Development Of Chronic Tolerance Research Articles

Nicotine chronic tolerance development and withdrawal in the planaria (Schmidtea mediterranea)

Chronic nicotine exposure reduces sensitivity to the effects of nicotine, which then results in behavioural changes and tolerance development. In the planaria, a valuable first-stage preclinical model for addictive behaviour, acute nicotine administration has been shown to steadily alter the motility of the animals, a result that has been interpreted as evidence of tolerance and withdrawal effects; however, chronic exposure - typically regarded as a condition for the development of tolerance - and the role of the contextual cues have not been systematically assessed. The present study assessed the acute and chronic effects of nicotine on the motility of planarians (Schmidtea mediterranea). The animals in the experimental groups received long chronic exposure to nicotine (ten daily 30 min exposures); a control group was exposed to water in the same context but in the absence of the drug. The motility of the animals was closely monitored on every exposure. Following this phase, all the animals were subject to three different tests: in the presence of the exposure context (without the drug, Test 1); in the presence of nicotine in the exposure context (Test 2); and in the presence of the drug in a novel context (Test 3). Exposure to nicotine consistently reduced motility; the motility in the presence of nicotine increased with repeated exposures to the drug, an instance of tolerance development. Tolerance development was dependent on nicotinic receptor activation, because it was blocked by the co-administration of mecamylamine. However, this tolerance was found to be independent of the contextual cues where the effects of the drug had been experienced. The results are discussed by reference to the existent theories of tolerance development to drugs.

Ligand-specific recycling profiles determine distinct potential for chronic analgesic tolerance of delta-opioid receptor (DOPr) agonists.

δ‐opioid receptor (DOPr) agonists have analgesic efficacy in chronic pain models but development of tolerance limits their use for long‐term pain management. Although agonist potential for inducing acute analgesic tolerance has been associated with distinct patterns of DOPr internalization, the association between trafficking and chronic tolerance remains ill‐defined. In a rat model of streptozotocin (STZ)‐induced diabetic neuropathy, deltorphin II and TIPP produced sustained analgesia following daily (intrathecal) i.t. injections over six days, whereas similar treatment with SNC‐80 or SB235863 led to progressive tolerance and loss of the analgesic response. Trafficking assays in murine neuron cultures showed no association between the magnitude of ligand‐induced sequestration and development of chronic tolerance. Instead, ligands that supported DOPr recycling were also the ones producing sustained analgesia over 6‐day treatment. Moreover, endosomal endothelin‐converting enzyme 2 (ECE2) blocker 663444 prevented DOPr recycling by deltorphin II and TIPP and precipitated tolerance by these ligands. In conclusion, agonists, which support DOPr recycling, avoid development of analgesic tolerance over repeated administration.

Downregulation of miR-219 enhances brain-derived neurotrophic factor production in mouse dorsal root ganglia to mediate morphine analgesic tolerance by upregulating CaMKIIγ.

BackgroundIncreasing evidence suggests that microRNAs are functionally involved in the initiation and maintenance of pain hypersensitivity, including chronic morphine analgesic tolerance, through the posttranscriptional regulation of pain-related genes. We have previously demonstrated that miR-219 regulates inflammatory pain in the spinal cord by targeting calcium/calmodulin-dependent protein kinase II gamma (CaMKIIγ). However, whether miR-219 regulates CaMKIIγ expression in the dorsal root ganglia to mediate morphine tolerance remains unclear.ResultsMiR-219 expression was downregulated and CaMKIIγ expression was upregulated in mouse dorsal root ganglia following chronic morphine treatment. The changes in miR-219 and CaMKIIγ expression closely correlated with the development of morphine tolerance, which was measured using the reduction of percentage of maximum potential efficiency to thermal stimuli. Morphine tolerance was markedly delayed by upregulating miR-219 expression using miR-219 mimics or downregulating CaMKIIγ expression using CaMKIIγ small interfering RNA. The protein and mRNA expression of brain-derived neurotrophic factor were also induced in dorsal root ganglia by prolonged morphine exposure in a time-dependent manner, which were transcriptionally regulated by miR-219 and CaMKIIγ. Scavenging brain-derived neurotrophic factor via tyrosine receptor kinase B-Fc partially attenuated morphine tolerance. Moreover, functional inhibition of miR-219 via miR-219-sponge in naive mice elicited thermal hyperalgesia and spinal neuronal sensitization, which were both suppressed by CaMKIIγ small interfering RNA or tyrosine receptor kinase B-Fc.ConclusionsThese results demonstrate that miR-219 contributes to the development of chronic tolerance to morphine analgesia in mouse dorsal root ganglia by targeting CaMKIIγ and enhancing CaMKIIγ-dependent brain-derived neurotrophic factor expression.

