The antinociceptive effect of Delta9-tetrahydrocannabinol in the arthritic rat involves the CB(2) cannabinoid receptor

Cannabinoid CB₂ receptors have been implicated in antinociception in animal models of both acute and chronic pain. We evaluated the role both cannabinoid CB₁ and CB₂ receptors in mechanonociception in non-arthritic and arthritic rats. The antinociceptive effect of Δ⁹-tetrahydrocannabinol (Δ⁹THC) was determined in rats following administration of the cannabinoid CB₁ receptor-selective antagonist, SR141716A, the cannabinoid CB₂ receptor-selective antagonist, SR144528, or vehicle. Male Sprague–Dawley rats were rendered arthritic using Freund’s complete adjuvant and tested for mechanical hyperalgesia in the paw-pressure test. Arthritic rats had a baseline paw-pressure of 83 ± 3.6g versus a paw-pressure of 177 ± 6.42g in non-arthritic rats. SR144528 or SR141716A (various doses mg/kg; i.p.) or 1:1:18 (ethanol:emulphor:saline) vehicle were injected 1 h prior to Δ⁹THC (4mg/kg; i.p) or 1:1:18 vehicle and antinociception determined 30min post Δ⁹THC. AD₅₀'s for both antagonists were calculated with 95% confidence limits. In addition, midbrain and spinal cord were removed for determination of cannabinoid CB₁ and CB₂ receptor protein density in the rats. SR144528 significantly attenuated the antinociceptive effect of Δ⁹THC in the arthritic rats [AD₅₀ = 3.3 (2.7–4) mg/kg], but not in the non-arthritic rats at a dose of 10/mg/kg. SR141716A significantly attenuated Δ⁹THC-induced antinociception in both the non-arthritic [AD₅₀ = 1.4 (0.8–2) mg/kg] and arthritic rat [AD₅₀ = 2.6 (1.8–3.1) mg/kg]. SR141716A or SR144528 alone did not result in a hyperalgesic effect as compared to vehicle. Our results indicate that the cannabinoid CB₂ receptor plays a critical role in cannabinoid-mediated antinociception, particularly in models of chronic inflammatory pain.     

Synergy between delta9-tetrahydrocannabinol and morphine in the arthritic rat

We have shown in past isobolographic studies that a small amount of Delta(9)-tetrahydrocannabinol (Delta(9)-THC) can enhance morphine antinociception in mice. However, previous studies of the Delta(9)-THC/morphine interaction were performed using normal mice or rats and evaluated acute thermal antinociception. Less is known about cannabinoid and opioid interactions involved in mechanical nociception and in chronic inflammatory pain models, such as Freund's complete adjuvant-induced arthritic model. One fixed-ratio combination was chosen for testing the interaction between Delta(9)-THC and morphine in the Freund's adjuvant-induced arthritic model. This combination represented a 1:1 ratio of the drugs and thus consisted of equieffective doses ranging from 0.1 to 5 mg/kg Delta(9)-THC and from 0.1 to 5 mg/kg morphine. The combination ED(50) value for the fixed ratios (total dose) in relation to the ED(50) value of the drugs alone was determined. The isobolographic analysis indicated a synergistic interaction between Delta(9)-THC and morphine in both the non-arthritic and the arthritic rats. Since Freund's adjuvant-induced alteration in endogenous opioid tone has been previously shown, our data indicate that such changes did not preclude the use of Delta(9)-THC and morphine in combination. As with acute preclinical pain models in which the Delta(9)-THC/morphine combination results in less tolerance development, the implication of the study for chronic pain conditions is discussed.     

