Enhancement of prepulse inhibition after blockade of GABA activity within the superior colliculus

The present study examined the effects of local microinjections of the GABA chloride channel blocker Picrotoxin into the superior colliculus (SC) on prepulse inhibition (PPI) of the acoustic startle response (ASR) in rats. PPI is a useful model for the investigation of the neuronal basis of sensorimotor gating which is deficient in some psychiatric disorders, such as schizophrenia. Blockade of GABA activity within the SC by Picrotoxin injections (leading to a moderate stimulation of the SC) significantly enhanced PPI without affecting the ASR baseline amplitude or the spontaneous motor activity. Based on these results we discuss the role of the SC in a hypothetical neuronal circuit mediating PPI of the ASR.     

Acute effect of an extract of Ambrosia paniculata (Willd.) O. E. Schultz (mugwort) in several models of experimental epilepsy

The acute effect of Ambrosia paniculata was studied in several animal models of epilepsy. Intraperitoneal injections (0.01 mL/g body wt) of a decoction of the dry leaves significantly enhanced the latency to the first convulsion and survival time in mice injected with picrotoxin (7 mg/kg) or isoniazid (210 mg/kg). Epileptic spikes were induced by topical application of penicillin through a glass electrode filled with a penicillin-agar-saline mixture and recorded in sensorimotor and occipital cortices, in rats immobilized with d-tubocurarine. The plant decoction reduced significantly the spike amplitude in both sites. The mentioned effects were elicited at doses that also reduced general motor activity (Irwin test) and exploratory behavior. The decoctions were not effective against electroshock-induced convulsions in mice. The convulsions induced by isoniazid, picrotoxin, and penicillin differed from those induced by electroshock implicating selective disruption of GABAergic neurotransmission. The results suggest that A. paniculata, like several conventional antiepileptic drugs, might act by enhancing GABAergic neurotransmission, a hypothesis that requires further demonstration. These results explain and justify the traditional use of the plant in epilepsy.     

An operant determination of the behavioral mechanism of benzodiazepine enhancement of food intake

Rationale A recent review paper by Cooper (Appetite 44:133–150, 2005) has pointed out that a role for benzodiazepines as appetite stimulants has been largely overlooked. Cooper’s review cited several studies that suggested the putative mechanism of enhancement of food intake after benzodiazepine administration might involve increasing the perceived pleasantness of food (palatability).Objectives The present study examined the behavioral mechanism of increased food intake after benzodiazepine administration.Materials and methods The cyclic-ratio operant schedule has been proposed as a useful behavioral assay for differentiating palatability from regulatory effects on food intake (Ettinger and Staddon, Physiol Behav 29:455–458, 1982 and Behav Neurosci 97:639–653, 1983). The current study employed the cyclic-ratio schedule to determine whether the effects on food intake of chlordiazepoxide (CDP) (5.0 mg/kg), sodium pentobarbital (5.0 mg/kg), and picrotoxin (1.0 mg/kg) were mediated through palatability or regulatory processes.Results The results of this study show that both the benzodiazepine CDP and the barbiturate sodium pentobarbital increased food intake in a manner similar to increasing the palatability of the ingestant, and picrotoxin decreased food intake in a manner similar to decreasing the palatability of the ingestant.Conclusions These results suggest that the food intake enhancement properties of benzodiazepines are mediated through a mechanism affecting perceived palatability.     

Reproductive and sexual behavior changes in male rats exposed perinatally to picrotoxin

Previous studies in rats suggested that picrotoxin, a GABA_A receptor antagonist, may cause long-term changes in male reproductive physiology and behavior in rats exposed during prenatal and postnatal periods. The present study has further examined this phenomenon. Wistar rat dams were dosed subcutaneously with 0.75 mg/kg picrotoxin in saline, or vehicle alone, during the perinatal period (day 19 of gestation, immediately after parturition, and once a day during the first 5 days of lactation). Birth weight and sexual maturation of pups were unchanged; however, plasma testosterone levels and sexual behavior was altered in male offspring. Although fertile, these males showed altered mating behavior in terms of a decrease in the mean number of mounts during a 30-min observation period with normal females. Some showed homosexual behavior when castrated and pretreated with exogenous estrogen. These findings suggest that perinatal exposure to picrotoxin alters sexual dimorphism in the developing rat brain, manifesting as altered reproductive performance and sexual behavior of males.     

