P. Leon Brown

Brain temperature fluctuation: a reflection of functional neural activation

Although it is known that relatively large increases in local brain temperature can occur during behaviour and in response to various novel, stressful and emotionally arousing environmental stimuli, the source of this heat is not clearly established. To clarify this issue, we monitored the temperature in three brain structures (dorsal and ventral striatum, cerebellum) and in arterial blood at the level of the abdominal aorta in freely moving rats exposed to several environmental challenges ranging from traditional stressors to simple sensory stimuli (cage change, tail pinch, exposure to another male rat, a female rat, a mouse or an unexpected sound). We found that brain temperature was consistently higher than arterial blood temperature, and that brain temperature increased prior to, and to a greater extent than, the increase in blood temperature evoked by each test challenge. Thus, the local metabolic consequences of widely correlated neural activity appear to be the primary source of increases in brain temperature and a driving force behind the associated changes in body temperature.

Brain edema and breakdown of the blood-brain barrier during methamphetamine intoxication : critical role of brain hyperthermia

To clarify the role of brain temperature in permeability of the blood–brain barrier (BBB), rats were injected with methamphetamine (METH 9 mg/kg) at normal (23 °C) and warm (29 °C) environmental conditions and internal temperatures were monitored both centrally (nucleus accumbens, NAcc) and peripherally (skin and nonlocomotor muscle). Once NAcc temperatures peaked or reached 41.5 °C (a level suggesting possible lethality), animals were administered Evans blue dye (protein tracer that does not normally cross the BBB), rapidly anaesthetized, perfused and had their brains removed. All METH‐treated animals showed brain and body hyperthermia associated with relative skin hypothermia, suggesting metabolic activation coupled with peripheral vasoconstriction. While METH‐induced NAcc temperature elevation varied from 37.60 to 42.46 °C (or 1.2–5.1 °C above baseline), it was stronger at 29 °C (+4.13 °C) than 23 °C (+2.31 °C). Relative to control, METH‐treated animals had significantly higher brain levels of water, Na+, K+ and Cl–, suggesting brain edema, and intense immunostaining for albumin, indicating breakdown of the BBB. METH‐treated animals also showed strong immunoreactivity for glial fibrillary acidic protein (GFAP), possibly suggesting acute abnormality or damage of astrocytes. METH‐induced changes in brain water, albumin and GFAP correlated linearly with NAcc temperature (r = 0.93, 0.98 and 0.98, respectively), suggesting a key role of brain hyperthermia in BBB permeability, development of brain edema and subsequent functional and structural neural abnormalities. Therefore, along with a direct destructive action on neural cells and functions, brain hyperthermia, via breakdown of the BBB, may be crucial for both decompensation of brain functions and cell injury following acute METH intoxication, possibly contributing to neurodegeneration resulting from chronic drug use.

Brain hyperthermia induced by MDMA (‘ecstasy’): modulation by environmental conditions

Drugs of abuse, such as 3,4‐methylenedioxymethamphetamine (MDMA), often have more powerful effects during states of increased activation and under specific environmental conditions. Because hyperthermia is a major complication of MDMA use and a factor potentiating neurotoxicity, we examined the effects of this drug (9 mg/kg, sc; approximately one‐fifth of the known LD50 in rats) on brain [nucleus accumbens (Nacc) and hippocampus (Hippo)] and muscle (musculus temporalis) temperatures in male rats under conditions that either model human drug use (social interaction with female, warm temperature) or restrict heat dissipation from the brain (chronic occlusion of jugular veins). Under quiet resting conditions at 23 °C, MDMA induced a moderate but prolonged hyperthermia. Both NAcc and Hippo showed more rapid and stronger temperature increases than muscle, suggesting metabolic neural activation as a primary cause of brain hyperthermia. During social interaction with a female, brain hyperthermia induced by MDMA was significantly potentiated (+89%). Brain hyperthermia induced by MDMA was also strongly potentiated (+188%) in animals with chronically occluded jugular veins, suggesting impaired cerebral outflow enhances intrabrain heat accumulation. At 29 °C, MDMA pushed temperatures in the brain to its biological limits (>41 °C; +268%), resulting in fatalities in most (83%) tested animals. Therefore, by inducing metabolic brain activation and restricting heat dissipation, MDMA use under ‘party’ conditions may be much more dangerous than under standard laboratory conditions.