Functional consequences of the repeated administration of Delta9-tetrahydrocannabinol in the rat

The repeated administration of Delta(9)-tetrahydrocannabinol (THC) results in tolerance to many of its behavioral and physiological effects. It also produces changes in the functionality of cannabinoid receptors. What is not completely understood is how these cellular events translate into the behavioral and physiological changes that are associated with repeated cannabinoid agonist treatment. The purpose of these studies was to determine the development of changes in the patterns of functional activity, as measured by the 2-[14C]deoxyglucose method (2DG), associated with repeated THC exposure. Male Sprague-Dawley rats (n=4-5) were administered THC (vehicle or 10 mg/kg, intraperitoneally), daily for 7 or 21 days. Fifteen minutes following the final THC treatment the 2DG procedure was initiated. In separate sets of rats similarly treated with THC, locomotor activity and core body temperature were measured at corresponding time points in order to establish the behavioral profile of repeated THC administration. The acute administration of THC following 7 or 21 days of drug exposure resulted in a significant attenuation of changes in rates of glucose utilization throughout the majority of brain regions analyzed when compared to the large global decreases observed following a single administration of THC. After 7 and 21 days of treatment, cerebral metabolic rates were no longer different from vehicle-treated controls in most cortical, thalamic and basal ganglia regions. This attenuation closely paralleled the development of tolerance to the effects of THC on locomotor activity and core body temperature. However, glucose utilization remained altered in the nucleus accumbens, mediodorsal thalamus, basolateral amygdala, portions of the hippocampus and median raphe. These data suggest that the development of tolerance to the cerebral metabolic effects of THC is regionally specific and temporally distinct. The persistence of effects in limbic areas as well as portions of the hippocampal complex, however, suggests that processes such as stress, reward, and aspects of memory mediated by these brain regions may continue to be affected by THC even after prolonged THC exposure.     

Repeated administration with Delta9-tetrahydrocannabinol regulates mu-opioid receptor density in the rat brain

Several studies have demonstrated reciprocal, as well as synergistic interactions between cannabinoid and opioid systems. The aim of this study was to explore the time-related effects of repeated administration of Delta9-tetrahydrocannabinol on mu-opioid receptor autoradiography in various brain regions of the rat. To this aim, the effects of Delta9-tetrahydrocannabinol (Delta9-THC, 5 mg/kg/day; i.p.) were examined after 1, 3, 7 and 14 days of repeated administration on regions containing mu-opioid receptors: (i) forebrain [caudate-putamen, nucleus accumbens (core and shell) and piriform cortex]; (ii) amygdala (medial pars and cortical posteromedial pars), hypothalamus (ventromedial and dorsomedial nuclei, zona incerta), hippocampal regions (CA1, CA2, CA3, dentate girus), hindbrain (substantia nigra and ventral tegmental area); and (iii) thalamus, including 12 thalamic nuclei. In most of these regions, repeated cannabinoid administration increases mu-opioid receptor density; however, the onset, degree of magnitude reached and time-related effects produced by administration with Delta9-tetrahydrocannabinol are dependent upon the brain region examined. It appears that the major increase in mu-opioid receptor density occurs 1 and 3 days after Delta9-THC administration. In some regions, this increase is maintained and, for most of the brain areas examined, this effect is no longer significant by 14 days of administration, suggesting tolerance to cannabinoid treatment. Taken together, the results of this study suggest that cannabinoids produce a time-related differential responsiveness in mu-opioid receptor density in several brain areas that may be relevant to an understanding of the alterations associated with cannabinoid exposure.     

Potentiation of Delta9-tetrahydrocannabinol (THC) effects by magnesium deficiency in the rat

Aggressive behavior can be classified into three major categories: defense, social attack and predatory behavior. The predatory behavior of rats, which prompts them to prey on mice (muricidal behavior) may be induced by injection of high doses (11 mg/kg) of Delta9-tetrahydrocannabinol (THC) or by acute magnesium deficiency. We have studied the effect of a single injection of low doses of THC (2, 4 or 8 mg/kg) in rats with a severe (50 ppm magnesium diet) or moderate (150 ppm) magnesium deficiency. The combination of moderate magnesium deficiency with low doses of THC induced muricidal behavior in all the rats and an increase in aggressiveness at the doses of 4 or 8 mg/kg of THC. Hyperaggressiveness increased with magnesium deficiency severity. Serotonin is probably involved in aggressiveness induced by both moderate magnesium deficiency and low doses of THC, but implication of other neurotransmitters and magnesium deficiency-induced alterations of CB1-and/or CB2-receptor expression are not excluded.     

