The neurosteroid pregnenolone sulfate neutralized the learning impairment induced by intrahippocampal nicotine in alcohol-drinking rats

Early postnatal neuroactive steroid manipulation differentially affects recognition memory and passive avoidance performance in male rats

Early postnatal neuroactive steroids (NAS) play a significant role in the neurodevelopment. Their alteration can modify adult behavior, such as anxiety or learning. For this reason, we set out to observe if neonatal NAS levels alteration affects two types of learning implying low or high levels of emotional content, such as recognition memory and aversive learning respectively. Thus, we tested allopregnanolone or finasteride administered from postnatal days 5-9. In adulthood, recognition memory was assessed using the object recognition test, as well as aversive learning throughout the passive avoidance test (PA). Because of the important emotional component of PA, which can be influencing learning, we evaluated anxiety-like behavior by means of the open field test (OF). The results indicated that those animals administered with finasteride showed higher recognition levels of a familiar object. On the other hand, they showed an impairment in a stressful learning, such as PA. However, no effects of finasteride were observed on anxiety-like behavior in OF, despite it has been reported that neonatal finasteride treatment can promote an anxiety-like profile in the elevated plus-maze test in adulthood. Regarding neonatal allopregnanolone, animals showed higher levels in OF exploration only when they were already familiar with the apparatus. Furthermore, neonatal allopregnanolone did not affect recognition memory or aversive learning. In conclusion, the neonatal NAS manipulation by means of finasteride differently affected two types of learning implying distinct stress levels. Altogether, the results show the importance of the emotional content to explain the effects of neonatal NAS manipulation on learning.

Neonatal allopregnanolone levels alteration: effects on behavior and role of the hippocampus

Several works have pointed out the importance of the neurosteroid allopregnanolone for the maturation of the central nervous system and for adult behavior. The alteration of neonatal allopregnanolone levels in the first weeks of life alters emotional adult behavior and sensory gating processes. Without ruling out brain structures, some of these behavioral alterations seem to be related to a different functioning of the hippocampus in adult age. We focus here on the different behavioral studies that have revealed the importance of neonatal allopregnanolone levels for the adult response to novel environmental stimuli, anxiety-related behaviors and processing of sensory inputs (prepulse inhibition). An increase in neonatal physiological allopregnanolone levels decreases anxiety and increases novelty responses in adult age, thus affecting the individual response to environmental cues. These effects are also accompanied by a decrease in prepulse inhibition, indicating alterations in sensory gating that have been related to that present in disorders, such as schizophrenia. Moreover, behavioral studies have shown that some of these effects are related to a different functioning of the dorsal hippocampus, as the behavioral effects (decrease in anxiety and locomotion or increase in prepulse inhibition) of intrahippocampal allopregnanolone infusions in adult age are not present in those subjects in whom neonatal allopregnanolone levels were altered. Recent data indicated that this hippocampal involvement may be related to alterations in the expression of gamma-aminobutyric-acid receptors containing α4 and δ subunits, molecular alterations that can persist into adult age and that can, in part, explain the reported behavioral disturbances.

Alteration of neonatal Allopregnanolone levels affects exploration, anxiety, aversive learning and adult behavioural response to intrahippocampal neurosteroids

Neurosteroids (NS) are well known to exert modulatory effects on ionotropic receptors. Recent findings indicate that NS could also act as important factors during development. In this sense, neonatal modifications of Allopregnanolone (Allop) levels during critical periods have been demonstrate to alter the morphology of the hippocampus but also other brain structures. The aim of the present work is to screen whether the alterations of Allop levels modify adult CA1 hippocampal response to NS administration. For this purpose, pups were injected with Allop (20 mg/kg s.c.), Finasteride (5α-reductase inhibitor that impedes Allop synthesis) (50 mg/kg s.c.) or Vehicle from postnatal day 5 (P5) to postnatal day 9 (P9). NS levels were tested at P5. To test the behavioural hippocampal response to NS in adulthood, animals were implanted with a bilateral cannula into the CA1 hippocampus at 80 days old and injected with Allop (0.2 μg/0.5 μl), Pregnenolone sulphate (5 ng/0.5 μl) or Vehicle in each hippocampus. After injections animals were tested in the Boisser test to assess exploratory behaviour, the elevated plus maze to assess anxiety and the passive avoidance to test aversive learning. Results indicate that alteration of neonatal Allop or pregnenolone levels (by Allop and Finasteride administration, respectively) suppressed intrahippocampal Allop anxiolytic effect in the EPM. Moreover our results also indicate that manipulation of neonatal Allop levels (Allop and Finast administration) alters exploratory and anxiety-like behaviour and impairs aversive learning in the adulthood. These data point out the role of Allop in the maturation of hippocampal function and behaviour.

