N-methyl-D-aspartate-evoked release of acetylcholine from the medial septum/diagonal band of rat brain

N-Methyl-D-aspartate (NMDA)-evoked release of [3H]acetylcholine (ACh) from slices of rat brain medial septum/vertical limb of the diagonal band (ms/vdB) was examined. NMDA increased the release of tritium in a concentration-dependent manner and the specific non-competitive NMDA antagonist, MK-801, and the competitive NMDA antagonist kynurenic acid inhibited this release. Tetrodotoxin inhibited the NMDA-evoked release suggesting the [3H]ACh released arises from collaterals of the cholinergic septohippocampal neurons. Basal release of tritium was significantly increased by glycine alone and strychnine inhibited this response while having no effect on NMDA-evoked release. However, glycine, although not affecting the NMDA-evoked release, did enhance release of tritium in the presence of NMDA and blocking concentrations of Kynurenic acid. Together, these findings suggest that under the conditions of these experiments sufficient concentrations of glycine permit the full expression of NMDA-evoked modulation of [3H]ACh release, and that the predominant actions of glycine were mediated by a specific, strychnine-sensitive receptor.     

GABAB receptors in the medial septum/diagonal band slice from 16-25 day rat

GABA(B) receptors are believed to play a role in rhythmic activity in the mammalian brain. The aim of our study was to examine the presynaptic and postsynaptic locations of these receptors in the medial septal diagonal band area (MS/DB), an area known to pace the hippocampus theta rhythm. Whole-cell patch recordings were made from parasagittal MS/DB slices obtained from the 16-25 day rat. Neurons were classified into GABAergic and cholinergic subtypes according to previous electrophysiological criteria. Bath application of the GABA(B) receptor agonist baclofen in the presence of tetrodotoxin, and brief tetanic fiber stimulation in the presence of ionotropic receptor antagonists, provided evidence for the presence of postsynaptic GABA(B) receptor transmission to GABAergic but not cholinergic neurons. Bath application of baclofen, at concentrations too low to elicit postsynaptic activity in MS/DB neurons, significantly reduced the amplitudes of stimulus-evoked ionotropic receptor inhibitory postsynaptic potentials (IPSPs) and excitatory postsynaptic potentials (EPSPs) and the paired pulse depression of these evoked potentials. Baclofen also significantly reduced the frequencies but not the amplitudes of miniature inhibitory postsynaptic currents (IPSCs) and excitatory postsynaptic currents (EPSCs), indicating the presence of presynaptic GABA(B) receptors on GABAergic and glutamatergic terminals in the MS/DB. Baclofen, also at a concentration too low to elicit postsynaptic activity, reduced the frequencies and amplitudes of spontaneous IPSCs and EPSCs recorded in the presence of 200-400 nM kainate. Rhythmic compound IPSCs at theta frequencies were recorded under these conditions in some neurons, and these rhythmic compound IPSCs were disrupted by the activation but not by the inhibition of GABA(B) receptors. These results suggest that GABA(B) receptors modulate rather than generate rhythmic activity in the MS/DB, and that this modulatory effect occurs via receptors located on presynaptic terminals.     

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.     

Parvalbumin-immunoreactive, fast-spiking neurons in the medial septum/diagonal band complex of the rat: intracellular recordings in vitro.

