In vitro studies of the role of gamma-aminobutyric acid in inhibition in the lateral septum of the rat

Neural Regulation of the Stress Response: The Many Faces of Feedback

The mammalian stress response is an integrated physiological and psychological reaction to real or perceived adversity. Glucocorticoids (GCs) are an important component of this response, acting to redistribute energy resources to both optimize survival in the face of challenge and restore homeostasis after the immediate threat has subsided. Release of GCs is mediated by the hypothalamo-pituitary-adrenocortical (HPA) axis, driven by a neural signal originating in the paraventricular nucleus (PVN). Stress levels of GCs bind to glucocorticoid receptors (GRs) in multiple body compartments, including brain, and consequently have wide-reaching actions. For this reason, GCs serve a vital function in feedback inhibition of their own secretion. Fast, non-genomic feedback inhibition of the HPA axis is mediated at least in part by GC signaling in the PVN, acting by a cannabinoid-dependent mechanism to rapidly reduce both neural activity and GC release. Delayed feedback termination of the HPA axis response is mediated by forebrain GRs, presumably by genomic mechanisms. GCs also act in the brainstem to attenuate neuropeptidergic excitatory input to the PVN via acceleration of mRNA degradation, providing a mechanism to attenuate future responses to stressors. Thus, rather than having a single defined feedback switch, GCs work through multiple neurocircuits and signaling mechanisms to coordinate HPA axis activity to suit the overall needs of multiple body systems.

Allopregnanolone reduces immobility in the forced swimming test and increases the firing rate of lateral septal neurons through actions on the GABAA receptor in the rat

Since allopregnanolone reduces the total time of immobility in rats submitted to the forced swimming test, we decided to explore whether this neuroactive steroid shares other antidepressant-like actions, such as increasing the neuronal firing rate in the lateral septal nucleus (LSN). In order to discard the influence of the oestrous cycle on immobility and on the firing rate of LSN neurons, all Wistar rats used in the study underwent ovariectomy before treatments. A group of rats received different doses of allopregnanolone (0.5, 1.0, 2.0 and 3.0 mg/kg, i.p.) 1 hour before being forced to swim in order to identify the minimum effective dose diminishing immobility. None of the tested doses of allopregnanolone produced significant changes in motor activity in the open-field test. The minimum dose of allopregnanolone producing a significant reduction in the total time of immobility (p<0.05) against the vehicle was 1.0 mg/kg, while 2.0 mg/kg and above also increased the latency to the first period of immobility (p<0.05). The minimum effective dose of allopregnanolone reducing immobility in the forced swimming test (1.0 mg/kg) significantly (p <0.05) produced a higher (twofold) neuronal firing rate in LSN neurons, but did not produce any change in septofimbrial nucleus neurons, which fired at a rate similar to that of vehicle-treated rats. The pretreatment with the non-competitive GABAA receptor antagonist, picrotoxin (1.0 mg/kg), blocked the aforementioned actions of allopregnanolone on both immobility and LSN firing rate. In conclusion, allopregnanolone produces an antidepressant-like effect in the forced swimming test, associated with an increase in the LSN neuronal firing rate, seemingly mediated by the GABAA receptor.

The effect of lateral septum corticotropin-releasing factor receptor 2 activation on anxiety is modulated by stress

Corticotropin-releasing factor (CRF), a 41 amino acid peptide, mediates endocrine, autonomic, and behavioral responses to stress. Whereas the CRF1 receptor appears to contribute to anxiety associated with stress, the role of the CRF2 receptor remains unclear and may depend on drug dose, brain location, or testing environment. Results involving treatments with selective CRF2 receptor agonists or antagonists and the behavior of CRF2 receptor knock-out mice suggest both anxiogenic and anxiolytic effects of CRF2 receptor activation. The present study tested the hypothesis that the effect of CRF2 receptor activation on anxiety depends on the stress level of the animal. The selective CRF2 receptor agonist urocortin 2 was infused into the lateral septum of mice under low- or high-stress (30 min of immobilization) testing conditions, and then behavior in the light-dark box, open-field, and novel-object tests was assessed. In the low-stress environment, 240 pmol of septal urocortin 2 increased anxiety, but lower doses (0.48, 4.8, and 48 pmol) did not have consistent effects. However, in the high-stress condition, 48 pmol of septal urocortin 2 significantly increased anxiety compared with control in wild-type but not CRF2 receptor knock-out mice in the light-dark box. Septal administration of the relatively selective CRF2 antagonist astressin-2B, but not the CRF1-selective antagonist antalarmin, blocked the anxiogenic effects of urocortin 2. Urocortin 2 infusion into the medial septum or lateral ventricle did not affect anxiety measures. These results indicate that the effect of septal CRF2 receptor activation on anxiety is dependent on stress level.

Regulation of affect by the lateral septum: implications for neuropsychiatry

Substantial evidence indicates that the lateral septum (LS) plays a critical role in regulating processes related to mood and motivation. This review presents findings from the basic neuroscience literature and from some clinically oriented research, drawing from behavioral, neuroanatomical, electrophysiological, and molecular studies in support of such a role, and articulates models and hypotheses intended to advance our understanding of these functions. Neuroanatomically, the LS is connected with numerous regions known to regulate affect, such as the hippocampus, amygdala, and hypothalamus. Through its connections with the mesocorticolimbic dopamine system, the LS regulates motivation, both by stimulating the activity of midbrain dopamine neurons and regulating the consequences of this activity on the ventral striatum. Evidence that LS function could impact processes related to schizophrenia and other psychotic spectrum disorders, such as alterations in LS function following administration of antipsychotics and psychotomimetics in animals, will also be presented. The LS can also diminish or enable fear responding when its neural activity is stimulated or inhibited, respectively, perhaps through its projections to the hypothalamus. It also regulates behavioral manifestations of depression, with antidepressants stimulating the activity of LS neurons, and depression-like phenotypes corresponding to blunted activity of LS neurons; serotonin likely plays a key role in modulating these functions by influencing the responsiveness of the LS to hippocampal input. In conclusion, a better understanding of the LS may provide important and useful information in the pursuit of better treatments for a wide range of psychiatric conditions typified by disregulation of affective functions.

