R. M. Wadsworth
Thanks for your comments. In other smooth muscle preparations, there is evidence that nitrergic nerves can release NO. So perhaps there can be a small component of neurogenic relaxation in cerebral arteries that is independent of VIP. Alternatively, perhaps these other nerves have their own mediator to switch on NOS, but the mediator has just not been identified yet !!
I assume that VIP activates nNOS in a similar way that other peptides (bradykinin, substance P)activate eNOS, ie by stimulating the influx and release of calcium.
With best wishes,