A seeding-based neuronal model of tau aggregation for use in drug discovery.

Intracellular accumulation of tau protein is a hallmark of Alzheimer's Disease and Progressive Supranuclear Palsy, as well as other neurodegenerative disorders collectively known as tauopathies. Despite our increasing understanding of the mechanisms leading to the initiation and progression of tau pathology, the field still lacks appropriate disease models to facilitate drug discovery. Here, we established a novel and modulatable seeding-based neuronal model of full-length 4R tau accumulation using humanized mouse cortical neurons and seeds from P301S human tau transgenic animals. The model shows specific and consistent formation of intraneuronal insoluble full-length 4R tau inclusions, which are positive for known markers of tau pathology (AT8, PHF-1, MC-1), and creates seeding competent tau. The formation of new inclusions can be prevented by treatment with tau siRNA, providing a robust internal control for use in qualifying the assessment of potential therapeutic candidates aimed at reducing the intracellular pool of tau. In addition, the experimental set up and data analysis techniques used provide consistent results in larger-scale designs that required multiple rounds of independent experiments, making this is a versatile and valuable cellular model for fundamental and early pre-clinical research of tau-targeted therapies.

P62 accumulates through neuroanatomical circuits in response to tauopathy propagation.

In Alzheimer's disease and related tauopathies, trans-synaptic transfer and accumulation of pathological tau from donor to recipient neurons is thought to contribute to disease progression, but the underlying mechanisms are poorly understood. Using complementary in vivo and in vitro models, we examined the relationship between these two processes and neuronal clearance. Accumulation of p62 (a marker of defective protein clearance) correlated with pathological tau accumulation in two mouse models of tauopathy spread; Entorhinal Cortex-tau (EC-Tau) mice where tau pathology progresses in time from EC to other brain regions, and PS19 mice injected with tau seeds. In both models and in several brain regions, p62 colocalized with human tau in a pathological conformation (MC1 antibody). In EC-Tau mice, p62 accumulated before overt tau pathology had developed and was associated with the presence of aggregation-competent tau seeds identified using a FRET-based assay. Furthermore, p62 accumulated in the cytoplasm of neurons in the dentate gyrus of EC-Tau mice prior to the appearance of MC1 positive tauopathy. However, MC1 positive tau was shown to be present at the synapse and to colocalize with p62 as shown by immuno electron microscopy. In vitro, p62 colocalized with tau inclusions in two primary cortical neuron models of tau pathology. In a three-chamber microfluidic device containing neurons overexpressing fluorescent tau, seeding of tau in the donor chamber led to tau pathology spread and p62 accumulation in both the donor and the recipient chamber. Overall, these data are in accordance with the hypothesis that the accumulation and trans-synaptic spread of pathological tau disrupts clearance mechanisms, preceding the appearance of obvious tau aggregation. A vicious cycle of tau accumulation and clearance deficit would be expected to feed-forward and exacerbate disease progression across neuronal circuits in human tauopathies.

Brain energy rescue: an emerging therapeutic concept for neurodegenerative disorders of ageing.

The brain requires a continuous supply of energy in the form of ATP, most of which is produced from glucose by oxidative phosphorylation in mitochondria, complemented by aerobic glycolysis in the cytoplasm. When glucose levels are limited, ketone bodies generated in the liver and lactate derived from exercising skeletal muscle can also become important energy substrates for the brain. In neurodegenerative disorders of ageing, brain glucose metabolism deteriorates in a progressive, region-specific and disease-specific manner - a problem that is best characterized in Alzheimer disease, where it begins presymptomatically. This Review discusses the status and prospects of therapeutic strategies for countering neurodegenerative disorders of ageing by improving, preserving or rescuing brain energetics. The approaches described include restoring oxidative phosphorylation and glycolysis, increasing insulin sensitivity, correcting mitochondrial dysfunction, ketone-based interventions, acting via hormones that modulate cerebral energetics, RNA therapeutics and complementary multimodal lifestyle changes.

