day 7 to 14 also failed to produce evidence of dependence on day 14. To address the question of whether the 14-day-old embryo is capable of expressing withdrawal, isobutylmethylxanthine (IBMX), a compound that produces quasi-opioid withdrawal, was injected directly into the embryo, resulting in a significant increase in motility. To determine whether the 14-day-old embryo could also express true opioid withdrawal, the embryos were injected with various doses of Meth or morphine (Morph), followed at different time intervals by injections of varying doses of Nx. A high dose of Morph followed 24 hours later by a low dose of Nx produced evidence of withdrawal, as did a low dose of Meth followed 1 hour later by a higher dose of Nx, U50488H, a selective kappa agonist, had no effect on motility in the 14-day-old embryo, suggesting that the decrease in motility seen after Meth was not mediated by a kappa receptor. Pretreatment with the irreversible mu antagonist, beta-funaltrexamine (B-FNA), blocked the decrease in motility seen after Meth and also prevented the overshoot in motility when Nx was given 1 hour post-Meth. We were also able to demonstrate dependence/withdrawal in the 12-day-old embryo, but higher doses of both Meth and Nx were required.(ABSTRACT TRUNCATED AT 250 WORDS)

Naloxone administration impairs autoshaped learning

Effects of naloxone on acquisition of autoshaped behavior were investigated. Rats deprived to 85% of free-feeding weights were trained to touch a retractable lever; delivery of a food pellet occurred on every trial following lever retraction. The lever was retracted immediately if a touch occurred within 15 s, or automatically after 15 s. Analyses were conducted on number and latencies of touches of the extended lever, nose-pokes (touches) directed at the retracted lever during intertrial intervals (a measure less constrained by ceiling effects than extended lever touches), and unconditioned exploratory rearing activity, measured as touches of a metal strip mounted above the grid floor of the apparatus. In an initial experiment, male Sprague-Dawley rats were given saline or naloxone (2.0 mg/kg, ip) 5 min before a training session of 12 trials. Two days later they were tested, in the absence of drug, in a session of 36 (three blocks of 12) trials. Naloxone depressed training levels of lever responding, in addition to slowing acquisition rate. No effect of naloxone was observed on rearing activity. Previous work showed that injection of saline 5 min before behavioral testing increases the rate of autoshaping compared to injections 30 min before (Messing & Sparber, 1984). Thus, effects of naloxone on acquisition of lever-directed behaviors may have been confounded by behavioral depressant effects and/or by an injection effect such a short time before testing. In a second experiment naloxone (0.5 or 2.0 mg/kg) was injected after five of seven training sessions (12 trials each) to male and female rats. A 6-s delay of reinforcement was inserted between lever retraction and food delivery, slowing acquisition rates and providing the opportunity to test the effects of naloxone throughout a multiple-session task. The low dose retarded acquisition of extended lever touching in both sexes; both doses retarded acquisition of interim lever touching in males. Thus, in some circumstances, post-training naloxone administration may impair learning. The results support the notion that low doses of naloxone may have agonist activity.

Modification of quasi-morphine withdrawal with serotonin agonists and antagonists: evidence for a role of serotonin in the expression of opiate withdrawal

Methylxanthines produce a quasi-morphine withdrawal syndrome (QMWS) in opiate naive rats. Additionally, methylxanthine-induced suppression of conditioned behavior in rats is reversed by the alpha 2 adrenergic agonist clonidine which also attenuates true opiate withdrawal and the QMWS. Therefore, the operant behavioral effects of 3-isobutyl-1-methylxanthine (IBMX) provide a model with which to study mechanisms involved in the expression of opiate withdrawal. In order to examine the role of serotonin (5-HT) in the rate-decreasing effects of IBMX on operant behavior, the 5-HT precursor 5-hydroxytryptophan, and 5-HT reuptake blocker fluoxetine were administered in combination with IBMX to rats performing a fixed-ratio 30 operant for food reinforcement. Both drugs failed to reverse the behavioral suppression caused by relatively low doses of IBMX, suggesting that elevated 5-HT neurotransmission contributes to, rather than attenuates, the QMWS. The relatively selective 5-HT2 antagonists mianserin and pirenperone blocked the IBMX-induced suppression, whereas the classic 5-HT antagonist methysergide had no effect. The results indicate that the operant behavioral effects of IBMX and possibly the QMWS may be mediated by serotonergic mechanisms.

