Excitatory actions of GABA in developing rat hypothalamic neurones.

Abstract

1. Gramicidin-perforated patch clamp recording was employed to study GABA-mediated responses in rat hypothalamic neurones (n = 102) with an intracellular Cl-concentration unaltered by the pipette solution. 2. In young cultures after 1-7 days in vitro (DIV), GABA induced depolarizing membrane potentials (+16-5 1-3 mV) that often surpassed the threshold for the firing of action potentials (-42 + mV) and resulted in an increase in neuronal activity. The depolarizing responses to GABA in young cultures were dose dependent. The concentration of GABA necessary to evoke the half-maximal depolarization (EC50) was 2-8 /M. In contrast, GABA induced hyperpolarizing membrane potentials (-12-0 1-4 mV) and a decrease in neuronal activity in older neurones (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33). Both the depolarization and the hyperpolarization induced by GABA were blocked by bicuculline, indicating a mediation by GABAA receptors. 3. The reversal potentials of the GABA-evoked currents were between -40 to -50 mV during the first week of culture, and shifted to below -70 mV after 3 weeks of culture. In parallel, neurones that were dissociated from older animals (postnatal day 5) had a more negative reversal potential for the GABA-evoked currents than cells from younger animals (embryonic day 15), suggesting that the negative shift of the reversal potential occurs both in vitro and in vivo. Our data suggest that the mechanism for GABA-induced depolarization is the depolarized Cl-reversal potential found in young but not older neurones.4. Consistent with the depolarizing response to exogenous application of GABA, some spontaneous depolarizing postsynaptic potentials in young cultures were insensitive to AP5-CNQX, but were eliminated by bicuculline, indicating that synaptically released GABA mediated excitatory synaptic transmission in early development.5. By combining a rapid computer-controlled delivery of GABA with subthreshold positive current injections into recorded neurones, we found in young cultures that the GABAevoked depolarization could directly trigger action potentials, facilitate some depolarizing input to fire action potentials, and shunt other depolarizing input. Whether the GABAinduced depolarization is excitatory or inhibitory would be determined by the reversal potential of the GABA-evoked current, and the temporal relationship between GABAevoked depolarizations and other excitatory events. 6. We conclude that the reversal potential of the GABA-evoked current shifts negatively from depolarizing to hyperpolarizing in developing hypothalamus. Consequently, GABA neurotransmission may serve both excitatory and inhibitory roles during early development.GABA is the primary inhibitory neurotransmitter through-conductance, which usually hyperpolarizes adult neurones out the mammalian central nervous system (CNS), and due to an influx of Cl-ions. In a recent study, we found that within the hypothalamus GABA is found in half of all GABA receptors were functional very early in hypothalamic presynaptic b...

Sex differences in estrogenic regulation of neuronal activity in neonatal cultures of ventromedial nucleus of the hypothalamus

“…Our data show an interesting pattern: that changes in the overall neuronal activity are negatively correlated to those of mIPSCs, consistent with the notion that GABA function switches into an inhibitory function in these hypothalamic neurons (14). Interactions between E2 and GABAergic neurotransmission system during the critical period are important mediators of sexual differentiation in the basal forebrain (2).…”

“…4B4, n ϭ 29, P Ͻ 0.005). Because our VMN neurons are cultured for Ϸ2 weeks from neonatal pups, GABA exerts inhibitory function at this stage (14). Thus, an increase of mIPSC frequency implies an increase of presynaptic GABA release, which will inhibit neuronal activity.…”

Excitatory Actions of Endogenously Released GABA Contribute to Initiation of Ictal Epileptiform Activity in the Developing Hippocampus

“…GABAergic synapses are established first (Tyzio et al, 1999) and exert an excitatory action as measured by the capacity for GABA A receptor activation to trigger action potentials in the postsynaptic cells. GABA A receptor-mediated synaptic currents with depolarizing reversal potentials are common in the embryonic and neonatal brain (Serafini et al, 1995;Chen et al, 1996;Warren and Jones, 1997) and are most likely explained by a high intracellular chloride concentration (Owens et al, 1996;Rivera et al, 1999). In addition to triggering action potentials, the depolarizing action of GABA in the neonatal brain may induce Ca 2ϩ entry through voltagedependent Ca 2ϩ channels (Yuste and Katz, 1991;Lin et al, 1994;Leinekugel et al, 1995;Owens et al, 1996), contribute to removal of the Mg 2ϩ block from NMDA channels, and facilitate network activity (Ben Ari et al, 1997) and may thereby trigger and modulate a wide range of developmental processes (LoTurco et al, 1995;Mitchell and Redburn, 1996).…”

Theta-bursts induce a shift in reversal potentials for GABA-A receptor-mediated postsynaptic currents in rat hippocampal CA1 neurons

“…In neonatal neurons, RPs for GABA-ergic PSCs are such that the amino acid is excitatory on central neurons (Chen et al, 1996;Luhmann and Prince, 1991;Misgeld et al, 1986). This excitatory action has been suggested to substitute for AMPA-receptor mediated depolarization of postsynaptic neurons in facilitating glutamatergic synaptic plasticity through the removal of the Mg 2+ block of the NMDA receptor (Cherubini et al, 1991).…”

“…The RP of GABA-ergic PSCs is also generally thought to stabilize in adult neurons making the PSC hyperpolarizing at resting membrane potentials. As can be seen in the current study, the RP for the PSC in adult neurons does shift following a theta-burst stimulation indicating that there is still plasticity in the RP of adult neuronal GABA-ergic PSCs.In neonatal neurons, RPs for GABA-ergic PSCs are such that the amino acid is excitatory on central neurons (Chen et al, 1996;Luhmann and Prince, 1991;Misgeld et al, 1986). This excitatory action has been suggested to substitute for AMPA-receptor mediated depolarization of postsynaptic neurons in facilitating glutamatergic synaptic plasticity through the removal of the Mg 2+ block of the NMDA receptor (Cherubini et al, 1991).…”