Increase In Spontaneous Firing Rate Research Articles

Microglial activation in the lateral amygdala promotes anxiety-like behaviors in mice with chronic moderate noise exposure.

Long-term non-traumatic noise exposure, such as heavy traffic noise, can elicit emotional disorders in humans. However, the underlying neural substrate is still poorly understood. We exposed mice to moderate white noise for 28 days to induce anxiety-like behaviors, measured by open-field, elevated plus maze, and light-dark box tests. Invivo multi-electrode recordings in awake mice were used to examine neuronal activity. Chemogenetics were used to silence specific brain regions. Viral tracing, immunofluorescence, and confocal imaging were applied to define the neural circuit and characterize the morphology of microglia. Exposure to moderate noise for 28 days at an 85-dB sound pressure level resulted in anxiety-like behaviors in open-field, elevated plus maze, and light-dark box tests. Viral tracing revealed that fibers projecting from the auditory cortex and auditory thalamus terminate in the lateral amygdala (LA). A noise-induced increase in spontaneous firing rates of the LA and blockade of noise-evoked anxiety-like behaviors by chemogenetic inhibition of LA glutamatergic neurons together confirmed that the LA plays a critical role in noise-induced anxiety. Noise-exposed animals were more vulnerable to anxiety induced by acute noise stressors than control mice. In addition to these behavioral abnormalities, ionized calcium-binding adaptor molecule 1 (Iba-1)-positive microglia in the LA underwent corresponding morphological modifications, including reduced process length and branching and increased soma size following noise exposure. Treatment with minocycline to suppress microglia inhibited noise-associated changes in microglial morphology, neuronal electrophysiological activity, and behavioral changes. Furthermore, microglia-mediated synaptic phagocytosis favored inhibitory synapses, which can cause an imbalance between excitation and inhibition, leading to anxiety-like behaviors. Our study identifies LA microglial activation as a critical mediator of noise-induced anxiety-like behaviors, leading to neuronal and behavioral changes through selective synapse phagocytosis. Our results highlight the pivotal but previously unrecognized roles of LA microglia in chronic moderate noise-induced behavioral changes.

Dynamics of coupled nonlinear neuronal-astrocyte model

More evidence indicates that central nervous system diseases are related to abnormal regulation of the reciprocal expression of nerve cells and astrocytes. In this paper, we investigated the dynamics of a model incorporating both a neuron and an astrocyte to elucidate the effective interaction between the two components. In this coupled system, external stimulation is applied to the neuron and the astrocyte, respectively. The results showed that after successive stimuli separated by 20 s, both neurons and astrocytes can exhibit the so-called hyperexcitability and nonlinear wave characterized by a decrease in the threshold for activation and an increase in spontaneous firing rate. Furthermore, this paper reveals the potential dynamic changes in the formation and disappearance of hyperexcitability and its transition mechanisms. It can be helpful for further understanding the bifurcation behavior during the progression of central nervous system dysfunction.

Non-ablative doses of focal ionizing radiation alters function of central neural circuits

BackgroundModulation of pathological neural circuit activity in the brain with a minimum of complications is an area of intense interest. ObjectiveThe goal of the study was to alter neurons' physiological states without apparent damage of cellular integrity using stereotactic radiosurgery (SRS). MethodsWe treated a 7.5 mm-diameter target on the visual cortex of Göttingen minipigs with doses of 40, 60, 80, and 100 Gy. Six months post-irradiation, the pigs were implanted with a 9 mm-wide, eight-shank multi-electrode probe, which spanned the radiation focus as well as the low-exposure neighboring areas. ResultsDoses of 40 Gy led to an increase of spontaneous firing rate, six months post-irradiation, while doses of 60 Gy and greater were associated with a decrease. Subjecting the animals to visual stimuli resulted in typical visual evoked potentials (VEP). At 40 Gy, a significant reduction of the P1 peak time, indicative of higher network excitability was observed. At 80 Gy, P1 peak time was not affected, while a minor reduction at 60 Gy was seen. No distance-dependent effects on spontaneous firing rate, or on VEP were observed. Post-mortem histology revealed no evidence of necrosis at doses below 60 Gy. In an in vitro assay comprising of iPS-derived human neuron-astrocyte co-cultures, we found a higher vulnerability of inhibitory neurons than excitatory neurons with respect to radiation, which might provide the cellular mechanism of the disinhibitory effect observed in vivo. ConclusionWe provide initial evidence for a rather circuit-wide, long-lasting disinhibitory effect of low sub-ablative doses of SRS.

