To specifically answer your question about the differences between WordTracker and the AdWords keyword tool, I examined the WordTracker site. I performed a keyword search for the phrase "depression and bipolar link". It showed 34 searches. To better understand what that result meant, I searched the site and located the following explanation:
"For the Wordtracker data, the Search count is the number of times each keyword appears in our database of searches over the past 365 days. This constitutes just under 1% of all US search, and the data is gathered from metacrawler.com and dogpile.com."
There are two key differences between AdWords data and WordTracker. AdWords clearly has a much larger data source so it should be more accurate. Also, AdWords data is presented based upon monthly searches, where WordTracker uses yearly searches. The AdWords result for "depression and bipolar link" would be 3 monthly searches. Since the result is less then 100, Google rounds the result to 0.
You are reaching for very long tail phrases. You will capture other keywords and shorter phrases in the process.
For example, while Adwords shows no traffic on "depression and bipolar link" the phrase "depression and bipolar" shows 165k monthly searches with medium competition. If I were to create a page, I would focus the article on "depression and bipolar". If you really wish to keep the focus on "depression and bipolar link", you can do such knowing you will capture traffic from other versions of the phrase.
