Modulation by different GABAB receptor types of voltage-activated calcium currents in rat thalamocortical neurones.

1. The effects of the GABAB receptor agonist baclofen on the voltage-dependent Ca2+ currents were studied in rat thalamocortical neurones with the use of whole cell voltage-clamp recordings in brain slices. 2. The contribution of N-, L- and P-types of Ca2+ channels to the total high voltage-activated Ca2+ (HVA Ca2+) current was assessed by the use of omega-conotoxin, nifedipine and omega-agatoxin IVA, respectively. No P-type current could be detected. Thus, the HVA Ca2+ current contained an N- and an L-type current (23 and 15% of the total current, respectively) and a residual current, which will be referred to as the 'R' component. 3. Baclofen (1-50 microM) had no effect on the low voltage-activated (LVA) Ca2+ current (IT). 4. At low concentrations (0.5-10 microM), baclofen decreased the HVA Ca2+ currents by about 10-20% without a marked modification on the kinetics, whereas 50 microM baclofen decreased the HVA Ca2+ currents by about 40% with a pronounced slowing down of the kinetics. 5. The 10-20% decrease of the total HVA Ca2+ currents produced by the low concentrations of baclofen occurred as the result of a 30% block of the 'R' component. The additional decrease observed with the dose of 50 microM was due to a full block of the N-type current. The L-type was unaffected by baclofen. 6. The effect of baclofen on the total HVA Ca2+ current was partially blocked by GABAB receptor antagonists indicating that it occurred through stimulation of GABAB receptors. 7. The effect of baclofen on the N-type current was abolished by CGP 35348 (100 microM) and CGP 55845A (100 nM). The effect on the 'R' component was also antagonized by CGP 55845A (100 nM) although with a lower potency, but was not blocked by CGP 35348 (100 microM). 8. We conclude that the effects of baclofen on the various components of the HVA Ca2+ currents occur through different types of GABAB receptors. One receptor has a high affinity for baclofen (i.e. saturated by concentrations as low as 0.5 microM), is insensitive to CGP 35348, is coupled to the 'R' component and is responsible for a maximum 20% decrease in the total HVA Ca2+ current. The other receptor has a lower affinity for baclofen (i.e. affected by a concentration of 50 microM), is sensitive to CGP 35348, is coupled to the N-type Ca2+ current and is responsible for the additional 20-30% decrease in the HVA Ca2+ current observed with 50 microM baclofen.