The medial septum/diagonal band complex is composed predominantly of cholinergic and GABAergic neurons, and it projects to the hippocampal formation. A proportion of the GABAergic neurons contain parvalbumin, a calcium-binding protein that has previously been localized in fast-spiking, non-accommodating GABAergic neurons in the cerebral cortex and neostriatum. The aim of the present study was to determine whether parvalbumin is localized preferentially in a similar electrophysiological class of neuron in the medial septum/diagonal band complex. The study was carried out using in vitro intracellular recording, intracellular biocytin filling and parvalbumin immunocytochemistry. Three main classes of neurons were identified according to standard criteria: burst-firing, slow-firing and fast-firing neuronal populations. The fast-firing neurons were subdivided into two subpopulations based on whether or not they displayed accommodation. The fast-spiking, non-accommodating cells were furthermore found to be spontaneously active at resting potentials, and to possess action potentials of significantly (P < 0.05) shorter duration (half width: 0.61 +/- 0.12 ms) than those of the regular-spiking, accommodating neurons (1.0 +/- 0.34 ms). Of the neurons that were successfully filled with biocytin and processed for parvalbumin immunoreactivity, 82% of the fast-spiking, non-accommodating cells possessed parvalbumin immunoreactivity, while none of the regular-spiking, accommodating neurons were found to be immunoreactive for parvalbumin. The slow-firing neurons, shown previously to be cholinergic, did not stain for parvalbumin immunoreactivity, in agreement with studies showing parvalbumin to be localized solely in GABAergic neurons in the medial septum/diagonal band complex. In conclusion, these findings suggest the presence of a previously uncharacterized population of neurons in the medial septum/diagonal band complex that generate high-frequency, non-adaptive discharge. This property correlates with the localization of parvalbumin in these neurons, which suggests that parvalbumin fulfils the same role in the medial septum/diagonal band complex that it does in other parts of the brain. The fast-spiking neurons in the medial septum/diagonal band complex may play an essential role in the GABAergic influence of the septum on the hippocampal formation.     

Muscarinic agonists block a late-afterhyperpolarization in medial septum/diagonal band neurons in vitro.

Intracellular recordings were made from neurons located in the medial septum (MS), and nucleus of the diagonal band (nDB) from slices of guinea pig brain. These forebrain nuclei contain both cholinergic and noncholinergic neurons that project to the cortex and hippocampus and are involved in many cortical functions. Muscarinic agonists (bethanechol, 2-30 microM) had the specific action to reduce a long-duration afterhyperpolarization (long-AHP) while leaving other shorter duration AHPs intact. Since the long-AHP was observed in both cholinergic and non-cholinergic neurons, muscarinic agonists were not selective for any one cell type. Block of a long-AHP was not associated with a consistent increase in cell excitability and therefore can not fully explain the excitatory actions of acetylcholine (ACh) observed in vivo within the MS/nDB.     

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.     

5-HT1A receptor mRNA and immunoreactivity in the rat medial septum/diagonal band of Broca-relationships to GABAergic and cholinergic neurons

Activation of 5-HT1A receptors results in a variety of physiological responses, depending on their localization on neurons with different phenotypes in the brain. This study investigated the localization of 5-HT1A receptor mRNA and 5-HT1A receptor immunoreactivity in cell bodies of the rat septal complex using in situ hybridization and immunohistochemistry. In adjacent sections of the medial septum/diagonal band of Broca (MSDB), the distribution of cell bodies expressing 5-HT1A receptor mRNA was closely related to cells labeled with oligonucleotide probes to GAD (glutamic acid decarboxylase), VAChT (vesicular acetylcholine transporter) or parvalbumin mRNA. Using antiserum to GAD and antibodies to GABA, 5-HT1A receptor immunoreactivity was demonstrated in a majority of GABAergic cells in the MSDB. 5-HT1A receptor-immunoreactive GABAergic cells in the MSDB were also demonstrated to contain the calcium-binding protein parvalbumin, a marker for septohippocampal projecting GABAergic neurons. In the lateral septum, 5-HT1A receptor immunoreactivity was colocalized with the calcium-binding protein calbindin D-28k, a marker for septal GABAergic somatospiny neurons. 5-HT1A receptor immunoreactivity was also detected in a subpopulation of VAChT-containing cholinergic neurons of the MSDB. In MSDB neurons, colocalization of 5-HT1A and 5-HT2A receptor immunoreactivities was demonstrated. These observations suggest that serotonin via 5-HT1A receptors may represent an important modulator of hippocampal transmission important for cognitive and emotional functions through actions on both GABAergic and cholinergic neurons of the rat septal complex. In addition, 5-HT may exert its effects in the MSDB via cells expressing both 5-HT1A and 5-HT2A receptors.     