Delta 9-tetrahydrocannabinol produced discrimination in pigeons.

In an operant situation pigeons learned to peck one response key 90 min after an injection of 0.25mg/kg delta9-tetrahydrocannabinol (delta9-THC) and another key when trained nondrugged. When tested with doses of delta9-THC lwer than the training dose the birds disciminated 0.20 mg/kg of the drug from the nondrugged state but not 0.15 mg/kg or lower doses. The animals were able to discriminate the drug state from the nondrugged 180 min but not 360 min after the injection At a shorter interval (45 min) both drug and nondrug responding appeared. Cannabinol and cannabidiol (4.0 - 8.0 mg/kg) did not elicit any drug responses, nor did pentobarbital, ditran or amphetamine. Tests with LSD resulted in both drug and nondrug responding. When administering noncannabinoid drugs in combination with delta9-THC 0.15 mg/kg the birds responded at the key associated with the drug state, suggesting interactional effects.     

Delta 9-tetrahydrocannabinol facilitates striatal dopaminergic transmission

We examined the effects of delta 9-tetrahydrocannabinol (THC) on striatal dopaminergic neurons in rats. THC inhibited the uptake of 3H-dopamine (DA) into striatal synaptosomes. THC facilitated the release of endogenous DA but not dihydroxyphenylacetic acid (DOPAC) from striatal slices. The concentration of DA in the dorsolateral striatum was reduced by THC. We propose that THC may stimulate nigrostriatal dopaminergic neurotransmission mainly by inhibiting uptake of DA and by facilitating release of DA.     

Delta9-tetrahydrocannabinol inhibits gastric motility in the rat through cannabinoid CB1 receptors.

We investigated involvement of the autonomic nervous system in gastric motor and cardiovascular responses to delta9-tetrahydrocannabinol (delta9-THC) in anesthetized rats. Intravenously administered delta9-THC evoked long-lasting decreases in intragastric pressure and pyloric contractility, bradycardia, and hypotension. The changes in gastric motor function and bradycardia were abolished by vagotomy and ganglionic blockade, whereas spinal cord transection prevented the hypotensive response. Administered intravenously alone, N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-met hyl-1H-pyrazole-3-carboxamide, a putative cannabinoid CB1 receptor antagonist, evoked transient decrease in intragastric pressure, and hypertension that was associated with bradycardia. However, this agent completely blocked the gastric motor and cardiovascular responses to intravenous delta9-THC. Application of delta9-THC to the dorsal surface of the medulla resulted in small and short-lasting decreases in gastric motor and cardiovascular function. We conclude that the decrease in gastric motor function and bradycardia are partially due to an action of delta9-THC in the dorsal medulla and that intact vagal nerves are required. The hypotension was mediated through sympathetic pathways. Both gastric motor and cardiovascular effects of peripherally administered delta9-THC seem to be mediated through cannabinoid CB1 receptors.     

The triple effect induced by delta 9-tetrahydrocannabinol on the rat blood pressure

The intravenous injection of delta 9-tetrahydrocannabinol (2-10 mg/kg) produced dose-related changes in the rat blood pressure. Three effects were detected: (1) an immediate and short-lasting hypotension related to bradycardia, blocked after atropine, vagotomy hexamethonium and pithing; (2) A rise in blood pressure 30 seconds after injection, insensitive to yohimbine, hexamethonium, pithing and reserpine treatment; (3) a slow and persistent hypotension, 5 min later, insensitive to atropine and vagotomy but inhibited by hexamethonium, pithing and reserpine treatment. It was concluded that intravenous injection of THC in rats may induce vagal stimulation and hypotension. This effect was reversed and followed by hypertension due to direct vasoconstriction not dependent on sympathetic activity. After this action a central decrease in sympathetic tonus led to a persistent hypotension an effect which is commonly reported for mammals.     

