Calcium currents and GABAB receptors in the dorsal sensory cells of the lamprey spinal cord.

Patch-clamp studies were performed on the isolated dorsal sensory cells of the spinal cords of three species of lamprey, Ichthyomyzon unicuspis, Petromyzon marinus, and Lampetra fluviatilis, to measure changes in the amplitudes of calcium current induced by GABA and its specific antagonists and agonists. The experiments showed that GABA (4 mM) reduced the peak amplitude of the calcium current by 28.5 +/- 4.9%, with subsequent recovery to 96.2 +/- 9.2% of control (n = 45). The GABAB agonist baclofen had similar effects. The GABAA agonists glycine and taurine had no effect on the Ca2+ current. The inhibitory effect of GABA was blocked by 2-hydroxysaclofen (a GABAB antagonist), but persisted in the presence of bicuculline (a GABAA antagonist). These results are evidence that the membranes of dorsal sensory cells contain GABAB receptors, which significantly increases our understanding of the mechanisms of presynaptic inhibition in the spinal cords of the cyclostomata.

The effects of baclofen on calcium channel currents in dorsal sensory cells of the spinal cord in the lamprey.

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.