Ligand-gated ion channels are activated by a ligand. Upon activation, the ion channels open and allow for passage of ions through the membrane. They are usually found on membranes such as plasma membrane and organelle membranes and facilitate the exchange of ions between cytoplasm and the extracellular matrix (or inside of organelles). Since all membranes found in a cell are phospholipid bilayers, ligand-gated ion channels are found on phospholipid bilayers.

To answer this question, we need to know the distribution of ion channels in the central nervous system. There are two kinds of ion channels: voltage-gated and ligand-gated. Voltage-gated channels respond to voltage changes whereas ligand-gated channels respond to ligand binding. Upon activation, both types of ion channels allow passage of ions across a membrane. In CNS, voltage-gated channels are typically found on the glial cells (supporting tissue of CNS) whereas ligand-gated channels are typically found on neurons.

A characteristic of ion channels is that they have a much quicker effect than receptors that utilize second messengers (like G protein-coupled receptors). Recall that depolarization in a neuron occurs when neurotransmitters (ligand) bind to receptors on the postsynaptic membrane. Upon binding of neurotransmitters, these receptors will open and allow for the flow of ions, which leads to depolarization.

G protein-coupled receptor (GPCR) is a receptor that is activated by the binding of a ligand; however, it acts through a second messenger molecule (cAMP). This means that the activation of GPCR activates cAMP, which activates subsequent signaling pathways. GPCRs are not involved in the influx and efflux of ions; therefore, they aren’t ion channels.

Insulin receptor is a type of a receptor tyrosine kinase. Upon binding, insulin activates the RTK which eventually leads to activation of genes for the glucose transporters. This increases the glucose uptake by the cells. Similar to GPCR's, insulin receptor does not allow movement of ions across the membrane; therefore, it isn't an ion channel.

Signaling receptors can be divided into two categories: ion channels and second-messenger utilizing receptors. Ion channels are activated by voltage changes (voltage-gated) or ligand binding (ligand-gated) and they tend to increase the flow of ions into and outside of cell. Receptors, such as G protein-coupled receptors, are activated by ligand binding; however, they signal the cell by activating second messenger molecules such as cAMP.

Receptor tyrosine kinase pathway utilizes second messenger molecules to activate molecules in the cell that, subsequently, activate cellular mechanisms. Ion channels allow for flow of ions between membranes; they do not directly activate second messenger molecules.

cAMP is a second messenger molecule that activates several molecules. Second messenger molecules often amplify the original signal, allowing for the signal to travel all across the cell. One of the molecules activated by cAMP is protein kinase C (PKC). This molecule, as the name implies, is a kinase; therefore, it phosphorylates other molecules. Note that this is a function of protein kinase C, not a direct function of cAMP.

PIP2 gets cleaved into two smaller molecules by phospholipase C, and these two molecules are DAG and IP3. The protein kinases are not produced from this reaction, nor is cAMP or phosphatidylcholine. This is simply a matter of knowing that DAG and IP3 are the two most important lipid-derived second messengers. 

Guanylyl cyclase is the enzyme responsible for catalyzing this reaction, and the reaction involves removing two phosphate groups from guanosine triphosphate to generate cyclic guanosine monophosphate. Adenylyl cyclase performs a similar reaction but the substrate is adenosine triphosphate and the product is cyclic adenosine monophosphate. cGMP protein kinase is a target that is activated by cGMP, but is not involved in this reaction. 

Once the conformation change has been induced by ligand binding, the GPCR can act as a guanine exchange factor (GEF) which exchanges out a bound GDP (lower energy) on the G-protein for a GTP (higher energy). This triggers the dissociation of the G-protein, and it goes on to activate various second messenger cascades within the cell. Generally, addition of phosphate groups in biochemistry signals "activation," and removal triggers "deactivation."

All of the hormones, and tissues, listed, have responses which can be mediated by cyclic AMP. Luteinizing hormone, however, does not target the heart. Rather, it targets organs of the reproductive system.