Sedative and anticonvulsant effects of zolpidem in adult and aged mice

To evaluate the possible age-related differences in the behavioral effects of zolpidem, a widely used hypnotic, we compared the effects of zolpidem on the locomotor activity and on the seizure threshold for pentylenetetrazole (PTZ) and picrotoxin (given by i.v. infusion) between adult (3 months) and aged (13 months) mice. Zolpidem (10 mg/kg) produced similar enhancements of the seizure threshold in adult and aged mice. The drug was more potent against PTZ- than against picrotoxin-induced seizures. Diazepam (1 mg/kg), which was taken for comparison, had a weaker effect on picrotoxin-induced tonic seizures in aged than in adult mice. Sedative effect of zolpidem (10 mg/kg), as assessed by its effect on the locomotor activity, was very strong in both groups of mice. The results suggest that sedative and antiseizure effects of zolpidem are not changed in aged mice.     

The effects of compounds related to γ-aminobutyrate and benzodiazepine receptors on behavioural responses to anxiogenic stimuli in the rat: Punished barpressing

Rats were trained to press a bar for sucrose reward on a random-interval (RI) schedule and footshock punishment was then introduced for 3-min intrusion periods (signalled by a tone) on an independent RI schedule. Shock intensity was individually adjusted to produce stable intermediate levels of response suppression during the tone for each animal. Groups of animals were then allocated to a number of separate experiments in which they were systemically injected with anxiolytics (chlordiazepoxide HCl or sodium amylobarbitone), GABA antagonists (picrotoxin or bicuculline), the GABA_A agonist muscimol, the GABA_B agonist baclofen, an antagonist (RO 15-1788) at the benzodiazepine receptor and, an inverse agonist (FG 7142) at this receptor. The results showed that the alleviation of punishment-induced suppression of barpressing produced by chlordiazepoxide was blocked or partially blocked by RO 15-1788, picrotoxin and bicuculline but not by FG 7142; that picrotoxin (but not FG 7142) increased the suppression of responding by punishment; that neither muscimol nor baclofen affected responding on their own, but their combination weakly but reliably released punished responding from suppression; and that the anti-punishment effect of amylobarbitone was unaffected by either picrotoxin or bicuculline, though the barbiturate reversed the punishment-enhancing effect of picrotoxin. These results are discussed in the light of the hypothesis that anxiolytic behavioural effects are due to increased GABAergic inhibition.     

Effect of Leu-enkephalin on the memory reactivation produced by blockade of the benzodiazepine/GABA-chloride ionophore receptor complex

This investigation sought to characterize the interaction between GABA/benzodiazepine and opioid systems in memory retrieval deficit induced by detaining an animal in the training apparatus after acquisition. Mice pretreated with saline or Leu-enkephalin (0.2 mg/kg) were trained in one-trial passive avoidance test with following detention. Pre-testing administration of bicuculline (1 mg/kg), picrotoxin (1 mg/kg), or flumazenil (10 mg/kg) produced the memory-enhancing effect in the saline-pretreated mice. Pretraining treatment with Leu-enkephalin blocked the reactivation of memory produced by bicuculline and picrotoxin, but not flumazenil. The present investigation suggest that both benzodiazepine/GABA and opioid systems are important modulators of memory retrieval and that a specific interaction between these systems is responsible for the observed recovery of impaired memory trace.     

Effect of antipsychotic drugs on the firing of dorsal raphe cells. II. Reversal by picrotoxin.

As reported in the preceding study, the ability of certain antipsychotic and adrenolytic agents to inhibit the spontaneous firing of serotonergic 5HT neurons in the dorsal raphe nucleus appeared to be related to adrenergic blocking efficacy. However, the interaction between adrenergic and serotonergic systems was apparently indirect. In this phase of the study we investigated the hypothesis that another transmitter system could mediate this interaction. We examined the effects of two inhibitory amino acid transmitters (GABA and glycine) for possible effects on dorsal raphe cell firing using single cell recording and microiontophoretic techniques. In addition, the ability of the GABA antagonist, picrotoxin and the glycine antagonist, strychnine to reverse the effects of the antipsychotic and alpha-blocking drugs on dorsal raphe firing was tested. Both GABA and glycine were found to inhibit raphe cell firing selectively, allowing for a possible neurotransmitter function for these amino acids within the dorsal raphe nucleus. However, picrotoxin but not strychnine was found to reverse the effects of the antipsychotic and alpha-blocking drugs on raphe firing. Based on these results, we propose that the adrenergic input may influence 5HT neurons indirectly via a GABAergic interneuron or interposed GABA neuron.     

