Fibrinogen primes the microglial NLRP3 inflammasome and propagates pro-inflammatory signaling via extracellular vesicles: Implications for blood-brain barrier dysfunction.

The brain's response to acute injury is characterized by increased permeability of the blood-brain barrier (BBB) and pro-inflammatory microglia signaling, both of which have been linked to poor cognitive outcomes and neurological disease. The damaged BBB has increased leakiness, allowing serum proteins like fibrinogen into the brain, which interacts with local cells in a deleterious manner. At the same time, in response to injury, microglia demonstrate increased NLRP3 inflammasome activity and heightened release of pro-inflammatory cytokines. The relationship between increased fibrinogen uptake and microglial inflammasome signaling in the injured brain has not been well described. In this work, we investigate fibrinogen mediated NLRP3 inflammasome priming of BV-2 cells and primary adult microglia and propose a role for extracellular vesicles (EVs) as propagators of this interaction. Following exposure to fibrinogen microglia significantly upregulate transcription of IL-1ß, IL-6, NLRP3 and other pro-inflammatory cytokines which was sustained by repeated fibrinogen exposure. Inhibition of fibrinogen mediated NLRP3 signaling was achieved at the transcriptional and assembly level using cannabidiol (CBD) and the NLRP3 inhibitor MCC950, respectively. EVs released following NLRP3 priming carry IL-1ß, IL-18 mRNA and fibrinogen, propagate inflammatory signaling and can be detected in the circulation following BBB disruption in a preclinical stroke model. In conclusion, the interplay between fibrinogen extravasation, microglial NLRP3 signaling, and EV release can perpetuate chronic pro-inflammatory signaling and represents a novel method of inflammatory propagation.

The roles of cannabinoid and dopamine receptor systems in neural emotional learning circuits: implications for schizophrenia and addiction.

Cannabinoids represent one of the most widely used hallucinogenic drugs and induce profound alterations in sensory perception and emotional processing. Similarly, the dopamine (DA) neurotransmitter system is critical for the central processing of emotion and motivation. Functional disturbances in either of these neurotransmitter systems are well-established correlates of the psychopathological symptoms and behavioral manifestations observed in addiction and schizophrenia. Increasing evidence from the anatomical, pharmacological and behavioral neuroscience fields points to complex functional interactions between these receptor systems at the anatomical, pharmacological and neural systems levels. An important question relates to whether these systems act in an orchestrated manner to produce the emotional processing and sensory perception deficits underlying addiction and schizophrenia. This review describes evidence for functional neural interactions between cannabinoid and DA receptor systems and how disturbances in this neural circuitry may underlie the aberrant emotional learning and processing observed in disorders such as addiction and schizophrenia.

Blockade of mesolimbic dopamine transmission dramatically increases sensitivity to the rewarding effects of nicotine in the ventral tegmental area.

Nicotine produces rewarding and aversive motivational effects in humans and other animal species. Here, we report that the mammalian ventral tegmental area (VTA) represents a critical neural substrate for the mediation of both the rewarding and aversive properties of nicotine. We demonstrate that direct infusions of nicotine into the VTA can produce both rewarding and aversive motivational effects. While the rewarding effects of higher doses of nicotine were not attenuated by dopamine (DA) receptor blockade, blockade of mesolimbic DA signalling with either systemic or intra-nucleus accumbens (NAc) neuroleptic pretreatment potentiated the sensitivity to nicotine's rewarding properties over a three-order-of-magnitude dose range. Furthermore, the behavioural effects of lower doses of intra-VTA nicotine were reversed, switching the motivational valence of nicotine from aversive to rewarding. Our results suggest that blockade of mesolimbic DA signalling induced by neuroleptic medications may block selectively the aversive properties of nicotine, thus increasing the vulnerability to nicotine's rewarding and addictive properties by inducing a unique, drug-vulnerable phenotype.