Brain and body temperature homeostasis during sodium pentobarbital anesthesia with and without body warming in rats

High-speed, multi-site thermorecording offers the ability to follow the dynamics of heat production and flow in an organism. This approach was used to study brain-body temperature homeostasis during the development of general anesthesia induced by sodium pentobarbital (50 mg/kg, ip) in rats. Animals were chronically implanted with thermocouple probes in two brain areas, the abdominal cavity, and subcutaneously, and temperatures were measured during anesthesia both with and without (control) body warming. In control conditions, temperature in all sites rapidly and strongly decreased (from 36-37 degrees C to 32-33 degrees C, or 3.5-4.5 degrees C below baselines). Relative to body core, brain hypothermia was greater (by 0.3-0.4 degrees C) and skin hypothermia was less (by approximately 0.7 degrees C). If the body was kept warm with a heating pad, brain hypothermia was three-fold weaker ( approximately 1.2 degrees C), but the brain-body difference was significantly augmented (-0.6 degrees C). These results suggest that pentobarbital-induced inhibition of brain metabolic activity is a major factor behind brain hypothermia and global body hypothermia during general anesthesia. These data also indicate that body warming is unable to fully compensate for anesthesia-induced brain hypothermia and enhances the negative brain-body temperature differentials typical of anesthesia. Since temperature strongly affects various underlying parameters of neuronal activity, these findings are important for electrophysiological studies performed in anesthetized animal preparations.

Dopamine-dependent and dopamine-independent actions of cocaine as revealed by brain thermorecording in freely moving rats

Brain temperature fluctuates biphasically in response to repeated, intravenous (i.v.) cocaine injections, perhaps reflecting cocaines inhibiting effect on both dopamine (DA) transporters and Na+ channels. By using a DA receptor blockade, one could separate these actions and determine the role of DA‐dependent and DA‐independent mechanisms in mediating this temperature fluctuation. Rats were chronically implanted with thermocouple probes in the brain, a non‐locomotor head muscle and subcutaneously. Temperature fluctuations associated with ten repeated i.v. cocaine injections (1 mg/kg with 8‐min inter‐injection intervals) were examined after a combined, systemic administration of selective D1‐like and D2‐like receptor blockers (SCH‐23390 and eticlopride) at doses that effectively inhibit DA transmission. In contrast to the initial temperature increases and subsequent biphasic fluctuations (decreases followed by increases) seen with repeated cocaine injections in saline‐treated control, brain and muscle temperatures during DA receptor blockade decreased with each repeated cocaine injection. DA receptor blockade had no effects on skin temperature, which tonically decreased and biphasically fluctuated (decreases followed by increases) during repeated cocaine injections in both conditions. DA receptor blockade by itself slightly increased brain and muscle temperatures, with no evident effect on skin temperature. DA antagonists also strongly decreased spontaneous movement activity and completely blocked the locomotor activation normally induced by repeated cocaine injections. Although our data confirm that cocaines inhibitory action on presynaptic DA uptake is essential for its ability to induce metabolic and behavioral activation, they also suggest that the physiological effects of this drug cannot be explained through this system alone. The continued hypothermic effect of cocaine points to its action on other central systems (particularly blockade of Na+ channels) that may be important for the development of cocaine abuse and adverse effects of this drug.

Modulation of physiological brain hyperthermia by environmental temperature and impaired blood outflow in rats

To study the role of ambient temperature and brain blood outflow in modulating physiological brain hyperthermia, temperatures in two brain structures (nucleus accumbens or NAcc and hippocampus or Hippo) and a non-locomotor head muscle (musculus temporalis) were monitored in rats exposed to three arousing stimuli (placement in the cage or environmental change, 3-min social interaction with a female rat, 3-min innocuous tail-pinch) under three conditions (intact animals at 23 degrees C or control, intact animals at 29 degrees C, animals with chronically occluded jugular veins at 23 degrees C). While each stimulus in each condition induced hyperthermia, with more rapid and stronger changes in brain structures than muscle, there were significant differences between conditions. At 29 degrees C, animal placement in the cage resulted in stronger temperature increase and larger brain-muscle differentials, while basal temperatures in Hippo and muscle (but not in NAcc) were higher than control. At 29 degrees C, hyperthermia during social interaction was smaller but more prolonged, while the response to tail-pinch was similar to that seen at normal environmental temperatures. Animals with chronically occluded jugular veins had similar basal temperatures but showed much weaker hyperthermia than intact animals during each stimulus presentation; temperature increases in brain structures, however, were much stronger than in the muscle. Our data suggest that the brain is able to decrease neural activation induced by environmental challenges under conditions of impaired blood outflow and restricted heat dissipation to the external environment.

