Zinc modulation of glycine receptors.

Glycine receptors are widely expressed in the mammalian central nervous system, and previous studies have demonstrated that glycine receptors are modulated by endogenous zinc. Zinc is concentrated in synaptic vesicles in several brain regions but is particularly abundant in the hippocampus and olfactory bulb. In the present study, we used patch-clamp electrophysiology of rat hippocampal and olfactory bulb neurons in primary culture to examine the effects of zinc on glycine receptors. Although glycine has been reported to reach millimolar concentrations during synaptic transmission, most previous studies on the effects of zinc on glycine receptors have used relatively low concentrations of glycine. High concentrations of glycine cause receptor desensitization. Our current results extend our previous demonstration that the modulatory actions of zinc are largely prevented when co-applied with desensitizing concentrations of glycine (300 µM), suggesting that the effects of zinc are dependent on the state of the receptor. In contrast, pre-application of 300 µM zinc, prior to glycine (300 µM) application, causes a slowly developing inhibition with a slow rate of recovery, suggesting that the timing of zinc and glycine release also influences the effects of zinc. Furthermore, previous evidence suggests that synaptically released zinc can gain intracellular access, and we provide the first demonstration that low concentrations of intracellular zinc can potentiate glycine receptors. These results support the notion that zinc has complex effects on glycine receptors and multiple factors may interact to influence the efficacy of glycinergic transmission.

Neuropeptide Y modulates excitatory synaptic transmission in the olfactory bulb.

Although the olfactory bulb contains one of the highest concentrations of neuropeptide Y in the CNS, its function in the bulb remains unclear. In this study, we used whole-cell electrophysiological, molecular, and primary culture techniques to investigate neuropeptide Y gene expression and neuromodulatory actions of neuropeptide Y on rat olfactory bulb neurons. Northern analysis showed that neuropeptide Y mRNA increases with animal age or time in culture, in a parallel manner. In electrophysiology experiments, agonists that activate neuropeptide Y receptors (whole neuropeptide Y) and the Y2 receptor subtype (neuropeptide Y 13-36) reduced spontaneous excitatory activity in bulb interneurons. In investigating potential presynaptic effects, both agonists reduced the amplitude of calcium channel currents in the presynaptic (mitral/tufted) cell. Also consistent with a presynaptic effect, both agonists reduced the frequency but not the amplitude of miniature excitatory postsynaptic currents (or "minis") in interneurons. In examining potential postsynaptic effects, both agonists slightly increased membrane resistance but had no effect on currents evoked by glutamate. Together, these data suggest that neuropeptide Y inhibits excitatory neurotransmission between olfactory bulb neurons via a presynaptic effect on transmitter (glutamate) release.

Evidence for Ca(2+)-permeable AMPA receptors in the olfactory bulb.

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors (AMPARs), a subtype of glutamate receptor, contribute to olfactory processing in the olfactory bulb (OB). These ion channels consist of various combinations of the subunits GluR1-GluR4, which bestow certain properties. For example, AMPARs that lack GluR2 are highly permeable to Ca(2+) and generate inwardly rectifying currents. Because increased intracellular Ca(2+) could trigger a host of Ca(2+)-dependent odor-encoding processes, we used whole cell recording as well as histological and immunocytochemical (ICC) techniques to investigate whether AMPARs on rat OB neurons flux Ca(2+). Application of 1-naphthylacetyl spermine (NAS), a selective antagonist of Ca(2+)-permeable AMPARs (CP-AMPARs), inhibited AMPAR-mediated currents in subsets of interneurons and principal cells in cultures and slices. The addition of spermine to the electrode yielded inwardly rectifying current-voltage plots in some cells. In OB slices, olfactory nerve stimulation elicited excitatory responses in juxtaglomerular and mitral cells. Bath application of NAS with d,l-2-amino-5-phosphonovaleric acid (AP5) to isolate AMPARs suppressed the amplitudes of these synaptic responses compared with responses obtained using AP5 alone. Co(2+) staining, which involves the kainate-stimulated influx of Co(2+) through CP-AMPARs, produced diverse patterns of labeling in cultures and slices as did ICC techniques used with a GluR2-selective antibody. These results suggest that subsets of OB neurons express CP-AMPARs, including functional CP-AMPARs at synapses. Ca(2+) entry into cells via these receptors could influence odor encoding by modulating K(+) channels, N-methyl-d-aspartate receptors, and Ca(2+)-binding proteins, or it could facilitate synaptic vesicle fusion.

