INTRODUCTION:

The World Health Organization estimated the smoking population that comes to nearly 1.1 billion. Previous studies have indicated that chronic cigarette smoking can directly damage neuronal and glial cell organelles due to cytotoxic compounds in cigarette smoke. [1,2] Additionally, the studies in the past reveal these compounds also increase the risk of atherosclerosis, which can cause decreased regional cerebral blood flow.

[3] These effects of smoking pave up to a direct or indirect impact on cognitive brain function. Pieces of the literature reveal that Brain-derived neurotrophic factor (BDNF) is involved in neuronal development, maintenance, and synaptic plasticity. [4] Chao et al., 2006 study reveals, that it has wide expression over hippocampus, septum, cortex, and in adrenergic brain stem nuclei. [5]

Previous studies have revealed that BDNF also modulates activity-dependent synaptic plasticity underlying learning and memory in the hippocampus by regulating the long-term potentiation (LTP) in both early phase (E-LTP) and late phase (L-LTP) of synaptic neurons. [6,7,8] Shreds of evidence in the past literature disclose, in animals, the brain and peripheral BDNF levels undergo similar changes during growth and developmental process, BDNF levels in serum correlate positively to cortical BDNF which indicates peripheral BDNF levels, may be reflective of BDNF levels in the brain. [9]

Various findings of previous literature reveal the cognitive function and BDNF level in smoking person, [10,11,12] although the results of some studies indicated poor cognitive performance in smokers. [13,14,15,16] Differential views reveled by a few other studies that smoking has positive effects on certain cognitive domains due to the facilitation effect of nicotine on the release of cholinergic neurotransmitters involved in cognitive processes.[17,18] An increasing array of researches suggests that chronic smoking is associated with multiple neurobiological deficits and decreased or increased BDNF levels. [19,20,21] The contrary views expressed by Jamal et al., 2014 that smoking enhanced some aspects of cognitive function and elevation of BDNF levels in smokers. [22]

Differences in the views of researchers, on the effects of smoking on cognitive performance and serum BDNF level, opens the scope for further researches to obtain more precise findings. The current study highlights the relationship between serum BDNF level and cognitive function in male chronic smokers.

OBJECTIVES:

To assess the relationship between serums BDNF level and cognitive function in middle-aged male chronic smokers.

METHODOLOGY:

The current study is hypothesized that chronic smoking affects the level of serum BDNF and cognitive function. This study was the cross-sectional comparative analytic study on 41 apparently healthy middle-aged (40–60 years) adult male chronic smokers and 41non-smokers. The subject least educational level was at least 8th standard and BMI were between 18.5-30kg/m². Chronic smokers are those who smoke at least 1 cigarette or cheroot per day for more than 5 years. Non-smokers are those who have never smoked before or smoked less than 20 cigarettes in a lifetime (STEPS instrument, WHO).

For assessing cognitive function:

General Memory Scale (GMS) originated from the Weschler Memory Scale 1956 [23] and adapted to Myanmar version by Ohn Hlaing 1980 [24]. The calculated combined score was used as the indicators of cognitive function for each subject. The tests include seven subtests General information, Orientation, Mental Control for attention and concentration, Logical Memory, Digit Span, Paired Associated Learning, and Visual Memory. It is a 20-minutes interview by the researcher.

Determination of serum BDNF level:

Serum BDNF level was measured by Enzyme-link Immunosorbent Assay by using Human BDNF Elisa Kit (Abbexa BDNF ELISA kit, Catalog No: abx570037) and the result was read by microtitration plate ELISA reader with absorbance measurement of 450nm (Stat Fax 4200, Awareness Technology, Inc., USA)

Study Procedure:

Apparently, healthy middle-aged (40-60) years male chronic smokers and non-smokers were recruited at Quarter Assembly Hall by a non-probability sampling method. The procedure was explained thoroughly to the participants. The informed consent was taken in writing from the study participants. Personal data, smoking history, and physical examination were performed according to the Pro forma.

