Rat lateral septum in slice preparation with viable transmission

Effects of temperature increase on the propagation of presynaptic action potentials in the pathway between the Schaffer collaterals and hippocampal CA1 neurons

Effects of temperature increase on the neuronal activity of hippocampal CA2-CA1 regions were examined by using optical and electrophysiological recording techniques. Stimulation of the Schaffer collaterals at the CA2 region evoked depolarizing optical signals that spread toward the CA1 region at 32 degrees C. The optical signal recorded by 49 pixels was characterized by fast and slow components that were closely related to presynaptic action potentials and excitatory postsynaptic responses, respectively. The optical signal was depressed by temperature increase to 38-40 degrees C. The temperature increase to 38 degrees C produced a hyperpolarization and a depression of the excitatory postsynaptic potential (EPSP) in single hippocampal CA1 pyramidal neurons. The depression of the neuronal activity induced by temperature increase was attenuated by application of glucose (22 mM) or pyruvate (22 mM). Adenosine (200 microM) did not block the presynaptic action potential but strongly depressed the excitatory postsynaptic response. 8-Cyclopentyl-1,3-dimethylxanthine (8-CPT) (10 microM), an antagonist for adenosine A(1) receptors, attenuated the depression of the excitatory postsynaptic response but not the inhibition of the presynaptic action potential at 38 degrees C. These results suggest that adenosine mediates the high-temperature-induced depression of the excitatory synaptic transmission but not that of action potential propagation in rat CA1 neurons.

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.

Effects of vasopressin and related peptides on neurons of the rat lateral septum and ventral hippocampus

The effects of vasopressin (VP), VP fragments and propressophysin glycopeptide on neuronal activities in the septum-hippocampus complex of rats were studied in vitro and in vivo. The frequency of the hippocampus theta rhythm in Brattleboro rats homozygous for diabetes insipidus was significantly slower than that of heterozygous litter mates and normal rats. Intracerebroventricular micro-injection of des-glycine-amide vasopressin corrected for several hours the frequency deficit of the theta rhythm in the homozygous Brattleboro rats and the centrally administered VP slowed down theta rhythm in normal rats. Microinotophoretically administered VP excited single neurons in the lateral septum of ventral hippocampus, and/or facilitated the responses of these neurons to glutamate and to stimulation of the glutamatergic afferent fibers in the fimbria bundle. The excitatory effects of VP vanished within seconds after termination of the peptide administration, however, the peptide-induced enhancement of glutamate and syntatically induced excitations were sustained for up to 60 min after the peptide administration. In vitro, pM concentrations of VP, VP 4-8 and C-terminus glycopeptide of propresophysin facilitated for 30-60 min the glutamate-mediated EPSPs in neurons of the lateral septum or the ventral hippocampus. The EPSPs increase in the lateral septum neurons was not prevented by pretreatment with antagonist of the V1a type of the vasopressin receptor. The resting membrane potential and input resistance were not affected by the peptides. A low-frequency electrical stimulation in the diagonal Band of Broca or in the Bed nucleus of the stria terminals, sources of the vasopressinergic innervation of the septum, facilitated the negative wave of the filed potentials responses evoked in the lateral septum by stimulating the fimbria bundle fibers in control Long-Evans and Brattleboro rats heterozygous for diabetes insipidus. The field potential increase was sustained for several hours after the stimulation, and it was not occluded by long-term potentiation elicited by high frequency stimulation of the fimbria bundle afferent fibers. Brattleboro rats homozygous for diabetes insipidus failed to show the filed potential increase after the diagonal band stimulation. It is suggested that the long-lasting facilitation of glutamate-mediated excitations might be a physiological action of the propressophysin-derived peptides in the septum-hippocampus complex which, in concert with other forms of synaptic plasticity like the long-term potentiation, facilitates the hippocampus-mediated forms of learning and memory. This action is presumably related to the memory enhancing effect of the propressophysin-derived peptides.

Long-lasting enhancement of synaptic excitability of CA1/subiculum neurons of the rat ventral hippocampus by vasopressin and vasopressin(4-8)

Vasopressin (VP) is axonally distributed in many brain structures, including the ventral hippocampus. Picogram quantities of VP injected into the hippocampus improve the passive avoidance response of rats, presumably by enhancing memory processes. Vasopressin is metabolized by the brain tissue into shorter peptides, such as [pGlu4,Cyt6]VP(4-9) and [pGlu4,Cyt6]VP(4-8), which preserve the behavioral activity but lose the peripheral activities of the parent hormone. Using brain slices, we investigated whether VP or VP(4-8) affects excitatory postsynaptic potentials (EPSPs) and/or membrane responses to depolarization in neurons of the CA1/subiculum of the ventral hippocampus. The EPSPs were evoked by stimulating the striatum radiatum of the CA1 field; the membrane responses were elicited by current injections. Exposure of slices for 15 min to 0.1 nM solution of these peptides resulted in an increase in the amplitude and slope of the EPSPs in 21 neurons (67%) tested. No consistent change in either the resting membrane potential or the input resistance of the neurons was observed. The peptide-induced increase in EPSPs reached a maximum 30-45 min after peptide application. In 14 of these neurons (66%), the peptide-induced increase in EPSPs remained throughout the entire 60-120 min washout period. In the remaining 7 neurons (33%), the initial increase in EPSPs amplitude was followed by a gradual decline to the pre-administration level. The increase in EPSP amplitude was often, but not always, associated with a decrease in the threshold and increase in the number of action potentials in response to depolarizing current injection. Suppression of GABAA receptor-mediated inhibition and N-methyl-D-aspartate (NMDA) receptor-mediated excitation did not prevent the effects of VP and VP(4-8) on the EPSP amplitude or the threshold for action potentials. The results demonstrate that 0.1 nM concentrations of these neuropeptides can elicit a long-lasting enhancement of the excitability of CA1/subiculum neurons of the ventral hippocampus to excitatory, glutamatergic synaptic input. This novel action of VP and its metabolite in the ventral hippocampus may be the physiological action, mediating the memory-enhancing effect of these peptides.

