Transient presence and functional interaction of endogenous GABA and GABAA receptors in developing rat optic nerve

Review : Glial Neuronal Interactions: A Physiological Perspective

Throughout the nervous system, neurons are closely surrounded by glial cells, leaving only a 20-nm wide extracellular space filled with interstitial fluid. Ions, transmitters, hormones, nutrients, and waste products all share this narrow diffusion pathway. Because the interstitial space occupies only a small volume, neuronal activity can lead to appreciable changes in the extracellular concentration of ions, protons, and neurotrans mitters. These changes can affect neuronal activity and are believed to be influenced by glial cells. The proximity of glial processes to synapses and axons make glial cells ideal partners to sequester ions and transmitters released by neurons. The failure of glial cells to perform such essential homeostatic functions can have profound effects, and these homeostatic activities may constitute one way in which glial cells can influence neuronal signaling. In addition, glial cells, which, unlike most neurons, are coupled to each other through gap-junctions, communicate with each other and possibly also with adjacent neurons through prop agated intracellular Ca2+waves. The importance of such interglial signaling is not understood. Additionally, glial cells and neurons mutually modulate their expression of ion channels, most likely through factors re leased into the extracellular space. The range of responses observed in glial cells and their intimate anatomical relationship with neurons suggest a broader role for glia than is currently appreciated. It also emphasizes the importance of a better understanding of glial-neuronal interactions to an understanding of brain function. The Neuroscientist 1:328-337, 1995

Neurotransmitter signaling in white matter

White matter (WM) tracts are bundles of myelinated axons that provide for rapid communication throughout the CNS and integration in grey matter (GM). The main cells in myelinated tracts are oligodendrocytes and astrocytes, with small populations of microglia and oligodendrocyte precursor cells. The prominence of neurotransmitter signaling in WM, which largely exclude neuronal cell bodies, indicates it must have physiological functions other than neuron-to-neuron communication. A surprising aspect is the diversity of neurotransmitter signaling in WM, with evidence for glutamatergic, purinergic (ATP and adenosine), GABAergic, glycinergic, adrenergic, cholinergic, dopaminergic and serotonergic signaling, acting via a wide range of ionotropic and metabotropic receptors. Both axons and glia are potential sources of neurotransmitters and may express the respective receptors. The physiological functions of neurotransmitter signaling in WM are subject to debate, but glutamate and ATP-mediated signaling have been shown to evoke Ca(2+) signals in glia and modulate axonal conduction. Experimental findings support a model of neurotransmitters being released from axons during action potential propagation acting on glial receptors to regulate the homeostatic functions of astrocytes and myelination by oligodendrocytes. Astrocytes also release neurotransmitters, which act on axonal receptors to strengthen action potential propagation, maintaining signaling along potentially long axon tracts. The co-existence of multiple neurotransmitters in WM tracts suggests they may have diverse functions that are important for information processing. Furthermore, the neurotransmitter signaling phenomena described in WM most likely apply to myelinated axons of the cerebral cortex and GM areas, where they are doubtless important for higher cognitive function.

Role of γ-aminobutyric acid and its receptors in carcinogenesis

Gamma-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain and it is also expressed in the central nervous system, peripheral nerves and peripheral non-neural tissues. Recent studies have shown that GABA is involved in the proliferation and migration of tumor cells and other processes of tumor development. According to different sensitivity to agonists and antagonists, GABA receptors have been classified into three types: A, B and C. GABA receptors and their receptor subunits are involved in complicated regulation of tumor cells. Many studies have demonstrated that GABA binding to its receptors can activate or inhibit the cAMP signaling pathway and the MAPK/ERK pathway, and regulate cancer cell proliferation and migration. The potential value of GABA in cancer diagnosis, prognostic prediction and biotherapy has been gradually revealed. In the present article, we reviewed the recent progress in understanding the role of GABA and its receptors in carcinogenesis.

Beyond faithful conduction: short-term dynamics, neuromodulation, and long-term regulation of spike propagation in the axon

Most spiking neurons are divided into functional compartments: a dendritic input region, a soma, a site of action potential initiation, an axon trunk and its collaterals for propagation of action potentials, and distal arborizations and terminals carrying the output synapses. The axon trunk and lower order branches are probably the most neglected and are often assumed to do nothing more than faithfully conducting action potentials. Nevertheless, there are numerous reports of complex membrane properties in non-synaptic axonal regions, owing to the presence of a multitude of different ion channels. Many different types of sodium and potassium channels have been described in axons, as well as calcium transients and hyperpolarization-activated inward currents. The complex time- and voltage-dependence resulting from the properties of ion channels can lead to activity-dependent changes in spike shape and resting potential, affecting the temporal fidelity of spike conduction. Neural coding can be altered by activity-dependent changes in conduction velocity, spike failures, and ectopic spike initiation. This is true under normal physiological conditions, and relevant for a number of neuropathies that lead to abnormal excitability. In addition, a growing number of studies show that the axon trunk can express receptors to glutamate, GABA, acetylcholine or biogenic amines, changing the relative contribution of some channels to axonal excitability and therefore rendering the contribution of this compartment to neural coding conditional on the presence of neuromodulators. Long-term regulatory processes, both during development and in the context of activity-dependent plasticity may also affect axonal properties to an underappreciated extent.

Central Role of GABA in Neuron–Glia Interactions

The major types of glial cells—astrocytes, microglia, and cells of the oligodendroglial lineage—are known to express functional metabotropic and ionotropic GABA receptors. Neuronal signaling mechanisms allowing for the activation of these receptors in glia are probably as complex as those described among neurons and involve synaptic and extrasynaptic transmission modes. In addition, astrocytes can signal back to neurons by releasing GABA, probably through unconventional nonvesicular mechanisms. The decryption of the roles played by GABAergic signaling in neuron–glia interactions is only beginning, but it has been suggested that activation of glial cells by GABA influences important functions of the brain such as neuronal activity, differentiation, myelination, and neuroprotection. This review discusses the cellular mechanisms allowing the major types of glial cells to sense and transmit GABAergic signals and gives an overview of potential roles of this signaling pathway in developing and mature brains.

Localization of a GABA transporter to glial cells in the developing and adult olfactory pathway of the moth Manduca sexta

Glial cells have several critical roles in the developing and adult olfactory (antennal) lobe of the moth Manduca sexta. Early in development, glial cells occupy discrete regions of the developing olfactory pathway and processes of gamma-aminobutyric acid (GABA)ergic neurons extend into some of these regions. Because GABA is known to have developmental effects in a variety of systems, we explored the possibility that the glial cells express a GABA transporter that could regulate GABA levels to which olfactory neurons and glial cells are exposed. By using an antibody raised against a characterized high-affinity M. sexta GABA transporter with high sequence homology to known mammalian GABA transporters (Mbungu et al. [1995] Arch. Biochem. Biophys. 318:489-497; Umesh and Gill [2002] J. Comp. Neurol. 448:388-398), we found that the GABA transporter is localized to subsets of centrally derived glial cells during metamorphic adult development. The transporter persists into adulthood in a subset of the neuropil-associated glial cells, but its distribution pattern as determined by light-and electron-microscopic-level immunocytochemistry indicates that it could not serve to regulate GABA concentration in the synaptic cleft. Instead, its role is more likely to regulate extracellular GABA levels within the glomerular neuropil. Expression in the sorting zone glial cells disappears after the period of olfactory receptor axon ingrowth, but may be important during ingrowth if GABA regulates axon growth. Glial cells take up GABA, and that uptake can be blocked by L-2,4-diaminobutyric acid (DABA). This is the first molecular evidence that the central glial cell population in this pathway is heterogeneous.