Time-Dependent Recruitment of Prelimbic Prefrontal Circuits for Retrieval of Fear Memory.

The long-lasting nature of fear memories is essential for survival, but the neural circuitry for retrieval of these associations changes with the passage of time. We previously reported a time-dependent shift from prefrontal-amygdalar circuits to prefrontal-thalamic circuits for the retrieval of auditory fear conditioning. However, little is known about the time-dependent changes in the originating site, the prefrontal cortex. Here we monitored the responses of prelimbic (PL) prefrontal neurons to conditioned tones at early (2 h) vs. late (4 days) timepoints following training. Using c-Fos, we find that PL neurons projecting to the amygdala are activated early after learning, but not later, whereas PL neurons projecting to the paraventricular thalamus (PVT) show the opposite pattern. Using unit recording, we find that PL neurons in layer V (the origin of projections to amygdala) showed cue-induced excitation at earlier but not later timepoints, whereas PL neurons in Layer VI (the origin of projections to PVT) showed cue-induced inhibition at later, but not earlier, timepoints, along with an increase in spontaneous firing rate. Thus, soon after conditioning, there are conditioned excitatory responses in PL layer V which influence the amygdala. With the passage of time, however, retrieval of fear memories shifts to inhibitory responses in PL layer VI which influence the midline thalamus.

Enhancement of synchronized activity between hippocampal CA1 neurons during initial storage of associative fear memory.

Learning and memory storage requires neuronal plasticity induced in the hippocampus and other related brain areas, and this process is thought to rely on synchronized activity in neural networks. We used paired whole-cell recording in vivo to examine the synchronized activity that was induced in hippocampal CA1 neurons by associative fear learning. We found that both membrane potential synchronization and spike synchronization of CA1 neurons could be transiently enhanced after task learning, as observed on day 1 but not day 5. On day 1 after learning, CA1 neurons showed a decrease in firing threshold and rise times of suprathreshold membrane potential changes as well as an increase in spontaneous firing rates, possibly contributing to the enhancement of spike synchronization. The transient enhancement of CA1 neuronal synchronization may play important roles in the induction of neuronal plasticity for initial storage and consolidation of associative memory. The hippocampus is critical for memory acquisition and consolidation. This function requires activity- and experience-induced neuronal plasticity. It is known that neuronal plasticity is largely dependent on synchronized activity. As has been well characterized, repetitive correlated activity of presynaptic and postsynaptic neurons can lead to long-term modifications at their synapses. Studies on network activity have also suggested that memory processing in the hippocampus may involve learning-induced changes of neuronal synchronization, as observed in vivo between hippocampal CA3 and CA1 networks as well as between the rhinal cortex and the hippocampus. However, further investigation of learning-induced synchronized activity in the hippocampus is needed for a full understanding of hippocampal memory processing. In this study, by performing paired whole-cell recording in vivo on CA1 pyramidal cells (PCs) in anaesthetized adult rats, we examined CA1 neuronal synchronization before and after associative fear learning. We first found in naive animals that there was a low level of membrane potential (MP) synchronization and spike synchronization of CA1 PCs. In conditioned animals, we found a significant enhancement of both MP synchronization and spike synchronization, as observed on day 1 after learning, and this enhancement was transient and not observed on day 5. Accompanying learning-induced synchronized activity was a decreased firing threshold and rise time of suprathreshold MP changes as well as an increased spontaneous firing rate, possibly contributing to the enhanced spike synchronization. The transiently enhanced CA1 neuronal synchronization may have important roles in generating neuronal plasticity for hippocampal storage and consolidation of associative memory traces.

