  We study the Laplacian in $L^2(\mathbb{R}^3)$ perturbed on an infinite curve
$\Gamma$ by a $\delta$ interaction defined through boundary conditions which
relate the corresponding generalized boundary values. We show that if $\Gamma$
is smooth and not a straight line but it is asymptotically straight in a
suitable sense, and if the interaction does not vary along the curve, the
perturbed operator has at least one isolated eigenvalue below the threshold of
the essential spectrum.