Glia as transmitter sources and sensors in health and disease

Glial cells express a bewildering array of neurotransmitter receptors. To illustrate the complexity of expression, we have assayed non-glutamatergic neurotransmitter receptor mRNA in isolated rat optic nerve, a preparation devoid of neurons and neuronal synapses and from which relatively pure "glial" RNA can be isolated. Of the 44 receptor subunits examined which span the GABA-A, nicotinic, adreno- and glycine receptor families, over three quarters were robustly expressed in this mixed population of white matter glial cells, with several expressed at higher levels than found in control whole brain RNA. In addition to the complexity of glial receptor expression, numerous neurotransmitter release mechanisms have been identified. We have focused on glutamate release from astrocytes, which can occur via at least seven distinct pathways and which is implicated in excitotoxic injury and are neurons and glia. Recent findings suggest that non-glutamatergic receptors can also mediate acute glial injury are also discussed.

GABA, a forgotten gliotransmitter

The amino acid gamma-aminobutiric acid (GABA) is a major inhibitory transmitter in the vertebrate central nervous system (CNS) where it can be released by neurons and by glial cells. Neuronal GABAergic signaling is well characterized: the mechanisms of GABA release, the receptors it targets and the functional consequences of their activation have been extensively studied. In contrast, the corresponding features of glial GABAergic signaling have attracted less attention. In this review, we first discuss evidence from the literature for GABA accumulation, production and release by glial cells. We then review the results of recent experiments that point toward functional roles of GABA as a "gliotransmitter".

A role for GABAA receptors in the modulation of Paramecium swimming behavior

The presence in Paramecium of gamma-aminobutyric acid A-type receptors (GABA(A)) and the capability of the protozoon to synthesize and release the GABA neurotransmitter into the environment have already been demonstrated. This study investigates the involvement of the GABA(A) complex in the swimming control of the ciliated protozoon. The GABA(A) receptors were pharmacologically activated by the selective agonist muscimol and the effect on Paramecium primaurelia swimming behavior was analyzed. Paramecium normally swims forward, but the activation of GABA(A) receptors induced a peculiar response, characterized by alternate periods of whirling and forward swim. This effect was inhibited by the GABA(A) selective antagonists bicuculline and picrotoxin in a dose-dependent manner. Moreover, the application of benzodiazepines did not enhance the agonist action but only reduced it. Response to muscimol was also suppressed by nimodipine, a selective antagonist of dihydropyridine-sensitive calcium channels. This inhibition suggests that an inflow of calcium ions through L-type channels mediates the muscimol-induced swimming behavior.

General mechanisms of axonal damage and its prevention

Axonal degeneration is a prominent pathological feature in multiple sclerosis observed over a century ago. The gradual loss of axons is thought to underlie irreversible clinical deficits in this disease. The precise mechanisms of axonopathy are poorly understood, but likely involve excess accumulation of Ca ions. In healthy fibers, ATP-dependent pumps support homeostasis of ionic gradients. When energy supply is limited, either due to inadequate delivery (e.g., ischemia, mitochondrial dysfunction) and/or excessive utilization (e.g., conduction along demyelinated axons), ion gradients break down, unleashing a variety of aberrant cascades, ultimately leading to Ca overload. During Na pump dysfunction, Na can enter axons through non-inactivating Na channels, promoting axonal Na overload and depolarization by allowing K egress. This will gate voltage-sensitive Ca channels and stimulate reverse Na-Ca exchange, leading to further Ca entry. Energy failure will also promote Ca release from intracellular stores. Neurotransmitters such as glutamate can be released by reverse operation of Na-dependent transporters, in turn activating a variety of ionotropic and metabotropic receptors, further exacerbating overload of cellular Ca. Together, this Ca overload will inappropriately stimulate a variety of Ca-dependent enzyme systems (e.g., calpains, phospholipases), leading to structural and functional axonal injury. Pharmacological interruption at key points in these interrelated injury cascades (e.g., at voltage-gated Na channels or AMPA receptors) may confer significant neuroprotection to compromised central axons and supporting glia. Such agents may represent attractive adjuncts to currently available immunomodulatory therapies.

The optic nerve: A model for axon–glial interactions

The rodent optic nerve is a model tissue for the physiological investigation of axonal-glial interactions in a typical CNS white matter tract. There is strong evidence that nerve transmission is maintained by a considerable degree of dynamic signalling between axons and glia through a variety of mechanisms, such as regulation of the ionic environment, energy metabolism and calcium signalling. This review focuses on the methods used to examine axonal and glial functions and interactions, primarily in the rodent optic nerve. Techniques encompass intracellular microelectrodes, sucrose- and grease-gap recordings of membrane potentials, suction electrode to measure compound action potentials, the use of ion-sensitive electrodes, patch clamping and imaging. An overview of the advantages and drawbacks of each technique is given and the application of each to the understanding glial and axonal physiology is briefly discussed.

Effects of N-methyl-d-aspartate, glutamate, and glycine on the dorsal column axons of neonatal rat spinal cord: in vitro study

The effects of N-methyl-D-aspartate (NMDA), glutamate, and glycine on the developmental axons of the neonatal rat spinal cord were investigated. Isolated dorsal column preparations from postnatal day (PN) 0 to 14 Long-Evans hooded rats (n = 119) were used in vitro. Compound action potentials (CAPs) were recorded from the cuneate and gracile fasciculi with a glass micropipette electrode. NMDA (100 microM) significantly increased CAP amplitude in PN 0-6 cords by 21.5 +/- 9.2% (mean +/- standard error of the mean, p < 0.001, n = 8) and in PN 7-14 cords by 6.7 +/- 6.6% (p < 0.001, n = 10). NMDA (10 microM) significantly increased the CAP amplitude by 6.3 +/- 2.9% in PN 0-6 cords (p < 0.01, n = 10). The increase of CAP amplitude induced by NMDA (100 microM) in PN 0-6 cords was significantly greater than that in PN 7-14 cords (p < 0.005). Glutamate (100 microM) significantly increased the CAP amplitude by 8.8 +/- 8.1% in PN 0-6 cords (p < 0.001, n = 29) and 6.7 +/- 7.5% in PN 7-14 cords (p < 0.01, n = 14), and glutamate (10 microM) significantly increased by 6.3 +/- 2.9% in PN 0-6 cords (p < 0.01, n = 21). The amplitudes induced by glutamate (100 microM or 10 microM) did not significantly differ between PN 0-6 and PN 7-14 cords. Application of glycine (100 microM) did not significantly alter CAP amplitudes induced by NMDA (100 microM or 10 microM) and glutamate (100 microM or 10 microM). D(-)-2-amino-5-phosphonopentanoic acid (NMDA receptor antagonist) blocked the effects of NMDA and glutamate. These results suggest that NMDA receptor is present on afferent dorsal column axons and may modulate axonal excitability, especially during the 1st week after birth.

