Marc Jones and Dr. Scott Steffensen, Psychology
Several recent studies have established a role for estrogens in ameliorating specific neurodegenerative disorders, mainly those associated with the cholinergic neurons of the basal forebrain and their targets in the cortex and hippocampus. We have previously demonstrated that endogenous and exogenous application of the neurosteroid dehydroepiandrosterone sulfate (DHEAS) markedly reduces GABA-mediated recurrent inhibition and synchronizes hippocampal unit activity to theta rhythm. In this study, I evaluated the role of muscarinic receptors in mediating the effects of the GABA-modulating neurosteroid, pregnenolone sulfate (PREGS) on monosynaptic evoked potential responses, paired-pulse inhibition, paired-pulse facilitation and GABA interneuron activity in the dentate gyrus and CA1 subfields of the rat hippocampus. Systemic administration of PREGS did not alter monosynaptic evoked potentials, but significantly increased PPI in CA1. These findings suggest that select GABA-modulating neurosteroids and neuroactive estrogen sulfates alter septohippocampal cholinergic modulation of hippocampal GABAergic interneurons mediating recurrent, but not feedforward, inhibition of hippocampal principal cell activity.
Due to its low aqueous solubility, PREGS could not be reliably administered microelectrophoretically. Thus, it was administered systemically. I compared the effects of PREGS to DMSO vehicle control on dentate and CA1 pEPSP slopes and PS amplitudes. In the dentate, intraperitoneal administration of 20 mg/kg PREGS not did significantly alter dentate pEPSP slopes (P>0.05, t(2,5)=0.1; mean 50% maximum pEPSP slope for DMSO vehicle control = 4.2 ± 0.3 mV/ms vs PREGS = 4.1 ± 0.2 mV/ms vs mean; n=6 each) or PS amplitudes (P>0.05, t(2,5)=0.3; mean 50% maximum PS amplitude for DMSO = 7.2 ± 0.9 mV vs PREGS = 6.9 ± 0.5 mV; n=6 each. In CA1, intraperitoneal administration of 20 mg/kg PREGS did not significantly alter CA1 pEPSP slopes (P>0.05, t(2,5)=0.7; mean PREGS pEPSP slope at 30 min = 2.1 ± 0.2 mV/msec vs mean vehicle 50% pEPSP amplitude at 30 min = 1.9 ± 0.3 mV/msec; n=6 each) or PS amplitudes (P>0.05, t(2,5)=0.6; mean 50% maximum PS amplitude for DMSO = 5.3 ± 0.2 mV vs mean PREGS = 5.6 ± 0.5 mV; n=6 each).
I evaluated the effects of PREGS on both paired-pulse facilitation (PPF) of pEPSPs and PPI in the dentate and CA1, as PREGS has been shown to enhance PPF in these two hippocampal subfields. Paired stimulation of the perforant path or contralateral commissural input facilitated pEPSP amplitudes and slopes for conditioned responses between ISIs 10-160 msec. Intraperitoneal administration of 20 mg/kg PREGS had no effect on dentate PPF responses (at 40 msec ISI: P>0.05, t(2,5)=0.14; mean DMSO control PPF = 114.7 ± 11 % PPF vs PREGS = 115.1 ± 18 % PPF; n=6 each), or CA1 PPF responses (at 60 ms ISI: P>0.05, t(2,5)=1.02; mean DMSO control PPF = 169.2 ± 18 % vs PREGS = 146.1 ± 12.5 %; n=6 each). I also evaluated the effects of PREGS on PPI in the dentate and CA1. Intraperitoneal administration of 20mg/kg PREGS did not significantly alter PPI in the dentate, but significantly enhanced PPI in CA1 (P<0.005, t(2,7)=4.5 at 160 msec ISI and P<0.01, t(2,7)=3.8 at 260 ms ISI; n=8).
