Ketamine and MK801 attenuate paired pulse inhibition in the olfactory bulb of the rat

fMRI study of olfactory processing in mice under three anesthesia protocols: Insight into the effect of ketamine on olfactory processing

Functional magnetic resonance imaging (fMRI) is a valuable tool for studying neural activations in the central nervous system of animals due to its wide spatial coverage and non-invasive nature. However, the advantages of fMRI have not been fully realized in functional studies in mice, especially in the olfactory system, possibly due to the lack of suitable anesthesia protocols with spontaneous breathing. Since mice are widely used in biomedical research, it is desirable to evaluate different anesthesia protocols for olfactory fMRI studies in mice. Dexmedetomidine (DEX) as a sedative/anesthetic has been introduced to fMRI studies in mice, but it has a limited anesthesia duration. To extend the anesthesia duration, DEX has been combined with a low dose of isoflurane (ISO) or ketamine (KET) in previous functional studies in mice. In this report, olfactory fMRI studies were performed under three anesthesia protocols (DEX alone, DEX/ISO, and DEX/KET) in three different groups of mice. Isoamyl-acetate was used as an odorant, and the odorant-induced neural activations were measured by blood oxygenation-level dependent (BOLD) fMRI. BOLD fMRI responses were observed in the olfactory bulb (OB), anterior olfactory nuclei (AON), and piriform cortex (Pir). Interestingly, BOLD fMRI activations were also observed in the prefrontal cortical region (PFC), which are most likely caused by the draining vein effect. The response in the OB showed no adaptation to either repeated odor stimulations or continuous odor exposure, but the response in the Pir showed adaptation during the continuous odor exposure. The data also shows that ISO suppresses the olfactory response in the OB and AON, while KET enhances the olfactory response in the Pir. Thus, DEX/KET should be an attractive anesthesia for olfactory fMRI in mice.

Existence of glia mitigated ketamine-induced neurotoxicity in neuron-glia mixed cultures of neonatal rat cortex and the glia-mediated protective effect of 2-PMPA

The present study compared ketamine-induced neurotoxicity in the neuron-glia mixed cultures and neuronal cultures and further explored the neuroprotective effect of the NAAG peptidase inhibitor 2-(phosphonomethyl) pentanedioic acid (2-PMPA). Firstly, Rosenfeld's staining and immunofluorescence staining of microtubule-associated protein 2 (MAP2) and glial fibrillary acidic protein (GFAP) were used to address the difference of morphology in the mixed cultures and neuronal cultures. Our results showed that neurons and astrocytes grew in good conditions. The ratio of neurons and astrocytes in the mixed cultures was around 1:1, and the purity of neurons in the neuronal cultures is 91.3%. Furthermore, ketamine was used to test the hypothesis that the presence of a higher proportion of glia in the mixed cultures would be protective against ketamine-induced neurotoxicity in the mixed cultures compared with neuronal cultures. The results showed that ketamine-induced morphological changes, cell viability decrease and lactate dehydrogenase (LDH) levels increase were significantly mitigated in neuron-glia mixed cultures compared with neuronal cultures. Furthermore, 2-PMPA was included to further explore efficient protective drug for ketamine-induced neurotoxicity. Our results showed that 2-PMPA reduced ketamine-induced decrease of cell viability and increase of LDH levels in the mixed cultures but not in the neuronal cultures. Further morphological changes of neurons and astrocytes also indicated that 2-PMPA could improve ketamine damaged neurons in the mixed cultures instead of neuronal cultures. These results indicate that glia protect neurons from ketamine-induced neurotoxicity. These data further suggest that glia mediate the neuroprotective effect of 2-PMPA and 2-PMPA has the potential to treat ketamine-induced neurotoxicity in vivo. Delineating the mechanisms underlying the communication between neurons and glia and the neuroprotective effects of 2-PMPA in the mixed cultures to ketamine-induced neurotoxicity require further investigation.

