An electrophysiological study of neurones in the rat median raphe and their projections to septum and hippocampus

Projection-defined median raphe Pet+ subpopulations are diversely implicated in seizure

The raphe nuclei, the primary resource of forebrain 5-HT, play an important but heterogeneous role in regulating subcortical excitabilities. Fundamental circuit organizations of different median raphe (MR) subsystems are far from completely understood. In the present study, using cell-specific viral tracing, Ca2+ fiber photometry and epilepsy model, we map out the forebrain efferent and afferent of different MR Pet+ subpopulations and their divergent roles in epilepsy. We found that PetMR neurons send both collateral and parallel innervations to different downstream regions through different subpopulations. Notably, CA3-projecting PetMR subpopulations are largely distinct from habenula (Hb)-projecting PetMR subpopulations in anatomical distribution and topological organization, while majority of the CA3-projecting PetMR subpopulations are overlapped with the medial septum (MS)-projecting PetMR subpopulations. Further, using Ca2+ fiber photometry, we monitor activities of PetMR neurons in hippocampal-kindling seizure, a classical epilepsy model with pathological mechanisms caused by excitation-inhibition imbalance. We found that soma activities of PetMR neurons are heterogeneous during different periods of generalized seizures. These divergent activities are contributed by different projection-defined PetMR subpopulations, manifesting as increased activities in CA3 but decreased activity in Hb resulting from their upstream differences. Together, our findings provide a novel framework of MR subsystems showing that projection-defined MR Pet+ subpopulations are topologically heterogenous with divergent circuit connections and are diversely implicated in seizures. This may help in the understanding of heterogeneous nature of MR 5-HTergic subsystems and the paradox roles of 5-HTergic systems in epilepsy.

Calcium, Bioenergetics, and Parkinson's Disease

Degeneration of substantia nigra (SN) dopaminergic (DAergic) neurons is responsible for the core motor deficits of Parkinson’s disease (PD). These neurons are autonomous pacemakers that have large cytosolic Ca²⁺ oscillations that have been linked to basal mitochondrial oxidant stress and turnover. This review explores the origin of Ca²⁺ oscillations and their role in the control of mitochondrial respiration, bioenergetics, and mitochondrial oxidant stress.

Median Raphe Stimulation Disrupts Hippocampal Theta Via Rapid Inhibition and State-Dependent Phase Reset of Theta-Related Neural Circuitry

Evidence has accumulated suggesting that the median raphe (MR) mediates hippocampal theta desynchronization. However, few studies have evaluated theta-related neural circuitry during MR manipulation. In urethane-anesthetized rats, we investigated the effects of MR stimulation on hippocampal field and cell activity using high-frequency (100 Hz), theta burst (TBS), and slow-frequency electrical stimulation (0.5 Hz). We demonstrated that high-frequency stimulation of the MR did not elicit deactivated patterns in the forebrain, but rather elicited low-voltage activity in the neocortex and small-amplitude irregular activity (SIA) in the hippocampus. Both hippocampal phasic theta-on and -off cells were inhibited by high-frequency MR stimulation, although MR stimulation failed to affect cells that had neither state or phase relationships with theta field activity. TBS of the MR-induced theta field activity phase locked to the stimulation. Slow-frequency stimulation elicited a state-dependent reset of theta phase through a short-latency inhibition (5 ms) in phasic theta-on cells. Subpopulations of phasic theta-on cells responded in either oscillatory or nonoscillatory patterns to MR pulses, depending on their intraburst interval. off cells exhibited a state-dependent modulation of cell firing occurring preferentially during nontheta. The magnitude of MR-induced reset varied as a function of the phase of the theta oscillation when the pulse was administered. Therefore high-frequency stimulation of the MR appears to disrupt hippocampal theta through a state-dependent, short-latency inhibition of rhythmic cell populations in the hippocampus functioning to switch theta oscillations to an activated SIA field state.