Long-Term Exposure to PFE-360 in the AAV-α-Synuclein Rat Model: Findings and Implications.

Parkinson's disease (PD) is a progressive neurodegenerative disorder associated with impaired motor function and several non-motor symptoms, with no available disease modifying treatment. Intracellular accumulation of pathological α-synuclein inclusions is a hallmark of idiopathic PD, whereas, dominant mutations in leucine-rich repeat kinase 2 (LRRK2) are associated with familial PD that is clinically indistinguishable from idiopathic PD. Recent evidence supports the hypothesis that an increase in LRRK2 kinase activity is associated with the development of not only familial LRRK2 PD, but also idiopathic PD. Previous reports have shown preclinical effects of LRRK2 modulation on α-synuclein-induced neuropathology. Increased subthalamic nucleus (STN) burst firing in preclinical neurotoxin models and PD patients is hypothesized to be causally involved in the development of the motor deficit in PD. To study a potential pathophysiological relationship between α-synuclein pathology and LRRK2 kinase activity in PD, we investigated the effect of chronic LRRK2 inhibition in an AAV-α-synuclein overexpression rat model. In this study, we report that chronic LRRK2 inhibition using PFE-360 only induced a marginal effect on motor function. In addition, the aberrant STN burst firing and associated neurodegenerative processes induced by α-synuclein overexpression model remained unaffected by chronic LRRK2 inhibition. Our findings do not strongly support LRRK2 inhibition for the treatment of PD. Therefore, the reported beneficial effects of LRRK2 inhibition in similar α-synuclein overexpression rodent models must be considered with prudence and additional studies are warranted in alternative α-synuclein-based models.

Parkinson's disease-like burst firing activity in subthalamic nucleus induced by AAV-α-synuclein is normalized by LRRK2 modulation.

Parkinson's disease (PD) affects motor function through degenerative processes and synaptic transmission impairments in the basal ganglia. None of the treatments available delays or stops the progression of the disease. While α-synuclein pathological accumulation represents a hallmark of the disease in its idiopathic form, leucine rich repeat kinase 2 (LRRK2) is genetically associated with familial and sporadic forms of PD. The genetic information suggests that LRRK2 kinase activity plays a role in the pathogenesis of the disease. To support a potential link between LRRK2 and α-synuclein in the pathophysiological mechanisms underlying PD, the effect of LRRK2 ablation or LRRK2 kinase pharmacological inhibition were studied in rats with adeno-associated virus-induced (AAV) α-synuclein overexpression in the nigrostriatal pathway. We first report that viral overexpression of α-synuclein induced increased burst firing in subthalamic neurons. Aberrant firing pattern of subthalamic neurons has also been reported in PD patients and neurotoxin-based animal models, and is hypothesized to play a key role in the appearance of motor dysfunction. We further report that genetic LRRK2 ablation, as well as pharmacological inhibition of LRRK2 kinase activity with PFE-360, reversed the aberrant firing pattern of subthalamic neurons induced by AAV-α-synuclein overexpression. This effect of LRRK2 modulation was not associated with any neuroprotective effect or motor improvement. Nonetheless, our findings may indicate a potential therapeutic benefit of LRRK2 kinase inhibition by normalizing the aberrant neuronal activity of subthalamic neurons induced by AAV-α-synuclein, a neurophysiological trait recapitulating observations in PD.

PFE-360-induced LRRK2 inhibition induces reversible, non-adverse renal changes in rats.

