Bidirectional synaptic plasticity is driven by sex neurosteroids targeting estrogen and androgen receptors in hippocampal CA1 pyramidal neurons

Neuroactive estrogenic and androgenic steroids influence synaptic transmission, finely modulating synaptic plasticity in several brain regions including the hippocampus. While estrogens facilitate long-term potentiation (LTP), androgens are involved in the induction of long-term depression (LTD) and depotentiation (DP) of synaptic transmission. To examine sex neurosteroid-dependent LTP and LTD in single cells, patch-clamp recordings from hippocampal CA1 pyramidal neurons of male rats and selective antagonists for estrogen receptors (ERs) and androgen (AR) receptors were used. LTP induced by high-frequency stimulation (HFS) depended on activation of ERs since it was prevented by the ER antagonist ICI 182,780 in most of the neurons. Application of the selective antagonists for ERα (MPP) or ERβ (PHTPP) caused a reduction of the LTP amplitude, while these antagonists in combination, prevented LTP completely. LTP was never affected by blocking AR with the specific antagonist flutamide. Conversely, LTD and DP, elicited by low-frequency stimulation (LFS), were impeded by flutamide, but not by ICI 182,780, in most neurons. In few cells, LTD was even reverted to LTP by flutamide. Moreover, the combined application of both ER and AR antagonists completely prevented both LTP and LTD/DP in the same neuron. The current study demonstrates that the activation of ERs is necessary for inducing LTP in hippocampal pyramidal neurons, whereas the activation of ARs is required for LTD and DP. Moreover, both estrogen- and androgen-dependent LTP and LTD can be expressed in the same pyramidal neurons, suggesting that the activation of sex neurosteroids signaling pathways is responsible for bidirectional synaptic plasticity

Involvement of estrogenic signal in the induction of LTP by the SIS protocol.

<p><b>A</b>. Averaged traces (n = 20) of EPSPs evoked before (thin traces: pre-SIS) and after SIS (dashed traces: post-SIS) in the presence of letrozole, ICI 182,780, flutamide and letrozole plus flutamide. (Left) Schematic drawing of stimulation pattern inducing LTP (SIS). <b>B</b> and <b>C</b>. Time courses of the responses induced by SIS under block of the E2 synthesising enzyme (<b>B</b>) and of ERs or ARs (<b>C</b>). <b>B</b>. Under letrozole SIS induces LTD (open square, n = 7) and no effect (open circle, n = 6), while under letrozole plus flutamide it has only no effect (filled triangles, n = 8). <b>C</b>. Under ICI 182,780 SIS has no effect (filled triangles, n = 10), while under flutamide it induces LTP (open circles, n = 6) or no effect (open square, n = 2). Note that induction of LTP is fully prevented under block of ERs (ICI 182,780) while in the majority of cases it is not affected by the block of ARs (flutamide). Moreover, under inhibition of the E2 synthesising enzyme (letrozole) LTP by SIS is prevented or reverted into LTD that is abolished by flutamide. <b>D</b>. Frequency occurrence (number of neurons) of LTD (black columns), LTP (grey columns) and no effect (NE, white columns) induced by SIS in the control condition and in the presence of letrozole, ICI 182,780, flutamide and letrzole plus flutamide (χ ² test: <i>*P</i><0.05, **<i>P</i><0.01, ***<i>P</i><0.005, **** <i>P</i><0.001 and n.s. = no significant). <b>E</b>. Comparisons between the amplitude of LTD obtained by SIS under letrozole (n = 7) and LTD normally induced by LIS in control condition (n = 8) and <b>F</b>. Comparison between the amplitudes of LTP induced by SIS in control condition (n = 8) and in the presence of flutamide (n = 6) (one-way ANOVA: n.s. = no significant).</p

Involvement of androgens in the induction of LTD by the LIS protocol.

<p><b>A</b>. Averaged traces (n = 20) of EPSPs evoked before (thin traces: pre-LIS) and after LIS (dashed traces: post-LIS) in the presence of flutamide, finasteride and ICI 182,780. (Left) Schematic drawing of stimulation pattern inducing LTD (LIS). <b>B</b> and <b>C</b>. Time courses of the responses induced by LIS under block of the DHT synthesizing enzyme (<b>B</b>) and of ARs or ERs (<b>C</b>). <b>B</b>. Under finasteride, LIS induces LTD (open square, n = 9) and no effect (open circle, n = 2). <b>C</b>. Under flutamide, LIS has no effect (open square, n = 11) or induces LTD (filled triangles, n = 2), while under ICI 182,780 it only induces LTD (open circles, n = 6). Note that under block of ARs (flutamide) LTD is prevented in the majority of cases, while it is still induced under block of DHT synthesising enzyme (finasteride) and of ERs (ICI 182,780). <b>D</b>. Frequency occurrence (number of neurons) of LTD (black columns), LTP (grey columns) and no effect (NE, white columns) induced by LIS in the control condition and in the presence of flutamide, finasteride and ICI 182,780 (χ² test: ***<i>P</i><0.005, **** <i>P</i><0.001, n.s. = no significant). <b>E</b>. In this and in the subsequent figure columns express the mean ± SE of the EPSP slopes (percentage of baseline) within 2.5-min intervals evaluated at 30 min post-stimulus. Here, comparison between the amplitudes of LTD induced by LIS in control condition (n = 8) and in the presence of finasteride (n = 9) or ICI 182,780 (n = 6) (one-way ANOVA: n.s = no significant). </p

Long-term potentiation and depression are induced by different stimulation patterns at the vestibular afferent synapses of the MVN type B neurons.

