Predicting addictive vulnerability: individual differences in initial responding to a drug's pharmacological effects

SCH 23390 inhibits the acquisition of nitrous oxide-induced conditioned place preference and the changes in ERK phosphorylation expression in nucleus accumbens of mice

Nitrous oxide (N₂O) has a long history of abuse, but its abuse mechanism has not been clear yet. This research aimed at the possibility of mesolimbic dopaminergic system (MLDS) involved in the rewarding effect of N₂O. In this work, the rewarding behavior of N₂O in mice was evaluated using a typical gas-administered conditioned place preference (CPP) procedure. SCH 23390, a Dopamine D1 receptor (D1R) antagonist, and Haloperidol, a Dopamine D2 receptor (D2R) antagonist were administered during CPP to evaluate the role of dopamine receptors in the N₂O-induced CPP. The accompanying changes in phosphorylation of extracellular signal-regulated kinase (ERK) in MLDS related brain regions, including the ventral tegmental area (VTA), caudate putamen (CPu), prefrontal cortex (PFC), and nucleus accumbens (NAc) were measured to assess the neural plasticity changes in the CPP mice by Western blot analysis. Results revealed that 60% N₂O induced CPP in the gas-administered mice and promoted the ERK phosphorylation (p-ERK) in the NAc and CPu during the test session of the CPP test. Pretreatment of SCH 23390 (0.5mg/kg) inhibited the acquisition of N₂O-induced CPP and the enhanced p-ERK in NAc.It suggested that Dopamine D1 receptor may play an important role in the acquisition of N₂O-induced CPP and the accompanied ERK activation in the NAc, which provide insight into the molecular mechanism in the rewarding properties of nitrous oxide.

Individual differences in biological regulation: Predicting vulnerability to drug addiction, obesity, and other dysregulatory disorders

Physiological regulation is so fundamental to survival that natural selection has greatly favored the evolution of robust regulatory systems that use both reactive and preemptive responses to mitigate the disruptive impact of biological and environmental challenges on physiological function. In good health, robust regulatory systems provide little insight into the typically hidden complex array of sensor-effector interactions that accomplish successful regulation. Numerous health disorders have been traced to defective regulatory mechanisms, and generations of scientists have worked to discover ways to correct these defects and restore normal physiological function. Despite progress, numerous chronic health disorders remain resistant to treatment, and indeed for some disorders the incidence is increasing. We propose that an individual's susceptibility to acquire certain persistent dysregulatory disorders can be traced to interindividual variation in how that individual's regulatory system responds to challenges. Preexisting reliable individual differences among regulatory systems are typically unrecognized until appropriate regulatory challenges (e.g., exposure to a drug of abuse) lead to dysregulation (e.g., drug addiction). Specific characteristics of an individual's regulatory responsiveness may include etiological factors that participate in the acquisition, escalation and maintenance of health disorders characterized by dysregulation. By appropriately challenging a healthy individual's regulatory systems to identify its underlying characteristics, it is possible to ascertain whether an individual has an elevated risk for acquiring a dysregulated health condition and thereby enable strategies designed to prevent, rather than treat, the condition. This model is applied to drug addiction, and in addition we relate this approach to other dysregulated conditions such as obesity. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Validation of an equation for energy expenditure that does not require the respiratory quotient

Background

Energy expenditure (EE) calculated from respirometric indirect calorimetry is most accurate when based on oxygen consumption (VO₂), carbon dioxide production (VCO₂) and estimated protein metabolism (PM). EE has a substantial dependence of ~7% on the respiratory quotient (RQ, VCO₂/VO₂) and a lesser dependence on PM, yet many studies have instead estimated EE from VO₂ only while PM has often been ignored, thus reducing accuracy. In 1949 Weir proposed a method to accurately calculate EE without using RQ, which also adjusts for estimated PM based on dietary composition. This RQ^- method utilizes the calorimeter airflow rate (FR), the change in fractional O₂ concentration (ΔFO₂) and the dietary protein fraction. The RQ^- method has not previously been empirically validated against the standard RQ⁺ method using both VO₂ and RQ. Our aim was to do that.

Methods

VO₂ and VCO₂ were measured repeatedly in 8 mice fed a high protein diet (HPD) during exposure to different temperatures (n = 168 measurements of 24h gas exchange). The HPD-adjusted RQ⁺ equation was: EE [kcal/time] = VO₂ [L/time]×(3.853+1.081RQ) while the corresponding RQ^- equation was: EE = 4.934×FR×ΔFO₂. Agreement was analyzed using the ratios of the RQ^- to RQ⁺ methods along with regression and Bland-Altman agreement analyses. We also evaluated the standard equation using the dietary food quotient (FQ) of 0.91 as a proxy for RQ (FQ⁺ method).

Results

Ratio analysis revealed that the mean error of the RQ^- method was only 0.11 ± 0.042% while the maximum error was only 0.21%. Error using the FQ⁺ method was 4 -and 10-fold greater, respectively. Bland-Altman analysis demonstrated that the RQ^- method very slightly overestimates EE as RQ decreases. Theoretically, this error can be eliminated completely by imposing an incurrent fractional oxygen concentration at a value only slightly greater than the atmospheric level.

Conclusions

The Weir ‘RQ-free’ method for calculating EE is a highly valid alternative to the ‘gold standard’ method that requires RQ. The RQ^- approach permits reduced cost and complexity in studies focused on EE and provides a way to rescue EE measurement in studies compromised by faulty CO₂ measurements. Practitioners of respirometry should consider adjusting EE calculations for estimated protein metabolism based on dietary composition.