The effects of Delta9-tetrahydrocannabinol in rat mesenteric vasculature, and its interactions with the endocannabinoid anandamide

1 Delta9-tetrahydrocannabinol (THC) produces varying effects in mesenteric arteries: vasorelaxation (third-order branches, G3), modest vasorelaxation (G2), no effect (G1) and vasoconstriction (the superior mesenteric artery, G0). 2 In G3, vasorelaxation to THC was inhibited by pertussis toxin, but was unaffected by the CB1 receptor antagonist, AM251 (1 microM), incubation with the TRPV1 receptor agonist capsaicin (10 microM, 1 h), the TRPV1 receptor antagonist capsazepine (10 microM) or de-endothelialisation. 3 In G3, vasorelaxation to THC was inhibited by high K+ buffer, and by the following K+ channel inhibitors: charybdotoxin (100 nM), apamin (500 nM) and barium chloride (30 microM), but not by 4-aminopyridine, glibenclamide or tertiapin. 4 In G3, THC (10 and 100 microM) inhibited the contractile response to Ca2+ in a Ca2+-free, high potassium buffer, indicating that THC blocks Ca2+ influx. 5 In G0, the vasoconstrictor responses to THC were inhibited by de-endothelialisation and SR141716A (100 nM), but not by the endothelin (ET(A)) receptor antagonist FR139317 (1 microM).THC (1 and 10 microM) antagonised vasorelaxation to anandamide in G3 but not G0. THC did not antagonise the noncannabinoid verapamil, capsaicin or the CB1 receptor agonist CP55,940. 6 THC (10 and 100 microM) inhibited endothelium-derived relaxing factor (EDHF)-mediated responses to carbachol in a manner similar to the gap junction inhibitor 18alpha-glycyrrhetinic acid. 7 These data show that THC causes vasorelaxation through activation of K+ channels and inhibition of Ca2+ channels, and this involves non-CB1, non-TRPV1 but G-protein-coupled receptors. In G0, THC does not cause relaxation and at high concentrations causes contractions. Importantly, THC antagonises the effects of anandamide, possibly through inhibition of EDHF activity.     

Rapid tolerance to Delta(9)-tetrahydrocannabinol and cross-tolerance between ethanol and Delta(9)-tetrahydrocannabinol in mice.

Motor incoordination in the rota-rod test was used to assess the development of rapid tolerance to Delta(9)-tetrahydrocannabinol and rapid cross-tolerance between ethanol and Delta(9)-tetrahydrocannabinol in mice. Further, the influence of the cannabinoid receptor antagonist SR 141716A (N-(piperidin-1-yl)-5-(4-chlorophenyl)-4-methyl-1H-pyrazole-3-carboxyamide) on the motor impairment induced by both drugs was examined. Mice were injected on day 1 with equipotent doses of Delta(9)-tetrahydrocannabinol (28 mg/kg, i.p.) and ethanol (2.25 g/kg, i.p.) and tested at 30, 60 and 90 min after the injections. On day 2, control groups received ethanol or Delta(9)-tetrahydrocannabinol, some groups received the same treatment as the day before, while the remaining groups switched the treatment. All groups were tested to evaluate tolerance. The development of rapid tolerance to Delta(9)-tetrahydrocannabinol was observed and pretreatment with ethanol resulted in rapid cross-tolerance to Delta(9)-tetrahydrocannabinol. SR 141716A (2 mg/kg, i.p.) failed to block the development of rapid tolerance to both drugs, ethanol and Delta(9)-tetrahydrocannabinol. These results suggest that Delta(9)-tetrahydrocannabinol, similarly to ethanol, can induce rapid tolerance to motor incoordination in mice. They also support the use of the 2-day protocol as an effective procedure to reduce the length of drug exposure necessary to induce tolerance.     