GABA(A) receptors in the ventral tegmental area control bidirectional reward signalling between dopaminergic and non-dopaminergic neural motivational systems.

In the midbrain ventral tegmental area (VTA), both dopaminergic and nondopaminergic neural substrates mediate various behavioural reward phenomena. VTA GABAergic neurons are anatomically positioned to influence the activity of both the mesolimbic dopamine system and nondopamine efferents from the VTA. In order to examine the possible functional role of VTA GABA(A) receptors in neural reward processes, we performed discrete, bilateral microinjections of the GABA(A) receptor agonist, muscimol, or the GABA(A) receptor antagonist, bicuculline, into the VTA. Using a fully counterbalanced, unbiased conditioned place-preference paradigm, we demonstrate that activation of VTA GABA(A) receptors, with the GABA(A) receptor agonist muscimol (5--50 ng/microL), or inhibition of VTA GABA(A) receptors, with the GABA(A) receptor antagonist bicuculline (5--50 ng/microL), both produce robust rewarding effects. Furthermore, these rewarding effects can be pharmacologically dissociated: blockade of dopamine receptors with a dopamine receptor antagonist, alpha-flupenthixol (0.8 mg/kg; i.p.), or concurrent activation of VTA GABA(B) receptors with a GABA(B) receptor agonist, baclofen (70 ng/microL), blocked the rewarding properties of the GABA(A) receptor agonist, but had no effect on the rewarding properties of the GABA(A) receptor antagonist. These results suggest that, within the VTA, a single GABA(A) receptor substrate controls bidirectional reward signalling between dopaminergic and nondopaminergic brain reward systems.

Role of the laterodorsal tegmental nucleus in scopolamine- and amphetamine-induced locomotion and stereotypy.

Scopolamine (1.5 mg/kg; i.p.) or amphetamine (3 mg/kg; i.p.) increases locomotion and stereotyped behavior patterns in rats. Previous studies suggest that scopolamine acts via muscarinic receptors near the midbrain-pons border. In this study, unilateral microinjections in N-methyl-scopolamine (2.5-10 microg) into the laterodorsal tegmental nucleus (LDT) increased locomotion. Bilateral ibotenate lesions of the LDT attenuated scopolamine-induced locomotion by 68% 7 days postlesion, and by 35% 28 days postlesion. LDT lesions reduced scopolamine-induced stereotypy less than locomotion. The sensitization to amphetamine observed on repeated tests was attenuated by LDT lesions for stereotypy, but not for locomotion. These findings suggest that scopolamine induces locomotion largely, but not exclusively, by blocking muscarinic receptors in LDT.

Locomotion and stereotypy induced by scopolamine: contributions of muscarinic receptors near the pedunculopontine tegmental nucleus.

In this study, we test whether blockade of muscarinic receptors near mesopontine cholinergic cell groups may contribute to locomotor activation induced by scopolamine. Unilateral or bilateral injections of scopolamine (10-150 micrograms) into the pedunculopontine tegmental nucleus (PPT) increased horizontal locomotion by 2-15 times in a dose-related way. Unilateral or bilateral injections of scopolamine into the PPT increased stereotypic behaviors (such as sniffing in one location or over large areas), self-biting and grooming. Carbachol (4 micrograms) injected into PPT reduced locomotion for 20 min, followed by 70 min of increased locomotion. When carbachol (4 micrograms) was injected into the PPT before scopolamine (3 mg/kg, i.p.), the activating effect of scopolamine was attenuated, but not when carbachol was injected after scopolamine. Therefore, carbachol appears to compete with scopolamine for muscarinic receptors near the PPT that mediate locomotor activating effects of systemic scopolamine. Haloperidol (0.1 mg/kg, i.p.) also attenuated the stereotypy and locomotion induced by scopolamine in the PPT. We hypothesize that scopolamine acts by blocking muscarinic receptors on mesopontine cholinergic neurons, thereby disinhibiting cholinergic neurons that can activate dopamine neurons.