MicroRNA expression profile and functional analysis reveal that miR-206 is a critical novel gene for the expression of BDNF induced by ketamine

Depression is a major social and health concern, and ketamine exerts a quick, remarkable and persistent anti-depressive effect. microRNAs (miRNAs) show remarkable potential in the treatment of clinical depression. Here, we determined the expression profile of miRNAs in the hippocampus of rats treated with ketamine (15 mg/kg). The results suggest that multiple miRNAs were aberrantly expressed in rat hippocampus after ketamine injection (18 miRNAs were significantly reduced, while 22 miRNAs were significantly increased). Among them, miR-206 was down-regulated in ketamine-treated rats. In both cultured neuronal cells in vitro and hippocampus in vivo, we identified that the brain-derived neurotrophic factor (BDNF) was a direct target gene of miR-206. Via this target gene, miR-206 strongly modulated the expression of BDNF. Moreover, overexpression of miR-206 significantly attenuated ketamine-induced up-regulation of BDNF. The results indicated that miRNA-206 was involved in novel therapeutic targets for the anti-depressive effect of ketamine.

MicroRNA expression profile and functional analysis reveal that miR-206 is a critical novel gene for the expression of BDNF induced by ketamine

Depression is a major social and health concern, and ketamine exerts a quick, remarkable and persistent anti-depressive effect. microRNAs (miRNAs) show remarkable potential in the treatment of clinical depression. Here, we determined the expression profile of miRNAs in the hippocampus of rats treated with ketamine (15 mg/kg). The results suggest that multiple miRNAs were aberrantly expressed in rat hippocampus after ketamine injection (18 miRNAs were significantly reduced, while 22 miRNAs were significantly increased). Among them, miR-206 was down-regulated in ketamine-treated rats. In both cultured neuronal cells in vitro and hippocampus in vivo, we identified that the brain-derived neurotrophic factor (BDNF) was a direct target gene of miR-206. Via this target gene, miR-206 strongly modulated the expression of BDNF. Moreover, overexpression of miR-206 significantly attenuated ketamine-induced up-regulation of BDNF. The results indicated that miRNA-206 was involved in novel therapeutic targets for the anti-depressive effect of ketamine.

In vitro dose-dependent inhibition of the intracellular spontaneous calcium oscillations in developing hippocampal neurons by ketamine

Spatial and temporal abnormalities in the frequency and amplitude of the cytosolic calcium oscillations can impact the normal physiological functions of neuronal cells. Recent studies have shown that ketamine can affect the growth and development and even induce the apoptotic death of neurons. This study used isolated developing hippocampal neurons as its study subjects to observe the effect of ketamine on the intracellular calcium oscillations in developing hippocampal neurons and to further explore its underlying mechanism using Fluo-4-loaded laser scanning confocal microscopy. Using a semi-quantitative method to analyze the spontaneous calcium oscillatory activities, a typical type of calcium oscillation was observed in developing hippocampal neurons. In addition, the administration of NMDA (N-Methyl-D-aspartate) at a concentration of 100 µM increased the calcium oscillation amplitude. The administration of MK801 at a concentration of 40 µM inhibited the amplitude and frequency of the calcium oscillations. Our results demonstrated that an increase in the ketamine concentration, starting from 30 µM, gradually decreased the neuronal calcium oscillation amplitude. The inhibition of the calcium oscillation frequency by 300 µM ketamine was statistically significant, and the neuronal calcium oscillations were completely eliminated with the administration of 3,000 µM Ketamine. The administration of 100, 300, and 1,000 µM NMDA to the 1 mM ketamine-pretreated hippocampal neurons restored the frequency and amplitude of the calcium oscillations in a dose-dependent manner. In fact, a concentration of 1,000 µM NMDA completely reversed the decrease in the calcium oscillation frequency and amplitude that was induced by 1 mM ketamine. This study revealed that ketamine can inhibit the frequency and amplitude of the calcium oscillations in developing hippocampal neurons though the NMDAR (NMDA receptor) in a dose-dependent manner, which might highlight a possible underlying mechanism of ketamine toxicity on the rat hippocampal neurons during development.