PREGS has been shown to increase PPF in the dentate and CA1 slices from mature rats.2 Based on this and other studies, it is suggested that PREGS enhances facilitated evoked release without affecting basal evoked release of glutamate. Notwithstanding these effects of PREGS on PPF in vitro, PREGS did not significantly affect PPF in vivo, either in the dentate or CA1. However, at this same dose, systemic PREGS increased PPI in vivo with an onset of 10 min and a peak response at 30-45 min. The lack of congruity for PREGS effects on PPF in vivo and in vitro may be due to differences in the circuit responses between preparations. There is considerably more GABA receptor mediated recurrent inhibition in vivo compared to in vitro, especially in CA1. It may be that PREGS enhancement of PPI seen in vivo is masking the effects of pregS on PPF. A possible explanation why PREGS had nearly opposite effects to DHEAS is that pregnenolone has been shown to interfere with ACh binding to muscarinic receptors.
In another study done in the lab of Scott Steffensen, they evaluated the role of muscarinic receptors in mediating the effects of the GABA-modulating neurosteroids dehydroepiandrosterone sulfate DHEAS and estrone sulfate (ES) on monosynaptic evoked potential responses, paired-pulse inhibition, paired-pulse facilitation and GABA interneuron activity in the dentate gyrus and CA1 subfields of the rat hippocampus. In situ microelectrophoretic application of the muscarinic M2 subtype cholinergic receptor agonist cis-dioxolane, DHEAS, and ES markedly reduced paired-pulse inhibition (PPI) in the dentate and CA1 that was blocked by the M2 receptor antagonist gallamine. Similar to DHEAS, microelectrophoretic administration of ES increased population spike amplitudes, without increasing monosynaptic excitatory transmission, but this effect was not blocked by gallamine.
In conclusion, activation of M2 ACh receptors excites and synchronizes the rhythmic activity of specific populations of GABA interneurons that inhibit separate populations of GABA interneurons that mediate recurrent inhibition in the hippocampus, thereby resulting in disinhibition of principal cell excitability in the dentate gyrus and CA1. The neurosteroid DHEAS and the neuroactive estrogen ES, but not the neurosteroid PREGS, appear to act via the same mechanism, as their suppression of recurrent inhibition is blocked by M2 ACh receptor antagonists. Taken together, these findings suggest that neurosteroids and neuroactive estrogens reduce recurrent inhibition in the hippocampus via enhancement of cholinergic neurotransmission, especially during theta rhythmic synchronization. We speculate that select neurosteroids and neuroactive estrogens may be involved in the moment-by-moment latching of hippocampal interneuronal excitability to extrahippocampal rhythmic input.
References
- Steffensen SC. 1995. Dehydroepiandrosterone sulfate suppresses hippocampal recurrent inhibition and synchronizes neuronal activity to theta rhythm. Hippocampus 5:320-328.
- Partridge LD, Valenzuela CF. 2001. Neurosteroid-induced enhancement of glutamate transmission in rat hippocampal slices. Neurosci Lett 301(2):103-6.
- Thomas MJ, Mameli M, Carta M, Valenzuela CF, Li PK, Partridge LD. 2005. Neurosteroid paradoxical enhancement of paired-pulse inhibition through paired-pulse facilitation of inhibitory circuits in dentate granule cells. Neuropharmacology 48(4):584-96.
- Schiess AR, Partridge LD. 2005. Pregnenolone sulfate acts through a G-protein-coupled sigma1-like receptor to enhance short term facilitation in adult hippocampal neurons. Eur J Pharmacol 518(1):22-9.
- Steffensen SC, editor. 1999. Ethanol suppression of hippocampal plasticity: Role of subcortical inputs. New York: 183-203 p.
- Horishita T, Minami K, Uezono Y, Shiraishi M, Ogata J, Okamoto T, Terada T, Sata T. 2005. The effects of the neurosteroids: pregnenolone, progesterone and dehydroepiandrosterone on muscarinic receptor-induced responses in Xenopus oocytes expressing M1 and M3 receptors. Naunyn Schmiedebergs Arch Pharmacol 371(3):221-8.