Differential expression of voltage-gated K+ currents in medial septum/diagonal band complex neurons exhibiting distinct firing phenotypes

The medial septum/diagonal band complex (MSDB) controls hippocampal excitability, rhythms and plastic processes. Medial septal neuronal populations display heterogeneous firing patterns. In addition, some of these populations degenerate during age-related disorders (e.g. cholinergic neurons). Thus, it is particularly important to examine the intrinsic properties of theses neurons in order to create new agents that effectively modulate hippocampal excitability and enhance memory processes. Here, we have examined the properties of voltage-gated, K(+) currents in electrophysiologically-identified neurons. These neurons were taken from young rat brain slices containing the MS/DB complex. Whole-cell, patch recordings of outward currents were obtained from slow firing, fast-spiking, regular-firing and burst-firing neurons. Slow firing neurons showed depolarization-activated K(+) current peaks and densities larger than in other neuronal subtypes. Slow firing total current exhibited an inactivating A-type current component that activates at subthreshold depolarization and was reliably blocked by high concentrations of 4-AP. In addition, slow firing neurons expressed a low-threshold delayed rectifier K(+) current component with slow inactivation and intermediate sensitivity to tetraethylammonium. Fast-spiking neurons exhibited the smaller I(K) and I(A) current densities. Burst and regular firing neurons displayed an intermediate firing phenotype with I(K) and I(A) current densities that were larger than the ones observed in fast-spiking neurons but smaller than the ones observed in slow-firing neurons. In addition, the prevalence of each current differed among electrophysiological groups with slow firing and regular firing neurons expressing mostly I(A) and fast spiking and bursting neurons exhibiting mostly delayer rectifier K(+) currents with only minimal contributions of the I(A). The pharmacological or genetic modulations of these currents constitute an important target for the treatment of age-related disorders.     

Medial septal beta-amyloid 1-40 injections alter septo-hippocampal anatomy and function

Degeneration of septal neurons in Alzheimer's disease (AD) results in abnormal information processing at cortical circuits and consequent brain dysfunction. The septum modulates the activity of hippocampal and cortical circuits and is crucial to the initiation and occurrence of oscillatory activities such as the hippocampal theta rhythm. Previous studies suggest that amyloid beta peptide (Abeta) accumulation may trigger degeneration in AD. This study evaluates the effects of single injections of Abeta 1-40 into the medial septum. Immunohistochemistry revealed a decrease in septal cholinergic (57%) and glutamatergic (53%) neurons in Abeta 1-40 treated tissue. Additionally, glutamatergic terminals were significantly less in Abeta treated tissue. In contrast, septal GABAergic neurons were spared. Unitary recordings from septal neurons and hippocampal field potentials revealed an approximately 50% increase in firing rates of slow firing septal neurons during theta rhythm and large irregular amplitude (LIA) hippocampal activities and a significantly reduced hippocampal theta rhythm power (49%) in Abeta 1-40 treated tissue. Abeta also markedly reduced the proportion of slow firing septal neurons correlated to the hippocampal theta rhythm by 96%. These results confirm that Abeta alters the anatomy and physiology of the medial septum contributing to septo-hippocampal dysfunction. The Abeta induced injury of septal cholinergic and glutamatergic networks may contribute to an altered hippocampal theta rhythm which may underlie the memory loss typically observed in AD patients.     

Electrophysiological properties of neurons recorded intracellularly in slices of the pigeon optic tectum