Effect of Inosine on Seizures Induced with Pentylenetetrazole, Bicuculline, or Picrotoxin

The effect in mice of inosine administered subcutaneously on the threshold of seizures induced with pentylenetetrazole (PTZ), bicuculline, and picrotoxin was studied, and brain inosine levels were measured. Following inosine, 1,000 mg/kg, the threshold to PTZ was increased at 10-30 min after injection as determined by a tail vein infusion method. Bicuculline and picrotoxin thresholds were significantly elevated only at 5 min. The time to the first myoclonic jerk after intraperitoneal administration of each of these convulsants was significantly prolonged by the lowest dose of inosine tested (250 mg/kg for PTZ, 100 mg/kg for bicuculline, and 500 mg/kg for picrotoxin). Inosine was given subcutaneously 10 min before the convulsant. The mean control brain inosine concentration was 2.9 microM. After subcutaneous inosine, 1,000, 500, and 250 mg/kg, the highest levels reached were 14.4, 7.9, and 4.3 microM, respectively. It is concluded that micromolar concentrations of inosine in brain are antiepileptic.     

Sequential mediation of norepinephrine- and dopamine-induced antinociception at the spinal level: Involvement of different local neuroactive substances

The effects of intrathecally (i.t.) administered opioid antagonist naloxone (Nal), adenosine antagonist aminophylline (Aph), and γ-aminobutyric acid (GABA_A)-receptor antagonist picrotoxin (PTX) or Bicuculline (BIC) on the antinociception produced by i.t. norepinephrine (NE), dopamine (DA), morphine (Mor), 5′-N-ethylcarboxamidoadenosine (NECA, an adenosine agonist) or muscimol (MUS, a selective GABA_A-receptor agonist) were studied and compared using the tail-flick test in rats. The results showed that: (1) both i.t. NE (0.3, 0.5 and 1.0 nmol) and DA (5.5, 8.3 and 16.5 nmol) produced significant and dose-dependent increases in tail-flick latencies (antinociception); (2) both Nal (240 nmol) and Aph (120 nmol) blocked the antinociception produced by NE (1.0 nmol); (3) both Nal (240 nmol) and Aph (120 nmol) blocked the antinociception produced by Mor (0.5 nmol), but only Aph (120 nmol) blocked the antinociception produced by NECA (0.5 nmol), while Nal (240 nmol) did not; (4) neither Nal (240 nmol) nor Aph (120 nmol) altered the antinociception produced by DA (16.5 nmol); (5) both i.t. PTX (1.5 nmol) and BIC (0.5 nmol) completely blocked the antinociception produced by NE (1.0 nmol); and (6) both PTX and BIC blocked the antinociception produced by MUS (1.0 nmol). These results suggest that: (a) endogenous opiate and adenosine may be involved in the mediation of NE-induced, but not DA-induced, antinociception; (b) NE, opioid and adenosine may act in a sequential order in NE-induced antinociception at the spinal level; (c) endogenous GABA may be involved in the mediation of DA-induced antinociception through the GABA_A-receptors, but is not involved in NE-induced antinociception at the spinal level.     

GABA receptors in the dorsal motor nucleus of the vagus influence feline lower esophageal sphincter and gastric function

