Pharmacological evidence for the existence of multiple functional pools of brain serotonin: analysis of brain perfusate from conscious rats

The serotonin reuptake inhibitor fluoxetine significantly reduced levels of endogenous 5-hydroxyindoleacetic acid (5-HIAA) in brain perfusate of rats implanted with push-pull cannulas. This occurred in conjunction with its suppressant effect upon fixed-ratio operant behavior. Behavior suppressed with the serotonin agonist lysergic acid diethylamide (LSD) occurred in conjunction with a reduction of 5-HIAA only after 5-HIAA was elevated, shortly before, by 5 mg/kg of the serotonin precursor 5-hydroxytryptophan. Our data demonstrate the likely existence of multiple functional pools of serotonin in brain and support the notion that LSD preferentially affects a newly synthesized pool of this transmitter.

Postnatal abstinence or acute toxicity can account for morbidity in developmental studies with opiates

The incidence of neonatal morbidity and mortality in rats exposed to opiates in utero is generally high. To determine the extent to which neonatal opioid intoxication and/or withdrawal contribute to this effect, addicted pups from dams treated chronically with the long-acting opioid levo-alpha-acetylmethadol (LAAM) and appropriate controls were injected within 12 h of birth with saline, an opioid agonist (LAAM and metabolites) or an antagonist (naloxone). The incidence of neonatal mortality for pups born to dams maintained on a high dose of LAAM was 52%. A single injection of agonist on the first day of life reduced mortality in this group to 29% while a single injection of the antagonist increased mortality to 88%. In contrast, administration of the agonist to control pups and pups born to dams maintained on lower doses of LAAM resulted in increased mortality. Naloxone was apparently innocuous in non-dependent neonates. These data show that, despite LAAM's long duration of action in the mature rat, newborn rats experience withdrawal soon after drug exposure is terminated. These data also indicate that continued opioid exposure is a highly effective means of treating/preventing severe spontaneous withdrawal in the newborn.

Separation of hepatic N-demethylase-inducing and opioid dependence-producing doses of levo-alpha-acetylmethadol in the pregnant rat

A 2.0 mg per kg oral dose of 1-alpha-acetylmethadol (LAAM) administered daily to female rats prior to mating and throughout pregnancy increased ethylmorphine N-demethylase activity in liver microsomes of the dams measured 24 h after parturition. This dose of LAAM decreased maternal weight gain during gestation and increased postnatal mortality. However, 0.05 mg LAAM per kg was sufficient to produce dependence in the dams without affecting hepatic drug metabolism, gestational weight gain or neonatal mortality. The data indicate that it is not necessary to use doses of LAAM which can affect drug metabolizing enzymes in dams and increase pup mortality to maintain opioid-type physical dependence.

Prenatal levo-alpha-acetylmethadol (LAAM) and/or naloxone: effects on brain chemistry and postweaning behavior

Female rats were treated daily with water or 0.2 mg LAAM/kg (PO) beginning 3 weeks before mating and continuing until parturition, after which addicted neonates were allowed to withdraw spontaneously. Beginning on the 14th day of gestation half of the animals in each group were injected (SC) 4 and 2 hours earlier with naloxone (Nx, 1 or 5 mg/kg) and the others with saline. Treatment with LAAM retarded maternal weight gain prior to mating but not during gestation. Evidence of dependence in the LAAM treated dams was manifest as a transient but robust weight loss following each dose of Nx. Offspring exposed to LAAM whose mothers were injected with Nx demonstrated significant perinatal mortality but survivors showed no evidence of growth retardation or biochemical and behavioral dysfunction. Pups exposed only to LAAM and withdrawn after birth showed some evidence of congenital effects. These include (1) lower 21 day postnatal body weights, (2) lower 1 and 3 day postnatal brain weights, (3) lower 19 day fetal brain protein concentrations, (4) lower 1 day postnatal brain DNA, RNA and protein content, and (5) lower levels of unconditioned exploratory activity at 1 1/2 and 10 months of age. The effects on growth and brain weight and biochemical parameters were transient, since control values were attained by 21 or 45 days of age. The absence of these effects in subjects exposed to both LAAM and Nx is interpreted as a screening effect of in utero withdrawal, with only the most resistant subjects surviving to be tested, the result of less severe spontaneous withdrawal because of displacement of cumulated LAAM metabolites by Nx or habituation to postnatal withdrawal stress by precipitated in utero withdrawal.

