Discriminative stimulus effects of tiagabine and related GABAergic drugs in rats

Rationale Tiagabine is an anticonvulsant drug which may also have sleep-enhancing properties. It acts by inhibiting reuptake at the gamma-aminobutyric acid (GABA) transporter (GAT-1). Objectives The aim of the study was to determine whether tiagabine acted as a discriminative stimulus and, if so, whether other GABAergic compounds would generalise to it. Materials and methods Rats were trained to discriminate tiagabine (30 mg/kg p.o.) from vehicle, and generalisation to drugs that modulate GABA was assessed. Results Gaboxadol (5–20 mg/kg p.o.), a selective extrasynaptic GABA_A agonist, generalised to tiagabine, although the extent of the generalisation was inconclusive. Indiplon (1 mg/kg p.o.), a benzodiazepine-like hypnotic, also partially generalised to tiagabine, although zolpidem and S-zopiclone did not. Baclofen, a GABA_B receptor agonist, and gabapentin, which increases synaptic GABA, did not generalise to tiagabine. (+)-Bicuculline (3 mg/kg i.p.), a GABA_A receptor antagonist, blocked the tiagabine cue, but the less brain-penetrant salt form, bicuculline methochloride, had no effect. Conclusions These data suggest that tiagabine generates a discriminative stimulus in rats, and provides a central GABA-mediated cue, but is distinct from the other GABAergic compounds tested.     

Fos expression in gonadotropin-releasing hormone neurons by naloxone or bicuculline in intact male rats

Double immunocytochemistry for Fos and GnRH was performed after the intravenous infusion of naloxone or bicuculline. Either naloxone or bicuculline significantly increased serum LH concentrations. After the naloxone infusion, some Fos-positive GnRH neurons were found from the preoptic area to the basal hypothalamic area, whereas those were found from the diagonal band of Broca to the preoptic area after the bicuculline infusion. These results suggest the distribution of GnRH neurons activated by naloxone and bicuculline in intact male rats.     

Synchronized neuronal activities in neocortical explant cultures

Summary Intracellular recordings revealed that in neocortical expiant cultures prepared on the day of birth and examined 3–6 weeks later, neurons mature and establish complex synaptic relationships that lead to spontaneous and triggered synchronous discharge. The spontaneous synchronous activity took several forms, including periodic generation of epileptiform depolarizing waves, prolonged periods of seizure-like discharge, and periodic, intense barrages of IPSPs. Synchronous depolarizations were associated with a marked increase in membrane conductance. Intracellular injection of currents of varying polarity and intensity affected their amplitudes and polarities without influencing the probability of their occurance, indicating that the discharge reflected the synchronous activities of a neuronal population. This conclusion was confirmed with simultaneous recordings from pairs of neurons. Effects of the GABAa receptor antagonist, bicuculline, and the NMDA receptor antgonist, 2-aminophosphonvalerate (2APV), were used to assess the contributions of impairment of inhibition and enhancement of excitation to the initiation of synchronous discharge. The frequency with which spontaneous depolarizations were generated in normal medium was markedly reduced by 2APV. Moreover, seizure-like activity was induced by removing Mg⁺⁺ from the medium, a condition that enhances conductance through NMDA receptor-coupled channels. This behavior was also attenuated by 2APV. Perfusion of bicuculline was potently epileptogenic. 2APV cut short the late, voltage-dependent phase of bicuculline-induced paroxysmal depolarizations, indicating a role of NMDA receptors in generating this component of the wave. Epileptiform activities induced by withdrawal of Mg⁺⁺ were greatly augmented by bicuculline, indicating that blockade of inhibition was not a prerequisite for seizure-like activity. This conclusion is supported by the finding that in many neurons in untreated cultures, paroxysmal generation of trains of IPSPs was the primary manifestation of spontaneous, synchronous population discharge.     

