When a peptide hormone reaches its target cell, which event happens next?

 They are unable to pass through the plasma membrane and have different methods of action. They attach to their receptors in the target cell surface and influence activity within the cell through cytoplasmic intermediates called second messengers. 

The two most important messengers are cAMP and inositol triphosphate. 

Cyclic AMP: ATP is converted into cAMP after a series of reactions on the plasma membrane following the attachment of the hormone to the membrane. cAMP relays the signal from the membrane to the metabolic machinery of the cytoplasm. 

Inositol Triphosphate: Involves the use of Ca+2 that regulates cellular protein activity.

Hope this helps