Promnestic effects of intranasally applied pregnenolone in rats

Neutral Sphingomyelinase is an Affective Valence-Dependent Regulator of Learning and Memory

Sphingolipids and enzymes of the sphingolipid rheostat determine synaptic appearance and signaling in the brain, but sphingolipid contribution to normal behavioral plasticity is little understood. Here we asked how the sphingolipid rheostat contributes to learning and memory of various dimensions. We investigated the role of these lipids in the mechanisms of two different types of memory, such as appetitively and aversively motivated memory, which are considered to be mediated by different neural mechanisms. We found an association between superior performance in short- and long-term appetitively motivated learning and regionally enhanced neutral sphingomyelinase (NSM) activity. An opposite interaction was observed in an aversively motivated task. A valence-dissociating role of NSM in learning was confirmed in mice with genetically reduced NSM activity. This role may be mediated by the NSM control of N-methyl-d-aspartate receptor subunit expression. In a translational approach, we confirmed a positive association of serum NSM activity with long-term appetitively motivated memory in nonhuman primates and in healthy humans. Altogether, these data suggest a new sphingolipid mechanism of de-novo learning and memory, which is based on NSM activity.

Neutral ceramidase is a marker for cognitive performance in rats and monkeys

Background

Ceramides are lipid molecules determining cell integrity and intercellular signaling, and thus, involved in the pathogenesis of several psychiatric and neurodegenerative disorders. However, little is known about the role of particular enzymes of the ceramide metabolism in the mechanisms of normal behavioral plasticity. Here, we studied the contribution of neutral ceramidase (NC), one of the main enzymes mediating ceramide degradation, in the mechanisms of learning and memory in rats and non-human primates.

Methods

Naïve Wistar rats and black tufted-ear marmosets (Callithrix penicillata) were tested in several tests for short- and long-term memory and then divided into groups with various memory performance. The activities of NC and acid ceramidase (AC) were measured in these animals. Additionally, anxiety and depression-like behavior and brain levels of monoamines were assessed in the rats.

Results

We observed a predictive role of NC activity in the blood serum for superior performance of long-term object memory tasks in both species. A brain area analysis suggested that high NC activity in the ventral mesencephalon (VM) predicts better short-term memory performance in rats. High NC activity in the VM was also associated with worse long-term object memory, which might be mediated by an enhanced depression-like state and a monoaminergic imbalance.

Conclusions

Altogether, these data suggest a role for NC in short- and long-term memory of various mammalian species. Serum activity of NC may possess a predictive role in the assessing the performance of certain types of memory.

Electronic supplementary material

The online version of this article (10.1007/s43440-020-00159-2) contains supplementary material, which is available to authorized users.

[New therapeutic avenues for neurosteroids in psychiatric diseases]

Discovered in the eighties by Pr Baulieu and colleagues, neurosteroids are a class of neuroactive brain-born steroids, which comprises the steroid hormones, their biosynthesis precursors and their metabolites. They can act through genomic as well as non-genomic pathways. Genomic pathways, only triggered by the neurosteroid hormones, are, in the brain, the same as those largely described in the periphery: the binding of these steroid hormones to nuclear receptors leads to transcription regulations. On the other hand, their precursors and metabolites, such as pregnenolone (PREG), dehydroepiandrosterone (DHEA), their respective sulfate esters, pregnenolone sulfate (PREG-S) and DHEA sulfate (DHEA-S) and allopregnanolone (ALLOP), are defined as neurosteroids, but no corresponding nuclear receptors have been identified so far. In fact, they trigger non-genomic pathways which consist in (i) inhibitory ionotropic receptors, (ii) excitatory ionotropic receptors and (iii) the microtubular system. Hence, inhibitory neurosteroids, whose mostly studied representative is ALLOP, positively modulate, or directly activate, the ionotropic GABA-A receptors. In contrast, excitatory neurosteroids, represented by PREG-S, DHEA-S and DHEA, inhibit the GABA-A receptors, and activate, directly or indirectly, through the sigma-1 receptors, the NMDA glutamate receptors. Neurosteroids of the third group, the microtubular neurosteroids, are able to bind microtubule associated proteins, in particular MAP2, to promote microtubule assembly, neurite outgrowth and in fine structural neuroplasticity. So far, PREG, DHEA and progesterone are the three identified microtubular neurosteroids. The pharmacological properties of neurosteroids have led to specific investigations for assessing their therapeutic potentialities in psychiatric diseases, using validated animal models. In some cases, clinical trials were also performed. These studies showed that ALLOP, the main inhibitory neurosteroid, displayed clear-cut anxiolytic-like and antidepressant-like efficacy in animals. It has been subsequently developed as Brexanolone and tested with success in phase III of clinical trials for the treatment of post-partum depression. Although showing pro-cognitive properties in animals, the sulfated neurosteroids, PREG-S and DHEA-S, were, in contrast, never tested in clinical trials, probably due to their poor stability and proconvulsivant side effects. Their respective non-sulfated forms, PREG and DHEA, showed antidepressant and antipsychotic efficacies in clinical trials, but these drugs never reached the phase III of clinical development because their therapeutic uses would have led to an overproduction of active metabolites responsible for intolerable side effects. The alternative strategy which has been selected consists of the development of non-metabolizable synthetic derivatives of these natural steroids, which keep the same neuroactive properties as their parent molecules, but are devoid of any hormonal side effects. An example of such innovative drugs is MAP4343, a synthetic derivative of PREG, which exhibits potent antidepressant-like efficacy in validated animal models. It is currently tested in depressed patients.

