5-HT3 receptor antagonism by anpirtoline, a mixed 5-HT1 receptor agonist/5-HT3 receptor antagonist

1. The aim of this study was to provide evidence that anpirtoline, which is an agonist at 5-HT1B and 5-HT1D receptors and also displays submicromolar affinity for 5-HT1A recognition sites, in addition, acts as an antagonist at 5-HT3 receptors. 2. In radioligand binding studies on rat brain cortical membranes, anpirtoline inhibited specific binding of [3H]-(S)-zacopride to 5-HT3 receptor recognition sites (pKi: 7.53). 3. In N1E-115 neuroblastoma cells in which [14C]-guanidinium was used as a tool to measure cation influx through the 5-HT3 receptor channel, the 5-HT-induced influx was concentration-dependently inhibited by anpirtoline. In this respect, anpirtoline mimicked other 5-HT3 receptor antagonists; the rank order of potency was ondansetron > anpirtoline > metoclopramide. 4. The concentration-response curve for 5-HT as a stimulator of [14C]-guanidinium influx was shifted to the right by anpirtoline (apparent pA2: 7.78). 5. In urethane-anaesthetized rats, anpirtoline inhibited (at lower potency than zacopride and tropisetron) the 5-HT- or phenylbiguanide-induced bradycardia (Bezold-Jarisch reflex), but did not induce this reflex by itself. 6. Intravenous infusion of cisplatin in the domestic pig caused a consistent emetic response which was antagonized by anpirtoline. 7. It is concluded that anpirtoline, which was previously characterized as a 5-HT1 receptor agonist also proved to be a 5-HT3 receptor antagonist in several experimental models and, hence, exhibits a unique pattern of properties at different 5-HT receptors.

Potentiation of amphetamine-induced locomotor activity following NMDA-induced retrohippocampal neuronal loss in the rat

The present experiment assessed the locomotor response to a low dose (1 mg/kg) of systemic D-amphetamine in rats with cytotoxic lesions of the retrohippocampus (entorhinal and extra-subicular cortices), compared with vehicle-operated shams and unoperated controls. Under spontaneous and saline conditions, both the sham and the lesioned animals were more active than unoperated controls, and they did not differ from each other. Systemic D-amphetamine produced increased locomotion in all groups, but this effect was potentiated in animals with retrohippocampal lesions; two control groups did not differ from each other in their response to the drug. The present results are consistent with the suggestion that cell loss within the retrohippocampal region could affect the functional response of nucleus accumbens to amphetamine. The results are discussed in terms of the interaction between the retrohippocampus and nucleus accumbens in the control of mesolimbic dopamine release and the possible implications for schizophrenia.

Correlation between the pharmacology of long-term potentiation and the pharmacology of memory

The pharmacology of memory has been recently studied by the infusion of drugs into the hippocampus (HIP), amygdala (AMY), medial septum (MS), and entorhinal cortex (EC) at various times after training or at the time of retention testing. It was found to be remarkably similar to that of long-term potentiation (LTP). Memory and LTP are blocked early on by antagonists of glutamate N-methyl-D-aspartate (NMDA) or metabotropic receptors (mGLUs), by the antagonist of the presynaptic membrane receptor to PAF, BN 52021, by the inhibitor of heme oxygenase, ZnPP, by the inhibitor of NO synthase, N-nitro-arginine, by GABA type A receptor agonists, or by muscarinic blockers. Both memory and LTP are enhanced, at this early stage, by glutamate, mGLU agonists, GABA-A antagonists, muscarinic agonists, and norepinephrine. In the next 1-3 h, memory and LTP are accompanied by enhanced activity of protein kinases and are blocked by specific inhibitors of calcium/calmodulin dependent protein kinase II and protein kinase C. At the time of expression, memory and LTP are blocked by antagonists of glutamate AMPA receptors and are accompanied by an enhanced sensitivity of these receptors. Memories that depend on HIP are affected by drugs given into the HIP but not the MS or AMY, memories that depend on the AMY are affected by drugs given into the AMY, and memories that depend on the HIP, AMY, and MS are affected by drugs given into the three structures.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of perforant path procaine on hippocampal type 2 rhythmical slow-wave activity (theta) in the urethane-anesthetized rat