The impact of electrophysiology of central dopaminergic neurons and the depression-state induced by chronic neuropathic pain

To explore the underlying electrophysiological mechanism of depression induced by chronic pain in dopaminergic neurons in midbrain ventral tegmental area (VTA) of rats. Twenty-four healthy adult rats were divided into two groups randomly(n=12):12 rats formed the sham group by exposing the spared nerve, and another 12 rats were served as the spared nerve injury (SNI)group by branchedness sciatic nerve injury surgery. The mechanical allodynia test were detected on the day of 3, 7, 14, 28, 42 and 56 after surgery, and the depressive-like behaviors such as open-field test, sucrose preference and forcedswim test were detected at the same time. Then we used the Multichannel Acquisition Processor (MAP) system to record the firing activity of neurons in VTA in both Sham rats and SNI rats. ①Comparing to sham rats, the paw withdrawalmechanical threshold of SNI rats was decreased significantly (P< 0.01);② According to depression-related behavioral test, SNI rats showed significant difference in open field text, sucrose preference, focus swim text comparing with Sham rats (P< 0.01);③ The firing rate and burst activity of dopaminergic neuronsin midbrain VTA are increased in depression rats compare to sham rats(P<0.05). Chronic pain could induce depression, and the increase of spontaneous firing rate of dopaminergic neurons in midbrain VTA might be contribute to the depression induced by the chronic neuropathic pain.

Amplitude modulation of audible sounds by non-audible sounds: Understanding the effects of wind turbine noise

Our research has suggested a number of mechanisms by which low-frequency noise could bother individuals living near wind turbines: causing endolymphatic hydrops, exciting subconscious pathways, and amplitude modulation of audible sounds. Here we focus on the latter mechanism, amplitude modulation. We measured single-auditory-nerve fiber responses to probe tones at their characteristic frequency in cats. A 50 Hz tone, which did not cause an increase in spontaneous firing rate (i.e., was not audible to the fiber when presented alone) was used to amplitude modulate responses to the probe tone. We found that as probe frequency decreased, a lower level of the low-frequency non-audible tone was needed to achieve criterion amplitude modulation. In other words, low-frequencies that are coded in the cochlear apex require less low-frequency sound pressure level to be amplitude modulated as compared to higher-frequencies that are coded in the cochlear base. This finding was validated, and extended to lower frequencies, by amplitude modulating gross measures of onset-synchronous (compound action potentials) and phase-synchronous (auditory nerve overlapped waveforms) in guinea pigs. Our results suggest that that infrasound generated by wind turbines may cause amplitude modulation of audible sounds, which is often the basis for complaints from those living near wind turbines.

Homeostatic Presynaptic Suppression of Neuronal Network Bursts

Spontaneous synchronized bursts of activity play an essential role in the maturation and plasticity of neuronal networks. To investigate the cellular properties that enable spontaneous network activity, we used dissociated cultures of hippocampal neurons that express prolonged network activity bursts. Acute exposure to a low concentration of N-methyl-d-aspartate (NMDA) caused an increase in spontaneous firing rates and intracellular calcium concentration ([Ca(2+)]i). However, in the course of a chronic (>1 day) exposure to NMDA, [Ca(2+)]i recovered back to normal baseline levels, and only sporadic asynchronous calcium transients were detected. Spontaneous network bursts were still absent 1 h after the removal of NMDA, indicating a persistent downregulation of network activity, which did recover eventually 2 days later. This effect of NMDA was activity-dependent as it was blocked by co-application of tetrodotoxin (TTX). The chronic NMDA-treated neurons expressed normal morphology and active membrane properties as well as spontaneous miniature excitatory postsynaptic currents and postsynaptic reactivity to glutamate. However, in response to trains of afferent stimulation in paired recordings, the treated neurons expressed synaptic depression as opposed to synaptic potentiation seen in control cells. Also, treated neurons did not respond to low-intensity electrical field stimulation as did control cells. Finally, Western blot analysis revealed that chronic exposure to NMDA altered presynaptic but not postsynaptic protein expression patterns, suggesting a presynaptic locus of effect. Thus a long-lasting increase in activity downregulates neurotransmitter release to prevent over-excitation of the network and, consequently, blocks the generation of network bursts.

