Beneficial effects of co-treatment with dextromethorphan on prenatally methadone-exposed offspring

Effect of prenatal and early post-natal oxycodone exposure on the reinforcing and antinociceptive effects of oxycodone in adult C57BL/6 J mice

Abuse of opioids (mu-opioid agonists such as oxycodone) among parents during the gestation and early post-natal period is a concern for the long-term health of the offspring, beyond potential neonatal withdrawal symptoms. However, there is only limited information on such effects.

OBJECTIVES

We examined how prenatal, and early-post natal oxycodone exposure affected opioid addiction behaviors.

METHODS

Adult male and female C57BL/CJ mice housed separately were first injected with ascending doses of oxycodone 1 time/day (1 mg/kg × 10 days, 1.5 mg/kg × 10 days, 2 mg/kg × 10 days, s.c.) whereas control mice were injected with saline. Newly formed parental dyads were then housed together and continued to receive ascending doses of oxycodone (3 mg/kg × 10 days, 4 mg/kg × 10 days, 5 mg/kg × 10 days, 6 mg/kg × 10 days or saline, s.c.) or saline during mating and gestation until the birth of the litter. The dams continued to receive oxycodone or saline through lactation, until F1 offspring were weaned. Upon reaching adulthood (12 weeks of age), male and female F1 offspring were examined in intravenous self-administration (IVSA) of oxycodone, on oxycodone-induced conditioned place preference (CPP) and oxycodone-induced antinociception.

RESULTS

Adult F1 male and female offspring of parental dyads exposed to oxycodone self-administered more oxycodone, compared to offspring of control parental dyads. Ventral and dorsal striatal mRNA levels of genes such as Fkbp5 and Oprm1 were altered following oxycodone self-administration.

CONCLUSION

Prenatal and early post-natal oxycodone exposure enhanced oxycodone self-administration during adulthood in the C57BL/6 J mice.

Opioid-induced dysbiosis of maternal gut microbiota during gestation alters offspring gut microbiota and pain sensitivity

There has been a rapid increase in neonates born with a history of prenatal opioid exposure. How prenatal opioid exposure affects pain sensitivity in offspring is of interest, as this may perpetuate the opioid epidemic. While few studies have reported hypersensitivity to thermal pain, potential mechanisms have not been described. This study posits that alterations in the gut microbiome may underly hypersensitivity to pain in prenatally methadone-exposed 3-week-old male offspring, which were generated using a mouse model of prenatal methadone exposure. Fecal samples collected from dams and their offspring were subjected to 16s rRNA sequencing. Thermal and mechanical pain were assessed using the tail flick and Von Frey assays. Transcriptomic changes in whole brain samples of opioid or saline-exposed offspring were investigated using RNA-sequencing, and midbrain sections from these animals were subjected to qPCR profiling of genes related to neuropathic and inflammatory pain pathways. Prenatal methadone exposure increased sensitivity to thermal and mechanical pain and elevated serum levels of IL-17a. Taxonomical analysis revealed that prenatal methadone exposure resulted in significant alterations in fecal gut microbiota composition, including depletion of Lactobacillus, Bifidobacterium, and Lachnospiracea sp and increased relative abundance of Akkermansia, Clostridium sensu stricto 1, and Lachnoclostridium. Supplementation of the probiotic VSL#3 in dams rescued hypersensitivity to thermal and mechanical pain in prenatally methadone-exposed offspring. Similarly, cross-fostering prenatally methadone-exposed offspring to control dams also attenuated hypersensitivity to thermal pain in opioid-exposed offspring. Modulation of the maternal and neonatal gut microbiome with probiotics resulted in transcriptional changes in genes related to neuropathic and immune-related signaling in whole brain and midbrain samples of prenatally methadone-exposed offspring. Together, our work provides compelling evidence of the gut-brain-axis in mediating pain sensitivity in prenatally opioid-exposed offspring.

