What is the generic term for a thing that is being 'replied to'?

When I 'reply' to something (a person, an email, etc.), the thing I produce through the action is a 'reply' (or a 'response'). Is there a generic term (noun) for the thing that the reply/response is replying to?

For some more context, I'm trying to choose a word for a database column for the 'parent' of a comment which is a reply to another comment (ie the next in the comment thread). This also implies a chain of comments rather than a bunch of comments all referring to a single 'conversation root'.

An example (that has a very obvious non-generic answer) would be: As you navigate through the conversation tree of comments, click the 'back' link to view the ___________ to which that particular reply was in response.

Some examples that I'm not (entirely) happy with:

• original_comment
• parent
• reply_to

Solution 1:

Is it not a request?

The person replying is a respondent, and the information they give is a response.

And to what do I respond, with a response - surely a request.