Activation of inhibitory pathways suppresses the induction of long-term potentiation in neurons of the rat lateral septal nucleus

Long-term potentiation of the hippocampal-septal pathway was examined by intracellular recording techniques. High frequency stimulation (two 100-Hz 1-s trains with a 20-s interval between them) of the hippocampal CA3 area resulted in a transient depolarization in rat lateral septal nucleus neurons. High frequency stimulation was followed by a facilitation of fast and slow inhibitory postsynaptic potentials, lasting for more than 2 h, but not by a long-lasting increase in the excitatory postsynaptic potential in the normal solution. Long-term potentiation (>2 h) of the excitatory postsynaptic potential did not appear in 74% of neurons tested, even when the fast inhibitory postsynaptic potential was blocked by bicuculline (30 microM), a GABA(A) receptor antagonist. High frequency stimulation produced long-term potentiation of the excitatory postsynaptic potential in the Mg(2+)-free solution containing bicuculline. When the fast and slow inhibitory postsynaptic potentials were blocked by GABA(A) and GABA(B) receptor antagonists (bicuculline and CGP 55845A respectively), high frequency stimulation produced a large and sustained depolarization followed by long-term potentiation of the excitatory postsynaptic potential. However, the excitatory postsynaptic potential was not enhanced by administration of these drugs after termination of high frequency stimulation. Pretreatment with 2-amino-5-phosphonopentanoate, a NMDA receptor antagonist, resulted in loss of long-term potentiation in both sets of experiments. Paired-pulse stimulation of the hippocampal CA3 region with interstimulus intervals between 200 and 800 ms depressed the second excitatory postsynaptic potential in the presence of bicuculline. CGP 35348, a GABA(B) receptor antagonist, reversed the depression of excitatory postsynaptic potentials to facilitation. These data suggest that high frequency stimulation of hippocampal CA3 neurons enhances the efficacy of GABAergic inhibitory circuits which, in turn, depress the ability of lateral septal nucleus neurons to express long-term potentiation.

Membrane and circuit properties of lateral septum neurons: relationships with hippocampal rhythms

The lateral septum receives the most important afferents from the hippocampus, has been proposed to contribute to theta (theta) rhythm generation. Our aim was to study the membrane and circuital properties of lateral septum neurons and their relationship with hippocampal rhythms. Extra- and intracellular recordings (n=81) were obtained in urethane-anesthetized rats. Two neuronal populations were found, one of them with hippocampal theta; rhythm dependence (theta-D, 68%), and the other group independent of hippocampal theta; rhythm (theta-I, 32%). Other differences were spontaneous firing rate (theta-D=13.20+/-2.09, theta-I=6.99+/-1.18; p<0.005) with a bursting pattern in the theta-D group and single discharges in the theta-I group. Intracellular recordings showed higher synaptic activity in theta-D than in theta-I neurons. Both groups showed fast spikes while only theta-D neurons had high-threshold and low-threshold slow spikes. theta-D neurons had theta-oscillations in their membrane potential. Stimulation of the contralateral lateral septum resets the hippocampal theta rhythm and the theta rhythm recorded intracellularly in the lateral septum neurons. Some neurons (10.6%) showed rhythmic oscillations lasting a few seconds, at a higher frequency than those of the simultaneously recorded hippocampal EEG. This high frequency appeared spontaneously or could be evoked by stimulations of the fornix and reticularis pontis oralis nucleus (RPO). Homologous high frequency oscillations appeared in the simultaneously recorded hippocampal EEG, synchronized with the neuronal activity, during fornix stimulation. RPO stimulation evoked increments (57%) or decrements (43%) in the firing rate of lateral septum neurons. Thus, they could participate in different septal networks to modulate the theta rhythm. The marked functional relationship between lateral septum neurons and hippocampal theta rhythm supports the proposal that lateral septum represents a feedback system for the improvement of medial septum theta rhythm. The hypothalamic projections could be a way to introduce theta and higher rhythms into this structure that control many biological rhythms.

The pharmacology of amino-acid responses in septal neurons

Septal cholinergic neurons are known to play an important role in cognitive processes including learning and memory through afferent innervation of the hippocampal formation and cerebral cortex. The septum contains not only cholinergic neurons but also various types of neurons including GABA (gamma-aminobutyric acid)-ergic neurons. Although synaptic transmission in the septum is mediated primarily by the activation of excitatory and inhibitory amino-acid receptors, it is possible that a distinct phenotype of neuron is endowed with a different type for each of the amino-acid receptors and thus they play different roles from each other, since it has been demonstrated within the septum that there is a regional distribution of various types of amino-acid receptor subunits, their expression as different combinations within a specific cell may produce receptor channels with disparate functional properties. As a first step towards knowing the various functions of septal cholinergic neurons, we characterized the functional properties of glutamate, GABA (type A; GABAA) and glycine receptor channels on cultured rat septal neurons which were histologically identified to be cholinergic. These were similar to those of receptor channels on other types of neurons, except for the actions of some neuromodulators. The septal N-methyl-D-aspartate receptor channel was distinct in being less sensitive to Mg2+ and in a voltage-dependent action of Zn2+. The septal GABAA receptor channel exhibited a lanthanide site whose activation resulted in a positive allosteric interaction with a binding site of pentobarbital. The septal glycine receptor channel was only positively modulated by Zn2+; this action of Zn2+ was not accompanied by an inhibitory effect. Our data suggest that the amino-acid receptors on septal cholinergic neurons may play a distinct role compared to other types of neurons; this difference depends on the actions of neuromodulators and metal cations. It would be interesting to compare these effects recorded in tissue culture to those observed with septal cholinergic neurons in slice preparations.

