Contractile properties and movement behaviour in neonatal rats with axotomy, treated with the NMDA antagonist DAP5

Abstract

Background: It is well known that axotomy in the neonatal period causes massive loss of motoneurons, which is reflected in the reduction of the number of motor units and the alteration in muscle properties. This type of neuronal death is attributed to the excessive activation of the ionotropic glutamate receptors (glutamate excitotoxicity). In the present study we investigated the effect of the NMDA antagonist DAP5 [D-2-amino-5-phosphonopentanoic acid] in systemic administration, on muscle properties and on behavioural aspects following peripheral nerve injury.

Methods: Wistar rats were subjected to sciatic nerve crush on the second postnatal day. Four experimental groups were included in this study: a) controls (injection of 0.9% NaCl solution) b) crush c) DAP5 treated and d) crush and DAP5 treated. Animals were examined with isometric tension recordings of the fast extensor digitorum longus and the slow soleus muscles, as well as with locomotor tests at four time points, at P14, P21, P28 and adulthood (2 months).

Results: 1. Administration of DAP5 alone provoked no apparent adverse effects. 2. In all age groups, animals with crush developed significantly less tension than the controls in both muscles and had a worse performance in locomotor tests (p < 0.01). Crush animals injected with DAP5 were definitely improved as their tension recordings and their locomotor behaviour were significantly improved compared to axotomized ones (p < 0.01). 3. The time course of soleus contraction was not altered by axotomy and the muscle remained slow-contracting in all developmental stages in all experimental groups. EDL, on the other hand, became slower after the crush (p < 0.05). DAP5 administration restored the contraction velocity, even up to the level of control animals 4. Following crush, EDL becomes fatigue resistant after P21 (p < 0.01). Soleus, on the other hand, becomes less fatigue resistant. DAP5 restored the profile in both muscles.

Conclusions: Our results confirm that contractile properties and locomotor behaviour of animals are severely affected by axotomy, with a differential impact on fast contracting muscles. Administration of DAP5 reverses these devastating effects, without any observable side-effects. This agent could possibly show a therapeutic potential in other models of excitotoxic injury as well.

Figure 1

Body Weight. No statistical significance was found among the experimental groups. Body weight increased with age in all groups.

Figure 2

Muscle Weight. Muscle weight was reduced by crush, after P14 in EDL and after P21 in soleus (p < 0.01). DAP-5 increased muscle weight, but not to the level of control animals.

Figure 3

EDL Force Generation. Bar chart showing the contractile force of the EDL muscle. Single twitch and maximum tetanic contraction at 100 Hz is depicted for all experimental groups in all ages. It is evident that crush animals developed significantly less tension than the controls and DAP5 administration clearly improved muscle performance.

Figure 4

Soleus Force Generation. Similar to Figure 1, showing the contractile force of soleus muscle.

Figure 5

EDL Time course of Contraction. Bar chart showing the evolution of the two parameters of the time course of contraction, among the different experimental groups. TTP:time-to-peak and HRT:Half-relaxation-time. Immature EDL is a slow muscle, but progressively its contraction is shortened in order to attain the fast profile of the adult animal. Crush disrupts this process and the muscle remains slow in all developmental stages.

Figure 6

Soleus Time course of Contraction. Bar chart showing the evolution of the two parameters of the time course of contraction, among the different experimental groups. TTP:time-to-peak and HRT:Half-relaxation-time. Soleus remains a slow muscle in all developmental stages, in all procedures.

Figure 7

Fatigue index. Bar chart showing the fatigue index for both muscles (A.EDL B.Soleus) in all developmental stages for all procedures. Fatigue index normally decreases by age. Crush renders EDL fatigue resistant after P21, while soleus becomes more fatiguable. DAP5 restored the profile in both muscles.

Figure 8

Rotarod test. Bar chart showing the duration that the animals achieved on the rotarod. Crush resulted in a definite reduction in time and DAP-5 increased the capacity.