Previous research has suggested that the entorhinal cortex plays a major role in the production of type 1 rhythmical slow-wave activity (RSA) recorded in the hippocampus of the freely moving preparation. In the present experiment we investigated the contribution of the entorhinal cortex to the type 2 fields recorded under urethane anesthesia. Rats had stimulating electrodes and cannulae filled with procaine positioned in the perforant pathway of one or both hemispheres. Recording electrodes were positioned in the dorsal hippocampus of each hemisphere to record perforant path and commissural/associational evoked potentials and RSA fields. Following unilateral procaine blockade, a decrease in RSA amplitude was observed in the stratum oriens and fissure regions of both hemispheres. Concomitant with this change in RSA, there was a loss of perforant path evoked responses, although commissural/associational control potentials remained unaltered. A greater reduction in RSA amplitude was observed following bilateral procaine microinfusion. RSA phase reversal also occurred more dorsally in microelectrode depth profiles conducted through the hippocampus during perforant path inactivation. In current source density analyses performed under baseline conditions, large rhythmic sinks were observed in stratum oriens, in stratum radiatum, and in strata adjacent to the hippocampal fissure. A rhythmic source was often observed in stratum pyramidale. Following perforant path inactivation decreases in the magnitude of the phasic sinks located near the fissure and stratum radiatum were observed. In contrast to the reduction in RSA amplitude observed in the stratum oriens region, the sink in this region and the source in stratum pyramidale remained relatively unaltered. These results demonstrate that the entorhinal region contributes to the production of RSA observed under urethane anesthesia. Furthermore, the CSD and amplitude changes following perforant path inactivation suggest that a substantial portion of RSA recorded in stratum oriens may result from ventrally located RSA dipoles.

Characterization of galanin and 5-HT1A receptor coupling to adenylyl cyclase in discrete regions of the rat brain

We have studied the coupling of galanin and 5-HT1A receptors with adenylyl cyclase in the hypothalamus, the entorhinal cortex and the hippocampus of the rat brain. Furthermore, we have evaluated the effects of simultaneous activation of galanin and 5-HT1A receptors on adenylyl cyclase activity. Galanin-(1-29) and galanin-(1-15) showed a dose-dependent inhibitory effect on forskolin-stimulated adenylyl cyclase activity in the hypothalamus and entorhinal cortex. No clear effects were observed in the hippocampus. Neither galanin-(1-29) nor galanin-(1-15) had any effect on the basal activity of adenylyl cyclase in these regions. The selective 5-HT1A receptor agonist 8-OH-2-(di-n-propylamino)-tetralin (8-OH-DPAT) induced a dose-dependent inhibition of forskolin stimulated adenylyl cyclase activity in the hippocampus and the entorhinal cortex. 5-HT induced an inhibition in the hypothalamus. In all regions the effects could be fully counteracted by methiothepin. 5-HT was shown to stimulate the basal activity of adenylyl cyclase in the hippocampus and the entorhinal cortex. The effects could be counteracted by methiothepin. When galanin-(1-29) and 5-HT/8-OH-DPAT were incubated simultaneously additive inhibitory effects, but no synergistic interactions, could be observed on the stimulated adenylyl cyclase activity. In conclusion, galanin and 5-HT1A receptors seem to be linked to different independent pools of G proteins, indicating that the previously demonstrated intramembrane interactions between galanin and 5-HT1A receptors involve a mechanism not directly related to adenylyl cyclase.

Muscarinic (M1) mediation of carbachol-induced theta in the cat entorhinal cortex in vitro

Entorhinal cortex slice preparations obtained from the cat exhibited theta rhythm during perfusion with 50 microM carbachol. The effect of carbachol was antagonized by the muscarinic blocker atropine sulphate, but not by hexamethonium and mecamylamine, which are antagonists of the nicotinic receptor. Further analysis of the pharmacological profile of these carbachol-induced theta oscillations showed that M1 receptor subtype to be involved in mediation of this EEG activity: the theta rhythm was antagonized by the M1 receptor blocker pirenzepine, but was unaffected by gallamine, an antagonist of the M2 receptor subtype.

Analysis of anatomical sites at which galanin impairs delayed nonmatching to sample in rats

Galanin, a neuropeptide that coexists with acetylcholine in the septohippocampal pathway of the rat, impairs operant delayed nonmatching to sample (DNMTS) when administered intracerebroventricularly. Microinjection experiments were conducted to determine the anatomical site or sites at which galanin acts to disrupt DNMTS. Galanin (0.1, 0.4, or 1.6 nmol) was microinjected into the ventral hippocampus, amygdala, nucleus basalis magnocellularis, prefrontal cortex, or entorhinal cortex. Galanin disrupted DNMTS in a dose-dependent manner when microinjected into the ventral hippocampus but not at the other sites tested. These findings are consistent with the ability of galanin to inhibit physiological and biochemical actions of acetylcholine in the ventral hippocampus.