Yohimbine exacerbates and clonidine attenuates acute morphine withdrawal in rats

The involvement of alpha 2-noradrenergic receptors in the expression of opiate withdrawal was studied using an operant behavioral model of acute morphine dependence. Clonidine, an alpha 2-agonist, attenuated and yohimbine, an alpha 2-antagonist, exacerbated the naloxone-induced suppression of fixed ratio 15 responding in rats pretreated several hours earlier with a single, moderate dose of morphine. These data indicate that the alpha 2-agonist action of clonidine is responsible for its amelioration of withdrawal symptoms. It also validates the acute dependence model for studying the pharmacology of opiates and associated adaptive processes.

Des-Gly-vasopressin improves acquisition and slows extinction of autoshaped behavior

The effects of a vasopressin analog (DGAVP) with minimal endocrinological activity, were assayed on acquisition and extinction of a discrete trial, food reinforced, autoshaped lever touch response. Magazine-trained rats, maintained at 80-85% of free-feeding body weights, were injected s.c. with saline, 5 or 10 micrograms/kg of DGAVP 1 h before each of two sessions in which they learned to touch a retractable lever, presented on a 45 s random interval (RI 45) schedule. Retracted lever contacts (nose-pokes) and unconditioned rearing activity were simultaneously monitored. After acquisition of the extended lever touch response, rats were reassigned to treatment groups, and again injected with saline, 5 or 10 micrograms/kg of DGAVP 1 h before each of two extinction sessions. DGAVP facilitated acquisition (5 micrograms/kg) and slowed extinction (5 and 10 micrograms/kg) of conditioned behavior, while having no effects on the other behaviors, thus demonstrating the specificity of the effect of a vasopressin-like compound on both tasks (enhanced acquisition and retarded extinction) used to study learning.

Behaviorally inactive doses of mianserin antagonize the suppressant effect of lysergic acid diethylamide on a fixed-ratio operant

Male Sprague-Dawley rats were trained to respond for food reinforcement on a fixed-ratio 15 schedule. Low, behaviorally inactive, doses of mianserin (0.1-1.0 mg/kg) administered 30 min before the operant session antagonized the behavioral suppression induced by lysergic acid diethylamide (LSD; 50 micrograms/kg) administered immediately before the session. Mianserin (0.5-1.0 mg/kg) also partially antagonized the behavioral suppressant effects of higher doses of LSD. At a dose which suppressed behavior to 70% of control when administered alone, mianserin (10 mg/kg) did not antagonize the behavioral suppression induced by LSD (100-200 micrograms/kg). The data suggest that the suppression of operant behavior produced by mianserin may be the result of nonselective or multiple actions, or possibly the emergence of an LSD-type partial agonist property. Specificity of drug action as an antagonist apparently occurs at the behaviorally inactive, low doses of mianserin which may act through a selective blockade of serotonin receptors stimulated by LSD.

Neonatal undernutrition alters responsiveness to morphine in mature rats: a possible source of epiphenomena in developmental drug studies

We studied the effect of neonatal undernutrition and its attendant stresses on the behavior and thermoregulation of adult rats in the absence and presence of morphine. Undernutrition was accomplished by fostering half the pups in each litter to a nonpregnant, nonlactating female rat every other day for the first 6 days of life. As a control, the remaining pups were fostered to lactating rats. Significant alterations in body and brain weight and in brain DNA, RNA and protein were noted in undernourished rats at 10 and 21 days of age. At 6 months of age, no difference was observed in unconditioned exploratory behavior or in the acquisition and performance of a conditioned autoshaped lever touch response. Differential sensitivity to morphine was observed in nourished and undernourished rats performing the autoshaped response at asymptotic levels. At 1 year of age, undernourished rats displayed lower rectal temperatures than controls and showed an altered thermic response to morphine. These data demonstrate that poor nutritional status and other nondrug factors may be responsible for the altered thermoregulation and opiate sensitivity observed in rodents perinatally addicted to opioids, effects generally regarded as specific consequences of early opiate exposure.

