Caillard O, McLean H A, Ben-Ari Y, Gaïarsa J L
Institut National de la Sant et de la Recherche M dicale U29, H pital de Port-Royal, 75014 Paris, France.
J Neurophysiol. 1998 Mar;79(3):1341-8. doi: 10.1152/jn.1998.79.3.1341.
gamma-Aminobutyric acid-B(GABAB) receptor-dependent and -independent components of paired-pulse depression (PPD) were investigated in the rat CA3 hippocampal region. Intracellular and whole cell recordings of CA3 pyramidal neurons were performed on hippocampal slices obtained from neonatal (5-7 day old) and adult (27-34 day old) rats. Electrical stimulation in the hilus evoked monosynaptic GABAA postsynaptic currents (eIPSCs) isolated in the presence of the ionotropic glutamate receptor antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) and D(-)2-amino-5-phosphovaleric acid (-AP5, 50 microM) with 2(triethylamino)-N-(2,6-dimethylphenyl) acetamine (QX314) filled electrodes. In adult CA3 pyramidal neurons, when a pair of identical stimuli was applied at interstimulus intervals (ISIs) ranging from 50 to 1,500 ms the amplitude of the second eIPSC was depressed when compared with the first eIPSC. This paired-pulse depression (PPD) was partially blocked by P-3-aminoprophyl -P-diethoxymethylphosphoric acid (CGP35348, 0.5 mM), a selective GABAB receptor antagonist. In neonates, PPD was restricted to ISIs shorter than 200 ms and was not affected by CGP35348. The GABAB receptor agonist baclofen reduced the amplitude of eIPSCs in a dose-dependent manner with the same efficiency in both adults and neonates. Increasing the probability of transmitter release with high Ca2+ (4 mM)/low Mg2+ (0.3 mM) external solution revealed PPD in neonatal CA3 pyramidal neurons that was 1) partially prevented by CGP35348, 2) independent of the membrane holding potential of the recorded cell, and 3) not resulting from a change in the reversal potential of GABAA eIPSCs. In adults the GABA uptake blocker tiagabine (20 microM) increased the duration of eIPSCs and the magnitude of GABAB receptor-dependent PPD. In neonates, tiagabine also increased duration of eIPSCs but to a lesser extent than in adult and did not reveal a GABAB receptor-dependent PPD. These results demonstrate that although GABAB receptor-dependent and -independent mechanisms of presynaptic inhibition are present onGABAergic terminals and functional, they do not operate at the level of monosynaptic GABAergic synaptic transmission at early stages of development. Absence of presynaptic autoinhibition of GABA release seems to be due to the small amount of transmitter that can access presynaptic regulatory sites.
在大鼠海马CA3区研究了γ-氨基丁酸B（GABAB）受体依赖性和非依赖性的双脉冲抑制（PPD）成分。对新生（5-7日龄）和成年（27-34日龄）大鼠海马切片上的CA3锥体神经元进行细胞内和全细胞记录。在离子型谷氨酸受体拮抗剂6-氰基-7-硝基喹喔啉-2,3-二酮（CNQX，10微摩尔）和D-（-）-2-氨基-5-磷酸戊酸（-AP5，50微摩尔）存在的情况下，用填充有2-（三乙氨基）-N-（2,6-二甲基苯基）乙胺（QX314）的电极刺激海马齿状回诱发单突触GABAA突触后电流（eIPSCs）。在成年CA3锥体神经元中，当以50至1500毫秒的刺激间隔（ISIs）施加一对相同的刺激时，与第一个eIPSC相比，第二个eIPSC的幅度降低。这种双脉冲抑制（PPD）被选择性GABAB受体拮抗剂P-3-氨基丙基-P-二乙氧基甲基磷酸（CGP35348，0.5毫摩尔）部分阻断。在新生大鼠中，PPD仅限于短于200毫秒的ISIs，且不受CGP35348影响。GABAB受体激动剂巴氯芬以剂量依赖性方式降低eIPSCs的幅度，在成年和新生大鼠中效率相同。用高钙（4毫摩尔）/低镁（0.3毫摩尔）细胞外溶液增加递质释放概率，揭示了新生CA3锥体神经元中的PPD，其具有以下特点：1）被CGP35348部分阻断；2）与记录细胞的膜钳制电位无关；3）不是由GABAA eIPSCs的反转电位变化引起的。在成年大鼠中，GABA摄取阻滞剂噻加宾（20微摩尔）增加了eIPSCs的持续时间和GABAB受体依赖性PPD的幅度。在新生大鼠中，噻加宾也增加了eIPSCs的持续时间，但程度小于成年大鼠，且未揭示GABAB受体依赖性PPD。这些结果表明，尽管GABAB受体依赖性和非依赖性的突触前抑制机制存在于GABA能终末且具有功能，但它们在发育早期的单突触GABA能突触传递水平上不起作用。GABA释放缺乏突触前自身抑制似乎是由于能够到达突触前调节位点的递质数量较少。