Interactions between carnosine and zinc and copper: implications for neuromodulation and neuroprotection.

This review examines interactions in the mammalian central nervous system (CNS) between carnosine and the endogenous transition metals zinc and copper. Although the relationship between these substances may be applicable to other brain regions, the focus is on the olfactory system where these substances may have special significance. Carnosine is not only highly concentrated in the olfactory system, but it is also contained in neurons (in contrast to glia cells in most of the brain) and has many features of a neurotransmitter. Whereas the function of carnosine in the CNS is not well understood, we review evidence that suggests that it may act as both a neuromodulator and a neuroprotective agent. Although zinc and/or copper are found in many neuronal pathways in the brain, the concentrations of zinc and copper in the olfactory bulb (the target of afferent input from sensory neurons in the nose) are among the highest in the CNS. Included in the multitude of physiological roles that zinc and copper play in the CNS is modulation of neuronal excitability. However, zinc and copper also have been implicated in a variety of neurologic conditions including Alzheimer's disease, Parkinson's disease, stroke, and seizures. Here we review the modulatory effects that carnosine can have on zinc and copper's abilities to influence neuronal excitability and to exert neurotoxic effects in the olfactory system. Other aspects of carnosine in the CNS are reviewed elsewhere in this issue.

Endogenous mechanisms of neuroprotection: role of zinc, copper, and carnosine.

Zinc and copper are endogenous transition metals that can be synaptically released during neuronal activity. Synaptically released zinc and copper probably function to modulate neuronal excitability under normal conditions. However, zinc and copper also can be neurotoxic, and it has been proposed that they may contribute to the neuropathology associated with a variety of conditions, such as Alzheimer's disease, stroke, and seizures. Recently, we demonstrated that carnosine, a dipeptide expressed in glial cells throughout the brain as well as in neuronal pathways of the visual and olfactory systems, can modulate the effects of zinc and copper on neuronal excitability. This result led us to hypothesize that carnosine may modulate the neurotoxic effects of zinc and copper as well. Our results demonstrate that carnosine can rescue neurons from zinc- and copper-mediated neurotoxicity and suggest that one function of carnosine may be as an endogenous neuroprotective agent.

Carnosine modulates zinc and copper effects on amino acid receptors and synaptic transmission.

Carnosine is a dipeptide which is highly concentrated in mammalian olfactory sensory neurons along with zinc and/or copper, and glutamate. Although carnosine has been proposed as a neurotransmitter or neuromodulator, no specific function for carnosine has been identified. We used whole-cell current- and voltage-clamp recording to examine the direct effects and neuromodulatory actions of carnosine on rat olfactory bulb neurons in primary culture. Carnosine did not evoke a membrane current or affect currents evoked by glutamate, GABA or glycine. Copper and zinc inhibited NMDA and GABA receptor-mediated currents and inhibited synaptic transmission. Carnosine prevented the actions of copper and reduced the effects of zinc. These results suggest that carnosine may indirectly influence neuronal excitability by modulating the effects of zinc and copper.

Vasopressin: a regulator of adrenal glucocorticoid production?

