Peripheral hypersensitivity to subthreshold stimuli persists after resolution of acute experimental disc-herniation neuropathy

Objective: While acute disc-herniation radiculopathy frequently resolves without clinical sequelae, some patients experience long-term sensory or motor dysfunction. This study examined chronic sensitivity of the rodent hindpaw after resolution of an acute inflammatory neuropathy. Methods: C57BL/6 mice underwent mid-thigh sciatic nerve exposure, with sham animals exposed and experimental animals injured by placement of littermate tail nucleus pulposus (NP). Animals were evaluated for mechanical allodynia (Von Frey), thermal sensitivity (heat withdrawal and acetone latency), and gait stability (RotaRod), until the acute nociceptive phenotype resolved. Thereafter, animals were injected with intraplantar subthreshold capsaicin or vehicle followed by the same testing. At sacrifice, sciatic nerves were assessed for macrophage infiltration by immunohistochemistry, and dorsal root ganglion (DRG) explants were assessed for capsaicin sensitivity using cobalt staining. Results: NP-treated animals were allodynic after subthreshold capsaicin delivery compared with sham-operated controls and NP-treated animals delivered vehicle only. Early intraneural macrophage infiltration at one week dissipated by this three week timepoint. DRGs derived from NP-treated animals exhibited greater cobalt staining upon capsaicin exposure compared with shams. Conclusion: Non-compressive disc herniation creates long-term sensitization in the sciatic nerve distribution. This persists despite resolution of acute intraneural macrophage migration, and the demonstrated role of TRPV1 provides insight into the transformation of acute inflammatory pain into chronic neuropathic pain.

Motor organization in pharynx of Helix pomatia

Identified motor neurons in the buccal ganglia of Helix pomatia and pharynx muscles innervated by them were studied with intracellular recording and cobalt staining. Retrograde cobalt staining via the buccal nerves indicated that neurons occupy relatively constant positions within the ganglia. With intracellular cobalt staining it was shown that the shape of a representative motor neuron (B4) is similar in different preparations. In some cases, however, deviations from the normal pattern of axon distribution were found. Presumed motor endings of neuron B4 in the muscle were also visualized with intracellular staining. Recordings from individual motor neurons show typical phase relationships of spontaneous spike activity. Most motor neurons are active in the retraction phase of the radula. Only excitatory motor neurons were found. Most neurons directly supply more than one muscle. Amplitude of excitatory junction potentials (EJP) and plasticity at neuromuscular junctions from one neuron are similar in different muscles. Single muscle fibers receive polyneuronal innervation. Activity of single motor neurons already leads to muscle contraction even without spiking of the muscle cells. Muscle tension depends on integrated EJP size. Most motor neurons supply typical combinations of a set of muscles. Thus, several muscles can be activated synchronously by activity of a single motor neuron. In this way muscle combinations are predetermined morphologically by the peripheral branching patterns of the respective neurons.