1. A neuron labeling method, comprising: contacting one or more primary neurons, each of which is connected by a plurality of synapses to a plurality of secondary neurons, with: (i) a transsynaptic transport (TST)-defective virus (TST-defective virus), wherein the TST-defective virus is defective for transport across the plurality of synapses and comprises a foreign envelope protein; (ii) at least one heterologous nucleic acid molecule that: (a) encodes one or more polypeptides that complements in trans the TST-defective phenotype of the TST-defective virus; wherein the one or more polypeptides are not substantially transported across the plurality of synapses; (b) encodes a receptor specific for the foreign envelope protein; and (c) is not substantially transported across the plurality of synapses; wherein the secondary neurons do not contain the heterologous nucleic acid molecules encoding one or more polypeptides that complement in trans the TST-defective phenotype of the TST-defective virus; under conditions that permit expression of the polypeptide(s) and the receptor in the primary neuron(s), rescue of the TST-defective phenotype of the primary neuron(s) by the polypeptide, and retrograde transport of the TST-defective virus from the primary neuron(s) to the plurality of secondary neurons across the plurality of synapses; and detecting the TST-defective virus.