Reformation of neuro-effector junctions in the regenerating rat submandibular gland

Objectives: following ligation of the main secretory duct the rat submandibular gland rapidly becomes atrophic with the loss of most acinar cells- the main target cell for parasympathetic nerves. Following ductal deligation there is a near-complete recovery of gland structure and function, as assessed by autonomimetic stimulation. The present study examines whether the parasympathetic nerves, which are required for fluid secretion, have re-attached to newly formed acinar cells to form functional neuro-effector junctions.

Methods: under recovery anaesthesia the right submandibular gland duct of rats (n= 4) was ligated with a clip via an intra-oral approach. Four weeks later rats were again anaesthetised and the ligation clip removed. Following a further 8 weeks rats were placed under a terminal anaesthesia and both submandibular ducts cannulated. Salivary flows were then stimulated electrically (chorda-lingual nerve at 2, 5 and 10 Hz) and subsequently by methacholine (whole body infusion at two doses). Glands were excised, weighed and fixed for histological examination. Rats were killed by an overdose of anaesthetic. All experiments were conducted under a Home Office license and with ethical approval.

Results: ligation caused a 50 % loss of gland weight with the loss of most acinar cells. Following deligation glands recovered half of their weight and had a normal morphology. When expressed per unit of glandular tissue methacholine-evoked flow rates were similar from control and regenerated glands. However parasympathetic nerve-evoked flows were significantly higher from regenerated glands compared to the unoperated contralateral glands.

Conclusions: In ligated/ deligated glands acinar cells have reintegrated with parasympathetic nerves at a greater ratio than occurs normally suggesting new nerve-acinar cell connections have been made. Thus parasympathetic nerves have inherent plasticity and can make new neuro-effector junctions in regenerated glands even after considerable target-cell loss.

The authors gratefully acknowledge Wellcome Trust funding.