Cellular mechanisms of neuronal population oscillations in the hippocampus in vitro

A variety of population oscillations, at frequencies approximately 5 Hz up to 200 Hz and above, can be induced in hippocampal slices either by (a) manipulation of the ionic environment, or (b) by stimulation of metabotropic receptors; brief oscillations can even occur spontaneously. In this review, we consider in vitro theta (4-12 Hz), gamma/beta (15-70 Hz), and very fast oscillations (VFO) (>70 Hz). Many in vitro oscillations are gated by synaptic inhibition but are influenced by electrical coupling as well; one type depends solely on electrical coupling. For some oscillations dependent upon inhibition, the detailed firing patterns of interneurons can influence long-range synchronization. Two sorts of electrical coupling are important in modulating or generating various in vitro oscillations: (a) between interneurons, primarily between dendrites; and (b) between axons of pyramidal neurons. VFO can exist in isolation or can act as generators of gamma frequency oscillations. Oscillations at gamma frequencies and below probably create conditions under which synaptic plasticity can occur, between selected neurons-even those separated by significant axonal conduction delays.

Nicotinic Acetylcholine Receptors in Mouse and Rat Optic Nerves

Receptor-mediated calcium signaling in axons of mouse and rat optic nerves was examined by selectively staining the axonal population with a calcium indicator. Nicotine (1-50 μM) induced an axonal calcium elevation that was eliminated when calcium was removed from the bath, suggesting that nicotine induces calcium influx into axons. The nicotine response was blocked by d-tubocurarine and mecamylamine but not α-bungarotoxin, indicating the presence of calcium permeable, non-α7 nicotinic acetylcholine receptor (nAChR) subtype. Agonist efficacy order for eliciting the axonal nAChR calcium response was cytisine ∼ nicotine >> acetylcholine. The nicotine-mediated calcium response was attenuated during the process of normal myelination, decreasing by approximately 10-fold from P1 (premyelinated) to P30 (myelinated). Nicotine also caused a rapid reduction in the compound action potential in neonatal optic nerves, consistent with a shunting of the membrane after opening of the nonspecific cationic nicotinic channels. Voltagegated calcium channels contributed little to the axonal calcium elevation during nAChR activation. During repetitive stimulations, the compound action potential in neonatal mouse optic nerves underwent a gradual reduction in amplitude that could be partially prevented by d-tubocurarine, suggesting an activity-dependent release of acetylcholine that activates axonal AChRs. We conclude that mammalian optic nerve axons express nAChRs and suggest that these receptors are activated in an activity-dependent fashion during optic nerve development to modulate axon excitability and biology.

Dynamic equilibrium of neurotransmitter transporters: not just for reuptake anymore

Many electrophysiologists view neurotransmitter transporters as tiny vacuum cleaners, operating continuously to lower extracellular neurotransmitter concentration to zero. However, this is not consistent with their known behavior, instead only reducing extracellular neurotransmitter concentration to a finite, nonzero value at which an equilibrium is reached. In addition, transporters are equally able to go in either the forward or reverse direction, and when they reverse, they release their substrate in a calcium-independent manner. Transporter reversal has long been recognized to occur in response to pathological stimuli, but new data demonstrate that some transporters can also reverse in response to physiologically relevant stimuli. This is consistent with theoretical calculations that indicate that the reversal potentials of GABA and glycine transporters are close to the resting potential of neurons under normal conditions and that the extracellular concentration of GABA is sufficiently high when the GABA transporter is at equilibrium to tonically activate high-affinity extrasynaptic GABAA receptors. The equilibrium for the GABA transporter is not static but instead varies continuously as the driving force for the transporter changes. We propose that the GABA transporter plays a dynamic role in control of brain excitability by modulating the level of tonic inhibition in response to neuronal activity.

Electrophysiology of optic nerve input to suprachiasmatic nucleus neurons in rats and degus

Neurons in the mammalian suprachiasmatic nucleus (SCN), the principal pacemaker of the circadian system, receive direct retinal input. Some SCN neurons respond to retinal illumination or optic nerve stimulation with changes in firing rates. In nocturnal rodents, retinal illumination increases firing rates of a large majority and decreases firing rates of a minority of responsive neurons. In two species of diurnal rodent, these proportions are altered or even reversed. Since retinal input to the SCN has been reported to involve release of the excitatory neurotransmitter glutamate, the mechanism mediating suppressions is unknown. We studied responses of neurons in SCN slices from diurnal degus and nocturnal rats to optic nerve stimulation. To test whether suppressions are mediated indirectly by release of the inhibitory neurotransmitter GABA from SCN neurons that are first activated by glutamate release, we attempted to block suppressions by adding to the bath either APV, an antagonist for excitatory glutamate receptors, or bicuculline, a GABA(A) receptor antagonist. If glutamate is the only neurotransmitter released by optic nerves in the SCN, APV should prevent both activations and suppressions in response to optic nerve stimulation. We found that APV had little effect on suppressions although it effectively blocked activations. Bicuculline blocked most suppressions. These findings are inconsistent with a model in which the retina provides only excitatory glutamate input to the SCN via NMDA receptors. Since some retinal fibers in adult mammals contain GABA, it is possible that the retinal innervation of the SCN includes both glutamate- and GABA-containing axons.

Excitatory actions of gaba during development: the nature of the nurture

In the immature brain, GABA (gamma-aminobutyric acid) is excitatory, and GABA-releasing synapses are formed before glutamatergic contacts in a wide range of species and structures. GABA becomes inhibitory by the delayed expression of a chloride exporter, leading to a negative shift in the reversal potential for choride ions. I propose that this mechanism provides a solution to the problem of how to excite developing neurons to promote growth and synapse formation while avoiding the potentially toxic effects of a mismatch between GABA-mediated inhibition and glutamatergic excitation. As key elements of this cascade are activity dependent, the formation of inhibition adds an element of nurture to the construction of cortical networks.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

GABA and development of the Xenopus optic projection

In the developing visual system of Xenopus laevis retinal ganglion cell (RGC) axons extend through the brain towards their major target in the midbrain, the optic tectum. Enroute, the axons are guided along their pathway by cues in the environment. In vitro, neurotransmitters have been shown to act chemotropically to influence the trajectory of extending axons and regulate the outgrowth of developing neurites, suggesting that they may act to guide or modulate the growth of axons in vivo. Previous work by Roberts and colleagues (1987) showed that populations of cells within the developing Xenopus diencephalon and mid-brain express the neurotransmitter gamma amino butyric acid (GABA). Here we show that Xenopus RGC axons in the midoptic tract grow alongside the GABAergic cells and cross their GABA immunopositive nerve processes. Moreover, RGC axons and growth cones express GABA-A and GABA-B receptors, and GABA and the GABA-B receptor agonist baclofen both stimulate RGC neurite outgrowth in culture. Finally, the GABA-B receptor antagonist CGP54626 applied to the developing optic projection in vivo causes a dose-dependent shortening of the optic projection. These data indicate that GABA may act in vivo to stimulate the outgrowth of Xenopus RGC axons along the optic tract.

