Pharmacological characterization and autoradiographic localization of dopamine receptors in the rat adrenal medulla

The pharmacological profile and the anatomical localization of dopamine D₁-like and D₂-like receptors were studied in sections of rat adrenal medulla, with radioligand binding and autoradiographic techniques, respectively. [³H]([R]-(+)-chloro-2,3,4,5-tetrahydro-5-phenyl-1H-3benzazepin-al hemimaleate) (SCH 23390) was used as a ligand for dopamine D₁-like receptors and [³H]spiperone was used as a ligand for dopamine D₂-like receptors. Radioligand binding and light microscope autoradiography did not show specific [³H]SCH 23390 binding in sections of rat adrenal medulla. This suggests that rat adrenal medulla does not express dopamine D₁-like receptors. [³H]Spiperone was specifically bound to sections of rat adrenal medulla. The binding was time-, temperature- and concentration-dependent, with a dissociation constant (K_d) of 1.05 nM and a maximum density of binding sites (B_max) of 100.2 ± 3.8 fmol/mg tissue. The pharmacological profile of [³H]spiperone binding to rat adrenal medulla was similar to that displayed by neostriatum, which is known to express dopamine D₂ receptors. Light microscope autoradiography showed the accumulation of specifically bound [³H]spiperone as silver grains within sections of adrenal medulla. Silver grains were found primarily over the cellular membrane of chromaffin cells. The above data indicate that chromaffin cells of the rat adrenal medulla express dopamine receptors belonging to the dopamine D₂ receptor subtype. These receptors are probably involved in the modulation of catecholamine release from chromaffin cells, as documented by functional studies.     

Endocannabinoids and food consumption: comparisons with benzodiazepine and opioid palatability-dependent appetite

The endocannabinoid system consists of several endogenous lipids, including anandamide and 2-arachidonoyl-glycerol (2-AG), and constitute a retrograde signalling system, which modulates neurotransmitter release and synaptic plasticity. Specific brain-type cannabinoid receptors (CB₁) are widely distributed in the central nervous system, and are localized presynaptically. Mounting evidence, reviewed here, indicates that cannabinoids can act to increase food consumption, and cannabinoid CB₁ receptor antagonists/inverse agonists reduce food intake and suppress operant responding for food rewards. Hence, endocannabinoids provide the first example of a retrograde signalling system, which is strongly implicated in the control of food intake. Benzodiazepine and opioid palatability-dependent appetite are well-established processes supported by several sources of convergent evidence; they provide pharmacological benchmarks against which to evaluate the endocannabinoids. To date, evidence that endocannabinoids specifically modulate palatability as an affective evaluative process is insufficient and not compelling. Endocannabinoids may have important clinical utility in the treatment of human obesity and forms of eating disorders.     

The endocannabinoid-CB(1) receptor system in pre- and postnatal life

Recent research suggests that the endogenous cannabinoids (“endocannabinoids”) and their cannabinoid receptors have a major influence during pre- and postnatal development. First, high levels of the endocannaboid anandamide and cannabinoid receptors are present in the preimplantation embryo and in the uterus, while a temporary reduction of anandamide levels is essential for embryonal implantation. In women accordingly, an inverse association has been reported between fatty acid amide hydrolase (the anandamide degrading enzyme) in human lymphocytes and miscarriage. Second, CB₁ receptors display a transient presence in white matter areas of the pre- and postnatal nervous system, suggesting a role for CB₁ receptors in brain development. Third, endocannabinoids have been detected in maternal milk and activation of CB₁ receptors appears to be critical for milk sucking by newborn mice, apparently activating oral–motor musculature. Fourth, anandamide has neuroprotectant properties in the developing postnatal brain. Finally, prenatal exposure to the active constituent of marihuana (Δ⁹-tetrahydrocannabinol) or to anandamide affects prefrontal cortical functions, memory and motor and addictive behaviors, suggesting a role for the endocannabinoid CB₁ receptor system in the brain structures which control these functions. Further observations suggest that children may be less prone to psychoactive side effects of Δ⁹-tetrahydrocannabinol or endocannabinoids than adults. The medical implications of these novel developments are far reaching and suggest a promising future for cannabinoids in pediatric medicine for conditions including “non-organic failure-to-thrive” and cystic fibrosis.     