Morphine blocks and naloxone enhances suppression of operant behavior by low doses of 3-isobutyl-1-methylxanthine

Methylxanthines administered in high doses or in combination with the opiate antagonist naloxone produce symptoms resembling opiate withdrawal. Administration of isobutyl-1-methylxanthine (IBMX, 1.25-5.00 mg/kg) to rats which were performing a fixed-ratio 30 operant for food reinforcement caused a dose-dependent suppression of responding and a decrease in colonic temperature. Administration of behaviorally inactive doses of the opiate antagonist naloxone (1.25-5.00 mg/kg) 10 min after the beginning of the fixed-ratio 30 session significantly increased the behavioral effects of IBMX (1.25-5.00 mg/kg) injected 30 min before the session, with no consistent effect upon IBMX-induced hypothermia. The coadministration of behaviorally inactive doses of morphine (0.05-1.50 mg/kg) with IBMX (5.00 mg/kg) 30 min before the fixed-ratio 30 session antagonized the operant behavioral effects of IBMX. These results suggest that the operant behavioral effects of IBMX provide a model with which to examine the expression of opiate withdrawal without the confounding changes associated with the acute pharmacological actions of opiates.

Evidence for antiserotonergic properties of yohimbine

Yohimbine (YOH) is a widely used pharmacological tool employed to produce a selective blockade of alpha 2-adrenergic receptors. In the present study operant behavior was used as a biobehavioral assay to determine the activity of YOH at serotonergic receptors, as indicated by its ability to antagonize the behavioral effects of a serotonergic agonist, lysergic acid diethylamide (LSD). Rats were trained to respond on a Fixed Ratio 15 schedule for food reinforcement. YOH (0.5-5.0 mg/kg) or vehicle and LSD (50 micrograms/kg) were administered (IP) 30 min and immediately prior, respectively, to the 30-min operant session. In a separate study, the ability of YOH (0.5-2.5 mg/kg) to antagonize a higher dose of LSD (100 micrograms/kg) was examined. Relatively low doses of YOH (0.5-1.0 mg/kg) were able to partially, but significantly antagonize the LSD-induced suppression and typical hallucinogen-induced disruption of schedule-controlled responding. These results suggest that YOH, even at moderate doses, may act nonselectively as an antagonist at 5-HT receptors, in addition to its antagonist action at alpha 2-adrenergic receptors. This study demonstrates the utility of operant behavior as a biobehavioral assay to study the receptor mediated action of drugs.

Parallel changes in operant behavioral adaptation and hippocampal corticosterone binding in rats treated with trimethyltin

Rats were given water vehicle or trimethyltin (TMT; 3.0, 6.0 or 7.5 mg/kg, p.o.). Lever responding for food was measured 3 months later, in a test in which the fixed ratio requirement was doubled daily (FR1-128). Response rates for all groups were inverted U-shaped functions of FR values. However, the effect of increasing ratio values was attenuated in the 6.0 mg/kg group, which responded less than controls when control rates were maximal (at FR16 and FR32). In contrast, rats given the high dose responded at higher rates (at FR4 and FR64). [3H]Corticosterone binding to hippocampal cytosolic protein was maximally reduced for the group given 6.0 mg TMT/kg. The greatest reduction in hippocampal weight resulted from injection of 7.5 mg TMT/kg, but a smaller reduction in [3H]corticosterone binding (i.e. 22%) was observed for this group. In the absence of an effect of 3.0 mg TMT/kg upon weight of hippocampus, there also was a reduction in steroid binding, indicating the sensitivity of this parameter for TMT toxicity. The results support the notion that hippocampal corticosteroid receptors are important for behavioral adaptation, and rats given moderate doses of TMT may be useful for studying functions of corticosterone receptors.