Ontogeny of gamma-aminobutyric acid-immunoreactive neuronal populations in the forebrain and midbrain of the sea lamprey

Although brain organization in lampreys is of great interest for understanding evolution in vertebrates, knowledge of early development is very scarce. Here, the development of the forebrain and midbrain gamma-aminobutyric acid (GABA)-ergic systems was studied in embryos, prolarvae, and small larvae of the sea lamprey using an anti-GABA antibody. Ancillary immunochemical markers, such as proliferating cell nuclear antigen (PCNA), calretinin, and serotonin, as well as general staining methods and semithin sections were used to characterize the territories containing GABA-immunoreactive (GABAir) neurons. Differentiation of GABAir neurons in the diencephalon begins in late embryos, whereas differentiation in the telencephalon and midbrain was delayed to posthatching stages. In lamprey prolarvae, the GABAir populations appear either as compact GABAir cell groups or as neurons interspersed among GABA-negative cells. In the telencephalon of prolarvae, a band of cerebrospinal fluid-contacting (CSF-c) GABAir neurons (septum) was separated from the major GABAir telencephalic band, the striatum (ganglionic eminence) primordium. The striatal primordium appears to give rise to most GABAir neurons observed in the olfactory bulb and striatum of early larval stages. GABAir populations in the dorsal telencephalon appear later, in 15-30-mm-long larvae. In the diencephalon, GABAir neurons appear in embryos, and the larval pattern of GABAir populations is recognizable in prolarvae. A small GABAir cluster consisting mainly of CSF-c neurons was observed in the caudal preoptic area, and a wide band of scattered CSF-c GABAir neurons extended from the preoptic region to the caudal infundibular recess. A mammillary GABAir population was also distinguished. Two compact GABAir clusters, one consisting of CSF-c neurons, were observed in the rostral (ventral) thalamus. In the caudal (dorsal) thalamus, a long band extended throughout the ventral tier. The nucleus of the medial longitudinal fascicle contained an early-appearing GABAir population. The paracommissural pretectum of prolarvae and larvae contained a large group of non-CSF-c GABAir neurons, although it was less compact than those of the thalamus, and a further group was found in the dorsal pretectum. In the midbrain of larvae, several groups of GABAir neurons were observed in the dorsal and ventral tegmentum and in the torus semicircularis. The development of GABAergic populations in the lamprey forebrain was similar to that observed in teleosts and in mouse, suggesting that GABA is a very useful marker for understanding evolution of forebrain regions. The possible relation between early GABAergic cell groups and the regions of the prosomeric map of the lamprey forebrain (Pombal and Puelles [ 1999] J. Comp. Neurol. 414:391-422) is discussed in view of these results and information obtained with ancillary markers.

GABA increases refractoriness of adult rat dorsal column axons

We applied randomized double pulse stimulation for assessing the effects of GABA and a GABAA antagonist on compound action potentials in dorsal column axons isolated from adult rat. We stimulated the axons with double pulses at 0.2 Hz and randomly varied interpulse intervals between 3, 4, 5, 8, 10, 20, 30, 50 and 80 ms. Action potentials were measured using glass micropipettes. The first pulse was used to condition the response activated by the second test pulse. Concentrations of GABA of 1 mM, 100 microM and 10 microM did not affect action potential amplitudes or latencies activated by conditioning pulses. In the control studies, before drug administration, test pulses induced response amplitudes that were significantly decreased at 3-, 4- and 5-ms interpulse intervals. The test action potential amplitudes were 84.6 +/- 2.5%, 89.0 +/- 3.9% and 93.3 +/- 3.6% (mean +/- S.E.M.) of conditioning pulse levels, respectively. At 3-ms interpulse intervals, test response latencies were prolonged to 104.3 +/- 1.0%, but were unchanged at the other interpulse intervals. The 10 microM, 100 microM and 1 mM concentrations of GABA affected test response amplitudes. Application of 100 microM GABA reduced the amplitudes of test responses at 3-, 4-, 5- and 8-ms interpulse intervals, to 59.2 +/- 3.0%, 70.0 +/- 3.0%, 80.2 +/- 1.1% and 88.6 +/- 3.6% of the conditioning pulse amplitudes, respectively. At both 100 microM and 1 mM concentrations, GABA significantly prolonged the latencies of test responses. Treatment with 100 microM GABA prolonged the latencies of test responses at 3-, 4- and 5-ms interpulse intervals, to 119.3 +/- 3.1%, 107.3 +/- 2.8% and 105.5 +/- 2.5% of conditioning pulse latencies, respectively. The addition of 100 microM bicuculline methochloride, a GABAA antagonist, eliminated the effects of 100 microM GABA. The combined application of GABA and bicuculline (both 100 microM) did not affect amplitudes or latencies of test responses. These results suggest that GABA(A) receptor subtypes are present on the spinal dorsal column axons of adult rat, and that they modulate the excitability of the axons. The randomized double pulse methods reveal that GABA increases refractoriness of adult rat dorsal column axons.

N-type calcium channels and their regulation by GABAB receptors in axons of neonatal rat optic nerve

Axons of neonatal rat optic nerves exhibit fast calcium transients in response to brief action potential stimulation. In response to one to four closely spaced action potentials, evoked calcium transients showed a fast-rising phase followed by a decay with a time constant of approximately 2-3 sec. By selective staining of axons or glial cells with calcium dyes, it was shown that the evoked calcium transient originated from axons. The calcium transient was caused by influx because it was eliminated when bath calcium was removed. Pharmacological profile studies with calcium channel subtype-specific peptides suggested that 58% of the evoked calcium influx was accounted for by N-type calcium channels, whereas L- and P/Q-type calcium channels had little, if any, contribution. The identity of the residual calcium influx remains unclear. GABA application caused a dramatic reduction of the amplitude of the action potential and the associated calcium influx. When GABAA receptors were blocked by bicuculline, the inhibitory effect of GABA on the action potential was eliminated, whereas that on the calcium influx was not, indicating involvement of GABAB receptors. Indeed, the calcium influx was inhibited by the GABAB receptor agonist baclofen. This baclofen effect was occluded by a previous block of N-type calcium channels and was unaffected by the broad-spectrum K+ channel blocker 4-AP. We conclude that neonatal rat optic nerve axons express N-type calcium channels, which are subjected to regulation by G-protein-coupled GABAB receptors. We suggest that receptor-mediated inhibition of axonal calcium channels plays a protective role in neonatal anoxic and/or ischemic injury.

