Behavioral and brain temperature responses to salient environmental stimuli and intravenous cocaine in rats: effects of diazepam

Basic metabolic and vascular effects of ketamine and its interaction with fentanyl

Ketamine is a short-acting general anesthetic with hallucinogenic, analgesic, and amnestic properties. In addition to its anesthetic use, ketamine is commonly abused in rave settings. While safe when used by medical professionals, uncontrolled recreational use of ketamine is dangerous, especially when mixed with other sedative drugs, including alcohol, benzodiazepines, and opioid drugs. Since synergistic antinociceptive interactions between opioids and ketamine were demonstrated in both preclinical and clinical studies, such an interaction could exist for the hypoxic effects of opioid drugs. Here, we focused on the basic physiological effects of ketamine as a recreational drug and its possible interactions with fentanyl-a highly potent opioid that induces strong respiratory depression and robust brain hypoxia. By using multi-site thermorecording in freely-moving rats, we showed that intravenous ketamine at a range of human relevant doses (3, 9, 27 mg/kg) dose-dependently increases locomotor activity and brain temperature, as assessed in the nucleus accumbens (NAc). By determining temperature differentials between the brain, temporal muscle, and skin, we showed that the brain hyperthermic effect of ketamine results from increased intracerebral heat production, an index of metabolic neural activation, and decreased heat loss due to peripheral vasoconstriction. By using oxygen sensors coupled with high-speed amperometry we showed that ketamine at the same doses increases NAc oxygen levels. Finally, co-administration of ketamine with intravenous fentanyl results in modest enhancement of fentanyl-induced brain hypoxia also enhancing the post-hypoxic oxygen increase. Therefore, in contrast to fentanyl, ketamine increases brain oxygenation but potentiates brain hypoxia induced by fentanyl.

Diazepam attenuates the effects of cocaine on locomotion, 50-kHz ultrasonic vocalizations and phasic dopamine in the nucleus accumbens of rats

BACKGROUND AND PURPOSE

Currently, there is no effective drug to treat cocaine-use disorder, which affects millions of people worldwide. Benzodiazepines are potential therapeutic candidates, as microdialysis and voltammetry studies have shown that they can decrease dopamine concentrations in the nucleus accumbens of rodents and block the increase in dopamine levels and appetitive 50-kHz ultrasonic vocalizations (USVs) induced by amphetamine in rats.

EXPERIMENTAL APPROACH

Here, we tested whether administration of 2.5-mg·kg^-1 diazepam (i.p.) in adult male rats could block the effects of 20-mg·kg^-1 cocaine (i.p.) on electrically evoked phasic dopamine signals in the nucleus accumbens measured by fast-scan cyclic voltammetry, as well as 50-kHz USV and locomotor activity.

KEY RESULTS

Cocaine injection increased evoked dopamine signals up to threefold within 5 min, and the increase was significantly higher than baseline for at least 75 min. The injection of diazepam, 5 min after cocaine, attenuated the cocaine effect by nearly 50%, and this attenuation was maintained for at least 40 min. Behaviourally, cocaine increased the number of appetitive 50-kHz calls by about 12-fold. Diazepam significantly blocked this effect for the entire duration of the session. Also, cocaine-treated rats were more active than controls and diazepam significantly attenuated cocaine-induced locomotion, by up to 50%.

CONCLUSION AND IMPLICATIONS

These results suggest that the neurochemical and psychostimulant effects of cocaine can be mitigated by diazepam.

Relationships between oxygen changes in the brain and periphery following physiological activation and the actions of heroin and cocaine

Using two-sensor electrochemical recordings in freely moving rats, we examined the relationship between physiological and drug-induced oxygen fluctuations in the brain and periphery. Animals chronically implanted with oxygen sensors in the nucleus accumbens (NAc) and subcutaneous (SC) space were subjected to several mildly arousing stimuli (sound, tail-pinch and social interaction) and intravenous injections of cocaine and heroin. Arousing stimuli induced rapid increases in NAc oxygen levels followed by and correlated with oxygen decreases in the SC space. Therefore, cerebral vasodilation that increases cerebral blood flow and oxygen entry into brain tissue results from both direct neuronal activation and peripheral vasoconstriction, which redistributes arterial blood from periphery to the brain. The latter factor could also explain a similar pattern of oxygen responses found in the substantia nigra reticulata, suggesting hyperoxia as a global phenomenon with minor structural differences during early time intervals following the stimulus onset. While arousing stimuli and cocaine induced similar oxygen responses in the brain and SC space, heroin induced a biphasic down-up brain oxygen fluctuation associated with a monophasic oxygen decrease in the SC space. Oxygen decreases occurred more rapidly and stronger in the SC space, reflecting a drop in blood oxygen levels due to respiratory depression.