The impaired long-term potentiation in the CA1 field of the hippocampus of cognitive deficient microencephalic rats is restored by D-serine

Rat embryos exposed on gestational day 15 to methyl-azoxymethanol acetate develop a microencephaly characterized primarily by a hypoplasia of the neocortex and CA fields of the hippocampus that in adulthood is associated with disturbances in learning. In brain slices prepared from microencephalic rats, we have examined the field excitatory postsynaptic potentials and population spike in the CA1 field of the hippocampus evoked by stimulation of the stratum radiatum. These parameters did not differ from those obtained in slices from control rats. High frequency stimulation of the stratum radiatum afferent fibres, which readily induced long-term potentiation of the field excitatory postsynaptic potentials and population spike in the CA1 field of the hippocampus of control rats, failed to induce long-term potentiation in that of microencephalic rats. High frequency stimulation of the perforant path readily elicited long-term potentiation in the dentate gyrus of both control and microencephalic rats. Picrotoxin had no apparent effect on field excitatory postsynaptic potentials and population spike in the CA1 field of the microencephalic rats, indicating that little GABAergic inhibition was present in slices from these rats. D-2-Amino-phosphonovalerate suppressed the field potentials in slices from microencephalic rats by more than 50%, suggesting that N-methyl-D-aspartate receptors contributed markedly to the synaptic responses evoked by single stimuli. D-Serine, but not picrotoxin, restored long-term potentiation in the CA1 field of the microencephalic rats. The D-serine effect was prevented by pretreating the slices with either 7-chloro-kynurenate or D-2-amino-phosphonovalerate. The failure to induce long-term potentiation, if also found in vivo, may be among the factors related to the learning deficits displayed by these rats.

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.

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.

The C-terminal glycopeptide of propressophysin potentiates excitatory transmission in the rat lateral septum

Effects of peptides synthesized from the same precursor as vasopressin, i.e. the C-terminal 39-amino acid long glycopeptide and neurophysin II, were investigated for biological activities in electrophysiological experiments in brain slices of the rat lateral septum. These slices contained the glycopeptide as the predominant form and a fragment of it, amino acid sequence 22-39, as a minor form (8% of the glycopeptide 1-39), as shown by high performance liquid chromatography of extracts and by radioimmunoassay. None of the peptides, neurophysin II, the glycopeptide 1-39 and the fragment 22-39, tested in a concentration of 10(-12) M, had measurable effects on the resting membrane potential of the neurons. The glycopeptide and the fragment 22-39, however, increased, in some cells, for tens of minutes the excitatory postsynaptic potentials evoked in these neurons by stimulation of the fimbria fibers. The increase in input resistance, seen in many septal neurons treated with either of the peptides was not correlated with the excitatory postsynaptic potential increase. Neurophysin II affected neither the excitatory postsynaptic potentials nor the input resistance of the neurons. It is concluded that the glycopeptide 1-39 and the fragment 22-39 possess biological activities amongst which the facilitation of excitatory amino acid transmission on lateral septum neurons. Therefore, these peptides derived from the vasopressin precursor may act in concert with vasopressin to establish facilitation of excitatory transmission in the brain.

Vasopressin maintains long-term potentiation in rat lateral septum slices

In brain slices of normal Wistar and Long-Evans rats, brief high frequency stimulation of the fimbria fibers induced long-term potentiation (LTP) in excitatory transmission between these fimbria fibers and neurons of the lateral septum (LS). Slices prepared from diabetes insipidus (DI) Brattleboro rats, that contained no vasopressin (VP), consistently failed to maintain LTP in this excitatory transmission. Exogenous VP, administered to slices from DI Brattleboro rats shortly prior to the experiment or released from a subcutaneous depot in DI Brattleboro rats for several days prior to decapitation, corrected this failure. The maintenance of LTP in the LS in slices from Wistar and Long-Evans rats was prevented by D(CH2)5-Tyr(Me)-arginine VP, an antagonist for the V1 type of VP receptors. These results indicate an important role of VP in the maintenance of LTP in excitatory transmission in the LS. It is conjectured that the effects of VP on LS neurons are related to the role of the peptide in the maintenance of LTP and that these processes play a role in memory formation.