Presynaptic modulation of 5-HT release in the rat septal region

5-HT released from serotonergic axon terminals in the septal nuclei modulates the activity of septal output neurons (e.g. septohippocampal cholinergic neurons) bearing somatodendritic 5-HT receptors. Therefore, we studied the mechanisms involved in the presynaptic modulation of 5-HT release in the lateral (LS) and medial septum (MS), and the diagonal band of Broca (DB). HPLC analysis showed that tissue concentrations of noradrenaline, dopamine and 5-HT were highest in DB (DB>MS>LS). Slices prepared from LS, MS and DB regions were preincubated with [(3)H]5-HT, superfused in the presence of 6-nitro-2-(1-piperazinyl)-quinoline (6-nitroquipazine) and electrically stimulated up to three times (first electrical stimulation period (S(1)), S(2), S(3); 360 pulses, 3 Hz, 2 ms, 26-28 mA). In all septal regions the Ca(2+)-dependent and tetrodotoxin-sensitive electrically-evoked overflow of [(3)H] was inhibited by the 5-HT(1B) agonist CP-93,129 and the alpha(2)-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline tartrate (UK-14,304). Also the mu- and kappa-opioid receptor agonists (d-Ala(2), N-Me-Phe(4), glycinol(5))-enkephalin (DAMGO) and [trans-(1S,2S(-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl) cyclohexyl]-benzenacetamide hydro-chloride] (U-50,488H), respectively, acted inhibitory (although less potently), whereas the delta-opioid receptor agonist (d-Pen(2), d-Pen(5))-enkephalin (DPDPE), the dopamine D(2) receptor agonist quinpirole and the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine were all ineffective; the GABA(B) receptor agonist baclofen had weak effects. All inhibitory effects of the agonists were antagonized by the corresponding antagonists (3-[3-(dimethylamino)propyl]-4-hydroxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride (GR-55,562), idazoxan, naloxone, nor-binaltorphimine), which also significantly enhanced the evoked release of 5-HT at S(1). It is concluded that 5-HT release in septal nuclei of the rat is modulated by presynaptic 5-HT(1B) autoreceptors, as well as by alpha(2)-, mu- and kappa-opioid heteroreceptors. All of these receptors seem to be under a tonic inhibitory influence of the corresponding endogenous agonists and show qualitatively comparable modulatory properties along the dorso-ventral distribution of the 5-HT terminals.

Hodological characterization of the septum in anuran amphibians: I. Afferent connections

On the basis of Nissl-stained sections, we subdivided the septum of the gray treefrog Hyla versicolor in the lateral, central, and medial septal complex. The afferent projections of the different septal nuclei were studied by combined retrograde and anterograde tracing with biotin ethylendiamine (Neurobiotin). The central and medial septal complex receives direct input from regions of the olfactory bulb and from all other limbic structures of the telencephalon (e.g., amygdalar regions, nucleus accumbens), whereas projections to the lateral septal complex are absent or less extensive. The medial pallium projects to all septal nuclei. In the diencephalon, the anterior thalamic nucleus provides the main ascending input to all subnuclei of the anuran septum, which can be interpreted as a limbic/associative pathway. The ventromedial thalamic nucleus projects to the medial and lateral septal complex and may thereby transmit multisensory information to the limbic system. Anterior preoptic nucleus, suprachiasmatic nucleus, and hypothalamic nuclei innervate the central and lateral septal complex. Only the nuclei of the central septal complex receive input from the brainstem. Noteworthy is the relatively strong projection from the nucleus raphe to the central septal complex, but not to the other septal nuclei.

Median raphe mediates estrogenic effects to the hippocampus in female rats

Subcortical regions such as the medial septum-diagonal band of Broca and supramammillary area have been shown to mediate indirect oestrogenic effects on hippocampal morphology and function. Here, the role of the median raphe (MR), a serotonergic subcortical structure, is studied. To this end, 17beta-estradiol-filled 30-gauge cannulae were implanted into the MR of female ovariectomized rats; cholesterol-filled cannulae served as controls. After seven days, using unbiased electron microscopic stereological calculations and semiquantitative analysis, the spine synapse density and surface density of glial fibrillary acidic protein-positive astrocyte processes, respectively, were determined in the stratum radiatum of the CA1 region of the hippocampus. Changes in the serotonergic innervation of the hippocampal CA1 region were determined by immunohistochemistry and subsequent morphometric analysis. In the stratum radiatum of the CA1 region, local estradiol application into the MR resulted in a 47% increase in spine synapse density. Simultaneously, the density of glial fibrillary acidic protein-positive fibers decreased by 16%. The density of serotonin (5-HT) innervation of the strata lacunosum moleculare and radiatum of the CA1 region of the hippocampus was reduced in response to estradiol, as shown by a decrease in the length of fibers (27.6 and 48.3% decrease, respectively) and the number of large varicosities (32.5 and 38.8% decrease, respectively). These observations suggest a major role of the MR in mediating oestrogenic effects on the hippocampus and an involvement of the serotonergic system.

Serotonin produces an enhanced outward current recorded at rat dorsal lateral septal neurons from the Flinders Sensitive Line of rats, a genetically-selected animal model of depression

Abnormalities in serotonin (5-HT), serotonin receptors, and serotonergic neurons have been reported in studies of brains from patients diagnosed clinically with depression. In this study, we examined a known cellular function of 5-HT(1A) receptor activation in dorsolateral septal nucleus (DLSN) neurons, namely, a concentration dependent 5-HT-induced outward current, and compared basic neuronal membrane properties and activities of DLSN neurons from two known genetic lines of rats. As compared to "control" rats (Flinders Resistant Line, FRL), DLSN neurons from Flinders Sensitive Line of rats (FSL) did not exhibit significant differences in resting membrane potential, membrane input resistance, or changes in typical spontaneous inhibitory or excitatory post-synaptic currents. FSL-rats exhibit a depressive phenotype and have been suggested to be rats with a genetic susceptibility to exhibit depressive behaviors. Exogenous application of 5-HT resulted in expected concentration-dependent outward currents; however, the amplitudes of these currents were enhanced significantly in 50% of DLSN neurons recorded from FSL rats compared to similar results recorded from FRL rats. Our results suggest that within a particular population of DLSN neurons from rats exhibiting a known phenotype of depression a post-synaptic 5-HT(1A) receptor is functionally hyper-responsive compared to controls.

