Abstract
Fifty-four naive, male, hooded rats were given a mild electric shock after stepping down from a small wooden platform. After this single trial, no animal was tested for 2 days. On the fourth day, all animals were returned to the platform and their step-down latencies recorded. Animals were injected with melanocyte-stimulating hormone (MSH), an MSH-release inhibiting factor (MIF), or saline on acquisition day only, on extinction day only, on both acquisition and extinction day, or on all 4 days. The data showed that regardless of when MSH was injected, MSH resulted in increased resistance to extinction as evidenced by greater inhibitory performance when compared with rats injected with either MIF or saline. The results obtained with this modified procedure confirm that MSH can cause increased resistance to extinction and extend these findings to a pigmented strain of rats. Thus, it is unlikely that the effects of MSH on behavior are due to the inability of rats to inhibit a response.
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