The intermediate substance in the synthesis of norepinephrine is dopamine.

Here's the breakdown of the synthesis pathway:

1. Tyrosine is converted to L-Dopa by the enzyme tyrosine hydroxylase.

2. L-Dopa is converted to dopamine by the enzyme aromatic L-amino acid decarboxylase.

3. Dopamine is converted to norepinephrine by the enzyme dopamine β-hydroxylase.

So, dopamine acts as a crucial intermediate in the production of norepinephrine.