Activation of presynaptic 5-hydroxytryptamine 2A receptors facilitates excitatory synaptic transmission via protein kinase C in the dorsolateral septal nucleus

Effects of 5-hydroxytryptamine (5-HT) on EPSPs and EPSCs in the rat dorsolateral septal nucleus (DLSN) were examined in the presence of GABA(A) and GABA(B) receptor antagonists. Bath application of 5-HT (10 microm) for 5-10 min increased the amplitude of the EPSP and EPSC. (+/-)-8-hydroxy-2-(di-N-propylamino)tetralin hydrobromide (10 microm), an agonist for 5-HT1A and 5-HT7 receptors, did not facilitate the EPSP. alpha-Methyl-5-HT (10 microm), a 5-HT2 receptor agonist, increased the amplitude of the EPSC. Alpha-methyl-5-(2-thienylmethoxy)-1H-indole-3-ethanamine (10 microm) and 6-chloro-2-(1-piperazinyl)pyrazine (10 microm), selective 5-HT2B and 5-HT2C receptor agonists, respectively, had no effect on the EPSP. The 5-HT-induced facilitation of the EPSP was blocked by ketanserin (10 microm), a 5-HT2A/2C receptor antagonist. However, N-desmethylclozapine (10 microm), a selective 5-HT2C receptor antagonist, did not block the facilitation of the EPSP induced by alpha-methyl-5-HT. The inward current evoked by exogenous glutamate was unaffected by 5-HT. 5-HT (10 microm) and alpha-methyl-5-HT (10 microm) increased the frequency of miniature EPSPs (mEPSPs) without changing the mEPSP amplitude. The ratio of the paired pulse facilitation was significantly decreased by 5-HT and alpha-methyl-5-HT. The 5-HT-induced facilitation of the EPSP was blocked by calphostin C (100 nm), a specific protein kinase C (PKC) inhibitor, but not by N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (10 microm), a protein kinase A inhibitor. Phorbol 12,13-dibutyrate (3 microm) mimicked the facilitatory effects of 5-HT. These results suggest that 5-HT enhances the EPSP by increasing the release of glutamate via presynaptic 5-HT2A receptors that link with PKC in rat DLSN neurons.

Neuronal activity of the septal pacemaker of theta rhythm under the influence of stimulation and blockade of the median raphe nucleus in the awake rabbit

The control of theta rhythm in neuronal activity of the medial septal area and hippocampal electroencephalogram by the brainstem structures was investigated in waking rabbits. In the first series of experiments stimulating electrodes were implanted into the midbrain reticular formation and median raphe nucleus. The standard frequency of theta-bursts in medial septal area neurons and in the electroencephalogram was uniformly and chronically decreased in all rabbits with electrodes implanted into the median raphe nucleus (4.7 +/- 0.5 Hz versus 5.2 +/- 0.19 Hz in animals without electrodes in median raphe nucleus). Weak electrical stimulation of the median raphe nucleus resulted in additional decrease of theta expression in the medial septal area neurons and its disappearance from the hippocampal electroencephalogram, where it was substituted by delta-waves and spindles. Stimulation of the reticular formation had the opposite effect, with an increase in theta frequency, regularity and expression in medial septal area neuronal activity and hippocampal electroencephalogram. In the second series of experiments reversible functional blockade of the median raphe nucleus by local microinjection of lidocaine was performed. This resulted in expression of theta-bursts in an additional group of medial septal area neurons, an increase in theta-burst frequency (by 0.5-2 Hz) and regularity with concomitant changes in the electroencephalogram. The effects of sensory stimuli on the background of increased theta activity were suppressed or significantly decreased. It is concluded that, in accordance with the data of other authors, the median raphe nucleus can be regarded as a functional antagonist of the reticular formation, powerfully suppressing theta-bursts of the medial septal area neurons and hippocampal theta rhythm. It is suggested that, in combination with the theta-enhancing influences of reticular formation, the median raphe nucleus may participate in termination of attention, its switching to other stimuli and stabilization of the effects of learning.