Procedure of rectal temperature measurement affects brain, muscle, skin, and body temperatures and modulates the effects of intravenous cocaine

Rectal probe thermometry is commonly used to measure body core temperature in rodents because of its ease of use. Although previous studies suggest that rectal measurement is stressful and results in long-lasting elevations in body temperatures, we evaluated how this procedure affects brain, muscle, skin, and core temperatures measured with chronically implanted thermocouple electrodes in rats. Our data suggest that the procedure of rectal measurement results in powerful locomotor activation, rapid and strong increases in brain, muscle, and deep body temperatures, as well as a biphasic, down-up fluctuation in skin temperature, matching the response pattern observed during tail-pinch, a representative stressful procedure. This response, moreover, did not habituate after repeated day-to-day testing. Repeated rectal probe insertions also modified temperature responses induced by intravenous cocaine. Under quiet resting conditions, cocaine moderately increased brain, muscle, and deep body temperatures. However, during repeated rectal measurements, which increased temperatures, cocaine induced both hyperthermic and hypothermic responses. Direct comparisons revealed that body temperatures measured by a rectal probe are typically lower (approximately 0.6 degrees C) and more variable than body temperatures recorded by chronically implanted electrodes; the difference is smaller at low and greater at high basal temperatures. Because of this difference and temperature increases induced by the rectal probe per se, cocaine had no significant effect on rectal temperatures compared to control animals exposed to repeated rectal probes. Therefore, although rectal temperature measurements provide a decent correlation with directly measured deep body temperatures, the arousing influence of this procedure may drastically modulate the effects of other arousing stimuli and drugs.

Sensory effects of intravenous cocaine on dopamine and non-dopamine ventral tegmental area neurons

Intravenous (iv) cocaine mimics salient somato-sensory stimuli in their ability to induce rapid physiological effects, which appear to involve its action on peripherally located neural elements and fast neural transmission via somato-sensory pathways. To further clarify this mechanism, single-unit recording with fine glass electrodes was used in awake rats to examine responses of ventral tegmental area (VTA) neurons, both presumed dopamine (DA) and non-DA, to iv cocaine and tail-press, a typical somato-sensory stimulus. To exclude the contribution of DA mechanisms to the observed neuronal responses to sensory stimuli and cocaine, recordings were conducted during full DA receptor blockade (SCH23390+eticloptide). Iv cocaine (0.25 mg/kg delivered over 10 s) induced significant excitations of approximately 63% of long-spike (presumed DA) and approximately 70% of short-spike (presumed non-DA) VTA neurons. In both subgroups, neuronal excitations occurred with short latencies (4-8 s), peaked at 10-20 s (30-40% increase over baseline) and disappeared at 30-40 s after the injection onset. Most long-(67%) and short-spike (89%) VTA neurons also showed phasic responses to tail-press (5-s). All responsive long-spike cells were excited by tail-press; excitations were very rapid (peak at 1 s) and strong (100% rate increase over baseline) but brief (2-3 s). In contrast, both excitations (60%) and inhibitions (29%) were seen in short-spike cells. These responses were also rapid and transient, but excitations of short-spike units were more prolonged and sustained (10-15 s) than in long-spike cells. These data suggest that in awake animals iv cocaine, like somato-sensory stimuli, rapidly and transiently excites VTA neurons of different subtypes. Therefore, along with direct action on specific brain substrates, central effects of cocaine may occur, via an indirect mechanism, involving peripheral neural elements, visceral sensory nerves and rapid neural transmission. Via this mechanism, cocaine, like somato-sensory stimuli, can rapidly activate DA neurons and induce phasic DA release, creating the conditions for DA accumulation by a later occurring and prolonged direct inhibiting action on DA uptake. By providing a rapid neural signal and triggering transient neural activation, such a peripherally driven action might play a crucial role in the sensory effects of cocaine, thus contributing to learning and development of drug-taking behavior.