Sensory information processing in the frog olfactory pathways. Experimental basis for modeling studies

In the frog, unitary electrophysiological recordings have been extensively used to investigate odor processing along the olfactory pathways. By comparing spontaneous and odor-evoked activities of neuroreceptor, mitral and cortical cells, we have collected fundamental data relating to coding abilities of the three olfactory levels, the olfactory mucosa, the bulb and the cortex. Based on a synthesis of our experimental data related to GABAergic and dopaminergic involvement in the olfactory bulb, this paper aims to match this information with computational data and to discuss some questions on bulbar processing. This paper is also devoted to further analyze original results on coding properties of two functionally evidenced neuron subpopulations in the olfactory cortex. Thus, the assumption according to which some cortical neurons may work as temporal integrators while others as coincidence detectors is presented. Moreover, the pertinence that the neural code may be carried by a single spike with varying latency was demonstrated.

Localization of the clustering protein gephyrin at GABAergic synapses in the main olfactory bulb of the rat

The tubulin-binding protein gephyrin is essential for the formation of postsynaptic glycine-receptor clusters in cultured spinal neurons. In addition, there is increasing evidence that gephyrin can also be present at nonglycinergic synapses. Here we analyzed immunocytochemically the subcellular localization of gephyrin in the main olfactory bulb of the rat and compared its distribution with that of gamma-aminobutyric acid (GABA) and of two major GABA(A)-receptor subunits. Gephyrin was selectively localized to the postsynaptic side of symmetric synaptic junctions, where the presynaptic terminals contained GABA. Moreover, gephyrin colocalized extensively with the alpha1 and gamma2 subunits of the GABA(A) receptor. In contrast, gephyrin was not detected at presumed glutamatergic synapses. These results indicate that gephyrin is not uniquely associated with glycine receptors, but can also be found at distinct GABAergic synapses. Thus, they raise the possibility that gephyrin is involved in anchoring certain GABA(A)-receptor subtypes in the postsynaptic membrane.

Odor processing in the frog olfactory system

In the frog, unitary electrophysiological recordings have been extensively used to investigate odor processing along the olfactory pathways. From the responses of primary second-order neurons, neuroreceptor and mitral cells, odor stimuli could be classified in qualitative groups, revealing that neuronal discriminative mechanisms are partly based on the structure of odor molecule. In the olfactory bulb, thanks both to the anatomical convergence of primary afferences and intrinsic network properties, mitral cells have been demonstrated to gain in odor discrimination and detection power abilities. GABAergic bulbar interneurons were found to be involved in the control of mitral cell excitability, adjusting response thresholds and duration and promoting a progressive increase of burst discharges with stimulus concentration. Otherwise, dopamine was observed to shunt off mitral cell spontaneous activity without altering their odor responsivity properties. Dopamine was demonstrated to act through D2 receptors. Matching anatomical and electrophysiological data, D2 receptors are assumed to be localized on mitral cells. The frog olfactory cortex neurons, silent at rest, could be segregated in two functional groups basing on their odor response properties. The first group shared most intensity coding properties with mitral cells while showing a lower discriminative power, similar to that of neuroreceptor cells. By contrast, the second group provided only minimal intensity coding and, basing on its high discrimination power, was assumed to be mainly devoted to odor discrimination. Thus, along the olfactory pathways, intensity and quality odor parameters which are simultaneously encoded by a neuroreceptor or mitral cell, become specified by two distinct populations in the cortex.

Mitral/tufted cell activity is attenuated and becomes uncoupled from respiration following naris closure

