Age-dependent differences in the thermoregulatory response of the immature rat to ethanol

Impact of acute ethanol exposure on body temperatures in aged, adult and adolescent male rats

The mean population age of the United States continues to increase, and data suggest that by the year 2060 the population of people over the age of 65 will more than double, providing a potentially massive strain on health care systems. Research demonstrates individuals 65 and older continue to consume ethanol, often at high levels. However, preclinical animal models are still being developed to understand how ethanol might interact with the aged population. The current experiments investigated differential body temperature responses in aged rats compared to adult rats and adolescent rats. Aged (19 months of age), adult (70 days of age), or adolescent (30 days of age) male Sprague Dawley rats were administered 1.0 g/kg, 2.0 g/kg, or 3.0 g/kg ethanol, intraperitoneally (i.p.), in a balanced Latin square design. Prior to ethanol administration, a core body temperature via an anal probe was obtained, and then repeatedly determined every 60 min following ethanol exposure for a total of 360 min. In addition, a blood sample was obtained from a tail nick 60, 180, and 300 min following the ethanol injection to investigate the relationship of ethanol levels and body temperature in the same animals. Aged rats had significantly greater reductions in body temperature compared to either adult or adolescent rats following both the 2.0 g/kg and 3.0 g/kg ethanol injection. Additionally, adolescent rats cleared ethanol significantly faster than aged or adult animals. These experiments suggest body temperature regulation in aged rats might be more sensitive to acute ethanol compared to adult rats or adolescent rats. Future studies are needed to identify the neurobiological effects underlying the differential sensitivity in aged rats to ethanol.

Neonatal experiences with ethanol intoxication modify respiratory and thermoregulatory plasticity and affect subsequent ethanol intake in rats

Different studies have focused on the deleterious consequences of binge-like or chronic exposure to ethanol during the brain growth spurt period (third human gestational trimester) that in the rat corresponds to postnatal days (PDs) 3-10. The present study analyzed behavioral and physiological disruptions caused by relatively brief binge-like exposures (PDs 3, 5, and 7) with an ethanol dose lower (3.0 g/kg) than those frequently employed to examine teratological effects during this stage in development. At PD 9, pups were exposed to ethanol doses ranging between .0-3.0 g/kg and tested in terms of breathing patterns and thermoregulation. At PDs 11 and 12, ethanol intake was examined. The main findings were as follows: i) pre-exposure to the drug resulted in brief depressions in breathing frequencies and an exacerbated predisposition toward apneic episodes; ii) these effects were not dependent upon thermoregulatory alterations; iii) early ethanol treatment increased initial consumption of the drug which also caused a marked hypothermia that appeared to regulate a subsequent decrement in ethanol consumption; and iv) ethanol exposure retarded overall body growth and even one exposure to the drug (PD 9) was sufficient to reduce brain weights although there were no indications of microcephaly. In conjunction with studies performed during the late gestational period in the rat, the results indicate that relatively brief binge-like episodes during a critical window of brain vulnerability disrupts the respiratory network and exacerbates initial acceptance of the drug. In addition, ethanol treatments were not found to induce tolerance relative to respiratory and thermal disruptions.

The Effects of NMDA and GABAA Pharmacological Manipulations on Acute and Rapid Tolerance to Ethanol During Ontogeny

BACKGROUND

Sensitivity to several ethanol effects increases during ontogeny, perhaps in part because of a notable decline in acute tolerance. In contrast, rapid tolerance to ethanol-induced sedation emerges slowly during ontogeny. This study tested the hypothesis that ontogenetic differences in glutamate and/or gamma-aminobutyric acid systems influence tolerance expression.

METHODS

Sprague-Dawley rats at postnatal day (P)26 or P70 received (+)MK-801, muscimol, or saline before ethanol (3.5 or 4.5 g/kg) or saline on day 1 and ethanol only on day 2. Loss of and time to regain the righting reflex and blood alcohol levels at recovery were recorded. The presence of acute tolerance was indicated as a positive slope of the linear regression of blood alcohol levels at recovery versus ethanol dose. Rapid tolerance was estimated on day 2 by comparing animals given ethanol only on day 2 with those given ethanol on both days.

RESULTS

Acute tolerance on day 1 only was observed at P26; this was disrupted by (+)MK-801 but not muscimol. Evidence for acute tolerance also emerged in adults on day 2. Whereas both drugs increased ethanol sedation at both ages, they did not facilitate ontogenetic expression of rapid tolerance: rapid tolerance was not evident at P26 regardless of pretreatment when indexed in terms of recovery time.