The electrical properties of pigeon's optic tectum neurons located in the non-retinorecipient region of layer II have been studied in vitro slice preparations by using intracellular recordings. As judged from the somatodendritic characteristics of cells intracellularly labeled with horseradish peroxidase recordings were obtained from pyramidal neurons, the main morphological type, as well as from ganglion cells. When stimulated with depolarizing current pulses of 300-500 ms duration, three distinct modes of firing were observed. Most neurons (Type I) responded with a continuous firing of fast action potentials whose frequency rate increased regularly when current strength was raised. Another group of cells (Type II) also exhibited sustained firing. However, in Type II cells, grouped discharges formed by 2-6 fast action potentials per group fired in rapid succession were elicited within a certain range of current intensity. Finally, another group of cells (Type III) responded at all intensities tested by a short train of fast action potentials only at the onset of the current step. At current strength close to threshold the spike undershoot of type I neurons was followed by a slow hyperpolarizing afterpotential while the spike undershoot of Type II cells was followed by a hump-like depolarization and a slow hyperpolarizing afterpotential. In Type II cells, we have also observed a pronounced increase of the hyperpolarizing afterpotential after a grouped discharge. Type III cells were characterized by a small amplitude and short duration hyperpolarizing afterpotential, barely visible in most of them. In Type I and II cells the slow hyperpolarizing afterpotential was blocked by replacing Ca2+ with Mg2+ or Cd2+ in the saline. These results support the idea that in these two types of neurons the slow hyperpolarizing afterpotential is primarily caused by a Ca2+-dependent K+ conductance. Furthermore, blocking the slow hyperpolarizing afterpotential provoked a pronounced increase of the firing frequency of Type I cells. In Type II cells blockade of the slow hyperpolarizing afterpotential had a greater effect on firing behavior: i.e. when Ca2+ was replaced with Mg2+ or Cd2+, Type II neurons exhibited repetitively fired action potentials at high frequency but were incapable of discharging repetitive grouped discharges. These observations indicate that the Ca2+-dependent K+ conductance involved in the generation of the slow hyperpolarizing afterpotential is the main modulator of the firing behavior of both types of cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Medial septal/diagonal band cells express multiple functional nicotinic receptor subtypes that are correlated with firing frequency

The medial septum-diagonal band (MS/DB) contains primarily cholinergic and GABAergic neurons that project to the hippocampus, and are important for learning and memory. Whole-cell patch clamp methods with brain slices from p11--p20 rats were used to measure MS/DB cell responses to focal somatic application of 1mM acetylcholine (ACh) and a series of current pulses was applied in order to assess firing frequencies and the presence of hyperpolarization-activated currents (Ih). We identified three types of cells: (1) cells with fast inward currents blocked by methyllycaconitine (MLA) with slow firing rates (3--12 Hz), accommodating action potentials, and no Ih (n=20); (2) cells with currents that had both fast (MLA-sensitive) and slow components that were blocked with mecamylamine (MEC) that showed fast firing (up to 60 Hz) and slow firing (up to 3 Hz), with accommodating and non-accommodating action potentials (n=46), 33% of which had Ih; and (3) cells not responsive to ACh with moderate firing rates (10--42 Hz), some with accommodating action potentials and some without (n=19), of which 92% had Ih. These results are among the first to demonstrate functional nicotinic receptors in the MS/DB. The data suggest that these receptors include alpha 7 and non-alpha 7 subtypes and that the expression of each is correlated with firing frequency and the presence of Ih. Responses to ACh were not affected by tetrodotoxin (TTX) and CdC l(2) but were blocked by MLA or MLA and MEC, suggesting that these currents involve direct activation of nicotinic receptors.     

A comparison of extracellular and intracellular recordings from medial septum/diagonal band neurons in vitro.

