Distribution of GABAergic neurons in late fetal and early postnatal rat hippocampus

Involvement of GABAA receptors in the outgrowth of cultured hippocampal neurons

Whereas GABA is a major inhibitory neurotransmitter in the adult central nervous system, recent experiments performed in our laboratory have shown that the activation of GABAA receptors in the hippocampus leads to excitatory effects during the early post-natal period. The possible consequence of a depolarizing effect of GABA was assessed on the neuritic outgrowth of embryonic hippocampal neurons in culture. No morphological alterations were observed when hippocampal neurons were cultured for three days in the presence of muscimol, a GABAA receptor agonist. In contrast, the neuritic outgrowth of cultured hippocampal neurons was profoundly affected by the presence of bicuculline in the culture medium. In the presence of this GABAA receptor antagonist neurons displayed a reduction in the number of primary neurites and branching points, resulting in a concomitant decrease of the total neuritic length. Thus, this study suggests that GABA, acting on GABAA subtype of receptors, is able to affect the development of the hippocampus.

Electrophysiological properties of identified postnatal rat hippocampal pyramidal neurons in primary culture

Electrophysiological experiments were performed on primary cell cultures of retrogradely labelled postnatal rat hippocampal neurons. Rhodamine microspheres injected into the dorsal fornix clearly labelled the pyramidal cell layer of the hippocampus while excluding the other hippocampal layers and the dentate gyrus. In dissociated cell culture, the labelled cells were easily identified by fluorescence microscopy. Anti-neuron-specific enolase and anti-glial fibrillary acidic protein antibody staining confirmed that the labelled cells were neurons. The input resistance decreased from 2 G omega to 450 M omega, the input capacitance increased from 25 pF to 75 pF, the percentage of cells showing repetitive action potentials increased from 6% to 30%, and both peak GABA and glutamate responses increased over 100% during the 0 to 10 days time period investigated. This increase in chemosensitivity can be accounted for by an increase in cell size without an increase in the specific amino acid gated channel density. The subset of hippocampal neurons identified by the retrograde tracer technique are similar to non-labelled neurons with respect to the electrophysiological and pharmacological variables investigated. Nevertheless, it is likely that identified neurons may possess unique properties not evident in this study and refinement of the dissociated cell culture system using identified neuronal subpopulations may facilitate investigations looking at neuronal interactions such as synapse formation.

Modulation of decay kinetics and frequency of GABAA receptor-mediated spontaneous inhibitory postsynaptic currents in hippocampal neurons

Inhibitory postsynaptic currents mediated by spontaneous activation of GABAA receptors were studied using whole-cell voltage-clamp recordings in granule cells of the adult rat (postnatal day 60+) dentate gyrus in 400-microns-thick coronal half-brain slices maintained at 34-35 degrees C. The average amplitude of spontaneous inhibitory postsynaptic currents remained constant during a given recording period (i.e. no rundown was noted). The spontaneous currents had an average conductance between 200-400 pS, were mediated by Cl- flux through GABAA receptor/channels since they reversed at the Cl- equilibrium potential and were blocked by bicuculline or picrotoxin. Their mono-exponential decay time-constants (range: 4.2-7.2 ms) were prolonged by midazolam and pentobarbital in a dose-dependent manner. The effect of midazolam was reversed by the benzodiazepine receptor antagonist flumazenil (RO 15-1788) which, by itself, had no effect on the decay time-constant. The decay time-constant was also dependent on membrane voltage and on temperature. A 132-mV change in membrane potential produced an e-fold prolongation of the decay while the Q10 (between 22-37 degrees C) of the decay rate was 2.1. Within a given neuron, the frequency of spontaneous GABAergic events was remarkably constant over long time-periods, though the mean frequency among different cells showed large variability. Spontaneous miniature inhibitory postsynaptic currents also persisted under experimental conditions such as the presence of extracellular tetrodotoxin (1 microM), Cd2+ (200 microM) or lowered extracellular Ca2+/elevated Mg2+, which effectively abolished all stimulus-evoked GABAergic neurotransmission. The frequency of tetrodotoxin-resistant miniature events was increased by elevating extracellular K+ concentration and was diminished by the GABAB receptor agonist (-)baclofen only at a dose (50 microM) which was an order of magnitude larger than that required to depress stimulus-evoked responses. These findings are consistent with different mechanisms being responsible for the spontaneous and stimulus-evoked release of GABA from interneuron terminals and also identify pre- and postsynaptic modulatory factors of the endogenous, action-potential-independent, GABAergic neurotransmission as being important determinants of the excitability level of mammalian CNS neurons.

