This morning, four stocks could make big moves today -- Xenoport (NASDAQ: XNPT), Questcor (NASDAQ: QCOR), NuVasive (NUVA), and Pain Therapeutics (NASDAQ: PTIE). Let's check in on the headlines moving these four stocks in pre-market trading.

Continued volatility expected for Xenoport
Xenoport looks set to drop sharply after falling 14% in after-hours trading yesterday. The stock has been very volatile over the past two weeks, after activist investing firm Clinton Group, one of the company's top shareholders, called for the rearrangement of the company's research pipeline and the replacement of its CEO Ronald Barrett.

The stock initially rose 14% after Clinton Group's statement, but since the rally was unprecedented, it's not surprising that it has declined in a similarly random manner.

Clinton Group now owns 1.3 million shares of Xenoport, which currently has one marketed product -- Horizant, a treatment for restless legs syndrome. Xenoport acquired Horizant's U.S. sales from GlaxoSmithKline after a marketing dispute. As a result, Xenoport reported $1.6 million in product sales last quarter, up from no product sales in the prior year quarter.

Clinton Group believes that Xenoport should focus most of its expenses to advance clinical trials of an experimental treatment, XP23829, for multiple sclerosis and psoriasis. XP23829 is similar to Biogen Idec's Tecfidera, which analysts believe could generate annual revenue of $3.5 billion by 2016.

In my opinion, investors should steer clear of Xenoport -- it's simply too volatile at the moment.

Unanswered questions about Questcor
Questcor Pharmaceuticals could also drop lower today, after it followed up impressive third-quarter results with an announcement that it was being probed by regulators. The stock fell 9% after hours yesterday and is trending lower in pre-market trading at the time of this writing.

For the third quarter, Questcor earned an adjusted $1.68 per share -- a big jump from the $0.97 per share it earned in the prior year quarter. Revenue soared 68% year-over-year to $236.3 million. Both Questcor's top and bottom line growth topped analyst estimates by a wide margin.

Questcor's top product is the H.P. Acthar Gel, which was recently expanded to treat 19 different indications by the Food and Drug Administration.

Yesterday, Questcor announced that its promotional practices were under investigation by the U.S. Attorney's Offices in Pennsylvania and New York and the SEC. The company plans to disclose further details about the investigation in a regulatory filing today.

This stock has soared more than 160% in the last 12 months, but investors should probably wait on the sidelines until the matter is resolved.

A bright new morning for NuVasive
On the bright side, shares of medical device maker NuVasive could pop higher today, after advancing 14% in after-hours trading yesterday following a positive third quarter earnings report.

Nuvasive reported earnings of $0.16 per share on a GAAP-adjusted basis, as revenue rose 14% year-over-year to $169.2 million. The company also offered rosy guidance across the board. Full-year GAAP earnings per share are now forecast at $0.07, up from a loss of $0.09 per share. Revenue is expected to come in at $670 million, up from its prior forecast of $655 million.

NuVasive primarily sells minimally invasive spinal products, along with several biologic drugs focused on spinal fusion surgery. However, the stock has been under pressure since July, after the company disclosed that it had been subpoenaed by the Office of the Inspector General of the U.S. Department of Health and Human Services, over possibly falsified or improper Medicare and Medicaid claims.

While that matter remains unresolved, investors were apparently willing to overlook that investigation after its solid earnings report. Shares of NuVasive are up 80% over the past 12 months.