Dopamine D1 vs D5 receptor-dependent induction of seizures in relation to DARPP-32, ERK1/2 and GluR1-AMPA signalling

Recent reports have shown that the selective dopamine D₁-like agonist SKF 83822 [which stimulates adenylate cyclase, but not phospholipase C] induces prominent behavioral seizures in mice, whereas its benzazepine congener SKF 83959 [which stimulates phospholipase C, but not adenylate cyclase] does not. To investigate the relative involvement of D₁ vs D₅ receptors in mediating seizures, ethological behavioral topography and cortical EEGs were recorded in D₁, D₅ and DARPP-32 knockout mice in response to a convulsant dose of SKF 83822. SKF 83822-induced behavioral and EEG seizures were gene dose-dependently abolished in D₁ knockouts. In both heterozygous and homozygous D₅ knockouts, the latency to first seizure was significantly increased and total EEG seizures were reduced relative to wild-types. The majority (60%) of homozygous DARPP-32 knockouts did not have seizures; of those having seizures (40%), the latency to first seizure was significantly increased and the number of high amplitude, high frequency polyspike EEG events was reduced. In addition, immunoblotting was performed to investigate downstream intracellular signalling mechanisms at D₁-like receptors following challenge with SKF 83822 and SKF 83959. In wild-types administered SKF 83822, levels of ERK1/2 and GluR1 AMPA receptor phosphorylation increased two-fold in both the striatum and hippocampus; in striatal slices DARPP-32 phosphorylation at Thr34 increased five-fold relative to vehicle-treated controls. These findings indicate that D₁, and to a lesser extent D₅, receptor coupling to DARPP-32, ERK1/2 and glutamatergic signalling is involved in mediating the convulsant effects of SKF 83822.     

Long-term effects of postnatal amphetamine treatment on striatal protein kinase A activity, dopamine D(1)-like and D(2)-like binding sites, and dopamine content

The purpose of the present study was to determine whether exposure to amphetamine during the preweanling period would alter dopaminergic functioning in the dorsal striatum of adult rats. In three experiments, we assessed the effects of repeated amphetamine treatment on striatal protein kinase A (PKA) activity, dopamine (DA) D₁-like and D₂-like binding sites, and DA content. Rats were pretreated with saline or amphetamine (2.5 mg/kg, ip) for 7 consecutive days starting on postnatal day (PD) 11. At PD 90, rats were killed and their dorsal striata (i.e., caudate–putamen) were removed and frozen until time of assay. Amphetamine pretreatment produced long-term reductions in both striatal PKA activity and DA content. Early amphetamine exposure also resulted in an upregulation of D₂-like binding sites, while leaving D₁-like binding sites unaffected. It is likely that the upregulation of D₂-like binding sites was stimulated by the persistent decline in striatal DA levels. Although speculative, it is possible that excess striatal D₂-like receptors were responsible for inhibiting PKA activity through actions on the cAMP signal transduction pathway. The behavioral relevance of these amphetamine-induced neurochemical changes has not yet be determined.     

Discriminative stimulus effects of (-)-ephedrine in rats: analysis with catecholamine transporter and receptor ligands

A drug discrimination procedure was used to examine the neuropharmacology of (−)-ephedrine (5 mg/kg), a sympathomimetic amine found in a variety of dietary supplements. (−)-Ephedrine has caused concern because of its use as a precursor in the manufacture of street drugs (e.g. methamphetamine) and its potential for abuse and toxicity. In the present study, the catecholamine reuptake inhibitors mazindol and nomifensine, the norepinephrine (NE) reuptake inhibitor desipramine, and the dopamine D₂-like (e.g. D₂, D₃ and D₄) agonist quinpirole substituted for (−)-ephedrine (80% (−)-ephedrine-lever responding). The NE reuptake inhibitor nisoxetine, the D₁-like (e.g. D₁ and D₅) agonists (±)-SKF 38393 and SKF 82958, and the mixed D₁-/D₂-like agonist apomorphine occasioned intermediate levels of responding (50–79% (−)-ephedrine-lever responding). The (−)-ephedrine cue was antagonized by the D₁-like antagonist SCH 23390 and the ₁-adrenoceptor antagonist prazosin as well as the D₂-like antagonists (−)-eticlopride and haloperidol, although only at doses that disrupted responding in some rats. The discriminative stimulus effects of a small dose of (−)-ephedrine (1.25 mg/kg) were enhanced by the ₂-adrenoceptor antagonist idazoxan and to a lesser extent by the β-adrenoceptor antagonist (−)-propranolol. However, the ₂-adrenoceptor agonist clonidine (0.04 mg/kg) did not attenuate the (−)-ephedrine stimulus. These results suggest that D₁-, D₂-like, and ₁-adrenergic receptors mediate the discriminative stimulus effects of (−)-ephedrine. Substitution of desipramine for (−)-ephedrine and not for some other stimulants suggests that NE transmission is a prominent feature of the (−)-ephedrine discriminative stimulus, and that NE underlies therapeutic and abuse-related effects of (−)-ephedrine that diverge from those of other stimulants.     

