In the current study, we demonstrated that salvinorin A is a potent pial artery dilator in piglets under normal and vessel-constricted conditions as induced by endothelin and hypocarbia. Dilatation effects were observed immediately after salvinorin administration, lasted less than 5 min for each test dose (10 nM and 1 μM), and were dose dependent. Sustained dilation effects were observed for 30 min with continual administration every 2 min. The activation of the opioid receptor, NOS, and KATP channel were involved in the signal pathway of dilation effects.
We previously demonstrated that U50488, an exogenous KOR agonist, and dynorphin, an endogenous KOR agonist, dilate the pial artery.12
Pei et al
demonstrated that U50488 relaxes the isolated aortic artery in rat in a dose-dependent manner. Different from other KOR agonists, the dilatation effect of salvinorin is very short-lived, lasting less than 5 min. Therefore, salvinorin is the shortest acting agent known among the κ agonists—presenting researchers with an opportunity to explore this novel agent that would allow for easy management and titration in perioperative and critical care settings.14
To achieve longer lasting effects, infusion is the most common technique used in clinical anesthesia. In the current study, we found that persistent vascular dilatation can be achieved for 30 min via
continual administration every 2 min.
Because continual administration every 2 min results in a sustained dilatory effects, the unique structure of salvinorin A, not tachyphylaxis, contributes to its short-acting character. Ester linkage in its structure can be easily metabolized by esterase in the blood and tissues. Tsujikawa et al
demonstrated that carboxylesterase mainly involved in the salvinorin A hydrolysis in rat plasma and the degradation products including salvinorin B which are not pharmacologically active.
Similar to other KOR agonists,12,13
the activation of NOS, KATP
channels, and opioid receptors mediated salvinorin A's dilation effects. Goyagi et al
demonstrated that selective KOR agonist BRL 52537 protected the ischemic brain by attenuating nitric oxide production in the ischemic striatum. Zeynalov et al
proved that the neuroprotection of BRL 52537 was lost in nNOS-null mice. Therefore, the KOR agonist BRL 52537 attenuated nNOS activity and ischemia-evoked nitric oxide production. However, in the current study, pial artery dilation to salvinorin A was abolished by the L-NNA, a nonspecific NOS inhibitor, but not 7-nitroindazole, a selective nNOS antagonist. This result indicates that nNOS is not involved in the dilative effective of salvinorin A. Because a subthreshold amount of the nitric oxide donor SNP failed to restore the dilation response, the elevation of cGMP in CSF is likely the result of stimulation, rather than inhibition, of nitric oxide production and direct activation of salvinorin on endothelial NOS rather than permissive enabling. The experimental paradigm used in this study does not allow us to determine the cellular site of origin for the cGMP detected in CSF, but the origin may include endothelial, vascular smooth muscle, and/or neuronal cells.
channel activation may result in hyperpolarization of the membrane of vascular smooth muscle cells. Membrane potential changes would then regulate muscle relaxation through alterations in Ca2+
influx through voltage-dependent Ca2+
To our knowledge, this is the first study to demonstrate that salvinorin A actives the KATP
channel directly or indirectly. Different from many other agents that can activate the KATP
channel, salvinorin A can easily penetrate the blood-brain barrier.7,8
Because the KATP
channel plays a crucial protective role against brain injury from hypoxia, ischemia, and metabolic inhibition,19–22
salvinorin A might be a potential neuroprotective agent for future clinical use.
Although salvinorin A shows a high affinity for dopamine D2
sulpiride, the dopamine D2
receptor selective antagonist, has no effects on the dilation effects of salvinorin A. On the contrary, naloxone and norbinaltorphimine, the KOR-selective antagonist, abolished the vascular dilative effects of salvinorin A, suggesting the important role of κ receptor rather than the dopamine D2
The vascular dilative effect of salvinorin A observed in normal and constricted cerebral vessels by hypocarbia and endothelin opens new possibilities for clinical applications. Likely targets include treatment of cerebral vessel spasm in clinical situations such as migraine and after subarachnoid hemorrhage, where increase of endothelin plays an important role.24
More studies are necessary to demonstrate the dilation effects of salvinorin A on other pathologic conditions. Similar to other short-acting agents, continuous infusion could be used to titrate and achieve sustained effects.
In the current study, newborn piglets were used as study subjects. The piglet brain is gyrencephalic, similar in maturity to human infant, and contains more white than grey matters, the former being selectively vulnerable to injury.25,26
The newborn piglet was also used because it is large enough for easy placement of a cranial window and vascular visualization. Cerebral vascular responses in the newborn piglet are similar to those of human subjects in many clinical situations; no reports indicate a difference in responses. However, it is advisable to retain caution when interpreting these data for use in adult subjects.
In conclusion, salvinorin A is a fast, short-acting, potent pial artery dilator in piglet in normal and vessel-constricted conditions induced by endothelin or hypocarbia. The mechanism involves the activation of NOS, KATP channels, and the κ opioid receptor. These findings suggest that salvinorin A might have clinical value in clinical settings that require cerebral vascular dilation.