Medial septal and median raphe innervation of vasoactive intestinal polypeptide-containing interneurons in the hippocampus

Vasoactive intestinal polypeptide-immunoreactive interneurons are known to form three anatomically and neurochemically well-characterized neuron populations in the hippocampus. Two of these establish synaptic contacts selectively with other GABAergic cells (interneuron-selective cells), whereas the third type innervates pyramidal cell bodies and proximal dendrites like a conventional basket cell. Our aim was to examine which of the vasoactive intestinal polypeptide-containing interneuron populations are among the targets of GABAergic septohippocampal and serotonergic raphe-hippocampal pathways. Anterograde tracing with Phaseolus vulgaris leucoagglutinin combined with double immunocytochemistry for vasoactive intestinal polypeptide was used at the light and electron microscopic levels. Our results show that both interneuron-selective cells and vasoactive intestinal polypeptide-containing basket cells receive synaptic input from the medial septum and median raphe nucleus. The GABAergic component of the septohippocampal pathway establishes multiple contacts on both cell types. In the case of the raphe-hippocampal projection, single or double contacts were more frequent on vasoactive intestinal polypeptide-positive interneuron selective cells (76%), whereas multiple contacts predominated on basket cells (83%). The extrinsic GABAergic innervation of interneuron-selective cells in the hippocampus indicates a complex interaction among GABAergic systems, which might ensure the timing and rhythmic synchronization of inhibitory processes in the hippocampus. On the other hand, our results suggest that the serotonergic effect on perisomatic inhibition is exerted via vasoactive intestinal polypeptide-containing basket cells that are functionally distinct from their parvalbumin-positive relatives, which appear to escape control of serotonergic as well as local interneuron-selective cells.

The fasciculus retroflexus controls the integrity of REM sleep by supporting the generation of hippocampal theta rhythm and rapid eye movements in rats

The fasciculus retroflexus (FR) fiber bundle comprises the intense cholinergic projection from the medial division of the habenula nucleus (Hbn) of the epithalamus to the interpeduncular nucleus (IPN) of the limbic midbrain. Due to the widespread connections of the Hbn and IPN, it could be surmised that the FR is integrated in the processings of various subsystems that are known to be involved in the sleep-wake mechanisms; relevant sites include the limbic forebrain and midbrain areas and more caudal pontine structures. Consequently, the present study addressed the significance of the FR in the spontaneous sleep-wake stage-associated variations of the different activity patterns of frontal cortex and hippocampal electroencephalograms (EEGs), the electrooculogram, and body movements, in freely behaving rats that had been subjected to either bilateral electrolytic lesioning of the FR or control operations. The evolution of different state combinations was assessed by the combinatory analysis of different activity stages appearing on the 6-h records. As compared to the control-operated group, the FR lesioning substantially reduced the time spent in rapid eye movement (REM) sleep by 79%, moderately decreased the duration of the intermediate state of sleep by 29%, and quiet waking state by 44%, but had virtually no effects on the durations of different types of non-REM sleep (i.e., drowsiness that which involved quiet sleep or slow-wave sleep containing delta and spindle state components) or on the times of active waking behavior that corresponded to the body movements. Quantitative decomposition analyses revealed marked variations in the frontal cortex and hippocampal activity as well as REM during the course of the extracted sleep-wake stages described and there were also some group differences. Of those individual features that were used to determine different sleep-wake stages, the overall hippocampal theta time (41% decrease) and single REM frequency (71% reduction during the REM sleep) were most affected. In contrast, the various properties of desynchronization/synchronization patterns of frontal cortex EEGs were consistently hardly influenced by the FR lesioning. Therefore, the present data suggest the involvement of the FR in the REM sleep processes by establishing prominent associations with the limbic and REM control mechanisms that involve the hippocampus and plausibly pontine ocular activity networks.

Physiological evidence for hippocampal disinhibition resulting from activation of the median raphe

The mechanism underlying median raphe (MR)-induced facilitation of hippocampal synaptic transmission was investigated by recording stimulus-evoked field potentials and unitary responses in urethane-anesthetized rats. Stimulation of the MR 40 ms prior to perforant path (PP) activation significantly increased the magnitude of PP-evoked granule cell population spikes (median increase = 78%) without affecting population EPSP slope. Injection of homocysteic acid into the vicinity of the MR also facilitated PP-evoked granule cell population spikes, in a dose-dependent manner. Nineteen dentate hilar units were characterized as putative interneurons on the basis of their waveform characteristics and their response to PP stimulation. Electrical activation of the MR inhibited spontaneous or PP-evoked activity in the majority (75%) of these cells; the remaining cells were unaffected. MR stimulation also inhibited spontaneous activity in a large proportion (60%) of putative interneurons in CA1. The present results provide evidence that neurons within the raphe modulate hippocampal throughput by altering discharge of non-principal cells. These data, thus, support the idea that disinhibition is a common mechanism by which extrahippocampal structures modulate information flow through the hippocampus.

