Effects of nicotine on odor-induced increases in regional blood flow in the olfactory bulb in rats

Influence of age on nicotinic cholinergic regulation of blood flow in rat's olfactory bulb and neocortex

The olfactory bulb receives cholinergic basal forebrain inputs as does the neocortex. With a focus on nicotinic acetylcholine receptors (nAChRs), this review article provides an overview and discussion of the following findings: (1) the nAChRs-mediated regulation of regional blood flow in the neocortex and olfactory bulb, (2) the nAChR subtypes that mediate their responses, and (3) their activity in old rats. The activation of the α4β2-like subtype of nAChRs produces vasodilation in the neocortex, and potentiates olfactory bulb vasodilation induced by olfactory stimulation. The nAChR activity producing neocortical vasodilation was similarly maintained in 2-year-old rats as in adult rats, but was clearly reduced in 3-year-old rats. In contrast, nAChR activity in the olfactory bulb was reduced already in 2-year-old rats. Thus, age-related impairment of α4β2-like nAChR function may occur earlier in the olfactory bulb than in the neocortex. Given the findings, the vasodilation induced by α4β2-like nAChR activation may be beneficial for neuroprotection in the neocortex and the olfactory bulb.

Nicotinic cholinergic regulation of olfactory bulb blood flow response in aged rats

In our previous research, we had demonstrated the crucial role of neuronal nicotinic acetylcholine receptors (nAChRs) in potentiation of the olfactory bulb blood flow response to olfactory stimulation in adult rats. The present study examined the effects of nAChR activation on the olfactory bulb blood flow response in rats aged 24-27 months. We found that, under urethane anesthesia, unilateral olfactory nerve stimulation (300 μA, 20 Hz, 5 s) increased blood flow within the ipsilateral olfactory bulb, without changes in the systemic arterial pressure. The increase in blood flow was dependent upon the current and frequency of the stimulus. Intravenous administration of nicotine (30 μg/kg) had little effect on the olfactory bulb blood flow response to nerve stimulation at either 2 Hz or 20 Hz. These results suggest a reduction in nAChR-mediated potentiation of the olfactory bulb blood flow response in aged rats.

Parkinsonian phenotypes induced by Synphilin-1 expression are differentially contributed by serotonergic and dopaminergic circuits and suppressed by nicotine treatment

Synphilin-1 is a protein encoded by the human SNCAIP gene whose function has yet to be fully understood. However, it has been linked to familial Parkinson's disease (PD). Synphilin-1 is a major component of the Lewy bodies found in neurons in the substantia nigra pars compacta of PD patients. Synphilin-1 expression in serotonergic and/or dopaminergic neurons of Drosophila melanogaster induces neurodegeneration, as well as motor and non-motor PD like symptoms. In this work, we examined the contribution of the serotonergic and dopaminergic circuits in the development of PD-like phenotypes. We found that olfactory and visual symptoms are majorly contributed by the serotonergic system, and that motor symptoms and reduction in survival are mainly contributed by the dopaminergic system. Chronic nicotine treatment was able to suppress several of these symptoms. These results indicate that both the serotonergic and dopaminergic systems contribute to different aspects of PD symptomatology and that nicotine has beneficial effects on specific symptoms.

Olfactory function and discrimination ability in the elderly: a pilot study

We recently reported that subjects with a higher olfactory identification threshold for rose odor declined more in attentional ability in the elderly. This study focuses on discrimination ability and olfactory identification threshold in twelve elderly subjects living in a community (age: 80.9 ± 1.6). Olfactory function was assessed by the rose odor identification threshold. We assessed the discrimination ability by distinguishing 5 similar odor pairs. Our results showed that the subjects with a higher olfactory identification threshold (≥ 5) declined more in discrimination ability (14% ± 14%, p = 0.03) compared to those with a lower threshold (≤ 4) (averaged value set at 100%). As discrimination ability is related to the basal forebrain cholinergic system, our results suggest that olfactory impairment links to the decline in cognitive function relating the cholinergic system.

The relationship between olfaction and cognitive function in the elderly

This study investigated the relationship between olfaction and cognitive function in 12 elderly people (age: 80.9 ± 1.6) living in the community. Olfactory function was assessed by the identification threshold for rose odor. Four cognitive measures consisting general cognitive ability assessed by Mini-Mental State Examination (MMSE), its sub-domains, and attentional ability assessed by drawing a line to connect the numbers consecutively (trail-making test part A; TMT-A), were assessed. Subjects with a higher olfactory threshold (≥ 5) declined more in the performance speed of TMT-A (73% ± 7%, p = 0.05) compared with those subjects with a lower threshold (≤ 4) (averaged value was set at 100%). Other cognitive statuses assessed by MMSE tended to decline in subjects with higher thresholds. Because attentional function relates to the basal forebrain cholinergic system, our results suggest that olfactory impairment links to the decline in cognitive function, particularly of attention-relating cholinergic function.

Effects of nicotine on regional blood flow in the olfactory bulb in response to olfactory nerve stimulation

This study examined the effect of olfactory nerve stimulation on regional cerebral blood flow and assessed the effect of intravenous nicotine administration on this response in anesthetized rats. Regional cerebral blood flow was measured with laser Doppler flowmetry or laser speckle contrast imaging. Unilateral olfactory nerve stimulation for 5 s produced current (≥ 100 μA) and frequency-dependent (≥ 5 Hz) increases in blood flow in the olfactory bulb ipsilateral to the stimulus. The increased olfactory bulb blood flow peaked at 30 ± 7% using stimulus parameters of 300 μA and 20 Hz. Nerve stimulation did not change frontal cortical blood flow or mean arterial pressure. The intravenous injection of nicotine (30 μg/kg) augmented the olfactory bulb blood flow response to nerve stimulation (20 Hz, 300 μA) by approximately 1.5-fold (60-s area after the stimulation). These results indicate that olfactory nerve stimulation increases olfactory bulb blood flow, and the response is potentiated by the activation of nicotinic cholinergic transmission.