The role of peripheral Na+ channels in triggering the central excitatory effects of intravenous cocaine

While alterations in dopamine (DA) uptake appear to be a critical mechanism underlying locomotor and reinforcing effects of cocaine (COC), many centrally mediated physiological and affective effects of this drug are resistant to DA receptor blockade and are expressed more quickly following an intravenous (i.v.) injection than expected based on the dynamics of drug concentration in the brain. Because COC is also a potent local anesthetic, its rapid action on Na+ channels may be responsible for triggering these effects. We monitored temperatures in the nucleus accumbens, temporal muscle and skin together with conventional locomotion during a single i.v. injection of COC (1 mg/kg), procaine (PRO, 5 mg/kg; equipotential anesthetic dose), a short‐acting local anesthetic drug that, like COC, interacts with Na+ channels, and cocaine methiodide (COC‐MET, 1.31 mg/kg, equimolar dose), a quaternary COC derivative that is unable to cross the blood–brain barrier. In this way, we explored not only the importance of Na+ channels in general, but also the importance of central vs. peripheral Na+ channels specifically. COC induced locomotor activation, temperature increase in the brain and muscle, and a biphasic temperature fluctuation in skin. Though PRO did not induce locomotor activation, it mimicked, to a greater degree, the temperature effects of COC. Therefore, Na+ channels appear to be a key substrate for COC‐induced temperature fluctuations in the brain and periphery. Similar to PRO, COC‐MET had minimal effects on locomotion, but mimicked COC in its ability to increase brain and muscle temperature, and induce transient skin hypothermia. It appears therefore that COCs interaction with peripherally located Na+ channels triggers its central excitatory effects manifested by brain temperature increase, thereby playing a major role in drug sensing and possibly contributing to COC reinforcement.

The role of peripheral and central sodium channels in mediating brain temperature fluctuations induced by intravenous cocaine

While cocaines interaction with the dopamine (DA) transporter and subsequent increase in DA transmission are usually considered key factors responsible for its locomotor stimulatory and reinforcing properties, many centrally mediated physiological and psychoemotional effects of cocaine are resistant to DA receptor blockade, suggesting the importance of other non-DA mechanisms. To explore the role of cocaines interaction with Na+ channels, rats were used to compare locomotor stimulatory and temperature (NAcc, temporal muscle and skin) effects of repeated iv injections of cocaine (1 mg/kg) with those induced by procaine (PRO 5 mg/kg), a short-acting local anesthetic with negligible effect on the DA transporter, and cocaine methiodide (COC-MET 1.31 mg/kg), a quaternary cocaine derivative that is unable to cross the blood-brain barrier. While PRO, unlike cocaine, did not induce locomotor activation, it mimicked cocaine in its ability to increase brain temperature following the initial injection and to induce biphasic, down-up fluctuations following repeated injections. This similarity suggests that both these effects of cocaine may be driven by its action on Na+ channels, a common action of both drugs. While COC-MET also did not affect locomotor activity, it shared with cocaine and PRO their ability to increase brain temperature but failed to induce temperature decreases after repeated injections. These findings point toward activation of peripheral Na+ channels as the primary mechanism of rapid excitatory effects of cocaine and inhibition of centrally located Na+ channels as the primary mechanism for transient inhibitory effects of cocaine. DA receptor blockade (SCH23390+eticlopride) fully eliminated locomotor stimulatory and temperature-increasing effects of cocaine, but its temperature-decreasing effects remained intact. Surprisingly, DA receptor blockade also altered the temperature fluctuations caused by PRO and COC-MET, suggesting that some of the central effects triggered via Na+ channels are in fact DA-dependent. Finally, repeated administration of PRO to animals that had previous cocaine experience led to conditioned locomotion and potentiated temperature-increasing effects of this drug. It appears, therefore, that, in addition to the central effects of cocaine mediated via interaction with the DA transporter and potentiation of DA uptake, interaction with peripheral and central Na+ channels is important for the initial physiological and, perhaps, affective effects of cocaine, likely contributing to the unique abuse potential of this drug.