Opiate withdrawal increases ornithine decarboxylase activity which is otherwise unaltered in brains of dependent chicken fetuses

We have used the developing chicken to determine if ornithine decarboxylase (ODC) activity is altered in fetuses chronically exposed to the opiate N-desmethyl-l-alpha-acetylmethadol (NLAAM) or rendered abstinent by acute injection of naloxone (Nx). Exposure to NLAAM from day 3 of embryogenesis did not significantly change brain ODC activity in 15, 17 or 19-day-old fetuses. Acute treatment of 17-day-old fetuses with a motility suppressant dose of NLAAM did not differentially affect ODC activity in NLAAM-dependent fetuses, but an additional treatment with Nx, which precipitated withdrawal, resulted in a significant increase in ODC activity in this group. We conclude that withdrawal can alter fetal ODC activity which otherwise appears normal, even though fetuses have been chronically exposed to and dependent upon an opiate.

A method for equating thermal stress parameters for neurobehavioral toxicity studies

Rats were immersed in 20 cm of cold (15 or 20 degrees C) water for 1, 2.5 or 5 min and were tested 15 min later on fixed ratio 15 (FR15) operant behavior for 30 min. Linear regression analyses predicted that 4 min exposure to 20 degrees C would suppress FR15 behavior 50%. Subsequent exposure to 20 degrees C water for this time period resulted in 53% suppression of FR15 responding. Rats were then immersed in heated water for 4 min and tested 15 min later, exactly as in the cold stress experiment. Exposure to 44, 45 or 46 degrees C water suppressed FR15 responding 21, 46 and 94%, respectively. Significant alterations in colonic temperature indicated that the manipulations disrupted thermoregulatory mechanisms and were probably stressful. The procedure is an alternative to other methods used to index the intensity of stress. By manipulating selected variables (e.g., water temperature or exposure duration), qualitatively different stressors can be matched behaviorally prior to studying interactions between stress and various other manipulations such as drugs or toxic compounds.

Behavioral toxicity of amphetamine is differentially affected by hot and cold water immersion stress

The interaction of two qualitatively different but behaviorally equivalent stressors with the behavioral toxicity induced by d-amphetamine (AMPH) was examined in rats. Stress, administered by immersing the rats in either hot or cold water, suppressed fixed ratio 15 (FR15) operant responding 50% and disrupted thermoregulation. AMPH (1, 2 and 3 mg/kg) administered immediately prior to stress, once every 7 days, in ascending order, did not modify stress-induced changes in colonic temperature (Tc) immediately after water immersion but induced a decrease in Tc in all groups 45 min later. The behavioral suppressant effect of AMPH observed in the Non-Stressed group was attenuated at all doses in rats exposed to cold water. Although both stressed groups had equivalent baseline rates, only the Cold-Stressed group's response rate was relatively unaffected by AMPH, compared to non-drug response rates, as would be predicted by the rate-dependency hypothesis. The hot stress appears to have activated processes antagonistic to those involved in the rate-dependent effect. The results indicate that AMPH interacts with hot and cold stressors differently when the intensity of the stressors are otherwise equivalent in their ability to alter the behavior which is used as the dependent variable to assess toxicity.

Differential interaction between stressors and chronic amphetamine or phencyclidine upon operant behavior in the rat

Three types of stress were examined for their effects upon fixed-ratio 15 (FR-15) operant behavior in Sprague-Dawley rats before and after chronic dextroamphetamine to determine if cumulated drug could interact with stress to produce a state of behavioral toxicity. Six daily s.c. injections of saline were given to rats following 30 min behavioral sessions. Thirty minutes of exposure to a cold environment (7 degrees C) or to electric footshock (2mA for 0.5 sec, 1 shock/min) had no significant effect upon FR-15 behaviour beginning 15 min afterward. Five daily s.c. injections of 2.5 mg dextroamphetamine/kg after behavioral sessions suppressed behavior during the following days an average of 12%. Rats were again stressed twenty-two hours after the sixth injection. Footshock further suppressed behavior to 26% of the unstressed, chronic amphetamine-treated control group. Cold exposure failed to suppress behavior significantly below control levels. A similar experiment employing cold water exposure (15 degrees C for 2 min) as the stressor showed that, although, this stressor suppressed FR-15 behavior to 53% of the previous day's rates and reduced body temperature 2.1 degree C, chronic amphetamine failed to interact with the cold water stress to suppress behavior further. In a third experiment, 4.5 mg phencyclidine/kg, s.c., given after 30-min FR-15 sessions for six days, failed to interact with the footshock stress to cause behavioral suppression different from control treatment. While stress can interact with chronic treatment with lipophilic drugs, the quality of the stress, and the physical and pharmacologic properties of the drug are important determinants of the outcome upon conditioned behavior.