A steroid modulatory domain in NR2A collaborates with NR1 exon-5 to control NMDAR modulation by pregnenolone sulfate and protons

NMDA receptor (NMDAR)-mediated excitatory synaptic transmission plays a critical role in synaptic plasticity and memory formation, whereas its dysfunction may underlie neuropsychiatric and neurodegenerative diseases. The neuroactive steroid pregnenolone sulfate (PS) acts as a cognitive enhancer in impaired animals, augments LTP in hippocampal slices by enhancing NMDAR activity, and may participate in the reduction of schizophrenia's negative symptoms by systemic pregnenolone. We report that the effects of PS on NMDAR function are diverse, varying with subunit composition and NR1 splice variant. While PS potentiates NR1-1a/NR2B receptors through a critical steroid modulatory domain in NR2B that also modulates tonic proton inhibition, potentiation of the NMDA response is not dependent upon relief of such inhibition, a finding that distinguishes it from spermine. In contrast, the presence of an NR2A subunit confers enhanced PS-potentiation at reduced pH, suggesting that it may indeed act like spermine does at NR2B-containing receptors. Additional tuning of the NMDAR response by PS comes via the N-terminal exon-5 splicing insert of NR1-1b, which regulates the magnitude of proton-dependent PS potentiation. For NR2C- and NR2D-containing receptors, negative modulation at NR2C receptors is pH-independent (like NR2B) while negative modulation at NR2D receptors is pH-dependent (like NR2A). Taken together, PS displays a rich modulatory repertoire that takes advantage of the structural diversity of NMDARs in the CNS. The differential pH sensitivity of NMDAR isoforms to PS modulation may be especially important given the emerging role of proton sensors to both learning and memory, as well as brain injury.

Neurosteroids infusion into the CA1 hippocampal region on exploration, anxiety-like behaviour and aversive learning

Neurosteroids (NS) are substances synthesised de novo in the brain that have rapid modulatory effects on ionotropic receptors. Specifically, NS can act as positive allosteric modulators of GABAA receptors as pregnanolone or allopregnanolone (Allop), or GABAA negative modulators and NMDA positive modulators as pregnenolone (PREG) or dehydroepiandrosterone (DHEA) and their sulphate esters (PREGS and DHEAS). Given this, their role in anxiety and emotional disturbances has been suggested. In addition, NS such as PREGS or DHEAS have demonstrated a promnesic role in several learning tests. The aim of the present work is to highlight the role that the dorsal (CA1) hippocampus plays in the behavioural profile of NS such as Allop and PREGS in tests assessing exploration, anxiety and aversive learning in rats. For this purpose, animals were administered intrahippocampally with Allop (0.2μg/0.5μl), PREGS (5ng/0.5μl) or vehicle in each hippocampus, and tested in the Boissier and elevated plus maze (EPM) tests. For learning test we have chosen the passive avoidance paradigm. Results indicate that intrahippocampal administration of Allop enhances exploration, reflected in an increase in the total and the inner number of head-dips. Allop-injected animals also showed an increase in the percentage of entries into the open arms of the EPM, suggesting an anxiolytic-like profile. In addition, post-acquisition PREGS administration enhanced passive avoidance retention, while post-acquisition Allop administration had no effects on aversive learning retention. These results point out the important role of the dorsal (CA1) hippocampus in several NS behavioural effects, such as exploration, anxiety, learning and memory.

Neuroactive steroid pregnenolone sulphate inhibits long-term potentiation via activation of alpha2-adrenoreceptors at excitatory synapses in rat medial prefrontal cortex

Pregnenolone sulphate (PREGS) is one of the most important neuroactive steroids. Previous study showed that PREGS enhanced long-term potentiation (LTP) via activation of post-synaptic NMDA receptors at excitatory synapses in the hippocampus. The present paper studied the effect of PREGS on LTP at excitatory synapses in the pyramidal cells of layers V-VI of the medial prefrontal cortex (mPFC) using whole-cell patch-clamp in slices and made a comparison with that in the hippocampus. We also studied the mechanism of the effect of PREGS in the mPFC. We found that PREGS inhibited induction of LTP in the mPFC and had no influence on NMDA currents, which was different from its effect in the hippocampus. Moreover, the effect of PREGS on LTP in the mPFC was cancelled by alpha2-adrenoreceptor antagonist, alpha2A-adrenoreceptor antagonist, Gi protein inhibitor, adenylate cyclase inhibitor and protein kinase A inhibitor. These results suggest that PREGS inhibits LTP via activation of the alpha2-adrenoreceptor-Gi protein-adenylate cyclase-protein kinase A signalling pathway in the mPFC.