Cocaine added to heroin fails to affect heroin-induced brain hypoxia

Heroin and cocaine are both highly addictive drugs that cause unique physiological and behavioral effects. These drugs are often co-administered and cocaine has been found in ~20% of cases of opioid overdose death. Respiratory depression followed by brain hypoxia is the most dangerous effect of high-dose opioids that could result in coma and even death. Conversely, cocaine at optimal self-administering doses increases brain oxygen levels. Considering these differences, it is unclear what pattern of oxygen changes will occur when these drugs are co-administered. Here, we used high-speed amperometry with oxygen sensors to examine changes in oxygen concentrations in the nucleus accumbens (NAc) induced by intravenous (iv) cocaine, heroin, and their mixtures in freely-moving rats. Cocaine delivered at a range of doses, both below (0.25 mg/kg) and within the optimal range of self-administration (0.5 and 1.0 mg/kg) modestly increased NAc oxygen levels. In contrast, heroin increased oxygen levels at a low reinforcing dose (0.05 mg/kg), but induced a biphasic down-up change at higher reinforcing doses (0.1 and 0.2 mg/kg), and caused a strong monophasic oxygen decrease during overdose (0.6 mg/kg). When combined at moderate doses, cocaine (0.25, 0.5 mg/kg) slightly increased and prolonged oxygen increases induced by heroin alone (0.5 and 0.1 mg/kg), but oxygen decreases were identical when cocaine (1 mg/kg) was combined with heroin at large doses (0.2 and 0.6 mg/kg). Therefore, health dangers of speedball may result from de-compensation of vital functions due to diminished intra-brain oxygen inflow induced by high-dose heroin coupled with enhanced oxygen use induced by cocaine.

Brain temperature effects of intravenous heroin: State dependency, environmental modulation, and the effects of dose

Here we examined how intravenous heroin at a dose that maintains self-administration (0.1 mg/kg) affects brain temperature homeostasis in freely moving rats under conditions that seek to mimic some aspects of human drug use. When administered under standard laboratory conditions (quiet rest at 22 °C ambient temperature), heroin induced moderate temperature increases (1.0-1.5 °C) in the nucleus accumbens (NAc), a critical structure of the brain motivation-reinforcement circuit. By simultaneously recording temperatures in the temporal muscle and skin, we demonstrate that the hyperthermic effects of heroin results primarily from inhibition of heat loss due to strong and prolonged skin vasoconstriction. Heroin-induced brain temperature increases were enhanced during behavioral activation (i.e., social interaction) and in a moderately warm environment (29 °C). By calculating the "net" effects of the drug in these two conditions, we found that this enhancement results from the summation of the hyperthermic effects of heroin with similar effects induced by either social interaction or a warmer environment. When the dose of heroin was increased (to 0.2, 0.4, 0.8, 1.6, 3.2, and 6.4 mg/kg), brain temperature showed a biphasic down-up response. The initial temperature decrease was dose-dependent and resulted from a transient inhibition of intra-brain heat production coupled with increased heat loss via skin surfaces-the effects typically induced by general anesthetics. These initial inhibitory effects induced by large-dose heroin injections could be related to profound CNS depression-the most serious health complications typical of heroin overdose in humans.

