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Literature highlights that serotonergic descending pathways [14] are implicated in somatosensory functions in the spinal cord and that serotonin (5-HT) in the dorsal horn might play a role in motor function through proprioceptive feedback [3, 8, 9, 13]. We hypothesized that 5-HT release in dorsal horn might represent an important factor in the completion of locomotion by facilitation of the spinocerebellar tract [8] and/or by modulation of spinal reflex pathways [9]. The present study demonstrates that during locomotor activity, 5-HT is released in layers II, III, IV, V of Rexed laminae. Microdialysis in combination with HPLC was used to measure concentrations of neurotransmitters in the lumbar dorsal horn before, during, and after a treadmill running exercise. Our results show a significant 41% increase of 5-HT release within the dorsal horn during a running exercise. 5-HT release is temporally related to exercise. The present study demonstrates that dorsal horn 5-HT release might modulate locomotion.

Glial cell-line derived neurotrophic factor (GDNF) gene therapy might offer new strategies for the treatment of Parkinson's disease (PD). GDNF is a potent dopaminergic (DA) neurotrophic factor. The effect of GDNF gene therapy was assessed using anatomical, behavioral, and neurochemical approaches. We examined the protective effect of increased striatal GDNF levels achieved by delivery of an adenoviral vector (Ad-) encoding human GDNF (Ad-GDNF). Animals were injected with Ad-GDNF prior to striatal lesion. Striatal DA concentration was measured by microdialysis. Animals receiving 6-hydroxydopamine (6-OHDA) only showed a significant decrease in rotation when compared to those receiving Ad-GDNF prior to the 6-OHDA neurotoxin. Under basal conditions, the Ad-GDNF group showed a significant (P < or = 0.05) increase (1880%) in DA concentration when compared to the 6-OHDA group. Amphetamine challenge induced a significantly (P < or = 0.05) higher release of DA in the Ad-GDNF group than in the 6-OHDA group. These findings show that long-term delivery of GDNF protein in the striatum provides significant cell, behavioral, and neurochemical protection.

The aim of the microdialysis technique is to reflect as closely as possible the status and fluctuations of substances contained in the extracellular space. Most often, microdialysis is performed with repetitively implanted probes. We have recently devised an experimental set-up which allows microdialysis to be performed in the spinal cord of unrestrained rats through chronically permanently implanted probes. In the present study, we have compared the in vitro recovery of a non-biogenic amine (DHBA) and of 5-HT, and the in vivo recovery of the former. Thus, we could extrapolate the in vivo recovery of endogenous 5-HT released. Moreover, we have found that the recovery does not vary irrespective of whether the animal is at rest or performing sustained physical exercise, and that it also remains stable with time, from 8 to 36 days after permanent implantation of the probe. We conclude that this simple method can be applied to standard experiments of microdialysis, and thus allow one to measure the actual rate of recovery for a given probe. Moreover, it permits control of the stability of dialysis parameters with time for long-term permanently implanted probes.

The ventral horn of the spinal cord is profusely innervated by serotonin (5-hydroxytryptamine, 5-HT) which presumably modulates locomotor activity through motoneurons. However, direct evidence of correlation between 5-HT release and activation of motoneurons is still lacking. In order to appreciate the functional characteristics of this innervation, we have used microdialysis to monitor the release of 5-HT in the spinal cord of rats spontaneously running on a treadmill. For this purpose, we developed an original surgical procedure adapted for the chronic implantation of a microdialysis probe in the lumbar spinal cord. The probe was kept in place for 40 days, and microdialysis experiments were carried out at days 8, 16, 20 and 32. 5-HT was detected with HPLC coupled to electrochemistry. This technique demonstrated that release of 5-HT is not increased during exercise. However, a significant decrease was measured during postexercise rest. 5-HT could still be detected 32 days after probe implantation. Detailed histological and immunocytochemical analysis based on glial fibrillary acidic protein and 5-HT immunocytochemistry showed minimal gliosis and the presence of serotonergic varicose fibers, respectively, at the probe contact. It thus appears that microdialysis can be performed through a probe implanted chronically in the spinal cord of unrestrained rat during an endurance running exercise.