Mapping membrane potential transients in crayfish (Procambarus clarkii) optic lobe neuropils with voltage-sensitive dyes

Voltage-sensitive dyes NK 2761 and RH 155 were employed (in conjunction with a 12 x 12 photodiode array) to study membrane potential transients in optic lobe neuropils in the eye stalk of the crayfish Procambarus clarkii. By this means we investigated a pathway linking deutocerebral projection neurons, via hemiellipsoid body local interneurons, to an unidentified target (most likely neurons processing visual information) in the medulla terminalis. Rapid (10- to 20-ms duration), transient changes in absorption with the characteristics of action potentials were recorded from the optic nerve and the region occupied by deutocerebral projection neurons after stimulation of the olfactory globular tract in the optic nerve and were blocked by 1 microM tetrodotoxin. Action potentials appeared to propagate to the glomerular layer of the hemiellipsoid body where synaptic responses were recorded from a restricted region of the hemiellipsoid body occupied by dendrites of hemiellipsoid body neurons. Action potentials were also recorded from processes of hemiellipsoid body neurons located in the medulla terminalis. Synaptic responses in the hemiellipsoid body and medulla terminalis were eliminated by addition to the saline of 500 microM Cd2+ or 20 mM Co2+, whereas the action potential attributed to branches of deutocerebral projection neurons in the hemiellipsoid body remained unaffected. Action potentials of hemiellipsoid body neurons in the medulla terminalis evoked postsynaptic potentials (50- to 200-ms duration) with an unidentified target in the medulla terminalis. Transient absorption signals were not detected in either the internal or external medulla nor were they recorded from other parts of the optic lobes in response to electrical stimulation of axons of the deutocerebral projection neurons. Functional maps of optical activity, together with electrophysiological and pharmacological findings, suggest that gamma-aminobutyric acid affects synaptic transmission in glomeruli of the hemiellipsoid body. Synapses of the olfactory pathway located in the medulla terminalis may act as a "filter," modifying visual information processing during olfactory stimulation.

GABAergic modulation of axonal conduction in the developing rat retinotectal pathway

The effect of gamma-aminobutyric acid (GABA) on nerve conduction was investigated in the developing rat optic nerves in vitro. Antidromic compound action potentials (CAP) were suppressed by GABA, with increasing attenuation in older preparations. Enhancement of CAP amplitude using bicuculline was observed in postnatal days 4-5 suggesting the presence of endogenous GABA activity. Our findings suggest that the role of electrical activity in the refinement of the retinotectal projection could be limited by the GABAergic action on axonal conduction.

GABAergic deafferentation hypothesis of brain aging and Alzheimer's disease revisited

Considering the mechanisms responsible for age- and Alzheimer's disease (AD)-related neuronal degeneration, little attention was paid to the opposing relationships between the energy-rich phosphates, mainly the availability of the adenosine triphosphate (ATP), and the activity of the glutamic acid decarboxylase (GAD), the rate-limiting enzyme synthesizing the gamma-amino butyric acid (GABA). Here, it is postulated that in all neuronal phenotypes the declining ATP-mediated negative control of GABA synthesis gradually declines and results in age- and AD-related increases of GABA synthesis. The Ca2+-independent carrier-mediated GABA release interferes with Ca2+-dependent exocytotic release of all transmitter-modulators, because the interstitial (ambient) GABA acts on axonal preterminal and terminal varicosities endowed with depolarizing GABA(A)-benzodiazepine receptors; this makes GABA the "executor" of virtually all age- and AD-related neurodegenerative processes. Such a role of GABA is diametrically opposite to that in the perinatal phase, when the carrier-mediated GABA release, acting on GABA(A)/chloride ionophore receptors, positively controls chemotactic migration of neuronal precursor cells, has trophic actions and initiates synaptogenesis, thereby enabling retrograde axonal transport of target produced factors that trigger differentiation of neuronal phenotypes. However, with advancing age, and prematurely in AD, the declining mitochondrial ATP synthesis unleashes GABA synthesis, and its carrier-mediated release blocks Ca2+-dependent exocytotic release of all transmitter-modulators, leading to dystrophy of chronically depolarized axon terminals and block of retrograde transport of target-produced trophins, causing "starvation" and death of neuronal somata. The above scenario is consistent with the following observations: 1) a 10-month daily administration to aging rats of the GABA-chloride ionophore antagonist, pentylenetetrazol, or of the BDZ antagonist, flumazenil (FL), each forestalls the age-related decline in cognitive functions and losses of hippocampal neurons; 2) the brains of aging rats, relative to young animals, and the postmortem brains of AD patients, relative to age-matched controls, show up to two-fold increases in GABA synthesis; 3) the aging humans and those showing symptoms of AD, as well as the aging nonhuman primates and rodents--all show in the forebrain dystrophic axonal varicosities, losses of transmitter vesicles, and swollen mitochondria. These markers, currently regarded as the earliest signs of aging and AD, can be reproduced in vitro cell cultures by 1 microM GABA; the development of these markers can be prevented by substituting Cl- with SO4(2-); 4) the extrasynaptic GABA suppresses the membrane Na+, K+-ATPase and ion pumping, while the resulting depolarization of soma-dendrites relieves the "protective" voltage-dependent Mg2+ control of the N-methyl-D-aspartate (NMDA) channels, thereby enabling Ca2+-dependent persistent toxic actions of the excitatory amino acids (EAA); and 5) in whole-cell patch-clamp recording from neurons of aging rats, relative to young rats, the application of 3 microM GABA, causes twofold increases in the whole-cell membrane Cl- conductances and a loss of the physiologically important neuronal ability to desensitize to repeated GABA applications. These age-related alterations in neuronal membrane functions are amplified by 150% in the presence of agonists of BDZ recognition sites located on GABA receptor. The GABA deafferentation hypothesis also accounts for the age- and AD-related degeneration in the forebrain ascending cholinergic, glutamatergic, and the ascending mesencephalic monoaminergic system, despite that the latter, to foster the distribution-utilization of locally produced trophins, evolved syncytium-like connectivities among neuronal somata, axon collaterals, and dendrites, to bidirectionally transport trophins. (ABSTRACT TRUNCATED)

Onset of optic nerve conduction and synaptic potentials in superior colliculus of fetal rats studied in vitro

This article describes the onset of electrical excitability and synaptic transmission in the retinocollicular pathway of the fetal and early postnatal rat, utilizing a novel in vitro preparation. Although the optic nerve is visible in embryonic day (E) 14 brain, its stimulation produced no response in the superior colliculus (SC) until E16 when a low voltage simple negative wave was evoked. At E17 these potentials were blocked rapidly, completely, and reversibly when choline was substituted for sodium or with the addition of cobalt ions. In the course of establishing the block with either of the above agents the latency of response increased, indicating an action on axonal transmission. By E20 the collicular evoked potential showed a short followed by a longer latency wave. The latter was blocked by the glutamate antagonist kynurenic acid, with latency unaffected. Further examination of potentials with the addition of glutamatergic receptor subtype blockers aminophosphonopentanoic acid (APV) and 6-cyano-7-nitroquinoxaline-2,3-dione/6,7-dinitroquinoxaline- 2,3-dione (CNQX/DNQX) showed a clear abolition of the elicited potentials by E20 and older. Thus, fetal rat optic nerve fibers are capable of conduction in response to electrical stimulation as soon as they reach the SC at E16. Both sodium and calcium are involved. GABA-mediated modulation of axonal conduction is evident by E18. Glutaminergic synaptic transmission is established by E20. The timetable of fetal onset of capability to conduct and support synaptic transmission in the retinocollicular pathway is earlier than had previously been reported in vivo in the rat in which the superior colliculus neurones are said not to be driven by the optic nerve until 6 days post natal. This has relevance to the possible role of impulse activity in development of the pathway.