Patterned neural activity helps to establish neuronal connectivity, produce coding of sensory information, and shape synaptic strengths. Here we demonstrate that normal olfactory bulb development might rely on spatial and temporal patterns of afferent neural activity. Neonatal naris occlusion profoundly impacts the development of the ipsilateral olfactory bulb, including reduced bulb volume, decreased protein synthesis, and increased cell death. Relatively few morphologic changes occur if closure is performed postweaning. We examined the immediate electrophysiological consequences of occlusion across this developmentally sensitive period by recording spontaneous and odor-driven mitral/tufted cell responses while the naris was open, closed, and then reopened. In 1-week-old animals, occlusion severely attenuated spontaneous activity, and presentation of the broad-spectrum odorant amyl acetate failed to evoke responses. In 2- and 4-week old rats, spontaneous activity was also reduced by naris closure. However, some cells remained responsive to concentrated odors, even in animals with transected anterior commissures, suggesting passage of odors across the septal window or retronasal pathways. In all age groups, cellular activity became uncoupled from the respiratory cycle. Approximately 47% (18 of 38) of the mitral/tufted cells exhibited activity that was correlated with respiration in the open-naris state, while only 5% (2 of 38) were coupled during naris closure. These data (a) indicate that naris closure reduces both spontaneous and odor-evoked responses, and (b) provide an electrophysiological correlate to a sensitive period in bulb development. The loss of respiration-related synchrony and the reduced activity of mitral/tufted cells may synergistically contribute to the diverse consequences of naris closure on bulb development.

Stimulus parameters affecting paired-pulse depression of dentate granule cell field potentials. II. Low-frequency stimulation

Low frequency (1 Hz) stimulation of the perforant path produces a depression in the population spike (PS) of dentate granule cell field potentials and also may affect the strength of paired pulse depression. The effects of 1 Hz stimulation (30 s train) on paired pulse depression (20 and 200 ms interpulse intervals, IPI) were evaluated in the unanesthetized rat under two conditions: (i) when the stimulus intensity of both pulses was increased simultaneously (5-100%); and (ii) when the stimulus intensity of the first (conditioning) pulse was increased (5-100%), while the stimulus intensity of the second (test) pulse was held constant (50%). The test PS amplitude was predicted based upon either the conditioning PS amplitude at the end of the 1 Hz train or upon the additive effects of paired pulse depression and 1 Hz stimulation. These predicted values then were assessed for the best fit to observed values following 1 Hz trains. Under both stimulus conditions, the 1 Hz depression in the conditioning PS amplitude exhibited characteristics that were identical to late paired pulse depression recorded before the train. A decrease in the test PS amplitude also was observed following 1 Hz stimulation at the 20 and 200 ms IPIs. The best fit to observed values of the test PS at the end of 1 Hz trains was provided by estimates based upon the additive effects of 1 Hz stimulation and paired pulse depression. These results indicate that the strength of paired pulse depression in the unanesthetized rat is unchanged following 1 Hz stimulation, and further, that the 1 Hz depression in dentate granule cell field potentials most likely reflects the cumulative influence of late paired pulse depression.

Stimulus parameters affecting paired-pulse depression of dentate granule cell field potentials. I. Stimulus intensity

Paired pulse stimulation of the perforant path provides a measure of inhibition of dentate granule cell field potentials that is reflected in the depression of the second (test) population spike (PS) relative to the first (conditioning) PS. The assumption that the strength of paired pulse depression is dependent upon the amplitude of the conditioning PS was investigated by increasing the stimulus intensity of both pulses (5-100% of maximum, Experiment 1), or by increasing the stimulus intensity of the conditioning pulse (5-100%) while maintaining a constant stimulus intensity of the test pulse (50%, Experiment 2). In both experiments, the threshold for early paired pulse depression (20 ms interpulse interval, IPI) was reached with moderate stimulation (30-40% of maximum). Above threshold, the test PS was depressed to a relatively constant amplitude in Experiment 1, in contrast to a nearly linear decrease observed in Experiment 2 with increasing strength of early paired pulse, relative to the conditioning reflects the lower stimulus intensity of the test pulse, relative to the conditioning pulse, in the second study, thereby allowing the increasing strength of early paired pulse depression to be detected more easily. The threshold for late paired pulse depression was reached near (20%, Experiment 1) or below (5%, Experiment 2) the PS threshold of dentate granule cells, and a paradoxical decrease in late paired pulse depression was detected with maximal stimulation in both studies. Together, these results suggest that early paired pulse depression exhibits a strong dependence upon the amplitude of the conditioning PS, whereas late paired pulse depression is marginally affected by the conditioning PS amplitude and is influenced by additional processes at both extremes of the stimulus intensity continuum.