Changes of cortical epileptic afterdischarges under the influence of convulsant drugs

Convulsant drugs picrotoxin (0.5 and/or 1 mg/kg, intraperitoneal (i.p.)) and pentylenetetrazol (10 and/or 20 mg/kg, i.p.) were used to compromise GABAergic inhibition, caffeine (75 and/or 150 mg/kg, i.p.) to antagonize adenosinergic system to study the role of inhibition in cortical epileptic afterdischarges. Rats with implanted cortical stimulation and registration electrodes were stimulated four times at 10-min intervals, drugs were injected between the first and second stimulation. Four different phenomena were evaluated: movements directly bound to stimulation were intensified by all three drugs, i.e., excitability of the cerebral cortex was increased. Incidence of two types of afterdischarges (spike-and-wave rhythm and "limbic" type) was not changed by any drug, i.e., the transition of epileptic activity into limbic structures was not increased. Afterdischarges were most efficiently prolonged by caffeine, i.e., caffeine probably interferes with mechanism(s) arresting cortical afterdischarges. The intensity of clonic seizures accompanying spike-and-wave afterdischarges, i.e., spread of epileptic activity into the motor system was only transiently increased by picrotoxin, the effects of caffeine did not reach the level of statistical significance. Our results indicate various mechanisms and diverse role of the two inhibitory systems in generation of evaluated phenomena.     

Further characterization of the anorexic action of orally-administered THIP, a GABA-analogue, in the rat

Sprague-Dawley rats were used to further characterize the anorexic action of orally-administered THIP, a GABA-analogue. The anorexic action of THIP (5 or 10 mg/kg) was antagonized by prior subcutaneous injection of bicuculline (1 mg/kg), but not by prior subcutaneous injection of bicuculline-methobromide (1.5 mg/kg), strychnine-SO4 (0.75 mg/kg), pentylenetetrazol (25 mg/kg), or picrotoxin (1 mg/kg). Orally-administered GABA (50-300 mg/kg), bicuculline (1-10 mg/kg) or picrotoxin (1-10 mg/kg) generally did not inhibit food intake. These results indicate that the anorexic action of THIP is mediated by central GABA-receptors.     

Mechanisms involved in the depolarization of cutaneous afferents produced by segmental and descending inputs in the cat spinal cord

Summary The relative contribution of specific and unspecific (potassium) components involved in the generation of primary afferent depolarization (PAD) of cutaneous fibres was analyzed in the spinal cord of the anaesthetized cat. To this end we examined the correlation between the intraspinal threshold changes of single afferent fibres in the sural nerve produced by segmental and descending inputs and the negative DC potential shifts produced by these same stimuli at the site of excitability testing, the latter taken as indicators of the changes in extracellular concentration of potassium ions. Stimulation of the ipsilateral brain-stem reticular formation and of the contralateral red nucleus with 100–200 Hz trains reduced very effectively the intraspinal threshold of sural nerve fibres ending in the dorsal horn practically without producing any negative DC potential shifts at the site of excitability testing. However, negative DC potential shifts were produced more ventrally, in the intermediate nucleus and/or motor nucleus. Stimulation of the sural and superficial peroneus nerves with pulses at 2 Hz and strengths below 2×T, also reduced the intraspinal threshold of single SU fibres without producing significant DC potential changes at the site of excitability testing. On the other hand, 100 Hz trains with strengths above 2×T produced negative DC potential shifts and a proportional reduction of the intraspinal threshold of the SU fibres. The PAD of sural fibres produced by stimulation of rubro-spinal and reticulospinal fibres as well as by stimulation of sensory nerves with low frequency trains was unaffected or slightly increased, by i.v. injection of strychnine (0.2 mg/kg), but was readily abolished 5–10 min after the i.v. injection of picrotoxin (2 mg/kg). The results suggest that activation of reticulo-spinal and rubrospinal fibres, as well as stimulation of cutaneous nerves with low frequencies and low strengths, produce PAD of cutaneous fibres involving activation of specific interneuronal pathways with interposed last-order GABAergic interneurons. The potassium component of the PAD produced by cutaneous fibres becames dominant with high stimulus frequencies and strengths.     

