Dorsal sensory cells isolated from the spinal cord of the lamprey species Ichthyomyzon unicuspis and Lampetra fluviatilis were used for whole-cell patch-clamp studies of the effects of baclofen on calcium channel currents, evoked in conditions in which Na+, K+ currents were blocked, by depolarizing membranes from constant holding potentials of -100 or -80 mV to +30 mV. Ba ions were used as carriers of currents through calcium channels. These studies demonstrated that baclofen (0.5 mM) decreased the peak amplitude of the Ba2+ current by an average of 22.5 +/- 4.2% (n = 12) in dorsal sensory cells of the lamprey Ichthyomyzon unicuspis and by 28.4 +/- 3.3% in the dorsal sensory cells of Lampetra fluviatilis (n = 25). The conductivity of dorsal sensory cell membranes in the presence of baclofen (and GABA) did not change. The blocking action of baclofen persisted in the presence of bicuculline (100 microM) and was lifted by addition of delta-aminovaleric acid and 2-hydroxysaclofen to the perfusing solution. These results are interpreted as evidence for the presence of GABAB receptors in dorsal sensory cell membranes. The data were compared with published results, and the question of the functional significance of GABAB receptors in the dorsal sensory cells (primary afferent cells) of cyclostomata is discussed.
Synaptic responses in motoneurons in the isolated spinal cord of the lamprey during stimulation of the reticulospinal axons were examined. Chemical transmission in the synapses was partially or completely blocked by temperature reduction of the perfusing solution, pentobarbitone application or substitution of Mn2+ ions for Ca2+ in the perfusate. Excitatory postsynaptic potentials (EPSPs) which were indifferent to the influences mentioned above, had an amplitude of 6-12 mV and were capable of evoking action potentials (APs) in motoneurons due to their high amplitude, the absence of a shunting effect at the postsynaptic membrane and the fast rise-time of the wave front. The suggestion is made that the electrical transmission is involved in functioning of the lamprey nervous system. Its stability and efficiency are likely to ensure functional connection between the brain and spinal cord under such unfavourable conditions when the chemical transmission does not operate and when the ability for locomotion would be prerequisite for the individual to survive.
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