Yes, there is an itemized deduction for investment interest. See
Schedule A (Line 9),
Form 4952, the instructions for these, and other publications mentioned therein.
My understanding from a brief read-through of these materials is that the loan needs to be used to purchase investments in order for the interest to be deductible. Interest on loans used for personal expenses, even if investment property is used as collateral, is not deductible. There's probably a bit of wiggle room here though, given that money is fungible. Suppose that this week you receive $10k of dividends in your brokerage account and you also take a $10k margin loan. The next week you purchase $10k of new shares. Did you use the loan dollars to purchase the shares, or the dividend dollars? Who can say? Seems like it would be more advantageous to claim you used the loan dollars, and you would then be able to deduct the interest every year until you either pay off the loan or cash out the shares you bought with the "loan dollars."