CONCLUSIONS

These data provide further evidence for an ontogenetic dissociation in the expression of acute and rapid tolerance to ethanol-induced sedation. Pharmacological attenuation of the expression of acute tolerance was sufficient but not necessary to delay recovery of righting after ethanol. The greater propensity of young animals to develop acute tolerance, seemingly modulated in part by NMDA receptors, may contribute to their relative resistance to ethanol, although other factors, including pharmacokinetic factors, also contribute to their more rapid recovery from ethanol sedation.

Sensitivity and tolerance to autonomic effects of ethanol in adolescent and adult rats during repeated vapor inhalation sessions

BACKGROUND

It is during adolescence that most drinkers initiate ethanol intake, with some of this use being excessive. One possible contributor to the increased ethanol consumption often seen during adolescence in humans and in various animal models is age differences in ethanol sensitivity and tolerance. The present study examined the impact of age on ethanol-related alterations in the autonomic nervous system.

METHODS

Sensitivity to the initial ethanol challenge and chronic tolerance as well as acute and protracted withdrawal-like phenomena were assessed in male adolescent and adult Sprague-Dawley rats, using implanted telemetry probes with ethanol delivered via vapor inhalation.

RESULTS

Both ages showed similar ethanol-induced tachycardia and activity suppression; however, adolescents were found to be more sensitive than adults to the hypothermic effect of ethanol, data opposite other results from our laboratory and elsewhere using intragastric intubations or intraperitoneal administrations of ethanol. Although little tolerance to ethanol's tachycardic or activity suppressant effects was seen after repeated ethanol inhalation sessions, chronic tolerance to ethanol's hypothermic effect developed faster in adults than in adolescents. A withdrawal-like syndrome, characterized by bradycardia and hypoactivity, typically emerged during the dark phase of the diurnal cycle after ethanol vapor exposure sessions. These effects were observed in animals of both ages, with the bradycardic effect more pronounced in adolescents.

CONCLUSIONS

In contrast to results indicating that adolescents may be less sensitive than adults to ethanol's hypothermic effect when ethanol is administered via bolus injection/intubation, adolescents appear more sensitive and develop tolerance to ethanol's hypothermic effects more slowly than adults when ethanol is administered at a more moderate rate via vapor inhalation.

Adolescent Rats Exposed to Repeated Ethanol Treatment Show Lingering Behavioral Impairments

BACKGROUND

Repeated ethanol treatment has been reported to differentially affect water maze performance in adolescent and adult rats. The present study was undertaken to determine the age-specific reversal of ethanol-induced deficit in water maze performance.

METHODS

Adolescent and adult male rats were subjected to repeated ethanol or saline treatments. Experimental rats were injected daily with 2 g/kg ethanol (intraperitoneally) for five consecutive days (Days 1-5) and tested in the hidden platform task of the Morris water maze 30 minutes after ethanol treatment; control rats received isovolumetric saline. On the last training day, all rats were tested in the probe trial and in the cued visual task. After an ethanol-free period of 4-25 days, rats were retested in the water maze.

RESULTS

Adolescent ethanol-treated rats had significantly higher latencies and swam greater distances to find the hidden platform, compared to age-matched saline control rats. Ethanol rats also showed increased hug time, i.e., spent significantly more time near the periphery of the pool than control rats. In the probe trial, compared to adolescent saline rats, ethanol rats spent less time in the target quadrant. However, there was no difference between ethanol- and saline-treated rats in the swim speed or in the visual task performance. Experimental and control rats were retested in the water maze 4 days (Day 9), 7 days (Day 12), and 25 days (Day 30) after the last ethanol/saline treatment; no injections were given on those days. Ethanol-treated rats continued to do poorly on all retest days. Ethanol treatment in adult male rats acutely increased latency and distance to find the hidden platform, but unlike adolescent alcohol rats, their performance in the probe trial did not differ from adult saline rats. Also, swim speed and visual task performance of adult rats were significantly affected by ethanol exposure. During retesting, their performance did not differ from adult control rats.

CONCLUSIONS

Adolescent rats exposed to ethanol showed deficits in water maze performance, had increased hug time, and failed to catch up with control rats during the weeks after the ethanol treatment period was over. Adult alcohol rats showed some behavioral dysfunction (increased latency and distance to find the hidden platform) but had problems swimming, and in the probe trial they performed as well as control rats. Also, in adult rats, ethanol-induced impairments were quickly reversed after the ethanol treatment was over, a finding that suggests impaired motor coordination more than a true learning deficit. Together, these data indicate that repeated ethanol treatment in adolescent rats, but not adult rats, show long-term impairments in maze performance.

