What are reciprocal inhibitory synapses?
It might be better understood if written with a comma as:


  reciprocal, inhibitory synapse


though the rules for this comma usage in English might discourage this because writing "inhibitory reciprocal synapse" doesn't sound quite right. I would say the author wrote it correctly even though it is not perfectly clear.

In this context, they are separate adjectives describing the synapse: inhibitory, because it suppresses firing, and reciprocal because Cell A gives input to Cell B, and Cell B also gives input to Cell A (where Cell A is the amacrine cell and Cell B is the bipolar cell).

The word reciprocal in this context is the adjective definition of the word reciprocal:


  given, felt, or done in return.


Note that it isn't the synapse itself that is reciprocal or in both directions, but that the cells have a reciprocal relationship.