Ultrastructural localisation of ATP-gated P2X2 receptor immunoreactivity in the rat hypothalamo-neurohypophysial system

The distribution of the P2X2 subtype of the purine receptor associated with the extracellular signalling activities of ATP was studied in the rat hypothalamo-neurohypophysial system at the electron microscope level. Receptors were labelled with ExtrAvidin-horseradish peroxidase preembedding immunocytochemistry using a polyclonal antibody against a fragment of an intracellular domain of the receptor. Immunoreactivity to P2X2 receptors was localised in: (i) paraventricular and supraoptic nuclei—in subpopulations of endocrine neurones, neurosecretory and non-neurosecretory axons and dendrites; and (ii) the neurohypophysis—in pituicytes and subpopulation of neurosecretory axons. In both the hypothalamic nuclei examined, labelled asymmetric axo-dendritic synapses were commonly observed. These synapses involved either P2X2-labelled axon terminals (synaptic buttons) and unlabelled dendrites or labelled dendrites and unlabelled axon terminals. Axo-somatic synapses established by P2X2-positive axons on P2X2-positive endocrine cell bodies as well as on P2X2-negative somata were also observed. The functional significance of these findings is discussed.