Effects of prenatal opioid exposure on synaptic adaptations and behaviors across development

In this review, we focus on prenatal opioid exposure (POE) given the significant concern for the mental health outcomes of children with parents affected by opioid use disorder (OUD) in the view of the current opioid crisis. We highlight some of the less explored interactions between developmental age and sex on synaptic plasticity and associated behavioral outcomes in preclinical POE research. We begin with an overview of the rich literature on hippocampal related behaviors and plasticity across POE exposure paradigms. We then discuss recent work on reward circuit dysregulation following POE. Additional risk factors such as early life stress (ELS) could further influence synaptic and behavioral outcomes of POE. Therefore, we include an overview on the use of preclinical ELS models where ELS exposure during key critical developmental periods confers considerable vulnerability to addiction and stress psychopathology. Here, we hope to highlight the similarity between POE and ELS on development and maintenance of opioid-induced plasticity and altered opioid-related behaviors where similar enduring plasticity in reward circuits may occur. We conclude the review with some of the limitations that should be considered in future investigations. This article is part of the Special Issue on 'Opioid-induced addiction'.

Perinatal oxycodone exposure causes long-term sex-dependent changes in weight trajectory and sensory processing in adult mice

RATIONALE

In utero opioid exposure is associated with lower weight and a neonatal opioid withdrawal syndrome (NOWS) at birth, along with longer-term adverse neurodevelopmental outcomes and mood disorders. While NOWS is sometimes treated with continued opioids, clinical studies have not addressed if long-term neurobehavioral outcomes are worsened with continued postnatal exposure to opioids. In addition, pre-clinical studies comparing in utero only opioid exposure to continued post-natal opioid administration for withdrawal mitigation are lacking.

OBJECTIVES

Here, we sought to understand the impact of continued postnatal opioid exposure on long term behavioral consequences.

METHODS

We implemented a rodent perinatal opioid exposure model of oxycodone (Oxy) exposure that included Oxy exposure until birth (short Oxy) and continued postnatal opioid exposure (long Oxy) spanning gestation through birth and lactation.

RESULTS

Short Oxy exposure was associated with a sex-specific increase in weight gain trajectory in adult male mice. Long Oxy exposure caused an increased weight gain trajectory in adult males and alterations in nociceptive processing in females. Importantly, there was no evidence of long-term social behavioral deficits, anxiety, hyperactivity, or memory deficits following short or long Oxy exposure.

CONCLUSIONS

Our findings suggest that offspring with prolonged opioid exposure experienced some long-term sequelae compared to pups with opioid cessation at birth. These results highlight the potential long-term consequences of opioid administration as a mitigation strategy for clinical NOWS symptomology and suggest alternatives should be explored.

Prenatal opioid exposure reprograms the behavioural response to future alcohol reward

As the opioid crisis has continued to grow, so has the number of infants exposed to opioids during the prenatal period. A growing concern is that prenatal exposure to opioids may induce persistent neurological changes that increase the propensity for future addictions. Although alcohol represents the most likely addictive substance that the growing population of prenatal opioid exposed will encounter as they mature, no studies to date have examined the effect of prenatal opioid exposure on future sensitivity to alcohol reward. Using a recently developed mouse model of prenatal methadone exposure (PME), we investigated the rewarding properties of alcohol and alcohol consumption in male and female adolescent PME and prenatal saline exposed (PSE) control animals. Conditioned place preference to alcohol was disrupted in PME offspring in a sex-dependent manner with PME males exhibiting resistance to the rewarding properties of alcohol. Repeated injections of alcohol revealed enhanced sensitivity to the locomotor-stimulating effects of alcohol specific to PME females. PME males consumed significantly more alcohol over 4 weeks of alcohol access relative to PSE males and exhibited increased resistance to quinine-adulterated alcohol. Further, a novel machine learning model was developed to employ measured differences in alcohol consumption and drinking microstructure to reliably predict prenatal exposure. These findings indicate that PME alters the sensitivity to alcohol reward in adolescent mice in a sex-specific manner and suggests prenatal opioid exposure may induce persistent effects on reward neurocircuitry that can reprogram offspring behavioural response to alcohol later in life.

