The literature search resulted in 43,354 manuscripts. Figure 2 highlights the continuous growth of research in the area that has occurred since the 1990s. The small number of publications before the 1990s is likely due to absent indexing of older manuscripts (prior to 1990s) by the online literature database. The majority of the manuscripts were original articles (n = 31,360) and reviews (n = 11,266). The remaining manuscripts included proceedings papers (n = 2,124), editorial materials (n = 385), etc. Most of the manuscripts were written in English (n = 41,527, 95.8%). The top five WoS categories associated with the analyzed manuscripts were Pharmacology/Pharmacy (n = 14,892, 34.3%), Neurosciences (n = 11,747, 27.1%), Clinical Neurology (n = 4,981, 11.5%), Psychiatry (n = 4,464, 10.3%), and Biochemistry/Molecular Biology (n = 4,337, 10.0%). The manuscripts were contributed by over 100,000 authors from 15,890 organizations in 161 countries/regions and published in 4,374 journals.

Seven of the top ten most prolific institutions are located in the USA, and one each in Canada, Italy and the UK, respectively (Table 1). These institutions had 37.7–66.6 citations per manuscript. Meanwhile, the USA has accounted for 38% of the total number of publications (Table 2) and the top 10 most prolific countries/regions have 14.1–44.3 citations per manuscript. NC data has shown that manuscripts contributed by the top 10 most prolific institutions generally have had higher than average citations. However, this was not applicable to the top 10 countries/regions. The large volume of contributions from the USA, Germany, England, and China is consistent with previous analyses pointing their contributions to neuroimaging (Yeung et al., 2017b; Yeung, 2018), ethnopharmacology (Yeung et al., 2018a), and nutraceutical studies (Yeung et al., 2018b). Most of the top ten most prolific journals were those dealing either with neuroscience or pharmacology (Table 3). The multidisciplinary journal PLOS One had fewer citations per manuscript (16.3)—at most one-half of the other nine journals (32.7–77.3). One potential confound for this finding was that, for example, the peer-reviewed journal PLOS One was just recently founded (in 2006), and therefore papers published in it might have had less time to accumulate more citations. In this line, when NC was considered, manuscripts published in PLOS One were only slightly less cited than the average manuscripts published from the same year of publication (0.8, i.e., 20% less than the average). Pearson's correlation test revealed that there was significant correlation between total publication count and averaged citations per manuscript in the institution level (r = 0.39, p < 0.001), country level (r = 0.31, p = 0.043), but not the journal level (r = 0.15, p = 0.209).

By analyzing the words in the titles and abstracts of the 43,354 manuscripts, a bubble map was generated to visualize the citation data (Figure 3). There were 2,276 terms that appeared in at least 100 of the included manuscripts. After excluding words from the 5,000 common word list (from the Corpus of Contemporary American English; https://www.wordfrequency.info/free.asp?s=y), 1,659 terms remained (provided as a Supplementary Data Sheet, are all the 1,659 terms and their citations per manuscript). Differential scientific attention has been given to several pathological conditions. For instance, schizophrenia (2,093 manuscripts; 48.1 citations per manuscript; NC = 1.3) had higher citations per manuscript than PD (1,854 manuscripts; 37.0 citations per manuscript; NC = 1.2), autism (159 manuscripts; 34.4 citations per manuscript; NC = 1.6), and AD (2,506 manuscripts; 34.2 citations per manuscript; NC = 1.2), which in turn had higher citations than neuropathic pain (1,349 manuscripts; 30.0 citations per manuscript; NC = 1.0) and seizure (1,915 manuscripts; 29.2 citations per manuscript; NC = 0.9). Interestingly, AD and autism have been identified as “hot topics” in the general neuroscience research field with continuously increasing citations rate during the recent years (2006–2015) (Yeung et al., 2017c). However, the current results confirm that other neuropsychiatric disorders (e.g., schizophrenia and PD) have also attracted big attention in the specific field of neuropharmacology. Meanwhile, manuscripts dealing with activation or inhibition pathways of receptors in neurons had both higher publication count and citations per manuscript than those dealing with adverse effect or adverse event of neuropharmacological drugs. This might be due to the universal and far-reaching relevance of molecular regulatory mechanisms characterization (receptor modulation, inhibition, or activation of pathways) as compared adverse effects/events reports that might be resulting in diminishing in the interest in the therapies that exhibited it.

