While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a look at particularly stock-shaking analyst upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Polypore International (UNKNOWN:PPO.DL) sank 5% this morning after Wedbush downgraded the microporous membrane manufacturer from outperform to neutral.

So what: Along with the downgrade, analyst Craig Irwin planted a price target of $38 on the stock, representing about 28% worth of downside to yesterday's close. So while momentum traders might be attracted to Polypore's sharp climb in recent weeks, Irwin's call could reflect a sense on Wall Street that its valuation is becoming a bit stretched.

Now what: According to Wedbush, Polypore's risk to reward trade-off is pretty unappealing at this point. "Downgrading shares of Polypore to NEUTRAL from OUTPERFORM following significant stock appreciation on the announcement of an agreement with Panasonic to supply separators for electric drive vehicle (EDV) batteries," said Irwin. "Our recent checks (from the industry supply chain) indicate Polypore is unlikely to displace Ube on their current EDV programs at Panasonic, including the Toyota Prius, and Ford C-Max and Ford Fusion, or to displace Sumitomo from Tesla. This means future EDV revenue Polypore receives from Panasonic must come from new car launches, and we hear the supply positions will be also bid competitively." When you couple that downbeat view with Polypore's red-hot price run of late, it's tough to disagree with Wedbush's cautious stance.