Comparing tonic and phasic dendritic calcium in cholinergic pedunculopontine neurons and dopaminergic substantia nigra neurons

Several brainstem nuclei degenerate in Parkinson's disease (PD). In addition to the well-characterized dopaminergic neurons of the substantia nigra pars compacta (SNc), the cholinergic neurons of the pedunculopontine nucleus (PPN) also degenerate in PD. One leading hypothesis of selective vulnerability is that pacemaking activity and the activation of low-threshold L-type calcium current are major contributors to tonic calcium load and cellular stress in SNc dopaminergic neurons. However, it is not yet clear whether the vulnerable PPN cholinergic neurons share this property. Therefore, we used two-photon dendritic calcium imaging and whole-cell electrophysiology to evaluate the role of L-type calcium channels in tonic and phasic dendritic calcium signals in PPN and SNc neurons. In addition, we investigated N- and P/Q-type calcium channel regulation of firing properties and dendritic calcium in PPN neurons. We found that blocking L-type channels reduces tonic firing rate and dendritic calcium levels in SNc neurons. By contrast, the tonic calcium load in PPN neurons did not depend on L-, N- or P/Q-type channels. However, we found that blocking either L-type (with nifedipine) or N- and P/Q-type (with omega-conotoxin MVIIC) channels reduces phasic calcium influx in PPN dendrites. Together, these findings show that L-type calcium channels play different roles in the activity of SNc and PPN neurons, and suggest that low-threshold L-type channels are not responsible for tonic calcium levels in PPN cholinergic neurons and are therefore not likely to be a source of selective vulnerability in these cells.

Comparing tonic and phasic calcium in the dendrites of vulnerable midbrain neurons

Several midbrain nuclei degenerate in Parkinson's Disease (PD). Many of these nuclei share the common characteristics that are thought to contribute to their selective vulnerability, including pacemaking activity and high levels of calcium influx. In addition to the well-characterized dopaminergic neurons of the substantia nigra pars compacta (SNc), the cholinergic neurons of the pedunculopontine nucleus (PPN) also degenerate in PD. It is well established that the low-threshold L-type calcium current is a main contributor to tonic calcium in SNc dopaminergic neurons and is hypothesized to contribute to their selective vulnerability. However, it is not yet clear whether the vulnerable PPN cholinergic neurons share this property. Therefore, we used two-photon dendritic calcium imaging and whole-cell electrophysiology to evaluate the role of L-type calcium channels in the tonic and phasic activity of PPN neurons and the corresponding dendritic calcium signal and directly compare these characteristics to SNc neurons. We found that blocking L-type channels reduces tonic firing rate and dendritic calcium levels in SNc neurons. By contrast, the calcium load in PPN neurons during pacemaking did not depend on L-type channels. However, we find that blocking L-type channels reduces phasic calcium influx in PPN dendrites. Together, these findings show that L-type calcium channels play different roles in the activity of SNc and PPN neurons, and suggest that low-threshold L-type channels are not responsible for tonic calcium levels in PPN cholinergic neurons and are therefore not likely to be a source of selective vulnerability in these cells.

Functional characterization of N-octyl 4-methylamphetamine variants and related bivalent compounds at the dopamine and serotonin transporters using Ca2+ channels as sensors

The early characterization of ligands at the dopamine and serotonin transporters, DAT and SERT, respectively, is important for drug discovery, forensic sciences, and drug abuse research. 4-Methyl amphetamine (4-MA) is a good example of an abused drug whose overdose can be fatal. It is a potent substrate at DAT and SERT where its simplest secondary amine (N-methyl 4-MA) retains substrate activity at them. In contrast, N-n-butyl 4-MA is very weak, therefore it was categorized as inactive at these transporters. Here, N-octyl 4-MA and other related compounds were synthesized, and their activities were evaluated at DAT and SERT. To expedite this endeavor, cells expressing DAT or SERT were co-transfected with a voltage-gated Ca2+ channel and, the genetically-encoded Ca2+ sensor, GCaMP6s. Control compounds and the newly synthesized molecules were tested on these cells using an automated multi-well fluorescence plate reader; substrates and inhibitors were identified successfully at DAT and SERT. N-Octyl 4-MA and three bivalent compounds were inhibitors at these transporters. These findings were validated by measuring Ca2+-mobilization using quantitative fluorescence microscopy. The bivalent molecules were the most potent of the series and were further characterized in an uptake-inhibition assay. Compared to cocaine, they showed comparable potency inhibiting uptake at DAT and higher potency at SERT. These observations support a previous hypothesis that amphetamine-related (and, here, N-extended alkyl and) bivalent arylalkylamine molecules are active at monoamine transporters, showing potent activity as reuptake inhibitors, and implicate the involvement of a distant auxiliary binding feature to account for their actions at DAT and SERT.

