Evidence for the inhibitory subunit of adenylate cyclase (Ni) in nervous and heart tissue of Aplysia.

Cyclic adenosine 3',5'-monophosphate (cAMP) plays a critical role in modulating a variety of neuronal responses in Aplysia californica. Previous studies have focused on the neurotransmitter activation of adenylate cyclase, which presumably occurs via the guanosine 5'-triphosphate (GTP)-regulated excitatory subunit (Ns). While adenylate cyclase has also been shown to be regulated by inhibitory neurotransmitters, coupled through the inhibitory GTP-regulated coupling protein Ni in some systems, the effects of Ni-mediated adenylate cyclase inhibition on neuronal processing in Aplysia have not previously been reported. In the present study Ni is detected in Aplysia by both protein chemistry and enzymatic activity. A 40 kdalton substrate for the enzymatic activity of Bordetella pertussis toxin is observed. Incubation of Aplysia nervous tissue homogenates with pertussis toxin (IAP) and 32P-nicotinamide-adenine dinucleotide labels a single protein, assessed by polyacrylamide gel electrophoresis and autoradiography. Furthermore, crude membrane suspensions of this tissue demonstrate biphasic adenylate cyclase activity in response to increasing concentrations of GTP, showing Ni and Ns functional activities. These findings provide evidence that Ni is present in Aplysia tissue. Ni may serve as an important site for the regulation of cAMP synthesis and neuronal plasticity.