Evaluation of the role of nicotinic acetylcholine receptor subtypes and cannabinoid system in the discriminative stimulus effects of nicotine in rats

Male Wistar rats were trained to discriminate (−)-nicotine (0.4 mg/kg) from saline under a two-lever, fixed-ratio 10 schedule of water reinforcement. During test sessions the following drugs were coadministered with saline (substitution studies) or nicotine (0.025–0.4 mg/kg; combination studies): the ₄β₂ nicotinic acetylcholine receptor subtype antagonist dihydro-β-erythroidine (DHβE), the non-selective nicotinic acetylcholine receptor subtype antagonist mecamylamine, the ₇ nicotinic acetylcholine receptor subtype antagonist methyllycaconitine (MLA), the ₄β₂ nicotinic acetylcholine receptor subtype agonist 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine (5-IA), the cannabinoid CB₁ receptor antagonist/partial agonist rimonabant, the cannabinoid CB₂ receptor antagonist N-[(1S)-endo-1,3,3-trimethylbicyclo-[2.2.1]heptan-2-yl]5-(4-chloro-3-methyl-phenyl)-1-(4-methybenzyl)pyrazole-3-carboxamide (SR 144528), the cannabinoid CB_1/2 receptor agonists (−)-cis-3-[2-hydroxy-4-(1,1-dimethylheptyl)-phenyl]-trans-4-(3-hydroxy-propyl)cyclohexanol (CP 55,940) or R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]-pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-(1-naphthalenyl)-methanone mesylate (WIN 55,212-2), the endogenous cannabinoid agonist and non-competitive ₇ nicotinic acetylcholine receptor subtype antagonist anandamide, the anandamide uptake and fatty acid amide hydrolase inhibitor N-(4-hydroxyphenyl)-5Z,8Z,11Z,14Z-eicosatetraenamide (AM-404), the fatty acid amide hydrolase inhibitor cyclohexylcarbamic acid 3′-carbamoyl-biphenyl-3-yl ester (URB 597), AM-404 + anandamide or URB 597 + anandamide. 5-IA (0.01 mg/kg) fully substituted for nicotine, while other drugs were inactive. In combination studies, DHβE and mecamylamine dose-dependently attenuated the discriminative stimulus effects of nicotine and the full substitution of 5-IA, while MLA, rimonabant, SR 144528, CP 55,940, WIN 55,212-2, and URB 597 did not alter the nicotine cue. Pretreatment with AM-404 + anandamide or URB 597 + anandamide weakly enhanced nicotine-lever responding. Our pharmacological analyses demonstrates that the expression of nicotine discrimination is under the control of nicotinic acetylcholine receptor subtypes composed of ₄β₂ (but not of ₇) subunits. Furthermore, we excluded the involvement of either cannabinoid CB₁ and CB₂ receptors or increases in the endocannabinoid tone in the nicotine discrimination.     

Hypolocomotor effects in rats of capsaicin and two long chain capsaicin homologues