Impact of menthol on nicotine intake and preference in mice: Concentration, sex, and age differences

Menthol has been shown to contribute to the appeal of tobacco products in humans. However, factors such as sex, age and menthol concentration remain unclear in the interaction between menthol and nicotine. To understand these factors, we utilized a mouse model to determine the impact of menthol on oral nicotine consumption. A range of menthol concentrations (oral and systemic) were tested with or without oral nicotine using the two-bottle choice paradigm in adolescent and adult female and male C57BL/6J mice. Moreover, genetically modified mice were used to investigate the role of α7 nicotinic acetylcholine receptors (nAChRs) on the effects of menthol. Menthol addition to nicotine solution increased oral nicotine consumption in C57BL/6J mice in a sex- and menthol concentration-dependent manner. At lower menthol concentrations, female mice demonstrated an enhancement of nicotine consumption and male mice showed a similar behavior at higher menthol concentrations. Menthol drinking alone was only significantly different by sex at 60 μg/ml menthol concentration where female mice had higher menthol intake than males. Menthol administered both orally and systemically (intraperitoneal) increased oral nicotine consumption. Adolescent female mice had a higher nicotine intake at lower menthol concentrations compared to their adult counterparts. While α7 nAChR wild type mice consumed more mentholated nicotine solution than nicotine only solution, this effect was abolished in KO mice. Effects of menthol are concentration-, sex-, age-, and α7 nAChR-dependent. Oral and intraperitoneal menthol increases nicotine intake, suggesting that sensory, peripheral, and/or central mechanisms are involved in effects of menthol on oral nicotine consumption.

Spreading Depression Promotes Astrocytic Calcium Oscillations and Enhances Gliotransmission to Hippocampal Neurons

Spreading depression (SD) is a pathophysiological phenomenon characterized by propagating waves of profound neuronal and glial depolarization in central nervous system gray matter. Although SD is primarily mediated by neurons with a subsequent astrocytic response, it remains unclear how astrocytic activity is modulated after SD and how altered astrocyte signaling contribute to neuronal excitability. Here, we report that after the concurrent Ca2+ wave, SD enhanced astrocytic activity by promoting a secondary period of Ca2+ oscillations. SD-induced Ca2+ oscillations did not require the activation of metabotropic glutamate receptors or purinergic receptors; instead, they were mediated by the activation of GABAB receptors and 1,4,5-trisphosphate (IP3) receptors. Furthermore, SD increased the number of NMDA receptor-mediated slow inward currents (SICs) in CA1 pyramidal neurons. The frequency of SD-induced SICs was reduced by blockade of GABAB receptors or by limiting Ca2+ efflux from the ER. Selective inhibition of astrocytic Ca2+ signals by dialysis of BAPTA into astrocytes or by knocking out the astrocytic type of IP3 receptors suppressed SICs after SD. These results demonstrated a causative link between the SD-induced Ca2+ oscillations and the enhanced glutamatergic astrocyte-neuron signaling. Therefore, we conclude that SD enhances the astrocyte Ca2+ signals and further promotes gliotransmission and neuronal excitability.

Luteolin inhibits GABAA receptors in HEK cells and brain slices

Modulation of the A type γ-aminobutyric acid receptors (GABA_AR) is one of the major drug targets for neurological and psychological diseases. The natural flavonoid compound luteolin (2-(3,4-Dihydroxyphenyl)- 5,7-dihydroxy-4-chromenone) has been reported to have antidepressant, antinociceptive, and anxiolytic-like effects, which possibly involve the mechanisms of modulating GABA signaling. However, as yet detailed studies of the pharmacological effects of luteolin are still lacking, we investigated the effects of luteolin on recombinant and endogenous GABA_AR-mediated current responses by electrophysiological approaches. Our results showed that luteolin inhibited GABA-mediated currents and slowed the activation kinetics of recombinant α1β2, α1β2γ2, α5β2, and α5β2γ2 receptors with different degrees of potency and efficacy. The modulatory effect of luteolin was likely dependent on the subunit composition of the receptor complex: the αβ receptors were more sensitive than the αβγ receptors. In hippocampal pyramidal neurons, luteolin significantly reduced the amplitude and slowed the rise time of miniature inhibitory postsynaptic currents (mIPSCs). However, GABA_AR-mediated tonic currents were not significantly influenced by luteolin. These data suggested that luteolin has negative modulatory effects on both recombinant and endogenous GABA_ARs and inhibits phasic rather than tonic inhibition in hippocampus.