Selected subjects were instructed to fast overnight from 10pm to 8am. On an experimental day they were requested to come to the Common Research Laboratory of the University of Medicine-2, Yangon at 7am. The venous blood samples 5 milliliters collected from a peripheral vein under aseptic condition, further serum obtained by centrifuging and stored at -20 BDNF level was analyzed.

Then cognitive function test was conducted to each subject by using a general memory scale (GMS) form. The researcher asked the questions of GMS form to the subjects individually at the room with minimal disturbances prepared for the cognitive function test.

The subjects were not allowed to meet each other so as not to discuss questions and answers. Scoring of GMS was done by the researcher by herself according to the instructions given so that the uniformity was assured.

Data analysis:

Data were analyzed using the statistical package for Social Sciences (SPSS) software version 16. For the descriptive purposes, the continuous data (such as age, serum BDNF level) were expressed by the mean and standard deviation. For statistical analysis, the ‘t’ test (unpaired) was applied to calculate the difference between the means of the parameters (serum BDNF level and GMS scores). The statistical significance of results was set at p < 0.05. Pearson’s correlation coefficient was calculated to assess the relationship between serum BDNF level and cognitive function of the study group.

RESULTS AND DISCUSSION:

Table (1) showed that general characteristic of 41 chronic smokers and 41 non-smokers. There was no significant difference between age and BMI.

Table 1. General characteristics of chronic smokers and non-smokers

Parameters	Chronic smokers (mean ± SD) (n= 41)	Non- smokers (mean ± SD) (n= 41)	P value
Age (year)	50.44 ± 5.97	49.46 ± 6.36	0.476
Weight (kg)	59.69 ± 10.17	62.06 ± 9.08	0.268
Height (m)	1.65 ± 0.07	1.65 ± 0.06	0.926
BMI (kg/m²)	22.40±2.51	23.17±2.09	0.137

Cognitive Function scores (GMS scores) in chronic smokers and non-smokers groups:

Total GMS scores of chronic smokers (n=41) was 56.88 ±12.24 and that of non-smokers (n=41) was 67.74±7.59. It was found that chronic smokers had significantly lowered total GMS scores than non-smokers (p<0.001) (Figure 1.1).

Figure 1. Cognitive function scores (GMS scores) (mean ±SD) in chronic smokers group and non-smokers group

* indicates statistically significance p<0.05 between two groups.

** indicates statistically significance p<0.001 between two groups.

Comparison was done by unpaired t Test.

Moreover, among seven subtest of General Memory Scale (GMS), the score of chronic smokers were significantly lower than of non-smokers in five subtests i.e. information (3.00±0.84 vs 3.37±0.58) (p=0.024), mental control (3.98±1.15 vs 4.71±0.96) (p=0.002), logical memory (15.49±5.27 vs 21.39±5.20) (p<0.001), digit span (9.59±1.53 vs 10.61±1.12) (p=0.001) and paired associated learning (14.46±6.32 vs 17.06±2.51) (p=0.017). There was no significant difference between the score of chronic smokers and non-smokers in the rest two tests i.e. orientation (4.80± 0.51 vs 4.90±0.37) and visual memory (5.56±0.87 vs 5.71±0.72) (p>0.05) (Figure 1.2).

Figure 1.2.

Solid lines indicate mean ± SD.

NS means no significant difference (p> 0.05) between two groups.

Comparison was done by unpaired t test.

Serum BDNF level in chronic smokers group and non- smokers group

There was no significant difference in serum BDNF level of chronic smokers (n=41) 19.19±6.05ng/ml and that of non-smokers (n=41) 20.83±8.63ng/ml (p>0.05) (Figure 2).

Figure 2. Serum BDNF level in chronic smokers’ group and non-smokers’ group

Solid lines indicate mean ± SD.

NS means no significant difference (p> 0.05) between two groups. Comparison was done by unpaired t test.