Potential involvement of a baclofen-sensitive autoreceptor in the modulation of the release of endogenous GABA from rat brain slices in vitro

Summary The effects of the GABA_A agonist, muscimol, and of the enantiomers of the GABA_B agonist, baclofen, on the release of endogenous GABA from slices of the rat cerebral cortex, striatum and hippocampus were measured by means of a HPLC method with electrochemical detection. Moreover, the effect of the GABA_A antagonist, bicuculline, and of the frequency of stimulation were studied in cortical slices. The amount of endogenous GABA released per impulse from cortical slices decreased by about 50% when the frequency was increased from 0.25 Hz to 1 Hz. This might indicate that GABA inhibited its own release. (–)-Baclofen at 1 and 10 M, but not its (+)-enantiomer, markedly inhibited the release of endogenous GABA, to a similar extent in all 3 areas investigated. The effect of (–)-baclofen was dependent on the frequency of stimulation: at lower frequencies (0.25 and 0.5 Hz) it was more marked than at a higher one (4 Hz). This would be expected from the results showing that the release of endogenous GABA decreases with increasing frequency, which suggests that this amino acid inhibits its own release. Muscimol at 10 M, on the other hand, was ineffective in all 3 areas at a stimulation frequency of 0.5 Hz. Bicuculline (10 M) at 4 Hz, at which autosuppression of GABA release is maximal did not enhance the release of endogenous GABA from cortical slices. With cerebellar or nigral slices, no adequate stimulation-induced release of endogenous GABA could be obtained under comparable conditions. These data are compatible with, but do not prove the existence of GABAB-type presynaptic autoreceptors modulating the release of this amino acid. More definite conclusions may possibly be drawn when a GABAB antagonist becomes available, which is expected to enhance GABA release under appropriate conditions.Presented in part at the 3rd Brit. Meeting on Electrochemical detection in Pharmacology and Neurochemistry, Cambridge, March 30–April 1, 1987Send offprint requests to P. C. Waldmeier     

Peptidergic regulation of norepinephrine induced feeding

The inhibitory effect of a variety of substances on feeding induced by norepinephrine (20 μg ICV) was studied. Subcutaneous administration of the opiate antagonist, naloxone, inhibited norepinephrine-induced eating at 10 and 5 mg/kg, but not a 1 mg/kg. Intraventricular administration of the GABA antagonist, bicuculline, produced a dose related decrease in food ingestion. The putative satiety hormones, bombesin (10 μg/kg; subcutaneously) and cholecystokinin octapeptide ( 10 gmg/kg; subcutaneously) also reduced norepinephrine induced eating, as did ICV administration of calcitonin (2 units). Neither thyrotropin-releasing hormone (1 μg ICV) nor its metabolits, histidyl-proline diketopiperazine (1 μg ICV) altered norepinephrine-induced feeding. The studies reported here suggest a neuromodulatory role of peptides in the central regulation of norepinephrine-induced feeding.     

MELATONIN REDUCES BLOOD PRESSURE IN RATS WITH STRESS-INDUCED HYPERTENSION VIA GABAA RECEPTORS

• 1 Several groups have reported that melatonin produces a significant decrease in blood pressure in mammals and that pinealectomy in rats causes hypertension. The purpose of the present study was to investigate the effects of melatonin and bicuculline methiodide on the blood pressure of rats, both in the developing and fully developed stage of stress‐induced hypertension (SIH).
• 2 Rats with SIH were generated by mild electric foot shocks for 15 days, after which tail arterial systolic pressure and plasma angiotensin (Ang) II levels were measured. The effects of melatonin injections (i.p. or i.c.v.) on mean arterial pressure (MAP) in rats with SIH were also determined.
• 3 Pretreatment with 1 mg/kg, i.p., melatonin significantly diminished the elevated tail arterial systolic pressure and plasma AngII levels caused by 15 days stress. The suppressive effects of melatonin were blocked by i.p. injection of 1 mg/kg bicuculline methiodide, an antagonist of the GABA_A receptor.
• 4 Intraperitoneal (0.2, 0.5 and 1 mg/kg) or i.c.v. (0.15 and 1.5 µg/3 µL) injection of melatonin produced a dose‐dependent lowering of MAP in rats with SIH. The antihypertensive response induced by melatonin was blocked by injection of both 1 mg/kg, i.p., and 1.5 × 10⁶ µg/3 µL, i.c.v., bicuculline methiodide.
• 5 In conclusion, melatonin not only prevents increases in blood pressure during the developing stage of SIH, but can also reduce the blood pressure of rats that have already developed SIH. The antihypertensive effect of melatonin may be mediated by GABA_A receptors through inhibition of plasma AngII levels.