Parkinson's disease (PD) is a progressive neurodegenerative disorder for which there is no existing therapeutic approach to delay or stop progression. Genetic, biochemical and pre-clinical studies have provided evidence that leucine-rich-repeat-kinase-2 (LRRK2) kinase is involved in the pathogenesis of PD, and small molecule LRRK2 inhibitors represent a novel potential therapeutic approach. However, potentially adverse target-related effects have been discovered in the lung and kidneys of LRRK2 knock-out (ko) mice and rats. It is unclear if the LRRK2 ko effect in the kidneys and lung is also induced by pharmacological inhibition of the LRRK2 kinase. Here, we show that treatment with the LRRK2 inhibitor PFE-360 in rats induces a morphological kidney phenotype resembling that of the LRRK2 ko rats, whereas no effects were observed in the lung. The PFE-360 treatment induced morphological changes characterised by darkened kidneys and progressive accumulation of hyaline droplets in the renal proximal tubular epithelium. However, no histopathological evidence of renal tubular injury or changes in the blood and urine parameters that would be indicative of kidney toxicity or impaired kidney function were observed after up to 12 weeks of treatment. Morphological changes were detected in the kidney after 2 weeks of treatment and were partially reversible within a 30 day treatment-free period. Our findings suggest that pharmacological LRRK2 inhibition may not have adverse consequences for kidney function.

α7-nAChR agonist enhances neural plasticity in the hippocampus via a GABAergic circuit.

Agonists of the α7-nicotinic acetylcholine receptor (α7-nAChR) have entered clinical trials as procognitive agents for treating schizophrenia and Alzheimer's disease. The most advanced compounds are orthosteric agonists, which occupy the ligand binding site. At the molecular level, agonist activation of α7-nAChR is reasonably well understood. However, the consequences of activating α7-nAChRs on neural circuits underlying cognition remain elusive. Here we report that an α7-nAChR agonist (FRM-17848) enhances long-term potentiation (LTP) in rat septo-hippocampal slices far below the cellular EC50 but at a concentration that coincides with multiple functional outcome measures as we reported in Stoiljkovic M, Leventhal L, Chen A, Chen T, Driscoll R, Flood D, Hodgdon H, Hurst R, Nagy D, Piser T, Tang C, Townsend M, Tu Z, Bertrand D, Koenig G, Hajós M. Biochem Pharmacol 97: 576-589, 2015. In this same concentration range, we observed a significant increase in spontaneous γ-aminobutyric acid (GABA) inhibitory postsynaptic currents and a moderate suppression of excitability in whole cell recordings from rat CA1 pyramidal neurons. This modulation of GABAergic activity is necessary for the LTP-enhancing effects of FRM-17848, since inhibiting GABAA α5-subunit-containing receptors fully reversed the effects of the α7-nAChR agonist. These data suggest that α7-nAChR agonists may increase synaptic plasticity in hippocampal slices, at least in part, through a circuit-level enhancement of a specific subtype of GABAergic receptor.

Hippocampal 5-HT7 receptors signal phosphorylation of the GluA1 subunit to facilitate AMPA receptor mediated-neurotransmission in vitro and in vivo.

BACKGROUND AND PURPOSE: The 5-HT7 receptor is a GPCR that is the target of a broad range of antidepressant and antipsychotic drugs. Various studies have demonstrated an ability of the 5-HT7 receptor to modulate glutamatergic neurotransmission and cognitive processes although the potential impact upon AMPA receptors has not been investigated directly. The purposes of the present study were to investigate a direct modulation of the GluA1 AMPA receptor subunit and determine how this might influence AMPA receptor function. EXPERIMENTAL APPROACH: The influence of pharmacological manipulation of the 5-HT7 receptor system upon phosphorylation of GluA1 subunits was assessed by Western blotting of fractionated proteins from hippocampal neurones in culture (or proteins resident at the neurone surface) and the functional impact assessed by electrophysiological recordings in rat hippocampus in vitro and in vivo. KEY RESULTS: 5-HT7 receptor activation increased cAMP and relative pCREB levels in cultures of rat hippocampal neurones along with an increase in phosphorylation (Ser845) of the GluA1 AMPA receptor subunit evident in whole neurone extracts and within the neurone surface compartment. Electrophysiological recordings in rat hippocampus demonstrated a 5-HT7 receptor-mediated increase in AMPA receptor-mediated neurotransmission in vitro and in vivo. CONCLUSIONS AND IMPLICATIONS: The 5-HT7 receptor-mediated phosphorylation of the GluA1 AMPA receptor provides a molecular mechanism consistent with the 5-HT7 receptor-mediated increase in AMPA receptor-mediated neurotransmission.

