Salvinorin A preserves cerebral pial artery autoregulation after forebrain ischemia via the PI3K/AKT/cGMP pathway

Dong and colleagues frame the study within the clinical problem of global cerebral ischemia, which occurs in scenarios such as cardiac arrest, drowning or profound systemic hypotension and is associated with high mortality and neurological impairment. Previous work indicates that cerebral ischemia alters vascular tone and impairs vessel responsiveness, so loss of autoregulation during hypercapnia and hypotension contributes to neuronal injury. The authors note that maintaining cerebrovascular autoregulation is therefore important for preserving cerebral blood supply and metabolism, but effective pharmacological strategies are lacking. The paper tests whether salvinorin A, a selective kappa opioid receptor (KOR) agonist that crosses the blood–brain barrier and has previously been reported to dilate vessels and reduce brain injury, can preserve cerebral pial artery autoregulation after transient forebrain ischemia. The investigators also set out to explore mechanisms, focusing on the PI3K/AKT pathway and downstream nitric oxide/cGMP signalling, because PI3K/AKT can regulate nitric oxide synthase (NOS) and cGMP-mediated vasodilation. Their primary aim was to determine if salvinorin A given at reperfusion preserves pial artery responses to hypercapnia and hypotension and whether PI3K/AKT/cGMP signalling mediates these effects.

The study used a rat model of transient global (forebrain) ischemia produced by bilateral common carotid artery occlusion (BCCAO) together with controlled haemorrhage to maintain arterial pressure at 25–30 mmHg for 10 minutes. The extracted text reports thirty Sprague–Dawley rats (250–300 g) were used and animals were acclimatised prior to surgery; however, the drug-treatment section lists group sizes of n=6 per group, and the text does not reconcile this apparent discrepancy. Anaesthesia was maintained with pentobarbital, and femoral artery and vein catheters were placed for monitoring and drug delivery. A closed cranial window permitted direct visualisation and topical treatment of cerebral pial arteries and allowed cerebrospinal fluid (CSF) sampling. Experimental groups included sham-operated controls, an ischaemia–reperfusion (IR) control receiving vehicle (DMSO, 1 mL/kg iv), two intravenous doses of salvinorin A (SA) administered at reperfusion (10 mg/kg and 20 mg/kg), and three groups receiving SA (20 mg/kg iv) plus one of the following agents applied topically via the cranial window: LY294002 (10 mM, PI3K inhibitor), L-NAME (10 mM, NOS inhibitor) or norbinaltorphimine (norBIN, 1 mM, KOR antagonist). The text describes these topical injections but does not detail the precise timing beyond administration after ischaemia. Primary physiological outcomes were dilation responses of cerebral pial arteries to hypercapnia (PaCO2 70–80 mmHg induced by 10% CO2 inhalation) and to hypotension (rapid withdrawal of 10–15 mL/kg blood to reduce arterial pressure by about 45%), measured before ischaemia and 1 hour after ischaemia using video microscopy through the cranial window. CSF was sampled at baseline and 1 hour post-ischaemia for cyclic GMP (cGMP) quantification by ELISA. Molecular and histological endpoints were assessed at 24 hours: phosphorylated AKT (p-AKT, Thr308) and total AKT by western blot in cortex/hippocampus/striatum, neuronal injury by haematoxylin–eosin staining, and apoptosis by TUNEL staining in the ischaemic-adjacent cortex. Motor function was evaluated on days 1, 2 and 5 post-ischaemia using a composite neuromotor score (screen clinging, horizontal bar, prehensile traction) with blinded assessment. Statistical analysis used one- or two-way ANOVA with Tukey multiple comparisons where appropriate; significance was set at P<0.05.