Prenatal Opioid Exposure Impairs Endocannabinoid and Glutamate Transmission in the Dorsal Striatum

The opioid crisis has contributed to a growing population of children exposed to opioids during fetal development; however, many of the long-term effects of opioid exposure on development are unknown. We previously demonstrated that opioids have deleterious effects on endocannabinoid plasticity at glutamate synapses in the dorsal striatum of adolescent rodents, but it is unclear whether prenatal opioid exposure produces similar neuroadaptations. Using a mouse model of prenatal methadone exposure (PME), we performed proteomics, phosphoproteomics, and patch-clamp electrophysiology in the dorsolateral striatum (DLS) and dorsomedial striatum (DMS) to examine synaptic functioning in adolescent PME offspring. PME impacted the proteome and phosphoproteome in a region- and sex-dependent manner. Many proteins and phosphorylated proteins associated with glutamate transmission were differentially abundant in PME offspring, which was associated with reduced glutamate release in the DLS and altered the rise time of excitatory events in the DMS. Similarly, the intrinsic excitability properties of DMS neurons were significantly affected by PME. Last, pathway analyses revealed an enrichment in retrograde endocannabinoid signaling in the DLS, but not in the DMS, of males. Electrophysiology studies confirmed that endocannabinoid-mediated synaptic depression was impaired in the DLS, but not DMS, of PME-males. These results indicate that PME induces persistent neuroadaptations in the dorsal striatum and could contribute to the aberrant behavioral development described in offspring with prenatal opioid exposure.

Consequences of Parental Opioid Exposure on Neurophysiology, Behavior, and Health in the Next Generations

Substance abuse and the ongoing opioid epidemic represents a large societal burden. This review will consider the long-term impact of opioid exposure on future generations. Prenatal, perinatal, and preconception exposure are reviewed with discussion of both maternal and paternal influences. Opioid exposure can have long-lasting effects on reproductive function, gametogenesis, and germline epigenetic programming, which can influence embryogenesis and alter the developmental trajectory of progeny. The potential mechanisms by which preconception maternal and paternal opioid exposure produce deleterious consequences on the health, behavior, and physiology of offspring that have been identified by clinical and animal studies will be discussed. The timing, nature, dosing, and duration of prenatal opioid exposure combined with other important environmental considerations influence the extent to which these manipulations affect parents and their progeny. Epigenetic inheritance refers to the transmission of environmental insults across generations via mechanisms independent of the DNA sequence. This topic will be further explored in the context of prenatal, perinatal, and preconception opioid exposure for both the maternal and paternal lineage.

Prenatal methadone exposure impairs adolescent cognition and GABAergic neurodevelopment in a novel rat model of maternal methadone treatment

Methadone maintenance treatment (MMT) is the most common treatment for opioid-dependent pregnant women worldwide. Despite its widespread use, MMT is associated with a variety of adverse neurodevelopmental outcomes in exposed offspring, particularly cognitive impairments. The neurobiological abnormalities underlying these cognitive impairments are, however, poorly understood. This is, in part, due to a lack of animal models that represents the standard of care that methadone is administered in the clinic, with inconsistencies in the timing, doses and durations of treatment. Here we describe the characterisation of a clinically relevant rat model of MMT in which the long-term behavioural and neurobiological effects of prenatal methadone exposure can be assessed in adolescent offspring. Female Sprague-Dawley rats were treated orally with an ascending methadone dosage schedule (5, 10, 15, 20, 25 and 30 mg/kg/day), self-administered in drinking water prior to conception, throughout gestation and lactation. Pregnancy success, maternal gestational weight gain, litter survival and size were not significantly altered in methadone-exposed animals. Methadone-exposed offspring body and brain weights were significantly lower at birth. Novel object recognition tests performed at adolescence revealed methadone-exposed offspring had impaired recognition memory. Furthermore, the rewarded T-maze alternation task demonstrated that methadone-exposed female, but not male, offspring also exhibit working memory and learning deficits. Immunoblots of the adolescent prefrontal cortex and hippocampus showed methadone-exposed offspring displayed reduced levels of mature BDNF, in addition to the GABAergic proteins, GAD67 and parvalbumin, in a sex- and brain region-specific fashion. This rat model closely emulates the clinical scenario in which methadone is administered to opioid-dependent pregnant woman and provides evidence MMT can cause cognitive impairments in adolescent offspring that may be underlined by perturbed neurodevelopment of the GABAergic system.