Lack of support for bexarotene as a treatment for Alzheimer's disease.

Bexarotene has been reported to reduce brain amyloid-ß (Aß) levels and to improve cognitive function in transgenic mouse models of Alzheimer's disease (AD). Four groups failed to fully replicate the primary results but the original authors claimed overall support for the general conclusions. Because of its potential clinical importance, the current work studied the effects of bexarotene using two animal species and highly relevant paradigms. Rats were tested for the ability of bexarotene to prevent changes induced by an Aß challenge in the form intracerebroventricular (i.c.v) administration of 7PA2 conditioned medium (7PA2 CM) which contains high levels of Aß species. Bexarotene had no effect on the long-term potentiation of evoked extracellular field excitatory postsynaptic potentials induced by i.c.v. 7PA2 CM. It also had no effect following subcutaneous administration of 2, 5, 10 and 15 mg/kg on behavioral/cognitive impairment using an alternating-lever cyclic-ratio schedule of operant responding in the rat. The effects of bexarotene were further tested using the APPSwFILon, PSEN1*M146L*L286V transgenic mouse model of AD, starting at the time Aß deposits first begin to develop. Mice were sacrificed after 48 days of exposure to 100 mg bexarotene per day. No significant difference between test and control mice was found using a water-maze test, and no significant difference in the number of Aß deposits in cerebral cortex, using two different antibodies, was apparent. These results question the potential efficacy of bexarotene for AD treatment, even if instigated in the preclinical period prior to the onset of cognitive deficits reported for human AD. This article is part of the Special Issue entitled 'Synaptopathy--from Biology to Therapy'.

MRZ-99030 - A novel modulator of Aß aggregation: II - Reversal of Aß oligomer-induced deficits in long-term potentiation (LTP) and cognitive performance in rats and mice.

ß-amyloid1-42 (Aß1-42) is a major endogenous pathogen underlying the aetiology of Alzheimer's disease (AD). Recent evidence indicates that soluble Aß oligomers, rather than plaques, are the major cause of synaptic dysfunction and neurodegeneration. Small molecules that suppress Aß aggregation, reduce oligomer stability or promote off-pathway non-toxic oligomerization represent a promising alternative strategy for neuroprotection in AD. MRZ-99030 was recently identified as a dipeptide that modulates Aß1-42 aggregation by triggering a non-amyloidogenic aggregation pathway, thereby reducing the amount of intermediate toxic soluble oligomeric Aß species. The present study evaluated the relevance of these promising results with MRZ-99030 under pathophysiological conditions i.e. against the synaptotoxic effects of Aß oligomers on hippocampal long term potentiation (LTP) and two different memory tasks. Aß1-42 interferes with the glutamatergic system and with neuronal Ca(2+) signalling and abolishes the induction of LTP. Here we demonstrate that MRZ-99030 (100-500 nM) at a 10:1 stoichiometric excess to Aß clearly reversed the synaptotoxic effects of Aß1-42 oligomers on CA1-LTP in murine hippocampal slices. Co-application of MRZ-99030 also prevented the two-fold increase in resting Ca(2+) levels in pyramidal neuron dendrites and spines triggered by Aß1-42 oligomers. In anaesthetized rats, pre-administration of MRZ-99030 (50 mg/kg s.c.) protected against deficits in hippocampal LTP following i.c.v. injection of oligomeric Aß1-42. Furthermore, similar treatment significantly ameliorated cognitive deficits in an object recognition task and under an alternating lever cyclic ratio schedule after the i.c.v. application of Aß1-42 and 7PA2 conditioned medium, respectively. Altogether, these results demonstrate the potential therapeutic benefit of MRZ-99030 in AD.

In vivo electrophysiological recording techniques for the study of neuropathic pain in rodent models.