Evidence for physiologically active axonal adenosine receptors in the rat corpus callosum

Several neurotransmitter receptors have been identified on axons, and emerging evidence suggests that central axonal conduction may be modulated by neurotransmitters. We have recently demonstrated the presence of extra-synaptic adenosine Al receptors along rat hippocampal axons. We now present immunocytochemical evidence for Al receptors on rat corpus callosum axons and show that these receptors actively modulate axon physiology. Using rat brain coronal slices, we stimulated the corpus callosum and recorded the evoked extracellular compound action potential. The lipid-soluble, Al-specific adenosine receptor agonist cyclopentyladenosine, dose-dependently decreased the compound action potential amplitude, an effect reversed by the specific Al antagonist 8-cyclopentyl-1, 3-dipropylxanthine. These data provide the first direct evidence that axonal Al adenosine receptors modulate axon physiology in the adult mammalian brain. Influencing axonal transmission is a potentially powerful mechanism of altering information processing in the nervous system.

Expression of the Na-K-2Cl cotransporter is developmentally regulated in postnatal rat brains: a possible mechanism underlying GABA's excitatory role in immature brain

An inhibitory neurotransmitter in mature brain, gamma-aminobutyric acid (GABA) also appears to be excitatory early in development. The mechanisms underlying this shift are not well understood. In vitro studies have suggested that Na-K-Cl cotransport may have a role in modulating immature neuronal and oligodendrocyte responses to the neurotransmitter GABA. An in vivo developmental study would test this view. Therefore, we examined the expression of the BSC2 isoform of the Na-K-2Cl cotransporter in the postnatal developing rat brain. A comparison of sections from developing rat brains by in situ hybridization revealed a well-delineated temporal and spatial pattern of first increasing and then diminishing cotransporter expression. Na-K-2Cl mRNA expression in the cerebral cortex and hippocampus was highest in the first week of postnatal life and then diminished from postnatal day (PND) 14 to adult. Cotransporter signal in white-matter tracts of the cerebrum, cerebellum, peaked at PND 14. Expression was detected in cerebellar progenitor cells of the external granular layer, in internal granular layer cells at least as early as PND 7, and in Purkinje cells beginning at PND 14. Double-labeling immunofluorescence of brain sections with anti-BSC2 antibody and cell type-specific antibodies confirmed expression of the cotransporter gene product in neurons and oligodendrocytes in the white matter in a pattern similar to that determined by in situ hybridization. The temporal pattern of expression of the Na-K-2Cl cotransporter in the postnatal rat brain supports the hypothesis that the cotransporter is the mechanism of intracellular Cl- accumulation in immature neurons and oligodendrocytes.

Electrophysiological changes that accompany reactive gliosis in vitro

An in vitro injury model was used to examine the electrophysiological changes that accompany reactive gliosis. Mechanical scarring of confluent spinal cord astrocytes led to a threefold increase in the proliferation of scar-associated astrocytes, as judged by bromodeoxyuridine (BrdU) labeling. Whole-cell patch-clamp recordings demonstrated that current profiles differed absolutely between nonproliferating (BrdU-) and proliferating (BrdU+) astrocytes. The predominant current type expressed in BrdU- cells was an inwardly rectifying K+ current (KIR; 1.3 pS/pF). BrdU- cells also expressed transient outward K+ currents, accounting for less than one-third of total K+ conductance (G). In contrast, proliferating BrdU+ astrocytes exhibited a dramatic, approximately threefold reduction in KIR (0.45 pS/pF) but showed a twofold increase in the conductance of both transient (KA) (0.67-1. 32 pS/pF) and sustained (KD) (0.42-1.10 pS/pF) outwardly rectifying K+ currents, with a GKIR:GKD ratio of 0.4. Relative expression of GKIR:GKD led to more negative resting potentials in nonproliferating (-60 mV) versus proliferating astrocytes (-53 mV; p = 0.015). Although 45% of the nonproliferating astrocytes expressed Na+ currents (0.47 pS/pF), the majority of proliferating cells expressed prominent Na+ currents (0.94 pS/pF). Injury-induced electrophysiological changes are rapid and transient, appearing within 4 hr postinjury and, with the exception of KIR, returning to control conductances within 24 hr. These differences between proliferating and nonproliferating astrocytes are reminiscent of electrophysiological changes observed during gliogenesis, suggesting that astrocytes undergoing secondary, injury-induced proliferation recapitulate the properties of immature glial cells. The switch in predominance from KIR to KD appears to be essential for proliferation and scar repair, because both processes were inhibited by blockade of KD.

Expression of GABA transporter mRNAs in the developing and adult rat optic nerve

There is electrophysiological evidence of a functional role for gamma-aminobutyric acid (GABA) and GABA transporters (GATs) in the neonatal rat optic nerve, and that they are down-regulated during development. The results of the present study demonstrate directly by reverse transcription-polymerase chain reaction (RT-PCR) that the mRNAs encoding for GAT-1, -2 and -3 are expressed in the optic nerves of both neonatal (5 day old) and adult rats. The results support a role for GABA in the developing rat optic nerve, a typical white matter tract which contains axons and glia, but neither neuronal cell bodies nor synapses. Significantly, the persistence of GAT mRNAs suggests an enduring function for both GABA and glial uptake mechanisms in the adult optic nerve.

Evidence for serotonin sensitivity of adult rat spinal axons: studies using randomized double pulse stimulation

We have recently shown both inhibitory and excitatory effects of serotonin on neonatal rat dorsal column axons. While neonatal rat dorsal column axons also respond to norepinephrine and GABA, adult rat dorsal columns are insensitive to the actions of both compounds. Therefore, we studied the effects of serotonin agonists on adult rat dorsal column axons using randomized double pulse stimuli at 0.2 Hz with random interpulse intervals of 3, 4, 5, 8, 10, 20, 30, 50 and 80 ms. The serotonin(1A) agonist, 8-hydroxy-dipropylaminotetralin-hydrobromide (8-OH-DPAT), significantly modulated test response amplitudes at 3, 4, 5 and 8 ms interpulse intervals by 29.6+/-4.0%, 17.4+/-2.1%, 9.6+/-2.3%, and 12.4+/-2.2% of conditioning pulse amplitudes, respectively. The mean latencies at 3, 4 and 5 ms interpulse intervals increased by 17.0+/-5.1%, 8.6+/-2.1%, and 5.1+/-1.4%, respectively (P<0.05). However, neither 10 microM 8-OH-DPAT nor 100 microM serotonin hydrochloride affected the compound action potentials evoked by conditioning or test pulses. In contrast, treatment with 100 microM quipazine dimaleate (a serotonin(2A) agonist) decreased the refractory period. While the response amplitudes to a 3-ms double pulse were reduced by 11.0+/-1.5% during the control period, the test response fell to only 2.4+/-1.8% of the conditioning response amplitudes after exposure to 100 microM quipazine. 8-OH-DPAT decreased the amplitude, prolonged the latency and increased the refractory periods of compound action potentials in the adult rat dorsal column, although a high concentration of the agonist (100 microM) was required for these effects. In contrast, the serotonin(2A) agonist, quipazine, decreased refractory periods. These results suggest that both serotonin(1A) and serotonin(2A) receptor subtypes are present on adult spinal dorsal column axons. Further, these receptors have opposing effects on axonal excitability, despite the fact that their sensitivities are relatively low.

