Diversity of oxytocin neurones: Beyond magno‐ and parvocellular cell types?

<jats:title>Abstract</jats:title><jats:p>The hypothalamic neuropeptide oxytocin (<jats:styled-content style="fixed-case">OT</jats:styled-content>), which is evolutionarily conserved among different species throughout the animal kingdom, is a key modulator of a variety of socio‐emotional behaviours such as fear, trust and empathy. <jats:styled-content style="fixed-case">OT</jats:styled-content> cells in the mammalian hypothalamus have been traditionally divided into two distinct types: agnocellular (magn<jats:styled-content style="fixed-case">OT</jats:styled-content>) and parvocellular (parv<jats:styled-content style="fixed-case">OT</jats:styled-content>) or preautonomic neurones. This distinction is based on <jats:styled-content style="fixed-case">OT</jats:styled-content> cell sizes and shapes, projections, electrophysiological activity and functions. Indeed, although neuroendocrine magn<jats:styled-content style="fixed-case">OT</jats:styled-content> neurones are known to primarily project their axons to the posterior pituitary and to a number of forebrain regions, non‐neuroendocrine parv<jats:styled-content style="fixed-case">OT</jats:styled-content> neurones have been seen as the main source of <jats:styled-content style="fixed-case">OT</jats:styled-content> innervation of the brainstem and spinal cord to control autonomic functions and pain perception. However, very recent findings have demonstrated distinct genetic profiles in <jats:styled-content style="fixed-case">OT</jats:styled-content> neurones, allowing the discrimination of at least four types of cells expressing <jats:styled-content style="fixed-case">OT</jats:styled-content>. Furthermore, unexpected axonal projections of parv<jats:styled-content style="fixed-case">OT</jats:styled-content> neurones to the forebrain and magn<jats:styled-content style="fixed-case">OT</jats:styled-content> neurones to the midbrain have been newly reported. In this review, we focus on the detailed analysis of methods of distinction between <jats:styled-content style="fixed-case">OT</jats:styled-content> cell types, in‐ and output sites, and morphology, as well as on the direct connectivity between <jats:styled-content style="fixed-case">OT</jats:styled-content> neurones and its physiological significance. Finally, we propose a hypothesis suggesting that the central <jats:styled-content style="fixed-case">OT</jats:styled-content> system is composed of more than just two <jats:styled-content style="fixed-case">OT</jats:styled-content> cell types, which needs to be confirmed by the application of available genetic and anatomical techniques.</jats:p>