Pain is a debilitating symptom of many diseases and causes significant disability and reduces the quality of life. We still do not have any effective treatment options and it remains major unmet global public health challenge. The difficulty in understanding and managing pain has also led to widespread overuse of, and addiction to opoids/morphine medications. This can lead to serious adverse outcomes [1, 2]. Pain can be classified into three major groups: Nonciceptive, Neuropathic and Nociplastic. Nociceptive pain results from activity in neural pathways and is the most common form of chronic pain encompassing arthritis and most forms of spinal pain. Neuropathic pain is caused by damage or disease affecting the somatosensory nervous system, typically associated with sensory abnormalities such as numbness and alloydynia. Approximately 15-25% of chronic pains are neuropathic in nature and include diabetic neuropathy, postherpetic neuralgia and radiculopathy. Nociplastic pain is the pain arising from the abnormal processing of the pain signals without any clear association with damage, injury or disease pathology [3, 4]. Pain may be either acute or chronic [5]. Acute pain is an essential sensory signal that protects us from harmful environmental stimuli, for instance, cold, heat, tissue damage or chemical irritants. Without acute pain, it would be hard to protect against harmful behavior [6]. On the contrary, chronic pain is “pain which has persisted beyond normal tissue healing time” [7]. Chronic pain is one of the common problems associated with anxiety, depression, opioids dependence, and restrictions in mobility and daily activities [8]. Ruan et al., gave evidence that the overall burden of disease of chronic pain along with its associated comorbidities is unparalleled at the global level [9]. Recently, Mills and the team reported that ~1.9 billion people are affected via recurrent “tension-type” headaches globally, which are the most common symptom of chronic pain [10]. Another systematic review revealed that chronic pain distresses affects ~50% of the total UK population [11]. Across Asia, the incidence of chronic pain in adults ranges from 7% (Malaysia) to 60% (North Iraq and Cambodia) [12]. Irrespective of the high incident rate, chronic pain remains ineffectively treated globally. If not treated early, chronic pain may affect individual emotional as well as physical conditions, thereby declining their work productivity as well as their quality of life. This, in turn, imposes a more significant threat to the socio-economic situation of the patients and their associated people [12].

Most of the studies performed to date suggest that the origin of every pain syndrome is inflammation and inflammatory responses [13]. Modification in gene expression, the voltage dependence of voltage-gated Na⁺, Ca²⁺ and K⁺ channels and ligand-gated ion channels, for instance, purinergic receptors and TR (transient receptor potential) channels, is reported to contribute to the enhanced sensory transduction witnessed during inflammation [14]. A group of these channels that are permeable to calcium ions mediates several signaling functions, for instance, neurotransmitters release, calcium-dependent enzyme activation and calcium-dependent gene expression modification. Hence, understanding calcium signaling is an essential step towards recognizing neural network activity associated with pain management [6]. Thus, in the present review, we attempted to understand the impact of calcium-permeable ion channels in the recognition, processing, transduction and modulation of pain signals. In the near future, this information will be helpful in pain management.

To understand how calcium channels are associated with different pain states, it is highly essential to understand the overall facets of the transmission as well as the regulation of pain signals throughout the nervous system. In recent years, several studies have been performed to unravel the mechanism via which different stimuli activate the pain sensors (nociceptors). One of the most critical findings is the identification of “Transient Receptor Potential” (TRP) channels. This ion channel family is highly conserved throughout evolution [15]. It is present in the cell membranes of a broad-spectrum of species, including invertebrate, vertebrate and fungi and is associated with several functions. Few members of this channel family that are situated within the mammal’s nociceptive nerve endings are initiated via thermal stimuli [16]. For instance, heat above 43°C stimulates the TRP vanilloid 1 (TRPV1) channel, which in turn causes an inflow of positive (Na⁺ and Ca²⁺) ions; thereby causing cell membrane depolarization. On the contrary, the Transient receptor potential Melastatin 8 channel (TRPM8) receptor retorts to both menthol as well as cold temperatures. This might be the main reason for feeling cold sensation after menthol ingestion. Additionally, numerous receptors are situated at the nerve ending and are activated via various biochemical compounds, e.g., bradykinin and prostaglandins, in response to tissue damage as well as inflammation. Few of these biochemical compounds can even directly modify neural excitability via stimulating ions channels present on the cell surfaces, e.g., ATP and serotonin. Other few compounds, for instance, nerve growth factor (NGF), modulate their effect via second messengers [17].