A ventrodorsal GABA gradient in the embryonic retina prior to expression of glutamate decarboxylase

GABA is known to function as a neurotransmitter in the mature nervous system, and in immature neurons it has been linked to neurotrophic actions. While most GABA is generated by glutamate decarboxylase (GAD), an alternative synthetic pathway is known to originate from putrescine, which is converted via gamma-aminobutyraldehyde in an aldehyde-dehydrogenase-requiring step to GABA. In a search for the role of two aldehyde dehydrogenases expressed in segregated compartments along the dorsoventral axis of the developing retina, we assayed dorsal and ventral retina fractions of the mouse for GABA by high performance liquid chromatography. We found GABA to be present in the embryonic retina, long before expression of GAD, and ventral GABA levels exceeded dorsal levels by more than three-fold. Postnatally, when GAD became detectable, overall GABA levels increased, and the ventrodorsal concentration difference disappeared. Our observations indicate that prior to the formation of synapses the embryonic retina contains a ventrodorsal GABA gradient generated by an alternate synthetic pathway.

Expression of GABA in the fetal, postnatal, and adult human retinas: an immunohistochemical study

The expression of GABA in the human fetal (12-25 weeks of gestation), postnatal (five-month-old), and adult (35-year-old) retinas was investigated by immunohistochemistry. GABA expression was seen as early as 12 weeks in the undifferentiated cells of the inner neuroblast zone; a few optic nerve fiber layer axons were clearly labeled, suggesting that some of the stained cell bodies were prospective ganglion cells, others could be displaced amacrine cells. From 16-17 to 24-25 weeks, intense labeling was found in the amacrine, displaced amacrine, and some ganglion cells. During this time period, horizontal cells (identified by calbindin immunohistochemistry), undergoing migration (periphery) and differentiation (center), expressed GABA prominently. In the postnatal retina, some horizontal cells were moderately labeled, but very weakly in a few cells, in the adult. The Müller cells developed immunoreactivity first weakly at 12 weeks and then moderately from 16-17 weeks onward. The staining was also evident in the postnatal and adult retinas, showing labeled processes of these glial cells. Virtually no axons in the adult optic nerve and nerve fiber layer were stained; the staining was restricted to a few, large ganglion cells and displaced amacrine cells: Some amacrines were also labeled. The possibility that GABA might play a role in horizontal cell differentiation and maturation is highlighted. Other evidences suggest that GABA might play a role in metabolism during retinal development.

A pacemaker current in dye-coupled hilar interneurons contributes to the generation of giant GABAergic potentials in developing hippocampus

The establishment of synaptic connections and their refinement during development require neural activity. Increasing evidence suggests that spontaneous bursts of neural activity within an immature network are mediated by gamma-aminobutyric acid via a paradoxical excitatory action. Our data show that in the developing hippocampus such synchronous burst activity is generated in the hilar region by transiently coupled cells. These cells have been identified as neuronal elements because they fire action potentials and they are not positive for the glial fibrillary acidic protein staining. Oscillations in hilar cells are "paced" by a hyperpolarization-activated current, with properties of Ih. Coactivated interneurons synchronously release GABA, which via its excitatory action may serve a neurotrophic function during the refinement of hippocampal circuitry.

GABA Levels in the Brain: A Target for New Antiepileptic Drugs

A basic strategy for the pharmacological treatment of epilepsy is to develop drugs that reduce the excitability of CNS neurons at times preceding or during the onset of seizure discharge with minimal effects on normal electrical activity. Several antiepileptic drugs currently in use exert their action by modulating sodium channels or receptors of the abundant inhibitory neurotransmitter, GABA. These approaches, which are often successful in reducing the number or severity of seizures, have some effects that limit their clinical use. More recently, a new class of antiepileptic drugs such as vigabatrin, which blocks GABA degradation enzymes, have been developed as effective antiepileptics and are associated with minimal side effects. Although these drugs do not display agonist or antagonist properties at GABA receptor sites, they do appear to interact with brain GABA systems because NMR spectroscopy studies indicate that subjects given these drugs have elevated brain GABA levels, and in vitro electrophysiological studies on CNS tissue reveal elevated GABA release. The precise cellular mechanisms of antiepileptic action of these GABA metabolic modulators are not clear, but current work on the cellular effects of these drugs suggests a model that may explain their action.

On the structure of ictal events in vitro

PURPOSE

To analyze the cellular and network mechanisms of sustained seizures, we reviewed the literature and present new data on in vitro epileptiform events. We considered single and recurring synchronized population bursts occurring on a time scale from tens of milliseconds to 1 min.

METHODS

We used intracellular and field potential recordings, together with computer network simulations, derived from three types of experimental epileptogenesis: gamma-aminobutyric-acidA (GABAA) blockade, low extracellular [Mg2+]o, and 4-aminopyridine (4-AP).

RESULTS

In all three models, sustained depolarizing synaptic currents developed, either through N-methyl-D-aspartate (NMDA) receptors, depolarizing GABAA receptors, or both. Ectopic action potentials (APs), probably originating in axonal structures, occurred in 4-AP and (as shown by other researchers) after tetanic stimulation; ectopic APs, occurring at sufficient frequency, should also depolarize dendrites, by synaptic excitation, enough to trigger bursts.

CONCLUSIONS

Ictal-like events appear to arise from two basic mechanisms. The first mechanism consists of sustained dendritic depolarization driving a series of dendritic bursts. The second mechanism consists of an increase in axonal and presynaptic terminal excitability driving a series of bursts analogous to interictal spikes.

Autoprotective mechanisms in the CNS: some new lessons from white matter

Anoxia/ischemia in the CNS is a common and devastating phenomenon. It is possible that the best hopes for protection against anoxic/ischemic injury may involve recruiting and/or augmenting any autoprotective systems that evolution has provided for the CNS. We describe here the existence of such an autoprotective system present in CNS white matter. White matter is both well suited to studying extrasynaptic systems, such as the system we describe here, and is a highly appropriate target for research into anoxic-ischemic injury in its own right. We show that white matter contains functional GABAB and adenosine receptors that respond to an anoxic efflux of GABA and adenosine by recruiting a convergent intracellular mechanism involving protein kinase C (PKC). The net result of this receptor-mediated cascade is an increase in resistance to anoxia, which presumably allows CNS white matter to tolerate better a common class of ischemic events that are located solely in white matter and that comprises approximately 25% of all strokes seen clinically.