Simultaneous GC-EI-MS determination of Delta9-tetrahydrocannabinol, 11-hydroxy-Delta9-tetrahydrocannabinol, and 11-nor-9-carboxy-Delta9-tetrahydrocannabinol in human urine following tandem enzyme-alkaline hydrolysis

A sensitive and specific method for extraction and quantification of Delta(9)-tetrahydrocannabinol (THC), 11-hydroxy-Delta(9)-tetrahydrocannabinol (11-OH-THC), and 11-nor-9-carboxy-Delta(9)-tetrahydrocannabinol (THCCOOH) in human urine was developed and fully validated. To ensure complete hydrolysis of conjugates and capture of total analyte content, urine samples were hydrolyzed by two methods in series. Initial hydrolysis was with Escherichia coli beta-glucuronidase (Type IX-A) followed by a second hydrolysis utilizing 10N NaOH. Specimens were adjusted to pH 5-6.5, treated with acetonitrile to precipitate protein, and centrifuged, and the supernatants were subjected to solid-phase extraction. Extracted analytes were derivatized with BSTFA and quantified by gas chromatography-mass spectrometry with electron impact ionization. Standard curves were linear from 2.5 to 300 ng/mL. Extraction efficiencies were 57.0-59.3% for THC, 68.3-75.5% for 11-OH-THC, and 71.5-79.7% for THCCOOH. Intra- and interassay precision across the linear range of the assay ranged from 0.1 to 4.3% and 2.6 to 7.4%, respectively. Accuracy was within 15% of target concentrations. This method was applied to the analysis of urine specimens collected from individuals participating in controlled administration cannabis studies, and it may be a useful analytical procedure for determining recency of cannabis use in forensic toxicology applications.     

Delta9-tetrahydrocannabinol content of commercially available hemp products

Delta9-Tetrahydrocannabinol (THC) is the main psychoactive compound present in marijuana. THC can also be found, as a contaminant, in some commercially available hemp products marketed in health food stores and on the internet as a good source of essential fatty acids. The products range from oil to alcoholic beverages to nutritional bars to candies, with oil being the most popular and commonly available. The analytical results are separated into two groups, products tested prior to and after publication of 21 CFR Part 1308, "clarification of listing of tetrahydrocannabinols." The data presented are a summary of 79 different hemp products tested for THC. THC was separated by a liquid-liquid or solid-liquid extraction, depending upon the product matrix. THC concentrations range from none detected to 117.5 microg THC/g material. Typical limits of detection for the assay (depending on matrix) are 1.0-2.5 microg THC/g material. Products that were of aqueous base (beer, tea) had much lower limits of detection (2.5 ng/mL). No THC was detected in 58% of the products from group 1 and 86% of the products from group 2. The amounts indicate that THC levels in currently marketed hemp products are significantly lower than in those products available before 2003 and reported in previous studies. The results reported here may be used as a general guideline for the THC content of hemp products recently found in the marketplace today.     

Regulation of DARPP-32 phosphorylation by Delta9-tetrahydrocannabinol

CB1 receptor agonists increase the state of phosphorylation of the dopamine and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) at the cAMP-dependent protein kinase site, Thr 34. This effect, which occurs in the medium spiny neurons of the striatum, has been proposed to mediate the motor depressant action of cannabinoids. In this study, we have examined the effect produced by systemic administration of Delta(9)-tetrahydrocannabinol (THC), the major component of marihuana and hashish, on DARPP-32. We show that THC increases DARPP-32 phosphorylation at Thr 34 both in dorsal striatum and nucleus accumbens. Time-course and dose-response experiments indicate that DARPP-32 phosphorylation is maximal 30 min following administration of 10mg/kg of THC. The THC-mediated increase in DARPP-32 phosphorylation is reduced by administration of the CB1 receptor antagonist, SR141716A (3mg/kg). A similar attenuation of the effect of THC is also exerted by suppression of cAMP signaling achieved using the dopamine D1 receptor antagonist, SCH23390 (0.125 mg/kg), or the adenosine A2A receptor antagonist, KW6002 (3mg/kg). These results indicate that, in the striatum, THC promotes PKA-dependent phosphorylation of DARPP-32 in striatal medium spiny neurons expressing dopamine D1 and adenosine A2A receptors.     