Capsaicin and its analogue N-arachidonoyl-vanillyl-amine (arvanil) are agonists of vanilloid VR₁ receptors, and suppress spontaneous activity in mice through an unknown mechanism. Here, we tested in rats the effect on motor behavior of: (1) capsaicin; (2) N-linoleoyl-vanillyl-amine (livanil) and N--linolenoyl-vanillyl-amine (linvanil), which, unlike arvanil, have very little affinity for cannabinoid CB₁ receptors; and (3) the endocannabinoid anandamide (N-arachidonoyl-ethanolamine), which is a full agonist at both cannabinoid CB₁ and vanilloid VR₁ receptors. All compounds, administered i.p., dose-dependently (0.1–10 mg/kg) inhibited ambulation and stereotypic behavior and increased inactivity in the open field test. The rank of potency observed in vivo (livanil>capsaicin>linvanil>anandamide) bore little resemblance with the relative potencies in a functional assay for rat vanilloid VR₁ receptors (livanil=linvanil>capsaicin>anandamide) and even less with the relative affinities in rat CB₁ receptor binding assays (anandamide>livanil>linvanil>capsaicin). The vanilloid VR₁ receptor antagonist capsazepine (10 mg/kg, i.p.) blocked the effect of capsaicin but not of livanil or anandamide, whereas the CB₁ receptor antagonist (N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide.HCl (SR141716A, 3 mg/kg, i.p.) antagonized the actions of the CB₁ receptor agonist Δ⁹-tetrahydrocannabinol, but not of livanil, anandamide or capsaicin. Anandamide occluded the effects of livanil on locomotion, possibly suggestive of a common mechanism of action for the two compounds. Finally, stimulation with capsaicin of cells expressing rat vanilloid VR₁ receptors led to anandamide formation. These data suggest that motor behavior can be suppressed by the activation of: (1) vanilloid receptors, possibly via the intermediacy of anandamide; or (2) capsazepine- and SR141716A-insensitive sites of action for anandamide, livanil and linvanil, possibly the same that were previously suggested to mediate arvanil hypokinetic effects in mice.     

Inhibition of fatty-acid amide hydrolase enhances cannabinoid stress-induced analgesia: sites of action in the dorsolateral periaqueductal gray and rostral ventromedial medulla

Recent research in our laboratory has demonstrated that stress activates an endogenous cannabinoid mechanism that suppresses sensitivity to pain [Nature 435 (2005) 1108]. In this work, CB₁ antagonists administered systemically blocked stress-induced analgesia induced by brief, continuous foot-shock. The present studies were conducted to examine the role of cannabinoid CB₁ receptors in the brainstem rostral ventromedial medulla (RVM) and midbrain dorsolateral periaqueductal gray (PAG) in cannabinoid stress-induced analgesia (SIA). Pharmacological blockade of vanilloid TRPV1 receptors with capsazepine, administered systemically, did not alter cannabinoid SIA, suggesting that cannabinoid SIA was not dependent upon TRPV1. Microinjection of the competitive CB₁ antagonist rimonabant (SR141716A) into either the RVM or dorsolateral PAG suppressed stress antinociception in this model. Rimonabant was maximally effective following microinjection into the dorsolateral PAG. The fatty-acid amide hydrolase (FAAH) inhibitor arachidonoyl serotonin (AA-5-HT) was subsequently used to block hydrolysis of endocannabinoids and enhance SIA. Systemic and site-specific injections of AA-5-HT into either the dorsolateral PAG or RVM induced CB₁-mediated enhancements of SIA. Palmitoyltrifluoromethylketone, a potent inhibitor of FAAH and phospholipase A2 activity, administered systemically, exerted similar effects. In all conditions, the antinociceptive effects of each FAAH inhibitor were completely blocked by coadministration of the CB₁ antagonist rimonabant. The present results provide evidence that a descending cannabinergic neural system is activated by environmental stressors to modulate pain sensitivity in a CB₁-dependent manner.     

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.     

Polymorphisms in genes involved in neurotransmission in relation to smoking

Smoking behavior is influenced by both genetic and environmental factors. The genetic contribution to smoking behavior is at least as great as its contribution to alcoholism. Much progress has been achieved in genomic research related to cigarette-smoking within recent years. Linkage studies indicate that there are several loci linked to smoking, and candidate genes that are related to neurotransmission have been examined. Possible associated genes include cytochrome P450 subfamily polypeptide 6 (CYP2A6), dopamine D₁, D₂, and D₄ receptors, dopamine transporter, and serotonin transporter genes. There are other important candidate genes but studies evaluating the link with smoking have not been reported. These include genes encoding the dopamine D₃ and D₅ receptors, serotonin receptors, tyrosine hydroxylase, trytophan 2,3-dioxygenase, opioid receptors, and cannabinoid receptors. Since smoking-related factors are extremely complex, studies of diverse populations and of many aspects of smoking behavior including initiation, maintenance, cessation, relapse, and influence of environmental factors are needed to identify smoking-associated genes. We now review genetic polymorphisms reported to be involved in neurotransmission in relation to smoking.     

