Neuroinflammation increases GABAergic tone and impairs cognitive and motor function in hyperammonemia by increasing GAT-3 membrane expression. Reversal by sulforaphane by promoting M2 polarization of microglia.

BACKGROUND: Hyperammonemia induces neuroinflammation and increases GABAergic tone in the cerebellum which contributes to cognitive and motor impairment in hepatic encephalopathy (HE). The link between neuroinflammation and GABAergic tone remains unknown. New treatments reducing neuroinflammation and GABAergic tone could improve neurological impairment. The aims were, in hyperammonemic rats, to assess whether: (a) Enhancing endogenous anti-inflammatory mechanisms by sulforaphane treatment reduces neuroinflammation and restores learning and motor coordination. (b) Reduction of neuroinflammation by sulforaphane normalizes extracellular GABA and glutamate-NO-cGMP pathway and identify underlying mechanisms. (c) Identify steps by which hyperammonemia-induced microglial activation impairs cognitive and motor function and how sulforaphane restores them. METHODS: We analyzed in control and hyperammonemic rats, treated or not with sulforaphane, (a) learning in the Y maze; (b) motor coordination in the beam walking; (c) glutamate-NO-cGMP pathway and extracellular GABA by microdialysis; (d) microglial activation, by analyzing by immunohistochemistry or Western blot markers of pro-inflammatory (M1) (IL-1b, Iba-1) and anti-inflammatory (M2) microglia (Iba1, IL-4, IL-10, Arg1, YM-1); and (e) membrane expression of the GABA transporter GAT-3. RESULTS: Hyperammonemia induces activation of astrocytes and microglia in the cerebellum as assessed by immunohistochemistry. Hyperammonemia-induced neuroinflammation is associated with increased membrane expression of the GABA transporter GAT-3, mainly in activated astrocytes. This is also associated with increased extracellular GABA in the cerebellum and with motor in-coordination and impaired learning ability in the Y maze. Sulforaphane promotes polarization of microglia from the M1 to the M2 phenotype, reducing IL-1b and increasing IL-4, IL-10, Arg1, and YM-1 in the cerebellum. This is associated with astrocytes deactivation and normalization of GAT-3 membrane expression, extracellular GABA, glutamate-nitric oxide-cGMP pathway, and learning and motor coordination. CONCLUSIONS: Neuroinflammation increases GABAergic tone in the cerebellum by increasing GAT-3 membrane expression. This impairs motor coordination and learning in the Y maze. Sulforaphane could be a new therapeutic approach to improve cognitive and motor function in hyperammonemia, hepatic encephalopathy, and other pathologies associated with neuroinflammation by promoting microglia differentiation from M1 to M2.

Pregnenolone sulfate restores the glutamate-nitric-oxide-cGMP pathway and extracellular GABA in cerebellum and learning and motor coordination in hyperammonemic rats.

Around 40% of cirrhotic patients show minimal hepatic encephalopathy (MHE), with mild cognitive impairment which reduces their quality of life and life span. Treatment of MHE is unsatisfactory, and there are no specific treatments for the neurological alterations in MHE. Hyperammonemia is the main contributor to neurological alterations in MHE. New agents acting on molecular targets involved in brain mechanisms leading to neurological alterations are needed to treat MHE. Chronic hyperammonemia impairs learning of a Y-maze task by impairing the glutamate-nitric-oxide (NO)-cGMP pathway in cerebellum, in part by enhancing GABA(A) receptor activation, which also induces motor in-coordination. Acute pregnenolone sulfate (PregS) restores the glutamate-NO-cGMP pathway in hyperammonemic rats. This work aimed to assess whether chronic treatment of hyperammonemic rats with PregS restores (1) motor coordination; (2) extracellular GABA in cerebellum; (3) learning of the Y-maze task; (4) the glutamate-NO-cGMP pathway in cerebellum. Chronic intracerebral administration of PregS normalizes motor coordination likely due to extracellular GABA reduction. PregS restores learning ability by restoring the glutamate-NO-cGMP pathway, likely due to both enhanced NMDA receptor activation and reduced GABA(A) receptor activation. Similar treatments would improve cognitive and motor alterations in patients with MHE.

Blocking NMDA receptors delays death in rats with acute liver failure by dual protective mechanisms in kidney and brain.

Treatment of patients with acute liver failure (ALF) is unsatisfactory and mortality remains unacceptably high. Blocking NMDA receptors delays or prevents death of rats with ALF. The underlying mechanisms remain unclear. Clarifying these mechanisms will help to design more efficient treatments to increase patient's survival. The aim of this work was to shed light on the mechanisms by which blocking NMDA receptors delays rat's death in ALF. ALF was induced by galactosamine injection. NMDA receptors were blocked by continuous MK-801 administration. Edema and cerebral blood flow were assessed by magnetic resonance. The time course of ammonia levels in brain, muscle, blood, and urine; of glutamine, lactate, and water content in brain; of glomerular filtration rate and kidney damage; and of hepatic encephalopathy (HE) and intracranial pressure was assessed. ALF reduces kidney glomerular filtration rate (GFR) as reflected by reduced inulin clearance. GFR reduction is due to both reduced renal perfusion and kidney tubular damage as reflected by increased Kim-1 in urine and histological analysis. Blocking NMDA receptors delays kidney damage, allowing transient increased GFR and ammonia elimination which delays hyperammonemia and associated changes in brain. Blocking NMDA receptors does not prevent cerebral edema or blood-brain barrier permeability but reduces or prevents changes in cerebral blood flow and brain lactate. The data show that dual protective effects of MK-801 in kidney and brain delay cerebral alterations, HE, intracranial pressure increase and death. NMDA receptors antagonists may increase survival of patients with ALF by providing additional time for liver transplantation or regeneration.