Adolescent rats are resistant to the development of ethanol-induced chronic tolerance and ethanol-induced conditioned aversion

The analysis of chronic tolerance to ethanol in adult and adolescent rats has yielded mixed results. Tolerance to some effects of ethanol has been reported in adolescents, yet other studies found adults to exhibit greater tolerance than adolescents or comparable expression of the phenomena at both ages. Another unanswered question is how chronic ethanol exposure affects subsequent ethanol-mediated motivational learning at these ages. The present study examined the development of chronic tolerance to ethanol's hypothermic and motor stimulating effects, and subsequent acquisition of ethanol-mediated odor conditioning, in adolescent and adult male Wistar rats given every-other-day intragastric administrations of ethanol. Adolescent and adult rats exhibited lack of tolerance to the hypothermic effects of ethanol during an induction phase; whereas adults, but not adolescents, exhibited a trend towards a reduction in hypothermia at a challenge phase (Experiment 1). Adolescents, unlike adults, exhibited ethanol-induced motor activation after the first ethanol administration. Adults, but not adolescents, exhibited conditioned odor aversion by ethanol. Subsequent experiments conducted only in adolescents (Experiment 2, Experiment 3 and Experiment 4) manipulated the context, length and predictability of ethanol administration. These manipulations did not promote the expression of ethanol-induced tolerance. This study indicated that, when moderate ethanol doses are given every-other day for a relatively short period, adolescents are less likely than adults to develop chronic tolerance to ethanol-induced hypothermia. This resistance to tolerance development could limit long-term maintenance of ethanol intake. Adolescents, however, exhibited greater sensitivity than adults to the acute motor stimulating effects of ethanol and a blunted response to the aversive effects of ethanol. This pattern of response may put adolescents at risk for early initiation of ethanol intake.

Repeated nitrous oxide exposure in rats causes a thermoregulatory sign-reversal with concurrent activation of opposing thermoregulatory effectors

Initial administration of 60% nitrous oxide (N2O) to rats at an ambient temperature of 21°C decreases core temperature (Tc), primarily via increased heat loss (HL). Over repeated N2O administrations, rats first develop tolerance to this hypothermia and subsequently exhibit hyperthermia (a sign-reversal) due primarily to progressive increases in heat production (HP). When rats initially receive 60% N2O in a thermal gradient, they become hypothermic while selecting cooler ambient temperatures that facilitate HL. This study investigated whether rats repeatedly administered 60% N2O in a thermal gradient would use the gradient to behaviorally facilitate, or oppose, the development of chronic tolerance and a hyperthermic sign-reversal. Male Long-Evans rats (N = 16) received twelve 3-h administrations of 60% N2O in a gas-tight, live-in thermal gradient. Hypothermia (Sessions 1–3), complete chronic tolerance (Sessions 4–6), and a subsequent transient hyperthermic sign-reversal (Sessions 7–12) sequentially developed. Despite the progressive recovery and eventual hyperthermic sign-reversal of Tc, rats consistently selected cooler ambient temperatures during all N2O administrations. A final 60% N2O administration in a total calorimeter indicated that the hyperthermic sign-reversal resulted primarily from increased HP. Thus, rats did not facilitate chronic tolerance development by moving to warmer locations in the gradient, and instead selected cooler ambient temperatures while simultaneously increasing autonomic HP. The inefficient concurrent activation of opposing effectors and the development of a sign-reversal are incompatible with homeostatic models of drug-adaptation and may be better interpreted using a model of drug-induced allostasis.