Depletion of glucose causes presynaptic inhibition of neuronal transmission in the rat dorsolateral septal nucleus

The role of glucose in synaptic transmission was examined in the rat dorsolateral septal nucleus (DLSN) with single-microelectrode voltage-clamp and slice-patch technique. Removal of glucose from the oxygenated Krebs solution caused an outward current associated with an increased membrane conductance. The current-voltage relationship (I-V curve) showed that the hypoglycemia-induced outward current was reversed in polarity at the equilibrium potential for K+. Exposure of DLSN neurons to the glucose-free solution for 5-20 min depressed the excitatory postsynaptic current (EPSC), the inhibitory postsynaptic current (IPSC), and the late hyperpolarizing current (LHC). Replacement of glucose with 2-deoxy-D-glucose (2DG), an antimetabolic substrate, mimicked the deprivation of glucose. Mannoheptulose (10 mM) and dinitrophenol, inhibitors of glucose metabolism, also depressed the PSCs, even in the presence of 10 mM glucose. Glucose-free perfusion did not significantly depress the glutamate-induced inward current, indicating that the inhibition of the EPSC by the glucose-free perfusion was presynaptic. gamma-Aminobutyric acid (GABA)-induced outward currents were depressed by the glucose-free solution. Intracellular dialysis of DLSN neurons with a patch-pipette solution containing 5 mM ATP attenuated the hypoglycemia-induced outward current. Glucose-free superfusion consistently inhibited the IPSC and the LHC without changing the GABA-induced outward current in ATP-treated DLSN neurons. It is suggested that glucose metabolism directly regulates the release of both excitatory amino acids and GABA from the presynaptic nerve terminals.

Electrophysiological actions of GABAB agonists and antagonists in rat dorso-lateral septal neurones in vitro

1. The actions of GABAB-receptor agonists and antagonists on rat dorso-lateral septal neurones in vitro were recorded with intracellular microelectrodes. 2. In the presence of 1 microM tetrodotoxin to prevent indirect neuronal effects caused by action potential-dependent neurotransmitter release, bath application of baclofen (0.1-30 microM) or SK&F 97541 (0.01-3 microM) evoked concentration-dependent hyperpolarizations which reversed close to the potassium equilibrium potential; the EC50S were 0.55 and 0.05 microM, respectively. No significant desensitization was observed during prolonged agonist exposure (< or = 10 min). 3. Hyperpolarizations induced by baclofen were antagonized in a competitive manner by the following GABAB-receptors antagonists (calculated pA2 values in parentheses): CGP 36742 (4.0), 2-OH saclofen (4.2), CGP 35348 (4.5), CGP 52432 (6.7) and CGP 55845A (8.3). Responses to SK&F 97541 were also antagonized by CGP 55845A (pA2 = 8.4). 4. The amplitude of the late, GABAB receptor-mediated inhibitory postsynaptic potential (i.p.s.p.) was reduced by the GABAB antagonists as follows (means +/- s.e.mean): CGP 55845A (1 microM) 91 +/- 5%, CGP 52432 (1 microM) 64 +/- 5%, CGP 35348 (100 microM) 82 +/- 5%, CGP 36742 (100 microM) 76 +/- 8%, and 2-OH saclofen (100 microM) 68 +/- 3%. 5. It is concluded that neurones in the rat dorso-lateral septal nucleus express conventional GABAB receptors, which are involved in the generation of slow inhibitory postsynaptic potentials. CGP 55845A is the most potent GABAB receptor antagonist described in this brain area.

A physiological role for GABAB receptors and the effects of baclofen in the mammalian central nervous system

The inhibitory neurotransmitter GABA acts in the mammalian brain through two different receptor classes: GABAA and GABAB receptors. GABAB receptors differ fundamentally from GABAA receptors in that they require a G-protein. GABAB receptors are located pre- and/or post-synaptically, and are coupled to various K+ and Ca2+ channels presumably through both a membrane delimited pathway and a pathway involving second messengers. Baclofen, a selective GABAB receptor agonist, as well as GABA itself have pre- and post-synaptic effects. Pre-synaptic effects comprise the reduction of the release of excitatory and inhibitory transmitters. GABAergic receptors on GABAergic terminals may regulate GABA release, however, in most instances spontaneous inhibitory synaptic activity is not modulated by endogenous GABA. Post-synaptic GABAB receptor-mediated inhibition is likely to occur through a membrane delimited pathway activating K+ channels, while baclofen, in some neurons, may activate K+ channels through a second messenger pathway involving arachidonic acid. Some, but not all GABAB receptor-gated K+ channels have the typical properties of those G-protein-activated K+ channels which are also gated by other endogenous ligands of the brain. New, high affinity GABAB antagonists are now available, and some pharmacological evidence points to a receptor heterogeneity. The pharmacological distinction of receptor subtypes, however, has to await final support from a characterization of the molecular structure. The function importance of post-synaptic GABAB receptors is highlighted by a segregation of GABAA and GABAB synapses in the mammalian brain.

