The epistatic interaction between the dopamine D3 receptor and dysbindin-1 modulates higher-order cognitive functions in mice and humans.

The dopamine D2 and D3 receptors are implicated in schizophrenia and its pharmacological treatments. These receptors undergo intracellular trafficking processes that are modulated by dysbindin-1 (Dys). Indeed, Dys variants alter cognitive responses to antipsychotic drugs through D2-mediated mechanisms. However, the mechanism by which Dys might selectively interfere with the D3 receptor subtype is unknown. Here, we revealed an interaction between functional genetic variants altering Dys and D3. Specifically, both in patients with schizophrenia and in genetically modified mice, concomitant reduction in D3 and Dys functionality was associated with improved executive and working memory abilities. This D3/Dys interaction produced a D2/D3 imbalance favoring increased D2 signaling in the prefrontal cortex (PFC) but not in the striatum. No epistatic effects on the clinical positive and negative syndrome scale (PANSS) scores were evident, while only marginal effects on sensorimotor gating, locomotor functions, and social behavior were observed in mice. This genetic interaction between D3 and Dys suggests the D2/D3 imbalance in the PFC as a target for patient stratification and procognitive treatments in schizophrenia.

Remote memories are enhanced by COMT activity through dysregulation of the endocannabinoid system in the prefrontal cortex.

The prefrontal cortex (PFC) is a crucial hub for the flexible modulation of recent memories (executive functions) as well as for the stable organization of remote memories. Dopamine in the PFC is implicated in both these processes and genetic variants affecting its neurotransmission might control the unique balance between cognitive stability and flexibility present in each individual. Functional genetic variants in the catechol-O-methyltransferase (COMT) gene result in a different catabolism of dopamine in the PFC. However, despite the established role played by COMT genetic variation in executive functions, its impact on remote memory formation and recall is still poorly explored. Here we report that transgenic mice overexpressing the human COMT-Val gene (COMT-Val-tg) present exaggerated remote memories (>50 days) while having unaltered recent memories (<24 h). COMT selectively and reversibly modulated the recall of remote memories as silencing COMT Val overexpression starting from 30 days after the initial aversive conditioning normalized remote memories. COMT genetic overactivity produced a selective overdrive of the endocannabinoid system within the PFC, but not in the striatum and hippocampus, which was associated with enhanced remote memories. Indeed, acute pharmacological blockade of CB1 receptors was sufficient to rescue the altered remote memory recall in COMT-Val-tg mice and increased PFC dopamine levels. These results demonstrate that COMT genetic variations modulate the retrieval of remote memories through the dysregulation of the endocannabinoid system in the PFC.

Stimulation of in vivo dopamine transmission and intravenous self-administration in rats and mice by JWH-018, a Spice cannabinoid.

The synthetic cannabinoid 1-pentyl-3-(1-naphthoyl)-indole (JWH-018) has been detected in about 140 samples of a smokable herbal mixture termed "Spice". JWH-018 is a CB1 and CB2 agonist with a higher affinity than Δ9-THC. In order to investigate the neurobiological substrates of JWH-018 actions, we studied by microdialysis in freely moving rats the effect of JWH-018 on extracellular dopamine (DA) levels in the nucleus accumbens (NAc) shell and core and in the medial prefrontal cortex (mPFC). JWH-018, at the dose of 0.25 mg/kg i.p., increased DA release in the NAc shell but not in the NAc core and mPFC. Lower (0.125 mg/kg) and higher doses (0.50 mg/kg) were ineffective. These effects were blocked by CB1 receptor antagonists (SR-141716A and AM 251) and were absent in mice lacking the CB1 receptor. Ex vivo whole cell patch clamp recordings from rat ventral tegmental area (VTA) DA neurons showed that JWH-018 decreases GABAA-mediated post-synaptic currents in a dose-dependent fashion suggesting that the stimulation of DA release observed in vivo might result from disinhibition of DA neurons. In addition, on the "tetrad" paradigm for screening cannabinoid-like effects (i.e., hypothermia, analgesia, catalepsy, hypomotility), JWH-018, at doses of 1 and 3 mg/kg i.p., produced CB1 receptor-dependent behavioural effects in rats. Finally, under appropriate experimental conditions, rats (20 µg/kg/inf i.v., FR3; nose-poking) and mice (30 µg/kg/inf i.v., FR1; lever-pressing) self-administer intravenously JWH-018. In conclusion, JWH-018 shares with the active ingredient of Marijuana, Δ9-THC, CB1-dependent reinforcing and DA stimulant actions.

Evidence for a role of a dopamine/5-HT6 receptor interaction in cocaine reinforcement.

