Dr Nichols writes in "LSD and Its Lysergamide Cousins":
The important thing to note from the table below, in the far right column, is the fact that LSD has a potency in rats in the drug discrimination behavioral assay of 48 nanomoles per kilogram of rat body weight. Only two other compounds have comparable activity: entries 6 and 16.
Curiously, entry 6 is a monoalkylamide that has the same molecular weight as LSD itself, that is, it has a total of four carbon atoms attached to the amide. Entry 16 has a five-carbon group attached to the amide.
We have no evidence as to whether either of these compounds would be active in man, but these rat data suggest that they might be.
...
Entry 16 (3-aminopentane) has a potency in the drug discrimination behavioral assay of 52 nanomoles per kilogram of rat body weight, this is very similar to the 48 nanomoles per kg of rat body weight observed with LSD.