Ethanol differentially modulates GABAA receptor-mediated chloride currents in hippocampal, cortical, and septal neurons in rat brain slices

Previous electrophysiological studies have reported conflicting results concerning the effects of ethanol on gamma-aminobutyric acid-A (GABAA) receptor-mediated responses in the brain. To examine the variables that might explain these inconsistencies, the present study was designed to determine whether ethanol modulation of synaptically evoked GABA responses is brain region dependent, to identify factors that might regulate ethanol sensitivity, and to investigate the mechanism(s) underlying ethanol modulation of GABA responses. Whole-cell voltage clamp methods were used to examine the effects of ethanol on synaptically evoked GABAA inhibitory postsynaptic currents (IPSCs) recorded from neurons in hippocampus, cerebral cortex, and intermediate lateral and medial septum from rat brain slice preparations. Bicuculline-sensitive IPSCs elicited by local stimulation were pharmacologically isolated by pretreatment with the glutamate specific antagonists, DL-(-)-2-amino-5-phosphonovaleric acid (APV) and 6, 7-dinitroquinoxaline-2, 3-dione (DNQX). Superfused ethanol (80 mM) potentiated evoked GABAA IPSCs in cortical neurons and in intermediate lateral and medial septal neurons but not in CA1 hippocampal neurons. However, the mechanism by which ethanol enhanced GABAA IPSC amplitudes differed between brain regions. In cortex, ethanol induced a hyperpolarizing shift in the GABAA IPSC reversal potential (EIPSC) without modifying the underlying GABAA receptor-mediated conductance (GIPSC). In contrast, ethanol enhanced GABAA IPSC amplitudes differed between brain regions. In cortex, ethanol induced a hyperpolarizing shift in the GABAA IPSC reversal potential (EIPSC) without modifying the underlying GABAA receptor-mediated conductance (GIPSC). In contrast, ethanol enhanced GABAA IPSC amplitudes in lateral and medial septal neurons by increasing the GIPSC without modifying the EIPSC. These results suggest that ethanol differentially modulates responses to endogenous GABA released during synaptic activation and that important differences between various brain regions may reflect multiple mechanisms of ethanol action.

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

A comparative study of age-related changes in inhibitory processes and long-term potentiation in the lateral septum of mice

Anaesthetized C57 BL/6 mice of different ages (young: 5 months; middle-aged: 15 months; and old: 21 months) were used to determine whether aging alters the efficiency of synaptic inhibition and long-term potentiation (LTP) in the lateral septum (LS). Electrical stimulation of the fimbria induced field potentials in the ipsilateral LS comprising two initial negative components (N2 and N3) followed by a positive wave of low amplitude. Paired-pulse experiments showed a facilitation of the N2 component and a concomitant depression of the N3 components. Facilitation of the N2 component was stronger in both middle-aged and old mice as compared to young mice, whereas an inverse pattern of changes was observed for inhibition of the N3 component. High-frequency stimulation of the fimbria produced a persistent increase in the N3 amplitude. This LTP was of significantly higher amplitude in both young and middle-aged mice as compared to old mice. These results suggest that aging impairs both inhibitory processes and synaptic plasticity in the mouse LS, but that inhibitory processes appear to be affected earlier.

Morphological and electrophysiological evidence for electrotonic coupling of rat dorsolateral septal nucleus neurons in vitro

Intracellular injections of Lucifer Yellow were utilized to evaluate the incidence of dye-coupling among dorsolateral septal nucleus (DLSN) neurons recorded from slice preparations of adult rat septal nuclei. Twenty percent of single injections of Lucifer Yellow resulted in pairs of labeled neurons. These dye-coupled cells were morphologically heterogeneous and did not exhibit any morphological characteristics that could be used to distinguish them from non dye-coupled neurons. The spatial separation of cell bodies and close apposition of dendrites within each pair indicated that the dye transfer site(s) were situated at dendrodendritic and/or dendrosomatic rather than somatosomatic junctions. The main axon of some dye-coupled neurons gave rise to intrinsic axon collaterals prior to exiting the nucleus indicating that these coupled neurons function as projection neurons as well as local circuit interneurons. Electrophysiological recordings of the passive membrane properties and spontaneous activity of individual dye-coupled neurons revealed no significant difference from non dye-coupled cells in the DLSN. Some neurons exhibited spontaneously occurring fast potentials which presumably represent electrotonic potentials. These fast potentials were often tightly coupled with action potentials but could be distinguished from synaptic potentials by their shape and their lack of voltage-dependent changes in amplitude. These morphological and supportive electrophysiological data provide the first indirect evidence for electrotonic coupling of dorsolateral septal neurons. The functional significance of this coupling may lie in the potential for synchronization of the output of the DLSN which could play an important role in the septal maintenance and modulation of hippocampal Theta rhythm.

Glucose regulation of synaptic transmission in the dorsolateral septal nucleus of the rat

Intracellular recordings were made from neurons in the dorsolateral septal nucleus (DLSN) of rat brain slices. Lowering the concentration of extracellular glucose resulted in a concentration-dependent membrane hyperpolarization associated with a cessation of spontaneous firing. The amplitude of the excitatory postsynaptic potential (EPSP), inhibitory postsynaptic potential (IPSP), and late hyperpolarizing potential (LHP) evoked by a single stimulus applied to the fimbrial/fornix pathway was decreased when the concentration of glucose was reduced to 0-2 mM. Substitution of glucose with 2-deoxy-D-glucose (11 mM), an antimetabolite of glucose substrate, mimicked the effects of glucose depletion. Mannoheptulose (10-20 mM), a potent hexokinase blocker, and dinitrophenol (50 microM), a potent inhibitor of oxidative phosphorylation, produced both the hyperpolarization and inhibition of postsynaptic potentials, even in the presence of 11 mM glucose. The sulphonylureas, glibenclamide (10 microM) and tolbutamide (1 mM), did not antagonize the hyperpolarization and the inhibition of the postsynaptic potentials produced by glucose depletion. The amplitude of membrane depolarizations produced by pressure application of glutamate (10 mM) and the membrane hyperpolarizations produced by pressure application of either muscimol (1 mM) or baclofen (1 mM) were almost unchanged, even when glucose was reduced to 1-2 mM. These results indicate that intracellular glucose metabolism regulates the function of septal neurons, not only by changing the resting membrane potential, but also by presynaptically affecting neurotransmission between the hippocampal formation and the lateral septum.

