Fluorescent Molecules That Help Reveal Previously Unidentified Neural Connections in Adult, Neonatal and Peripubertal Mammals

One hundred and twenty-five years ago there was a lively discussion between Hungarian and Spanish neuroscientists on the nature of neural connections. The question was whether the neurofibrils run from one neuron to the next and connect neurons as a continuous network or the fibrils form an internal skeleton in the neurons and do not leave the cell; however, there is close contact between the neurons. About 50 years later, the invention of the electron microscope solved the problem. Close contacts between individual neurons were identified and named as synapses. In the following years, the need arose to explore distant connections between neuronal structures. Tracing techniques entered neuroscience. There are three major groups of tracers: (A) non-transsynaptic tracers used to find direct connections between two neuronal structures; (B) tracers passing gap junctions; (C) transsynaptic tracers passing synapses that are suitable to explore multineuronal circuits. According to the direction of the transport mechanism, the tracer may be ante- or retrograde. In this review, we focus on the ever-increasing number of fluorescent tracers that we have also used in our studies. The advantage of the use of these molecules is that the fluorescence of the tracer can be seen in histological sections without any other processes. Genes encoding fluorescent molecules can be inserted in various neuropeptide or neurotransmitter expressing transcriptomes. This makes it possible to study the anatomy, development or functional relations of these neuronal networks in transgenic animals.