5-Aminolevulinic acid inhibits [3H]muscimol binding to human and rat brain synaptic membranes

The interaction of 5-aminolevulinic acid (ALA) with GABA_A receptors has been proposed to underlie the neurological dysfunctions of ALA-accumulating disorders, such as acute intermittent porphyria. The effects of ALA on [³H]muscimol binding to human and rat cerebral cortical membranes were compared. ALA (0.1–10 mM) significantly inhibited the binding of [³H]muscimol (12 nM), with a similar potency in rat and human membranes (IC₅₀ = 199 vs. 228 M, respectively). Kinetical analysis revealed that ALA (1 mM) significantly increased the Kd and decreased the Bmax of [³H]muscimol to both rat (100 and 50%, respectively) and human (200 and 40%, respectively) membranes, indicating a mixed-type inhibition. The similarity in the potency and mechanism of the ALA-induced inhibition of muscimol binding in rat and human membranes indicate that rat studies are useful to evaluate the neurotoxic properties of ALA towards the human GABAergic system, and may help to understand the pathophysiology of porphyria.     

Cerebellar GABAA receptor binding and function in vitro in two rat lines developed for high and low alcohol sensitivity

TheB _max of the [³H]muscimol binding in the cerebellum of ethanol-naive alcohol-sensitive ANT (Alcohol Non-Tolerant) rats was only about 70% of that in the alcohol-insensitive AT (Alcohol Tolerant) rats. There were no line differences in the muscimol binding to cerebrocortical and hippocampal membranes. In the alcohol-sensitive rats, the cerebellar [³H]muscimol binding (5 nM) negatively correlated with the ethanol-induced motor-impairment measured in the tilting plane test. Muscimol stimulated the flux of³⁶Cl^– in cerebellar synaptoneurosomes and non-filtered microsacs to the same extent in both rat lines. Ethanol produced only a small, although statistically significant, enhancement of the muscimol-stimulated chloride flux in both rat lines. The present data confirms our earlier finding of a low level of muscimol binding in the cerebellar membranes of alcohol-sensitive rats as compared to alcohol-insensitive rats. Further studies are needed to determine the relationship between the Cl^– flux stimulation by muscimol and the differential muscimol binding in the cerebellum of these rat lines, and its importance for alcohol sensitivity.     

Changes of [<Superscript>3</Superscript>H]Muscimol, [<Superscript>3</Superscript>H]Flunitrazepam and [<Superscript>3</Superscript>H]MK-801 Binding in Rat Brain by Prolonged Ventricular Infusion of 7-Nitroindazole

In the present study, we have investigated the effects of prolonged inhibition of nitric oxide synthase (NOS) by infusion of neuronal NOS (nNOS) inhibitor, 7-nitroindazole (7-NI), to examine modulation of NMDA and GABA_A receptor binding in rat brain. The duration of sleeping time was significantly increased by the pre-treatment with 7-NI (100 mg/kg) 30 min before pentobarbital (40 mg/kg) treatment in rats. However, the duration of pentobarbital-induced sleep was shortened by the prolonged infusion of 7-NI into cerebroventricle for 7 days. We have investigated the effect of NOS inhibitor on NMDA and GABA_A receptor binding characteristics in discrete areas of brain regions by using autoradiographic techniques. The GABA_A receptors were analyzed by quantitative autoradiography using [³H]muscimol and [³H]flunitrazepam binding, and NMDA receptor binding was analyzed by using [³H]MK-801 binding in rat brain slices. Rats were infused with 7-NI (500 pmol/10 l/ h, i.c.v.) for 7 days, through pre-implanted cannula by osmotic minipumps. The levels of [³H]muscimol were markedly elevated in cortex, caudate putamen, and thalamus while the levels of [³H]flunitrazepam binding were only elevated in cerebellum by NOS inhibitor. However, there was no change in the level of [³H]MK-801 binding except decreasing in the thalamus. These results show that the prolonged inhibition of NOS by 7-NI-infusion highly elevates [³H]muscimol binding in a region-specific manner and decreases the pentobarbital-induced sleep.     

Cortical control of tongue protrusion and lateral movements in the cat

Based on area P lesion experiments, we hypothesized that tongue protrusion adapted for licking might be regulated by the lateral wall of the presylvian sulcus (bilateral areas P) of the cerebral cortex (Hiraba H, Sato T, Nakakawa K, Ueda K. . Cortical control of appropriate tongue protrusion during licking in cats—Increase in regional cerebral blood flow (rCBF) of the contralateral area P and in tongue protrusion after the unilateral area P lesion. Somatosens Mot Res 26:82–89). We propose that the right and left lingual muscles are dominated by the right and left hypoglossal nucleus, respectively, and that right and left pyramidal cells projecting to the right and left hypoglossal nucleus, respectively, exist in unilateral area P. These cells project via an inhibitory interneuron relay to the lateral branches toward the left or right pyramidal cells in contralateral area P. In this study, we aimed to demonstrate the existence of inhibitory interneurons using injections of a gamma-aminobutyric acid (GABA) agonist (muscimol), a GABA antagonist (bicuculline), and kainic acid into unilateral area P, followed by examination of tongue protrusion and lateral movements during trained licking and changes in regional cerebral blood flow (rCBF) values in the contralateral area P. We found disordered protrusion toward both sides and a marked decrease in rCBF values in the contralateral area P after bicuculline injection. We also found abnormal tongue protrusion toward the front and a marked increase in rCBF values after muscimol and kainic acid injections. These results suggest that cortical networks between the bilateral areas P are relayed by inhibitory interneurons.     