Transduction, the release of transmitter and transmission of the action potential are the three major physiological processes required for the peripheral transmission of sensory stimuli. Specialized ion channels and receptors (such as transient receptor potential vanilloid 1 (TRPV 1)) are present in the peripheral ends of nociceptive neurons and are the potential in transducing painful stimuli into an electrical signal. The amplification of these impulses is facilitated by voltage-gated sodium channels (Na_vS) till the threshold for action potential generation is met. The transmission of action potentials to the central terminals of the spinal cord is aided by voltage gated-potassium channels (VGKCs) and Na_vs [18]. Nav1.1, Nav1.6, Nav1.7, Nav1.8, and Nav1.9 are the most prominent Na_vs found in sensory neurons [19]. Nav1.1 and Nav1.6 are primarily expressed by myelinated A-fiber neurons, whereas Nav1.7, Nav1.8, and Nav1.9 are primarily expressed by unmyelinated C fibers.

TRPM8 is a calcium-permeable, nonselective cation channel. The TRPM8 channel is significant among the thermal-sensing TRP channels for sensing cold temperatures (8–26°C) and adding to the cooling sensation through compounds like menthol and icilin. TRPM8 is expressed on a δ- and C- sensory nerve fibers, as well as dorsal root ganglion (DRG) and trigeminal ganglia (TG) neurons [20]. TRPM8 activation on macrophages has been shown to cause an anti-inflammatory response, with enhanced interleukin 10 (IL-10) and reduced tumor necrosis factor release. TRPM8 activation on pulmonary epithelial cells, on the other hand, promotes the expression of pro-inflammatory cytokines such as interleukin 1 (IL-1) and tumor necrosing factor (TNF) [21].

Out of all biochemical compounds, glutamate is the key transmitter released via the primary afferent neuron terminals. Two forms of glutamate receptors are required for modulating these signals. They function as cation channels within the spinal cord dorsal horn and activate second-order neurons. Α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors are involved primarily in the conduction of day-to-day physiologic pain. N-Methyl-D-aspartic acid or N-Methyl-D-aspartate (NMDA) receptors are distinctive in that they act as both voltages gated and ligand gated. These receptors play a special role in the development of chronic pain and -wind-up states [22].

For initiating short term pain, glutamate initiates AMPA-receptors on neurons present in the spinal trigeminal complex and dorsal horn. In prolonged pain, glutamate initiates the NMDA-receptors that are mainly associated with sensitization. A few other critical biochemical compounds, namely, substance P, ATP and calcitonin gene-related peptide, are also released from the primary afferent terminals. These compounds are mainly associated with sensitization as well as pain prolongation [15]. However, it is still unclear if the receptors responsible for modulating the function of both peripheral as well as central nerve terminals of the primary afferents are the same. For instance, bradykinin-, ATP- prostaglandin-receptors and calcium & TRPA1 channels are responsible for enhancing the release of the transmitter (e.g. Glutamate) and, hence, increasing the pain intensity. Additionally, presynaptic opioids, glycine, cannabinoid and γ-amino butyric acid (GABA) receptors are associated with reduced transmitter release from the central nociceptive nerve terminals, thereby causing less pain [15].

Another significant pathway that is associated with the pain matrix is the “descending pain modulatory system” with noradrenaline- and serotonin-comprising projections from the locus coeruleus and raphe nuclei, respectively. During normal scenarios, initiation of these pathways impedes pain signaling either by inhibitory interneurons present in the spinal trigeminal nucleus and dorsal horn or via direct effects. However, in chronic pain, the balance within the descending pathways as well as the pain matrix gets misbalanced, plausibly via pain itself, or through numerous precipitating factors. Under these circumstances there might also be an alteration of receptor subtypes; thereby making opioids and serotonin for functioning as proalgesic substances [23].

Calcium, being one of the most ubiquitous secondary messengers, play a significant role in modulating numerous biological process, including cell proliferation, muscle contraction gene transcription [24]. Calcium ions (Ca²⁺) play a vital role in cellular organelle function and homeostasis and act as secondary messengers in a variety of cellular activities [25]. Broadly, Ca²⁺ channels can be categorized into “high-voltage-activated “and “low-voltage-activated” channels. Further, “high-voltage-activated” channels can be subcategorized as L, P/Q, N, R-types. “Low-voltage-activated” channels are also known as T (“transient”)-type [26].

High voltage-activated calcium channels cause extreme primary afferents excitation, which in turn causes neurogenic inflammation and releases neuropeptides, e.g., substance P, from their peripheral endings. This, in turn, stimulates events associated with inflammation, including vasodilatation as well as enhanced vascular permeability (Fig. 1) [60]. Though almost all subtypes of calcium channels are highly expressed in the CNS [37], the “N-type calcium channels” play a key role in the release of the neurotransmitter from the afferent terminals within the spinal dorsal. Hence, they serve as a key therapeutic target during the treatment of analgesics [37]. Few other studies have also reported that the Ca_v2.2 expression in microglia is associated with the pathophysiology of neuropathic pain subsequent to spinal nerve injury [61]. As the “N-type calcium channels” are mainly expressed at nociceptive primary afferents synapse, an increased nociceptive function is observed subsequent to inflammation as well as neuropathy [62] and genetic deletion at this site reduces inflammatory pain behaviors [63]. Additionally, “N-type calcium channels” blockers diminish behavioral hyperalgesia and dorsal horn neurons’ hyperexcitability during inflammatory & neuropathic pain [64]. Few splice variants, especially e37a and e37b, of the “N-type calcium channel”, are reported to have different current densities and G-protein sensitivities and distinct electrophysiological properties. These variants are associated with the development of mechanical and thermal hyperalgesia at the time of inflammatory and neuropathic pain [65].