Directing attention to a location in space results in retinotopic activation in primary visual cortex

It is well-known that directing attention to a location in space enhances the processing efficiency of stimuli presented at that location. Previous studies have shown that directing spatial attention manifests itself as an increase in spontaneous firing rate of neurons (the baseline signal) in extrastriate cortex at the retinotopic corresponding location. There has been considerable debate as to whether this preparatory effect of attention also occurs in human striate cortex (area V1). In the present study, participants had to direct attention to a cued location in space, while changes in blood oxygen level dependent (BOLD) signals were measured. We show that in conditions in which there was no change in sensory stimulation, modulations due to flexibly shifting spatial attention were present throughout early visual cortex (areas V1, V2 and V3). In all early visual areas, the increased BOLD signal in response to the cue was retinotopically specific. The present study shows that voluntary top–down attentional control modulates activity not only in extrastriate but also in striate cortex. This modulation occurs quickly and flexibly in a retinotopic fashion, and serves to facilitate target processing in a continuously changing environment.

Seasonal changes in intrinsic electrophysiological activity of song control neurons in wild song sparrows

Song behavior and its underlying neural substrate can change seasonally in adult songbirds. To test whether environmental cues induce seasonal changes in electrophysiological characteristics of song control neurons, we measured in vitro intrinsic neuronal activity in the song control nucleus RA of adult male song sparrows (Melospiza melodia) in both the fall non-breeding and spring breeding seasons. We found that RA neurons in spring-captured birds show a more than threefold increase in spontaneous firing rate compared to those from fall-captured birds. We conclude that environmental cues are sufficient to induce seasonal changes in electrophysiological properties of song control neurons, and that changes in these properties may underlie seasonal changes in song behavior.

Medial forebrain bundle stimulation evokes endocannabinoid-mediated modulation of ventral tegmental area dopamine neuron firing in vivo

Endocannabinoid-mediated forms of transient synaptic depression have been described in several brain structures, including the dopaminergic ventral tegmental area (VTA). However, their functional and/or behavioural correlates are yet to be determined. The present study was designed to investigate whether back-propagating action potentials in dopamine (DA) neurons, evoked by the stimulation of the medial forebrain bundle (MFB), could trigger endocannabinoid-mediated forms of synaptic modulation. The MFB contains axons ascending from DA neurons to the nucleus accumbens and other forebrain structures, and its stimulation is rewarding because it elicits intra-cranial self-stimulation. Single cell extracellular recordings were carried out from anti-dromically identified VTA DA neurons in chloral hydrate anesthetized rats. DA neurons responded to MFB stimulation (1 s, 20-80 Hz) with a frequency-dependent increase in spontaneous firing rate, which was enhanced by the cannabinoid type-1 receptor antagonist SR141716A (1 mg/kg) and depressed by the agonist WIN55212-2 (0.125 mg/kg). Increasing brain levels of the endocannabinoid anandamide by blocking its major hydrolysing enzyme, fatty-acid amide hydrolase, with URB597 (0.1 mg/kg) was ineffective, whereas blockade of the endocannabinoid membrane transporter with UCM707 (1 mg/kg) enhanced post-stimulus firing rate. Our study indicates that stimulation of the MFB evokes an endocannabinoid-mediated short-term modulation of DA neuron activity. Thus, endocannabinoids might play an important role in the mechanisms underlying the rewarding properties of MFB stimulation.