Physiological monitoring indicated no significant between-group differences in arterial blood pressure, blood gas tensions or pH at the measured time points (before ischaemia, onset of reperfusion, and 1 hour after ischaemia). Baseline pial artery autoregulation did not differ among groups prior to ischaemia. Ischaemia impaired pial artery dilation responses to both hypercapnia and hypotension at 1 hour post-ischaemia. Intravenous salvinorin A at both 10 mg/kg and 20 mg/kg given at reperfusion preserved the dilation responses of cerebral pial arteries to hypercapnia and to hypotension. The preservation of autoregulatory responses by salvinorin A was abolished when LY294002, L-NAME, or norBIN were administered topically in combination with salvinorin A. Histological assessment showed that salvinorin A reduced ischaemia-induced neuronal death in hippocampus/cortex/striatum compared with the IR (DMSO) control; the protective effect on neuronal injury was reversed by LY294002, L-NAME or norBIN. TUNEL staining indicated that salvinorin A significantly reduced apoptosis in the ischaemic-adjacent cortex at 24 hours, and again the anti-apoptotic effect was abolished by each of the three inhibitors. At the molecular level, p-AKT levels in cortical tissue were not reduced by ischaemia compared with sham in the extracted text, but salvinorin A administration increased p-AKT expression: the lower dose (SA10) produced a statistically significant increase versus IR (P<0.05) and the higher dose (SA20) produced a larger increase (P<0.01 versus IR). The PI3K inhibitor LY294002 abolished the salvinorin A-induced increase in p-AKT; L-NAME and norBIN did not. CSF cGMP concentrations were reported to be attenuated 1 hour after ischaemia, although the IR versus sham difference was not statistically significant in the extracted text. Salvinorin A elevated CSF cGMP levels, and this increase was blocked by LY294002, L-NAME and norBIN. Functionally, salvinorin A improved composite motor scores on post-ischaemia days 1, 2 and 5 compared with IR control; the motor improvements were negated when LY294002, L-NAME or norBIN were co-administered. Throughout, the extracted text indicates statistical significance for these comparisons but does not present full numerical values for most endpoints. The extracted material also does not reconcile the reported total animal number with the per-group n values given in the drug-treatment description.

Using the BCCAO transient global ischaemia model, the investigators interpret their findings to indicate that salvinorin A administered at reperfusion preserves cerebral pial artery autoregulation to hypercapnia and hypotension and reduces ischaemia-associated neuronal death and apoptosis, with concomitant improvement in motor function. They emphasise that loss of autoregulation contributes to disturbed brain metabolism, oedema and worsened injury, and that maintaining vascular responsiveness is therefore neuroprotective. Mechanistically, the authors highlight increased AKT phosphorylation and elevated CSF cGMP after salvinorin A as key biochemical correlates of the protective effects. Because the PI3K inhibitor LY294002 abolished salvinorin A-induced increases in p-AKT and cGMP, and because both the NOS inhibitor L-NAME and the KOR antagonist norBIN blocked the salvinorin A-mediated elevation of cGMP and the physiological protection, the study team infers involvement of a PI3K/AKT/NOS/cGMP pathway downstream of KOR activation. They also note that LY294002 abolished the p-AKT increase while L-NAME and norBIN did not, suggesting that salvinorin A may regulate PI3K/AKT in ways not solely dependent on NOS or KOR signalling; the authors mention prior reports of salvinorin A acting via ERK/MAPK and propose additional hypotheses such as platelet-mediated effects, which they say require further study. The investigators acknowledge limitations: only two doses of salvinorin A were tested and potential adverse effects at higher doses were not assessed, and timing of administration after ischaemia was not varied so the optimal therapeutic window remains unknown. They call for further work to delineate upstream and downstream mechanisms for PI3K/AKT modulation by salvinorin A and to clarify how these molecular changes translate into preserved vessel function and decreased neuronal injury. In conclusion, the authors propose that salvinorin A can preserve cerebrovascular autoregulation after forebrain ischaemia via the PI3K/AKT/cGMP pathway, with associated reductions in neuronal death and improvements in motor outcome.