and DAG can act as intracellular signaling molecules, moving about
and activating other processes. Thus, these cascades create further
diversity and amplification in the effects of metabotropic receptors.
The various targets of activated G-proteins are sometimes collectively
called effector enzymes because they have various effects within the
cell. Molecules such as cAMP, cGMP, IP3 and DAG are sometimes called
intracellular messengers or second messengers, with the neurotransmit-
ter being understood as the extracellular or first messenger.
Finally, because G-proteins have been found to be a central feature
in all of these scenarios, metabotropic receptors are generally referred
to as G-protein-coupled receptors, or GPCRs. These receptors occur
widely in the brain as neurotransmitter receptors. And because of
their sensitivity and versatility, GPCRs also occur throughout the body
playing other roles, several of which are discussed later in this book.
Highlighting again the poetic molecular beauty of all this, here is a
sonnet, in Shakespearean form—a lyric poem to the GPCR:Neurotransmitter binds, receptor shifts,
Within the cell G-protein attaches,
G-protein changes, and we get new gifts,
Like from GDP, GTP hatches.Now G-protein transmutes from whole to parts,
Each subunit starts down a differ’nt road;
Adenylate cyclase labor surge starts,
Cyclic AMP now increased or slowed.Protein kinase attaches phosphate group
To the neuron’s ion channel protein,
Open channel or closed gives the cell’s soup
The potential to fire or stay serene.