The effect of δ 9-tetrahydrocannabinol on sleep

Five volunteers slept 8 to 15 consecutive nights in the laboratory with electroencephalogram, chin electromyogram, and eye movements monitored by the method originated by Dement and Kleitman. ⁹-tetrahydrocannabinol (THC), 20 mg administered at bedtime decreased the amount of time spent in the REM or paradoxical phase of sleep. Abrupt withdrawal of THC after 4 to 6 consecutive nights of use produced a mild insomnia characterized by difficulty in falling and staying asleep but did not produce a marked REM rebound.     

The psychoactive plant cannabinoid, Delta9-tetrahydrocannabinol, is antagonized by Delta8- and Delta9-tetrahydrocannabivarin in mice in vivo

BACKGROUND AND PURPOSE: To follow up in vitro evidence that Delta(9)-tetrahydrocannabivarin extracted from cannabis (eDelta(9)-THCV) is a CB(1) receptor antagonist by establishing whether synthetic Delta(9)-tetrahydrocannabivarin (O-4394) and Delta(8)-tetrahydrocannabivarin (O-4395) behave as CB(1) antagonists in vivo. EXPERIMENTAL APPROACH: O-4394 and O-4395 were compared with eDelta(9)-THCV as displacers of [(3)H]-CP55940 from specific CB(1) binding sites on mouse brain membranes and as antagonists of CP55940 in [(35)S]GTPgammaS binding assays performed with mouse brain membranes and of R-(+)-WIN55212 in mouse isolated vasa deferentia. Their ability to antagonize in vivo effects of 3 or 10 mg kg(-1) (i.v.) Delta(9)-tetrahydrocannabinol in mice was then investigated. KEY RESULTS: O-4394 and O-4395 exhibited similar potencies to eDelta(9)-THCV as displacers of [(3)H]-CP55940 (K (i)=46.6 and 64.4 nM, respectively) and as antagonists of CP55940 in the [(35)S]GTPgammaS binding assay (apparent K (B)=82.1 and 125.9 nM, respectively) and R-(+)-WIN55212 in the vas deferens (apparent K (B)=4.8 and 3.9 nM respectively). At i.v. doses of 0.1, 0.3, 1.0 and/or 3 mg kg(-1) O-4394 and O-4395 attenuated Delta(9)-tetrahydrocannabinol-induced anti-nociception (tail-flick test) and hypothermia (rectal temperature). O-4395 but not O-4394 also antagonized Delta(9)-tetrahydrocannabinol-induced ring immobility. By themselves, O-4395 and O-4394 induced ring immobility at 3 or 10 mg kg(-1) (i.v.) and antinociception at doses above 10 mg kg(-1) (i.v.). O-4395 also induced hypothermia at 3 mg kg(-1) (i.v.) and above. CONCLUSIONS AND IMPLICATIONS: O-4394 and O-4395 exhibit similar in vitro potencies to eDelta(9)-THCV as CB(1) receptor ligands and as antagonists of cannabinoid receptor agonists and can antagonize Delta(9)-tetrahydrocannabinol in vivo.     

Cellular mechanisms underlying the anxiolytic effect of low doses of peripheral Delta9-tetrahydrocannabinol in rats.

