Cerebrospinal Fluid Neurotransmitter Concentrations After Subarachnoid Hemorrhage in Dogs: Effect of Treatment
Subarachnoid hemorrhage (SAH) is characterized by decreased cerebral blood flow, subsequent cerebral vasospasm and ischemia, and high mortality in people. Impaired endothelial and neuronal nitric oxide (NO) release further lead to inflammation and excitoxicity triggered by excitatory amino acids, glutamate and aspartate. Immunosuppression using cyclosporine A ameliorates cerebral vasospasm, as well as upregulation of NO synthase using statin treatment. We hypothesized that dogs with SAH have alterations in CSF concentrations of glutamate, aspartate, GABA and glycine which are indirectly but positively affected by use of cyclosporine and simvastatin. CSF concentrations of neurotransmitters were investigated as biomarkers of neuronal injury and indicators of beneficial treatments for CNS ischemia.
A double SAH model was induced in dogs by 2 injections (3 mls) of autologous blood into the cerebellomedullary cistern (CMC) 24 hours apart. Dogs were assigned to one of three groups: Control-untreated (n=4); simvastatin (Zocor, Merck Inc., 20 mg/kg SID PO) only (n=4); simvastatin (20 mg/kg SID PO) and cyclosporine A (Sandimmune, Sandoz Inc., 6 mg/kg SID PO) (n=4). Drugs were administered 24 hours after the second injection for 10 days. CSF was collected from the CMC before each injection and on days 3, 7, and 10 and immediately stored at -80°C. The CSF samples were reacted to produce 1-cyanobenz[f]isoindole derivatives. Derivatives were analyzed with electrochemical HPLC. Data analysis used repeated measures model and included factors for treatment, time and a treatment time interaction term (PROC MIXED in SAS; p<0.05). Multiple comparisons were adjusted for using Tukey's test.
In the control group, glutamate significantly increased to highest levels by day 3 and then returned to baseline, whereas glycine, GABA and aspartate were not significantly altered from baseline at any time point. There was significant decremental effect of simvastatin alone (p=0.0004) and in combination with cyclosporine (p=0.0007) on day 3 glutamate concentrations when compared to the control group. A significant incremental effect of combination treatment on day 3 glycine levels was noted compared to control group (p=0.006). No significant differences in GABA and aspartate levels were noted between treatment groups on any of the sample days.
Although precise roles of these neurotransmitters have not been elucidated in pathophysiology of canine CNS ischemia, their alterations from baseline suggest further investigation. A combination of immunosuppression and NO synthase upregulation may be useful in ameliorating elevated glutamate levels in CNS ischemia. Results may be confounded by small sample size and concurrent independent effects of hemorrhage and inflammation notably present in SAH models.