Neonatal finasteride induces anxiogenic-like profile and deteriorates passive avoidance in adulthood after intrahippocampal neurosteroid administration

Recent findings indicate that neurosteroids could act as important keys during the brain development. Fluctuations in neonatal allopregnanolone (AlloP) could result in altered pharmacological properties of the GABA(A) receptor system in adulthood. Recent studies demonstrated that neurosteroids play a critical role in regulating normal neurodevelopment in the hippocampus. The aim of the present work is to screen whether developmentally altered neurosteroid levels influence the behavioral response to adult intrahippocampal administration of AlloP, a GABA(A) positive modulating neurosteroid, and pregnenolone sulfate (PregS), a GABA(A) negative modulator in rats. For this purpose, pups received AlloP (10 mg/kg, s.c.), a 5alpha-reductase inhibitor (finasteride, 50 mg/kg, s.c.) or vehicle from the fifth to the ninth postnatal day. At maturity (i.e. 90 days old) a bilateral cannula was implanted into the hippocampus. After recovery from surgery, animals received an administration of AlloP (0.2 microg/0.5 microl), PregS (5 ng/0.5 microl) or vehicle in each hippocampus 5 min before they were tested in the elevated plus maze (EPM) and immediately after the passive avoidance training session, and retention was tested 24 h later. Results indicated that neonatal finasteride treatment deteriorated passive avoidance retention and elicited an anxiogenic-like effect in the EPM test in adulthood, as seen by the reduction of open arm entries and in the time spent in the open arms. Intrahippocampal PregS administration also disrupted passive avoidance, possibly related to its anxiogenic profile. Fluctuations in neonatal AlloP affect the aversive learning and the anxiety-related behavior in adulthood, and this effect could be in part mediated by alterations of the mature functions of the hippocampus, possibly via the GABA(A) receptor. These data point to the role of GABAergic neurosteroids in critical periods of vulnerability that influence normal development of GABAergic pathways in the CNS.

Effects of moderate chronic ethanol consumption on hippocampal nicotinic receptors and associative learning

A number of studies have reported that ethanol exposure induces changes in different brain systems. The hippocampus is a brain region that is very vulnerable to ethanol exposition, which functionally results in impairment of learning and memory processes reported in heavy drinkers. Hippocampal nicotinic receptors are involved in learning and memory. In this study, we determined the effects of ethanol on the main hippocampal subtypes of neural nicotinic receptors (alpha7 and alpha4beta2) in rats non-selected for alcohol consumption, in order to check for possible changes on these receptors that could be linked with alterations in learning acquisition. Binding assays were carried out with [3H]methyllycaconitine ([3H]MLA) to study the alpha7 and [3H]nicotine to study alpha4beta2 receptors. Auto-shaping, continuous ratio and extinction procedures were used as behavioral tests. The results show that moderate chronic ethanol consumption for 10 weeks produces: (a) a decrease of both hippocampal nicotinic receptor subtypes without alterations in affinity; (b) no differences in behavioral performance between control rats and ethanol-drinking rats in auto-shaping and continuous ratio; (c) an improvement of performance of extinction paradigm. These results indicate that chronic ethanol consumption, at moderate levels, induces changes in hippocampal nicotinic receptors but does not impair acquisition and performance of new associative learning and even improves some kind of paradigms. These results may have implications in the biochemical basis of interactions between alcohol and nicotine and the effects of these drugs on behavior.