We investigated the effect of low doses of intraperitoneal Delta(9)-tetrahydrocannabinol (THC) on anxiety behavior in rats using the elevated plus maze (EPM). An anxiolytic effect was obtained in a range of doses between 0.075 and 1.5 mg/kg, the 0.75 dose being the most effective. Pretreatment with the CB1 receptor antagonist AM251 fully reversed THC's effect, suggesting CB1 receptors were involved. In order to elucidate the neuroanatomical substrates underlying the effect of the maximal effective dose of THC, we investigated cFos expression in anxiety-related brain regions (prefrontal cortex, nucleus accumbens, amygdala, and hippocampus) of rats exposed to the EPM. THC significantly lowered the amount of cFos in prefrontal cortex and amygdala without affecting the other cerebral areas. As there is increasing evidence that CREB function regulates anxiety-like behavior in rats, the second biochemical parameter we measured was phosphorylated CREB in the same brain areas. Rats treated with THC showed a significant increase in CREB activation in the prefrontal cortex and hippocampus. In the prefrontal cortex this increased activation was linked to an increase in ERK activation, whereas in the hippocampus there was a drop in the activity of CAMKII, a kinase with inhibitory effect on CREB activation. All these effects were reversed by AM251 pretreatment, suggesting that stimulation of CB1 receptors is fundamental for triggering the biochemical events. Our results suggest that the stimulation of these receptors in the prefrontal cortex, amygdala, and hippocampus with the subsequent activation of different signaling pathways is the first event underlying the effects of cannabinoids on anxious states.     

Delta 9-tetrahydrocannabinol alters cerebral metabolism in a biphasic, dose-dependent manner in rat brain.

delta 9-tetrahydrocannabinol (THC)-induced alterations in limbic and neocortical function are associated with deficiencies in short-term memory and recall. The 2-deoxy-D-glucose (2DG) autoradiographic method was used to examine the effect of acute THC administration (0, 0.2, 0.5, 2.0, 10.0 mg/kg) on regional brain metabolism in limbic and cortical brain regions of male rats. THC altered 2DG uptake in a biphasic, dose-dependent manner in most limbic and cortical structures, however most diencephalic and brainstem structures examined were unaffected. The 0.2 mg/kg THC dose significantly increased 2DG uptake relative to vehicle treatment in all cortical and selected limbic regions, whereas the 2.0 and 10.0 mg/kg THC doses decreased 2DG uptake in most of these regions. Certain limbic regions, particularly the hippocampus, are more sensitive to THC suggesting a selective regional action of the drug at lower doses. The incidence of enhanced metabolic activity in limbic and cortical regions is consistent with the occurrence of high density cannabinoid receptors in these regions.     

Chronic Delta9-tetrahydrocannabinol treatment produces antinociceptive tolerance in mice without altering protein kinase A activity in mouse brain and spinal cord

The present study investigated the effect of different levels of Delta-9-tetrahydrocannabinol (Delta(9)-THC) antinociceptive tolerance on Protein Kinase A (PKA) activity in mouse brain and spinal cord. To strengthen this investigation, a positive control was developed to demonstrate the assay utilized in this study was sensitive enough to detect an increase in PKA activity in the anatomical regions utilized in this study. The membrane-permeant and phosphodiesterase-resistant cAMP analog 8-Bromoadenosine-3',5'-cyclic monophosphorothioate, Sp-isomer (Sp-8-Br-cAMPS) was utilized for the development of this positive control and this compound produced an increase in PKA activity in several mouse brain regions (i.c.v.) and lumbar spinal cord (i.t.) following its administration. Models were then developed in which mice expressed either a 13-fold or 49-fold level of Delta(9)-THC antinociceptive tolerance following chronic treatment with 10mg/kg Delta(9)-THC or 80mg/kg Delta(9)-THC for 6.5 days. Basal and total cytosolic and particulate PKA activities were measured directly in homogenates from the striatum, hippocampus, cerebellum, cortex and lumbar spinal cord. Results from this study indicate that chronic exposure to Delta(9)-THC does not produce an increase in PKA activity in these mouse brain regions or spinal cord. Future work is needed to determine the role of PKA in cannabinoid tolerance in mice.