Nicotine smoking concentrations modulate GABAergic synaptic transmission in murine medial prefrontal cortex by activation of α7* and ß2* nicotinic receptors.

Nicotine is the major addictive component of cigarettes, reaching a brain concentration of ~300 nM during smoking of a single cigarette. The prefrontal cortex (PFC) mechanisms underlying temporary changes of working memory during smoking are incompletely understood. Here, we investigated whether 300 nM nicotine modulates γ-aminobutyric acid (GABA) ergic synaptic transmission from pyramidal neurons of the output layer (V) of the murine medial PFC. We used patch clamp in vitro recording from C57BL/6 mice in the whole-cell configuration to investigate the effect of nicotine on pharmacologically isolated GABAergic postsynaptic currents (IPSCs) in the absence or presence of methyllycaconitine (MLA) or dihydro-ß-erythroidine (DHßE), selective antagonists of α7- and ß2-containing (α7* and ß2*) nicotinic acetylcholine receptors (AChRs), respectively. Our results indicated that nicotine, alone or in the presence of MLA, decreases electrically evoked IPSC (eIPSC) amplitude, whereas in the presence of DHßE, nicotine elicited either an eIPSCs amplitude increase or a decrease. In the presence of DHßE, nicotine increased membrane conductance leaving the paired pulse ratio unchanged in all conditions, suggesting a non-ß2* mediated effect. In the presence of MLA, nicotine decreased the mean spontaneous IPSC (sIPSC) frequency but increased their rise time, suggesting a non-α7* AChR-mediated synaptic modulation. Also, in the presence of DHßE, nicotine decreased both eIPSC rise and decay times. No receptors other than α7* and ß2* appear to be involved in the nicotine effect. Our results indicate that nicotine smoking concentrations modulate GABAergic synaptic currents through mixed pre- and post-synaptic mechanisms by activation of α7* and ß2* AChRs.

Cerebrolysin prevents deficits in social behavior, repetitive conduct, and synaptic inhibition in a rat model of autism.

Autism spectrum disorder (ASD) is a syndrome of diverse neuropsychiatric diseases of growing incidence characterized by repetitive conduct and impaired social behavior and communication for which effective pharmacological treatment is still unavailable. While the mechanisms and etiology of ASD are still unknown, a consensus is emerging about the synaptic nature of the syndrome, suggesting a possible avenue for pharmacological treatment with synaptogenic compounds. The peptidic mixture cerebrolysin (CBL) has been successfully used during the last three decades in the treatment of stroke and neurodegenerative disease. Animal experiments indicate that at least one possible mechanism of action of CBL is through neuroprotection and/or synaptogenesis. In the present study, we tested the effect of CBL treatment (daily injection of 2.5 mL/Kg i.p. during 15 days) on a rat model of ASD. This was based on the offspring (43 male and 51 female pups) of a pregnant female rat injected with valproic acid (VPA, 600 mg/Kg) at the embryonic day 12.5, which previous work has shown to display extensive behavioral, as well as synaptic impairment. Comparison between saline vs. CBL-injected VPA animals shows that CBL treatment improves behavioral as well as synaptic impairments, measured by behavioral performance (social interaction, Y-maze, plus-maze), maximal response of inhibitory γ-amino butyric acid type A receptor (GABAA R)-mediated synaptic currents, as well as their kinetic properties and adrenergic and muscarinic modulation. We speculate that CBL might be a viable and effective candidate for pharmacological treatment or co-treatment of ASD patients. © 2017 Wiley Periodicals, Inc.

Interleukin 6 trans-signaling regulates basal synaptic transmission and sensitivity to pentylenetetrazole-induced seizures in mice.

The pro-inflammatory cytokine interleukin 6 (IL-6) interacts with the central nervous system in a largely unknown manner. We used a genetically modified mouse strain (GFAP-sgp130Fc, TG) and wild type (WT) mice to determine whether IL-6 trans-signaling contributes to basal properties of synaptic transmission. Postsynaptic currents (PSCs) were studied by patch-clamp recording in cortical layer 5 of a mouse prefrontal cortex brain slice preparation. TG and WT animals displayed differences mainly (but not exclusively) in excitatory synaptic responses. The frequency of both action potential-independent (miniature) and action potential-dependent (spontaneous) excitatory PSCs (EPSCs) were higher for TG vs. WT animals. No differences were observed in inhibitory miniature, spontaneous, or tonic inhibitory currents. The pair pulse ratio (PPR) of electrically evoked inhibitory as well as of excitatory PSCs were also larger in TG animals vs. WT ones, while no changes were detected in electrically evoked excitatory-inhibitory synaptic ratio (eEPSC/eIPSC), nor in the ratio between the amino-propionic acid receptor (AMPAR)-mediated and N-methyl D aspartate-R (NMDAR)-mediated components of eEPSCs (IAMPA /INMDA ). Evoked IPSC rise times were shorter for TG vs. WT animals. We also compared the sensitivity of TG and WT animals to pentylenetetrazole (PTZ)-induced seizures. We found that TG animals were more sensitive to PTZ injections, as they displayed longer and more severe seizures. We conclude that the absence of basal IL-6 trans-signaling contributes to increase the basal excitability of the central nervous system, at the system level as well at the synaptic level, at least in the prefrontal cortex.

Activation of the anti-inflammatory reflex blocks lipopolysaccharide-induced decrease in synaptic inhibition in the temporal cortex of the rat.

