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Early in their careers, students and postdoctoral researchers in neuroscience have to make important decisions that might have long-lasting consequences for their success as researchers. In this Viewpoint, four high-profile neuroscientists provide their thoughts on some of the issues they might want to consider.

The genetic interactions that pattern the embryonic telencephalon are highly complex. Fishell and Hébert bring clarity to these events by describing the key genetic interactions that underlie the patterning of the early telencephalon into distinct proliferative zones.

The role of sonic hedgehog as a morphogen involved in establishing ventral cell identity in the CNS is well known. Fishell and colleagues outline the current understanding of the molecular pathways involved in patterning and describe several recently identified and unrelated roles for hedgehog signalling.

In contrast to typical strengthening of developing circuits, thalamocortical inputs are transient on somatostatin neurons. Here the authors show this process involves transcriptional regulation from metabotropic signaling, which controls the proper development of mouse behaviors.

How genetic and environmental factors contribute to the generation of various subtypes of inhibitory neurons called interneurons in the brain is unclear. A study in mice provides new insight into this process.

Debate continues over how best to classify cortical interneurons. Standardizing the nomenclature used to describe these cells might aid classification efforts. Here, a committee of interneuron researchers propose a standardized set of terms to describe the morphological, molecular and physiological features of an interneuron.

New work identifies a latent population of neuronal progenitor cells in neocortical layer 1 of adult rats. These cells proliferate in response to forebrain ischemia, then integrate into the cortical network as interneurons.

Our growing understanding of cortical interneuron diversity has been matched by increasing interest in the underlying developmental mechanisms. Wamsley and Fishell describe current models of interneuron specification, highlighting the contribution of activity-dependent mechanisms to this process.

Single-nucleus RNA-sequencing analyses of brain from humans, macaques, marmosets, mice and ferrets reveal diverse ways that interneuron populations have changed during evolution.

The embryonic emergence of interneuron subtypes in mice is revealed by integrated single-cell transcriptomic analysis along a developmental time course.

The transcriptional programs and chromatin accessibility changes that cause two related interneuronal subtypes to diverge from common developmental origins and acquire specialized properties are elucidated.

This study describes a series of new gene-regulatory sequences that restrict expression of viral transgenes to specific interneuron subtypes, allowing for selective monitoring and manipulation of their activity from mice to humans.

Chen, Deister et al. show that Shank3B-knockout mice display hypersensitivity to tactile sensory stimulation and that dysfunction of interneurons in somatosensory cortex contributes to the sensory hyper-reactivity in this mouse model of autism.

Neurogliaform interneurons constitute an essential component of cortical circuits. This paper provides evidence that, during early development, superficial neurogliaform cells of the primary somatosensory barrel field cortex receive prominent innervation from the thalamus. These afferents also activate postsynaptic NR2B-containing NMDA receptors, which are essential for the neurons' proper cortical integration.

The authors find that long-range axons from primary motor cortex (vM1) preferentially recruit vasointestinal peptide (VIP)-expressing interneurons in somatosensory cortex (S1). VIP neurons in turn inhibit somatostatin-expressing interneurons that target the distal dendrites of pyramidal cells in S1. This dis-inhibitory circuit is active during voluntary movement, suggesting that it participates in the modulation of primary cortical sensory processing by motor cortex.

The ability to target and manipulate specific neuronal populations is crucial for understanding brain function. In this report, the authors describe a novel virus that restricts gene expression to telencephalic GABAergic interneurons, allowing for morphological visualization, activity monitoring and functional manipulation of interneurons in mice and in non-genetically tractable species.

Can the adult neocortex in primates generate new neurons? A new study uses a clever way of determining the age of postmortem human neurons to conclude that there is very little postnatal cortical neurogenesis—or none at all.

The neocortex contains a large diversity of interneurons but the subtype-specific roles of these cells in establishing excitatory and inhibitory circuits are not well understood. Here the authors genetically delete parvalbumin- and somatostatin-positive interneurons during mouse development and study the functional effects in postnatal cortical circuits.

Single-cell RNA sequencing with parallel tagging of progenitor cells is used to track clonal relationships and transcriptomic signatures during development of the mouse forebrain.

Cortical excitatory/inhibitory (E/I) imbalance is a feature of autism spectrum disorder (ASD). Here, the authors show that FoxG1 regulates the formation of cortical GABAergic circuits affecting social behaviour during a specific postnatal time window in mouse models of ASD.

A subset of adult neural stem cells, responsive to sonic hedgehog, are more effective at remyelination when the transcription factor Gli1 is inhibited.

Oxytocin is shown to sharpen neuronal network activity by increasing fast-spiking interneuron activity.

The classification of cortical neurons, including interneurons, remains a thorny issue in neuroscience. This Analysis article presents and tests a possible taxonomical solution for classifying cortical GABAergic interneurons based on a web-based interactive system that allows experts to classify neurons with pre-determined morphological criteria.