Sildenafil Treatment Eliminates Pruritogenesis and Thermal Hyperalgesia in Rats with Portacaval Shunts.

Pruritus is a common symptom in chronic liver diseases, which may also alter thermal sensitivity. The underlying mechanisms remain unclear and treatments are not satisfactory. Portal-systemic shunting has been proposed to alter thermal sensitivity in cirrhotics. Inflammation-induced enhanced activity of the Transient Receptor Potential Vanilloid 1 (TRPV1) may contribute to pruritus and thermal hyperalgesia. Sildenafil reduces neuroinflammation in portacaval shunt (PCS) rats. The aims were to assess whether: (1) PCS rats show enhanced scratching or thermal sensitivity; (2) TRPV1 activity is enhanced in PCS rats; (3) treatment with sildenafil reduces TRPV1 activation, scratching and thermal hyperalgesia. Rats were treated with sildenafil beginning 3 weeks after surgery. The number of scratches performed were counted. Thermal hyperalgesia was analyzed using the Hargreaves' Plantar Test. TRPV1 activation by measuring the increase in Ca2+ induced by capsaicin in dorsal root ganglia neurons. PCS rats show enhanced scratching behavior, reaching 66 ± 5 scratches/h (p < 0.01) at 21 days after surgery, while controls show 37 ± 2 scratches/h. PCS rats show thermal hyperalgesia. Paw withdrawal latency was reduced (p < 0.05) to 10 ± 1 s compared to controls (21 ± 2 s). Capsaicin-induced calcium increase was higher in dorsal root ganglia cultures from PCS rats, indicating TRPV1functional increase. PCS rats show enhanced scratching behavior and thermal sensitivity and are a good model to study these alterations in chronic liver diseases. Enhanced sensitivity and activity of TRPV1 channel underlies these alterations. Treatment with sildenafil reduces TRPV1 channel sensitivity and activity and normalizes scratching behavior and thermal sensitivity.

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.

Interplay between glutamatergic and GABAergic neurotransmission alterations in cognitive and motor impairment in minimal hepatic encephalopathy.

The cognitive and motor alterations in hepatic encephalopathy (HE) are the final result of altered neurotransmission and communication between neurons in neuronal networks and circuits. Different neurotransmitter systems cooperate to modulate cognitive and motor function, with a main role for glutamatergic and GABAergic neurotransmission in different brain areas and neuronal circuits. There is an interplay between glutamatergic and GABAergic neurotransmission alterations in cognitive and motor impairment in HE. This interplay may occur: (a) in different brain areas involved in specific neuronal circuits; (b) in the same brain area through cross-modulation of glutamatergic and GABAergic neurotransmission. We will summarize some examples of the (1) interplay between glutamatergic and GABAergic neurotransmission alterations in different areas in the basal ganglia-thalamus-cortex circuit in the motor alterations in minimal hepatic encephalopathy (MHE); (2) interplay between glutamatergic and GABAergic neurotransmission alterations in cerebellum in the impairment of cognitive function in MHE through altered function of the glutamate-nitric oxide-cGMP pathway. We will also comment the therapeutic implications of the above studies and the utility of modulators of glutamate and GABA receptors to restore cognitive and motor function in rats with hyperammonemia and hepatic encephalopathy.