Burst-firing activity of presumed 5-HT neurones of the rat dorsal raphe nucleus: electrophysiological analysis by antidromic stimulation

We recently reported raphe neurones which frequently fired spikes in short bursts. However, the action potentials were broad and the neurones fired in a slow and regular pattern, suggesting they were an unusual type of 5-hydroxytryptamine (5-HT) neurone. In the present study, we investigated whether these putative burst-firing 5-HT neurones project to the forebrain and whether all spikes fired in bursts propagate along the axon. In anaesthetised rats, electrical stimulation of the medial forebrain bundle evoked antidromic spikes in both burst-firing neurones and in single-spiking, classical 5-HT neurones recorded in the dorsal raphe nucleus. Although the antidromic spike latency of the single-spiking and burst-firing neurones showed a clear overlap, burst-firing neurones had a significantly shorter latency than single-spiking neurones. For both burst-firing neurones and classical 5-HT neurones, antidromic spikes made collisions with spontaneously occurring spikes. Furthermore, in all burst-firing neurones tested, first, second and third order spikes in a burst could be made to collide with antidromic spike. Interestingly, in a small number of burst-firing neurones, antidromic stimulation evoked spike doublets, similar to those recorded spontaneously. From these data we conclude that burst-firing neurones in the dorsal raphe nucleus project to the forebrain, and each spike generated by the burst propagates along the axon and could thereby release transmitter (5-HT).

Regulation of the septal pacemaker theta rhythm by the cervical nuclei of the midbrain

Neuronal activity in the medial septal region (the medial nucleus and the diagonal band nucleus, MN-DBN) was recorded along with hippocampal EEG traces in conscious rabbits with stimulatory electrodes implanted in the median cervical nucleus (MCN) and the reticular formation (RF) of the midbrain and pons. In all animals with electrodes in the MCN, the background theta activity frequency was low (4.6 +/- 0.15 Hz) as compared with intact rabbits or those with electrodes implanted only in the RF (5.2 +/- 0.19 Hz, p < 0.5). Stimulation of the MCN with weak low-frequency impulses reduced theta volleys from MN-DBN cells, reducing their frequency and regularity and inducing the appearance or strengthening of low-frequency delta modulation. The number of spikes in a volley decreased, and the duration of inter-volley intervals increased. Stimulation of the MCN led to a gradual decrease in the frequency and amplitude of theta waves, induced irregular delta waves and spindles of 12 Hz in the hippocampal EEG. Stimulation of the RF produced the opposite changes in volley activity in the MN-DBN and hippocampal EEG, with increases in theta and decreases in delta components. These results support a role for the midbrain cervical nuclei as structures limiting the generation of theta activity by the reticular-septal system, but do not support the existence of an MN-DBN-independent high-frequency serotoninergic theta rhythm. It is proposed that the effect of the MCN may be important for suppression and switching of attention.

Excitatory actions of serotonin on GABAergic neurons of the medial septum and diagonal band of Broca

The physiological and pharmacological actions of serotonin (5-HT) on neurons in the medial septum and diagonal band of Broca (MSDB) were examined using extracellular and intracellular recording techniques in an in vitro rat brain-slice preparation. In addition to previously described inhibitory effects, novel excitatory actions of 5-HT on GABA-type cells were observed. In intracellular recordings with KCl-containing electrodes, bath-applied 5-HT induced a bicuculline and tetrodotoxin-sensitive increase in the number of reverse IPSPs in both cholinergic- and noncholinergic-type neurons (presumably GABAergic). In brain slices where all structures neighboring the MSDB, including the lateral septum, had been excised, a similar increase in 5-HT-induced IPSPs occurred, indicating that 5-HT-induced IPSPs in both cholinergic- and noncholinergic-type neurons originate from GABAergic neurons within the MSDB itself. Accordingly, GABA-type neurons in the MSDB were found to be directly excited by 5-HT. MDL 100,907, a selective 5-HT2A antagonist, blocked 5-HT-induced excitations in a majority of neurons (58%). ICS 205-930, a 5-HT3/5-HT4 antagonist, or mianserin, a nonselective 5-HT antagonist, blocked most MDL-resistant responses, indicating a role for multiple 5-HT receptor subtypes. This study also provides the first electrophysiological evidence for synaptic interactions between 5-HT-activated GABAergic neurons and cholinergic neurons and amongst GABAergic neurons in the MSDB. The implications of the findings vis-à-vis intraseptal circuitry and septohippocampal circuitry are discussed.

Expression, control, and probable functional significance of the neuronal theta-rhythm