Intranasal pregnenolone increases acetylcholine in frontal cortex, hippocampus, and amygdala-Preferentially in the hemisphere ipsilateral to the injected nostril

This study determined the effects of intranasal pregnenolone (IN-PREG) on acetylcholine (ACh) levels in selected areas of the rat brain, using in vivo microdialysis. Previous studies showed that PREG rapidly reaches the rodent brain after intranasal administration and that direct infusion of PREG and PREG-S into the basal forebrain modulates ACh release in frontal cortex, amygdala, and hippocampus. In the present study, we investigated the effects of IN-PREG on the cholinergic system in the rat brain. In the first experiment, IN-PREG (5.6 and 11.2 mg/ml) or vehicle was applied bilaterally, and we hypothesized that IN-PREG would increase ACh levels in amygdala, hippocampus, and frontal cortex, relative to baseline and vehicle. Dialysate was collected for 100 min, based on pilot data of duration of effect. Bilateral IN-PREG (5.6 and 11.2 mg/ml) increased frontal cortex and hippocampal ACh relative to both baseline and vehicle. Moreover, 11.2 mg/ml PREG increased ACh in the amygdala relative to baseline, the lower dose, and vehicle. Therefore, in the second experiment, IN-PREG (11.2 mg/ml) was applied only into one nostril, with vehicle applied into the other nostril, in order to determine whether ACh is predominantly increased in the ipsilateral relative to the contralateral amygdala. Unilateral application of IN-PREG increased ACh in the ipsilateral amygdala, whereas no effect was observed on the contralateral side, suggesting that PREG was transported from the nostrils to the brain via the olfactory epithelial pathway, but not by circulation. The present data provide additional information on IN-PREG action in the cholinergic system of frontal cortex, amygdala, and hippocampus. This may be relevant for therapeutic IN application of PREG in neurogenerative and neuropsychiatric disorders.

Neurosteroid Actions in Memory and Neurologic/Neuropsychiatric Disorders

Memory dysfunction is a symptomatic feature of many neurologic and neuropsychiatric disorders; however, the basic underlying mechanisms of memory and altered states of circuitry function associated with disorders of memory remain a vast unexplored territory. The initial discovery of endogenous neurosteroids triggered a quest to elucidate their role as neuromodulators in normal and diseased brain function. In this review, based on the perspective of our own research, the advances leading to the discovery of positive and negative neurosteroid allosteric modulators of GABA type-A (GABA_A), NMDA, and non-NMDA type glutamate receptors are brought together in a historical and conceptual framework. We extend the analysis toward a state-of-the art view of how neurosteroid modulation of neural circuitry function may affect memory and memory deficits. By aggregating the results from multiple laboratories using both animal models for disease and human clinical research on neuropsychiatric and age-related neurodegenerative disorders, elements of a circuitry level view begins to emerge. Lastly, the effects of both endogenously active and exogenously administered neurosteroids on neural networks across the life span of women and men point to a possible underlying pharmacological connectome by which these neuromodulators might act to modulate memory across diverse altered states of mind.

Neurosteroids: non-genomic pathways in neuroplasticity and involvement in neurological diseases

Neurosteroids are neuroactive brain-born steroids. They can act through non-genomic and/or through genomic pathways. Genomic pathways are largely described for steroid hormones: the binding to nuclear receptors leads to transcription regulation. Pregnenolone, Dehydroepiandrosterone, their respective sulfate esters and Allopregnanolone have no corresponding nuclear receptor identified so far whereas some of their non-genomic targets have been identified. Neuroplasticity is the capacity that neuronal networks have to change their structure and function in response to biological and/or environmental signals; it is regulated by several mechanisms, including those that involve neurosteroids. In this review, after a description of their biosynthesis, the effects of Pregnenolone, Dehydroepiandrosterone, their respective sulfate esters and Allopregnanolone on their targets will be exposed. We then shall highlight that neurosteroids, by acting on these targets, can regulate neurogenesis, structural and functional plasticity. Finally, we will discuss the therapeutic potential of neurosteroids in the pathophysiology of neurological diseases in which alterations of neuroplasticity are associated with changes in neurosteroid levels.