1. The effect of spinal GABAergic neurons on the segmental neuronal network generating locomotion has been analyzed in the lamprey spinal cord in vitro. It is shown that γ-aminobutyric acid (GABA)(A)- and GABA(B)-mediated effects influence the burst frequency and the intersegmental coordination and that the GABA system is active during normal locomotor activity. 2. Fictive locomotor activity was induced by superfusing the spinal cord with a Ringer solution containing N-methyl-D-aspartate (NMDA, 150 μM). The efferent locomotor activity was recorded by suction electrodes from the ventral roots or intracellularly from interneurons or motoneurons. If a GABA uptake blocker was added to the perfusate, the burst rate decreased. This effect was counteracted by GABA(B) receptor blockade by phaclofen or 2-(OH)-saclofen. If instead a GABA(B) receptor agonist (baclofen) was added during fictive locomotion, a depression of the burst rate occurred. It was concluded that a GABA(B) receptor activation due to an endogenous release of GABA caused a depression of the burst activity with a maintained well-coordinated locomotor activity. 3. If a GABA(A) receptor antagonist (bicuculline) is applied during fictive locomotion elicited by NMDA, a certain increase of the burst rate occurred. Conversely, if a selective GABA(A) agonist (muscimol) was administered, the burst rate decreased. Similarly, if the GABA(A) receptor activity was potentiated by activation of a benzodiazepine site by diazepam, the burst rate was reduced. If, however the GABAergic effect was first enhanced by an uptake blocker (nipecotic acid), an administration of a GABA(A) antagonist (bicuculline) increased the burst rate, but in addition, the burst pattern became less regular with recurrent shorter periods without clear reciprocal burst activity. The GABA(A) receptor activity appears important for the rate control and for permitting a regular burst pattern. 4. The intersegmental coordination in the lamprey is characterized by a rostrocaudal constant phase lag of ~1% of the cycle duration between the activation of consecutive segments during forward swimming. This rostrocaudal phase lag can be reversed during backward swimming, which can be induced also experimentally in the isolated spinal cord by providing a higher excitability to the caudal segments. In a split-bath configuration, a GABA uptake blocker or a GABA(B) agonist was administered to the rostral part of the spinal cord, which caused a reversal of the phase lag as during backward swimming. If GABA(A) receptors were blocked under similar conditions, the intersegmental coordination became irregular. It is concluded that an increased GABA activity in a spinal cord region can modify the intersegmental coordination. 5. The cellular effects of GABA are due to a phasic modulation of spinal GABA neurons during fictive locomotion that acts at both a presynaptic and a postsynaptic level on the network interneurons. The lowest level of the GABA(B) agonist (baclofen, 5 μM) caused a significant reduction of the locomotor-driven excitatory postsynaptic potentials and the late afterhyperpolarization due to calcium-dependent potassium channels. The latter effect is induced and due to a modulation of voltage-dependent calcium channels. The effects known to be exerted by GABA(A) and GABA(B) receptors on the neurons of the network can account for the effects reported here on the segmental and intersegmental level.
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