Metabolism of methadone by chicken embryos prevents induction of chronic opioid-type dependence after a single injection: use of osmotic pumps for continuous infusion

Unlike N-desmethyl-1-alpha-acetylmethadol (NLAAM), a single injection of methadone (METH), near domestic chicken embryos early during development, cannot induce and sustain opioid-type dependence in the older embryo (i.e., days 14-17 of development). Injection of [3H]-METH near the 14-day-old embryo, followed by differential extraction, indicated that significant quantities of unmetabolized METH gained entrance to the brain, peaking at about 1 hr and declining with a half-life of about 2.8 hr. Thus, it is probably not practical to use a single injection of this shorter-acting opioid for studying biobehavioral effects of sustained dependence and withdrawal during development in this species. Chronic infusion of METH for 7 days via an externalized Alza osmotic mini-pump resulted in significant, dose-dependent brain concentrations of [3H]-METH on day 14. Even though the opioid antagonist naloxone (Nx) was unable to induce withdrawal, manifest as a significant increase in embryonic motility above that of controls, it partially reversed the depressed motility caused by the chronic infusion of [3H]-METH. Since 7-day-old embryos exposed to NLAAM, at doses which can be demonstrated to produce dependence by precipitating withdrawal on day 17 of development, were also unable to express withdrawal on day 14, it is possible that either 14-day-old chicken embryos cannot yet fully respond with an adaptive process (i.e., dependence) or its expression (i.e., withdrawal).

Different methods for producing neonatal undernutrition in rats cause different brain changes in the face of equivalent somatic growth parameters

Similar degrees of undernutrition were induced in neonatal rat pups using a modified Slob technique and a large-litter manipulation. From birth, large litters and Slob litters were comprised of 20 and 10 pups, respectively. On days 2, 4 and 6, half of the pups in the Slob group were cross-fostered to nonlactating foster aunts, while the remaining half of each litter (controls), as well as the large-litter rats were cross-fostered to a lactating dam. On day 10, undernourished Slob and large-litter pups showed similar body weight and body length deficits compared with controls, but differences between the two undernourished groups emerged when regional brain weight and nucleic acids and proteins were examined. The acute intermittent undernutrition of the modified Slob technique had more severe consequences than the large-litter manipulation for brain growth and development. Therefore, two methods of inducing early postnatal undernutrition may yield equivalent somatic effects, but very different brain growth parameters.

Reversal of a trimethyltin-induced learning deficit by desglycinamide-8-arginine vasopressin

Trimethyltin (TMT) is an organometal neurotoxin which produces lesions primarily in the limbic system. Selectivity seems to depend upon the dose, but the hippocampus and related entorhinal cortical structures, of importance for learning and memory, are most often described as target sites. We have previously demonstrated that subjects treated with a moderate dose of TMT prior to acquisition sessions, are unable to learn a forward autoshaping task with a 6 sec delay of reinforcement, but are capable of acquiring the same task when no delay of reinforcement is used. These data suggested that the performance deficit is one of learning (i.e. consolidation) rather than of memory (i.e. storage), retrieval, or sensorimotor impairment. To more rigorously test this hypothesis, we determined if performance of a task already learned would be impaired by the neurotoxin. Adult male Long Evans rats were given 10 acquisition sessions of 24 trials, following which TMT (6.0 mg/kg, p.o.) was administered. One month later, these rats performed the lever-touching behavior as well as controls, despite the fact that the same dose of TMT interfered with learning if given one month prior to acquisition sessions, thus confirming our hypothesis. In a second experiment we determined if the peptide analog of vasopressin, desglycinamide-8-arginine vasopressin (DGAVP), could reverse a learning deficit in a population of non-learners. Rats were treated with TMT or water vehicle one month prior to autoshaping. TMT significantly retarded acquisition. After 10 sessions of 12 trials each, non-learners (i.e. rats treated with TMT that failed to associate the lever with delivery of a reinforcer) were administered saline or DGAVP (7.5 micrograms/kg, s.c.) 1 hr before sessions 11-13; treatment was discontinued prior to sessions 14 and 15. Peptide treated subjects showed evidence of acquisition and exhibited higher levels of lever-directed behavior than saline treated nonlearners. Performance was maintained after DGAVP treatment was discontinued, indicating that the learning-enhancing action of DGAVP was not transient or state-dependent.

