A voltammetric and mathematical analysis of histaminergic modulation of serotonin in the mouse hypothalamus

Abstract

Histamine and serotonin are neuromodulators which facilitate numerous, diverse neurological functions. Being co-localized in many brain regions, these two neurotransmitters are thought to modulate one another's chemistry and are often implicated in the etiology of disease. Thus, it is desirable to interpret the in vivo chemistry underlying neurotransmission of these two molecules to better define their roles in health and disease. In this work, we describe a voltammetric approach to monitoring serotonin and histamine simultaneously in real time. Via electrical stimulation of the axonal bundles in the medial forebrain bundle, histamine release was evoked in the mouse premammillary nucleus. We found that histamine release was accompanied by a rapid, potent inhibition of serotonin in a concentration-dependent manner. We developed mathematical models to capture the experimental time courses of histamine and serotonin, which necessitated incorporation of an inhibitory receptor on serotonin neurons. We employed pharmacological experiments to verify that this serotonin inhibition was mediated by H3 receptors. Our novel approach provides fundamental mechanistic insights that can be used to examine the full extent of interconnectivity between histamine and serotonin in the brain. Histamine and serotonin are co-implicated in many of the brain's functions. In this paper, we develop a novel voltammetric method for simultaneous real-time monitoring of histamine and serotonin in the mouse premammillary nucleus. Electrical stimulation of the medial forebrain bundle evokes histamine and inhibits serotonin release. We show voltammetrically, mathematically, and pharmacologically that this serotonin inhibition is H3 receptor mediated.

Keywords: H3 autoreceptor; carbon fiber microelectrodes; fast-scan cyclic voltammetry; heteroreceptor; thioperamide.

Fig. 1

(ai & aii) The position of electrodes (stimulation and carbon fiber microelectrodes) in the mouse brain. (bi & bii) Representative color plots of the stimulated release of histamine and serotonin in the premammillary nucleus (PM) and stimulated release of serotonin in the substantia nigra pars reticulata (SNr), respectively. (ci & ii) Superimposed cyclic voltammograms of in vivo and in vitro histamine and serotonin signals taken from vertical dashed lines in the PM. (ciii) Comparison of normalized cyclic voltammograms of in vivo serotonin signals taken from vertical dashed lines in both PM and SNr. HA, histamine; 5-HT, serotonin.

Fig. 2

(a) Representative color plot of the stimulated release of histamine and serotonin inhibition in the premammillary nucleus. (b) Correlation plot between [histamine] and [serotonin] for all stimulation parameters. (c) Averaged current versus time traces along the two horizontal dashed lines of histamine and serotonin with respect to different stimulation frequencies (n = 5). (d) Averaged current responses to various stimulation pulse widths of histamine and serotonin (n = 5). (e) Averaged current responses to various stimulation amplitudes of histamine and serotonin (n = 5). HA, histamine; 5-HT, serotonin.

Fig. 3

(a) [Histamine] versus time plots comparing in vivo (solid traces) and the results of the mathematical model (large dots) in the control case. (b) [Serotonin] versus time plots comparing in vivo (solid traces) and the results of the mathematical model (large dots) in the presence of thioperimide (20 mg/kg) (c) Firing rate of the histamine neurons as a function of time in the two cases control (blue) and drug (green), respectively. (d) Assumed fractional release of histamine from the histamine neurons as a function of time in the two cases. HA, histamine; 5-HT, serotonin.

Fig. 4

[Histamine] versus time traces are shown in blue and green for pre- and post-drug administration, respectively. [Serotonin] versus time traces are shown in red and orange for before and after the drug respectively. Error bars showing SEM (n = 5 ± SEM) are lighter versions of these respective colors. (a) thioperamide 2 mg/kg (b) thioperamide 20 mg/kg (c) thioperamide 50 mg/kg. HA, histamine; 5-HT, serotonin.