Leucokinin neuropeptide in the tick Ixodes ricinus: Neurohaemal release site, salivary gland innervation, and receptor distribution

Leucokinin (LK) has been functionally characterised in ticks, but its anatomical organisation remains poorly understood. Here, we investigate the anatomical and molecular organisation of LK and its receptor (LKR) in the tick Ixodes ricinus using molecular and anatomical approaches. We functionally identified the I. ricinus LKR and confirmed its activation by LK. Quantitative PCR revealed lkr transcripts across life stages and in peripheral tissues, including salivary glands. Within the synganglion, LK was localised to a defined population of medial protocerebral neurosecretory neurons by immunohistochemistry and confirmed by in situ hybridisation. Their axons project to the dorsolateral perineurium of the synganglion, suggesting a neurohaemal release site supported by ultrastructural evidence. LKR immunoreactivity was detected in a limited subset of neuronal somata within the synganglion. In peripheral tissues, LK-immunoreactive axons innervate salivary glands, mainly type II acini and occasionally type III acini. LKR immunoreactivity was detected in the corresponding acinar cells. This spatial organisation of axonal projections and receptor localisation suggests a role for LK in the regulation of acinar activity. Together, these findings refine the anatomical organisation of LK in I. ricinus and provide a framework for future functional studies.