Firing patterns, action potential characteristics and some active membrane properties of guinea-pig medial septum/diagonal band neurons were studied in an in vitro slice preparation. A comparison was made between several types of cells classified according to either extracellularly recorded (n = 130) or intracellularly recorded (n = 30) electrophysiological characteristics. Using multi-barrel extracellular electrodes, three principal cell types were distinguished: slow rhythmic firing cells (29%), fast rhythmic firing cells (65%) and burst-firing cells (6%). Most slow firing cells could also be distinguished from other cell types by their relatively longer action potential duration and a characteristic cadmium-sensitive "hump" in the repolarization phase of the action potential. These characteristics of slow firing cells matched well with the characteristics of cholinergic, slow afterhyperpolarization cells previously identified with intracellular recordings. The action potential shape, firing rate and firing pattern characteristics of about 60% of extracellularly recorded fast rhythmic firing cells matched those of previously identified non-cholinergic fast afterhyperpolarization cells. The remaining extracellularly recorded, rhythmic firing cells (about 10% of slow firing and 40% of fast firing cells) had a mixture of characteristics which precluded unequivocal classification as to cholinergic or non-cholinergic cell type. Using intracellular recording, the bee venom toxin, apamin, was shown to attenuate the characteristic post spike slow afterhyperpolarization of cholinergic cells and greatly enhanced their firing rate to depolarizing pulses. Apamin often attenuated a smaller and more transient afterhyperpolarization found in identified non-cholinergic cells, but firing rate was increased only slightly. Extracellular recordings from slow and fast rhythmic firing cells in the presence of apamin showed that excitability of slow firing cells was enhanced significantly more than fast firing cells. The apamin data support the hypothesis that extracellularly recorded slow firing cells are cholinergic. We conclude that extracellularly recorded medial septum/diagonal band cells characterized by broad action potentials, slow rhythmic firing under microiontophoresed glutamate and a signature "hump" in the falling phase of the action potential are cholinergic cells. Extracellularly recorded fast rhythmic firing cells with a narrow action potential and no "hump" in the action potential are likely to be non-cholinergic cells. This extracellular electrophysiological "fingerprint" for cholinergic medial septum/diagonal band cells in vitro may now be extended to studies in vivo where controversy remains as to the neurochemical identity of basal forebrain cells involved in control of hippocampal slow rhythmic activity.     

Physiological evidence for subpopulations of cortically projecting basal forebrain neurons in the anesthetized rat

Sixty-three cortically projecting basal forebrain neurons were identified in chloral hydrate anesthetized rats by antidromic activation from the cerebral cortex. Two subpopulations were noted: type I neurons exhibited two antidromic action potentials of constant latency and identical waveform in response to double pulse cortical stimulation. In contrast, type II neurons exhibited two antidromic action potentials of constant latency but differing waveforms in response to the double pulse paradigm. The phenomenon exhibited by type II cortically projecting basal forebrain neurons is interpreted as evidence for loss of the somatodendritic portion of the antidromic action potential with high frequency stimulation. The median latency to antidromic activation of type II neurons (13.5 ms) was significantly longer than that of type I neurons (3.9 ms). Spontaneous firing rates varied over a wide range (0-49 Hz), and there was no significant difference between the rates of type I and type II neurons. These data underscore the physiological heterogeneity of this presumptive cholinergic cortical afferent system. Anatomical studies have shown that most, but possibly not all cortically projecting basal forebrain neurons are cholinergic. The relative proportions of type I (87%) and type II (13%) neurons encountered in this study suggest that type I neurons might be cholinergic and type II neurons non-cholinergic. If substantiated, this hypothesis would permit cholinergic and non-cholinergic cortically projecting basal forebrain neurons to be distinguished using a simple test of antidromicity.     

A distinct group of non-cholinergic neurons along the mid-line of the septum and within the rat medial septal nucleus

The septum is a critical and integral component of the limbic brain that serves as a link between diverse brain structures while being necessary for human cognition and emotionality. A major anatomical component of the septum is designated as the medial septum/diagonal band of Broca complex (MS/DB). A primary focus of much research has been to investigate cholinergic neurons within the MS/DB, as these are the rodent brain's main source of acetylcholine to the cortex and hippocampus. On the other hand, we have chosen to investigate a specific group of neurons that lie on the midline of the MS/DB in an area distinguished anatomically as the medial septal nucleus (MSN). Based on somatic morphology and electrophysiological characteristics we conclude that these neurons, characterized into three different types, are non-cholinergic.     