Immunocytochemical evidence for an axonal localization of GABA in the optic nerves of rabbits, rats, and cats

We have examined, by light-microscopic immunocytochemistry, the distribution of GABA in the optic nerves of adult rabbits, rats, and cats. Within the optic nerves, immunoreactivity for GABA was restricted to a small subset of axons; some axons were strongly labelled, others weakly labelled, whilst most axons were unlabelled. Glia and other non-neuronal elements were always unlabelled. Our ability to detect GABA in optic nerve axons of adult mammals contrasts with previous reports that indicate a lack of GABA immunoreactivity in such axons. We suggest that this discrepancy may be due to the sensitivity of our immunocytochemical techniques which enable us to detect low concentrations of GABA.

Early appearance and transient expression of putative amino acid neurotransmitters and related molecules in the developing rabbit retina: an immunocytochemical study

We have studied, by immunocytochemistry, the ontogeny of GABA, glycine, glutamate, glutamine, and taurine-containing cells in the rabbit retina. Amacrine cells show GABA immunoreactivity by embryonic day 25 (E25) and throughout postnatal life. By contrast, ganglion cells and horizontal cells are only transiently GABA-immunoreactive (-IR); few appear GABA-IR by the third postnatal week. At maturity, glycine is present in amacrine cells and in some bipolar cells. During development, putative ganglion cells transiently contained glycine between E25 and postnatal day 3 (P3), whereas immunolabelling in presumed amacrine cells and bipolar cells persists after birth. Ganglion cells, bipolar cells, photoreceptors, and some amacrine cells are glutamate-IR in the adult retina. Glutamate immunoreactivity first appears in the somata and processes of cytoblastic cells by E20 and is prominent by E25. Surprisingly, ganglion cells are not strongly glutamate-IR until just before eye-opening, at postnatal day 10 (P10), coincident with the appearance of glutamine in their somata and in Müller glial cells. Bipolar cells are glutamate-IR before they or Müller cells contain high levels of glutamine (at P10). Glutamate immunoreactivity in photoreceptors is progressively restricted to the inner segments by eye-opening. At no stage are presumed horizontal cells glutamate-IR or glutamine-IR, but some amacrine cells show glutamate- and glutamine-IR by P10. Taurine is localized to photoreceptors and Müller glial in the adult retina. Some cytoblasts are taurine-IR at E20; with ensuing development, taurine labelling becomes restricted primarily to Müller cells and photoreceptors; some putative bipolar cells may also be labelled. However, for a few days around birth, cells resembling horizontal cells, also show taurine immunoreactivity. The early appearance and often transient expression of these amino acids in retinal cells suggests that these neuroactive molecules may be involved in the structural and functional development of the retina.

The astrocyte response to gamma-aminobutyric acid attenuates with age in the rat optic nerve

There is increasing evidence that glial cells respond to the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), and astrocytes have been shown to possess GABAA receptors both in vivo and in vitro. A recent study by Sakatani et al. (Proc. R. Soc. Lond. B247, 155 (1992)) demonstrated the transient expression of functional GABAA receptors in the developing rat optic nerve, but axonal and glial components of the response were not distinguished. To help address this problem, we have determined the electrophysiological response to GABA in astrocytes of the isolated intact optic nerves from neonatal rats, identified morphologically following intracellular injection of horseradish peroxidase. Astrocytes responded to GABA by a GABAA receptor-mediated depolarization which attenuated gradually during post-natal development; astrocytes in 21-day-old nerves were not observed to respond to GABA. The results indicate the transient presence of functional GABAA receptors in developing rat optic nerve astrocytes in situ, and we speculate upon a role for GABA in glial signalling and the organization of axonglial interrelations during development.

Ontogeny of GABAA receptor subunit mRNAs in rat spinal cord and dorsal root ganglia

Relatively little is known about the development of GABAA receptor subunits and their gene expression in mammalian spinal cord. The expression of mRNAs encoding 13 GABAA receptor subunits (alpha 1-6, beta 1-3, gamma 1-3, and delta) in embryonic, postnatal, and adult rat spinal cord and dorsal root ganglia (DRG) cells were studied by in situ hybridization and reverse transcription-polymerase chain reaction (RT-PCR) analysis. Both techniques revealed the presence of all subunit mRNAs originally found in the rat brain, except for alpha 6, which was not detectable, and delta, which was weakly detected only by RT-PCR. Two anatomically distinctive sets of subunit mRNAs were found by in situ hybridization within the ventricular zone (VZ) and mantle zone (MZ). The trio of alpha 4, beta 1, and gamma 1 subunit mRNAs emerged exclusively in neuroepithelial cells at embryonic day 13 (E13) and remained detectable in the VZ until E17. In the MZ, beta 3 subunit mRNA was first detected at E12, while alpha 2, alpha 3, alpha 5, beta 2, gamma 2, and gamma 3 transcripts appeared at E13. Expressions of the subunit mRNAs in the MZ rapidly increased and expanded in a ventrodorsal sequence from motoneurons to dorsal horn neurons before reaching a peak in the late embryonic/early postnatal period. The mRNA expressions declined during postnatal development, by region-selective depletion, with alpha 4, alpha 5, beta 1, beta 2, gamma 1, and gamma 3 subunit mRNAs becoming barely detectable. In contrast, alpha 2, alpha 3, beta 3, and gamma 2 transcripts persisted into adulthood with distinct anatomical distributions. RT-PCR analysis revealed unique developmental patterns in the intensities of PCR products, most of which were in good agreement with developmental changes in the densities of hybridized mRNA signals. However, RT-PCR amplified minute amounts of mRNAs for alpha 1, alpha 4, alpha 5, beta 1, beta 2, gamma 1, gamma 3, and delta subunits in adults, which were not found in film autoradiograms, but could be detected in a few grain-positive cells in emulsion-dipped sections. DRG cells expressed alpha 2, alpha 3, alpha 5, beta 2, beta 3, and gamma 2 subunit mRNAs during embryogenesis but only alpha 2, beta 3, and gamma 2 subunit mRNAs were reliably detected in the adult.(ABSTRACT TRUNCATED AT 400 WORDS)

Transient presence of GABA in astrocytes of the developing optic nerve

Immunostaining and high-pressure liquid chromatography (HPLC) were used to study the developmental time course of astrocytic gamma-aminobutyric acid (GABA) expression in rat optic nerve. GABA immunostaining was carried out on cultured astrocytes, and on whole optic nerve. Confocal scanning laser microscopy was used to obtain optical sections in excised whole tissue in order to localize the cellular origins of GABA within the relatively intact optic nerve. GABA immunoreactivity was localized in astrocytes identified by GFAP staining; GABA staining was most intense in early neonatal optic nerve and attenuated over 3 weeks of postnatal development. The staining was pronounced in the astrocyte cell bodies and processes but not in the nucleus. There was a paucity of GABA immunoreactivity by postnatal day 20, both in culture and in whole optic nerve. A biochemical assay for optic nerve GABA using HPLC indicated a relatively high concentration of GABA in the neonate, which rapidly attenuated over the first 3 postnatal weeks. Immunoreactivity for the GABA synthesis enzyme glutamic acid decarboxylase (GAD) was pronounced in neonates but also attenuated with development. These results indicate that GABA and the GABA synthesis enzyme GAD are localized in astrocytes of optic nerve, and that their expression is transient during postnatal development.