Control of the Cutaneous Circulation by the Central Nervous System

The central nervous system (CNS), via its control of sympathetic outflow, regulates blood flow to the acral cutaneous beds (containing arteriovenous anastomoses) as part of the homeostatic thermoregulatory process, as part of the febrile response, and as part of cognitive-emotional processes associated with purposeful interactions with the external environment, including those initiated by salient or threatening events (we go pale with fright). Inputs to the CNS for the thermoregulatory process include cutaneous sensory neurons, and neurons in the preoptic area sensitive to the temperature of the blood in the internal carotid artery. Inputs for cognitive-emotional control from the exteroceptive sense organs (touch, vision, sound, smell, etc.) are integrated in forebrain centers including the amygdala. Psychoactive drugs have major effects on the acral cutaneous circulation. Interoceptors, chemoreceptors more than baroreceptors, also influence cutaneous sympathetic outflow. A major advance has been the discovery of a lower brainstem control center in the rostral medullary raphé, regulating outflow to both brown adipose tissue (BAT) and to the acral cutaneous beds. Neurons in the medullary raphé, via their descending axonal projections, increase the discharge of spinal sympathetic preganglionic neurons controlling the cutaneous vasculature, utilizing glutamate, and serotonin as neurotransmitters. Present evidence suggests that both thermoregulatory and cognitive-emotional control of the cutaneous beds from preoptic, hypothalamic, and forebrain centers is channeled via the medullary raphé. Future studies will no doubt further unravel the details of neurotransmitter pathways connecting these rostral control centers with the medullary raphé, and those operative within the raphé itself. © 2016 American Physiological Society. Compr Physiol 6:1161-1197, 2016.

Brain temperature and its fundamental properties: a review for clinical neuroscientists

Brain temperature, as an independent therapeutic target variable, has received increasingly intense clinical attention. To date, brain hypothermia represents the most potent neuroprotectant in laboratory studies. Although the impact of brain temperature is prevalent in a number of common human diseases including: head trauma, stroke, multiple sclerosis, epilepsy, mood disorders, headaches, and neurodegenerative disorders, it is evident and well recognized that the therapeutic application of induced hypothermia is limited to a few highly selected clinical conditions such as cardiac arrest and hypoxic ischemic neonatal encephalopathy. Efforts to understand the fundamental aspects of brain temperature regulation are therefore critical for the development of safe, effective, and pragmatic clinical treatments for patients with brain injuries. Although centrally-mediated mechanisms to maintain a stable body temperature are relatively well established, very little is clinically known about brain temperature's spatial and temporal distribution, its physiological and pathological fluctuations, and the mechanism underlying brain thermal homeostasis. The human brain, a metabolically "expensive" organ with intense heat production, is sensitive to fluctuations in temperature with regards to its functional activity and energy efficiency. In this review, we discuss several critical aspects concerning the fundamental properties of brain temperature from a clinical perspective.

Reduced brown adipose tissue thermogenesis during environmental interactions in transgenic rats with ataxin-3-mediated ablation of hypothalamic orexin neurons

Thermogenesis in brown adipose tissue (BAT) contributes to substantial increases in body temperature evoked by threatening or emotional stimuli. BAT thermogenesis also contributes to increases in body temperature that occur during active phases of the basic rest-activity cycle (BRAC), as part of normal daily life. Hypothalamic orexin-synthesizing neurons influence many physiological and behavioral variables, including BAT and body temperature. In conscious unrestrained animals maintained for 3 days in a quiet environment (24-26°C) with ad libitum food and water, we compared temperatures in transgenic rats with ablation of orexin neurons induced by expression of ataxin-3 (Orx_Ab) with wild-type (WT) rats. Both baseline BAT temperature and baseline body temperature, measured at the onset of BRAC episodes, were similar in Orx_Ab and WT rats. The time interval between BRAC episodes was also similar in the two groups. However, the initial slopes and amplitudes of BRAC-related increases in BAT and body temperature were reduced in Orx_Ab rats. Similarly, the initial slopes and amplitudes of the increases in BAT temperatures induced by sudden exposure to an intruder rat (freely moving or confined to a small cage) or by sudden exposure to live cockroaches were reduced in resident Orx_Ab rats. Constriction of the tail artery induced by salient alerting stimuli was also reduced in Orx_Ab rats. Our results suggest that orexin-synthesizing neurons contribute to the intensity with which rats interact with the external environment, both when the interaction is "spontaneous" and when the interaction is provoked by threatening or salient environmental events.