Cortical tonotopic map reorganization and its implications for treatment of tinnitus

Conclusion: There appears to be a definite link between reorganization of the cortical tonotopic map and increased spontaneous firing rates. The results have implications for the reduction of noise-induced hearing loss and in the prevention of noise-induced tinnitus in humans. Objectives: To review animal and human studies related to neural correlates of tinnitus. Among those are increased spontaneous firing rate, enhanced neural synchrony, and reorganization of the cortical frequency-place (tonotopic) map. Materials and methods: To separate these issues one would want to have a situation where hearing loss is present but without reorganization of the cortical frequency-place map. For that purpose, noise-exposed cats were placed, immediately after the trauma and for at least 3 weeks, either in a quiet or in a high-frequency or low-frequency enriched acoustic environment. Results: In exposed cats that were placed in the quiet environment there was an increase in spontaneous firing rate and synchrony of neurons in primary auditory cortex. In contrast, exposed cats placed in the high-frequency-enriched acoustic environment did not show any significant difference in spontaneous firing rate or synchrony compared to the non-traumatized controls.

Seasonal‐like plasticity of spontaneous firing rate in a songbird pre‐motor nucleus

Many animals exhibit seasonal changes in behavior and its underlying neural substrates. In seasonally breeding songbirds, the brain nuclei that control song learning and production undergo substantial structural changes at the onset of each breeding season, in association with changes in song behavior. These changes are largely mediated by photoperiod-dependent changes in circulating concentrations of gonadal steroid hormones. Little is known, however, about whether changes in the electrophysiological activity of neurons accompany the dramatic morphological changes in the song nuclei. Here we induced seasonal-like changes in the song systems of adult white-crowned sparrows and used extracellular recording in acute brain slices from those individuals to study physiological properties of neurons in the robust nucleus of the arcopallium (RA), a pre-motor nucleus necessary for song production. We report that: RA neurons from birds in breeding condition show a more than twofold increase in spontaneous firing rate compared to those from nonbreeding condition; this change appears to require both androgenic and estrogenic actions; and this change is intrinsic to the RA neurons. Thus, neurons in the song circuit exhibit both morphological and physiological adult seasonal plasticity.

Glucose sensitivity in mouse substantia nigra pars reticulata neurons in vitro

Glucose sensitivity of substantia nigra pars reticulata (SNr) GABAergic neurons was investigated by extracellular recording in acute slice. Approximately two thirds of the GABAergic SNr neurons tested exhibited a significant increase in spontaneous firing rate as the extracellular glucose concentration was lowered from 10 to 4–6 mM. At lower glucose concentrations, a small proportion of these glucose-sensitive GABAergic SNr neurons exhibited multiple, robust increases in spontaneous firing rate with periods ranging in minutes. Similar changes in firing rate of SNr neurons in response to lowered glucose were detected under blockade of GABA A, NMDA, and non-NMDA receptors, indicating that mechanisms other than those mediated by the major synaptic transmissions in the SNr are involved. These findings suggest involvement of previously unknown glucose dependent alterations of GABAergic SNr neuronal activity in the central regulation of glucose homeostasis.

Multiminute oscillations in mouse substantia nigra pars reticulata neurons in vitro

In acute slice of substantia nigra pars reticulata (SNr), a small proportion (6.6%) of GABAergic neurons exhibited abrupt increases in spontaneous firing rate from baseline frequency (∼40 Hz) to peak (>100 Hz) with periods ranging in minutes when GABA A receptors were blocked by 20 μM bicuculline. The combination of GABA B, non-NMDA, and NMDA blockers, SCH50911 (10 μM), 6,7-dinitro-quinoxaline-2,3-dione (20 μM), and dl-2-amino-5-phosphonovalerate (50 μM), respectively, did not affect the incidence or properties of these multiminute oscillations, indicating that disinhibition induced by blockade of GABA A receptors is crucial in their generation. Incidence of oscillatory activity was increased to 16% by elevation of the K + concentration to 8 mM from basal level (6.24 mM). The SNr neurons exhibiting oscillatory activity with the addition of bicuculline had shown irregular fluctuations in basal firing rate, while the non-oscillatory neurons had shown a more regular baseline firing pattern. This is the first in vitro report of oscillations in firing rate of multiminute range in basal ganglia.