The putative 5-HT6 receptor agonist ST1936 has been shown to increase extracellular dopamine (DA) in the n.accumbens (NAc) shell and in the medial prefrontal cortex (PFCX). These observations suggest that 5-HT6 receptors modulate DA transmission in mesolimbic and mesocortical terminal DA areas. To investigate the behavioral counterpart of this interaction we studied in rats 1) the ability of ST1936 to maintain i.v. self-administration in fixed ratio (FR) and progressive ratio (PR) schedules of reinforcement; 2) the effect of 5-HT6 receptor blockade on cocaine stimulated overflow of DA in dialysates from the PFCX and from the NAc shell and on cocaine i.v. self-administration. ST1936 was i.v. self-administered at unitary doses of 0.5-1 mg/kg on an FR1 and PR schedule of reinforcement, with breaking point of about 4. Pretreatment with the 5-HT6 antagonist SB271046 reduced by about 80% responding for ST1936. SB271046 also reduced cocaine-induced increase of dialysate DA in the NAc shell but not in the PFCX and impaired i.v. cocaine self-administration. These observations indicate that ST1936 behaves as a weak reinforcer and suggest that 5-HT6 receptors play a role in cocaine reinforcement via their facilitatory interaction with DA projections to the NAc shell. This novel 5-HT/DA interaction might provide the basis for a new pharmacotherapeutic strategy of cocaine addiction.

Cannabinoid facilitation of behavioral and biochemical hedonic taste responses.

Cannabinoid receptor agonists are known to stimulate feeding in humans and animals and this effect is thought to be related to an increase in food palatability. On the other hand, highly palatable food stimulates dopamine (DA) transmission in the shell of the nucleus accumbens (NAc) and this effect undergoes one trial habituation. In order to investigate the relationship between the affective properties of tastes and the response of NAc shell DA we studied the effect of delta-9-tetrahydrocannabinol (THC) on behavioral taste reactivity to intraoral infusion of appetitive (sucrose solutions) and aversive (quinine and saturated NaCl solutions) tastes and on the response of in vivo DA transmission in the NAc shell to intraoral sucrose. Rats were implanted with intraoral cannulae and the effect of systemic administration of THC on the behavioral reactions to intraoral infusion of sucrose and of quinine or saturated NaCl solutions were scored. THC increased the hedonic reactions to sucrose but did not affect the aversive reactions to quinine and NaCl. The effects of THC were completely blocked by the CB1 receptor inverse agonist/antagonist rimonabant given at doses that do not affect taste reactivity to sucrose. In rats implanted with microdialysis probes and with intraoral cannulae, THC, made sucrose effective in raising dialysate DA in the shell of the NAc. As in the case of highly palatable food (Fonzies, sweet chocolate), the stimulatory effect of sucrose on shell DA under THC underwent one trial habituation. Altogether, these findings demonstrate that stimulation of CB1 receptors specifically increases the palatability of hedonic taste without affecting that of aversive tastes. Consistent with the ability of THC to increase sucrose palatability is the observation that under THC pretreatment sucrose acquires the ability to induce a release of DA in the shell of the NAc and this property undergoes adaptation after repeated exposure to the taste (habituation). This article is part of a Special Issue entitled 'Central Control of Food Intake'.

Caffeine and accumbens shell dopamine.

It has been reported that caffeine (1.5-30 mg/kg i.p.) as well as specific A1 (DPCPX, 8-cyclopentyl-1,3-dipropylxanthine) receptor antagonists fail to increase extracellular dopamine (DA) in the shell of the nucleus accumbens (NAc). However, it has also been reported that caffeine (10 and 30 mg/kg i.p.) and the A1 antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT) increases NAc shell DA. To clarify this issue rats were implanted with microdialysis probes at different sites in the NAc shell, in the medial prefrontal cortex (PFCX, infralimbic cortex), and at the border between those areas. Irrespective of probe placement within the NAc shell and of the use of different surgical anesthetics (chloral hydrate and ketamine), we failed to observe changes in dialysate DA after 10 and 30 mg/kg i.p. of caffeine. Similarly negative results were obtained with DPCPX and CPFPX, two potent and selective A1 receptor antagonists. A significant increase of DA was obtained after caffeine when probes were located at the border between the NAc shell and the PFCX (10 and 30 mg/kg) or in the PFCX (10 mg/kg). In view of this and of our previous report that caffeine increases dialysate DA in the medial PFCX, we conclude that the increase in dialysate DA by caffeine observed by others arises from the medial PFCX rather than from the NAc shell as a result of placement of microdialysis probes at the border between the NAc shell and the PFCX.

Fetal mouse heart in organ structure: ultrastructure.

The fetal mouse heart can be maintained in organ culture for days while it retains its capacity to beat rhythmically; however, degenerative changes eventually occur. In this study, the ultrastructure of the hearts fixed immediately after removal from the fetus without incubation often revealed some swelling of organelles,suggesting that early ischemic alterations may occur during the time required for removaland fixation. Within 2 days in organ culture, most of the cells reverted to normal, indicating that the injury was reversible. However, a few cells had undergone necrosis.An interesting finding in some cells was the presence of autophagic vacuoles containing damaged mitochondria and occasionally,myrofibrils. These vacuoles were usually surrounded by a single layered membrane, but occasionally a double membrane was present. The occurrence of these vacuoles in an otherwise normal appearing cell suggests that focal cytoplasmic injury had occurred and the damaged elements were being sequestered anddigested within the vacuoles. An alternative possibility is that these cells were adjusting their cytoplasmic contents to a new level of homostasis. In as much as this phenomenon is not often seen in adult myocytes, it may suggest that fetal cells have agreater capacity to resist injury than adult cells and thus can isolate and digest damaged components without undergoing complete necrosis.