Gamma-aminobutyric acid (GABA) antagonist (bicuculline methiodide, BIC; picrotoxin, PIC) or agonist (muscimol, MUS) microinjections were made into the dorsal motor nucleus of the vagus nerve (DMV), and effects on lower esophageal sphincter pressure (LESP), gastric motility, and gastric acid secretion were determined in chloralose-anesthetized cats. Right or left DMV sites were microinjected with BIC, PIC, MUS, or isotonic saline (140 nl) through a glass micropipette having a tip diameter of 15–21 μm. Esophageal body, LESP, and gastric fundic pressures were measured manometrically. Circular smooth muscle contractions of the antrum and pylorus were recorded with strain-gauge force transducers. Gastric acid secretion was measured every 15 min through a gastric cannula and titrated to pH 7.0. DMV microinjection sites were verified histologically. Direct BIC microinjections (0.275 or 0.550 nmol) into the DMV primarily produced a decrease in LESP (71% of all sites tested), with mean LESP changing from 23.2 ± 1.7 mmHg to 3.7 ± 0.7 mmHg (p < 0.01). Tonic LESP increases and phasic LESP contractile activity occurred less frequently. BIC-induced LESP responses were abolished by vagotomy or by microinjections of MUS (0.5 to 10 nmol) into the DMV. Direct PIC microinjection (0.232 nmol) into the DMV produced a pattern of responses similar to those observed with BIC (which were also abolished by vagotomy or by MUS microinjections into the DMV). The antrum and pylorus were also responsive to DMV microinjections of both GABA antagonists. Microinjections of BIC or PIC into the DMV produced increases in gastric circular muscle activity that occurred less frequently than LESP effects, but also were eliminated by vagotomy. The high (0.550 nmol) dose of BIC increased gastric motility significantly more often than the low dose of BIC (p < 0.05). In addition, BIC (0.550 nmol) microinjections into the DMV increased gastric secretary volume (from 0.6 ± 0.2 to 6.0 ± 2.5 ml/15 min; p < 0.01) and total titratible acid (from 34.4 ±8.9 to 86.0 ± 19.1 mEq/15 min; p < 0.01), and decreased gastric pH (from 4.63 ± 0.44 to 3.50 ± 0.49; p < 0.05). Vagotomy also eliminated the gastric secretory effects of DMV BIC. Direct microinjections of MUS into the DMV also blocked BIC- or PIC-induced changes in gastric motility and/or gastric acid secretion. Isotonic saline microinjected into the DMV did not increase basal or decrease stimulated gastric esophageal motility or gastric secretion. These data indicate that LESP, gastric motility, and gastric secretion are influenced by a tonic DMV inhibition mediated by GABA_A receptor stimulation of the DMV. Because disinhibition of these receptors clearly activates the upper gut, future work should focus on identifying the nuclei providing this synaptic input to the DMV that might be involved in the functional regulation of upper gut motor and secretory function.     

Chromogranin A applied to the nucleus accumbens decreases locomotor activity induced by activation of the mesolimbic dopaminergic system in the rat

The aim of the study was to obtain supporting evidence, using a behavioral paradigm, of the hypothesis that chromogranin A attenuates transmitter release in the CNS. We studied the effects of chromogranin A injected into the nucleus accumbens on locomotor activity triggered by application of picrotoxin into the ventral tegmental area of rats. Injection of picrotoxin into the ventral tegmental area, which is known to disinhibit dopaminergic mesolimbic neurons, caused a significant increase in horizontal activity. Distance covered during locomotion and movement time increased more than twofold, whereas stereotypy time and number, indices of nonlocomotor behavior, were not significantly affected by picrotoxin. Pressure injection of Chromogranin A into the nucleus accumbens prior to injection of picrotoxin into the ventral tegmental area prevented these locomotor effects and had little or no effect on nonlocomotor behavior. Similarly, the picrotoxin-induced activity was prevented by injecting cobalt chloride into the nucleus accumbens. The results show that chromogranin A has an attenuating effect, either directly or indirectly, on dopaminergic neurotransmission in the nucleus accumbens that can be exemplified by inhibiting picrotoxin-induced locomotor activity. Further studies are needed to determine the mechanism of chromogranin A action in the nucleus accumbens.     

Characteristics of an atypical benzodiazepine,Ro 5-4864

Ro 5-4864 is a 1,4 benzodiazepine which, atypically, does not bind to the classical CNS benzodiazepine receptors, but has high affinity for the peripheral type of binding site found both in the periphery and in the brain. Biochemical evidence for alternative sites of action for this compound is discussed. We review the behavioral profile of Ro 5-4864 (sedative, convulsant and anxiogenic in rodents) and also describe the behavioral effects of combining Ro 5-4864 treatment with benzodiazepines (e.g., diazepam, chlordiazepoxide) and with other drugs that modify the activity of benzodiazepines (Ro 15-1788, CGS 8216, picrotoxin, PK 11195, phenytoin). In the light of these interactions and electrophysiological evidence we conclude that the actions of Ro 5-4864 are most likely to be mediated at the GABA-benzodiazepine receptor complex in the CNS.