Intracellular recordings from medial septal neurons during hippocampal theta rhythm

 The electrophysiological properties of neurons of the medial septal nucleus and the nucleus of the diagnonal band of Broca (MS/DB) were studied using intracellular methods in urethane-anesthetized rats. Three types of rhythmically bursting neurons were identified in vivo on the basis of their action potential shapes and durations, afterhyperpolarizations (AHPs), membrane characteristics, firing rates and sensitivities to the action of muscarinic antagonist: (1) Cells with short-duration action potentials and no AHPs (2 of 34 rhythmic cells, 6%) had high firing rates and extremely reliable bursts with 6–16 spikes per theta cycle, which were highly resistant to scopolamine action. (2) Cells with short-duration action potentials and short-duration AHPs (8 of 34 rhythmic cells, 24%) also had high firing rates and reliable bursts with 4–13 spikes per theta cycle, phase-locked to the negative peak of the dentate theta wave. Hyperpolarizing current injection revealed a brief membrane time constant, time-dependent membrane rectification and a burst of firing at the break. Depolarizing current steps produced high-frequency repetitive trains of action potentials without spike frequency adaptation. The action potential and membrane and characteristics of this cell type are consistent with those described for GABAergic septal neurons. Many of these neurons retained their theta-bursting pattern in the presence of muscarinic antagonist. (3) Cells with long-duration action potentials and long-duration AHPs (24 of 34 rhythmic cells, 70%) had low firing rates, and usually only 1–3 spikes per theta cycle, locked mainly to the positive peak of the dentate theta rhythm. Hyperpolarizing current injection revealed a long membrane time constant and a break potential; a depolarizing pulse caused a train of action potentials with pronounced spike frequency adaptation. The action potential and membrane properties of this cell type are consistent with those reported for cholinergic septal neurons. The theta-related rhythmicity of this cell type was abolished by muscarinic antagonists. The phasic inhibition of ”cholinergic” MS/DB neurons by ”GABAergic” MS/DB neurons, followed by a rebound of their firing, is proposed as a mechanism contributing to recruitment of the whole MS/DB neuronal population into the synchronized rhythmic bursting pattern of activity that underlies the occurrence of the hippocampal theta rhythm. Received: 5 February 1996 / Accepted: 6 November 1996     

Electrophysiological characteristics of non-bursting, glutamate decarboxylase messenger RNA-positive neurons of the medial septum/diagonal band nuclei of guinea-pig and rat

Nuclei of the medial septum/diagonal band region of the mammalian forebrain contain neurons that give rise to the septohippocampal pathway, which has separate cholinergic and GABAergic components. This pathway is known to influence hippocampal-dependent memory and learning processes, but the precise role of each component is unclear. In this study, we tested the hypothesis that fast-firing, non-bursting medial septum/diagonal band neurons are GABAergic. We used brain slice preparations from young adult guinea-pigs and rats, or from weanling rats, to perform current-clamp recordings from medial septum/diagonal band neurons. Recorded neurons were injected with biocytin for subsequent visualization with fluorescent avidin, and then hybridized with a 35S-labeled riboprobe for glutamate decarboxylase-67 messenger RNA. As a positive control, guinea-pig cerebellar Purkinje cells were labeled and hybridized with the riboprobe. As expected, labeled Purkinje cells were glutamate decarboxylase-67 messenger RNA positive. Slow-firing, cholinergic (choline acetyltransferase-positive) guinea-pig medial septum/diagonal band neurons were glutamate decarboxylase-67 messenger RNA negative. Contrary to our hypothesis, of the guinea-pig neurons, only three of 11 fast-firing neurons were glutamate decarboxylase-67 positive. Of the rat medial septum/diagonal band neurons, three of four were positive for glutamate decarboxylase-67 messenger RNA.These data suggest that fast-firing, non-bursting neurons of the medial septum/diagonal band, as sampled by sharp-electrode intracellular recordings in brain slices, may be a heterogeneous group of neurons, some of which are GABAergic. Together with recent data demonstrating the presence of another GABAergic marker, parvalbumin, in fast-firing septal neurons, we conclude that GABAergic septohippocampal neurons include a population of fast-firing, non-bursting neurons. The influence of these neurons on the hippocampus is likely to occur on a shorter time-scale and over a wider range of firing frequencies as compared to slowly firing cholinergic septohippocampal neurons.     