Neuropathic pain develops following nerve injury, and is a chronic pain syndrome that can persist long after repair of a wound or removal of the neurological insult. This condition remains poorly treated, not least because of a lack of mechanism-based therapeutics. Clinically, neuropathic pain is characterized by three major symptoms: thermal or mechanical allodynia (pain sensation in response to previously non-noxious stimuli); hyperalgesia (enhanced pain sensation to noxious stimulation); and spontaneous, ongoing pain. These clinical symptoms can be modeled in rodent neuropathic pain models using behavioral and electrophysiological readouts. This unit describes techniques designed to record pathophysiological electrical activity associated with neuropathic pain at the level of the periphery, in single fibers of primary sensory neurons, and from wide dynamic range (WDR) neurons of the dorsal horn of the spinal cord. These techniques can be employed in both naïve animals and in animal models of neuropathy to investigate fundamental mechanisms contributing to the neuropathic pain state and the site, mode, and mechanism of action of putative analgesics.

Novel 5-aryloxypyrimidine SEN1576 as a candidate for the treatment of Alzheimer's disease.

Prefibrillar assembly of amyloid-ß (Aß) is a major event underlying the development of neuropathology and dementia in Alzheimer's disease (AD). This study determined the neuroprotective properties of an orally bioavailable Aß synaptotoxicity inhibitor, SEN1576. Binding of SEN1576 to monomeric Aß 1-42 was measured using surface plasmon resonance. Thioflavin-T and MTT assays determined the ability of SEN1576 to block Aß 1-42-induced aggregation and reduction in cell viability, respectively. In vivo long-term potentiation (LTP) determined effects on synaptic toxicity induced by intracerebroventricular (i.c.v.) injection of cell-derived Aß oligomers. An operant behavioural schedule measured effects of oral administration following i.c.v. injection of Aß oligomers in normal rats. SEN1576 bound to monomeric Aß 1-42, protected neuronal cells exposed to Aß 1-42, reduced deficits in in vivo LTP and behaviour. SEN1576 exhibits the necessary features of a drug candidate for further development as a disease modifying treatment for the early stages of AD-like dementia.

Electrophysiological Techniques for Studying Synaptic Activity In Vivo.

Understanding the physiology, pharmacology, and plasticity associated with synaptic function is a key goal of neuroscience research and is fundamental to identifying the processes involved in the development and manifestation of neurological disease. A diverse range of electrophysiological methodologies are used to study synaptic function. Described in this unit is a technique for recording electrical activity from a single component of the central nervous system that is used to investigate pre- and post-synaptic elements of synaptic function. A strength of this technique is that it can be used on live animals, although the effect of anesthesia must be taken into consideration when interpreting the results. This methodology can be employed not only in naïve animals for studying normal physiological synaptic function, but also in a variety of disease models, including transgenic animals, to examine dysfunctional synaptic plasticity associated with neurological pathologies.

Effects of D-amino acid oxidase inhibition on memory performance and long-term potentiation in vivo.

N-methyl-d-aspartate receptor (NMDAR) activation can initiate changes in synaptic strength, evident as long-term potentiation (LTP), and is a key molecular correlate of memory formation. Inhibition of d-amino acid oxidase (DAAO) may increase NMDAR activity by regulating d-serine concentrations, but which neuronal and behavioral effects are influenced by DAAO inhibition remain elusive. In anesthetized rats, extracellular field excitatory postsynaptic potentials (fEPSPs) were recorded before and after a theta frequency burst stimulation (TBS) of the Schaffer collateral pathway of the CA1 region in the hippocampus. Memory performance was assessed after training with tests of contextual fear conditioning (FC, mice) and novel object recognition (NOR, rats). Oral administration of 3, 10, and 30 mg/kg 4H-furo[3,2-b]pyrrole-5-carboxylic acid (SUN) produced dose-related and steady increases of cerebellum d-serine in rats and mice, indicative of lasting inhibition of central DAAO. SUN administered 2 h prior to training improved contextual fear conditioning in mice and novel object recognition memory in rats when tested 24 h after training. In anesthetized rats, LTP was established proportional to the number of TBS trains. d-cycloserine (DCS) was used to identify a submaximal level of LTP (5× TBS) that responded to NMDA receptor activation; SUN administered at 10 mg/kg 3-4 h prior to testing similarly increased in vivo LTP levels compared to vehicle control animals. Interestingly, in vivo administration of DCS also increased brain d-serine concentrations. These results indicate that DAAO inhibition increased NMDAR-related synaptic plasticity during phases of post training memory consolidation to improve memory performance in hippocampal-dependent behavioral tests.