Congenital behavioral effects in mature rats prenatally exposed to levo-alpha-acetylmethadol (LAAM)

Male and female rats exposed prenatally to LAAM (0.2 mg/kg maternal body weight per day, PO) or water were tested for congenital effects on their performance of various unconditioned and conditioned behaviors. No differences were found in neuromuscular development or in exploratory activity. Although they acquired and performed an autoshaped lever-touch response like controls, the LAAM rats responding under this paradigm were less affected by dextroamphetamine than were controls, suggesting an alteration in catecholamine neurotransmission in these subjects. Furthermore, when required to respond on a progressive fixed-ratio (FR) procedure, in which the response requirement for reinforcement was doubled each day from FR1 to a maximum of FR128, LAAM rats made more responses at the higher ratios than did controls. Although these data suggest that LAAM may be a behavioral of functional teratogen, the possibility that early postnatal toxicity or withdrawal are responsible for these findings is discussed.

Phencyclidine retards autoshaping at a dose which does not suppress the required response

Four groups of five food-deprived hooded Long-Evans rats were injected subcutaneously with saline (vehicle) or 2, 4 or 8 mg phencyclidine (PCP) hydrochloride/kg fifteen minutes before being placed for the first time into operant chambers modified to detect exploratory behaviors. Rearing was found to be more sensitive to disruption by phencyclidine than was unconditioned level touching (a measure of floor-level exploratory activities). In an autoshaping session immediately following, the group of animals given the low dose of PCP made as many lever-touch responses as the group given saline, but consumed fewer of the food pellets delivered. In addition, none of the animals in the low-dose group showed within-session shortening of the latency to respond which was observed in four of five control animals. The two other groups given higher doses of PCP demonstrated dose-related decrements in responding as well as a reduction in food pellet consumption during the first session of autoshaping. Over the next two daily autoshaping sessions, performance improved in those groups initially suppressed. Performance converged in all group by the third autoshaping session.

Antagonism of morphine-induced behavioral suppression by opiate receptor alkylators

Experiments were conducted to test the in vivo opiate specificity and long-lasting effects of two non-equilibrium opiate antagonists: beta-chlornaltrexamine (beta-CNA) and the beta-fumarate methyl ester derivative of naltrexone (beta-FNA). beta-CNA (2.5 or 5.0 micrograms, ICV) partially antagonized suppression of conditioned autoshaped behavior by morphine, when morphine was administered 48-72 hr after beta-CNA. beta-CNA had no effect on amphetamine-induced suppression of autoshaped responding, nor did it antagonize the suppression in rearing activity induced by either morphine or amphetamine. Similarly, beta-FNA (5 mg/kg, IP) antagonized the suppression of conditioned behavior by morphine, for up to 48 hr, while having no effect on amphetamine-induced suppression of autoshaped responding, or on the suppression of rearing activity induced by morphine or amphetamine. Further peripherally administered beta-FNA acts in the brain, since it antagonized analgesia following ICV morphine administration.

Cumulation of active metabolites of levo-alpha-acetylmethadol in the rat fetus and neonate

Levo-alpha-acetylmethadol (LAAM, 0.2 or 2.0 mg/kg/day) was orally administered to female Sprague-Dawley rats for one month prior to and throughout pregnancy. The rats were killed on the 18th day of gestation along with a group of 18-day pregnant females given a single oral 2.0 mg/kg dose of LAAM 24 hours earlier. Although cumulation of LAAM or its active metabolites was not seen in plasma or brain of pregnant rats given drug chronically, significant cumulation was observed in whole fetus and in fetal brain. In addition, a 2-3 fold elevation in the concentrations, and an even greater elevation of total content, was noted in the newborn pup. These data suggest that opiate intoxication soon after birth may be a factor responsible for the increased morbidity and mortality of rat pups prenatally exposed to LAAM.