Membrane properties of mesopontine cholinergic neurons studied with the whole-cell patch-clamp technique: implications for behavioral state control.

1. The whole-cell patch-clamp technique was used to study the membrane properties of identified cholinergic and noncholinergic laterodorsal tegmental neurons in slices of rat brain maintained in vitro. 2. On the basis of their expression of the transient outward potassium current IA and the transient inward calcium current IT, three classes of neurons were observed: type I neurons exhibited a large IT; type II neurons exhibited a prominent IA; and type III neurons exhibited both IA and IT. 3. Combining intracellular deposition of biocytin with NADPH diaphorase histochemistry revealed that the vast majority of type III neurons were cholinergic, whereas only a minority of type I and type II neurons were cholinergic. Thus mesopontine cholinergic neurons possess intrinsic ionic currents capable of inducing burst firing. 4. Delineation of the intrinsic membrane properties of identified mesopontine cholinergic neurons, in concert with recent results regarding the responses of these neurons to neurotransmitter agents, has led us to present a unifying and mechanistic hypothesis of brain stem cholinergic function in the control of behavioral states.     

Electrophysiological evidence for cholinoceptive neurons in the medial vestibular nucleus: studies on rat brain stem in vitro

Electrophysiological studies were performed to determine whether or not cholinoceptive neurons are present in the rat medial vestibular nucleus (MVN) using brainstem slice preparations. Fifty-three MVN neurons, whose activities were extracellularly recorded, fired spikes spontaneously and regularly with an interspike interval of 180 +/- 27 ms (mean +/- S.E.M.) and a coefficient of variation of 0.11 +/- 0.02. Intracellularly recorded neurons also exhibited similar spontaneous and regular generation of action potentials. Carbachol dose-dependently increased the spontaneous firing, although the firing rate was decreased in a few neurons. The addition of atropine reduced the firing rate, and dose-dependently attenuated the carbachol-induced excitation of the neurons. In a low Ca2+ and high Mg2+ medium, carbachol also increased the firing rate. These results indicate that the MVN contains neurons with spontaneous and regular firing, and that the excitability of these neurons is regulated by a cholinergic muscarinic mechanism.     

Septal GABAergic neurons are selectively vulnerable to pilocarpine-induced status epilepticus and chronic spontaneous seizures

The septal region of the basal forebrain plays a critical role modulating hippocampal excitability and functional states. Septal circuits may also play a role in controlling abnormal hippocampal hyperexcitability in epilepsy. Both lateral and medial septal neurons are targets of hippocampal axons. Since the hippocampus is an important epileptogenic area in temporal lobe epilepsy, we hypothesize that excessive excitatory output will promote sustained neurodegeneration of septal region neurons. Pilocarpine-induced status epilepticus (SE) was chosen as a model to generate chronic epileptic animals. To determine whether septal neuronal populations are affected by hippocampal seizures, immunohistochemical assays were performed in brain sections obtained from age-matched control, latent period (7 days post-SE) and chronically epileptic (more than one month post-SE survival) rats. An anti-NeuN (neuronal nuclei) antibody was used to study total neuronal numbers. Anti-ChAT (choline acetyltransferase), anti-GAD (glutamic acid decarboxylase) isoenzymes (65 and 67), and anti-glutamate antibodies were used to reveal cholinergic, GABAergic and glutamatergic neurons, respectively. Our results revealed a significant atrophy of medial and lateral septal areas in all chronically epileptic rats. Overall neuronal density in the septum (medial and lateral septum), assessed by NeuN immunoreactivity, was significantly reduced by approximately 40% in chronically epileptic rats. The lessening of neuronal numbers in both regions was mainly due to the loss of GABAergic neurons (80-97% reduction in medial and lateral septum). In contrast, populations of cholinergic and glutamatergic neurons were spared. Overall, these data indicate that septal GABAergic neurons are selectively vulnerable to hippocampal hyperexcitability, and suggest that the processing of information in septohippocampal networks may be altered in chronic epilepsy.     