Vasopressin infusion has been shown to decrease plasma adrenocorticotropic hormone (ACTH) concentration and transiently increase plasma cortisol concentration in conscious dogs. In the present study, one experiment tested the hypothesis that vasopressin infusion decreases ACTH by activation of a V1 receptor mechanism, e.g., by increasing atrial pressures and stimulating the low-pressure baroreceptor reflex. Administration of a vasopressin V1 antagonist eliminated the increases in atrial pressure and decreases in heart rate with vasopressin infusion (1 ng.kg-1.min-1), as it eliminated the decrease in ACTH, which is consistent with baroreflex-mediated inhibition of ACTH by vasopressin. A second experiment evaluated the role of ACTH in the increase in glucocorticoids. Dexamethasone pretreatment, which inhibits ACTH secretion, abolished the increase in glucocorticoid concentration with vasopressin infusion, indicating that ACTH is necessary for the glucocorticoid response. A third experiment was performed to determine whether the glucocorticoid response could be restored in dexamethasone-treated dogs, when ACTH concentration was maintained near control levels by intravenous infusion of synthetic alpha-ACTH-(1-24) (0.3 ng.kg-1.min-1). In these dogs, vasopressin infusion produced a sustained increase in plasma glucocorticoid concentration from 22 +/- 3 to 49 +/- 8 ng/ml (P less than 0.001). Infusing higher levels of ACTH (0.5 ng.kg-1.min-1) enhanced basal glucocorticoid levels but did not enhance the response to vasopressin. Vasopressin infusion did not alter clearance of glucocorticoids. Collectively, these results suggest that vasopressin directly stimulates adrenal glucocorticoid production, provided that background levels of ACTH are present.

Intravenous vasopressin infusion decreases plasma ACTH concentration in conscious dogs.

Vasopressin infusion increases arterial and atrial pressures, which could stimulate arterial and cardiac baroreceptors to inhibit adrenocorticotropin (ACTH) secretion. Therefore, the current experiments were performed to test the hypothesis that vasopressin infusion decreases plasma ACTH concentration in conscious dogs. Vasopressin was infused for 90 min in three doses (0.5, 1.0, and 2.0 ng.kg-1.min-1) that produced increases in plasma levels within the physiological range. Only the highest dose of vasopressin increased mean arterial pressure, but left atrial pressure increased with all doses, and right atrial pressure increased with the two highest doses. A bradycardia was produced with all doses of vasopressin. Plasma ACTH concentration decreased from 44 +/- 12 to 25 +/- 7 (P less than 0.01), from 50 +/- 11 to 26 +/- 9 (P less than 0.001), and from 70 +/- 15 to 28 +/- 4 pg/ml (P less than 0.001) with infusion of 0.5, 1.0, and 2.0 ng.kg-1.min-1 vasopressin, respectively. In contrast, plasma cortisol concentration first increased (P less than 0.05) with each vasopressin dose, but after 15-30 min it decreased back to control levels. These results demonstrate that intravenous infusion of vasopressin decreases plasma ACTH concentration. Because the inhibition is associated with increases in atrial pressure and decreases in heart rate, it may be mediated via activation of the baroreceptor reflex.

Timing of 6-hydroxydopamine administration influences its effects on visual cortical plasticity.

We recorded from the visual cortex of 4 groups of monocularly deprived kittens. Three groups were treated with intraventricular 6-hydroxydopamine (6-OHDA) at different times relative to monocular deprivation (MD). One group received only vehicle solution and MD. 6-OHDA caused the greatest decrease in plasticity in the kittens receiving 6-OHDA throughout the deprivation period; that is, these kittens were the least affected by MD. 6-OHDA caused a smaller decrease in plasticity in kittens receiving 6-OHDA just prior to eyelid suture and a still smaller decrease in kittens waiting a week between 6-OHDA treatment and eyelid suture. The kittens in all groups receiving 6-OHDA were equally depleted of norepinephrine (NE). We conclude that 6-OHDA decreases plasticity in the visual cortex; however, the time course of this decrease is better related to the time course of the 6-OHDA treatment than to the time course of NE depletion.

Effect of desmethylimipramine on norepinephrine content and plasticity of kitten visual cortex.

Because norepinephrine (NE) reuptake inhibitors have been reported to block 6-hydroxydopamine (6-OHDA) induced NE depletion, we wondered whether a reuptake inhibitor would block 6-OHDA's effects on visual cortical plasticity. We found, however, that desmethylimipramine (DMI) did not reduce 6-OHDA-induced NE depletion at 6-OHDA doses sufficient to prevent the effects of monocular deprivation. We also found that DMI itself induced transient NE depletion. We used this last result to further examine the NE hypothesis of depletion. In contrast to 6-OHDA-induced NE depletion, DMI-induced NE depletion was not accompanied by changes in visual cortical plasticity.