Chronic intermittent ethanol exposure during adolescence: Effects on social behavior and ethanol sensitivity in adulthood

This study assessed long-lasting consequences of repeated ethanol exposure during two different periods of adolescence on 1) baseline levels of social investigation, play fighting, and social preference and 2) sensitivity to the social consequences of acute ethanol challenge. Adult male and female Sprague-Dawley rats were tested 25 days after repeated exposure to ethanol (3.5 g/kg intragastrically [i.g.], every other day for a total of 11 exposures) in a modified social interaction test. Early-mid adolescent intermittent exposure (e-AIE) occurred between postnatal days (P) 25 and 45, whereas late adolescent intermittent exposure (l-AIE) was conducted between P45 and P65. Significant decreases in social investigation and social preference were evident in adult male rats, but not their female counterparts following e-AIE, whereas neither males nor females demonstrated these alterations following l-AIE. In contrast, both e-AIE and l-AIE produced alterations in sensitivity to acute ethanol challenge in males tested 25 days after adolescent exposure. Ethanol-induced facilitation of social investigation and play fighting, reminiscent of that normally seen during adolescence, was evident in adult males after e-AIE, whereas control males showed an age-typical inhibition of social behavior. Males after l-AIE were found to be insensitive to the socially suppressing effects of acute ethanol challenge, suggesting the development of chronic tolerance in these animals. In contrast, females showed little evidence for alterations in sensitivity to acute ethanol challenge following either early or late AIE. The results of the present study demonstrate a particular vulnerability of young adolescent males to long-lasting detrimental effects of repeated ethanol. Retention of adolescent-typical sensitivity to the socially facilitating effects of ethanol could potentially make ethanol especially appealing to these males, therefore promoting relatively high levels of ethanol intake later in life.

The Mammalian Circadian Clock Exhibits Acute Tolerance to Ethanol

Tolerance to ethanol is observed over a variety of time courses, from minutes to days. Acute tolerance, which develops over 5 to 60 minutes, has been observed for both behavioral and neurophysiological variables and may involve changes in signaling through NMDA, GABA, or other receptors. Previous work has shown that both acute and chronic ethanol treatments modulate photic and nonphotic phase resetting of the mammalian circadian clock located in the suprachiasmatic nucleus (SCN). Although not specifically tested, the data thus far do not point to the development of chronic tolerance to the modulatory effects of ethanol. Here we investigated whether acute tolerance the ethanol occurs with respect to in vitro phase modulation of the SCN clock. Mouse brain slices containing the SCN were pretreated with ethanol for varying lengths of time, followed by treatment concurrent with either glutamate or the serotonin agonist, 8-hydroxy-DPAT (DPAT). The phase of the SCN circadian clock was assessed the following day through extracellular recordings of SCN neuronal activity. SCN neuronal activity normally peaks during mid-day, and this rhythm can be shifted by treatment with either glutamate or DPAT. While concurrent treatment of SCN-containing brain slices with ethanol and glutamate blocks glutamate-induced phase delays of the SCN clock, pretreating the slices with ethanol for > or =15 minutes prevents this inhibition. Likewise, while concurrent treatment with ethanol and DPAT enhances DPAT-induced phase advances of the SCN clock, pretreating the slices with ethanol for > or =30 minutes prevents this enhancement. Both the inhibiting and enhancing effects of ethanol on in vitro SCN clock phase resetting show acute tolerance. Additional experiments are needed to determine whether more slowly developing forms of tolerance also occur with respect to the SCN circadian clock.

In vivo pharmacology of MDMA and its enantiomers in rhesus monkeys.

The chiral nature of the MDMA molecule gives rise to two enantiomers, each of which is biologically active. This review attempts to cover the author's research into the in vivo effects of MDMA and its enantiomers, as well as other relevant publications which pertain to this topic. No particular differences between the capacities of racemic MDMA and its enantiomers to maintain behavior were noted, but antagonism of the 5-HT2A receptor produces a parallel rightward shift in the dose-effect function for the S(+)-enantiomer, but insurmountably reduces the reinforcing effects of R(-)-MDMA. Long-term self-administration of MDMA may lead to the development of chronic tolerance to the reinforcing effects of MDMA, but S(+)-MDMA is somewhat less susceptible to this effect than the racemate or the R(-)-enantiomer. Using PET neuroimaging, negligible occupancy at the dopamine transporter (DAT) was observed following administration of R(-)-MDMA, but reasonable DAT interaction was quantified following injection of S(+)-MDMA. The non-human primate studies reviewed herein caution that any results obtained in vivo with the MDMA enantiomers may not be particularly informative with regards to the racemate and vice versa.