Distribution and role of Kv3.1b in neurons in the medial septum diagonal band complex

The medial septum diagonal band complex (MS/DB) projects via cholinergic and GABAergic pathways to the hippocampus and plays a key role in the hippocampal theta rhythm. In the MS/DB we have previously described a population of fast spiking GABAergic neurons that contain parvalbumin and mediate theta frequency activity in vitro. The Kv3.1 potassium channel is a delayed rectifier channel that plays a major role in fast spiking neurons in the CNS, and has previously been localized in the MS/DB. To determine which cell types in the MS/DB express the Kv3.1b ion channel subunit, transgenic mice in which the expression of GABAergic and glutamate markers are associated with the expression of green fluorescent protein (GFP; GAD67-GFP and VGluT2-GFP mice, respectively) were used for immunofluorescence and axonal tract tracing. Electrophysiological studies were also carried out on rat MS/DB slices to examine the role of the Kv3.1 channel in theta frequency oscillations. The results for the MS/DB were as follows: (1) cholinergic cells did not express GFP in either GAD67-GFP or VGluT2-GFP mice, and there was GAD67 immunoreactivity in GFP-positive neurons in GAD67-GFP mice and in a small proportion (6%) of GFP-positive neurons in VGluT2-GFP mice. (2) Kv3.1b immunofluorescence was associated with the somata of GABAergic neurons, especially those that contained parvalbumin, and with a minority of glutamatergic neurons, but not with cholinergic neurons, and with GABAergic axonal terminal-like processes around certain GABAergic neurons. (3) Both Kv3.1b-positive and -negative GABAergic neurons were septo-hippocampal, and there was a minor projection to hippocampus from VGluT2-GFP neurons. (4) Kainate-induced theta oscillations in the MS/DB slice were potentiated rather than inhibited by the Kv3.1 blocker 4-aminopyridine, and this agent on its own produced theta frequency oscillations in MS/DB slices that were reduced by ionotropic glutamate and GABA receptor antagonists and abolished by low extracellular calcium. These studies confirm the presence of heterogeneous populations of septo-hippocampal neurons in the MS/DB, and suggest that presence of Kv3.1 in the GABAergic neurons does not contribute to theta activity through fast spiking properties, but possibly by the regulation of transmitter release from axonal terminals.     

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.     

Retrograde cell changes in medial septum and diagonal band following fimbria-fornix transection: quantitative temporal analysis

Complete unilateral fimbria-fornix transections, including the overlying cingulate cortex, were administered to female rats. At time points from 1 day to 6 weeks, the septal-diagonal band region was examined using acetylcholinesterase histochemistry, Cresyl Violet cell staining, and choline acetyltransferase biochemistry. As early as 1 day following the transection a decrease in acetylcholinesterase positive cell body staining was observed in the medial septum; however, no loss of Nissl-stained neurons was measured in Cresyl Violet stained sections until 1 week after the lesion. Maximal loss of acetylcholinesterase-positive cells, as visualized after irreversible acetylcholinesterase inhibition, was measured at 1 week, and no further change was observed at time points up to 6 weeks after operation. The loss of acetyltransferase-positive cells was greatest in the medial septal area (-65%) and the vertical limb of the diagonal band (-55%). Little cell loss was measured in the horizontal limb of the diagonal band. This is consistent with the known projections of these cell bodies. Remaining acetylcholinesterase-positive cell bodies in the medial septum had shrunk by about 20% (measured as the diameter along the major axis). A marked neuronal cell loss (about 50%) was demonstrable in the medial septum and vertical limb of the diagonal band in the Cresyl Violet-stained sections, too. A pile-up of acetylcholinesterase-stained material was observed in the dorsal-lateral quadrant of the septal area just proximal to the lesion at 1 day following transection. This pile-up occurred in the medial septum and diagonal band area up to 1 week following the transection, and had nearly disappeared by 2 weeks post-transection. Choline acetyltransferase biochemical activity, measured in samples of whole septum, decreased significantly at 1 day but subsequently returned to control levels. By 2 weeks following transection, an increase in acetylcholinesterase-positive stained fibers was observed in the dorsal-lateral quadrant of the septum, ipsilateral to the lesion relative to the contralateral septum. This response, which was interpreted as sprouting from the lesioned axons proximal to the transection, probably accounted for the rise in choline acetyltransferase biochemical activity in the whole septum following the reduction on the first day.     