Topographical evaluation of the phenotype of spontaneous behaviour in mice with targeted gene deletion of the D1A dopamine receptor: paradoxical elevation of grooming syntax

The phenotype of spontaneous behaviour in mice with targeted gene deletion of the D_1A dopamine receptor was investigated topographically. Via direct visual observation, individual elements of behaviour were resolved and quantified using an ethologically-based, rapid time-sampling behavioural check-list procedure. Relative to wildtypes (D_1A ^+/+), D_1A-null (^−/−) mice evidenced over initial exploration significant reductions in rearing free, sifting and chewing, but significant increases in locomotion, grooming and intense grooming. Sniffing and rearing to a wall habituated less readily in D_1A-null mice such that these behaviours occurred subsequently to significant excess: increases in locomotion were persistent. The ethogram of spontaneous behaviour in D_1A-null mice was characterised by neither ‘hypoactivity’ or ‘hyperactivity’ but, rather, by prominent topographical shifts between individual elements of behaviour that could not be encapsulated by either term. Given the substantial body of evidence that grooming and particularly intense grooming constitute the most widely accepted behavioural index of D₁-like receptor function, the elevation of such behaviour in D_1A-null mice was paradoxical; it may reflect (over)compensatory processes subsequent to developmental absence of D_1A receptors and/or the involvement of a D₁-like receptor other than/additional to the D_1A subtype.     

Effects of stimulation of putative dopamine autoreceptors on electroencephalographic power spectrum in comparison with effects produced by blockade of postsynaptic dopamine receptors in rats

Alterations in cortical EEG activity in male rats produced by putative agonists at dopamine (DA) autoreceptors and by antagonists at postsynaptic DA receptors were compared in order to study, whether an impairment in dopaminergic neurotransmission via two different mechanisms might result in similar or different effects. Simultaneously to the EEG recordings, gross behaviour was observed. Putative agonists at DA autoreceptors (apomorphine 0.05 mg/kg, quinpirole 0.05 mg/kg, or talipexole 0.02 mg/kg s.c.) produced increases in the power iin all of the frequency bands, except beta-2, with the most pronounced increase in the delta band. These EEG alterations were accompanied by hypokinesia, ptosis and yawning. In contrast, antagonists at DA receptors (haloperidol 0.1 mg/kg i.p., D₂ blocker) or SCH 23390 (0.2 mg/kg i.p., D₁ blocker) led to little increases in the delta band, but more pronounced increases in the alpha-2 band. Behavioural signs were hypokinesia, but little ptosis and yawning. The combination of both blockers produced, in addition, strong increases in the delta band and behavioural signs of ptosis and yawning. These results suggest that activation of putative dopamine autoreceptors produced EEG patterns and behavioural patterns different from those produced by blockade of either D₁ or D₂ postsynaptic dopamine receptors. In contrast, the effects following a stimulation of putative DA autoreceptors, which are expected to decrease the release of the agonist and its action at postsynaptic D₁ and D₂ receptors, were very similar to those found after a combined blockade of both types of postsynaptic dopamine receptors.     

Involvement of serotonergic and dopaminergic mechanisms in hyperthermia induced by a serotonin-releasing drug, p-chloroamphetamine in mice.

Serotonergic and dopaminergic involvement in hyperthermia induced by a serotonin (5-hydroxytryptamine, 5-HT)-releasing drug, p-chloroamphetamine, was investigated in mice. Neither the 5-HT transporter inhibitor fluoxetine nor the 5-HT depleter p-chlorophenylalanine affected p-chloroamphetamine-induced hyperthermia. The dopamine depleter -methyl-p-tyrosine significantly reduced p-chloroamphetamine-induced hyperthermia. The dopamine D₁ receptor antagonist 7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH 23390) antagonized p-chloroamphetamine-induced hyperthermia, although the dopamine D₂ receptor antagonist sulpiride was without effect. These results indicate that p-chloroamphetamine-induced hyperthermia in mice is mediated by dopamine release followed by activation of the dopamine D₁ receptor.     