Manuscripts mentioning certain chemicals or pharmaceuticals displayed high citations per manuscript, such as those reporting about anandamide, THC, L-glutamate, clozapine, and curcumin (Figure 1 and Table 4). The chemicals or pharmaceuticals with at least 50 citations per manuscript were mostly related to cannabis and anti-psychotic drugs, as well as some dealing with anti-epilepsy or AD (Table 4). Cannabis and cannabinoids have long been known to exert multiple effects on the CNS, cardiorespiratory system, eye, immune system, and reproductive system (Pertwee, 2000; Kumar et al., 2001). Perhaps the most important effect associated with cannabis and cannabinoids is their use as analgesics (Miller and Miller, 2017). Cannabis pharmacology has firstly attracted the attention in the 1960s. Nowadays, researchers have been also exploring its use in the treatment of psychiatric disorders and cancer (Russo and Marcu, 2017). Besides, two natural products, curcumin and galantamine, were on the list of the most mentioned molecules. It was also reported that curcumin and its derivatives have anti-inflammatory properties that may have a potential role in treating neurodegenerative diseases such as AD and PD (Lee et al., 2013). Meanwhile, galantamine is used to treat AD as it has the effects of blocking acetylcholinesterase and modulating nicotinic acetylcholine receptors (Arias et al., 2004). It is worth noting that galantamine is the only current drug of plant origin against dementia (Tewari et al., 2018). Continuous pre-clinical research efforts are also focused on neuropharmacology effects mediated from a range of other natural products derived from medicinal plants or dietary sources (Baur et al., 2014; Ajami et al., 2017; Khan et al., 2017; Nabavi et al., 2018; Valenti et al., 2018). The presence of natural products as a key theme in a large volume of neuropharmacology manuscripts should come as no surprise, taking into consideration the importance of natural product pharmacology, both historically as well as in modern drug discovery (Wang et al., 2014; Atanasov et al., 2015; Waltenberger et al., 2016; Uhrin et al., 2018).

While this bibliometric analysis outlines the prevailing publication and citation trends of the neuropharmacology field, it should be noted that the choice of the keywords had an influence on the body of literature that was identified and analyzed, and therefore, all relevant neuropharmacology manuscripts could not be covered. For example, while our analysis includes manuscripts mentioning “neuro^*” AND “pharma^*” AND “compound^*” manuscripts mentioning “neuro^*” in combination with a specific drug name (e.g., “neuro^*” AND “clozapine”) might not necessarily be included (depending on the entire sets of terms used in the respective manuscripts). Another important limitation for readers to consider as they interpret the presented data of citation per manuscript is that a few very highly cited articles might have skewed the results, and geometric or truncated means should perform better to account for that. However, using geometric or truncated means with VOSviewer is not possible. With consideration of this, in the main text and tables we have only listed the terms that have appeared in at least 100 manuscripts. Besides, we report NC that account for citation performance in comparison to the average number of citations of all neuropharmacology publications (within the data set) from the same period (the same year of publication).

In conclusion, a bibliometric analysis was performed to evaluate manuscripts focused on neuropharmacology. The obtained data indicated that the USA is the major contributor to this research field and seven of the top 10 most prolific institutions are located in the USA. The most prolific journals were mainly specialized in neuropharmacology. Manuscripts involving cannabis, cannabinoid, antipsychotic drugs, anti-epileptic drugs, and curcumin had more than 50 citations per manuscript. Manuscripts dealing with schizophrenia and AD also displayed a substantial number of citations per manuscript. With virtually all of the articles published since the 1990s, the research area of neuropharmacology has been growing steadily. With the ever-growing neuroscience research, it is hoped that new pharmaceutical molecules or new therapeutic applications of discovered molecules will continue to be introduced and will continuously contribute for the future improvement of patient's health and well-being.