The effect of anosmia on MK-801-induced behaviour in mice

Systemic administration of N-methyl-D-aspartate (NMDA) receptor antagonists induces a well defined behaviour in rodents characterized by, for example increased locomotion and ataxia. It is not clear in what brain region(s) NMDA antagonists induce this behaviour. We have studied the possible involvement of olfactory pathways by making adult mice anosmic via intranasal injection of zinc sulphate, a procedure that is known to destroy the olfactory epithelium. The NMDA antagonist MK-801 was given intraperitoneally (0.1-1.0 mg/kg) and the animals were scored for locomotion and ataxia 60-90 min later. Before MK-801 administration, olfactory-lesioned mice did not differ from non-lesioned controls with regard to locomotion or ataxia. MK-801 caused locomotor activation (> or = 0.2 mg/kg) and ataxia (> or = 0.5 mg/kg) in both groups. In general, olfactory-lesioned animals showed more locomotion and less ataxia after MK-801 administration than non-lesioned animals. Lesioned animals displayed 2.0- (P < 0.05) and 3.7-fold (P < 0.05) more extensive locomotor activation than non-lesioned animals after 0.5 and 1.0 mg/kg of MK-801, respectively. No difference in the degree of ataxia was seen between the two groups at 0.5 mg/kg, whereas non-lesioned animals showed a 2.1-fold higher degree of ataxia after 1.0 mg/kg of MK-801, indicating that the enhanced MK-801-induced locomotor activation in olfactory-lesioned mice was not simply due to less ataxia. These results suggest that olfactory input is involved in NMDA antagonist-induced behaviour.

Unilateral naris closure and olfactory system development

In most animals there is bilateral access of odorants to the olfactory sensory epithelium. Air enters the nose through two external nares and passes back through the nasal cavity, which is divided down the midline by a cartilaginous nasal septum. The olfactory mucosa, a sheet of ciliated bipolar receptor cells, is found in the caudal two thirds of the nasal cavity. Axons from the sensory cells project to an ipsilateral extension of the telencephalon known as the olfactory bulb. If a single external naris of a rat pup is surgically closed (usually via brief cauterization) on the day after the day of birth (P1) and the subject is examined on P30, the size of the ipsilateral olfactory bulb is reduced by approximately 25%. The large reduction in size, coupled with the clear lamination and other features of the olfactory system, indicates that the manipulation is an ideal preparation for examining the regulation of early growth. We know that both olfactory bulbs are of equal size at the time of occlusion, but that 30 days later there is a large discrepancy. What series of events produces the changes? The present paper outlines what is known about the anatomical, biochemical and physiological changes introduced by naris occlusion in order to lay a framework for further work.

GABAergic control of odour-induced activity in the frog olfactory bulb: possible GABAergic modulation of granule cell inhibitory action

In the olfactory bulb, the activity of the output neurons, the mitral cells, is under inhibitory control exerted by GABAergic interneurons, the granule cells. Although the mechanisms of this inhibition are well known from in vitro studies, its physiological role in controlling mitral cell activity in response to odours has never been investigated. This study planned to improve understanding of the involvement of granule cells. To do so, GABAA-synaptic mechanisms were altered using GABAA antagonists in order to observe the consequences on mitral cell electrophysiological responses to odours, delivered over a wide concentration range. Due to the laminar organization of bulbar cell populations, the antagonists picrotoxin or bicuculline were injected into the bulbar ventricle in order to block granule cell inhibitory action at first. Surprisingly, the early consequence of the antagonist injection was a decrease in cell responsivity: response spike frequencies were lowered while thresholds were occasionally shifted toward higher concentrations. This initial depressive effect was followed by a recovery of control excitability and, later, by an increase in excitability: spike bursts became more sustained in frequency and in duration. At the same time, in most of the cells studied, spontaneous activity became bursting. The early depressive effect of GABAA antagonists is discussed in terms of an enhancement of the inhibitory influence of granule cells on mitral cells. This might reflect a blocking action of the antagonists at the level of GABAergic synapses located on granule cells themselves. The late effect, an increase in excitability, is explained as the consequence of the alteration of the functioning of dendrodendritic synapses between granule and mitral cells leading to a disinhibition of the latter. The comparison of the present findings with others obtained when antagonists were applied on to glomerular layers led us to infer that granule cell inhibition would be devoted to limiting mitral cell responses in frequency and in duration rather than to adjusting their response threshold. The chronology of the effects observed strongly supports the fact that, following the intraventricular injection, the antagonists acted primarily in the deep layers of the bulb. Nevertheless, due to free diffusion starting from the injection site, the possibility that drugs act later in the glomerular layer can not be rejected. It can be concluded that, in addition to its extensive involvement through intrinsic interneurons, GABA might also control the strength of the inhibition exerted by granule cells on mitral cells via centrifugal fibres.