Orally bioavailable small molecule drug protects memory in Alzheimer's disease models.

Oligomers of beta-amyloid (Aß) are implicated in the early memory impairment seen in Alzheimer's disease before to the onset of discernable neurodegeneration. Here, the capacity of a novel orally bioavailable, central nervous system-penetrating small molecule 5-aryloxypyrimidine, SEN1500, to prevent cell-derived (7PA2 [conditioned medium] CM) Aß-induced deficits in synaptic plasticity and learned behavior was assessed. Biochemically, SEN1500 bound to Aß monomer and oligomers, produced a reduction in thioflavin-T fluorescence, and protected a neuronal cell line and primary cortical neurons exposed to synthetic soluble oligomeric Aß(1-42). Electrophysiologically, SEN1500 alleviated the in vitro depression of long-term potentiation induced by both synthetic Aß(1-42) and 7PA2 CM, and alleviated the in vivo depression of long-term potentiation induced by 7PA2 CM, after systemic administration. Behaviorally, oral administration of SEN1500 significantly reduced memory-related deficits in operant responding induced after intracerebroventricular injection of 7PA2 CM. SEN1500 reduced cytotoxicity, acute synaptotoxicity, and behavioral deterioration after in vitro and in vivo exposure to synthetic Aß and 7PA2 CM, and shows promise for development as a clinically viable disease-modifying Alzheimer's disease treatment.

The N-methylated peptide SEN304 powerfully inhibits Aß(1-42) toxicity by perturbing oligomer formation.

Oligomeric forms of ß-amyloid (Aß) have potent neurotoxic activity and are the primary cause of neuronal injury and cell death in Alzheimer's disease (AD). Compounds that perturb oligomer formation or structure may therefore be therapeutic for AD. We previously reported that d-[(chGly)-(Tyr)-(chGly)-(chGly)-(mLeu)]-NH(2) (SEN304) is able to inhibit Aß aggregation and toxicity, shown primarily by thioflavin T fluorescence and MTT (Kokkoni, N. et al. (2006) N-Methylated peptide inhibitors of ß-amyloid aggregation and toxicity. Optimisation of inhibitor structure. Biochemistry 45, 9906-9918). Here we extensively characterize how SEN304 affects Aß(1-42) aggregation and toxicity, using biophysical assays (thioflavin T, circular dichroism, SDS-PAGE, size exclusion chromatography, surface plasmon resonance, traveling wave ion mobility mass spectrometry, electron microscopy, ELISA), toxicity assays in cell culture (MTT and lactate dehydrogenase in human SH-SHY5Y cells, mouse neuronal cell death and synaptophysin) and long-term potentiation in a rat hippocampal brain slice. These data, with dose response curves, show that SEN304 is a powerful inhibitor of Aß(1-42) toxicity, particularly effective at preventing Aß inhibition of long-term potentiation. It can bind directly to Aß(1-42), delay ß-sheet formation and promote aggregation of toxic oligomers into a nontoxic form, with a different morphology that cannot bind thioflavin T. SEN304 appears to work by inducing aggregation, and hence removal, of Aß oligomers. It is therefore a promising lead compound for Alzheimer's disease.

Translational PK-PD modelling of molecular target modulation for the AMPA receptor positive allosteric modulator Org 26576.