Ontogenetic Oxycodone Exposure Affects Early Life Communicative Behaviors, Sensorimotor Reflexes, and Weight Trajectory in Mice

Nationwide, opioid misuse among pregnant women has risen four-fold from 1999 to 2014, with commensurate increase in neonates hospitalized for neonatal abstinence syndrome (NAS). NAS occurs when a fetus exposed to opioids in utero goes into rapid withdrawal after birth. NAS treatment via continued post-natal opioid exposure has been suggested to worsen neurodevelopmental outcomes. We developed a novel model to characterize the impact of in utero and prolonged post-natal oxycodone (Oxy) exposure on early behavior and development. Via subcutaneous pump implanted before breeding, C57BL/6J dams were infused with Oxy at 10 mg/kg/day from conception through pup-weaning. At birth, in utero oxy-exposed pups were either cross-fostered (paired with non-Oxy exposed dams) to model opioid abstinence (in utero Oxy) or reared by their biological dams still receiving Oxy to model continued post-natal opioid exposure (prolonged Oxy). Offspring from vehicle-exposed dams served as cross-fostered (in utero Veh) or biologically reared (prolonged Veh) controls. In utero Oxy exposure resulted in sex-dependent weight reductions and altered spectrotemporal features of isolation-induced ultrasonic vocalization (USV). Meanwhile, prolonged Oxy pups exhibited reduced weight and sex-differential delays in righting reflex. Specifically, prolonged Oxy female offspring exhibited increased latency to righting. Prolonged Oxy pups also showed decreases in number of USV calls and changes to spectrotemporal USV features. Overall, ontogenetic Oxy exposure was associated with impaired attainment of gross and sensorimotor milestones, as well as alterations in communication and affective behaviors, indicating a need for therapeutic interventions. The model developed here will enable studies of withdrawal physiology and opioid-mediated mechanisms underlying these neurodevelopmental deficits.

Prenatal opioid exposure and vulnerability to future substance use disorders in offspring

The heightened incidence of opioid use during pregnancy has resulted in unprecedented rates of neonates prenatally exposed to opioids. Prenatal opioid exposure (POE) results in significantly adverse medical, developmental, and behavioral outcomes in offspring. Of growing interest is whether POE contributes to future vulnerability to substance use disorders. The effects of POE on brain development is difficult to assess in humans, as the timing, dose, and route of drug exposure together with complex genetic and environmental factors affect susceptibility to addiction. Preclinical models of POE have allowed us to avoid methodological difficulties and confounding factors of POE in humans. Here, we review the effects of maternal opioid exposure on the developing brain with an emphasis on the neurobiological basis of drug addiction and on preclinical models of POE and their limitations. These studies have indicated that POE increases self-administration of drugs, reward-driven behaviors in the conditioned place paradigm, and locomotor sensitization. While addiction is multifaceted and vulnerability to drug addiction is still inconclusive in human studies of prenatally exposed infants, animal studies do provide a noteworthy corroboration of negative behavioral outcomes.

Prenatal Opioid Exposure Enhances Responsiveness to Future Drug Reward and Alters Sensitivity to Pain: A Review of Preclinical Models and Contributing Mechanisms

The opioid crisis has resulted in an unprecedented number of neonates born with prenatal opioid exposure (POE); however, the long-term effects of POE on offspring behavior and neurodevelopment remain relatively unknown. The advantages and disadvantages of the various preclinical POE models developed over the last several decades are discussed in the context of clinical and translational relevance. Although considerable and important variability exists among preclinical models of POE, the examination of these preclinical models has revealed that opioid exposure during the prenatal period contributes to maladaptive behavioral development as offspring mature including an altered responsiveness to rewarding drugs and increased pain response. The present review summarizes key findings demonstrating the impact of POE on offspring drug self-administration (SA), drug consumption, the reinforcing properties of drugs, drug tolerance, and other reward-related behaviors such as hypersensitivity to pain. Potential underlying molecular mechanisms which may contribute to this enhanced addictive phenotype in POE offspring are further discussed with special attention given to key brain regions associated with reward including the striatum, prefrontal cortex (PFC), ventral tegmental area (VTA), hippocampus, and amygdala. Improvements in preclinical models and further areas of study are also identified which may advance the translational value of findings and help address the growing problem of POE in clinical populations.