GABA-induced responses in Purkinje cell dendrites of the rabbit cerebellar slice

Pressure applications of GABA localized to Purkinje cell somas in a rabbit cerebellar slice produced uniphasic hyperpolarizing responses, whereas applications of GABA that were directed at the Purkinje cell dendrites produced complex, triphasic responses with hyperpolarizing and depolarizing components. Both somatic and dendritic application of GABA elicited fast hyperpolarization (GABAhf), but dendritic application also elicited a slower depolarization (GABAd) and a later, long-lasting hyperpolarization (GABAhl). All three types of responses were accompanied by increased conductance. Use of either GABA antagonist, bicuculline or picrotoxin, eliminated the GABAhf and GABAd responses but left the GABAhl response intact. Pressure delivery of the GABA agonist, baclofen, to the dendrites but not the soma elicited a GABAhl response. Application of baclofen paired with membrane depolarization sufficient to elicit local, calcium-dependent dendritic spiking produced a persistent reduction in the GABAhl response, whereas alternating presentations of baclofen and membrane depolarization or presentations of baclofen alone could not. The fact that GABA and baclofen inhibited Purkinje cell activity in the rabbit cerebellar slice and that picrotoxin and bicuculline eliminated some, but not all of the components of the GABA response suggests the presence of both GABAA and GABAB receptors. The ability of baclofen to inhibit Purkinje cells if it was applied to the dendrites but not if applied to the soma suggests that GABAB receptors are located predominantly on Purkinje cell dendrites. The pairing-specific change in the baclofen response suggests the existence of GABAB-mediated modifiability of Purkinje cell dendrites.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabotropic glutamate receptors are required for the induction of long-term potentiation

Recent observations have led to the suggestion that the metabotropic glutamate receptor may play a role in the induction or maintenance of long-term potentiation (LTP). However, experimental evidence supporting a role for this receptor in the induction of LTP is still inconclusive and controversial. Here we report that, in rat dorsolateral septal nucleus (DLSN) neurons, which have the highest density of metabotropic receptors and show functional responses, the induction of LTP is not blocked by the NMDA receptor antagonist 2-amino-5-phosphonovalerate, but is blocked by two putative metabotropic glutamate receptor antagonists, L-2-amino-3-phosphonopropionic acid and L-2-amino-4-phosphonobutyrate. Furthermore, superfusion of (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid, a selective metabotropic glutamate agonist, resulted in a long-lasting potentiation of synaptic transmission similar to that induced by tetanic stimuli. Our results demonstrated that activation of postsynaptic metabotropic receptors is both necessary and sufficient for the induction of LTP in the DLSN, and we suggest that such a mechanism may be important at other CNS synapses.

Adenosine inhibits the synaptic potentials in rat septal nucleus neurons mediated through pre- and postsynaptic A1-adenosine receptors

Intracellular and voltage-clamp recordings were made from neurons in rat brain slices containing dorsolateral septal nucleus (DLSN), in vitro. Bath application of adenosine (100 microM) produced a hyperpolarization (2-15 mV) in 46% of DLSN neurons (AH-neurons); in the remaining 54% neurons (non-AH-neurons), no hyperpolarization to adenosine was observed. Adenosine (1-300 microM) depressed not only the excitatory postsynaptic potential (EPSP) but also the inhibitory postsynaptic potential (IPSP) and the late hyperpolarizing potential (LHP) evoked by stimulation of the hippocampal CA3 area or the fimbria/fornix pathway in both AH- and non-AH-neurons. In non-AH-neurons, adenosine did not block current responses resulting from glutamate, muscimol or baclofen applied directly to DLSN neurons. In AH-neurons, adenosine partially depressed the baclofen-induced outward current. Adenosine did not block the directly-evoked IPSP (monosynaptic IPSP) as well as the glutamate-induced (hyperpolarizing) postsynaptic potential (PSP) that is mediated by GABA released from interneurons. These results suggest that adenosine does not directly inhibit the release of GABA. The effects of adenosine was mimicked by selective A1-receptor agonists and was blocked by selective A1-receptor antagonists. Pertussis toxin (PTX) blocked the hyperpolarization induced by adenosine or baclofen applied exogenously. Adenosine consistently produced presynaptic inhibition of the EPSP even in DLSN neurons treated with PTX. We conclude that adenosine inhibits neurotransmission between the hippocampus and septum through activation of pre- and postsynaptic A1-receptors which couple with G-proteins of different PTX-sensitivity or with distinct transduction processes at pre- vs. postsynaptic sites.

Spontaneous bursting and non-bursting activity in morphologically identified neurons of the rat dorsolateral septal nucleus, in vitro

Membrane potential-dependent changes in the repetitive firing properties of morphologically identified rat dorsolateral septal nucleus neurons were investigated in a submerged slice preparation using intracellular recording techniques and lithium acetate-Lucifer Yellow-filled microelectrodes. The results indicate that the majority of dorsolateral septal nucleus neurons are capable of burst firing and suggest, moreover, the existence of neuronal subtypes with distinct differences in spike waveform and the pattern of spontaneous activity. In the largest proportion of neurons, single spike activity predominated at membrane potentials near rest while burst-like discharges prevailed at more hyperpolarized membrane potentials. Less frequently observed were neurons exhibiting different burst waveforms at various membrane potentials. In a few neurons, hyperpolarization slowed neuronal firing but did not elicit burst-like discharges. Characteristics such as the presence of burst or single spike discharges, spike afterpotentials, and the membrane potential dependence of repetitive firing patterns did not appear to be closely associated with membrane time constant, membrane resistance, or resting membrane potential. A detailed examination of the somatodendritic and axonal morphology of the Lucifer Yellow-filled cells revealed that these electrophysiologically identified neurons in the dorsolateral septal nucleus are morphologically heterogeneous. However, there did not appear to be any correlation between a particular somatodendritic morphology and the expression of a distinct spontaneous firing pattern. The present findings demonstrate that neurons in the rat dorsolateral septal nucleus are morphologically diverse and capable of intrinsically generating rhythmic neuronal activity. Similar patterns of rhythmic neuronal firing in vivo may provide a substrate for the integration of afferent neuronal activity and have a central role in intraseptal circuitry necessary for generation of hippocampal theta rhythm.