Exciting Inhibition

Some neurons exhibit high levels of spontaneous activity, allowing synaptic input to "tune" the system either up or down, that is, to increase or decrease the activity of these spontaneously firing neurons (see Häusser and Monsivais). Nelson et al. examined the plasticity of the spontaneously active neurons of the medial vestibular nucleus (MVN), which is responsible for the vestibulo-ocular reflex that stabilizes images on the retina during head movement by altering the rate of eye movements. In MVN slices, transient activation of inhibitory synaptic input leads to prolonged increase in spontaneous firing rate and in the "gain" of the firing response (slope of the relationship between the input current and the firing rate) once inhibitory input is terminated, a response termed firing rate potentiation (FRP). FRP was also produced in response to application of the inhibitory transmitters γ-aminobutyric acid and glycine, and in response to direct hyperpolarization by current injection. Synaptic inhibition produced a decrease in afterhyperpolarization (AHP), a change in membrane potential mediated by potassium currents; thus, it implicated potassium channels as participating in FRP. Stimuli that produced FRP also decreased intracellular calcium concentration, and transient depletion of calcium by application of a calcium chelator or by transient removal from the extracellular medium also triggered FRP. Inhibition of calcium-activated potassium channels could account for the change in AHP and contribute to the induction of FRP. Pharmacological inhibition of either the SK or BK channels stimulated FRP; however, only when BK channels were inhibited was hyperpolarization of the neurons unable to trigger further FRP. Thus, decreased calcium leading to inhibition of BK channels appears to be central to the inhibition-mediated stimulation of spontaneous activity. A. B. Nelson, C. M. Krispel, C. Sekirnjak, S. du Lac, Long-lasting increases in intrinsic excitability triggered by inhibition. Neuron 40 , 609-620 (2003). [Online Journal] M. Häusser, P. Monsivais, Less means more: Inhibition of spontaneous firing triggers persistent increases in excitability. Neuron 40 , 449-450 (2003). [Online Journal]

Exciting Inhibition

Some neurons exhibit high levels of spontaneous activity, allowing synaptic input to "tune" the system either up or down, that is, to increase or decrease the activity of these spontaneously firing neurons (see Häusser and Monsivais). Nelson et al. examined the plasticity of the spontaneously active neurons of the medial vestibular nucleus (MVN), which is responsible for the vestibulo-ocular reflex that stabilizes images on the retina during head movement by altering the rate of eye movements. In MVN slices, transient activation of inhibitory synaptic input leads to prolonged increase in spontaneous firing rate and in the "gain" of the firing response (slope of the relationship between the input current and the firing rate) once inhibitory input is terminated, a response termed firing rate potentiation (FRP). FRP was also produced in response to application of the inhibitory transmitters γ-aminobutyric acid and glycine, and in response to direct hyperpolarization by current injection. Synaptic inhibition produced a decrease in afterhyperpolarization (AHP), a change in membrane potential mediated by potassium currents; thus, it implicated potassium channels as participating in FRP. Stimuli that produced FRP also decreased intracellular calcium concentration, and transient depletion of calcium by application of a calcium chelator or by transient removal from the extracellular medium also triggered FRP. Inhibition of calcium-activated potassium channels could account for the change in AHP and contribute to the induction of FRP. Pharmacological inhibition of either the SK or BK channels stimulated FRP; however, only when BK channels were inhibited was hyperpolarization of the neurons unable to trigger further FRP. Thus, decreased calcium leading to inhibition of BK channels appears to be central to the inhibition-mediated stimulation of spontaneous activity.A. B. Nelson, C. M. Krispel, C. Sekirnjak, S. du Lac, Long-lasting increases in intrinsic excitability triggered by inhibition. Neuron 40, 609-620 (2003). [Online Journal]M. Häusser, P. Monsivais, Less means more: Inhibition of spontaneous firing triggers persistent increases in excitability. Neuron 40, 449-450 (2003). [Online Journal]

Dendritic synchrony and transient dynamics in a coupled oscillator model of the dopaminergic neuron.