Prenatal exposure to methadone or buprenorphine alters µ-opioid receptor binding and downstream signaling in the rat brain

There is a growing concern related to the use of opioid maintenance treatment during pregnancy. Studies in both humans and animals have reported reduced cognitive functioning in offspring prenatally exposed to methadone or buprenorphine; however, little is known about the neurobiological mechanisms underlying these impairments. To reveal possible neurobiological effects of such in utero exposure, we examined brain tissue from methadone- and buprenorphine-exposed rat offspring previously shown to display impaired learning and memory. We studied µ-opioid receptor (MOR) and N-methyl-D-aspartate receptor (NMDAR) binding in the rat offspring cerebrum during development and in the hippocampus at young adulthood. Moreover, we examined activation of the Ca2+ /calmodulin-dependent protein kinase II (CaMKII) and the extracellular signal-regulated kinase (ERK), which are central in the downstream signaling of these receptors. The methadone- and buprenorphine-exposed rat pups displayed reduced MOR binding up to two weeks after birth, whereas the NMDAR binding was unaffected. Prenatal exposure to methadone or buprenorphine also resulted in decreased activation of CaMKII and/or ERK during development, while young adult offspring displayed increased hippocampal ERK activation. In conclusion, our findings suggest that prenatal exposure to exogenous opioids, such as methadone or buprenorphine, may disturb the endogenous opioid system during development, with long-term effects on proteins important for cognitive functioning.

Prenatal exposure to methadone or buprenorphine impairs cognitive performance in young adult rats

BACKGROUND

Concerns have been raised about the use of opioid maintenance treatment (OMT) during pregnancy and negative effects for the offspring. While neonatal outcomes and short-term effects are relatively well described, studies examining long-term effects in adolescents and adults are absent. The aim of the present study was to examine effects on learning and memory in young adult rats prenatally exposed to methadone or buprenorphine.

METHODS

Female rats were implanted with a 28-day osmotic minipump delivering methadone (10 mg/kg/day), buprenorphine (1 mg/kg/day) or vehicle 5 days prior to mating. To examine possible effects on cognitive functioning, young adult offspring were included in three different behavioral tests that examine recognition memory, nonspatial, and spatial learning and memory. In addition, offspring growth and maternal behavior after birh were investigated.

RESULTS

Prenatal exposure to methadone or buprenorphine caused impaired recognition memory and nonspatial reference learning and memory in young adult rats compared with the vehicle-treated group. Methadone-exposed offspring, but not the buprenorphine-exposed, also showed reduced long-term spatial memory. We did not observe any changes in maternal behavior or offspring growth after prenatal exposure to methadone or buprenorphine, suggesting that the impaired cognitive functioning is due to the opioid exposure rather than reduced maternal caregiving.

CONCLUSION

The present findings of long-term cognitive impairments in methadone- and buprenorphine-exposed offspring points to a negative impact of OMT on neurobiological development.

Prenatal exposure to methadone or buprenorphine and long-term outcomes: A meta-analysis

AIM

To combine meta-analyses of multiple long-term outcomes in children prenatally exposed to methadone or buprenorphine through their mothers' Opioid maintenance therapy (OMT) with a systematic review of similar outcomes in experimental animals.

METHOD

The Medline, Embase, Web of Science, CINAHL, Cochrane and Epistemonikos databases were searched through August 30, 2018. Clinical studies measuring effects on cognitive, behavioral or visual outcomes in 3 months or older children prenatally exposed to OMT and control group(s) were included for meta-analyses. Experimental animal studies with similar exposures and outcomes were included in a systematic review. The three authors independently performed abstract screenings and full-text reviews, and extracted the data. One author performed the meta-analyses.

RESULTS

The pooled results of the meta-analyses showed worse cognitive, psychomotor, behavioral, attentional and executive functioning, and affected vision in children born to mothers who were in OMT during pregnancy compared to children without prenatal drug exposure (overall effect size = 0.49, 95% confidence interval = 0.38, 0.59, p < 0.00001). Many of the experimental animal studies showed impaired outcomes after prenatal exposure to methadone or buprenorphine. The clinical results may be biased, e.g., with the OMT group having more concurrent risk factors than the unexposed comparison group. There are few studies of older children.