Systems-level adaptations explain chronic tolerance development to nitrous oxide hypothermia in young and mature rats

Nitrous oxide (N(2)O) can initially lower core temperature (T (core)), but hypothermic tolerance develops with chronic administration. Therefore, one or both of T (core)'s controlling determinants, heat production (HP) and heat loss (HL), must adapt across repeated N(2)O administrations. Simultaneous measurements of HP, HL, and T (core) during chronic N(2)O administrations will elucidate this adaptive process and constitute a rigorous model for studying the systems-level dynamics of tolerance in both mature and young animals. This approach is justified by the need to better understand the increased vulnerability to addiction associated with adolescent drug use. The objective of the study was to measure HL and HP across repeated steady-state administrations of 60% N(2)O in young and mature rats. Synchronous measurements of HP (indirect calorimetry), HL (direct calorimetry), and T (core) (telemetry) were obtained during 60% N(2)O administrations in adolescent (28-45 days, n = 11) and mature rats (>90 days, n = 8). Rats received five 90-min drug exposures (every other day). Compared to mature rats, adolescents initially exhibited greater hypothermia, but acquired tolerance more rapidly and actually developed hyperthermia during the fifth administration. In both groups, N(2)O consistently increased HL, but progressive increases of intrasessional HP over repeated administrations prevented hypothermia and subsequently promoted hyperthermia in adolescent rats. Adolescent rats hyper-adapt to N(2)O hypothermia. Increases of intrasessional HP across N(2)O administrations explained both tolerance to N(2)O hypothermia and the unexpected hyperthermia observed in adolescents. These findings raise the possibility that the increased vulnerability to addiction associated with adolescent drug use involves a hyper-adaptive tolerance mechanism.

Chronic tolerance to the social consequences of ethanol in adolescent and adult Sprague-Dawley rats

Adolescence is a time when experimentation with ethanol becomes normative, with high levels of use becoming apparent in some adolescents. Little is known, however, as to whether ethanol adaptations emerging in adolescents with repeated ethanol use are similar to those emerging in adults. The presents study used a rodent model to investigate the development of chronic tolerance to ethanol-induced alterations in social behavior. The study focused both on ethanol-induced social facilitations, typically evident in adolescents but not adult animals at low doses of ethanol, as well as the inhibition of social behavior occurring at higher doses in both adolescent and adult rats. Adolescent and adult male and female Sprague-Dawley rats were injected intraperitoneally with either isotonic saline or 1 g/kg ethanol for 7 consecutive days: postnatal day (P) 27–33 for adolescents and P62–68 for adults. Acute effects of ethanol (0, 0.25, 0.5, 0.75, and 1 g/kg) on social behavior, social motivation (measured in terms of social preference), and locomotor activity were assessed 48 h after the last chronic exposure using a modified social interaction test in a familiar environment. Adolescents chronically exposed to ethanol developed tolerance to ethanol-induced social facilitation. Animals of both ages likewise developed chronic tolerance to ethanol-induced social inhibition. Metabolic tolerance emerged in adults, as indexed by a decrease in blood ethanol concentrations after chronic ethanol at this age, whereas only functional tolerance was evident in adolescents. Unexpectedly, chronic ethanol diminished baseline levels of social preference in adolescents, but made them more responsive to ethanol-induced enhancement of social preference. Chronic ethanol exposure in adulthood, however, only induced tolerance to the suppressing effects of higher ethanol doses on social preference. Thus, whereas adolescents and adult both develop adaptations following repeated exposure to ethanol, adolescents are more vulnerable to the disruptive effects of chronic ethanol exposure on social preference than their more mature counterparts.