The effects of morphine and delta-9-tetrahydrocannabinol on motor activity in rats

Acute treatment of rats either by high doses of morphine or delta 9-tetrahydrocannabinol (THC) decreased locomotor activity. Naloxone reversed morphine-induced depression completely and reversed THC-induced depression only partially. On day 3 of treatment, tolerance developed to the locomotor inhibitory action of THC or morphine and partial cross-tolerance was observed to the depressant action of THC. Naloxone slightly depressed locomotor activity in THC-tolerant rats, but increased motor activity in morphine-tolerant rats.

Outcome of pregnancy in rats chronically exposed to 1-alpha-acetylmethadol (LAAM)

Repeated oral administration of 0.2, 0.5 or 2.0 mg of LAAM per kg to mature female rats for 21 days resulted in diminished weight gain and was sufficient to induce opiate dependence. Chronic oral administration of 0.2 and 2.0 mg/kg/day to female rats before mating and throughout gestation gave rise to a population of pups which were shown to be dependent at birth by precipitated weight loss after a s.c. injection of 2.5 mg of naloxone per kg. Among the litters born to dams maintained on the 2.0 mg/kg/day dose of LAAM, there was a high incidence of stillbirths and infanticide by biological mothers or by surrogate mothers to which they were transferred within a few hours of birth. Surviving pups from this group lost weight during the next 24 hr and weighed less than controls at the time of weaning (21 days). In contrast, pups born to dams maintained on the low dose of LAAM were not cannibalized and weighed the same as or more than controls from birth until weaning. Although this group had a higher incidence of mortality during the 1st week after fostering, mortality was significantly less than for the higher dose group.

Distribution of methionine-enkephalin immunoreactivity in the chick brain: an immunohistochemical study

The distribution of Met-enkephalin-like immunoreactivity in the brains of 2-week-old domestic chicks was studied with immunohistofluorescence and the unlabeled antibody peroxidase-antiperoxidase technique, using antibodies to Met-enkephalin generated in rabbits. Immunoreactive cell bodies were found in the telencephalon, diencephalon, and mesencephalon in areas as yet uncharacterized as discrete nuclei in birds (E-1, E-2, E-3, E-4, E-5); further cells were located in the diencephalic nucleus spiriformis lateralis, the midbrain medial intercollicular nucleus (E-6), the nucleus mesencephalicus lateralis, pars dorsalis, the Edinger-Westphal nucleus and dorsal occulomotor nucleus; and in the nucleus of cranial nerve X and an uncharacterized area in the dorsolateral medulla (E-7). Immunoreactive fibers and/or terminals were located around the immunoreactive cell bodies and, in addition in the lateral septal area of the telencephalon; in the preoptic and hypothalamic areas of the diencephalon; in the anterior intercollicular area, periaqueductal central gray, area C, and the midventral tegmentum of the mesencephalon; in the nucleus solitarius, nucleus IX-X, nucleus intercalatus, nucleus intermedius, and ventrolateral areas of the rhombencephalon. The pattern of distribution of met-enkephalin in the chick is compared with that in the rat. A possible functional role for Met-enkephalin in neural mechanisms mediating some behaviors of the chick is suggested.

Prenatal withdrawal from opiates interferes with hatching of otherwise viable chick fetuses

Fetal chicks were made opiate-dependent by injections of N-desmethyl-1-alpha-acetylmethadol into the chorioallantois on day 3 of embryogenesis. The injections had no effect on subsequent hatchability; however, spontaneous fetal motility was significantly depressed. Injection of naloxone caused a significant increase in the motility of the opiate-exposed fetuses but had no effect on control fetuses. That naloxone's effect was an expression of opiate withdrawal and not due to antagonism of depressed motility is also supported by the observation that naloxone significantly reduced the hatchability of opiate-exposed chicks and not of control chicks. Thus the withdrawal of a developing organism from a narcotic may be more deleterious to its survival than continued exposure.