Transient increases in spontaneous firing rate of mesencephalic dopaminergic neurons have been suggested to act as a reward prediction error signal. A mechanism previously proposed involves subthreshold calcium-dependent oscillations in all parts of the neuron. In that mechanism, the natural frequency of oscillation varies with diameter of cell processes, so there is a wide variation of natural frequencies on the cell, but strong voltage coupling enforces a single frequency of oscillation under resting conditions. In previous work, mathematical analysis of a simpler system of oscillators showed that the chain of oscillators could produce transient dynamics in which the frequency of the coupled system increased temporarily, as seen in a biophysical model of the dopaminergic neuron. The transient dynamics was shown to be consequence of a slow drift along an invariant subset of phase space, with rate of drift given by a Lyapunov function. In this paper, we show that the same mathematical structure exists for the full biophysical model, giving physiological meaning to the slow drift and the Lyapunov function, which is shown to describe differences in intracellular calcium concentration in different parts of the cell. The duration of transients was long, being comparable to the time constant of calcium disposition. These results indicate that brief changes in input to the dopaminergic neuron can produce long lasting firing rate transients whose form is determined by intrinsic cell properties.

Orexin a preferentially excites glucose-sensitive neurons in the lateral hypothalamus of the rat in vitro.

Falls in blood glucose induce hunger and initiate feeding. The lateral hypothalamic area (LHA) contains glucose-sensitive neurons (GSNs) and orexin neurons, both of which are stimulated by falling blood glucose and are implicated in hypoglycemia-induced feeding. We combined intracellular electrophysiological recording with fluorescein labeling of GSNs to determine their neuroanatomic and functional relationships with orexin neurons. Orexin A (1 micromol/l) caused a 500% increase (P < 0.01) in spontaneous firing rate and rapid and lasting depolarization that was tetrodotoxin-resistant and thus a direct postsynaptic effect. Orexin A altered the intrinsic neuronal properties of GSNs, consistent with increased excitability. Confocal microscopy showed that GSNs were intimately related to orexin neurons: orexin-immunoreactive axons were frequently entwined around GSN dendrites, establishing close and putatively synaptic contacts. Orexin-cell axons also passed in close proximity to glucose-responsive neurons, which are inhibited by low glucose, but orexin A caused smaller depolarization than on GSNs and only a 200% increase in spontaneous firing rate (P < 0.05 vs. GSN). We conclude that GSNs are specific target neurons for orexin A and suggest that they may mediate, at least in part, the acute appetite-stimulating effect of orexin A. Orexin neurons may regulate GSNs so as to control the onset and termination of hypoglycemia-induced feeding.

Spontaneous and auditory-evoked activity of medial agranular cortex as a function of arousal state in the freely moving rat: interaction with locus coeruleus activity

To characterize the electrophysiological properties of neurons in the medial agranular frontal cortex (Fr2) with respect to arousal level and locus coeruleus (LC) activity, we recorded spontaneous and auditory-evoked single unit activity in these areas simultaneously during different states of arousal in the rat. In the low-arousal state, as determined by EEG, 14/56 Fr2 neurons showed a tonic increase in spontaneous firing rate and 9/56 presented a clear inhibitory response to tone (onset latency 37 ms, duration 200 ms). The inhibitory response was not clear during the high-arousal state. Cross-correlation analysis of pairs of Fr2 and LC units, excluding activity during the actual tone, also showed a negative correlation from 120 ms before, to 170 ms after, Fr2 discharge in 5/63 pairs, only during low arousal. Significantly, 4/5 of the Fr2 neurons having this negative correlation with LC were included in that population which showed a tonic increase in spontaneous firing rate and inhibited to tone during low arousal. LC neurons, on the other hand, all showed excitation to tone stimulation (peak latency 30 ms), and response amplitude was not affected by changes in arousal level. The negative correlation in spontaneous activity, as well as their differential responses to tone, suggests an interaction between a select population of Fr2 neurons and the LC during the low-arousal state. Future studies with lesion or pharmacological manipulations would be necessary to confirm the presence of this interaction.