Projections from the medial septum and diagonal band of Broca to the dorsal and central superior raphe nuclei: a non-cholinergic pathway

Summary The anatomical organization of projections from the medial septal nucleus (MS), and the vertical (VDB) and horizontal limb (HDB) of the diagonal band of Broca to the dorsal raphe nucleus (NRD) and the central superior raphe nucleus (RCS) of the rat were studied by anterograde [³H]-leucine, and True Blue and Fluoro Gold fluorescent retrograde tracing. Projections from the MS were found to enter the basal mesencephalon at the rostro-medial aspect of the pontine nuclei, curve dorsally and terminate throughout the RCS and in the caudal portion of the NRD. Fibers from the VDB were found to enter these raphe nuclei by two separate routes; some fibers reached the basal mesencephalon, curved dorsally and terminated in the RCS and NRD. Other fibers entered the pedunculopontine nucleus, curved medially and reached the NRD. Presumed terminal labelling was found overlaying the RCS and NRD throughout their rostro-caudal extensions. The brain stem projections from HDB entered the mesencephalon by the same routes as those from VDB, but the labelling over RCS was sparse, and the NRD labelling was preferentially distributed to the rostral portion of the nucleus. The present data indicate a crude topographic organization of the projections from the septal region to the NRD and RCS. In general, the distribution of presumed terminal labelling appeared to be more closely associated with the distribution of NRD and RCS 5-HT immunoreactive cell bodies, than with the cytoarchitectonically defined extensions of these raphe nuclei. By sequential evaluation of the distribution of retrogradely labelled and acetylcholine esterase-stained cells on the same section, and by selective tracing with radiolabelled choline, it appears that the vast majority, if not all, of the neurons in MS and diagonal band which project to the rostral raphe are non-cholinergic.Abbreviations CLi caudal linear raphe nucleus - DTg dorsal tegmental nucleus - flm medial longitudinal fasciculus - HDB horizontal limb of the diagonal band of Broca - IP interpeduncular nucleus - MS medial septal nucleus - NRD dorsal raphe nucleus - PAG periaqueductal gray - Pn pontine nucleus - R red nucleus - RCS central superior raphe nucleus - RMg raphe magnus nucleus - RPn pontine raphe nucleus - SN substantia nigra - VDB vertical limb of the diagonal band of Broca - VTg ventral tegmental nucleus - 5 trigeminal motor nucleus     

淋巴滞留脑病模型大鼠内侧隔核-斜角带垂直支一氧化氮合酶神经元的改变

目的 :探讨淋巴滞瘤脑病模型大鼠内侧隔核 斜角带垂直支 (Septummedialis verticaldiagonalbandcomplex ,SM vDB)神经元一氧化氮合酶 (Nitricoxidesynthase,NOS)表达与空间参考记忆变化的相关性。方法 :用免疫细胞化学方法显示记忆获得后及淋巴滞留脑病期间 ,大鼠SM vDB的NOS阳性神经元 ,并进行定量分析。结果 :(1 )训练组阳性神经元数比对照组分别增加 96.91 % ,62 .96% (P <0 .0 1 ) ,与逃避潜伏期呈明显负相关 (SMr=-0 .7646,P <0 .0 1 ;vDBr=-0 .81 5 2 ,P <0 .0 1 ) ;细胞面积分别增加 42 .5 1 % ,3 5 .0 0 % (P <0 .0 1 ) ;免疫反应灰度值分别增加1 3 0 .5 1 % ,1 1 5 .41 % (P <0 .0 1 )。 (2 )脑病模型组与假手术组相比 ,阳性神经元数量分别减少 2 5 .91 % ,2 0 .1 3 % (P <0 .0 1 ) ,与逃避潜伏期呈明显负相关 (SMr=-0 .65 3 8,P <0 .0 1 ;vDBr=-0 .72 42 ,P <0 .0 1 ) ;细胞面积分别减少1 4.92 % ,1 2 .97% (P <0 .0 1 ) ,免疫反应灰度值分别减少 3 3 .3 4% ,41 .5 4% (P <0 .0 1 )。结论 :淋巴滞留性脑病神经元NOS表达减少 ,可能损害空间参考记忆的保持     