Long-term potentiation of excitatory and slow inhibitory synaptic potentials in the hippocampal-septal projection of the rat

The hippocampal projection to the lateral septum was examined for use-dependent plasticity in a brain slice preparation using intracellular recording. Paired-pulse facilitation and posttetanic potentiation were present. Long-term potentiation (LTP) following high-frequency stimulation was observed after treatment with the GABAA antagonist bicuculline. A slow inhibitory synaptic potential also exhibited long-lasting potentiation after high-frequency stimulation. LTP of both components of the response was sensitive to NMDA antagonists.

Bursting discharges in disinhibited amygdala slices: the role of excitatory amino acid receptors

The involvement of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the epileptiform activity, induced by bicuculline, was studied in slices of amygdala in the rat, using intracellular recording techniques. Stimulation of the ventral endopyriform nucleus evoked an excitatory postsynaptic potential (EPSP). After exposure to bicuculline (20 microM), the same stimulus evoked burst firing. Occasionally, spontaneous bursts similar in waveform to synaptically triggered bursts also occurred in disinhibited slices. Superfusion of DL-2-amino-5-phosphonovalerate (DL-APV, 50 microM) or 3-((+)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acids (CPP, 10 microM), rapidly blocked the late component of the paroxysmal depolarizing shift. The spontaneous and evoked bursts were never completely abolished in the presence of DL-APV or CPP. These results suggest that NMDA receptors may contribute to but are not required for the generation of these bursts. In contrast, application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM) largely abolished the bursts, indicating that activation of non-NMDA receptors is of primary importance in this model of epilepsy.

Somatostatin depresses GABA receptor-mediated inhibition in the rat dorsolateral septal nucleus

The effect of somatostatin-14 (SS-14) on gamma-aminobutyric acid (GABA)-mediated inhibitory neurotransmission in the dorsolateral septal nucleus (DLSN) was investigated using a submerged slice preparation and intracellular recording techniques. Somatostatin-14 applied by superfusion or by pressure ejection from micropipettes predominantly inhibited the intracellularly recorded fast inhibitory postsynaptic potential (fIPSP) and late hyperpolarizing potential (LHP) elicited by focal electrical stimulation of the DLSN. The decreases in LHP and fIPSP amplitude occurred at low concentrations of peptide, in the absence of appreciable changes in the passive-membrane properties of postsynaptic neurons, and outlasted the membrane hyperpolarizing effect produced by SS-14 at higher concentrations. The ability of SS-14 to modulate postsynaptic GABA receptor responses underlying the fIPSP and LHP were investigated by applying baclofen, a selective GABAB receptor agonist, and isoguvacine, a selective GABAA receptor agonist, by pressure ejection. Hyperpolarizing responses to GABAA and GABAB receptor stimulation were significantly decreased during superfusion of SS-14. Tetrodotoxin applied by superfusion blocked electrically evoked synaptic potentials but not the depressant effect of SS-14 on baclofen- or isoguvacine-induced hyperpolarization. Facilitation of the fIPSP or LHP by SS-14 also occurred but less frequently and consistently than the depressant action. Excitatory postsynaptic potentials and membrane response to NMDA or quisqualate appeared unaltered by bath-applied SS-14. These findings suggest a novel postsynaptic action of SS-14 leading to depression of synaptic responses mediated by GABAA and GABAB receptors. Synaptically released SS-14 in the DLSN may participate in modulation of feedforward and/or feedback inhibitory mechanisms coordinating DLSN function in the septo-hippocampal system.

Vasopressin facilitates excitatory transmission in slices of the rat dorso-lateral septum

The effect of vasopressin on neurons of the rat dorso-lateral septum (DLS) was studied in brain slices with intracellular microelectrodes. Two out of 13 neurons showed a small depolarization, spontaneous activity, and increased input resistances following a 15 min exposure to 10(-6) to 10(-8) M vasopressin (VP). These membrane effects disappeared completely within 3-5 min after the application. The remaining DLS neurons treated with these vasopressin concentrations showed an increase in glutamate-mediated excitatory postsynaptic potentials (EPSPs), evoked by stimulation of the fimbria fibers. As little as 10(-12) MVP increased these EPSPs markedly in nearly 80% of the cells studied. This increase in most of the cells disappeared within 15 min after the application period, whereas the increase in EPSPs induced by 10(-10) M VP outlasted the peptide application period for more than 30 min. Neither the blockade of GABA-ergic synaptic inhibition nor the pre-treatment of the neurons with d(CH2)5-Tyr(Me)-arginine vasopressin or 2-amino-5-phosphonovaleric acid (2-APV), antagonists for the V1 type of vasopressin receptor and NMDA receptors, respectively, interfered with the EPSPs potentiating effect of the peptide. It is concluded that a type of vasopressin receptor other then the V1 type is involved in the long-lasting potentiation of the primarily non-NMDA receptor mediated transmission in DLS neurons.

