punctuation

I suggest that you move the section in brackets so that it does not interrupt the sentence "I recently read ... to be remembered", and then remove the brackets. Otherwise, excellently punctuated. A few suggestions in bold.

I recently read that when a person is very shocked ['by something' is redundant], a ['certain' not needed] chemical is produced in the brain which causes the incident to be remembered. I forget the name of the chemical. I [no need for 'can only'] remember that it starts with 'n'. Is this related to depression?

Many thanks.

Some more points.

I recently read that when a person is very shocked,(1) a substance (2) is produced in the brain which causes the incident to be remembered. I think its name starts with an 'N'(3)(4)(5). Is this related to depression?

(1) A comma here

(2) Avoid "chemical". It makes people think of reagent bottles.

(3) In seeking to avoid the repetition of the word "chemical" (or its replacement) we see a way to shorten a sentence still further.

(4) Best to be modest here. You forgot its name, don't insist upon the first letter. Better still, don't even mention forgetting. Just imply it.

In conversation a person might say "I think it's called N something".

(5) We can refer thus to letters of the alphabet with a preceding indefinite article (dependent on the sound of the letter) thus...

an "A", a "b", an "f", a "g", an "N", an "m".

Gimme me an "F"!
Gimme me a "U"!
Gimme me a "C"!
(etc)

-from a famous song by Country Joe Macdonald