Correlation between serum BDNF level and Cognitive function scores (GMS scores) in chronic smokers’ group

In chronic smokers’ group (n=41), serum BDNF level was positively correlated with the total GMS scores (r=0.453, p =0.003). (Figure 1.3)

Figure 3. Cognitive function in chronic smokers’ group and non-smokers group:

Cognitive function and BDNF level greatly vary in different ages, sex, educational standard, or BMI. [25] In this study, apparently healthy male subjects with matching age, education, and BMI, were selected where most participants have finished high school basic education. In the chronic smokers' group, 17 out of 41 (41.47%) were high school leavers, and 3 out of 41 (7.32%) were graduated. In the non-smokers' group, 15 out of 41 (36.59%) were high school leavers, and 4 out of 41 (9.76%) were graduated. In the chronic smokers' group (n=41), the youngest age of first use of cigarettes was 15 years and the duration of smoking was ranged from 10 to 42 years. The mean amount of cigarettes per day was 7.37±5.29 and pack-year was 11.30±9.52. Accordingly, the smokers’ group in this study is meant to be active in chronic smokers.

It was found that chronic smokers’ group had significantly lowered total GMS scores than non- smokers’ group (p<0.001) in the present study. Among the seven subtests of GMS, the total GMS score in five subtests: information (p=0.024), mental control (p=0.002), logical memory (p<0.001), digit span (p=0.001) and paired associated learning (p=0.017) were significantly lower in chronic smokers group than that of non-smokers group. Orientation scores and visual memory scores of GMS were used to assess attention and short-term memory.

In the present study, smokers were active chronic smokers and they might not have a considerable decrease in attention and short-term memory probably due to nicotinic enhancement of dopamine release. Amitai et al (2009) explored in their study that nicotine stimulation of dopamine in the striatum or the thalamus or other brain regions associated with attention or arousal can improve attention and short-term memory. [26] A growing body of research evidence suggested that chronic cigarette smoking adversely affects neurocognition such as immediate memory, attention, language, and general cognitive performance [21,27]

Researches in the past had revealed the causal relationship between chronic smoking, an increased risk of cognitive decline, and dementia [28, 29, 30, 31, 32]. The finding of lower cognitive performance in chronic smokers in the present study is concordant with these above studies. Among the various mechanisms, direct and indirect mechanisms are proposed to explain the relationship between chronic smoking and cognitive decline. [33] One of the literature studies show evidence that some of the cognitive, intellectual, and behavioral impairments in the non-demented elderly. [34] Chronic smoking increases the risk for atherosclerosis, as well as abnormalities in vascular endothelial morphology and function, which may alter cerebral perfusion. [28, 3]. This may impact the functional integrity (e.g., vasomotor reactivity/responsively) of the cerebrovasculature and may contribute to decreased regional cerebral blood flow [34]. Furthermore, the result of the present study found relationship between smoking severity and cognitive function such as positive correlation between age of first use and total GMS scores (r =0.668 and p < 0.001), negative correlation between duration of cigarettes smoking and total GMS scores (r = -0.505 and p = 0.001) and negative correlation between pack year and total GMS scores (r = -0.412 and p = 0.007). These findings indicated that the earlier the smoking started, the more cognitive decline occurred and that the longer the duration of cigarettes smoking and pack year, the more cognitive decline occurred. These finding were similar with some other literature studies. [21, 28, 36] In the study of Zhang et al, the age of smoking initiation was positively associated with immediate memory and attention in the smoking group. [21]

Serum Brain-Derived Neurotrophic Factor (BDNF) Level in Chronic Smokers Group and Non-smokers Group:

Although the cognitive function of chronic smokers (n=41) was significantly lower than that of non-smokers (n=41), serum brain-derived neurotrophic factor (BDNF) level of chronic smokers (19.19±6.05ng/ml) was not significantly lower than that of non-smokers (20.83± 8.63ng/ml) in the present study (p>0.05). It has stated, in one of the previous literature healthy subjects, serum BDNF levels ranged from 12.2±2.4ng/ml to 64.1±13.1 ng/ml. [36,37] Hence serum BDNF level of both chronic smokers and non-smokers in the present study was within the normal range.