Noradrenaline reduces synaptic responses in normal and tottering mouse entorhinal cortex via alpha 2 receptors

The effects of noradrenaline (NA) on synaptic responses in layer II of the entorhinal cortex (EC) were studied in normal and spontaneously epileptic mutant mice tottering using intracellular recording in a slice preparation. Neither the membrane properties of neurones nor the responses to NA differed between normal and tottering mice. NA (50 microM) hyperpolarized most (29/54) of the neurones via alpha 2 adrenergic receptors. Synaptic responses of EC neurones were complex. NA (10-100 microM) reduced all the components of the synaptic response in a concentration-dependent and reversible manner. The pharmacological properties of the inhibitory effect of NA were characterised and quantified on one component of the complex synaptic response, the fast excitatory postsynaptic potential. The effect of NA was mimicked by the alpha 2 agonist UK 14,304 and blocked by the alpha 2 antagonist yohimbine. It is concluded that NA can inhibit via an alpha 2 receptor-mediated action synaptic responses in the superficial layers of the EC.

Impairment in the acquisition of passive and active avoidance learning tasks due to bilateral entorhinal cortex lesions

The relationship between the entorhinal cortex and learning behavior was examined. The initial stage of Alzheimer's disease has been shown to be characterized by neuropathological alteration in the entorhinal cortex, with the appearance of the greatest number of neuronal tangles and severe neuronal loss in comparison with other brain regions involved. This entorhinal cortex, because of its anatomical relationship to the hippocampus, may play a crucial role in memory formation. In this study, rats with bilateral ibotenic acid-induced lesions of the entorhinal cortices were tested for acquisition of passive and active avoidance learning tasks. These animals displayed no sensorimotor disturbances as shown by evaluation of locomotor activity and shock sensitivity. However, they did show impair acquisition of passive and active avoidance responses. On the other hand, when the lesions were induced after training, there was no extinction of the acquired passive and active avoidance responses. The results demonstrate the importance of the entorhinal cortex in learning acquisition and indicate that rats with partial neuronal loss in the entorhinal cortex may be a useful model for studying the memory disturbance of Alzheimer's disease.

Multifocal spontaneous epileptic activity induced by restricted bicuculline ejection in the piriform cortex of the isolated guinea pig brain

1. We investigated the development of epileptic activity in the limbic lobe of the in vitro isolated brain preparation after induction of a restricted epileptic focus in the rostral portion of the anterior piriform cortex (APC) by means of local bicuculline ejection. 2. The modifications of the local responses evoked by stimulation of the lateral olfactory tract were consistent with an increase in excitability of layer II pyramidal neurons. The abnormal discharge output of the epileptic focus propagated throughout the piriform cortex (PC) and generated late synaptic stimulus-evoked epileptiform potentials in layer Ib, where corticocortical associative fibers contact the distal dendrites of pyramidal cells. 3. Spontaneous epileptiform potentials (SEPs) originated in the primary focus at the bicuculline ejection site 10-15 min after drug application and propagated via associative fibers to the posterior PC (PPC). 4. Autonomous secondary foci generating SEPs independently from the primary anterior focus developed in the PPC within 2 h after the bicuculline application. 5. Ictal events seldom occurred spontaneously in PC but could be induced by tetanic afferent stimulation. Evoked ictal events in PC had the characteristics of afterdischarges, being short in duration and localized in space and showing a progressively decreasing firing rate. 6. Self-sustained spontaneous ictal discharges were observed in the entorhinal cortex and hippocampus but not in PC after activation of the APC focus. 7. We conclude that in an acute pharmacological model of focal epilepsy, associative corticocortical connections hold a primary role 1) in the propagation of interictal epileptiform activity throughout the limbic lobe and 2) in the generation of secondary epileptic foci. We also demonstrate that self-sustained ictal events are not spontaneously generated in PC, although paroxysmal seizure-like discharges occur in hippocampus. We discuss the relevance of the rostral-to-caudal-directed associative fiber system in generation of limbic seizures.