The data on theta-modulation of neuronal activity in the hippocampus and related structures, obtained by the author and her colleagues have been reviewed. Analysis of extracellularly recorded neuronal activity in alert rabbits, intact and after various brain lesions, in slices and transplants of the hippocampus and septum allow one to make the following conclusions. Integrity of the medial septal area (MS-DB) and its efferent connections are indispensable for theta-modulation of neuronal activity and EEG of the hippocampus. The expression of hippocampal theta depends on the proportion of the MS-DB cells involved in the rhythmic process, and its frequency in the whole theta-range, is determined by the corresponding frequencies of theta-burst in the MS-DB. The neurons of the MS-DB have the properties of endogenous rhythmic burst and regular single spike oscillators. Input signals ascending to the MS-DB from the pontomesencephalic reticular formation increase both the frequency of the MS-DB theta-bursts and the proportion of neurons involved in theta-activity; serotonergic midbrain raphe nuclei have the opposite effect on the MS-DB rhythmic activity and hippocampal EEG theta. Increase of endogenous acetylcholine (by physostigmine) also increases the proportion of the MS-DB neurons discharging in theta-bursts (both in intact and basally-undercut septum), but does not influence the theta-frequency. The primary effect of the MS-DB on hippocampal neurons (pyramidal and non-pyramidal) consists in GABAergic reset inhibition. Reset inhibition, after which theta-modulation follows in constant phase relation, is triggered also by sensory stimuli. About two-thirds of the hippocampal pyramidal neurons are tonically inhibited by sensory stimuli which evoke EEG theta, while others are excited, or do not change their activity. Anticholinergic drugs restrict the population of rhythmic neurons but do not completely suppress theta-bursts in the MS-DB and hippocampus. Under their action, EEG theta can be evoked (presumably through GABAergic MS-DB influences) by strong reticular or sensory stimuli with corresponding high frequency. However information processing in this condition is defective: expression of reset is increased, responses to electrical stimulation of the perforant path and to sensory stimuli are often augmented, habituation to sensory stimuli is absent and tonic responses are curtailed. On a background of continuous theta induced by increase of endogenous acetylcholine, reset is absent or reduced, responsiveness of the hippocampal neurons to electrical and sensory stimulation is strongly reduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonin-containing terminals synapse on septohippocampal neurons in the rat

Serotonin [5-hydroxytryptamine (5-HT)] is thought to be involved in mnemonic functions and dysfunctions possibly by directly contacting neurons in the medial septal and diagonal band nuclei (i.e., the septal complex) that project to the hippocampal formation. However, there is no cellular substrate for this modulation. Thus, we examined the ultrastructure and synaptic associations of 5-HT-containing terminals in relation to septohippocampal neurons in the septal complex of the rat brain. Projection neurons were identified by retrograde transport of wheat germ agglutinated apo-horseradish peroxidase conjugated to colloidal gold particles (WAHG) following an injection into the ventral hippocampal formation of anesthetized adult rats. After a 1 day survival, sections through the septal complex were labeled with antibodies to 5-HT. By light microscopy, numerous processes with 5-HT immunoreactivity (5-HT-I) were observed in close proximity to neurons containing retrogradely transported WAHG. By electron microscopy, 5-HT-I was found exclusively in axons and axon terminals. Axons were primarily unmyelinated. Terminals with 5-HT-I were 0.35-1.2 microns in diameter and contained numerous small, clear vesicles and 0-4 large, dense-core vesicles. The 5-HT-labeled terminals: 1) contacted perikarya and dendrites (220 of 349); 2) were closely apposed to other terminals (25 of 349); or 3) had no neuronal contacts in the plane of section analyzed (104 of 349). The 5-HT-labeled terminals formed exclusively symmetric synapses on perikarya; some of these perikarya as well as some large dendrites similarly contacted by the 5-HT-labeled terminals also contained WAHG affiliated with lysosomes and multivesicular and "sequestration" bodies in the cytoplasm. However, the majority of terminals with 5-HT-I formed contacts on the shafts of small unlabeled dendrites (69% of 220); most of these were characterized as either asymmetric synapses or appositions not separated by astrocytes in the plane of section analyzed. We conclude that 5-HT-containing terminals in the rat septal complex: 1) directly modulate septohippocampal and other neurons through symmetric (potentially inhibitory) synapses on soma and proximal dendrites; and 2) form primarily asymmetric (potentially excitatory) synapses with distal (small) dendrites from neurons of unidentified origin. These findings suggest that serotonin may affect learning and memory through modulation of septal efferents to the hippocampal formation and may have direct relevance to the neuropathological basis for Alzheimer's disease.

Calretinin is present in non-pyramidal cells of the rat hippocampus--III. Their inputs from the median raphe and medial septal nuclei

The subcortical innervation of a recently described subpopulation of non-pyramidal neurons, containing the calcium binding protein, calretinin, was investigated in the rat hippocampus using the anterograde tracer Phaseolus vulgaris-leucoagglutinin and double immunocytochemistry for calretinin and serotonin at the light and electron microscopic levels. Our results show that the GABAergic component of the septohippocampal pathway and the serotonergic raphe afferents establish multiple synaptic contacts with the calretinin-immunoreactive interneurons. The majority of the targets of both pathways were spine-free calretinin neurons known to innervate the dendritic region of the principal cells, but the GABAergic septal pathway was found to terminate also on the spiny neurons of stratum lucidum of the CA3 region and in the dentate hilus. The present results demonstrate that the serotonergic raphe-hippocampal and the GABAergic septohippocampal pathways are able to modulate dendritic inhibition of principal cells via calretinin-containing GABAergic interneurons.