Calcium-Acting Drugs Modulate Expression and Development of Chronic Tolerance to Nicotine-Induced Antinociception in Mice

Initial studies in our laboratory suggested that tolerance to nicotine is thought to involve neuronal adaptation not only at the level of the drug-receptor interaction but at postreceptor events such as calcium-dependent second messengers. The present study was undertaken to investigate the hypothesis that L-type calcium channels and calcium-dependent calmodulin protein kinase II are involved in the development and expression of nicotine tolerance. To that end, the effects of modulation of L-type calcium channels (through the use of inhibitors or activators) as well as calcium-dependent calmodulin protein kinase II inactivation were studied in a mouse model of tolerance where mice were infused with nicotine in minipumps (24 mg/kg/day) for 14 days. In addition, the activity of calcium-dependent calmodulin protein kinase II in the lumbar spinal cord region obtained from nicotine-tolerant mice was measured. Our data showed that chronic administration of L-type calcium channel antagonists nimodipine (1 and 5 mg/kg) and verapamil (10 mg/kg) prevented the development of tolerance to nicotine-induced antinociception. In contrast, chronic exposure of BAYK8644 [(+/-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyridine carboxylic acid methyl ester], a calcium channel activator, enhanced nicotine's tolerance. Moreover, a significant increase in both dependent and independent calcium-dependent calmodulin protein kinase II activity was seen in the spinal cord in nicotine-tolerant mice. Finally, spinal administration of 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-tyrosyl]-4-phenylpiperazine (KN-62), a calcium-dependent calmodulin protein kinase II antagonist, reduced the expression of tolerance to nicotine-induced antinociception in mice. In conclusion, our data indicate that calcium-dependent mechanisms such as L-type calcium channels and calcium-dependent calmodulin protein kinase II activation are involved in the expression and development of nicotine tolerance.

Ketamine enhances the efficacy to and delays the development of tolerance to electroacupuncture-induced antinociception in rats

Our previous studies have shown that 100 Hz electroacupuncture (EA) produced antinociception through the release of endogenous opioids (mainly dynorphin) and the activated κ-opioid receptors in normal rats. Acupuncture is an effective treatment in relieving pain, but it develops tolerance after repeated administration. It has been reported that N-methyl- d-aspartate (NMDA) receptor antagonists could increase the antinociceptive effects induced by morphine and delay the development of tolerance to morphine but nothing has yet been described to reduce EA tolerance. Here we test whether ketamine, a non-competitive NMDA receptor antagonist, would enhance 100 Hz EA antinociception as well as prevent or delay the development of chronic tolerance to 100 Hz EA in normal rats. The results are as follows: (1) ketamine injected intraperitoneally (i.p.) 15 min prior to EA enhanced the antinociceptive effects of 100 Hz EA at a dose of 5.0 mg/kg, but not 0.2 or 1.0 mg/kg. However, ketamine at either dose did not affect the basal nociceptive threshold (represented by tail-flick latency). (2) Ketamine at a dose of 5.0 mg/kg delayed the development of chronic tolerance to 100 Hz EA antinociception. We conclude that ketamine can enhance antinociception of 100 Hz EA and delay the tolerance to 100 Hz EA in rats. These results suggest that the development of 100 Hz EA tolerance to antinociception was mediated, at least in part, through peripheral NMDA receptors, which may be useful in improving the therapeutic effects of EA in the treatment of pain when EA tolerance occurs.

Assessment of heat production, heat loss, and core temperature during nitrous oxide exposure: a new paradigm for studying drug effects and opponent responses

Studies using core temperature (T(c)) have contributed greatly to theoretical explanations of drug tolerance and its relationship to key features of addiction, including dependence, withdrawal, and relapse. Many theoretical accounts of tolerance propose that a given drug-induced psychobiological disturbance elicits opponent responses that contribute to tolerance development. This proposal and its theoretical extensions (e.g., conditioning as a mechanism of chronic tolerance) have been inferred from dependent variables, such as T(c), which represent the summation of multiple underlying determinants. Direct measurements of determinants could increase the understanding of opponent processes in tolerance, dependence, and withdrawal. The proximal determinants of T(c) are metabolic heat production (HP) and heat loss (HL). We developed a novel system for simultaneously quantifying HP (indirect calorimetry), HL (direct gradient layer calorimetry), and T(c) (telemetry) during steady-state administrations of nitrous oxide (N(2)O), an inhalant with abuse potential that has been previously used to study acute and chronic tolerance development to its hypothermia-inducing property. Rats were administered 60% N(2)O (n = 18) or placebo gas (n = 16) for 5 h after a 2-h placebo baseline exposure. On average, N(2)O rapidly but transiently lowered HP and increased HL, each by approximately 16% (P < 0.001). On average, rats reestablished and maintained thermal equilibrium (HP = HL) at a hypothermic T(c) (-1.6 degrees C). However, some rats entered positive heat balance (HP > HL) after becoming hypothermic such that acute tolerance developed, i.e., T(c) rose despite continued drug administration. This work is the first to directly quantify the thermal determinants of T(c) during administration of a drug of abuse and establishes a new paradigm for studying opponent processes involved in acute and chronic hypothermic tolerance development.