CONCLUSION

Children born to mothers in OMT show worse outcomes for a number of different behaviors and impaired vision compared to children born to nonusers. Experimental animal studies indicate that there might be a causal relationship between prenatal methadone or buprenorphine exposure and subsequent negative outcomes.

High Accumulation of Methadone Compared with Buprenorphine in Fetal Rat Brain after Maternal Exposure

Experimental animal studies are valuable in revealing a causal relationship between prenatal exposure to opioid maintenance treatment (OMT) and subsequent effects; however, previous animal studies of OMT during pregnancy have been criticized for their lack of clinical relevance because of their use of high drug doses and the absence of pharmacokinetic data. Hence, the aim of this study was to determine blood and brain concentrations in rat dams, fetuses, and offspring after continuous maternal exposure to methadone or buprenorphine during gestation and to examine the offspring for neonatal outcomes and withdrawal symptoms. Female rats were implanted with a 28-day osmotic minipump delivering methadone (10 mg/kg per day), buprenorphine (1 mg/kg per day) or vehicle 5 days before mating. Continuous exposure to methadone or buprenorphine induced stable blood concentrations in the dams of 0.25 ± 0.02 µM and 5.65 ± 0.16 nM, respectively. The fetal brain concentration of methadone (1.89 ± 0.35 nmol/g) was twice as high as that in the maternal brain, whereas the fetal brain concentration of buprenorphine (20.02 ± 4.97 pmol/g) was one-third the maternal brain concentration. The opioids remained in the offspring brain several days after the exposure ceased. Offspring prenatally exposed to methadone, but not buprenorphine, displayed reduced body weight and length and increased corticosterone levels. No significant changes in ultrasonic vocalizations were revealed. Our data in rat fetuses and neonates indicate that OMT with buprenorphine may be a better choice than methadone during pregnancy. SIGNIFICANCE STATEMENT: Concern has been raised about the use of opioid maintenance treatment during pregnancy because of the important role of the endogenous opioid system in brain development. Here, we show that the methadone concentration in the fetal rat brain was twice as high as that in the maternal brain, whereas the buprenorphine concentration was one-third the maternal concentration. Furthermore, buprenorphine allowed more favorable birth outcomes, suggesting that buprenorphine may be a better choice during pregnancy.

Modeling prenatal opioid exposure in animals: Current findings and future directions

The past decade has seen a drastic rise in the number of infants exposed to opioids in utero. It is unclear what lasting effect this exposure may have on these children. Animal models of prenatal opioid exposure may provide insight into potential areas of vulnerability. The present review summarizes the findings across animal models of prenatal opioid exposure, including exposure to morphine, methadone, buprenorphine, and oxycodone. Details regarding the drug, doses, and duration of treatment, as well as key findings, are summarized in tables with associated references. Finally, significant gaps in the current preclinical literature and future directions are discussed.

Will Widespread Synthetic Opioid Consumption Induce Epigenetic Consequences in Future Generations?

A growing number of evidence demonstrates that ancestral exposure to xenobiotics (pollutants, drugs of abuse, etc.) can perturb the physiology and behavior of descendants. Both maternal and paternal transmission of phenotype across generations has been proved, demonstrating that parental drug history may have significant implications for subsequent generations. In the last years, the burden of novel synthetic opioid (NSO) consumption, due to increased medical prescription of pain medications and to easier accessibility of these substances on illegal market, is raising new questions first in term of public health, but also about the consequences of the parental use of these drugs on future generations. Besides being associated to the neonatal abstinence syndrome, in utero exposure to opioids has an impact on neuronal development with long-term repercussions that are potentially transmitted to subsequent generations. In addition, recent reports suggest that opioid use even before conception influences the reactivity to opioids of the progeny and the following generations, likely through epigenetic mechanisms. This review describes the current knowledge about the transgenerational effects of opioid consumption. We summarize the preclinical and clinical findings showing the implications for the subsequent generations of parental exposure to opioids earlier in life. Limitations of the existing data on NSOs and new perspectives of the research are also discussed, as well as clinical and forensic consequences.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.