Blockade of the spinal excitatory effect of cAMP on the startle reflex by intrathecal administration of the isoquinoline sulfonamide H-8: comparison to the protein kinase C inhibitor H-7.

Drugs thought to inhibit the actions of protein kinase C (PKC) and cAMP dependent protein kinase (A-kinase) were infused intrathecally into the subarachnoid space of the lumbar region of the spinal cord, and the effects on acoustic startle were measured. Previous work has shown that intrathecal infusion of drugs thought to increase cAMP increase the startle response. The present experiment evaluated whether inhibition of A-kinase would prevent this effect. Rats were infused with the isoquinoline sulfonamide, H-8 (360 nmol) or vehicle (50% dimethyl sulfoxide), 30 min prior to infusion of 100 nmol of dibutyryl cAMP. By itself, H-8 had little effect on startle, but completely blocked the normal excitatory effect of dibutyryl cAMP on startle. In contrast, the isoquinoline sulfonamide, H-7, which is less active in blocking A-kinase, but more active in blocking PKC, did not block dibutyryl cAMP. Moreover, H-8 did not block the excitatory effect of intrathecal infusion of the 5-HT1A receptor agonist, 8-OH-dipropylaminotetraline (8-OH-DPAT). Thus, the blockade of dibutyryl cAMP by H-8 appears somewhat specific and suggests an involvement of A-kinase in the excitatory effects of dibutyryl cAMP on the acoustic startle response. In a second experiment, it was found that administration of the isoquinoline sulfonamide H-7 caused a marked, dose-dependent (150-800 nmol) facilitation of the startle reflex in comparison with its vehicle. Tris buffer (0.1 M). Like H-7, another PKC inhibitor, GT1b (20 nmol) produced a marked increase in the startle reflex versus its vehicle, 0.01 M phosphate buffer.(ABSTRACT TRUNCATED AT 250 WORDS)