New perspectives on the mechanisms through which nitric oxide may affect learning and memory processes

Tonic and stimulus-evoked nitric oxide production in the mouse olfactory bulb

Nitric oxide (NO) has been long assumed to play a key role in mammalian olfaction. This was based largely on circumstantial evidence, i.e. prominent staining for nitric oxide synthase (NOS) and cyclic guanosine 3',5'-cyclic monophosphate (cGMP) or soluble guanylyl cyclase, an effector enzyme activated by NO, in local interneurons of the olfactory bulb. Here we employ innovative custom-fabricated NO micro-sensors to obtain the first direct, time-resolved measurements of NO signaling in the olfactory bulb. In 400 microm thick mouse olfactory bulb slices, we detected a steady average basal level of 87 nM NO in the extracellular space of mitral or granule cell layers. This NO 'tone' was sensitive to NOS substrate manipulation (200 microM L-arginine, 2 mM N(G)-nitro-L-arginine methyl ester) and Mg(2+) modulation of N-methyl-D-aspartate (NMDA) receptor conductance. Electrical stimulation of olfactory nerve fibers evoked transient (peak at 10 s) increments in NO levels 90-100 nM above baseline. In the anesthetized mouse, NO micro-sensors inserted into the granule cell layer detected NO transients averaging 55 nM in amplitude and peaking at 3.4 s after onset of a 5 s odorant stimulation. These findings suggest dual roles for NO signaling in the olfactory bulb: tonic inhibitory control of principal neurons, and regulation of circuit dynamics during odor information processing.

Pharmacological attenuation of electroconvulsive therapy--induced cognitive deficits: theoretical background and clinical findings

Electroconvulsive therapy (ECT) is an effective treatment for depression and other psychiatric disorders. However, the practice of ECT is limited by memory and nonmemory cognitive adverse effects. Technical strategies such as a preference for unilateralover bilateral ECT and low-dose over high-dose stimulation reduce these cognitive adverse effects but may also be associated with lesser treatment efficacy or slower treatment response. This article therefore reviews the use of psychopharmacological agents in the attenuation of ECT-induced cognitive deficits with 2 objectives: the identification of implicated mechanisms and the identification of putative efficacy in both animal and human studies. Drugs examined include N-methyl-d-aspartate receptor antagonists, cyclooxygenase inhibitors, calcium channel blockers, cholinesterase inhibitors, glucocorticoid receptor antagonists, thyroid hormones, opioid antagonists, NO donors, nootropic agents, and other medications. Although the clinical data at present are sparse and inconsistent, many recently opened lines of research improve our understanding of the mechanisms involved as well as suggest possible avenues for the testing of new treatments with the potential to attenuate the cognitive adverse effects of ECT.

The distribution of nitric oxide synthase in the inferior colliculus of guinea pig

The modulation of neuronal activity by the gas nitric oxide is one of the most novel discoveries in neuroscience. In the auditory pathway, the highest expression of nitric oxide synthase is found in the inferior colliculus (IC), an important center for the convergence of parallel ascending pathways traveling in the brainstem, and descending projections from the auditory cortex. Here we use immunocytochemistry with an antibody for neuronal nitric oxide synthase (nNOS), or NOS Type 1, to map the distribution of nNOS expression in the IC of the guinea pig. The results show that nNOS is differentially expressed by both cell bodies and neuropil across its different subdivisions. The highest levels of neuronal staining are seen in the dorsal and lateral cortices, and the commissural nucleus, making them readily distinguishable from the ventro-lateral part of the central nucleus where nNOS expression in neuropil and somata is minimal. Dorso-medially, and caudally, however, the region of nNOS expression extends from the dorsal cortex into the area normally designated as the central nucleus, and nNOS is expressed by neurons characteristic of this subdivision. Our findings support the idea of a gradual transition in cell properties rather than a distinct boundary between the central nucleus and the dorsal cortex. This transition zone may provide a cytoarchitectonic substrate for functional interaction between these two subdivisions.

Mesenchymal stem cell-mediated immunosuppression occurs via concerted action of chemokines and nitric oxide

Mesenchymal stem cells (MSCs) can become potently immunosuppressive through unknown mechanisms. We found that the immunosuppressive function of MSCs is elicited by IFNgamma and the concomitant presence of any of three other proinflammatory cytokines, TNFalpha, IL-1alpha, or IL-1beta. These cytokine combinations provoke the expression of high levels of several chemokines and inducible nitric oxide synthase (iNOS) by MSCs. Chemokines drive T cell migration into proximity with MSCs, where T cell responsiveness is suppressed by nitric oxide (NO). This cytokine-induced immunosuppression was absent in MSCs derived from iNOS(-/-) or IFNgammaR1(-/-) mice. Blockade of chemokine receptors also abolished the immunosuppression. Administration of wild-type MSCs, but not IFNgammaR1(-/-) or iNOS(-/-) MSCs, prevented graft-versus-host disease in mice, an effect reversed by anti-IFNgamma or iNOS inhibitors. Wild-type MSCs also inhibited delayed-type hypersensitivity, while iNOS(-/-) MSCs aggravated it. Therefore, proinflammatory cytokines are required to induce immunosuppression by MSCs through the concerted action of chemokines and NO.

Hippocampal GABAergic synapses possess the molecular machinery for retrograde nitric oxide signaling

Nitric oxide (NO) plays an important role in synaptic plasticity as a retrograde messenger at glutamatergic synapses. Here we describe that, in hippocampal pyramidal cells, neuronal nitric oxide synthase (nNOS) is also associated with the postsynaptic active zones of GABAergic symmetrical synapses terminating on their somata, dendrites, and axon initial segments in both mice and rats. The NO receptor nitric oxide-sensitive guanylyl cyclase (NOsGC) is present in the brain in two functional subunit compositions: alpha1beta1 and alpha2beta1. The beta1 subunit is expressed in both pyramidal cells and interneurons in the hippocampus. Using immunohistochemistry and in situ hybridization methods, we describe that the alpha1 subunit is detectable only in interneurons, which are always positive for beta1 subunit as well; however, pyramidal cells are labeled only for beta1 and alpha2 subunits. With double-immunofluorescent staining, we also found that most cholecystokinin- and parvalbumin-positive and smaller proportion of the somatostatin- and nNOS-positive interneurons are alpha1 subunit positive. We also found that the alpha1 subunit is present in parvalbumin- and cholecystokinin-positive interneuron terminals that establish synapses on somata, dendrites, or axon initial segments. Our results demonstrate that NOsGC, composed of alpha1beta1 subunits, is selectively expressed in different types of interneurons and is present in their presynaptic GABAergic terminals, in which it may serve as a receptor for NO produced postsynaptically by nNOS in the very same synapse.