Differential effects of dopamine and opioid receptor blockade on motivated Coca-Cola drinking behavior and associated changes in brain, skin and muscle temperatures

Although pharmacological blockade of both dopamine (DA) and opiate receptors has an inhibiting effect on appetitive motivated behaviors, it is still unclear which physiological mechanisms affected by these treatments underlie the behavioral deficit. To clarify this issue, we examined how pharmacological blockade of either DA (SCH23390+eticlopride at 0.2 mg/kg each) or opioid receptors (naloxone 1 mg/kg) affects motor activity and temperature fluctuations in the nucleus accumbens (NAcc), temporal muscle, and facial skin associated with motivated Coca-Cola drinking behavior in rats. In drug-free conditions, presentation of a cup containing 5 ml of Coca-Cola induced locomotor activation and rapid NAcc temperature increases, which both transiently decreased during drinking, and phasically increased again after the cup was emptied. Muscle temperatures followed this pattern, but increases were weaker and more delayed than those in the NAcc. Skin temperature rapidly dropped after cup presentation, remained at low levels during consumption, and slowly restored during post-consumption behavioral activation. By itself, DA receptor blockade induced robust decrease in spontaneous locomotion, moderate increases in brain and muscle temperatures, and a relative increase in skin temperatures, suggesting metabolic activation coupled with adynamia. Following this treatment (approximately 180 min), motor activation to cup presentation and Coca-Cola consumption were absent, but rats showed NAcc and muscle temperature increases following cup presentation comparable to control. Therefore, DA receptor blockade does not affect significantly central and peripheral autonomic responses to appetitive stimuli, but eliminates their behavior-activating effects, thus disrupting appetitive behavior and blocking consumption. Naloxone alone slightly decreased brain and muscle temperatures and increased skin temperatures, pointing at the enhanced heat loss and possible minor inhibition of basal metabolic activity. This treatment (approximately 60 min) had minimal effects on the latencies of drinking, but increased its total duration, with licking interrupted by pauses and retreats. This behavioral attenuation was coupled with weaker than in control locomotor activation and diminished temperature fluctuations in each recording location. Therefore, attenuation of normal behavioral and physiological responses to appetitive stimuli appears to underlie modest inhibiting effects of opiate receptor blockade on motivated behavior and consumption.

Cocaine-induced breakdown of the blood-brain barrier and neurotoxicity

Role of cocaine in influencing blood-brain barrier (BBB) function is still unknown. Available evidences suggest that cocaine administration results in acute hyperthermia and alterations in brain serotonin metabolism. Since hyperthermia is capable to induce the breakdown of the BBB either directly or through altered serotonin metabolism, a possibility exists that cocaine may induce neurotoxicity by causing BBB disruption. This hypothesis is discussed in this review largely based on our own laboratory investigations. Our observations in rats demonstrate that cocaine depending on the dose and routes of administration induces profound hyperthermia, increased plasma and brain serotonin levels leading to BBB breakdown and brain edema formation. Furthermore, cocaine was able to enhance cellular stress as seen by upregulation of heat shock protein (HSP 72 kD) expression and resulted in marked neuronal and glial cell damages at the time of the BBB dysfunction. Taken together, these observations are the first to suggest that cocaine-induced BBB disruption is instrumental in precipitating brain pathology. The possible mechanisms of cocaine-induced BBB breakdown and neurotoxicity are discussed.

Brain temperature responses to salient stimuli persist during dopamine receptor blockade despite a blockade of locomotor responses