Excitatory amino acid-receptor-mediated EPSPs in rat dorsolateral septal nucleus neurones in vitro

1. Intracellular recordings were made from rat dorsolateral septal nucleus (DLSN) neurones in vitro. We investigated depolarizations resulting from pressure application of excitatory amino acids and compared these to synaptically evoked excitatory postsynaptic potentials (EPSPs). 2. EPSPs evoked by focal fimbrial afferent stimulation in saline with 30-50 microM-bicuculline and 1.2 mM-Mg2+ yielded a linear amplitude-voltage relationship: their reversal potential was -3 mV. These EPSPs exhibited little sensitivity to 2-amino-5-phosphonopentanoate (APV), an N-methyl-D-aspartate(NMDA)-receptor-specific antagonist, but were markedly depressed by kynurenic acid, a broad-spectrum excitatory amino acid antagonist. 3. In Mg2(+)-free solution, the amplitude and the duration of EPSPs were increased markedly masking the following inhibitory postsynaptic potential (IPSP) and the late hyperpolarizing potential (LHP). These facilitated and broadened EPSPs were sensitive to APV or Mg2+. The APV or Mg2(+)-sensitive component of the EPSP obtained by digital subtraction suggests a slower time course for the NMDA-receptor-mediated EPSP compared to the non-NMDA-receptor-mediated EPSP. On the other hand, in normal Mg2+ solution an EPSP evoked by either a single strong stimulus or by repetitive stimuli had APV-sensitive components. 4. The depolarizing potentials induced by pressure application of glutamate, kainate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), quisqualate or NMDA were compared. The amplitude-voltage relationship of depolarizations induced by NMDA obtained in a normal Mg2+ solution was non-linear, but approached linearity when the same responses were recorded in a Mg2(+)-free solution. Depolarizations induced by kainate, AMPA and quisqualate were linear in their amplitude-voltage relationship in the presence or absence of Mg2+. APV blocked NMDA-induced depolarizations specifically, while kynurenic acid blocked all the depolarizations induced by NMDA, quisqualate, or kainate. 5. Our data demonstrate the existence of NMDA-receptor-mediated synaptic potentials in the rat DLSN, the characteristics of which are similar to those in other central nervous system regions.

Bicuculline- and phaclofen-sensitive components of N-methyl-D-aspartate-induced hyperpolarizations in rat dorsolateral septal nucleus neurones

1. Intracellular recordings were made from rat dorsolateral septal nucleus (DLSN) neurones in vitro. Pressure application (puff) of N-methyl-D-aspartate (NMDA) usually produced a hyperpolarization followed by a depolarization in normal Mg2+ solution. 2. A hyperpolarizing response and a depolarizing response could be evoked separately by appropriate positioning of the puff pipette. The NMDA-induced hyperpolarization was blocked by a low-Ca2+ solution or a tetrodotoxin (TTX)-containing solution, but under these conditions the NMDA-induced depolarization was spared. The amplitude of NMDA-induced hyperpolarizations was 7.7 +/- 2.7 mV (n = 13) at the resting membrane potential level. 3. An NMDA-induced hyperpolarization had two components. A fast component reversed at about -76 mV and a slow component reversed at -90 mV. The reversal potential of the slow component shifted in the depolarizing direction of hyperpolarizing direction when the slice was bathed in a high or low-K+ solution, respectively. 4. The reversal potentials of NMDA-induced hyperpolarizations were similar to the synaptic potentials evoked by fimbrial stimulation. The reversal potentials of the fast and slow components were close to the IPSP reversal potential (-70 mV) and the late hyperpolarizing potential (LHP) reversal potential (-95 mV), respectively. 5. NMDA-induced hyperpolarizations were blocked by the specific NMDA receptor antagonist 2-amino-5-phosphonopontanoate (APV). The fast component of an NMDA-induced hyperpolarization was blocked by the specific GABAA receptor antagonist, bicuculline, and the slow component was depressed by the specific GABAB receptor antagonist, phaclofen. 6. Glutamate receptor subtype-specific agonists, such as kainate or quisqualate, could induce similar hyperpolarizations which had bicuculline-sensitive and insensitive components. These non-NMDA-type agonist-induced hyperpolarizations were not affected by APV (50 microM) but were blocked by kynurenic acid (1 mM). 7. We conclude that these excitatory amino acid-induced hyperpolarizations observed in the rat DLSN are mediated by GABAergic interneurones which have both non-NMDA-type and NMDA-type receptors.

A direct nicotinic receptor-mediated inhibition recorded intracellularly in vitro

Acetylcholine activates both nicotinic and muscarinic receptors in the central nervous system. Although the action of acetylcholine at muscarinic receptor has been well characterized, relatively little is known at the cellular level concerning nicotinic receptor stimulation in brain. Central nicotinic receptors have been implicated in Alzheimer's disease, seizure activity, the generation of slow-wave theta rhythm in the hippocampus and the potential abuse liability of nicotine. At the neuronal level, nicotinic agonists have been most often associated with postsynaptically mediated excitation and membrane depolarization at various sites, including Renshaw spinal motoneurons, locus coeruleus and the medial habenular nucleus. Nicotine acting presynaptically can produce either excitation or inhibition indirectly through the release of endogeneous transmitters or modulators. Whereas a direct inhibitory effect of nicotine has been suggested by one in vivo extracellular recording study in rat cerebellar Purkinje neurons, the mechanism(s) underlying this action is not yet known. We now report our findings obtained using in vitro intracellular methods in a submerged brain slice preparation in which application of nicotinic agonists to rat dorsolateral septal neurons reveal a direct membrane hyperpolarization mediated by an increase in potassium conductance.

Somatostatin hyperpolarizes neurons and inhibits spontaneous activity in the rat dorsolateral septal nucleus

Intracellular recordings were made from rat brain neurons in a submerged slice preparation containing the dorsolateral septal nucleus (DLSN). Somatostatin-14 (SS-14) was applied to these neurons by superfusing solutions containing known concentrations of the peptide or by pressure ejection from micropipettes. With either method of treatment, SS-14 produced membrane hyperpolarization and decreased membrane resistance in a concentration-dependent manner. The hyperpolarizing response to SS-14 occurred in virtually all neurons tested and appeared to result from a direct action on DLSN neurons mediated by an increased permeability to potassium ions. The SS-14-induced membrane hyperpolarization was not blocked by naloxone, bicuculline, tetrodotoxin, or calcium-free, high-magnesium superfusion media. In a small number of neurons, SS-14 application produced a membrane depolarization which did not exhibit clear concentration-dependence and was blocked by superfusion of calcium-free, high-magnesium media indicating an indirect action. These findings reveal that SS-14 is a potent inhibitor of DLSN neurons in vitro and provide the first evidence that receptors for this putative neurotransmitter are located on postsynaptic neurons in this nucleus. Synaptically released SS-14 may play an important role in the modulation of septohippocampal function.

