GABA neurotransmission in the hypothalamus: developmental reversal from Ca2+ elevating to depressing

Abstract

GABA is the primary inhibitory transmitter of the adult hypothalamus, synthesized by many neurons and found in 50% of the presynaptic boutons. GABA causes a decrease in Ca2+ in mature hypothalamic neurons in vitro by depressing cellular activity through opening Cl- channels. Despite the early expression of GABAA receptors in the embryonic hypothalamus (E15), the cellular function of GABA in the developing hypothalamus has received little attention. In the present study the role of GABA in modulating intracellular Ca2+ in developing hypothalamic neurons was studied with fura-2 digital imaging. GABA (0.5- 500 microM) applied to embryonic hypothalamic neurons elicited a dramatic and rapid increase in intracellular Ca2+ This Ca2+ rise could be completely blocked by the GABAA antagonist bicuculline (20 microM) and persisted in the presence of tetrodotoxin (1 microM). The Ca2+ elevation induced by GABA was greater than that of equimolar concentrations of the excitatory transmitter glutamate in early development. The number of E15 neurons that responded to GABA with a Ca2+ rise increased during the first few days of culture, reaching 78% after 4 d in vitro. The Ca2+ rise was 87% blocked by cadmium (100 microM) and 85% blocked by nimodipine (1 microM), indicating that the mechanism of Ca2+ increase was primarily via L-type voltage operated Ca2+ channels. Addition of bicuculline to synaptically coupled cultures caused a significant decrease in Ca2+ 4–10 d after culturing, indicating hypothalamic neurons were secreting GABA at an early age of development, and that sufficient GABA was released to elicit an increase in Ca2+. This effect was seen even after blocking all glutamatergic activity with glutamate receptor antagonists. In contrast, GABA elicited no Ca2+ rise in older neurons (> 18 d in vitro), and the action of bicuculline reversed and caused a large increase in Ca2+ in spontaneously active neurons. Similar findings were obtained in cultures enriched in GABAergic neurons from the suprachiasmatic nucleus. To determine if the Ca2+ stimulating role of GABA on developing neurons was restricted to the hypothalamus and a few other regions, or whether it might exist throughout the brain, we examined the Ca2+ responses in cultured olfactory bulb, cortex, medulla, striatum, thalamus, hippocampus, and colliculus. The majority (75%) of developing neurons from each region showed a Ca2+ rise in response to GABA. Together these data suggest that GABA elevates Ca2+ in developing, but not mature, neurons from the hypothalamus and all other brain regions examined.

Response to neonatal anesthesia: Effect of sex on anatomical and behavioral outcome

“…In the neonatal mammalian hippocampus, activation of the GABA A receptor leads to chloride efflux and membrane depolarization sufficient to open voltage sensitive calcium channels (Leinekugel et al, 1995;LoTurco et al, 1995;Obrietan and van den Pol, 1995). This cellular excitation is in stark contrast to chloride influx and membrane hyperpolarization (cellular inhibition) that occurs following GABA A receptor activation in the hippocampus in adulthood (Duggan, 1978).…”

“…Many commonly used anesthetics act on the GABA A receptor, the predominant inhibitory neurotransmitter system in the adult CNS . However, GABA A receptor activation in the developing brain, including the hippocampus, cerebral cortex and hypothalamus, leads to chloride efflux and membrane depolarization sufficient to open voltage sensitive calcium channels (Leinekugel et al, 1995;LoTurco et al, 1995;Obrietan and van den Pol, 1995). GABA mediated excitation persists through the first postnatal week in the rodent hippocampus (Nuñez et al, 2005;Nuñez and McCarthy, 2007).…”

Contributions of Voltage- and Ca2+-Activated Conductances to GABA-Induced Depolarization in Spider Mechanosensory Neurons

“…In some, the GABA current reversal potentials are less negative than the resting potentials (Obrietan and van den Pol 1995;Panek et al 2002;Reichling et al 1994); in others they are closer to the resting membrane potential (Connors et al 1987). Because of this, there is some controversy about the mechanism of [Ca 2ϩ ] increase.…”

“…For instance, GABA depolarized cultured rat dorsal horn neurons and produced a [Ca 2ϩ ] increase that could be prevented by removing Ca 2ϩ from the extracellular solution (Reichling et al 1994). In rat embryonic hippocampal neurons GABA induced a [Ca 2ϩ ] increase that could be removed with Ca-channel blockers (Obrietan and van den Pol 1995).…”

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

“…Specifically, the mRNA level of GAD is twice as high in some steroid-concentrating regions of the hypothalamus in the neonatal ( brain compared with & (30). Early in development, GABA is actually excitatory and induces depolarization in neurons (14), resulting in elevated intracellular calcium levels (35). The enhanced and prolonged excitatory GABAergic input should result in substantially higher levels of neuronal excitation in the ( brain (2).…”