Intrahippocampal allopregnanolone decreases voluntary chronic alcohol consumption in non-selected rats

We have recently shown that 0.2 microg of the neurosteroid allopregnanolone (AlloP) administered to the hippocampus induced an anxiolytic-like profile and also reduced alcohol withdrawal symptoms in voluntary and chronic alcohol-drinking rats. The aim of the present work was to study whether the administration of this dose of AlloP could affect alcohol consumption in non-selected rats that have been voluntarily ingesting high doses of alcohol for long periods of time in a limited access procedure. We used a free-choice drinking procedure that involved providing the rats with an alcoholic solution (10% ethanol) at an early age. Alcohol and control rats were assigned randomly to three groups that received an intrahippocampal (dorsal CA1) injection before the period of alcohol consumption after a long history of chronic alcohol intake. The injection groups were AlloP (0.2 microg, 1.26 microM), pregnenolone sulfate (PregS) (5 ng, 24 microM) or vehicle. Blood alcohol concentrations (BAC) were assessed before testing the effects of injections on alcohol consumption. Although AlloP did not eliminate alcohol ingestion, it significantly decreased alcohol consumption. The intrahippocampal administration of PregS, at the dose tested, did not effectively modify alcohol consumption levels. These results indicate that the positive modulation of hippocampal GABA(A) receptors induced by neurosteroids can be an important neurobiological target for reducing chronic alcohol consumption.

Effects of Intrahippocampal Nicotine and Neurosteroid Administration on Withdrawal in Voluntary and Chronic Alcohol-Drinking Rats

BACKGROUND

Previous studies have shown that 4.6 mug of nicotine administered to the hippocampus can deteriorate learning acquisition in alcohol-drinking rats. The aim of the present study was to research whether this nicotine dose can alter the alcohol withdrawal syndrome and whether the two neurosteroids, allopregnanolone (AlloP) and pregnenolone sulfate (PregS), at doses previously reported as anxiolytic and promnesic, respectively, can modulate these effects.

METHODS

We used a free-choice drinking procedure that involved providing the rats with an alcoholic solution (10% ethanol) at an early age. Alcohol and control rats were assigned randomly to six groups that received two consecutive intrahippocampal (dorsal CA1) injections once per week during three consecutive weeks after one hour of ethanol drinking. The first injection was nicotine (4.6 microg, 20 mM) or saline and the second injection was PregS (5 ng, 24 microM), AlloP (0.2 microg, 1.26 microM) or saline. Blood alcohol concentrations were assessed one week before the withdrawal testing. Locomotor activity and audiogenic seizures were tested during withdrawal after 110 days of voluntary ethanol consumption. Rats were injected immediately before the withdrawal testing.

RESULTS

AlloP induced a decrease in horizontal and vertical activities, suggesting that the dose tested has sedative effects. AlloP reversed the seizures induced by ethanol withdrawal and also the spontaneous audiogenic seizures induced by the acoustic stimulation in control rats. Moreover, AlloP decreased other alcohol withdrawal signs, such as tail stiffening and body rigidity. Intrahippocampal administration of nicotine or PregS, at the doses tested, did not effectively modify the expression of audiogenic seizures induced by alcohol withdrawal.

CONCLUSIONS

These results show that hippocampal GABAergic activity and AlloP have an important role in preventing convulsive behavior. The results also highlight the therapeutic potential of AlloP for reducing the alcohol withdrawal syndrome.

The intrahippocampal administration of the neurosteroid allopregnanolone blocks the audiogenic seizures induced by nicotine

Allopregnanolone (AlloP), GABA(A) positive modulator, has efficacy as anticonvulsant. In contrast, nicotine and pregnenolone sulfate (PregS) act as potent convulsants. The present study aims to evaluate whether a promnesic dose of PregS and/or an anxiolytic dose of AlloP administered in the hippocampus can affect the audiogenic seizures induced by nicotine administration. Rats were assigned at random to six groups that received two consecutive intrahippocampal (dorsal CA1) injections once a week during three consecutive weeks. First injection: nicotine (4.6 microg, 20 mM) or saline, second injection: PregS (5 ng, 24 microM), AlloP (0.2 microg, 1.26 microM) or saline. After the last injections, locomotor activity and audiogenic seizures were tested. AlloP decreased the horizontal and vertical activity, suggesting sedative effects. Nicotine induced behavioral convulsions and AlloP acted as an anticonvulsant. AlloP reversed the seizures induced by nicotine and decreased the audiogenic convulsions in comparison with the controls. PregS also reversed the nicotine-induced audiogenic seizures in the nicotine group but not in the control group. These results suggest that anticonvulsive effects of AlloP and PregS are mediated by different action mechanisms such as GABA(A) positive modulation, or negative modulatory action on neural nicotinic acetylcholine receptors. Even though several brain structures could be involved, these results highlight the important role played by hippocampal cholinergic and GABAergic activities, as well as neurosteroids, especially AlloP, in preventing convulsive behavior.