Increased binding of GABA to its post-synaptic carrier sites on the plasma membrane of Deiters' neurons after a learning experiment in rats

Permanent alteration of muscarinic acetylcholine receptor binding in rat striatum after circling training during development

We evaluated the effect of circling training (CT) in the expression of muscarinic acetylcholine receptor (mAchR) in developing rat striatum. For this, male and female rats were subjected to CT at 20, 30, 40 and 60 days of age during 7 days. Animals trained at 30 days but not at other ages showed an average decreased binding to mAchR of 33% in males and 24% in females, representing a significant difference with respect to control non-trained animals (males P < 0.001, females P < 0.005), and showing also a differential response between sex (P < 0.01). mAchR drop was found invariably either 2 months or 1 year after training indicating a long term plastic change due to circling training. Scatchard analysis showed that altered binding represents a variation of the total receptor number instead of its binding affinity, with no significant differences found among Kd (P > 0.1). mAchR variation was correlated with the motor performance accomplished in the test. Regarding total distance run, male rats trained for 3 days (300 meters. run), for 5 days (600 meters) and for 7 days (900 meters) showed a drop of 19, 28 and 33% respectively (r2 = 0.91, P < 0.001), while female changes were of 21, 23 and 24% (r2 = 0.78, P < 0.001). Nevertheless, no correlation with running speed was found (r2 = 0.13 male, r2 = 0.02 female; P > 0.1). In summary, these results demonstrate the presence of a limited sensitivity period during striatum development where mAchR expression may be affected by the activity performed during CT, representing a permanent alteration of the receptor levels.

Quantitative autoradiographic analysis of the distribution of [3H]muscimol binding to GABA receptors in chick brain

Quantitative receptor autoradiography was used to investigate the distribution of high-affinity GABA receptors (GABAA) in left and right hemispheres of the brains of 3-week-old chicks. The receptors were labelled with the potent GABA agonist [3H]muscimol. High levels of [3H]muscimol labelling were found throughout the fore-, mid-, and hindbrain, though considerable variation was found in different regions. In the telencephalon the highest concentration of specific binding was found in the hyperstriatum ventrale followed by the neostriatum, and then the lobus parolfactorius of the paleostriatal complex, whilst in the diencephalon highest levels of labelling were present in the infundibulum. In the midbrain distinct lamination was observed in the high levels of [3H]muscimol binding in the optic tectum and in the hind brain the highest density of labelling occurred in the granular layers of the cerebellum. Levels of labelling were generally low in the brainstem regions. The distribution of [3H]muscimol binding in the optic tectum and in the hind brain the highest density of labelling occurred in the granular layers of the cerebellum. Levels of labelling were generally low in the brainstem regions. The distribution of [3H]muscimol binding sites is in good agreement with our previous work on the distribution of GABA-immunoreactivity in the chick brain.

Micromethods for the study of GABA biochemistry and function at single GABA acceptive membranes

Three different micromethods for studying GABA biochemistry and function at single microdissected GABA-acceptive neuronal membranes are discussed. The basis for such studies is the possibility of obtaining by microdissection single Deiters' neurons from the lateral vestibular nucleus of the rat and the rabbit. From these isolated cells the plasma membrane may be prepared and studied. The first micromethod allows the study of the Na+ independent diffusion of GABA through such a plasma membrane which is postsynaptic to GABA-ergic boutons. A modification of such method allows also the study of the effects of GABA-ergic drugs on Cl- permeability. The second method allows the study by microelectrophoresis in capillaries of GABA catabolism by GABA-T associated with microdissected single Deiters' membranes. The third one was developed in order to study the characteristics of Na+ dependent GABA carrier activity present on such membranes.

Asymmetric diffusion into the postsynaptic neuron: an extremely efficient mechanism for removing excess GABA from synaptic clefts on the Deiters' neurone plasma membrane

Microdissected Deiters' neuron plasma membranes have been used for studying the passage of GABA through the membrane both in the inward and outward direction. Working with 0.2 mM GABA in the compartment simulating the outside of the neurone and with 2.0 mM GABA in the one simulating the inside we found a net transport of GABA towards the inside. This mechanism does not require a Na+ ion gradient across the membrane. The nature of the transport process involved was studied by determining the rate of [3H]-GABA inward passage as a function of GABA concentration (1 nM - 800 microM) on the outward side of the membrane. The results have shown that until 50 microM a diffusion process (v = D1 X C, where D1 = 3.1 X 10(-11) 1/micron 2 X sec) is the sole mechanism involved. Above 50 microM a second diffusion process is activated v = D2 X (C - 50 X 10(-6), where D2 = 2.8 X 10(-11) 1/micron 2 X sec. Taking in account both inward and outward directed diffusion, one can calculate 16 microM as the equilibrium concentration of GABA on the outward side of the membrane. From a kinetic point of view, these diffusion processes are able to reduce GABA concentration in a synaptic cleft from 3 mM to 20 microM within 3 mu sec. These diffusion systems are discussed as extremely efficient in removing the excess of released GABA in the synaptic cleft.

gamma-Aminobutyric acid (GABA) removal from the synaptic cleft: a postsynaptic event?

In the present commentary we discuss the adequacy of Na+ transport-coupled presynaptic gamma-aminobutyric acid (GABA) uptake systems for the removal of GABA from the synaptic cleft. This discussion is based on the accepted stoichiometry for GABA presynaptic internalization, GABAout + 3Na+out + K+in in equilibrium GABAin + 3Na+in + K+out, on the parameters reported in the literature for typical synaptosomal preparations, and on the assumption that GABA removal must be a quick event (less than or equal to 2 msec), as derived from electrophysiological studies. On these bases, we have developed a calculation in order to evaluate the time course of synaptic cleft GABA removal by presynaptic systems and ended up with an overall value (t approximately 0.3 sec) which does not fit with the data derived from electrophysiological recordings. Moreover, we calculated that if such systems had the function of removing GABA within 2 msec, as it should be, a large depolarization would be brought about in GABAergic boutons, resulting ultimately in further GABA release. These considerations together with biochemical and pharmacological experimental results seem to exclude that presynaptic uptake systems have the function of removing GABA from the synaptic cleft. Our experimental data on the ability of a GABA-acceptive postsynaptic membrane (Deiters' neuron membrane) to transport GABA indicate that this system may have the correct characteristics for removing the neurotransmitter. This refers to both the kinetics and the electrophysiological consequences of the phenomenon.

An inhibitory protein of intestinal fluid secretion reverses neuronal GABA transport

Intestinal challenge with cholera toxin induces the synthesis of a hormone-like protein which counteracts intestinal hypersecretion. This study shows that the protein also inhibits GABA transport across the plasma membrane of Deiters' cells in rabbits. The inhibitory action of the protein was dose dependent, and 10(3) times more potent than met 5-enkephalin, hitherto the most effective known inhibitor of GABA transport in vitro. The influence of the protein on the plasma membrane was reversible, and did not affect either postsynaptic binding or uptake of GABA.