Cell biologist in need of stats help!

I am using stata to analyse my data and have two questions:

1. I have a continuous outcome (hippocampal volume) and exposures grouped either as categorical (4 levels) or binary. Is linear regression an appropriate analysis for this data?

2. When I construct basic scatter plots between my exposure and outcome, there looks to be no or very little correlation. Am I right in assuming you need to see some kind of relationship to conduct linear regression? In that case, is there anything I can do to my data to allow me to analyse it?

Any advice would be gratefully received