Pharmacological enhancement of cannabinoid CB1 receptor activity elicits an antidepressant-like response in the rat forced swim test

These experiments aimed to assess whether enhanced activity at the cannabinoid CB₁ receptor elicits antidepressant-like effects. To examine this we administered 1 and 5 mg/kg doses of the endocannabinoid uptake inhibitor AM404; 5 and 25 μg/kg doses of HU-210, a potent CB₁ receptor agonist; 1, 2.5 and 5 mg/kg of oleamide, which elicits cannabinoidergic actions; 1 and 5 mg/kg doses of AM 251, a selective CB₁ receptor antagonist, as well as 10 mg/kg desipramine (a positive antidepressant control) and measured the duration of immobility, during a 5-min test session of the rat Porsolt forced swim test. Results demonstrated that administration of desipramine reduced immobility duration by about 50% and that all of AM404, oleamide and HU-210 administration induced comparable decreases in immobility that were blocked by pretreatment with AM 251. Administration of the antagonist AM 251 alone had no effect on immobility at either dose. These data suggest that enhancement of CB₁ receptor signaling results in antidepressant effects in the forced swim test similar to that seen following conventional antidepressant administration.     

Evidence for regulation of body temperature in rats by dopamine D2 receptor and possible influence of D1 but not D3 and D4 receptors

The dopamine D₃ receptor agonist PD 128907 decreased body temperature in the rat. The selective dopamine D₃ and D₄ receptor antagonists, A-437203 and L-745,870, respectively, did not prevent this effect. In contrast, PD 128907-induced hypothermia was antagonized by SCH 23390, a selective D₁ receptor antagonist, and by L-741,626, a selective D₂ receptor antagonist. Moreover, the selective D₂ receptor agonist trihydroxy-N-n-propylnoraporphine (TNPA) elicited a robust hypothermia which was prevented by pretreatment with L-741,626 but not by A-437203. In agreement with previous data obtained in D₃ knock-out mice, present results suggest that D₂ rather than D₃ receptors mediate dopamine receptor agonist-induced hypothermia in rats. In addition, it appears that both D₁ and D₂ receptors may be involved in a cooperative manner.     

Involvement of cannabinoid CB(2) receptor-mediated response and efficacy of cannabinoid CB(2) receptor inverse agonist, JTE-907, in cutaneous inflammation in mice

Involvement of cannabinoid CB₂ receptor and effect of cannabinoid CB₂ receptor antagonist/inverse agonists on cutaneous inflammation were investigated. Mice ears topically exposed to an ether-linked analogue of 2-arachidonoylglycerol (2-AG-E) or selective cannabinoid CB₂ receptor agonist, {4-[4-(1,1-dimethylheptyl)-2,6-dimethoxy-phenyl]-6.6-dimethyl-bicyclo[3.1.1]hept-2-en-2-yl}-methanol (HU-308), had early and late ear swelling (0–24 h and 1–8 days after exposure, respectively). Both types of responses induced by 2-AG-E were significantly suppressed by oral administration of cannabinoid CB₂ receptor antagonist/inverse agonists, [N-(benzo[1,3]dioxol-5-ylmethyl)-7-methoxy-2-oxo-8-pentyloxy-1,2-dihydroquinoline-3-carboxamide] (JTE-907) and {N-[(1S)-endo-1,3,3-trimethylbicyclo[2.2.1]heptan-2yl]5-(4-chloro-3-methyl-phenyl)-1-(4-methylbenzyl)pyrazole-3-carboxamide}} (SR144528). In contrast, JTE-907 did not affect arachidonic acid-induced swelling. Orally administered JTE-907 (0.1–10 mg/kg) and SR144528 (1 mg/kg) also produced significant inhibition of dinitrofluorobenzene-induced ear swelling, with increased cannabinoid CB₂ receptor mRNA expression observed in the inflamed ear. These results suggest that cannabinoid CB₂ receptor is partially involved in local inflammatory responses and cannabinoid CB₂ receptor antagonist/inverse agonist has beneficial effects on ear swelling.     