Plasma corticosterone, epinephrine, and norepinephrine levels increase during administration of nitrous oxide in rats

Nitrous oxide (N2O) is a gaseous drug with abuse potential. Despite its common clinical use, little is known about whether N2O administration activates the HPA axis and/or the sympathetic adrenomedullary system. The goal of this study was to determine whether 60% N2O alters plasma concentrations of corticosterone (CORT), epinephrine (EPI), and norepinephrine (NE) in male Long-Evans rats. A gas-tight swivel assembly in the lid of a gas administration chamber allowed the remote collection of blood samples from an indwelling jugular vein catheter at four time-points: baseline and at 30, 60, and 120 min during a two-hour administration of 60% N2O. Relative to baseline, plasma CORT (n = 9) was significantly elevated at all three time-points during N2O inhalation (mixed model analysis, p = .001) and plasma EPI and NE levels were each significantly elevated (n = 8, p ≤ .001) at the 30 min assessment. EPI then declined and did not differ from baseline at the 60 and 120 min assessments (p > .05) whereas NE remained elevated (120 min, p = .001). Administration of 60% N2O increases circulating CORT, EPI, and NE, supporting N2O as a physiological stressor. An N2O-induced increase in CORT is consistent with the observation that addictive drugs typically activate the HPA axis causing increased plasma levels of glucocorticoids. Allostatic models of drug addiction typically involve stress systems and the possible role of stress hormones in N2O-induced allostatic dysregulation is discussed.

Concentration-related metabolic rate and behavioral thermoregulatory adaptations to serial administrations of nitrous oxide in rats

Background

Initial administration of ≥60% nitrous oxide (N₂O) to rats evokes hypothermia, but after repeated administrations the gas instead evokes hyperthermia. This sign reversal is driven mainly by increased heat production. To determine whether rats will behaviorally oppose or assist the development of hyperthermia, we previously performed thermal gradient testing. Inhalation of N₂O at ≥60% causes rats to select cooler ambient temperatures both during initial administrations and during subsequent administrations in which the hyperthermic state exists. Thus, an available behavioral response opposes (but does not completely prevent) the acquired hyperthermia that develops over repeated high-concentration N₂O administrations. However, recreational and clinical uses of N₂O span a wide range of concentrations. Therefore, we sought to determine the thermoregulatory adaptations to chronic N₂O administration over a wide range of concentrations.

Methods

This study had two phases. In the first phase we adapted rats to twelve 3-h N₂O administrations at either 0%, 15%, 30%, 45%, 60% or 75% N₂O (n = 12 per group); outcomes were core temperature (via telemetry) and heat production (via respirometry). In the second phase, we used a thermal gradient (range 8°C—38°C) to assess each adapted group’s thermal preference, core temperature and locomotion on a single occasion during N₂O inhalation at the assigned concentration.

Results

In phase 1, repeated N₂O administrations led to dose related hyperthermic and hypermetabolic states during inhalation of ≥45% N₂O compared to controls (≥ 30% N₂O compared to baseline). In phase 2, rats in these groups selected cooler ambient temperatures during N₂O inhalation but still developed some hyperthermia. However, a concentration-related increase of locomotion was evident in the gradient, and theoretical calculations and regression analyses both suggest that locomotion contributed to the residual hyperthermia.

Conclusions

Acquired N₂O hyperthermia in rats is remarkably robust, and occurs even despite the availability of ambient temperatures that might fully counter the hyperthermia. Increased locomotion in the gradient may contribute to hyperthermia. Our data are consistent with an allostatic dis-coordination of autonomic and behavioral thermoregulatory mechanisms during drug administration. Our results have implications for research on N₂O abuse as well as research on the role of allostasis in drug addiction.

Brown adipose tissue thermogenesis does not explain the intra-administration hyperthermic sign-reversal induced by serial administrations of 60% nitrous oxide to rats

Initial administration of ≥60% nitrous oxide (N2O) to rats promotes hypothermia primarily by increasing whole-body heat loss. We hypothesized that the drug promotes heat loss via the tail and might initially inhibit thermogenesis via brown adipose tissue (BAT), major organs of thermoregulation in rodents. Following repeated administrations, N2O inhalation evokes hyperthermia underlain by increased whole-body heat production. We hypothesized that elevated BAT thermogenesis plays a role in this thermoregulatory sign reversal. Using dual probe telemetric temperature implants and infrared (IR) thermography, we assessed the effects of nine repeated 60% N2O administrations compared to control (con) administrations on core temperature, BAT temperature, lumbar back temperature and tail temperature. Telemetric core temperature, telemetric BAT temperature, and IR BAT temperature were reduced significantly during initial 60% N2O inhalation (p≤0.001 compared to con). IR thermography revealed that acute N2O administration unexpectedly reduced tail temperature (p=0.0001) and also inhibited IR lumbar temperature (p<0.0001). In the 9th session, N2O inhalation significantly increased telemetric core temperature (p=0.007) indicative of a hyperthermic sign reversal, yet compared to control administrations, telemetric BAT temperature (p=0.86), IR BAT temperature (p=0.85) and tail temperature (p=0.47) did not differ significantly. Thus, an initial administration of 60% N2O at 21°C may promote hypothermia via reduced BAT thermogenesis accompanied by tail vasoconstriction as a compensatory mechanism to limit body heat loss. Following repeated N2O administrations rats exhibit a hyperthermic core temperature but a normalized BAT temperature, suggesting induction of a hyperthermia-promoting thermogenic adaptation of unknown origin.