Reorganization of the area dentata serotoninergic plexus after lesions of the median raphe nucleus

Serotoninergic projections from the dorsal and median raphe nuclei to the area dentata of the hippocampal formation terminate mainly in the molecular layer and hilus, respectively. Consequently, a reduction in the density of the hilar serotoninergic plexus is seen by immunocytochemistry 2 weeks after lesions of the median raphe nucleus. Hippocampal serotonin concentration and serotonin high affinity uptake are also significantly reduced. Six weeks after lesion, surviving serotoninergic axons form a dense band in the inner molecular layer of the dorsal area dentata, a region that usually contains a sparse serotoninergic plexus. Moreover, serotoninergic fibers transverse the molecular layer and pass through the granule cell layer to reinnervate the hilus. Serotonin concentration and high affinity uptake have recovered to near normal levels by 6 weeks postlesion. Changes in the anatomical distribution of the area dentata serotoninergic plexus have not been reported in cases in which serotoninergic sprouting follows axotomy of serotoninergic projections. Thus direct lesions of serotoninergic neurons can produce a homotypic compensatory response that is qualitatively different from that generated by axotomy. The mechanistic basis for this reorganization is unclear, but the apparent extension of serotoninergic axon collaterals toward the hilus suggests that the denervated hilar neuropil is guiding reinnervation. Finally, anatomical evidence from animals studied 10 weeks postlesion suggests that the compensatory proliferation of serotoninergic axons observed 6 weeks after median raphe lesion is a transient event.

Do septal neurons pace the hippocampal theta rhythm?

The hippocampal theta rhythm (rhythmical slow activity, RSA) is one of the most thoroughly studied EEG phenomena. Much of this experimental interest has been stimulated by suggestions that the mnemonic functions of the hippocampus may depend upon theta-related neuronal activity. Inputs from the medial septal nuclei to the hippocampus were shown to be essential for the theta rhythm in the 1950s, but the role of these basal forebrain projections has not been clearly defined. Four models of the septo-hippocampal connections involved in theta rhythm production are reviewed as the precise roles of these projections are discussed. In our final, consolidated model both cholinergic and GABAergic septal projection cells fire in rhythmic bursts that entrain hippocampal interneurons. The resulting rhythmic inhibition of hippocampal projection cells, together with their excitatory interconnections, generates at least one component of the theta rhythm.

Detection of an atropine-resistant component of the hippocampal theta rhythm in urethane-anesthetized rats

An important pharmacological feature of the hippocampal theta rhythm in urethane-anesthetized animals is its apparent sensitivity to antimuscarinic drugs. This sensitivity may be partly due to a masking of the theta frequency by increases in both higher and lower frequency EEG components that are unrelated to any residual theta rhythm. The discovery of atropine-resistant, rhythmic medial septal neurons has provided a physiological trigger for averaging EEG and unit activity after large atropine doses. Such averaging has permitted the detection of an atropine-resistant component of the hippocampal theta rhythm in urethane-anesthetized rats. The postatropine theta activity recorded from both CAl (superficial to the pyramidal cell layer) and dentate (near the hippocampal fissure) in 15 rats was typically reduced in amplitude, but the recordings from the two locations maintained their phase relations to the septal units and to each other. The presence of this residual theta component after doses as large as 100 mg/kg indicates that it cannot be mediated by muscarinic cholinergic receptors. The coupling of the signal to the atropine-resistant septal cells strengthens our previous suggestion that these septo-hippocampal neurons are not cholinergic, and are therefore probably GABAergic.

Autoradiographic analysis of ascending projections from the pontine and mesencephalic reticular formation and the median raphe nucleus in the rat

Ascending projections from the medial pontine reticular formation, the mesencephalic reticular formation, and the median raphe nucleus were examined using the autoradiographic technique. The majority of the ascending fibers labeled after injections of [3H]-leucine into the nucleus pontis caudalis (RPC) course through the brainstem within the tracts of Forel (tractus fasciculorum tegmenti of Forel) and directly ventral to them. At the caudal diencephalon, Forel's bundle divides into dorsal and ventral components bound primarily for the dorsal thalamus and the subthalamus, respectively. RPC fibers project to several regions involved in oculomotor/visual functions. These include the abducens nucleus, the intermediate gray layer of the superior colliculus (SCi), the anterior pretectal nucleus (APN), the ventral lateral geniculate nucleus (LGNv), and regions of the central gray directly bordering the oculomotor nucleus, the interstitial nucleus of Cajal, and the nucleus of Darkschewitsch. Few, if any, fibers from RPC (or from nucleus pontis oralis-RPO) terminate within the oculomotor nucleus proper. Other sites receiving heavy projections from the RPC include adjacent regions of the pontomesencephalic reticular formation (RF), the parafascicular (PF) and central lateral (CL) nuclei of the thalamus and the fields of Forel/zona incerta (FF-ZI). RPO fibers also ascend predominantly in Forel's bundle. Other ascending tracts for these fibers are the medial longitudinal fasciculus and the central tegmental tract (CTT). RPO fibers distribute significantly to the same structures of the oculomotor/visual system receiving projections from RPC. The RPO projections to the SCi and the APN are particularly pronounced. RPO fibers terminate heavily in several nuclei located ventrally within the rostral midbrain/caudal diencephalon. These include major dopamine-containing cell groups (the retrorubral nucleus, the ventral tegmental area, and the substantia nigra-pars compacta) as well as the interpeduncular nucleus, the lateral mammillary nucleus, and the supramammillary nucleus. Other prominent targets for RPO fibers include the mesencephalic RF, specific regions of the central gray, the PF, the CL, the paracentral and central medial nuclei of the thalamus, and the FF/ZI. The major bundle of the ascending fibers labeled after injections of the mesencephalic reticular formation (MRF) travels within the CTT in a position just lateral to the central gray, but a significant number of labeled axons also course in Forel's bundle.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotoninergic innervation of the cat cerebral cortex