Morphology of intracellularly labeled interneurons in the dentate gyrus of the immature rat

Although many aspects of the morphological development of interneurons in the dentate gyrus have been described, the full extent of their dendrites and local axon projections in immature rodents has not been examined. Here intracellular labeling was used to assess the branching patterns of interneurons in the dentate gyrus of rat pups between 7 and 9 days of age. Labeled neurons were located within or just below the granule cell layer, and most were classified as GABAergic basket neurons on the basis of their dendritic morphologies. All labeled interneurons exhibited immature characteristics. Spines were present on cell bodies and dendrites, and growth cones were visible on some dendrites and axons. In spite of these immature features, the dendrites and axon arbors of the labeled neurons were extensive. Many apical dendrites reached the top of the molecular layer, and a number of basal dendrites extended to the CA3 pyramidal cell layer of the hippocampus. Elaborate axon plexuses were present within the dentate gyrus itself, and axon collaterals of several neurons extended beyond the dentate gyrus to branch within regions CA3 and CA1 of the hippocampus. These results indicate that the dendrites and axon collaterals of dentate interneurons are extensive at a time when the principal neurons, the granule cells, are still proliferating. These data are consistent with the idea that GABAergic interneurons may influence granule cell development in the dentate gyrus, as well as pyramidal cell maturation in the hippocampus proper.

A burst-dependent hippocampal excitability defect elicited by potassium at the developmental onset of spike-wave seizures in the Tottering mutant

Hippocampal CA3 pyramidal neurons in the adult epileptic mutant mouse tottering (tg) show normal intrinsic membrane properties, yet fire abnormally prolonged paroxysmal depolarizing shifts (PDS) during in vitro exposure to elevated extracellular potassium solutions. Intracellular recordings in immature mutants reveal that this network burst abnormality is present during the developmental period that coincides with the onset of seizures in the mutant (19-20 postnatal days), and is significantly more pronounced at this age than at adulthood. These data are inconsistent with the hypothesis that the mutant PDS prolongation represents a secondary consequence of a prolonged history of repeated seizures and suggest that it may reflect a cellular epileptogenic phenotype more directly related to the primary neuropathological expression of the tg gene.

Region-specific expression of GABAA receptor alpha 3 and alpha 4 subunits mRNAs in the rat brain

The expression of mRNAs encoding the alpha 3 and alpha 4 subunits of the gamma-aminobutyric acid A (GABAA) receptor in the rat brain was investigated by in situ hybridization histochemistry. Both subunits showed a wide but uneven distribution, which did not coincide with the distribution of any other subunit so far reported. The cerebral cortex, anterior olfactory nucleus, lateral septum, subiculum, lateral and medial nuclei of the amygdaloid complex, anterior nuclei of the thalamus, pars compacta of the substantia nigra, trigeminal sensory nuclei, and cochlear nucleus were some of the areas where strong expression of mRNA for both the alpha 3 and alpha 4 subunits was detected. In the mitral cell layer of the olfactory bulb, the preoptic area and locus coeruleus, strong expression of only the alpha 3 subunit was detected. In the granular cell layer of the olfactory bulb, caudate-putamen, tenia tecta, pyramidal cell layer of the CA region and granular cell layer of the dentate gyrus in the hippocampal formation, dorsomedial and ventrolateral nuclei of the thalamus, dorsal part of the lateral geniculate body, preolivary nuclei and pontine nuclei, only the alpha 4 subunit showed strong expression. The diverse distribution of these two subunits is considered to indicate that each has a different role in the central nervous system.