Attenuation of isobutylmethylxanthine-induced suppression of operant behavior by pretreatment of rats with clonidine

Administration of 3-isobutyl-1-methylxanthine (IBMX) to rats performing a FR30 operant for food reinforcement produces a dose-dependent suppression of behavior. Operant behavior suppressed by 5 mg IBMX/kg is attenuated by pretreatment, 30 min before the operant session, with the alpha 2 adrenergic agonist clonidine (5-30 micrograms/kg). Clonidine itself causes a dose-dependent reduction in FR30 responding prior to the administration of IBMX. However, doses of clonidine which also suppressed responding were not more effective than lower doses in attenuating the suppression of operant behavior caused by IBMX, perhaps due to postsynaptic or nonspecific actions of clonidine. Methylxanthines, alone or in combination with the opiate antagonist naloxone, produce signs of opiate withdrawal. This quasi-morphine withdrawal syndrome may be useful in studies of either the development or expression of opiate withdrawal. Since clonidine attenuates the rate-suppressant effect of IBMX, it is likely that a significant component of IBMX's behavioral effects are due to increases in NE neurotransmission. These results are similar to those obtained with true opiate withdrawal in rats, strengthening the idea that suppression of operant behavior by IBMX involves mechanisms in common with opiate withdrawal. It may be a useful way of objectively studying the expression of the withdrawal syndrome in the absence of opiates and/or a way of determining if a drug can selectively block withdrawal.

Learning enhancement and behavioral arousal induced by yohimbine

Learning of a food motivated delayed reinforcement autoshaping task was investigated in rats treated with water vehicle or the prototypical anxiogenic agent and alpha 2-adrenergic antagonist yohimbine (0.5 or 1.5 mg/kg, i.p. 30 min before behavioral testing). Unconditioned exploratory rearing activity was monitored concomitantly with acquisition of a lever touch response. The low dose of yohimbine enhanced learning, but it also increased unconditioned behavioral arousal. The high dose retarded acquisition, but when it was withdrawn the animals learned but exploratory activity increased beyond control levels prior to acquisition. Learning thus appeared to be related to the behavioral arousal produced by yohimbine, suggesting that learning enhancement by anxiogenic substances is not due to a direct effect on processes intrinsic to information storage and retrieval; rather, anxiogenic substances may be important modulators of vigilance and performance variables.

Selective learning impairment of delayed reinforcement autoshaped behavior caused by low doses of trimethyltin

The organometal neurotoxin trimethyltin (TMT), induces impaired learning and memory for various tasks. However, administration is also associated with other "non-specific" behavioral changes which may be responsible for effects on conditioned behaviors. To determine if TMT treatment causes a specific learning impairment, three experiments were done using variations of a delay of reinforcement autoshaping task in which rats learn to associate the presentation and retraction of a lever with the delivery of a food pellet reinforcer. No significant effects of TMT treatment were found with a short (4 s) delay of reinforcement, indicating that rats were motivated and had the sensorimotor capacity for learning. When the delay was increased to 6 s, 3.0 or 6.0 mg TMT/kg produced dose-related reductions in behaviors directed towards the lever. Performance of a group given 7.5 mg TMT/kg, while still impaired relative to controls, appeared to be better than the performance of groups given lower doses. This paradoxical effect was investigated with a latent inhibition paradigm, in which rats were pre-exposed to the Skinner boxes for several sessions without delivery of food reinforcement. Control rats showed retardation of autoshaping when food reinforcement was subsequently introduced. Rats given 7.5 mg TMT/kg exhibited elevated levels of lever responding during pre-exposure and autoshaping sessions. The results indicate that 7.5 mg TMT/kg produces learning impairments which are confounded by hyperreactivity to the environment and an inability to suppress behavior toward irrelevant stimuli. In contrast, low doses of TMT cause learning impairments which are not confounded by hyperreactivity, and may prove to be useful models for studying specific associational dysfunctions.

Ketanserin and pirenperone attenuate acute morphine withdrawal in rats

The involvement of serotonin2 (5-HT2) receptors in the expression of opiate withdrawal was examined using a behavioral test for acute morphine dependence. The 5-HT2 antagonists, ketanserin and pirenperone, injected shortly before naloxone, attenuated the naloxone-induced suppression of an autoshaped lever-touch response in rats treated 4 h earlier with a moderate dose of morphine. A low dose of pirenperone was also effective in blocking withdrawal-induced hypothermia. These data support the hypothesis that 5-HT is involved in the expression of opiate withdrawal.