Role of brain thromboxane A2 in the release of noradrenaline and adrenaline from adrenal medulla in rats

The thrifty genotype hypothesis postulates that the genetically determined ability to grow obese and insulin resistant in times of food abundance confers a survival advantage in times of famine. Obviously, this ability poses a major health threat in modern times, where food is always available in large quantities. In the last 10–15 years, many genes encoding pathways that orchestrate energy balance and fuel flux have been discovered. This paper summarizes the evidence that diminished dopaminergic tone in hypothalamic nuclei contributes to the “thrifty” genotype/phenotype. Reduced dopaminergic neurotransmission in the suprachiasmatic nucleus of seasonally obese animals appears to drive noradrenalin and NPY mediated transmissions in other nuclei to induce the obesity syndrome at the appropriate time of year. Treatment with dopamine D₂ receptor agonists can fully reverse the metabolic syndrome in these animals. Similar mechanisms are operative in non-seasonal obese animal models. In man, treatment with dopamine D₂ receptor antagonists induces obesity and type 2 diabetes mellitus, whereas dopamine D₂ receptor activation ameliorates the metabolic profile in obese nondiabetic and diabetic humans. Various loss of function mutations of the dopamine D₂ receptor gene are associated with overweight in humans. In concert, the data support the notion that diminution of dopaminergic (dopamine D₂ receptor mediated) transmission in relevant hypothalamic nuclei sets the stage for efficient partitioning of ingested nutrients to contribute to a phenotype that is not so thrifty anymore.     

Bidirectional cannabinoid modulation of social behavior in adolescent rats

Rationale Marijuana use in adolescents is a highly social activity, and interacting endocannabinoid and opioid systems may modulate social reward. However, cannabinoid exposure has been reported to reduce social behavior. Objectives The aim of this study was to elucidate the mechanisms underlying the paradoxical relationship between cannabinoid exposure and sociability. Materials and methods We investigated the effect of cannabinoid agonists with a different mechanism of action on social play behavior in adolescent rats. In addition, we examined whether endocannabinoid neurotransmission interacts with opioid and dopaminergic neurotransmission in the modulation of social play behavior. Results The direct CB₁ cannabinoid receptor agonist WIN55,212-2 reduced social play. However, the indirect cannabinoid agonist URB597, which inhibits the hydrolysis of the endocannabinoid anandamide, enhanced social play. This effect of URB597 depended upon stimulation of opioid and dopamine receptors. The well-known stimulatory effect of morphine on social play was attenuated by the CB₁ cannabinoid receptor antagonist SR141716A, but independent of dopamine receptor stimulation. Combined treatment with ineffective doses of URB597 and morphine increased social play. Conclusions Cannabinoid neurotransmission can both enhance and inhibit social interaction in adolescent rats depending on how the endocannabinoid system is stimulated. Activation of cannabinoid receptors throughout the brain, which occurs during cannabis use, inhibits sociability. In contrast, on-demand release of endocannabinoids facilitates social interaction, which is magnified by indirect cannabinoid agonists through an interaction with opioid and dopaminergic neurotransmission. These results shed light on the paradoxical relationship between cannabis exposure and sociability and suggest that endocannabinoid degradation inhibitors hold promise for the treatment of social dysfunctions.     

Effect of cerebral ischemia on dopamine receptors and uptake sites in the gerbil hippocampus

Dopamine D₁ and D₂ receptors and uptake sites were studied in the gerbil hippocampus, parietal cortex and thalamus 1 h to 7 days after 10 min of cerebral ischemia using the occlusion of bilateral common carotid arteries. [³H]SCH23390 ([N-methyl-³H]R[+]-8-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-7-ol-benzazepine) and [³H]mazindol were used as markers of dopamine D₁ receptors and uptake sites, respectively. [³H]Nemonapride was used to label dopamine D₂ receptors. No obvious alteration in [³H]SCH23390 and [³H]mazindol binding was found in the hippocampus up to 48 h after ischemia. These bindings showed a significant reduction in the hippocampus after 7 days of recirculation. In contrast, [³H]nemonapride binding was unaffected in the hippocampus during the recirculation periods. The parietal cortex and thalamus also exhibited no significant changes in [³H]SCH23390, [³H]nemonapride and [³H]mazindol binding after ischemia. MAP2 (microtubule-associated protein 2) immunoreactivity was unchanged in all regions up to 48 h after ischemia. Thereafter, a marked loss of MAP2-immunoreactive neurons was observed in the hippocampal CA1 and CA3 neurons 7 days after recirculation. These findings were consistent with histological observations with cresyl violet staining. Our results demonstrate that dopamine D₁ receptors and dopamine uptake sites in the hippocampus are susceptible to cerebral ischemia, whereas dopamine D₂ receptors in this region are particularly resistant. Furthermore, these findings suggest that dopamine transmission may not be major factor in producing ischemic hippocampal damage.