Development of GABA-mediated, chloride-dependent inhibition in CA1 pyramidal neurones of immature rat hippocampal slices

1. gamma-Aminobutyric acid (GABA)-mediated, Cl(-)-dependent inhibitory postsynaptic potentials (IPSPs) and GABA currents in immature rat hippocampal CA1 neurones were studied using the whole-cell recording technique in brain slices. 2. IPSPs evoked by electrical stimulation were observed in postnatal 2- to 5- (PN2-5), 8- to 13-(PN8-13) and 15- to 20-(PN15-20)day-old CA1 neurones. In the presence of glutamate receptor blockers 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D-2-amino-5-phosphonovaleric acid (APV), the reversal potential for the IPSP (EIPSP) was near the resting membrane potential (RMP) in the PN2-5 neurones, but 13 and 25 mV more negative than the RMP in PN8-13 and PN15-20 neurones respectively. IPSPs and GABA currents were blocked by the GABAA-receptor antagonists bicuculline or picrotoxin. 3. The reversal potential for somatic GABA currents (EGABA) was examined in the presence of tetrodotoxin (TTX). There was a strong dependence of the EGABA upon the patch pipette [Cl-] ([Cl-]p). indicating that the GABA currents were mediated by a Cl- conductance. In PN2-5 neurones, EGABA agreed with the value predicted by the Goldman-Hodgkin-Katz equation at given concentrations of internal and external anions permeable through GABA-activated Cl- channels, whereas EGABA in older neurones was 8-18 mV more negative. 4. Examination of the relations between EGABA, holding potential, [Cl-]p and resting conductance indicated that the membrane of the PN2-5 neurones was readily permeable to Cl- which followed a passive Donnan equilibrium. Passive distribution of Cl- played a decreasing role in PN8-13 neurones and in PN15-20 neurones. 5. To assess the contribution of outward Cl- co-transport, bath applications of high K+ or furosemide were performed. High K+ and furosemide caused a reversible positive shift of EGABA in PN15-20 neurones. Raising the temperature moved EGABA to a more negative potential, with a Q10 of 5 mV. A similar change of EGABA in response to high K+, but not to furosemide, was found in PN8-13 neurones. 6. The present data indicate the existence of GABAA-mediated inhibitory synaptic connections in CA1 neurones at the earliest stages of postnatal life. During the first postnatal week, Cl- ions are passively distributed and the EIPSP and EGABA are near the RMP.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparison of the ontogeny of excitatory and inhibitory neurotransmission in the CA1 region and dentate gyrus of the rat hippocampal formation

In vivo electrophysiological experiments were used to chart the ontogeny of excitatory and inhibitory neurotransmission in the hippocampal formation of rats. Using standardized protocols, responses in the dentate gyrus were quantified and systematically compared to similar measurements obtained in the CA1 region. Measurements were taken at numerous ages, ranging from postnatal day (PN) 6 to adults (PN 60). Excitation was monitored by two parameters recorded with extracellular electrodes in response to monosynaptic inputs to CA1 pyramidal cells or to dentate gyrus granule cells: maximum population spike (PSmax) amplitudes and maximum population excitatory postsynaptic potential slopes (pEPSP slopemax). Inhibition was assessed by a paired-pulse protocol to measure maximal inhibition (the potency of inhibition at an interpulse interval of 20 ms) and duration of inhibition (the interpulse interval at which paired-pulse inhibition changed to paired-pulse facilitation). Excitatory parameters matured later in the dentate gyrus than in CA1, consistent with the later appearance of granule cells. Until PN 21, pEPSPmax values in the dentate gyrus paralleled those in CA1; thereafter they diverged with far larger values in the dentate gyrus. Inhibitory parameters reached adult values between PN 14 and 18. In both regions paired-pulse responses consisted of three phases: (1) an initial inhibition; (2) a second facilitatory phase; and (3) a later inhibition. The maximal inhibition in the initial phase was comparable in both regions, but lasted longer in the CA1 region. The facilitation in the second phase was greater in the dentate gyrus, and the inhibition in the third phase was greater in the dentate gyrus. Results are discussed in terms of neurogenesis of principal cells and GABAergic cells in the regions of interest.