Increased forebrain beta-adrenergic ligand binding induced by trimethyltin

Systemic injection of the organometal neurotoxin trimethyltin (TMT) into rats causes impairments in learning and memory. However, there is a discontinuity in dose-response functions, such that the deficit which emerges with a higher, acutely toxic dose, is qualitatively different from the impairment induced after lower doses. To investigate synaptic receptor changes associated with behavioral deficits, neurotransmitter-receptor ligand binding assays were done in forebrain areas of rats given TMT (3.6 or 7.5 mg/kg). Binding of the beta-adrenergic ligand, dihydroalprenolol in frontal cortex and amygdala/pyriform cortex was an inverted U-shaped function of TMT dose, with rats given the median dose exhibiting increased binding. The curvilinear dose-response functions in behavioral and biochemical assays suggest that altered forebrain noradrenergic neurotransmission could play a role in behavioral deficits.

Studies on the specificity of neurochemical and behavioral effects of LSD-25

Brain perfusion experiments of conscious rats engaged in operant behavior and administered fluoxetine or LSD, with or without prior injection of 5-HTP, indicate there is probably more than one functional pool of 5-HT in the CNS. Furthermore, the fact that prior loading with the precursor is necessary before unmasking an effect of LSD suggests the LSD-sensitive pool is newly synthesized and represents only a small fraction of total CNS serotonin. Separating the effects of LSD's behavioral action into pausing (disruption) and depressed responding rate, with or without pausing, enabled us to demonstrate blockade of the disruption by methysergide without blockade of the decreased responding rate. Mianserin blocks both effects of LSD's action. We suggest that behavioral effects of low doses of LSD are due to sympathetic arousal and may offer a model for agitated depression and/or anxiety and that drugs of the mianserin-type may prove useful for treating some forms of anxiety, as well as depression.

Mianserin attenuates naloxone-precipitated withdrawal signs in rats acutely or chronically dependent upon morphine

The effects of the atypical antidepressant and serotonin antagonist mianserin on the expression of opiate withdrawal was examined using an acute and a chronic model of morphine dependence. In the first experiment, rats, trained to perform a food-reinforced, autoshaped lever touch response, were injected with naloxone (5 mg/kg) 4 hr after treatment with a single moderate dose of morphine (15 mg/kg). Mianserin (0.25, 1.0 and 2.5 mg/kg) attenuated the naloxone-induced suppression of autoshaped responding. Colonic temperatures were also monitored. Morphine treatment resulted in significant hyperthermia, while precipitation of withdrawal by naloxone produced hypothermia. Mianserin also attenuated the naloxone-induced hypothermia. In the second experiment, rats were implanted s.c. with a single 75-mg morphine or placebo pellet. Withdrawal was precipitated with naloxone (5 mg/kg) 24, 48 and 120 hr post implantation. Mianserin (2.5 mg/kg) blocked or attenuated signs of withdrawal precipitated by naloxone. Naloxone-precipitated weight loss was also attenuated 48 and 120 hr post implantation. At 120 hr post implantation, rats were decapitated 1 hr after the administration of naloxone and trunk blood was collected. Mianserin did not block the naloxone-induced rise in plasma corticosterone levels. Thus, several signs of withdrawal (e.g., behavioral effects, weight loss and hypothermia) seem to involve serotonergic mediation and can be blocked by mianserin, while others (e.g., rise in plasma corticosterone), which may be unaffected by mianserin, may be a reflection of a compensatory response to withdrawal stress, rather than a mediator of maladaptive consequences of withdrawal that are not mediated by serotonin.

Greater task difficulty amplifies the facilitatory effect of des-glycinamide arginine vasopressin on appetitively motivated learning

Rats were trained in a discrete-trial forward autoshaping paradigm to touch an extended lever to earn food pellets. Reinforcement was delivered either simultaneously with or 6 s after lever retraction which occurred either non-contingently after 15 s or when the animal touched the lever. Treatment with des-glycinamide arginine vasopressin (DGAVP; 15 micrograms/kg, sc) 1 hr before sessions increased the rate of acquisition of the extended-lever-touch response and also facilitated development of intertrial (adjunctive) nose poking. Effects of the peptide were more robust in the more difficult, delayed reinforcement task. DGAVP lacks the classical peripheral activity of vasopressin. In both experiments, peptide treatment was terminated before asymptotic levels of performance were attained; the continued facilitation of acquisition in treated groups suggests a specific enhancement of learning and/or enhanced memory retrieval.

