Where Do Talking Points Come From?

Bernstein wants to know "how the partisan press functions":

My guess is that most constraints [on partisan commentators] are real, but not especially heavy-handed or, in most cases, top-down.  They work the way a lot of things work in our party system: through networks, and through informal pressure and influence.  In other words, Rachel Maddow starts talking about filibuster reform because the liberal guests she has on are all interested right now in filibuster reform, and activists in her audience are interested in filibuster reform -- not because the White House or the DNC or the Majority Leader's office told MSNBC to tell her to push filibuster reform.  Although I should add: we do know that both parties do send out talking points, and presumably talk show hosts and their producers are reading them.  More likely, Maddow has other very real, if informal, constraints; if she suddenly revealed she was secretly pro-life and began dedicating a segment every night to how Democrats should have more diversity of opinion on abortion, her credibility with her audience would disappear rapidly, and MSNBC would soon replace her with someone liberals could love and trust.