The adolescent brain and age-related behavioral manifestations

To successfully negotiate the developmental transition between youth and adulthood, adolescents must maneuver this often stressful period while acquiring skills necessary for independence. Certain behavioral features, including age-related increases in social behavior and risk-taking/novelty-seeking, are common among adolescents of diverse mammalian species and may aid in this process. Reduced positive incentive values from stimuli may lead adolescents to pursue new appetitive reinforcers through drug use and other risk-taking behaviors, with their relative insensitivity to drugs supporting comparatively greater per occasion use. Pubertal increases in gonadal hormones are a hallmark of adolescence, although there is little evidence for a simple association of these hormones with behavioral change during adolescence. Prominent developmental transformations are seen in prefrontal cortex and limbic brain regions of adolescents across a variety of species, alterations that include an apparent shift in the balance between mesocortical and mesolimbic dopamine systems. Developmental changes in these stressor-sensitive regions, which are critical for attributing incentive salience to drugs and other stimuli, likely contribute to the unique characteristics of adolescence.

Ontogeny of ethanol elimination and ethanol-induced hypothermia

Ontogeny of ethanol elimination rates and ethanol-induced hypothermia were examined as possible mechanisms contributing to the marked reduction in ethanol sensitivity early in life (Little et al., 1996; Silveri & Spear, 1998) and the notable gender difference in ethanol sleep-time seen in adult animals (Silveri & Spear, 1998). Elimination rates and brain/blood ethanol levels were determined following doses of 1.5 or 4.5 g/kg ethanol in male and female Sprague-Dawley rats at postnatal days (P)16, 26, 36, or 56. Animals were sacrificed at 40, 80, or 160 min post-injection, with ethanol elimination rates estimated from the slope of the regression of blood and brain alcohol levels across the three sampling periods. P16 animals exhibited the slowest rate of ethanol metabolism, while no gender effects were evident at any age. Observed ontogenetic increases in ethanol hypothermia were not systematically related to the ontogeny of ethanol metabolism. Factors other than ontogenetic changes in ethanol metabolism, hypothermia, or the distribution of ethanol between brain and blood must underlie the relative insensitivity to ethanol often reported in young and adolescent organisms, a fruitful area for future studies given the frequent use and misuse of alcohol by human adolescents.

Developmental differences in the acquisition of tolerance to ethanol

This study was designed to compare the development of tolerance to ethanol in adolescent and adult rats. Rats were pretreated with ethanol (4 g/kg) twice daily by intragastric gavage for 3 or 7 days, and then challenged with a single IP dose of ethanol (5 g/kg). Throughout the pretreatment period body temperature was measured before and after the morning dosage. During the IP challenge test we measured body temperature, duration of the loss of righting reflex, and blood ethanol level upon regaining the righting reflex. The adolescent rats that were pretreated for 7 days developed greater tolerance to the effects of ethanol on body temperature during the pretreatment period. No tolerance was observed in animals that received only 3 days of pretreatment. Ethanol decreased body temperature to a greater extent in adolescent animals than adults in response to the IP challenge dose, but there was no significant difference between control and pretreated animals in either age group. The time to regain the righting reflex was lower in adolescent rats than in adults. Post hoc analyses indicated that 7 days of ethanol pretreatment diminished the effects of the IP ethanol challenge on the loss of righting reflex in adolescent but not adult rats. Although adolescent rats regained the righting reflex with higher blood ethanol levels than adults, there was no significant effect of ethanol pretreatment on that measure. These findings indicate that the development of tolerance to the temperature regulatory effects of ethanol is more marked in the adolescent rat than in the adult. This developmental difference in ethanol sensitivity is consistent with other recent findings and suggests that adolescence may be a period of unique sensitivity to a number of the behavioral and physiological effects of ethanol.

Differential effects of ethanol in adolescent and adult rats

Alcohol use in children and adolescents is widespread. However, very little is known about the effects of alcohol exposure during this period of postnatal development. The goal of the present study was to compare the relative sensitivity to the sedative effects of alcohol in periadolescent and adult rats. After treatment with either 4 or 5 g/kg ethanol, both 20- and 30-day-old rats regained their righting reflex significantly earlier than 60-day old rats. In 30-day-old rats, serum ethanol concentrations (SECs) were significantly greater at the time of the recovery of the righting reflex than 60-day-old rats. Developmental differences in the effects of ethanol on locomotor activity were also observed. In 60-day-old rats, 2.5 g/kg ethanol generally decreased locomotor activity. Ethanol did not significantly alter locomotor activity in 20- and 30-day-old rats. Finally there were significant developmental differences in the pharmacokinetics of ethanol with a significant delay in the time to peak SECs in 60-day-old rats relative to 20- and 30-day-old rats. These findings indicate that peri-adolescent rats are less sensitive to the sedative effects of ethanol as they recovered their righting reflex earlier and at significantly higher SECs than adult rats.