Relationship between calcium absorption and plasma dehydroepiandrosterone sulphate (DHEAS) in healthy males

CONTEXT

Impaired gut sensitivity to 1,25-dihydroxyvitamin D (1,25(OH)(2)D), leading to reduced intestinal calcium absorption, has been reported in older men and women. While this phenomenon in postmenopausal women has been attributed to oestrogen deficiency, it is unclear whether the same observation in older men correlates with the age-related decline in androgen concentrations.

OBJECTIVE

To examine the relationship between androgens and intestinal calcium absorption in older men.

DESIGN

Cross-sectional study on 55 healthy male volunteers, divided into younger (n = 27) and older (n = 28) groups separated according to the median age of 59 years.

MAIN OUTCOME MEASURES

Calcium absorption, total and free (calculated) testosterone, dehydroepiandrosterone sulphate (DHEAS), SHBG, and 1,25(OH)(2)D, among others, were measured.

RESULTS

Calcium absorption, free testosterone and DHEAS, but not 1,25(OH)(2)D, declined significantly with age. After adjusting for age and body mass index, stepwise regression showed that 1,25(OH)(2)D and serum albumin were the only significant determinants of calcium absorption in younger men, while the sole determinant in older men was DHEAS, not testosterone. Residual deviations from the regression of calcium absorption on 1,25(OH)(2)D, reflecting the efficiency of 1,25(OH)(2)D-induced calcium absorption, was positively correlated with DHEAS (r = 0.27, P = 0.027).

CONCLUSIONS

DHEAS is an independent determinant of calcium absorption in older men, although its manner of influence is, as yet, undefined. The age-related decline of DHEAS may, partly, account for the observed 'intestinal resistance to 1,25(OH)(2)D' in older men.

Relative androgen deficiency in relation to obesity and metabolic status in older men

BACKGROUND

Although androgen deficiency in men has been linked with obesity and the metabolic syndrome, whether it predisposes to, or is a consequence of, type 2 diabetes mellitus (T2DM) is still unclear.

OBJECTIVE

To determine the relationship between plasma androgen levels, obesity, metabolic status and T2DM in men of 70 years or older.

DESIGN AND METHODS

A sample of 195 men from the Australian Longitudinal Study of Ageing with a mean age of 76.2 +/- 0.3 years were followed up for 8 years. Total testosterone (TT), fasting plasma glucose (FPG), urate, serum creatinine, total cholesterol (TC), HDL cholesterol (HDL-C), LDL cholesterol (LDL-C), triglycerides (TG), blood pressure (BP), body mass index (BMI), waist circumference (WC) and diabetic status were assessed at baseline. Self-reported diabetic status was obtained after 8 years. Metabolic syndrome was diagnosed based on the Third National Cholesterol Education Program Adult Treatment Panel clinical criteria.

RESULTS

TT levels were lower in diabetic men compared with non-diabetic men (12.1 +/- 0.7 vs. 14.2 +/- 0.4 nmol/l, p = 0.026). TT levels in healthy, non-diabetic men over 80 years of age were lower (11.9 +/- 0.8 vs. 15.0 +/- 0.5 nmol/l, p = 0.002) than TT levels in those aged 70-79 years, inversely related to BMI (r = -0.26, p = 0.001), WC (r = -0.30, p < 0.001) and TG (r = -0.22, p = 0.005) and positively related to LDL-C (r = 0.25, p = 0.002). Men with the metabolic syndrome had significantly lower levels of TT and HDL-C, and higher values of BP, FPG, TG, BMI and WC, compared with those without. However, no significant difference in plasma TT levels was noted between men with incident T2Dm and healthy men. Stepwise linear regression analysis revealed that only LDL-C and WC related significantly to the variance of TT. Multiple logistic regression revealed FPG to be the only independent predictor of incident diabetes (odds ratio = 60.2, p = 0.003).

CONCLUSIONS

Testosterone levels continue to decline even in healthy men over the age of 80 years. Although TT levels were inversely related to visceral obesity and several components of the metabolic syndrome, our data do not support a predictive or causative role for decreasing TT levels in the development of incident T2Dm. Androgen deficiency is consequent upon, rather than a cause of, poor metabolic status.