INTRODUCTION: The α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor potentiator Org 26576 represents an interesting pharmacological tool to evaluate the utility of glutamatergic enhancement towards the treatment of psychiatric disorders. In this study, a rat-human translational pharmacokinetic-pharmacodynamic (PK-PD) model of AMPA receptor modulation was used to predict human target engagement and inform dose selection in efficacy clinical trials. METHODS: Modelling and simulation was applied to rat plasma and cerebrospinal fluid (CSF) pharmacokinetic and pharmacodynamic measurements to identify a target concentration (EC(80)) for AMPA receptor modulation. Human plasma pharmacokinetics was determined from 33 healthy volunteers and eight major depressive disorder patients. From four out of these eight patients, CSF PK was also determined. Simulations of human CSF levels were performed for several doses of Org 26576. RESULTS: Org 26576 (0.1-10 mg/kg, i.v.) potentiated rat hippocampal AMPA receptor responses in an exposure-dependant manner. The rat plasma and CSF PK data were fitted by one-compartment model each. The rat CSF PK-PD model yielded an EC(80) value of 593 ng/ml (90% confidence interval 406.8, 1,264.1). The human plasma and CSF PK data were simultaneously well described by a two-compartment model. Simulations showed that in humans at 100 mg QD, CSF levels of Org 26576 would exceed the EC(80) target concentration for about 2 h and that 400 mg BID would engage AMPA receptors for 24 h. CONCLUSION: The modelling approach provided useful insight on the likely human dose-molecular target engagement relationship for Org 26576. Based on the current analysis, 100 and 400 mg BID would be suitable to provide 'phasic' and 'continuous' AMPA receptor engagement, respectively.

The role of central 5-HT3 receptors in vagal reflex inputs to neurones in the nucleus tractus solitarius of anaesthetized rats.

Brainstem 5-hydroxytryptamine (5-HT, serotonin)-containing neurones modulate cardiovascular reflex responses but the differing roles of the many 5-HT receptors have not been thoroughly investigated. The present experiments on anaesthetized rats investigated the role of 5-HT3 receptors in modulating vagal afferent evoked activity of nucleus tractus solitarius (NTS) neurones. Recordings were made from 301 NTS neurones receiving an input at long (> 20 ms) minimum onset latency from stimulation of the vagus nerve. These included 140 neurones excited by activating non-myelinated cardiopulmonary afferents by right atrial injection of phenylbiguanide (PBG). Ionophoretic application of PBG, a highly selective 5-HT3 receptor agonist, significantly increased activity (from 2.4 +/- 0.4 to 5.5 +/- 0.8 spikes s(-1)) in 96 of 106 neurones tested and in all 17 neurones tested the increase in activity (3.4 +/- 1.1 to 7.0 +/- 1.9 spikes s(-1)) was significantly attenuated (3.0 +/- 0.9 to 3.8 +/- 1.1 spikes s(-1)) by the selective 5-HT3 receptor antagonist granisetron. Ionophoretic application of PBG potentiated responses to vagus nerve and cardiopulmonary afferent stimulation, and granisetron significantly attenuated this cardiopulmonary input (20.2 +/- 5.7 to 10.6 +/- 4.1 spikes burst(-1)) in 9 of 10 neurones tested. Ionophoretic application of AMPA and NMDA also excited NTS neurones and these excitations could be selectively antagonized by the non-NMDA and NMDA receptor antagonists DNQX and AP-5, respectively. At these selective currents, DNQX and AP-5 also attenuated PBG- and cardiopulmonary input-evoked increases in NTS activity. These data are consistent with the hypothesis that vagal inputs, including non-myelinated cardiopulmonary inputs to the NTS, utilize a 5-HT-containing pathway which activates 5-HT3 receptors. This excitatory response to 5-HT3 receptor activation may be partly a direct postsynaptic action but part may also be due to facilitation of the release of glutamate which in turn acts on either non-NMDA or NMDA receptors to evoke excitation.