Effects of acute ethanol administration on polyphosphoinositide turnover and levels of inositol 1,4,5-trisphosphate in mouse cerebrum and cerebellum

Although ethanol is known for its central depressant action, its effect on the polyphosphoinositide (poly-PI) signal transduction activity in brain has not been examined in detail. In this study, C57Bl/6J mice were injected intracerebrally with [3H]inositol, and poly-PI turnover in brain was assessed by determining the levels of labeled inositol monophosphates (IP1) accumulated after intraperitoneal injection of LiCl (6 meq/kg body weight) 4 hr before killing. Using this experimental protocol, acute ethanol administration (by gavage) resulted in time- and dose-dependent decreases in the levels of labeled IP1 in both cerebrum and cerebellum as compared with controls. The ethanol-induced decrease in labeled IP1 correlated well with the decrease in levels of inositol 1,4,5-triphosphate (as measured by the radioreceptor assay) and the increase in blood ethanol concentration. Despite a 4-fold higher accumulation of labeled IP1 in the cerebrum compared with the cerebellum, there were no major differences in the steady-state levels of inositol 1,4,5-triphosphate (based on tissue weight) in either brain region. Intraperitoneal injection of atropine (50 mg/kg) (a muscarinic cholinergic receptor antagonist) to the lithium-treated mice resulted in a 34% decrease in labeled IP1 as compared with controls. This result suggests that a substantial proportion of the signals transduced were due to activation of the muscarinic cholinergic receptor. Administration of ethanol (5 g/kg) to the atropine-treated mice resulted in a further decrease in labeled IP1 and longer sleep time as compared with those given ethanol alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic administration of alcohol in the developing rat: expression of functional tolerance and alcohol olfactory aversions

Three experiments were designed to evaluate the acquisition and expression of tolerance to ethanol in preweanling rats. Administrations were begun on Postnatal Day 13 (PD13) and continued, on alternating days, through PD23. Following either three or five administrations of a 2.5 g/kg dose of ethanol, animals expressed tolerance to the motor-impairing consequences of acute ethanol, as measured through attenuated latencies to perform a righting reflex. However, no tolerance was evident when it was assessed through the hypothermic response to ethanol delivery (Experiment 1). The results of subsequent experiments suggested that this tolerance was not due to lower levels of ethanol in the blood and brain of the chronically exposed subjects (Experiment 2) nor to the development of behavioral tolerance, such as learning to perform the motor task in the presence of intoxication (Experiment 3). Additional tests determined that tolerance level did not alter the infant rats' ability to learn about the orosensory cues associated with ethanol delivery. Specifically, tests confirmed previous studies that an aversion to ethanol odor was formed as a result of the chronic administration procedure, presumably due to the animals associating the orosensory properties of eliminated ethanol during intoxication with the aversive consequences of this state. These results provide evidence that tolerance can develop in the preweanling rat, and that at least one measure (righting reflex) can be used to assess its development early in life.

Delayed thermoregulatory changes in the immature rat following a single injection of ethanol

Adult rats exhibit rebound hyperthermia within 24 hr following a single injection of ethanol (EtOH). Tests were conducted to determine whether similar changes in thermoregulatory ability occur in the immature rat. Animals were administered saline or EtOH (4 g/kg BW; intraperitoneally) at 2 to 3, 8 to 9, or 14 to 15 days of age. Littermates were handled or left undisturbed with the dams to serve as controls. All rats were tested at 24 or 48 hr post-treatment to measure steady-state colonic temperature (Tco), tail skin temperature and metabolic rate (MR) at both thermoneutral and cold ambient temperatures (Tas). The youngest group exhibited no delayed change in body temperature or MR at 24 or 48 hr post-treatment with EtOH. Likewise, thermoregulatory ability of rats pretreated with EtOH at 8 or 9 or 14 to 15 days of age was not significantly different from controls when tested 24 hr post-treatment at thermoneutral Ta. In contrast, Tco of EtOH-treated rats in the two older age groups was 1 degree C above control level when tested 24 hr post-treatment at cold Ta. This Tco response can be explained by differences in heat transfer to the tail and MR. No altered response to cold Ta was found at 48 hr postinjection, indicating recovery from the EtOH effect. A single injection with EtOH at 2 to 15 days of age results in a change in Tco, which is dependent on postinjection time, age, and Ta.