成年大鼠基底前脑存在一个Nestin免疫阳性神经元簇

目的　观察Nestin免疫活性在成年大鼠基底前脑中的表达和分布 ,并探讨其与胆碱能神经元之间的关系。方法　采用免疫组化方法对成年大鼠基底前脑的切片进行nestin免疫组化染色及其与ChAT ,NADPH d双标染色。结果　在成年大鼠基底前脑的隔斜角带复合体有一个连续的nestin免疫阳性细胞带 ,胞体较大 ,梭形或多极形 ,有 2～ 4个突起。双标染色显示 ,Nestin阳性神经元与ChAT ,NADPH d阳性神经元间杂分布 ,大多数不呈交叉反应 ,只有少数 ,约 10 %呈双标染色。结论　成年大鼠基底前脑存在一个有别于胆碱能神经元的nestin免疫阳性神经元簇 ,其化学属性和生物学意义有待进一步研究。     

Modulation of theta rhythmicity in the medial septal neurons and the hippocampal electroencephalogram in the awake rabbit via actions at noradrenergic alpha2-receptors

The modulation of the firing discharge of medial septal neurons and of the hippocampal electroencephalogram (EEG) mediated by actions on alpha2-adrenoreceptors (ARs) was investigated in awake rabbits. Bilateral i.c.v. infusion of a relatively low dose (0.5 microg) of the alpha2-AR agonist clonidine produced a reduction in the theta rhythmicity of both medial septal neurons and the hippocampal EEG. In contrast, a high dose of clonidine (5 microg) increased the percentage and degree of rhythmicity of theta bursting medial septal neurons as well as the theta power of the hippocampal EEG. On the other hand, administration of alpha2-AR antagonist idazoxan produced the opposite dose-dependent effect. While a low dose of the antagonist (20 microg) produced an increase in both the theta rhythmicity of medial septal neurons and the theta power of the hippocampal EEG, a high dose (100 microg) caused a reduction of theta rhythmicity in both the medial septum and hippocampus. These results suggest that low doses of alpha2-ARs agents may act at autoreceptors regulating the synaptic release of noradrenaline, while high doses of alpha2-ARs drugs may have a predominant postsynaptic action. Similar results were observed after local injection of the alpha2-AR drugs into the medial septum suggesting that the effects induced by the i.c.v. infusion were primarily mediated at the medial septal level. We suggest that noradrenergic transmission via the postsynaptic alpha2-ARs produces fast and strong activation of the septohippocampal system in situations that require urgent selective attention to functionally significant information (alert, aware), whereas the action via the presynaptic alpha2-ARs allows a quick return of the activity to the initial level.     

The hippocampal control of neuronal discharges in the septum of the rat

1. Records of field potentials and of the firing patterns of single neurones evoked by stimulation of the fimbria and fornix have been obtained from the septal nuclei of the rat.2. Fimbrial stimulation caused orthodromic activation of lateral septal neurones which was followed by a lengthy inhibition. In the medial septum such stimulation could elicit an antidromic response; but inhibition whose duration depended upon whether the neurones were firing irregularly or in synchronized bursts was obtained without prior activation.3. Stimulation of the medial septum evoked an antidromic response in the lateral septal neurones, which was followed by inhibition.4. For all of these inhibitory phenomena, bursts of action potentials of small amplitude and correlated with the start of the inhibitory periods were observed, and are believed to indicate the firing of inhibitory interneurones.5. Stimulation of the fornix caused excitation of medial septal neurones but was without effect on those in the lateral septum.6. A scheme is proposed in which the direct inhibition of medial septal neurones from the fimbria is suggested to act as a ;reset' mechanism, while the phasic input from lateral septum resulting from the recurrent inhibitory pathways regulates the frequency of bursting in the medial septum.     

Arousal-dependent properties of medial septal neurons in the unanesthetized rat.