     

Dynamic differentiation of GABAA-sensitive influences on orientation selectivity of complex cells in the cat striate cortex

The influence of GABA_A receptors on orientation selectivity of cat complex cells was tested by iontophoresis of the GABA_A receptor blockers bicuculline and N-methyl-bicuculline while stimulating with drifting sinusoidal gratings. Reduction of orientation tuning was markedly less than reported in previous studies that used drifting bars as visual stimuli. Only 3/31 cells lost orientation selectivity, with an average increase in bandwidth of 33%, as opposed to half the cells losing selectivity and a bandwidth increase for the remainder of 47% as reported previously. Infusion of GABA_A blockers revealed a prominent stimulus onset transient response, lasting about 120 ms, that showed a broadening of orientation selectivity comparable to that found using drifting bars under similar circumstances. We believe that drifting gratings emphasize a steady-state response component that retains, in the presence of GABA_A blockers, significant orientation selectivity. Because the onset transient is initially unselective for orientation, we suggest that the steady-state, orientation-selective response component develops from an alternate inhibitory mechanism, possibly mediated by GABA_B receptors.     

Electrophysiological actions of the plant alkaloid 6-benzoylheteratisine in rat hippocampal slices

The effects of the Aconitum alkaloid 6-benzoylheteratisine on neuronal activity was investigated in the in vitro slice preparation of rat hippocampus by extracellular recording of the stimulus-evoked population spike. 6-Benzoylheteratisine (0.01-10 μΜ) depressed the orthodromic and antidromic population spike in a concentration-dependent manner.The action of the drug was activity-dependent. The latency of onset of the inhibition was accelerated when the frequency of electrical stimulation had been increased. Furthermore, the effect of 6-benzoylheteratisine was evaluated in two different models of epileptiform activity induced either by blockade of GABA receptors by bicuculline (10 μΜ) or by a nominal Mg²⁺-free bathing medium. Due to the activity-dependent mode of action, this drug effectively reduced the number and the size of the synaptically evoked population spikes in the presence of bicuculline or nominal Mg²⁺-free bathing medium, respectively. Received: 23 September 1996 / Accepted: 2 January 1997     

Genotype-dependent effects of GABAergic agents on sedative properties of ethanol

Two lines of mice, selectively bred for differential sensitivity to the soporific effects of ethanol (ETOH), were administered GABAergic drugs in an effort to evaluate a role for GABA in ETOH sensitivity. ETOH sensitive Long-Sleep mice (LS) showed potentiated ETOH sedation when administered bicuculline, muscimol and aminooxyacetic acid (AOAA). ETOH-insensitive SS mice exhibited reduced ETOH sedation in the presence of the antagonists, bicuculline and picrotoxin, and potentiated sedation in the presence of muscimol and AOAA. These changes in narcosis duration were interpreted as central effects, since blood ethanol levels at waking from ETOH sedation varied with GABAergic drug treatment. Picrotoxin antagonized pentobarbital-induced nacrosis in both lines, but to a greater extent in SS mice. These and other experiments with a genetically heterogeneous stock suggest GABA involvement in genotype-dependent ETOH sensitivity, but do not support a simple role of GABA receptor involvement.     

2［对－（二甲氨基）苯乙烯］碘化甲基砒啶对豚鼠乳头肌慢反应动作电位和慢内向离子流的影响

用细胞内微电级和电压钳技术研究了２［对－（二甲氨基）苯乙烯］碘化甲基砒啶（ＤＳＰＭ）对豚鼠乳头肌慢反应动作电位（ＳＲＡＰ，）和慢内向电流（Ｉｓｉ）的影响。用ＤＳＰＭ液灌流５０分钟后，高Ｋ￣＋所致ＳＲＡＰ的振幅和零相最大上升速率均显著降低，Ｉｓｉ的峰值由８．８±１．６μＡ降至５．７±１．８μＡ。上述结果表明ＤＳＰＭ具有选择性阻断钙通道的作用