Infusion of GAT1-saporin into the medial septum/vertical limb of the diagonal band disrupts self-movement cue processing and spares mnemonic function

Degeneration of the septohippocampal system is associated with the progression of Dementia of the Alzheimer’s type (DAT). Impairments in mnemonic function and spatial orientation become more severe as DAT progresses. Although evidence supports a role for cholinergic function in these impairments, relatively few studies have examined the contribution of the septohippocampal GABAergic component to mnemonic function or spatial orientation. The current study uses the rat food-hoarding paradigm and water maze tasks to characterize the mnemonic and spatial impairments associated with infusing GAT1-Saporin into the medial septum/vertical limb of the diagonal band (MS/VDB). Although infusion of GAT1-Saporin significantly reduced parvalbumin-positive cells in the MS/VDB, no reductions in markers of cholinergic function were observed in the hippocampus. In general, performance was spared during spatial tasks that provided access to environmental cues. In contrast, GAT1-Saporin rats did not accurately carry the food pellet to the refuge during the dark probe. These observations are consistent with infusion of GAT1-Saporin into the MS/VDB resulting in spared mnemonic function and use of environmental cues; however, self-movement cue processing was compromised. This interpretation is consistent with a growing literature demonstrating a role for the septohippocampal system in self-movement cue processing.     

The role of cholinergic and GABAergic medial septal/diagonal band cell populations in the emergence of diencephalic amnesia

The septohippocampal pathway, which is mostly composed of cholinergic and GABAergic projections between the medial septum/diagonal band (MS/DB) and the hippocampus, has an established role in learning, memory and disorders of cognition. In Wernicke-Korsakoff's syndrome (WKS) and the animal model of the disorder, pyrithiamine-induced thiamine deficiency (PTD), there is both diencephalic damage and basal forebrain cell loss that could contribute to the amnesic state. In the current experiment, we used the PTD animal model to access both cholinergic (choline acetyltransferase [ChAT] immunopositive) and GABAergic (parvalbumin [PV]; calbindin [CaBP]) neuronal loss in the MS/DB in relationship to midline-thalamic pathology. In addition, to gain an understanding about the role of such neuropathology in behavioral dysfunction, animals were tested on a non-rewarded spontaneous alternation task and behavioral performance was correlated to neuropathology. Unbiased stereological assessment of neuronal populations revealed that ChAT-positive neurons were significantly reduced in PTD rats, relative to control pair-fed rats, and thalamic mass and behavioral performance correlated with ChAT neuronal estimates. In contrast, both the PV- and CaBP-positive neurons in the MS/DB were not affected by PTD treatment. These results support an interactive role of both thalamic pathology and cholinergic cell loss in diencephalic amnesia.     

Somato-dendritic nicotinic receptor responses recorded in vitro from the medial septal diagonal band complex of the rodent

