Although neuroscience holds many mysteries, there are some things of which we feel we can be sure. For example, the 'classical' amino-acid neurotransmitters, GABA (γ-aminobutyric acid) and glutamate, carry information from one neuron to the next; GABA inhibits activity of the postsynaptic neuron, whereas glutamate causes excitation. But even here, where things seem so simple, there is more to the story than meets the eye.

Glutamate, for example, is a key metabolite that is found not only in synaptic vesicles, but also in the cytosol of both neurons and glia. And glutamate receptors are expressed by astrocytes and by many non-excitable cells outside the central nervous system, as well as by neurons. On page 748 of this issue, Maiken Nedergaard, Takahiro Takano and Anker Hansen look beyond the role of glutamate as a neurotransmitter to investigate the potential diversity of its non-synaptic functions and mechanisms of action.

Like glutamate, GABA is not simply a neurotransmitter. Two articles this month discuss the role of GABA in development, when it can act as an excitatory neurotransmitter before switching to inhibition later in life. On page 715, David Owens and Arnold Kriegstein consider the evidence that GABA is an important developmental signal in the brain. They discuss possible functions of GABA in controlling the proliferation, migration and differentiation of neurons. Then, on page 728, Yehezkel Ben-Ari considers the importance of maintaining a balance between excitatory and inhibitory activity in the developing brain, and the involvement of excitatory GABA signalling in the mechanisms that make this possible.