<p><b>A</b>. Schematic drawing of rat brainstem slice showing position of stimulating electrode (black area) and the recording loci (grey area) in the ventral portion of the MVN. Abbreviations: D, descending vestibular nucleus; Md, dorsal part of the MVN; Mv, ventral part of the MVN; L, lateral vestibular nucleus; S, superior vestibular nucleus; R, recording and St, stimulating electrodes. <b>B</b>. Representative action potential recorded from type B neuron displaying an early fast and a late slow AHP (arrows), and the ADP (asterisk). <b>C</b>. LTD of EPSP induced by long-interval stimulation (LIS). (Top-left) Averaged traces (n = 20) of EPSPs evoked in type B neurons before (thin trace: pre-LIS) and after LIS (dashed traces: post-LIS). (Top-right) Schematic drawing of LIS protocol with burst duration (BD) = 0.55 s, burst number (BN) = 30 and inter-burst interval (IBI) = 10 s. (Bottom) Time courses of the LIS effects on the slope of EPSP in control condition (circles) and in the presence of AP-5 (squares). In this and following figures each point represents the mean ± SE (from the number of neurons reported in the graph legend) of EPSP slope measured every 15 s and expressed as percentage of EPSP baseline values. <b>D</b>. LTP of EPSP induced by SIS. (Top-left) Averaged traces (n = 20) of EPSPs evoked before (thin trace: pre-SIS) and after SIS (dashed traces: post-SIS). (Top-right) Schematic drawing of SIS protocol with BD = 0.55 s, BN = 4 and IBI = 1 s. (Bottom) Time courses of the SIS effects on the EPSP slope in control condition (circles) and in the presence of AP-5 (squares). In this and the subsequent figures the bars indicate stimulation delivery time, so that the negative values represent the times before the start of stimulation and the positive ones those after the end of stimulation.</p

Different metabotropic glutamate receptors play opposite roles in synaptic plasticity of the rat medial vestibular nuclei

In the medial vestibular nuclei (MVN) of rat brainstem slices, the role of group II and III metabotropic glutamate receptors (mGluRs) and of the subtypes of group I mGluRs: mGluR1, mGluR5, was investigated in basal synaptic transmission and in the induction and maintenance of long-term potentiation (LTP). We used selective antagonists and agonists for mGluRs and we analysed the field potentials evoked by vestibular afferent stimulation before and after high-frequency stimulation (HFS) to induce LTP. The group II and III mGluR antagonist, (R,S)-α-2-methyl-4sulphonophenylglycine (MSPG), induced LTP per se and caused a reduction of the paired-pulse facilitation (PPF) ratio indicating an enhancement of glutamate release. This suggests that group II and III mGluRs are activated under basal conditions to limit glutamate release. Both the group II and III mGluR selective antagonists, 2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoate (LY341495) and (R,S)-α-methylserine-O-phosphate (MSOP), induced LTP, and the selective agonists, (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4) depressed the field potentials and prevented HFS-LTP, with a prevailing contribution of group II mGluRs over that of group III mGluRs. The mGluR1 antagonist, 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) prevented the full development and maintenance of HFS-LTP. By contrast, the mGluR5 antagonist, 2-methyl-6-phenylethynylpyridine (MPEP) induced LTP per se, which was impeded by CPCCOEt, and it had no effect on LTP once induced by HFS. The PPF analysis showed an enhancement of glutamate release during MPEP potentiation. The group I mGluR agonist, (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced LTP per se, which was blocked by CPCCOEt. By contrast the mGluR5 agonist, (R,S)-2-chloro-5-hydroxypheylglycine (CHPG) prevented LTP elicited by HFS and DHPG as well. In conclusion vestibular LTP is inhibited by group II and III mGluRs during the early induction phase while it is facilitated by mGluR1 for achieving its full expression and consolidation. An additional inhibitory control is exerted by mGluR5 at the level of this facilitatory phase

Developmental shift from long-term depression to long-term potentiation in the rat medial vestibular nuclei: role of group I metabotropic glutamate receptors

The effects of high frequency stimulation (HFS) of the primary vestibular afferents on synaptic transmission in the ventral part of the medial vestibular nuclei (vMVN) were studied during postnatal development and compared with the changes in the expression of the group I metabotropic glutamate receptor (mGluR) subtypes, mGluR1 and mGluR5. During the first stages of development, HFS always induced a mGluR5- and GABAA-dependent long-term depression (LTD) which did not require NMDA receptor and mGluR1 activation. The probability of inducing LTD decreased progressively throughout the development and it was zero at about the end of the second postnatal week. Conversely, long-term potentiation (LTP) appeared at the beginning of the second week and its occurrence increased to reach the adult value at the end of the third week. Of interest, the sudden change in the LTP frequency occurred at the time of eye opening, about the end of the second postnatal week. LTP depended on NMDA receptor and mGluR1 activation. In parallel with the modifications in synaptic plasticity, we observed that the expression patterns and localizations of mGluR5 and mGluR1 in the medial vestibular nuclei (MVN) changed during postnatal development. At the earlier stages the mGluR1 expression was minimal, then increased progressively. In contrast, mGluR5 expression was initially high, then decreased. While mGluR1 was exclusively localized in neuronal compartments and concentrated at the postsynaptic sites at all stages observed, mGluR5 was found mainly in neuronal compartments at immature stages, then preferentially in glial compartments at mature stages. These results provide the first evidence for a progressive change from LTD to LTP accompanied by a distinct maturation expression of mGluR1 and mGluR5 during the development of the MVN