Constitutive expression of the mature array of neurofilament proteins by a CNS neuronal cell line

Neurofilament protein expression was examined immunochemically in a neuronal cell line derived from postnatal day 21 septal tissue. The SN48.1p cell line was found to constitutively synthesize an array of neurofilament proteins typical of a mature neuron. All three neurofilament subunits (NF-L, NF-M, and NF-H) as well as differentially phosphorylated isoforms (P-, P+, P++, and P ) of NF-M and NF-H were identified by immunoblot analysis. Immunofluorescence studies revealed that the neurofilament proteins were components of discrete, filamentous structures. Abnormal intracellular aggregations of neurofilament proteins were never observed. Some SN48.1p cells apportioned specific isoforms into selected intracellular regions based on the molecular weight and phosphorylation level of the protein. NF-L was preferentially localized to perikarya and proximal neurites; NF-M[P++] and NF-H[P ] were distributed to distal aspects of neurites. The expression of these differentiated features of neurofilament proteins and, presumably, the synthesis of the kinases and phosphatases required for normal neurofilament metabolism occurred in the absence of growth factors, differentiating agents, and specialized culture substrates. In addition, the non-neuronal intermediate filaments glial fibrillary acidic protein and epithelial cytokeratin proteins were absent. These data demonstrate that SN48.1p cells exhibit a neurofilament phenotype characteristic of mature neurons and provide a unique model to examine the expression and function of neurofilaments in differentiated neuronal cells.

GABAergic mechanisms in the CA3 hippocampal region during early postnatal life

The developmental pattern of GABAergic neurons in the rat hippocampus during the first week of postnatal life shows several particularities both from a morphological and physiological point of view: (1) GABA immunoreactive neurons which are initially localized in a deep and superficial layer, progressively disappear from these two layers. From the end of the first postnatal week, GABAergic neuronal somata appear throughout the whole hippocampus, but GABA immunoreactive terminal structures are not frequent until the second postnatal week. (2) Intracellular observations in slices reveal the presence in CA3 pyramidal neurons between P0 and P6 (postnatal days) of spontaneous giant depolarizing potentials (GDPs); these are mediated by GABA acting on GABAA receptors and modulated presynaptically by NMDA receptors. During this period of development, GABA and GABAA analogues have a depolarizing action at resting membrane potential. Bicuculline at this developmental stage blocks completely spontaneous and evoked synaptic potentials. During the second postnatal week, when GABA responses shift from depolarizing to hyperpolarizing, bicuculline induces spontaneous interictal discharges. It is suggested that the positive feedback of the GABAergic interneuron on the pyramidal neuron during the first week of life may account for the generation of GDPS which may play an important role in synaptogenesis.

GABA mediated excitation in immature rat CA3 hippocampal neurons

Intracellular recordings from rat hippocampal neurons in vitro during the first postnatal week revealed the presence of spontaneous giant depolarizing potentials (GDPs). These were generated by the synchronous discharge of a population of neurons. GDPs reversed polarity at -27 and -51 mV when recorded with KCl or K-methylsulphate filled electrodes, respectively. GDPs were blocked by the GABAA receptor antagonist bicuculline (10 microM). Iontophoretic or bath applications of GABA (10-300 microM) in the presence of tetrodotoxin (1 microM), induced a membrane depolarization or in voltage clamp experiments an inward current which reversed polarity at the same potential as GDPs. The response to GABA was blocked in a non-competitive manner by bicuculline (10 microM) and did not desensitize. GABA mediated GDPs were presynaptically modulated by N-methyl-D-aspartate (NMDA) and non-NMDA receptors. Their frequency was reduced or blocked by NMDA receptor antagonists and by the rather specific non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). The frequency of GDPs was enhanced by glycine and D-serine (10-30 microM) in a strychnine insensitive manner. This effect was blocked by AP-5, suggesting that it was mediated by the allosteric modulatory site of the NMDA receptor. These observations suggest that most of the 'excitatory' drive in immature neurons is mediated by GABA acting on GABAA receptors; furthermore excitatory amino acids modulate the release of GABA by a presynaptic action on GABAergic interneurons.