Genetic analysis of rapid tolerance to ethanol's incoordinating effects in mice: inbred strains and artificial selection.

Ethanol tolerance, a decrease in drug responsiveness with repeated administrations, is an important diagnostic criterion for alcoholism. Rapid tolerance develops within 8-24 hours of an initial ethanol exposure and shares many similarities with chronic tolerance. The genetic contribution to rapid tolerance to ethanol-induced ataxia was estimated using a panel of inbred strains of mice. Strains differed significantly in the degree of rapid tolerance development, which had a broad-sense heritability estimate of 0.11. Artificial selection was carried out to develop lines of mice that would show High (HRT) and Low (LRT) levels of Rapid Tolerance. Starting with HS/Npt mice, derived from a systematic cross of eight inbred strains, a significant response to selection was seen in replicate 1 by the third selection generation. No difference was found in replicate 2. Heritability estimates after the fourth generation were 0.25 for HRT-1 mice and 0.06 for LRT-1 mice. HRT-1 and LRT-1 mice also differed significantly in chronic tolerance development to four doses of ethanol. These studies provide evidence for a genetic contribution to rapid tolerance and support a genetic link between rapid and chronic tolerance to ethanol's ataxic effects.

Nocistatin potentiates electroacupuncture antinociceptive effects and reverses chronic tolerance to electroacupuncture in mice

Nocistatin (NST) and nociception/orphanin FQ (OFQ) are peptides derived from the same precursor that play opposing roles in pain modulation. OFQ antagonizes morphine analgesia and electroacupuncture (EA)-induced antinociceptive effect. The present study investigates whether NST potentiates EA-induced antinociceptive effect and reverses chronic tolerance to EA in mice. Injection of NST (0.5, 5.0 and 50.0 ng) intracerebroventricularly had no effect on basal thermal latency, but produced a dose-dependent potentiation of EA-induced antinociceptive effect in mice with the maximum response at 5.0 ng. NST (5.0 ng) partly reversed chronic tolerance to EA. These results suggest that NST in the brain might play roles in EA-induced antinociceptive effect and the development of chronic tolerance to EA in mice.

Sensitivity and tolerance to ethanol-induced incoordination and hypothermia in HAFT and LAFT mice

Acute functional tolerance (AFT) manifests as rapid adaptation during a single ethanol exposure, leading to a decrease in the behavioral response to ethanol. In order to investigate the genetic and environmental components of the development of AFT, mice were selectively bred in replicate from HS/Ibg mice. High (HAFT) and low (LAFT) acute functional tolerance selected lines were bred to differ in the rate of development and magnitude of AFT to ethanol's intoxicating effects using a static dowel-balancing task. In the present set of experiments, HAFT and LAFT mice were tested for development of AFT on a fixed-speed rotarod using a protocol similar to that for which they were selected. HAFT mice developed greater AFT to ethanol than did LAFT mice. In a separate experiment, other mice from these lines were tested for initial sensitivity and the development of chronic tolerance to ethanol-induced hypothermia, and ethanol-induced incoordination in the grid test. Previous research has detected possible common genetic control of these phenotypes. No differences between lines were found in initial sensitivity to ethanol or in the development or magnitude of chronic tolerance in either test. These experiments show that genetic factors influencing the development of acute tolerance to ethanol-induced intoxication are at least partially distinct from those influencing initial sensitivity and the development of chronic tolerance to ethanol-induced hypothermia and incoordination. Furthermore, these experiments show that AFT measured by the stationary dowel generalizes to AFT measured by the fixed-speed rotarod.

