GABAergic inhibition of the pituitary release of adrenocorticotropin and alpha-melanotropin is impaired in dogs with hepatic encephalopathy

Hepatic encephalopathy in dogs and cats

OBJECTIVE

To comparatively review the pathogenesis, clinical presentation, diagnosis, and management of hepatic encephalopathy (HE) in dogs and cats.

DATA SOURCES

The Medline database was searched for articles related to HE in people, dogs, and cats. Articles published within the last 5 years were given special importance.

HUMAN DATA SYNTHESIS

The pathogenesis of HE is complex and incompletely understood, but ammonia appears to play a central role. Hyperammonemia leads to accumulation of glutamine in astrocytes, with subsequent astrocyte swelling and neurological dysfunction. The development of HE in patients with hepatic cirrhosis is a poor prognostic indicator. The fermentable disaccharide lactulose and the antimicrobial rifaximin are US Food and Drug Administration approved treatments for human HE. Severe protein restriction is no longer recommended for patients with this condition.

VETERINARY DATA SYNTHESIS

HE is often associated with portosystemic shunting in dogs and cats. Ammonia plays a central role in the pathogenesis of HE in dogs and cats, but other factors such as manganese and endogenous benzodiazepines may also contribute. Recently, a soy protein-based diet was found to be beneficial in treating canine HE. Severe dietary protein restriction is likely to be detrimental in affected animals. There have been no clinical trials of drugs routinely used in the management HE in veterinary medicine, but lactulose and antimicrobials such as metronidazole are well-established treatments.

CONCLUSIONS

HE is a potentially life-threatening condition that is probably underdiagnosed in companion animals. Although various treatment recommendations have been proposed, there is a lack of evidence in the veterinary literature regarding optimal strategies for the management of this condition. As our understanding of the pathogenesis of HE in dogs and cats evolves, novel diagnostic tests and therapeutic agents may become available.

Improvement of chronic hepatic encephalopathy in dogs by the benzodiazepine-receptor partial inverse agonist sarmazenil, but not by the antagonist flumazenil

Therapeutic modulation of the increased GABAergic tone in chronic hepatic encephalopathy (HE) by the benzodiazepine receptor (BR) antagonist flumazenil (F) has led to conflicting results in humans and animal models for HE. The BR inverse agonist sarmazenil (S) has only been used in animal models of acute HE. Therefore we investigated the effects of intravenous injection of F and S in dogs with chronic HE 8 to 12 weeks after placement of a portocaval shunt and 40% hepatectomy (n=7), compared to sham-operated pair-fed controls (n=7). The HE dogs had hyperammonemia (298 +/- 48 microM v 33 +/- 3 before surgery (mean +/- SEM)) and signs of HE at the start of the experiments (0.9 +/- 0.1 (scale 0-4)). Three (S3) and 8 (S8) mg/kg of S resulted in a significant improvement of encephalopathy (grade 0.9 +/- 0.2 immediately before v 0.5 +/- 0.1 after injection (S3) and 0.7 +/- 0.1 v 0.3 +/- 0.1 (S8)) and increase in mean dominant frequency of the EEG (MDF; 9.1 +/- 0.7 Hz v 11.1 +/- 0.3 (S3) and 8.9 +/- 0.5 v 11.0 +/- 0.3 (S8)) in HE dogs, whereas 15 mg/kg of S, 3 and 8 mg/kg of F, and the vehicle had no significant effects. The efficacy of S in these dogs is consistent with an increased GABAergic tone in the pathogenesis of chronic HE. The lack of effects of F makes a role for endogenous benzodiazepines herein unlikely.