Serum BDNF concentrations showed a considerable variability between different studies in smokers. In the previous study by El Zalabany et al., (2010), [38] it was found that there was no significant difference in serum BDNF between 33 non-smokers and 55 smokers. [39] Zhang et al (2014), studied the BDNF level and cognitive function in 191 healthy male subjects aged 22-

70 years, (47 isolated smokers, 31 isolated chronic alcohol users, 58 combined smokers, and chronic alcohol users and 55 non-smokers, and non-alcohol users), and also reported that no significant difference in serum BDNF level between smokers and non-smokers although the smoking group had significant low total scores of RBANS. [Repeatable Battery for the assessment of neuropsychological status.[21]

If there was impairment or damage in neuronal cells of brain resulted from consequences of smoking, BDNF level might be decreased [40] BDNF level of chronic smokers was not significantly lowered than that of non- smokers in this study probably due to that smokers were healthy normal adult males and might not have considerable changes in function of neurons in central nervous system. The limitation of the present study was nicotine dependence test and carbon monoxide test which check the severity of smoking could not done.

The relationship between serum BDNF Level and Cognitive Function:

In the present study, serum BDNF level has a positive correlation with cognitive function (total GMS scores) in both chronic smokers’ group and non-smokers group. BDNF is one of the important factors, which are essential for cognitive performance, and a number of studies were performed to investigate the relationship between BDNF level and cognitive performance. Previous studies have shown that BDNF can modulate activity-dependent synaptic plasticity underlying learning and memory in the hippocampus. [7,8] One other previous literature reveals that BDNF levels in serum correlate positively to cortical BDNF, [9] which indicates that peripheral BDNF levels may be reflective of BDNF levels in the brain.

Taken together upon the findings of the present study, chronic smoking can compromise cognitive function even in middle-aged adults, and the smokers who started smoking in their early part of life, the more severity in cognitive decline. Despite the cognitive function lowered in chronic smokers in this study, serum BDNF level was comparable with the non-smokers. Thus, the lowered cognitive function of the smokers might not be due to lowered BDNF levels in the brain. Moreover, the BDNF level is not the only influencing factor for cognitive function, and there would be other adverse effects of smoking on the cognitive function of the human brain.

Chronic smoking also can increase the risk of impaired lung function [41] or cerebral perfusion [3] which impact the functional integrity of the cerebro-vasculature and may contribute to poor neurocognition or increased subcortical atrophy. Therefore, cognitive decline in the smoking group might be due to both direct and indirect effects of smoking on cognitive function including, impaired cerebrovascular or pulmonary function in the present study.

CONCLUSION:

Although the cognitive function of chronic smokers was significantly lower than that of non-smokers, serum BDNF levels of both groups were not significantly different. Nevertheless, there was a significantly positive correlation between serum BDNF level and cognitive function in both chronic smokers’ group and non- smokers group indicating the role of BDNF in cognitive function. It can be concluded that chronic smoking can cause cognitive decline even in middle-aged adults, the earlier the initiation of smoking in life, the more severity in cognitive decline occurred. Ultimately, with or without chronic smoking, serum BDNF level was

positively correlated with cognitive function in the present study. Further researches in this area are recommended by the authors to gain more clarity. The resultant analysis positively corroborated with the objective described in the study. The serum BDNF levels of both smokers and nonsmoker groups were not significantly different, thus the study results pave the way to come up with other related researches, with regard to smoking and its impact on the neurobiology of humans, the more awareness in this field has its significance in curtailing this social menace at large.

CONFICT OF INTEREST:

There is no conflict of interest among authors.

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Received on 29.07.2020 Modified on 18.08.2020

Research J. Pharm. and Tech. 2020; 13(10):4925-4930.

DOI: 10.5958/0974-360X.2020.00865.3