Behaviorally equivalent stressors differentially modify the monoamine altering property of d-amphetamine

We previously demonstrated that behaviorally equivalent heat and cold stressors interacted with d-amphetamine (AMPH) treatment to produce different effects in rats responding for food on a fixed ratio 15 (FR15) schedule of reinforcement [25]. The present study was carried out to determine if these stressors differentially affect the disposition of AMPH to brain and/or if the stressors alone or in combination with AMPH affect CNS monoamines in a dissimilar manner. Exposure to either heat or cold stressor produced equivalent elevations of [3H]-AMPH in brain following 3 mg AMPH/kg but not 1 mg AMPH/kg. Neither stressor alone significantly altered any of the neurochemical parameters measured in any of the brain regions studied. In forebrain, heat and cold stressors interacted with AMPH treatment in different manners. Thus, although [3H]-AMPH was equally elevated in stressed groups following the high dose, cold-induced stress was not associated with an increase in dopamine (DA) levels, which was observed in Nonstressed and Heat-Stressed subjects. Although serotonin (5-HT) levels were not changed by any manipulation, 5-hydroxyindoleacetic acid (5-HIAA) levels were lowered in Nonstressed and Cold-Stressed subjects following both doses of AMPH. This effect was not associated with heat-induced stress. The apparent attenuation of AMPH behavioral toxicity observed in Cold-Stressed and/or exacerbation in Heat-Stressed rats observed in the earlier study may involve a pharmacodynamic interaction of AMPH and stress with transmitter substances, including DA and/or 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Indomethacin facilitates acute tolerance to and dependence upon morphine as measured by changes in fixed-ratio behavior and rectal temperature in rats

The effects of indomethacin, a prostaglandin (PG) synthetase inhibitor, on acute tolerance to and dependence on morphine were investigated. Twelve mature, male Long-Evans rats were trained to lever press for food reinforcement on a fixed-ratio 30 schedule (FR 30 behavior) and have their rectal temperature taken. The experimental protocol began with taking the rat's temperature followed by a 30 minute behavioral session. Immediately after this session the animal was injected with indomethacin or its vehicle. Two-and-a-half hours later this procedure was repeated, except that morphine or saline was administered. After an additional 2.5 hours had elapsed, a 60 minute behavioral session occurred. Half-way through the session the rat was injected with morphine (tolerance), naloxone (dependence), or saline. Immediately after the session the rat's temperature was recorded. Indomethacin potentiated the acute tolerance to the behavioral suppressant and hyperthermic effects of morphine. Indomethacin pretreatment also greatly enhanced the capacity of naloxone to decrease temperature and suppress FR 30 behavior in morphine-treated rats. These effects were not due to indomethacin altering the acute effects of morphine or the amount of morphine in the brain. These data suggest that indomethacin is inhibiting synthesis of PGs which are important in morphine tolerance and dependence.

Central administration of prostaglandin E2 facilitates while F2 alpha attenuates acute dependence upon morphine rats

The effects of prostaglandin D2 (PGD2), E2 (PGE2), and F2 alpha (PGF2 alpha) on acute dependence on morphine were investigated. Five mature, male Long-Evans rats were trained to lever press for food reinforcement on a fixed-ratio 30 schedule (FR 30 behavior) and implanted with permanent guide cannulas with the tips of the cannulas in their right lateral brain ventricles. The experimental protocol began with a 45 minute behavioral session and brain infusion (1 microliter/minute of a solution containing 2.3 mM CaCl2 in 0.9% saline, ICV). Fifteen minutes into the session the rats were injected with 7.5 mg morphine/kg (IP). Beginning 2.25 hours later the brain infusion was reinitiated during a second 45 minute behavioral session which was interrupted after 15 minutes to inject 1.0 mg naloxone/kg (IP). In several experiments a dose of PG, which did not in-and-of-itself affect behavior, was added to the infusion medium. Prior to the naloxone injection it was ascertained that the behavioral effects of morphine had dissipated. The injection of naloxone or saline did not alter behavior of the rats while they were being infused with a PG or PG vehicle. Injection of naloxone, 3 hours after the injection of morphine, resulted in a significant suppression of FR 30 behavior (withdrawal). A dose of PGE2, which did not alter the initial suppressant action of morphine, potentiated the naloxone effect. A dose of PGF2 alpha, which likewise did not alter the initial action of morphine, antagonized the naloxone effect. However, a higher dose of PGF2 alpha which enhanced the initial morphine effect, caused an enhanced naloxone effect as well.(ABSTRACT TRUNCATED AT 250 WORDS)