We have performed a qualitative and quantitative analysis of the electrophysiological properties of medial septal neurons in the unanesthetized rat. The rat's head was held in a stereotaxic apparatus by a painless head-restrained system that was implanted seven days prior to the recording sessions. Extracellular recordings were made in a mixed population of antidromically identified septohippocampal neurons and unidentified medial septal neurons in different states of arousal and in response to peripheral and reticular stimulations. The spontaneous activity as well as the percentage of rhythmically bursting septal neurons varied significantly according to the state of arousal. Higher values were noted in paradoxical sleep (28 imp/s and 94% of bursting neurons) as compared with wakefulness with hippocampal theta rhythm (17.4 imp/s and 64.2% of bursting neurons) and slow wave sleep (12.3 imp/s and 8% of bursting neurons). The frequency of the bursts was significantly higher during paradoxical sleep. In individual medial septal neurons, arousing stimuli and paradoxical sleep could induce rhythmic bursting activity in previously non-bursting neurons provided that they were fast-firing neurons. No differences were noted in the functional characteristics of neurons in the medial septal nucleus as compared with the diagonal band of Broca. When the unanesthetized rats were compared with a group of urethane-anesthetized rats, the spontaneous activity was higher and more irregular in the absence of anesthesia. The percentage of the bursting neurons was significantly lower in the unanesthetized rats (32.3% vs 43.3%). However, the frequency of the bursts was higher (5.9 +/- 0.1 Hz vs 3.5 +/- 0.1 Hz). Since the patterns of activity of medial septal neurons fluctuate in different physiologically relevant states, previous classifications of these neurons made by ourselves and other authors, in urethane-anesthetized rats, may not be appropriate.     

Functional organization of premotor neurons in the cat medial vestibular nucleus related to slow and fast phases of nystagmus

Summary Extracellular spikes were recorded from secondary vestibular neurons in the cat medial vestibular nucleus (MVN) and were identified as type I or II neurons by horizontal rotation. Type I neurons were further classified as excitatory or inhibitory premotor neurons on the basis of their axonal termination in the contralateral or ipsilateral abducens nucleus, demonstrated by spike-triggered averaging of abducens nerve discharges, or by antidromic activation using systematic microstimulation within the abducens nucleus.Both excitatory and inhibitory premotor type I MVN neurons exhibited a rhythmic modulation of their firing rate in association with nystagmus elicited by rotation or electrical stimulation of the vestibular nerve. Their tonic activity during the slow phase was suppressed at the quick phase directed to the ipsilateral side.Excitatory type I MVN neurons terminating in the contralateral abducens nucleus sent collateral axons to the contralateral MVN. These commissural neurons also showed a nystagmus-related discharge pattern.Type II MVN neurons activated at short latency by stimulation of the contralateral vestibular nerve exhibited burst discharges when the activity of ipsilateral type I neurons was suppressed at the quick phase. These type II neurons made monosynaptic inhibitory connections with type I neurons as shown by the post-spike average of the membrane potential of secondary MVN neurons triggered from spikes of single type II neurons. Thus, the inhibitory action originating from burst activity of type II MVN neurons contributes to suppression of type I premotor MVN neurons during fast eye movements.Supported by Grant-in-Aid for Scientific Research No. 248106 from Japan Ministry of Education, Science and Culture. Dr. Schor was supported by the Research Fellowship of Japan Society for the Promotion of Science (JSPS)     

Firing activity and postsynaptic properties of morphologically identified neurons of ventral oral pontine reticular nucleus