Synaptic transmission and modulation in the olfactory bulb

Recent work in molecular biology and synaptic physiology has significantly increased our understanding of inhibitory and excitatory mechanisms in the olfactory bulb. Multiple subtypes of amino acid receptors with different functional and neuromodulatory properties are likely to play key roles in processing odor information transduced and relayed to the olfactory bulb by the olfactory sensory neurons, and in modulating that information during olfactory learning.

Identification of specific pathways of communication between the CNS and NK cell system

The specific signals and pathways utilized by the natural killer (NK) cell system and the central nervous system (CNS) that results in the conditioned response (CR) is not clearly understood. Single trial conditioning of the NK cell activity provides us with a model to probe the mechanisms of communication between two major systems (Immune and CNS) which are involved in the health and disease of the individual. The studies show that the IFN-beta molecules possess the properties attributed to the unconditioned stimulus (US). IFN-beta can penetrate the CNS and evoke the elevation of NK cell activity in the spleen. This unconditioned response (UR) can be linked to a specific conditioned stimulus (CS). Specific odors such as camphor provide a neural pathway for the CS to associate with the US. Evidence is presented that in conditioning there are two locations where memory develops. The CS/US association is made centrally and its memory is stored at a central location, but the memory for the specificity of the odor is presumably stored in the olfactory bulbs. The CS recalls the CR by triggering the olfactory neural pathway which, in turn, signals the hypothalamic-pituitary axis to release mediators that modulate the activity of NK cells in the spleen. These results imply that through conditioning one has direct input into the regulatory hypothalamus that controls the internal environment of the organism and the health and disease of the individual. Consequently, it is not inconceivable that through this approach we might be able to alter the course of a disease process.

Acquisition of enhanced natural killer cell activity under anesthesia

An increase in natural killer (NK) cell activity can be conditioned with a one trial learning paradigm to demonstrate the interaction between the central nervous system (CNS) and the immune system. In order to demonstrate learning possibilities during 'non-conscious' state, mice were anesthetized with a ketamin/rompun mixture and underwent one trial learning with odor cue as the conditioned stimulus (CS) preceding the unconditioned stimulus (US). The results indicated that mice that were exposed to camphor odor cue under the influence of anesthesia can associate the signal with the poly I:C unconditioned stimulus and were able to recall the conditioned response upon reexposure to the CS. Secondly, the conditioned association made in a conscious state can be recalled by exposure to the same olfactory odor cue in a 'non-conscious' state. The increase in the conditioned change in NK cell activity of both situations was significantly higher than the control group. The results demonstrate that learning can take place and the learned response can be recalled under the reduced awareness caused by anesthesia. The findings we report are unusual and novel in that they demonstrate that the CNS can learn new associations under conditions where the host is apparently unaware of the signals being linked. Anesthesia combined with the long interstimulus interval indicates that certain neuronal pathways in the CNS are receptive to second signals (elicited by the US) even when the second signal is separated by one day. This means the conditioned learning of a physiological response can take place unconsciously at a separate level and under situations where the host is totally unaware of the events which the brain is processing and linking as incoming information.