Stress is a potential trigger for a number of neuropsychiatric conditions, including anxiety syndromes and schizophrenic psychoses. The temporal neocortex is a stress-sensitive area involved in the development of such conditions. We have recently shown that aseptic inflammation and mild electric shock shift the balance between synaptic excitation and synaptic inhibition in favor of the former in this brain area (Garcia-Oscos et al., 2012), as well as in the prefrontal cortex (Garcia-Oscos et al., 2014). Given the potential clinical importance of this phenomenon in the etiology of hyperexcitable neuropsychiatric illness, this study investigates whether inactivation of the peripheral immune system by the "anti-inflammatory reflex" would reduce the central response to aseptic inflammation. For a model of aseptic inflammation, this study used i.p. injections of the bacterial toxin lipopolysaccharide (LPS; 5 µM) and activated the anti-inflammatory reflex either pharmacologically by i.p. injections of the nicotinic α7 receptor agonist PHA543613 or physiologically through electrical stimulation of the left vagal nerve (VNS). Patch-clamp recording was used to monitor synaptic function. Recordings from LPS-injected Sprague Dawley rats show that activation of the anti-inflammatory reflex either pharmacologically or by VNS blocks or greatly reduces the LPS-induced decrease of the synaptic inhibitory-to-excitatory ratio and the saturation level of inhibitory current input-output curves. Given the ample variety of pharmacologically available α7 nicotinic receptor agonists as well as the relative safety of clinical VNS already approved by the FDA for the treatment of epilepsy and depression, our findings suggest a new therapeutic avenue in the treatment of stress-induced hyperexcitable conditions mediated by a decrease in synaptic inhibition in the temporal cortex.

Interleukin 6 Dependent Synaptic Plasticity in a Social Defeat-Susceptible Prefrontal Cortex Circuit.

The role of the pro-inflammatory cytokine interleukin-6 (IL-6) in the etiology of stress-induced synaptic plasticity is yet unknown. We took advantage of a genetically modified mouse (TG) in which IL-6 trans-signaling via the soluble IL-6 receptor was blocked, to determine the role of IL-6 trans-signaling in the effects of a Social Defeat protocol (SD) on synaptic function of the medial prefrontal cortex (mPFC). Synaptic function in stress-sensitive (S) and stress-resilient (R) animals was studied in a mPFC slice preparation with whole-cell patch-clamp recording. SD altered numerous synaptic properties of the mPFC: R WT (but not TG) displayed a decreased ratio between N methyl-D-aspartate receptor (NMDAR-) dependent and amino propionic acid receptor (AMPAR-) dependent-current (INMDA/IAMPA), while S WT animals (but not TG) showed a reduced ratio between AMPA and γ-amino-butyric acid receptor type A (GABAAR)-dependent currents (IAMPA/IGABA). Also, SD induced an increase in the frequency but a decrease in the amplitude of excitatory action-potential dependent PSCs (sEPSCs), both in an IL-6 dependent manner, as well as a generalized (S/R-independent) decrease in the frequency of action potential independent (miniature) excitatory (IL-6 dependent) as well as inhibitory (IL-6 independent) postsynaptic current frequency. Interestingly, corner preference (measuring the intensity of social defeat) correlated positively with INMDA/IAMPA and eEPSC frequency and negatively with IAMPA/IGABA. Our results suggest that SD induces behaviorally-relevant synaptic rearrangement in mPFC circuits, part of which is IL-6 dependent. In particular, IL-6 is necessary to produce synaptic plasticity leading to stress resilience in some individuals, but to stress sensitivity in others.

Locus Ceruleus Norepinephrine Release: A Central Regulator of CNS Spatio-Temporal Activation?

Norepinephrine (NE) is synthesized in the Locus Coeruleus (LC) of the brainstem, from where it is released by axonal varicosities throughout the brain via volume transmission. A wealth of data from clinics and from animal models indicates that this catecholamine coordinates the activity of the central nervous system (CNS) and of the whole organism by modulating cell function in a vast number of brain areas in a coordinated manner. The ubiquity of NE receptors, the daunting number of cerebral areas regulated by the catecholamine, as well as the variety of cellular effects and of their timescales have contributed so far to defeat the attempts to integrate central adrenergic function into a unitary and coherent framework. Since three main families of NE receptors are represented-in order of decreasing affinity for the catecholamine-by: α2 adrenoceptors (α2Rs, high affinity), α1 adrenoceptors (α1Rs, intermediate affinity), and ß adrenoceptors (ßRs, low affinity), on a pharmacological basis, and on the ground of recent studies on cellular and systemic central noradrenergic effects, we propose that an increase in LC tonic activity promotes the emergence of four global states covering the whole spectrum of brain activation: (1) sleep: virtual absence of NE, (2) quiet wake: activation of α2Rs, (3) active wake/physiological stress: activation of α2- and α1-Rs, (4) distress: activation of α2-, α1-, and ß-Rs. We postulate that excess intensity and/or duration of states (3) and (4) may lead to maladaptive plasticity, causing-in turn-a variety of neuropsychiatric illnesses including depression, schizophrenic psychoses, anxiety disorders, and attention deficit. The interplay between tonic and phasic LC activity identified in the LC in relationship with behavioral response is of critical importance in defining the short- and long-term biological mechanisms associated with the basic states postulated for the CNS. While the model has the potential to explain a large number of experimental and clinical findings, a major challenge will be to adapt this hypothesis to integrate the role of other neurotransmitters released during stress in a centralized fashion, like serotonin, acetylcholine, and histamine, as well as those released in a non-centralized fashion, like purines and cytokines.