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

doi:10.1016/0306-4522(85)90122-8

Neuroscience

Volume 15, Issue 1, May 1985, Pages 47-53, 55-60

https://doi.org/10.1016/0306-4522(85)90122-8 Get rights and content

Abstract

Extracellular single unit recordings were made in the median raphe nucleus from rats anaesthetized with urethane. Spontaneous firing as well as orthodromic and antidromic responses to stimulation of the fornix and the medial septum were studied.

One hundred and twelve units (out of a total of 355) with a regular spontaneous firing rate of 0.2–3 spikes/s were classified as serotonin-containing neurons. Fifty nine of them were antidromically invaded from either the fornix or the medial septum (conduction velocity, 0.8 m/s) and 7 additional neurones from both the fornix and the medial septum. Antidromic action potentials were followed by a period of decreased probability of firing, that was already present below threshold for antidromic invasion, were proportional to the stimulation intensity and had a latency similar to orthodromic inhibition. No preferential topographical distribution within the median raphe nucleus was observed for the serotonin neurones, even those invaded antidromically.

Twenty six neurones with a clear-cut anatomical location around the borders of the median raphe nucleus showed a spontaneous rhythmic activity (4–20 spikes/s) characterized by the presence of extremely prolonged silent periods (up to 5 min). Only one of these neurones was invaded antidromically from the medial septum and none from the fornix.

Of the remaining non-serotonin neurones, 28 showed a very low firing rate consisting of single action potentials every 10–60 s while 189 had a spontaneous activity of 6–30 spikes/s. Regardless of their firing rate they were all antidromically invaded from the fornix and/or the medial septum and had a conduction velocity of 5 m/s.

These experiments demonstrate the electrophysiological heterogeneity of the neuronal population of the median raphe nucleus, the presence of strong projections of both putative serotonin and non-serotonin neurones to the medial septum and, via the fornix, to the hippocampus, and the existence of axonal branching in both types of neurones.

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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 Pet^MR neurons send both collateral and parallel innervations to different downstream regions through different subpopulations. Notably, CA3-projecting Pet^MR subpopulations are largely distinct from habenula (Hb)-projecting Pet^MR subpopulations in anatomical distribution and topological organization, while majority of the CA3-projecting Pet^MR subpopulations are overlapped with the medial septum (MS)-projecting Pet^MR subpopulations. Further, using Ca²⁺ fiber photometry, we monitor activities of Pet^MR neurons in hippocampal-kindling seizure, a classical epilepsy model with pathological mechanisms caused by excitation-inhibition imbalance. We found that soma activities of Pet^MR neurons are heterogeneous during different periods of generalized seizures. These divergent activities are contributed by different projection-defined Pet^MR 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.
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The “consciousness system” encompasses brain areas that are crucial for maintaining the diverse behavioral components of consciousness. Both subcortical and cortical networks contribute to the content and level of consciousness. Although small in number, the subcortical neuromodulatory systems exert a powerful effect on our behavior and electroencephalogram while we are both awake and asleep. This chapter aims to provide the reader with an introduction to the seven principal subcortical arousal systems organized by neurotransmitters (acetylcholine, norepinephrine, histamine, serotonin, orexin, glutamate, and dopamine). These parallel systems exhibit a diverse range of firing patterns across the sleep–wake cycle and govern both overlapping and unique behaviors. Their stimulation or inhibition affects behavior, as well as the encephalogram, by complex and multifactorial means. It is our hope that this chapter will provide an adequate foundation on which to understand central nervous system disorders that disrupt consciousness via subcortical structures.
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Citation Excerpt :
Although we did not test this hypothesis, this effect is most probably caused by an increase in the spontaneous firing of serotonergic afferents in the presence of low extracellular Ca2+-concentrations. In all septal subregions the evoked release of 5-HT was significantly reduced by the selective 5-HT1B receptor agonist CP-93,129 (see Hoyer et al., 1994) suggesting that, as in other brain regions (Göthert et al., 1996), the axon terminals of serotonergic raphe neurons projecting to the septal region (Segal and Landis, 1974; Köhler et al., 1982; Crunelli and Segal, 1985; Vertes, 1988, 1991; Leranth and Vertes, 1999; Waselus et al., 2006) are endowed with 5-HT1B autoreceptors. This notion is supported by 5-HT1B mRNA in situ hybridization studies in the dorsal and median raphe neurons (Doucet et al., 1995).
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 [³H]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 (S₁), S₂, S₃; 360 pulses, 3 Hz, 2 ms, 26–28 mA). In all septal regions the Ca²⁺-dependent and tetrodotoxin-sensitive electrically-evoked overflow of [³H] was inhibited by the 5-HT_1B agonist CP-93,129 and the α₂-adrenoceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline tartrate (UK-14,304). Also the μ- and κ-opioid receptor agonists (d-Ala², N-Me-Phe⁴, glycinol⁵)-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 δ-opioid receptor agonist (d-Pen², d-Pen⁵)-enkephalin (DPDPE), the dopamine D₂ receptor agonist quinpirole and the adenosine A₁ receptor agonist N⁶-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 S₁. 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 α₂-, μ- and κ-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.
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
2003, Neuroscience Letters
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.
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
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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
1999, Neuroscience
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.

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^∗: Please address all correspondence to: Dr. V. Crunelli, Department of Pharmacology and Clinical Pharmacology, St George's Hospital Medical School, Cranmer Terrace, London SW17 0RE U.K.

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

Abstract

Brain Res.

Brain Res.

Brain Res.

Neurosci. Lett.

Brain Res.

Neurosci. Lett.

Neurosci. Lett.

Brain Res.

Brain Res.

Neuroscience

Neuroscience

Brain Res.

Brain Res.

Brain Res.

Cervical branching of lumbar vestibulospinal axons

J. Physiol., Lond.

Intracellular identification of central noradrenergic and serotoninergic neurones by a new double labelling procedure

J. Neurosci.

Axonal branching patterns and location of nigrothalamic and nigrocollicular neurons in the cat

J. Neurophysiol.

An autoradiographic analysis of the differential ascending projections of the dorsal and median raphe nuclei in the rat

J. comp. Neurol.

Microelectrophoretic studies of neurons in the cat hippocampus

J. Physiol., Lond.

The use of neurotoxic dihydroxytryptamines as tools for morphological studies and localized lesioning of central indoleamine neurons

Z. Zellforsch. mikrosk. Anat.