Changes of [3H]Muscimol Binding and GABAA Receptor β2-Subunit mRNA Level by Tolerance to and Withdrawal from Pentobarbital in Rats

Effects of continuous pentobarbital administration on binding characteristics of [³H]muscimol were examined by autoradiography, and levels of GABA_A receptor 2-subunit mRNA were investigated by in situ hybridization histochemistry in the rat brain. In order to eliminate the induction of hepatic metabolism by systemic administration of pentobarbital, an i.c.v. infusion model of tolerance to and withdrawal from pentobarbital was used. An experimental model of barbiturate tolerance and withdrawal was developed using i.c.v. infusion of pentobarbital (300 g/10 l/hr for 7 days) by osmotic minipumps and abrupt withdrawal from pentobarbital. The levels of [³H]muscimol binding were elevated in cingulate of frontal cortex (46%) and granule layer of cerebellum (32%) of rats 24-hr after withdrawal from pentobarbital, while it was only elevated in cingulate (58%) of tolerant rats. The GABA_A receptor 2-subunit mRNA was increased in the withdrawal rats only: in the cortex (9–14%), hippocampus (15–21%), inferior colliculus (21%), and granule layer of cerebellum (24%). These results show the involvement of GABA_A receptor and its 2-subunit up-regulations in pentobarbital withdrawal rats, and suggest that the levels of [³H]muscimol binding and GABA_A receptor 2-subunit mRNA are altered in a region-specific manner during pentobarbital withdrawal.     

The Bindings of [3H]Muscimol and [3H]Flunitrazapam Are Elevated in Discrete Brain Regions of Butorphanol-Withdrawal Rats

We have investigated the effects of continuous infusion of butorphanol on the modulation of GABA_A receptor binding. Butorphanol was infused continuously into intracerebroventricle (ICV) at a constant rate of 26 nmol/l/h for 3 days, and the withdrawal from opioid was rendered 7 h after the cessation of infusion. The GABA_A receptor bindings in rat brain slices were analyzed by quantitative autoradiography using [³H]muscimol and [³H]flunitrazepam. In the rats withdrawn from butorphanol, the levels of [³H]muscimol binding were significantly elevated in cortex, thalamus, and part of the hippocampus. The levels of [³H]flunitrazepam binding were elevated in almost all of brain regions including cortex, caudate putamen, thalamus, hippocampus, brainstem, and cerebellum in the rats withdrawn from butorphanol. The levels of binding of either [³H]muscimol or [³H]flunitrazepam were not changed in the rats tolerant to butorphanol. However, the activity of GABAergic neuron was not found to have been modulated by butorphanol withdrawal, because the level of glutamic acid decarboxylase was not changed markedly either in rats that were tolerant to or withdrawn from butorphanol by Western blot and immunohistochemical data. These results suggest that the withdrawal from butorphanol infusion markedly elevates the binding of [³H]muscimol and [³H]flunitrazepam throughout the brain in a region-specific manner, and that the regulatory mechanisms in butorphanol tolerance and withdrawal may be different.     

GABAA Receptors in the Pars Compacta and GABAB Receptors in the Pars Reticulata of Rat Substantia Nigra Modulate the Striatal Dopamine Release

The nigral GABAergic regulation of striatal dopamine release was investigated using voltammetry in freely moving rats. The local administration of muscimol (1 nM) in the substantia nigra pars compacta, but not in the substantia nigra pars reticulata, increased the striatal dopamine release. In contrast, the administration of baclofen (10 nM) in the substantia nigra pars reticulata, but not in the substantia nigra pars compacta, produced a decrease of the striatal dopamine release. Opposite effects were respectively observed after administration of GABA_A and GABA_B antagonists. These data lead us to suggest a differential presynaptic GABAergic control of the dopaminergic neurotransmission through GABA_A receptors in the substantia nigra pars compacta, and GABA_B receptors in the substantia nigra pars reticulata.     

Early Activation of Extracellular Signal-Regulated Kinase Signaling Pathway in the Hippocampus is Required for Short-Term Memory Formation of a Fear-Motivated Learning

1. According to its duration there are, at least, two major forms of memory in mammals: short term memory (STM) which develops in a few seconds and lasts several hours and long-term memory (LTM) lasting days, weeks and even a lifetime. In contrast to LTM, very little is known about the neural, cellular and molecular requirements for mammalian STM formation.2. Here we show that early activation of extracellular signal-regulated kinases 1/2 (ERK1/2) in the hippocampus is required for the establishment of STM for a one-trial inhibitory avoidance task in the rat. Immediate posttraining infusion of U0126 (a selective inhibitor of ERK kinase) into the CA1 region of the dorsal hippocampus blocked STM formation.3. Reversible inactivation of the entorhinal cortex through muscimol infusion produced deficits in STM and a selective and rapid decrease in hippocampal ERK2 activation.4. Together with our previous findings showing a rapid decrease in ERK2 activation and impaired STM after blocking BDNF function, the present results strongly suggest that ERK2 signaling in the hippocampus is a critical step in STM processing.Lionel Muller Igaz and Milena Winograd contributed equally to this work