Greater task difficulty amplifies the facilitatory effect of des-glycinamide arginine vasopressin on appetitively motivated learning

Rats were trained in a discrete-trial forward autoshaping paradigm to touch an extended lever to earn food pellets. Reinforcement was delivered either simultaneously with or 6 s after lever retraction which occurred either non-contingently after 15 s or when the animal touched the lever. Treatment with des-glycinamide arginine vasopressin (DGAVP; 15 micrograms/kg, sc) 1 hr before sessions increased the rate of acquisition of the extended-lever-touch response and also facilitated development of intertrial (adjunctive) nose poking. Effects of the peptide were more robust in the more difficult, delayed reinforcement task. DGAVP lacks the classical peripheral activity of vasopressin. In both experiments, peptide treatment was terminated before asymptotic levels of performance were attained; the continued facilitation of acquisition in treated groups suggests a specific enhancement of learning and/or enhanced memory retrieval.

A comparative study of the effects of prostaglandins and d-amphetamine on the metabolism of 3H-dopamine continuously presented to rat brain in vivo

Unanesthetized rats with chronic indwelling cannulas, engaged in food reinforced operant behavior, were infused intracerebroventricularly with a solution containing a trace concentration of 3H-dopamine (3H-DA) with or without prostaglandins (PGs). Approximately 45 minutes after the infusion was started, the procedure was changed to a push-pull perfusion. Perfusate from the ventricles contained significant quantities of the 3H-DA metabolites 3H-3,4-dihydroxyphenylacetic acid (3H-DOPAC), 3H-3-methoxy-4-hydroxyphenylacetic acid (3H-homovanillic acid, 3H-HVA), 3H-3-methoxytyramine (3H-3-MT), and the 3H-noradrenaline (3H-NA) metabolite 3H-3-methoxy-4-hydroxy-phenylethyleneglycol (3H-MHPG). The presence of PGF2 alpha decreased the amount of 3H-DOPAC, 3H-HVA, and 3H-3-MT in perfusate, while PGE1 had the opposite effects. d-Amphetamine (0.5 mg/kg, 1P) affected the recovery of these metabolites from perfusate in a manner similar to PGF2 alpha and opposite to PGE1. PGF2 alpha and the highest (seizure-inducing) dose of PGE1 significantly decreased, while d-amphetamine significantly increased, the quantity of 3H-MHPG in perfusate. Therefore, PGs affect central dopaminergic and noradrenergic activity in vivo, as reflected by changes in their metabolic profiles, and may play a role in the response of the central nervous system to drugs which act through catecholaminergic mechanisms.

d-Amphetamine antagonizes prostaglandin E1-induced hyperthermia and suppression of fixed interval operant behavior in rats

The experiments reported herein were designed to study the effects of prostaglandin F2 alpha (PGF2 alpha) and PGE1 on operant behavior and rectal temperature of rats. A solution containing PGF2 alpha or PGE1 was infused intracerebroventricularly into rats trained to press a lever for food reward on a fixed interval 75 second (FI 75 sec) schedule. PGF2 alpha (10, 100 or 1000 ng/min) had no effect on FI 75 sec operant behavior. Only the highest dose increased temperature. PGE1 (100 ng/min) had no effect, whereas higher doses (250 and 500 ng/min) produced a rate-dependent effect on behavior, increasing low rates and decreasing high rates. The two higher doses also produced convulsions after about 25 min or 20 min infusions, respectively. PGE1 also increased temperature in a dose-dependent manner. Systemic administration of a low dose of d-amphetamine (0.5 mg/kg IP) had little or no effect on behavior or temperature. d-Amphetamine did not alter hyperthermia induced by the highest dose of PGF2 alpha, but antagonized the PGE1-induced hyperthermia. d-Amphetamine also antagonized all of the behavioral effects of PGE1, including convulsions. The results are discussed in relation to the actions of PGs and d-amphetamine on catecholamine neurons in the central nervous system.

