Change caption style of listings package without "caption" package

Change caption style of listings package without "caption" package

Is there a way to change style of a caption in listings without the
"caption" package (make it smaller and non-bold)? I am using
\lstnewenvironment to create my own customized environment and then use
listings via this new environment.
I tried to do it with the "caption" package with
\captionsetup[lstlisting]{font={scriptsize,tt}} and it worked, but the
problem is that I cannot use caption package as it messes up captions in
the rest of the document (as soon as I "include" it), and I have to use
predefined class.
Thank you for your help!