Potentiation of the transient receptor potential vanilloid 1 channel contributes to pruritogenesis in a rat model of liver disease.

Persistent pruritus is a common disabling dermatologic symptom associated with different etiologic factors. These include primary skin conditions, as well as neuropathic, psychogenic, or systemic disorders like chronic liver disease. Defective clearance of potential pruritogenic substances that activate itch-specific neurons innervating the skin is thought to contribute to cholestatic pruritus. However, because the underlying disease-specific pruritogens and itch-specific neuronal pathways and mechanism(s) are unknown, symptomatic therapeutic intervention often leads to no or only limited success. In the current study, we aimed to first validate rats with bile duct ligation (BDL) as a model for hepatic pruritus and then to evaluate the contribution of inflammation, peripheral neuronal sensitization, and specific signaling pathways and subpopulations of itch-responsive neurons to scratching behavior and thermal hypersensitivity. Chronic BDL rats displayed enhanced scratching behavior and thermal hyperalgesia indicative of peripheral neuroinflammation. BDL-induced itch and hypersensitivity involved a minor contribution of histaminergic/serotonergic receptors, but significant activation of protein-activated receptor 2 (PAR2) receptors, prostaglandin PGE2 formation, and potentiation of transient receptor potential vanilloid 1 (TRPV1) channel activity. The sensitization of dorsal root ganglion nociceptors in BDL rats was associated with increased surface expression of PAR2 and TRPV1 proteins and an increase in the number of PAR2- and TRPV1-expressing peptidergic neurons together with a shift of TRPV1 receptor expression to medium sized dorsal root ganglion neurons. These results suggest that pruritus and hyperalgesia in chronic cholestatic BDL rats are associated with neuroinflammation and involve PAR2-induced TRPV1 sensitization. Thus, pharmacological modulation of PAR2 and/or TRPV1 may be a valuable therapeutic approach for patients with chronic liver pruritus refractory to conventional treatments.

Chronic hyperammonemia, glutamatergic neurotransmission and neurological alterations.

This mini-review focus on our studies on alterations in glutamatergic neurotransmission and their role in neurological alterations in rat models of chronic hyperammonemia and hepatic encephalopathy (HE). Hyperammonemia impairs the glutamate-nitric oxide (NO)-cGMP pathway in cerebellum, which is responsible for reduced learning ability. We studied the underlying mechanisms and designed treatments to restore the pathway and learning. This was achieved by treatment with: phosphodiesterase 5 inhibitors, cGMP, anti-inflammatories (ibuprofen), p38 inhibitors or GABAA receptor antagonists (bicuculline). Hyperammonemia alters signal transduction associated to metabotropic glutamate receptors (mGluRs). Hypokinesia in hyperammonemia and HE is due to increased extracellular glutamate and mGluR1 activation in substantia nigra; blocking this receptor restores motor activity. The motor responses to mGluRs activation in nucleus accumbens (NAcc) are altered in hyperammonemia and HE, with reduced dopamine and increased glutamate release. This leads to activation of different neuronal circuits and enhanced motor responses. These studies show that altered responses to activation of NMDA receptors and mGluRs play essential roles in cognitive and motor alterations in hyperammonemia and HE and provide new treatments restoring cognitive and motor function.

Hyperammonemia alters the modulation by different neurosteroids of the glutamate-nitric oxide-cyclic GMP pathway through NMDA- GABAA - or sigma receptors in cerebellum in vivo.

Several neurosteroids modulate the glutamate-nitric oxide (NO)-cGMP pathway in cerebellum through modulation of NMDA- GABAA - or sigma receptors. Hyperammonemia alters the concentration of several neurosteroids and impairs the glutamate-NO-cGMP pathway, leading to impaired learning ability. This work aimed to assess whether chronic hyperammonemia alters the modulation by different neurosteroids of GABAA, NMDA, and/or sigma receptors and of the glutamate-NO-cGMP pathway in cerebellum. Neurosteroids were administered through microdialysis probes, and extracellular cGMP and citrulline were measured. Then NMDA was administered to assess the effects on the glutamate-NO-cGMP pathway activation. Hyperammonemia completely modifies the effects of pregnanolone and pregnenolone. Pregnanolone acts as a GABAA receptor agonist in controls, but as an NMDA receptor antagonist in hyperammonemic rats. Pregnenolone does not induce any effect in controls, but acts as a sigma receptor agonist in hyperammonemic rats. Hyperammonemia potentiates the actions of tetrahydrodeoxy-corticosterone (THDOC) as a GABAA receptor agonist, allopregnanolone as an NMDA receptor antagonist, and pregnenolone sulfate as an NMDA receptor activation enhancer. Neurosteroids that reduce the pathway (pregnanolone, THDOC, allopregnanolone, DHEAS) may contribute to cognitive impairment in hyperammonemia and hepatic encephalopathy. Pregnenolone would impair cognitive function in hyperammonemia. Neurosteroids that restore the pathway in hyperammonemia (pregnenolone sulfate) could restore cognitive function in hyperammonemia and encephalopathy.