Serotoninergic axons in the cat cerebral cortex were demonstrated immunohistochemically with a monoclonal antibody to serotonin (5-HT). Three types of 5-HT axons are distinguished at the light microscopic level by differences in their morphology. Small varicose axons are fine (less than 0.5 micron) and bear fusiform varicosities that are generally less than 1 micron in diameter. These axons extend throughout the width of the cortex and branch frequently, giving rise to widely spreading collaterals. Nonvaricose axons are smooth, show a relatively large and constant caliber (about 1 micron), travel in straight, horizontal trajectories, and branch infrequently. Large varicose axons are distinguished by large round or oval varicosities (1 micron or more in diameter) borne on fine-caliber fibers. These axons often form basket-like arbors around the somata of single neurons. In the simplest basket-like arbors, several large, round varicosities from a small number of axons contact the soma. In complex baskets intertwining collaterals contact the soma and apparently climb along and outline the cell's major dendrites. The patterns revealed by the climbing axons suggest that a variety of nonpyramidal cell types selectively receive dense 5-HT innervation. Serial reconstructions of the 5-HT axons within the cortex show that the large varicose axons arise as infrequent collaterals from the nonvaricose axons. A single nonvaricose parent axon gives rise to several large varicose axon collaterals that may contribute to different basket-like arbors. Conversely, a single basket-like arbor may be formed by large varicose axon collaterals from more than one nonvaricose parent axon. The small varicose axons do not appear to be related within the cortex to either the nonvaricose or large varicose axon types. The results support the hypothesis that the 5-HT projection to the cortex is organized into two subsystems, one of which may exert widespread influence in the cortex via highly divergent branches, while the other, with a more restricted distribution, acts on specific classes of cortical neurons.

Brainstem afferents to the basal forebrain in the rat

Brainstem afferents to various nuclei of the basal forebrain of the rat were examined using the retrograde transport of wheat germ agglutinin-horseradish peroxidase. These forebrain nuclei included the medial septum-vertical limb of the diagonal band nucleus, the lateral septum, the nucleus of the horizontal limb of the diagonal band, the medial preoptic area and the magnocellular preoptic nucleus/substantia innominata. Medial septal-vertical limb of the diagonal band injections produced dense cell labeling in: raphe obscurus, nucleus incertus, central gray-pars alpha, locus coeruleus, raphe pontis, median raphe, nucleus of Darkschewitsch, a compact cell group within the mesencephalic gray dorsolateral to the nucleus of Darkschewitsch and the supramammillary nucleus. Lateral septal injections produced the heaviest cell labeling in the A1 and A2 areas (of Dahlstrom and Fuxe), the lateral parabrachial nucleus, the Kolliker-Fuse nucleus, the ventral tegmental area and the supramammillary nucleus. There were considerably fewer labeled cells overall with lateral septal as compared with medial septal injections. Brainstem projections to the horizontal limb of the diagonal band were pronounced. The most heavily labeled nuclei were A1, locus coeruleus, laterodorsalis (dorsolateral tegmental nucleus of Castaldi), raphe pontis, median raphe, lateral parabrachial nucleus, ventral tegmental area, nucleus of Darkschewitsch and the supramammillary nucleus. Medial preoptic area injections produced pronounced labeling in: A1 and A2 areas, raphe magnus, locus coeruleus, laterodorsalis, lateral parabrachial nucleus, pedunculopontine nucleus, peripenduncular nucleus and the supramammillary nucleus. The pattern of brainstem labeling obtained with magnocellular preoptic/substantia innominata injections was considerably different from the patterns seen with the other injections. Specifically, relatively few cell groups, essentially confined to the upper brainstem (rostral pons and midbrain), were densely labeled following magnocellular preoptic/substantia innominata injections. These included the medial parabrachial nucleus, the pedunculopontine nucleus, the dorsal raphe nucleus, the ventral tegmental area and the supramammillary nucleus. With the exception of the supramammillary nucleus, each of these cell groups was more heavily labeled with magnocellular preoptic/substantia innominata injections than with others of this series. The above describes the major brainstem projections to each of the forebrain sites.(ABSTRACT TRUNCATED AT 400 WORDS)