Cholinergic Receptors

• 2 types
  • Nicotinic receptors
    • Ligand gated channels
  • Muscarinic receptors
    • G-protein coupled receptors  

Nicotinic Receptors 

(Ligand gated channels)

• Autonomic ganglia (N_n)
• Neuromuscular junction (N_m)

Muscarinic receptors 

(G-protein coupled receptors)

• Found on most effector organs
• M1
• M2
• M3
• M4- presynaptic terminals and CNS- G_i/G_o protein-inhibition

M2 receptors

• Predominately in heart (SA & AV nodes, some atrial tissue, minimal in ventricles)
• Presynaptic
• CNS
• G_i/G_o protein-inhibition

M3 receptors

• Vascular (endothelial cells) and nonvascular (epithelial cells) smooth muscle
• Secretory glands
• CNS
• G_q/11 protein- contraction or relaxation

Adrenergic receptors

• All are G-protein coupled receptors
• 2 types
  • α
    
    • There are 2 subtypes of α receptors
  • β
    
    • There are 3 subtypes of β receptors
    • β₃ receptors are found on adipose tissue (G_s-lipolysis)

Alpha one receptors

• Vascular and nonvascular smooth muscles
• G_q protein-contraction

Alpha two receptors

• Presynaptic nerve terminal
• Pancreatic beta cells
• Vascular smooth muscle
• G_i/G_o protein-inhibitory most of the time (except on vascular smooth muscle)

Beta one receptors

• Heart
• J-G cells within kidneys
• G_s protein-excitatory

Beta two receptors

• Smooth muscle (vascular, bronchial, GI & UT)
• G_s protein-inhibitory

α_1A

• Contraction of smooth muscle 
• High density in prostate
• Also found on arteries and veins

α_1B

• Most abundant type in heart (function ???)
• May be involved with α_1A in cardiac growth & structure
• May be more abundant in blood vessels as we get older

• α_{1C =} α_1B

• Found on coronary blood vessels and aorta

• Inhibitory autoreceptor found on presynaptic nerve endings of sympathetic and also parasympathetic nerves
• Found in CNS and stimulation associated with hypotension and antinociceptive responses
• Same affect as α_1A on smooth muscle

• On peripheral blood vessels
• Low density
• Can produce constriction

• Predominately inhibitory- found in adrenal medulla and on nerve endings to inhibit release of Epi and Dopamine

M2 receptor intrinsic mechanism

• G_i/G_o- inhibits adenylyl cyclase
• Inactivates Ca²⁺ channels
• Increase K⁺ efflux-hyperpolarization
• Inhibitory

• G_s protein- increase activity of adenylyl cyclase activity
• Increase intracelllular Ca²⁺ 
• Excitatory- contraction of the heart

• G_s protein- increase activity of adenylyl cyclase activity in most smooth muscle cells
• Decrease intracellular Ca²⁺
• Inhibitory-cause relaxation of smooth muscles