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.

Activation of 5-HT receptors inhibits GABAergic transmission by pre-and post-synaptic mechanisms in layer II/III of the juvenile rat auditory cortex.

The specific mechanisms by which serotonin (5-HT) modulates synaptic transmission in the auditory cortex are still unknown. In this work, we used whole-cell recordings from layer II/III of pyramidal neurons in rat brain slices to characterize the influence of 5-HT on inhibitory synaptic activity in the auditory cortex after pharmacological blockade of excitatory glutamatergic transmission. We found that bath application of 5-HT (5 µM) reduced the frequency and amplitude of both spontaneous and miniature inhibitory postsynaptic currents (IPSCs), reduced the amplitude of evoked IPSCs, and enhanced facilitation of paired pulse ratio (PPR), suggesting presynaptic inhibition. To determine which the serotonin receptors were involved in this effect, we studied the influence of specific 5-HT receptor agonists and antagonists on É£-aminobutyric acid (GABA)ergic synaptic transmission. The inhibiting influence of 5-HT in the GABAergic synaptic activity was mimicked by using the selective agonists of the 5-HT1A and 5-HT2A receptors, 8(OH)-DPAT (10 µM) and DOI (10 µM), respectively; and it was prevented by their respective antagonists NAN-190 (1 µM) and ritanserin (1 µM). Furthermore, the application of the selective agonist of 5-HT1A receptors, 8-(OH)-DPAT (10 µM), produced PPR facilitation, while DOI application (5-HT2A agonist) did not change the PPR. Moreover, the 5-HT2A agonist reduced the amplitude of the IPSCs evoked by application of the selective GABA agonist, muscimol. These results suggest a presynaptic and postsynaptic reduction of GABAergic transmission mediated by 5-HT1A and 5-HT2A serotonergic receptors, respectively.

Presynaptic α4ß2 nicotinic acetylcholine receptors increase glutamate release and serotonin neuron excitability in the dorsal raphe nucleus.

Several behavioral effects of nicotine are mediated by changes in serotonin (5-HT) release in brain areas that receive serotonergic afferents from the dorsal raphe nucleus (DRN). In vitro experiments have demonstrated that nicotine increases the firing activity in the majority of DRN 5-HT neurons and that DRN contains nicotinic acetylcholine receptors (nAChRs) located at both somata and presynaptic elements. One of the most common presynaptic effects of nicotine is to increase glutamate release. Although DRN receives profuse glutamatergic afferents, the effect of nicotine on glutamate release in the DRN has not been studied in detail. Using whole-cell recording techniques, we investigated the effects of nicotine on the glutamatergic input to 5-HT DRN neurons in rat midbrain slices. Low nicotine concentrations, in the presence of bicuculline and tetrodotoxin (TTX), increased the frequency but did not change the amplitude of glutamate-induced EPSCs, recorded from identified 5-HT neurons. Nicotine-induced increase of glutamatergic EPSC frequency persisted 10-20 min after drug withdrawal. This nicotinic effect was mimicked by exogenous administration of acetylcholine (ACh) or inhibition of ACh metabolism. In addition, the nicotine-induced increase in EPSC frequency was abolished by blockade of α4ß2 nAChRs, voltage-gated calcium channels, or intracellular calcium signaling but not by α7 nAChR antagonists. These data suggest that both nicotine and endogenous ACh can increase glutamate release through activation of presynaptic α4ß2 but not α7 nAChRs in the DRN. The effect involves long-term changes in synaptic function, and it is dependent on voltage-gated calcium channels and presynaptic calcium stores.

Serotoninergic dorsal raphe neurons possess functional postsynaptic nicotinic acetylcholine receptors.

Very few neurons in the telencephalon have been shown to express functional postsynaptic nicotinic acetylcholine receptors (nAChRs), among them, the noradrenergic and dopaminergic neurons. However, there is no evidence for postsynaptic nAChRs on serotonergic neurons. In this study, we asked if functional nAChRs are present in serotonergic (5-HT) and nonserotonergic (non-5-HT) neurons of the dorsal raphe nucleus (DRN). In rat midbrain slices, field stimulation at the tegmental pedunculopontine (PPT) nucleus evoked postsynaptic currents (eEPSCs) with different components in DRN neurons. After blocking the glutamatergic and GABAergic components, the remaining eEPSCs were blocked by mecamylamine and reduced by either the selective alpha7 nAChR antagonist methyllycaconitine (MLA) or the selective alpha4beta2 nAChR antagonist dihydro-beta-eritroidine (DHbetaE). Simultaneous addition of MLA and DHbetaE blocked all eEPSCs. Integrity of the PPT-DRN pathway was assessed by both anterograde biocytin tracing and antidromic stimulation from the DRN. Inward currents evoked by the direct application of acetylcholine (ACh), in the presence of atropine and tetrodotoxin, consisted of two kinetically different currents: one was blocked by MLA and the other by DHbetaE; in both 5-HT and non-5-HT DR neurons. Analysis of spontaneous (sEPSCs) and evoked (eEPSCs) synaptic events led to the conclusion that nAChRs were located at the postsynaptic membrane. The possible implications of these newly described nAChRs in various physiological processes and behavioral events, such as the wake-sleep cycle, are discussed.