The medial septal diagonal band area (MS/DB), made up of GABAergic and cholinergic neurones, plays an essential role in the generation and modulation of the hippocampal theta rhythm. To understand the part that the cholinergic neurones might play in this activity, we sought to determine whether postsynaptic nicotinic receptor responses can be detected in slices of the rodent MS/DB by puffing on acetylcholine (ACh). Neurones were characterized electrophysiologically into GABAergic and cholinergic neurones according to previous criteria. Responses of the MS/SB neurones to ACh were various combinations of fast depolarizations (1.5–2.5 s), fast hyperpolarizations (3–4 s) and slow depolarizations (20–30 s), the latter two being blocked by atropine. The fast depolarizations were partially or not blocked with cadmium and low calcium, tetrodotoxin, and antagonists of other ionotropic receptors, and were antagonized with 25 μ m mecamylamine. Pharmacological investigation of the responses showed that the α7* nicotinic receptor type is associated with cholinergic neurones and 10% of the GABAergic neurones, and that nonα7* nicotinic receptor subtypes are associated with 50% of the GABAergic neurones. Pharmacological dissection of evoked and spontaneous postsynaptic responses, however, did not provide evidence for synaptic nicotinic receptor transmission in the MS/DB. It was concluded that nicotinic receptors, although prevalent on the somatic and/or dendritic membrane compartments of neurones in the MS/DB, are on extrasynaptic sites where they presumably play a neuromodulatory role. The presence of α7* nicotinic receptors on cholinergic neurones may also render these cells specifically vulnerable to degeneration in Alzheimer's disease.     

Induction by kainate of theta frequency rhythmic activity in the rat medial septum–diagonal band complex in vitro

The medial septum–diagonal band (MSDB) complex, via the septohippocampal pathway, is thought to be critical for the generation and/or maintenance of the hippocampal theta rhythm in vivo . The aim was to determine whether the MSDB is capable of generating and maintaining its own rhythmic firing activity, a mechanism by which it could impose a theta frequency oscillatory activity on the hippocampus. Bath application of 50–300 n m kainate to an in vitro preparation of 20- to 25-day-old rat MSDB elicited rhythmic extracellular field activity primarily within the theta frequency band (4–12 Hz). This activity was observed both at 33°C and at 37°C, and was localized to the midline part of the MSDB that is rich in parvalbumin-containing neurones. The application of neurotransmitter receptor antagonists and putative gap junction blockers showed that the oscillatory field activity was dependent upon the activation of GABA_A receptors and possibly gap junctions, but not on the activation of NMDA, GABA_B, muscarinic or nicotinic receptors. The frequency of the oscillatory activity was reduced by the application of diazepam or low doses of baclofen. Intracellular recording showed that concomitant action potential firing activity in putative GABAergic and cholinergic neurone populations was of a single spiking rather than a bursting firing nature, and was coherent with extracellularly recorded oscillatory field activity. We conclude that kainate activation of neuronal circuitry in the MSDB is capable of synchronization of rhythmic activity in the MSDB, and that this may underlie the mechanism for phase-locking rhythmic burst activity in the MSDB in vivo .     

Regulation of endogenous acetylcholine release from mammalian brain slices by opiate receptors: hippocampus, striatum and cerebral cortex of guinea-pig and rat

The effects of opiate agonists on acetylcholine release from hippocampal, striatal and cerebral cortical slices were tested; tissue from rat was compared to that from guinea-pig. The results show that opiate receptors in each of these areas can alter the evoked release of acetylcholine from nerve terminals; however, there are species and tissue differences with respect to the apparent subtype of opiate receptor effective. In the hippocampus and striatum of the two species studied, opiates caused a dose-dependent decrease in evoked acetylcholine release from tissue slices but in the guinea-pig kappa-selective agonists were effective, and mu or delta agonists were not, whereas in the rat, mu-, but not delta- or kappa-selective drugs were effective. Opiates also altered acetylcholine release from the frontal, parietal and occipital cortex of both of these species. In all three regions of the guinea-pig cortex, kappa and delta agonists were active and in the parietal cortex mu agonists were also active; rat cortical slices showed similar results except that delta agonists were not effective. The inhibitory effects of the opiate agonists were effectively antagonized by the non-selective opiate antagonist naloxone and by the calcium channel agonists, BAY K 8644 or YC-170. In addition, the effects of the opiate drugs tested in this study on acetylcholine release were confined to evoked release, that is, spontaneous acetylcholine release was not affected. The results suggest that in guinea-pig and rat brain, opiate receptors regulate acetylcholine release, and that, although the subtypes of opiate receptors involved in this effect are different in the two species and in different tissues from the same species, the effect results from a common mechanism that involves alterations of calcium influx into the nerve terminals during depolarization.