Projection neurons in the rat dorsolateral septal nucleus possess recurrent axon collaterals

Projection neurons in the rat dorsolateral septal nucleus (DLSN) were labeled intracellularly with horseradish peroxidase (HRP) in an in vitro slice preparation. The labeled neurons exhibited widespread 'isodendritic' type dendritic fields. Each of the neurons was identified as a projection neuron by the tracing of its main axon out of DLSN. The axons of these neurons gave rise to intrinsic collaterals which branched to form an extensive axon plexus which was confined to DLSN. These axon collaterals exhibited numerous en passant swellings suggestive of boutons. It is proposed that the recurrent axon collaterals of DLSN projection neurons may form an anatomical substrate for local inhibition within DLSN.

Comparison of antagonism by phaclofen of baclofen induced hyperpolarizations and synaptically mediated late hyperpolarizing potentials recorded intracellularly from rat dorsolateral septal neurons

Phaclofen has recently been described as an antagonist to baclofen at both peripheral and central receptors. We have applied phaclofen in known concentrations to an isolated rat brain slice preparation containing the septal nuclei. Our data demonstrate that phaclofen antagonizes responses to exogenously applied baclofen in a competitive manner. On the other hand, phaclofen is not as effective in antagonizing competitively the synaptically mediated late hyperpolarizing response (LHP) recorded from the same or similar dorsolateral septal nucleus (DLSN) neurons from which baclofen responses were recorded. Our data support the usefulness of phaclofen as a competitive antagonist of baclofen, and suggest that when larger stimulus intensities are applied, the LHP in the dorsolateral septum of the rat may be mediated by a transmitter in addition to gamma-aminobutyric acid (GABA).

A physiological role for GABAB receptors in the central nervous system

The role of GABA in synaptic transmission in the mammalian central nervous system is more firmly established than for any other neurotransmitter. With virtually every neuron studied, the synaptic action of GABA is mediated by bicuculline-sensitive GABAA receptors which selectively increase chloride conductance. However, it has been shown that GABA has a presynaptic inhibitory action on transmitter release that is insensiive to bicuculline and is selectively mimicked by baclofen. The receptors involved in this action are referred to as GABAB receptors, to distinguish them from the classic bicuculline-sensitive GABAA receptors. In hippocampal pyramidal cells an additional postsynaptic action of GABA and baclofen has been reported that is also insensitive to GABAA antagonists, and may be mediated by GABAB receptors on the postsynaptic neuron. This action of GABA and baclofen involves an increase in potassium conductance. Synaptic activation of pathways converging on hippocampal pyramidal cells results in a slow inhibitory postsynaptic potential which involves an increase in potassium conductance, and it has been suggested that GABAB receptors might be responsible for this synaptic potential. However, to establish convincingly that GABAB receptors are physiologically important in the central nervous system, a selective GABAB antagonist is required. Here we provide this missing evidence. Using the hippocampal slice preparation, we now report that the phosphonic acid derivative of baclofen, phaclofen, is a remarkably selective antagonist of both the postsynaptic action of baclofen and the bicuculline-resistant action of GABA, and that it selectively abolishes the slow inhibitory postsynaptic potential in pyramidal cells.

Disinhibition in the rat septum mediated by M1 muscarinic receptors

Acetylcholine (ACh) has been documented as an important central neurotransmitter. We have investigated the actions of ACh within the dorsolateral septal nucleus of the rat to examine its actions within this nucleus, specifically how it may interact to modulate the inhibitory action of gamma-aminobutyric acid (GABA), the known inhibitory transmitter in this area. Our results demonstrate that ACh, acting on M1 muscarinic receptors leads to disinhibition by decreasing GABA release.

Actions of serotonin recorded intracellularly in rat dorsal lateral septal neurons

The actions of serotonin (5HT) on passive and active membrane properties of neurons in the rat dorsal lateral septal nucleus (LSN) were studied by using intracellular recordings in transverse, septal slices. Superfusion with 10 microM 5HT induced a hyperpolarization of the membrane in almost all neurons tested in the dorsolateral part of the LSN. The hyperpolarization was accompanied by a decrease in membrane resistance. These effects of 5HT persisted in a low-Ca2+/high-Mg2+-containing medium or medium with tetrodotoxin, indicating a post-synaptic site of action for 5HT. The reversal potential for the hyperpolarizing effect was ca. -95 mV. If the extracellular K+-concentration was raised, the reversal potential became less negative. These data suggest that 5HT hyperpolarizes LSN neurons by increasing a K+-conductance. Spontaneous, synaptically evoked action potentials and action potentials induced in LSN neurons by a depolarizing current step typically display a fast Na+-spike with a subsequent K+-afterhyperpolarization, followed by a much slower Ca2+-dependent afterdepolarization. The amplitude of the K+-afterhyperpolarization was decreased by 5HT, while at the same time the afterdepolarization became more pronounced. The Ca2+-spike of LSN neurons was not affected by 5HT. Synaptic responses that were evoked in LSN neurons by stimulation of the dorsal part of the LSN consisted of a fast EPSP or spike, followed by a Cl(-)-dependent fast IPSP and a K+-dependent late IPSP. Of these synaptic responses, 5HT suppressed particularly the late IPSP. The present data indicate that 5HT affects the conductance for active and passive K+-channels in LSN neurons.(ABSTRACT TRUNCATED AT 250 WORDS)