Effect of epipregnanolone and pregnenolone sulfate on chronic tolerance to ethanol

The aim of the present study was to investigate the influence of neurosteroids on the development of tolerance to ethanol. Male Swiss mice were injected daily with the positive allosteric modulator of the gamma amino butyric acid-A (GABA A) receptor epipregnanolone (5β-pregnan-3β-ol-20-one; 0.15 mg/kg i.p.) or pregnenolone sulfate (5-pregnen-3β-ol-20-one sulfate sodium; 0.08 mg/kg i.p.) — considered a negative allosteric modulator of this receptor and/or positive allosteric modulator of the N-methyl- d-aspartate (NMDA) receptor — 30 min before ethanol (2.5 g/kg i.p.). They were tested on the rota-rod apparatus, under continuous acceleration (1rpm/s), at 30, 60 and 90 min after ethanol injections for 5 days. The results showed that tolerance to the motor incoordinating effect of ethanol occurred on the fifth day of treatment when this effect was blocked by pretreatment with epipregnanolone. On the other hand, ethanol tolerance was enhanced by pretreatment with pregnenolone sulfate from the second to the fifth days of treatment. Taken together, our results suggest that neurosteroids can either stimulate or block the development of chronic tolerance to ethanol. Moreover, since neurosteroids can interact with GABA A or NMDA receptor systems, our results suggest the involvement of these systems in the actions of neurosteroids upon ethanol tolerance.

Background genotype modulates the effects of γ‐PKC on the development of rapid tolerance to ethanol‐induced hypothermia

The role of γ-PKC in initial sensitivity and in the development of rapid tolerance to the hypothermic effects of ethanol were investigated in γ-PKC null mutant mice. Effects of the single gene mutation were evaluated on three different genetic backgrounds. Null mutants from a C57BL/6J X 129/SvJ mixed genetic background failed to develop rapid tolerance after 4 days of i.p. ethanol injections. However, when the null mutation was introgressed onto a C57BL/6J background for six generations to create a congenic line, the expression of rapid tolerance unexpectedly reoccurred in the null mutant mice. Subsequent outcrossing of the γ-PKC null mutation to a C57BL/6J X 129/SvEvTac mixed background did not restore the no tolerance phenotype. These observations, taken together with similar results reported previously concerning the development of chronic tolerance to ethanol in these same genotypes, ¹ indicate that the gene coding for gamma-PKC has pleiotropic effects in the expression of both rapid and chronic tolerance to ethanol-induced hypothermia. However, the impact of γ-PKC is modulated by the background genotype. These results stress the necessity of understanding interactions with genetic background when interpreting the effects of single gene mutations on complex behavioral traits.

Nitrous Oxide-Induced Hypothermia in the Rat: Acute and Chronic Tolerance

Although inhalation of nitrous oxide (N2O) causes hypothermia in rats, there is a paucity of information as to whether tolerance develops to this effect. The purpose of this study was to determine whether tolerance to N2O hypothermia develops within a single administration as well as over repeated administrations. Temperature was measured telemetrically by implanting intraperitoneal thermal sensors/transmitters in male Long–Evans rats. Experimental rats received an initial 2-h exposure to 60% N2O and became hypothermic relative to controls breathing placebo gas. Only a few rats demonstrated evidence of acute tolerance over the 120 min. Over the next 10 days, the experimental rats received five additional 30-min exposures to 60% N2O and five 30-min exposures to placebo while the control rats received only placebo gas exposures. Chronic tolerance developed to N2O hypothermia over these repeated administrations. A test for Pavlovian drug conditioning found no evidence that conditioned temperature effects contributed to chronic tolerance development. In a second experiment, naive rats were given a 380-min exposure to 60% N2O and a 380-min exposure to placebo gas in a counterbalanced order. Acute tolerance did develop to N2O hypothermia, with the recovery of temperature beginning after a mean of 141 min of gas administration. Hence, both acute and chronic tolerance develop to N2O’s hypothermic effects in rats.

Vasoactive intestinal polypeptide induces analgesia and impairs the antinociceptive effect of morphine in mice

Vasoactive intestinal polypeptide (VIP) has numerous regulatory roles in peripheral, endocrine organs and in the central nervous system. The present study related to the effects of centrally (intracerebroventricularly) administered VIP on pain sensitivity and on opiate tolerance and dependence in intact male CFLP mice. VIP was analgesic when administered alone centrally. Naloxone treatment abolished this analgesic effect. VIP decreased the analgesic effect of a single subcutaneous morphine injection and the development of chronic tolerance to morphine. Morphine withdrawal signs were not significantly affected after VIP pretreatment. These findings indicate that VIP may play a role in pain sensitivity and that an opiate component may participate in this effect.