The ventral part of the oral pontine reticular nucleus (vRPO) is an important region for the generation and maintenance of REM sleep. Firing activity and synaptic response properties of morphologically identified vRPO neurons have been investigated in urethane-anaesthetized cats. Extracellular recordings were performed through recording micropipettes and neurons were extracellularly stained with biocytin. Two types of neurons were identified under spontaneous conditions: type I neurons (77%) are characterized by non-rhythmic firing; type II neurons (23%) display single spikes firing rhythmically at between 7 and 22 Hz. Type I neurons displayed ellipsoid somata with thick dendritic trunks and axons that arose from either the soma or the initial dendritic segment; these axons could not be clearly followed. Type II neurons showed polygonal somata with radial dendrites; their axons branched at a small distance from the soma. Electrical stimulation of the contralateral vRPO elicited responses in both neuron types (57% and 31%, respectively); this effect was blocked by the non-NMDA glutamatergic receptor antagonist CNQX. Electrical stimulation of the PpT evoked orthodromic responses in type I neurons (41%) and inhibited the firing rate of all type II neurons for 50-100 ms. Both effects were blocked by the muscarinic receptor antagonist atropine. The cholinergic agonist, carbachol, increased the firing rate in most type I neurons and inhibited most type II neurons in these animals.The results demonstrated that the activity of vRPO neurons is modulated through the postsynaptic activation exerted by extrinsic afferents on cholinergic and glutamatergic receptors.     

Frequency-related inhibitory mechanisms controlling rhythmical activity in the septal area.

1. The response patterns of identified neurones in the medical and lateral septal regions to varying rates of repetitive stimulation of the fimbria were investigated in rats anaesthetized with urethane. 2. Neurones in the lateral septum which characteristically respond to single pulse stimulation of the fimbria with an activation-inhibition sequence, exhibited a reduction or complete elimination of the inhibitory component both during and following tetanic volleys delivered at 7-12 HZ. Stimulation at lower frequencies did not alter the response. 3. Concurrently with this effect on the inhibitory component of the response exhibited by lateral septal cells, repetitive volleys eliminate the small amplitude burst discharges which are correlated with the onset of the inhibitory period and are considered to indicate the firing of inhibitory interneurones. 4. Tetanic stimulation of the fimbria at rates which eliminate this interneuronal response in the lateral septum, produce an irregular pattern of firing in medial septal neurones which previously exhibited a synchronized bursting discharge to single pulses. 5. Ipsilateral section of the fimbrial input to the septum resulted in the elimination of the burst discharge pattern exhibited by medial septal neurones. 6. The results suggest that a frequency gating mechanism in the lateral septum, activation of which is dependent upon the level of hippocampal output, is responsible for controlling the firing pattern of medical septal neurones.     

Effects of gonadectomy on immunoreactivity for choline acetyltransferase in the cortex, hippocampus, and basal forebrain of adult male rats

Androgens are known to affect cognitive and mnemonic aspects of spatial processing. The cholinergic system is thought to play an important role in cognition and memory, but little is known about the interaction between androgen and cholinergic neurons. The present study focused on the effects of testosterone on the cholinergic neurons in the anterior cingulate cortex, the posterior parietal cortex, the hippocampus, and the basal forebrain including the medial septum, i.e., regions related to spatial processing. We examined choline acetyltransferase (ChAT) immunoreactivity in three groups of adult male rats: sham-operated (Sham), 28-day gonadectomized (GDX), and 28-day gonadectomized with immediate implantation of testosterone propionate (GDX+TP). Comparison of the Sham and GDX+TP groups demonstrated that the GDX group had significantly decreased cell counts of ChAT-immunoreactive neurons in anterior cingulate cortex layer II/III, posterior parietal cortex layer II/III, and the medial septum, but not in the other basal forebrain subregions examined (the horizontal part of the diagonal band of Broca and the substantia innominata). The GDX group also had significantly reduced hippocampal ChAT-immunoreactive fiber pixel density. The GDX+TP group maintained ChAT-immunoreactive cell counts in the anterior cingulate cortex, posterior parietal cortex, and medial septum equivalent to those in the Sham group. Less than 1% of identified cells showed colocalization of immunoreactivity for ChAT and androgen receptor in the cell bodies of the cortex and basal forebrain.Our observations demonstrate that the presence or absence of testosterone for 4 weeks influenced the cholinergic population region-specifically in the adult rat brain.