Indomethacin potentiates the operant behavior suppressant and rectal temperature lowering effects of low doses of d-amphetamine in rats

Prostaglandins (PGs) may be important in modulating the actions of d-amphetamine. To test this hypothesis male rats were pretreated with indomethacin to inhibit PG synthesis before d-amphetamine was injected. Indomethacin (5 mg/kg, IP) was found to be without direct effect upon fixed-ratio behavior or rectal temperature, but significantly enhanced the capacity of low doses of d-amphetamine to suppress behavior and lower temperature. These effects were not due to partial food deprivation or to an increase in the concentration of d-amphetamine in brain. It is concluded that one or more of the PGs modulate the central actions of d-amphetamine, perhaps by modifying the release and/or reuptake of catecholamines, or through a postsynaptic action.

A kindling-like effect induced by repeated exposure to heated water in rats

Hyperthermia was induced in mature rats by immersing them in 20 cm of 45 degrees C water for 4 min. Rats were made hyperthermic once every 4 days for six exposures. A significant number had at least one convulsion by the third exposure. During the seventh exposure, 2 weeks after the sixth exposure, twice as many rats experienced convulsions. Two weeks later, four rats that had convulsed following exposure to heated water at an earlier time convulsed on exposure to a strobe light flashing at 25 Hz. The percentage of rats having experienced at least one convulsion increased dramatically when tested 2 and then an additional 3 months later. Spontaneous (handling-induced) convulsions also occurred in a few rats that had been exposed to heated water previously. The data indicate that repeated exposure to this type of hyperthermia can result in an increase in convulsive susceptibility in mature rats and may be a useful, noninvasive model for studying kindling, febrile convulsions, and epilepsy in rodents.

Neonatal opiate withdrawal alters the reactivity of adult rats to the hot-plate

Prenatal exposure of rats to 0.2 mg LAAM/kg/day but not to 0.05 mg LAAM/kg/day resulted in faster hot-plate escape latencies in 6 mo old offspring. No differences in tail-flick latencies were observed at 7 mo of age in offspring exposed to either dose of LAAM prenatally. Subsequent testing of littermates at 16 mo of age revealed that the greater sensitivity to the hot-plate observed in rats prenatally exposed to LAAM is apparently a result of neonatal withdrawal rather than a primary consequence of the drug. The data are discussed in relation to possible effects of drug or withdrawal on central nervous system development.

Amphetamine cumulation and tolerance development: concurrent and opposing phenomena

Whether diminished or augmented behavioral effects are observed after repeated amphetamine administration may reflect the relative balance between tolerance and drug cumulation. To investigate this, we measured the distribution of d-amphetamine in various tissues and its effects on performance of a conditioned behavior after acute or chronic treatment. Rats trained to lever press under a fixed ratio 5 schedule for food-reinforcement were tested daily for 4 min epochs in each of 6 consecutive hours. After responding was stable, animals were injected for 16 days with saline or 1.0, 2.5 or 5.0 mg 3H-d-amphetamine sulfate/kg IP 15 min before the second daily behavioral epoch. On the 17th day, animals which had been receiving 3H-d-amphetamine were given their usual dose and those which had been receiving saline were given one of the doses of 3H-d-amphetamine; all animals were decapitated approximately 2 1/4 hours after this final injection, immediately after the 4th behavioral epoch. Brain, heart, muscle, epididymal fat, and kidney were removed for subsequent analysis of unchanged 3H-d-amphetamine. The experiment was carried out in two phases, 3 1/2 months apart, which inadvertently resulted in shipment of rats from different buildings on the supplier's campus. Acute treatment produced dose-related effects on operant responding, the lowest dose increasing responding and the highest dose suppressing it. Chronic injection of the highest dose of d-amphetamine resulted in significant attenuation of its acute suppressant effect. Additionally, chronic treatment suppressed responding of rats 23 1/4 hours after injection (i.e., before the subsequent daily injection). Tissue levels of d-amphetamine were dose related and d-amphetamine cumulated after chronic treatment with the highest dose. When d-amphetamine was administered acutely, the behavioural effect immediately before decapitation was highly correlated with the concentration of d-amphetamine in brain and in heart. This was not the case after chronic treatment, since rats given the higher doses showed less behavioural effect than would have been predicted from the concentrations of d-amphetamine in their tissues. Besides evidence of tolerance and cumulation of drug in one or more tissues, a significant phase or colony difference emerged, which could have been due to seasonal or other factors. Additional, different experiments, performed concurrently on a new shipment of rats from each colony, allowed us to conclude that the original observations of phase differences were not due to seasonal differences or chance.(ABSTRACT TRUNCATED AT 400 WORDS)