Nociceptive spinothalamic tract and postsynaptic dorsal column neurons are modulated by paraventricular hypothalamic activation

Previously, we demonstrated that stimulation of the paraventricular hypothalamic nucleus diminishes the nociceptive dorsal horn neuronal responses, and this decrease was mediated by oxytocin in the rat. In addition, we have proposed that oxytocin indirectly inhibits sensory transmission in dorsal horn neurons by exciting spinal inhibitory GABAergic interneurons. The main purpose of the present study was to identify which of the neurons projecting to supraspinal structures to transmit somatic information are modulated by the hypothalamic‐spinal descending activation. In anaesthetized rats, single‐unit extracellular and juxtacellular recordings were made from dorsal horn lumbar segments, which receive afferent input from the toe and hind‐paw regions. The projecting spinothalamic tract and postsynaptic dorsal column system were identified antidromically. Additionally, in order to label the projecting dorsal horn neurons, we injected fluorescent retrograde neuronal tracers into the ipsilateral gracilis nucleus and contralateral ventroposterolateral thalamic nucleus. Hence, juxtacellular recordings were made to iontophoretically label the recorded neurons with a fluorescent dye and identify the recorded projecting cells. We found that only nociceptive evoked responses in spinothalamic tract and postsynaptic dorsal column neurons were significantly inhibited (48.1 ± 4.6 and 47.7 ± 8.2%, respectively) and non‐nociceptive responses were not affected by paraventricular hypothalamic nucleus stimulation. We conclude that the hypothalamic‐spinal system selectively affects the transmission of nociceptive information of projecting spinal cord cells.