Involvement of GABAergic mechanisms in the optokinetic nystagmus of the frog

Summary Frog optokinetic nystagmus (OKN) was studied before and after an intravitreal injection of picrotoxin, a specific non-competitive GABA antagonist. In monocular vision, the OKN displayed a directional asymmetry favouring the Temporal-Nasal (T-N) stimulation. In that case, the nystagmus extinction frequency (NEF) is low, about 2 frames/s. In binocular vision, the OKN is symmetrical with a facilitation of performances compared to monocular vision (NEF = 3 frames/s). In monocular as in binocular vision, an intravitreal injection of picrotoxin (between 1×10^–4 and 5×10^–3 M) provoked the disappearance of the injected eye OKN and a spectacular facilitation in the performances of the intact eye, with the appearance of a N-T component and the increase of the NEF value reaching 7 or 11.5 frames/s according to the experimental conditions. This contralateral facilitation was no longer observed after the optic nerve of the injected eye had been cut, indicating that such a facilitation can only be explained by alterations of a central process triggered by the visual input. It is concluded that GABAergic mechanisms might be responsible for the inhibition of the N-T component in the frog OKN and might be involved in the control of the power of temporal resolution in this animal.     

Cerebellar inhibitory control of the vestibulo-ocular reflex investigated in rabbit IIIrd nucleus

Summary In anaesthetized rabbits, the vestibulo-ocular reflex was evoked by electric stimulation of VIIIth nerve and was observed by recording postsynaptic potentials and relevant field potentials in Illrd nucleus. The electric stimulation of flocculus produced a prominent inhibition of the vestibulo-ocular reflex in both the inhibitory component relayed by the superior vestibular nucleus and the excitatory component mediated by the brachium conjunctivum. The excitatory component mediated by the medial vestibular nucleus appeared to be free of the flocculus inhibition. The flocculus inhibition was blocked very effectively by systemic injection of picrotoxin. That the flocculus inhibitory action is due to monosynaptic postsynaptic inhibition of secondary vestibular neurones was demonstrated by direct stimulation of, and also by recording from, the superior nucleus. Recording from the superior nucleus was also performed in anaesthetized cats. All of these above results indicate that Purkinje cells in flocculus projecting to vestibular and cerebellar nuclei cells have inhibitory synaptic action. Flocculus stimulation produced also an excitatory effect upon vestibular nuclei neurones. However, this effect could be attributed to intracerebellar activation of the primary vestibular fibers which pass into the flocculus.     

Response of amygdalar norepinephrine to footshock and GABAergic drugs using in vivo microdialysis and HPLC

These studies used in vivo microdialysis and high-performance liquid chromatography (HPLC) to examine levels of norepinephrine (NE) within the amygdala in response to both footshock and GABAergic compounds. In Experiment 1, microdialysis probes were inserted into a previously implanted guide cannula aimed at the amygdala and the level of NE was examined in response to footshock. A 0.55 mA (2 s) footshock induced a significant increase in NE levels when the microdialysis probe was located within the amygdala; levels of NE did not differ from baseline in rats with microdialysis probes located outside the amygdala. Experiment 2 examined the effects of the GABAergic antagonist, picrotoxin, the GABAergic agonist, muscimol, and saline on NE levels in the amygdala. Rats treated systemically with picrotoxin showed a dramatic increase in levels of NE within the amygdala. In contrast, systemic injection of muscimol resulted in decreased levels of NE. These findings are consistent with the hypothesis that drugs that are capable of modulating memory do so by altering levels of NE within the amygdala.