Abstract
The overall objective of this thesis is to characterize the mechanisms involved in the generation of locomotor activity in the spinal cord. To this end, we initially used the lamprey spinal cord to determine the transmitter phenotype of commissural interneurons (CINs). In addition, we developed a novel preparation of the brainstem-spinal cord of the adult zebrafish and used it to determine the mechanisms of locomotor pattern generation. The results obtained show:
(1) That the left–right alternation is maintained because commissural glycinergic interneurons outnumber the glutamatergic ones. It was also shown that CINs display a graded rostrocaudal distribution and are immunoreactive to both glycine and glutamate. The difference in the proportion of inhibitory and excitatory CINs represents an anatomical substrate that ensures the predominance of alternating activity during locomotion.
(2) That adult zebrafish spinal cord can produce locomotor activity and be used to study the organization of the locomotor circuitry. In this study we developed both a semi-intact and an in vitro preparation of the juvenile/adult zebrafish spinal cord that are able to generate a rhythmic motor pattern with characteristics similar to swimming in intact animals. In the in vitro preparation, spinal cord neurons were accessible for patch-clamp recordings to study their pattern of activation during fictive locomotion.
(3) That 5-HT is released within the locomotor circuitry and acts as an intrinsic modulator to set the baseline locomotor activity. 5-HT decreases the frequency of the locomotor rhythm by increasing the mid-cycle inhibition and delaying the onset of the following on- phase excitation. Thus endogenous 5-HT sets the balance between excitation and inhibition and set the baseline locomotor frequency.
(4) That a brief stimulation of descending inputs at a defined region located at the first segments of the spinal cord induces long-lasting coordinated swimming activity. The burst amplitude, frequency and duration of the episode can increase by changing the frequency and strength of the stimulus pulses. The descending inputs seems to act as a switch to turn on the activity of the spinal locomotor network in the caudal spinal cord that relies mostly on iontropic glutamate receptors.
In summary, our results provide anatomical evidence underlying the dominance of reciprocal inhibition over excitation during locomotion. We also showed that our newly developed in vitro adult zebrafish spinal cord preparation can be used to study spinal circuitry underlying locomotion.