We examined how an acute dopamine (DA) receptor blockade affects locomotor and brain (nucleus accumbens or NAcc), muscle and skin temperature responses to three arousing stimuli (procedure of sc injection, tail-pinch and social interaction with another male rat) and intravenous cocaine (1 mg/kg). DA receptor blockade was induced by mixture of D1- (SCH23390) and D-2 selective (eticlopride) DA antagonists at 0.2 mg/kg doses. Each arousing stimulus and cocaine caused locomotor activation, prolonged increase in NAcc and muscle temperature (0.6-1.0 degrees C for 20-50 min) and transient skin hypothermia (-0.6 degrees C for 1-3 min) in drug-naive conditions. DA receptor blockade strongly decreased basal locomotor activity, but moderately increased brain, muscle and skin temperatures. Therefore, selective interruption of DA transmission does not inhibit the brain, making it more metabolically active and warmer despite skin vasodilatation and the enhanced heat loss to the body and the external environment. DA antagonists strongly decreased locomotor responses to all stimuli and cocaine, had no effects on acute skin vasoconstriction, but differentially affected stimuli- and drug-induced changes in NAcc and muscle temperatures. While brain and muscle temperatures induced by cocaine were fully blocked and both temperatures slightly decreased, temperature increases induced by tail-pinch and social interaction, despite a significant attenuation, persisted during DA receptor blockade. These data are discussed to define the role of the DA system in regulating the central activation processes and behavioral responsiveness to natural arousing and drug stimuli.

Diazepam alters cocaine self-administration, but not cocaine-stimulated locomotion or nucleus accumbens dopamine

Cocaine is known to enhance nucleus accumbens dopamine (NAcc DA), to serve as a positive reinforcer and to produce negative effects, such as anxiety. The influence of diazepam on cocaine intake, cocaine-stimulated behavioral activity and NAcc DA was investigated using self-administration and experimenter-administered intravenous (i.v.) cocaine. In Experiment 1, rats were pretreated with diazepam (0.25 mg/kg) or saline (0.1 ml) 30 min prior to 20 daily 1-hour cocaine (0.75 mg/kg/injection) self-administration sessions. Cocaine intake increased for all animals across sessions, but was highest in diazepam-pretreated animals. Diazepam rats also self-administered their first cocaine injection of each session faster than controls. Experiment 2 utilized in vivo microdialysis to assess NAcc DA levels before and after experimenter-administered i.v. cocaine injections (0.75 mg/kg/injection x 2; 10-min interval) in diazepam- and saline-pretreated rats. Group differences were not revealed across basal and cocaine-stimulated NAcc DA assessments, indicating that diazepam did not decrease NAcc DA during cocaine self-administration. Findings that diazepam enhances cocaine self-administration and decreases cocaine response latency support the notion that cocaine-induced anxiety limits voluntary cocaine intake. It is further suggested that individual variations in cocaine-induced aversive effects may determine whether cocaine use is avoided or repeated.

Behavioral and temperature effects of delta 9-tetrahydrocannabinol in human-relevant doses in rats

Marijuana smoking dramatically alters responses to various environmental stimuli. To study this phenomenon, we assessed how delta-9-tetrahydrocannabinol (THC), a primary psychoactive ingredient of marijuana, affects locomotor and brain (nucleus accumbens or NAcc), muscle and skin temperature responses to natural arousing stimuli (one-minute tail-pinch and one-minute social interaction with another male rat) and iv cocaine (1 mg/kg) in male rats. THC was administered at three widely varying doses (0.5, 2.0 and 8.0 mg/kg, ip), and the drug-induced changes in basal values and responses to stimuli were compared to those occurring following ip vehicle injections (control). Each stimulus in control conditions caused acute locomotor activation, a prolonged increase in brain and muscle temperature (0.6-1.0 degrees C for 20-50 min) and transient decrease in skin temperature (-0.6 degrees C for 1-3 min). While THC at any dose had a tendency to decrease spontaneous locomotion as well as brain and muscle temperatures, true hypothermia and hypoactivity as well as clearly diminished locomotor and temperature responses to all stimuli were only seen following the largest dose. In this case, temperature decreases in the NAcc were stronger than in the muscle, suggesting metabolic brain inhibition as the primary cause of hypoactivity, hypothermia and hyporesponsiveness. While weaker in strength and without associated vasodilatation, this response pattern is mimicked by general anesthetics, questioning to what extent the hypothermic action of THC is specific (i.e., mediated via endogenous cannabinoid receptors) or non-specific, reflecting drug interaction with membrane lipids or other receptors. In contrast, weaker behavioral and temperature effects of THC at lower doses resemble those of diazepam, whose locomotion- and temperature-decreasing effects are evident only in activated conditions, when rats are moving and basal temperatures are elevated.