Migraine is also reported to involve neurogenic inflammation [66]. “P/Q-type calcium channels” is reported to play a key role in migraine. For instance, gain-of-function mutations in CACNA1A are a risk factor for familial hemiplegic migraine type 1 [36]. It enhances open channel probability and channel activation even at a lesser voltage [67]. Though the relation between the “P/Q-type calcium channels” and migraine is well established, their involvement in pain signaling is a topic of debate. The Nagoya mutant mouse with a loss-of-function mutation in “P/Q-type calcium channels” experiences less inflammatory pain phenotype [67]. Although complete “P/Q-type calcium channels” deletion causes hyposensitivity to neuropathic pain, they paradoxically enhance acute thermal nociception [68]. Thus, though “P/Q calcium channels” might be associated with pain signaling, their role is very limited in comparison to “N-type” as well asset-type calcium channels” [68].

All types of “L-type calcium channels” are present in the lymphocytes. Ca_v1.2 channels are highly expressed in the Type-2 helper T cells, where they play a vital role in modulating calcium signaling as well as cytokine production that is associated with allergic asthma [69]. Earlier studies have also reported that enhanced expression of Ca_v1.2 in murine microglial BV2 cells increases intracellular calcium concentration and TNFα and nitric oxide (NO) production, thereby supporting the association between Ca_v1.2 and CNS inflammation [70]. Few studies have reported that reduced Ca_v1.2 and Ca_v1.3 expression in dorsal root ganglion is associated with hyperexcitability during neuropathic pain [71]. One study reported that the expression of miR-103 gets reduced during neuropathic pain, which in turn enhances Ca_v1.2, thereby causing hyperexcitability as well as allodynia [72]. Ca_v1.2 post-translational modification in smooth muscle cells is thought to have a role in aberrant gastrointestinal smooth muscle motility after colonic inflammation and may contribute to intestinal dysmotility in inflammatory bowel disease patients (IBD) [73].

Few other studies have reported that-type calcium channels”, namely Ca_v3.2, is highly expressed in the primary sensory neurons and is responsible for modulating the excitation of neuron nearby peripheral axons, as well as the release of spontaneous neurotransmitter towards the central endings, thereby supporting their involvement in nociceptive processing, specifically in neuropathic pain [74]. The expression and function of Ca_v3.2 are reported to be modulated via various inflammatory mediators, for instance, bradykinin, hydrogen sulfide (H₂S) as well as through the neuronal activity itself [47]. Ca_v3.2's enhanced activity is involved in colonic, bladder, and pancreatic pain, as well as regulating visceral inflammation. Inflammation and inflammatory pain are caused by VGCCs, and Ca_v3.2 T-type VGCC is a promising therapeutic target for the treatment of visceral inflammatory pain in patients with irritable bowel syndrome, interstitial cystitis/bladder pain syndrome, pancreatitis, and other conditions, as well as neuropathic pain [75].

Fig. (1). Regulation of calcium channels in neuroinflammation .

Inhibition of calcium channels through various analgesics, for instance, pregabalin & gabapentin, are considered as the first-line treatment against neuropathic pain [76]. Protein kinase C (PKC) translocation caused by the pronociceptive peptides bradykinin and prokineticin 2, which are involved in both inflammatory and chronic pain, was dramatically reduced by gabapentin. After nerve injury, these drugs distinctly reverse the neural sensitization without distrusting the normal physiological pain [77]. Another drug, namely gabapentinoids, is reported to exercise their analgesic effect via blocking α2δ1 trafficking towards synaptic terminals within the spinal cord of neuropathic rats [78]. Nilvadipine, a calcium channel blocker, is reported to inhibit ocular inflammation in endotoxin-induced uveitis [79]. Amlodipine, lacidipine and verapamil also have an anti-inflammatory effect on calcium channels [80].

Agents that control the activity of high voltage-activated Calcium channels (HVCCs) relieve visceral pain in both experimental and clinical models. The antagonists of HVCCs and Transisent receptor potential cation channels receptor (TRPA1), Phα1β and CTK 01512-2 toxins showed an excellent response profile in the modulation of nociception and inflammatory process in the acute pancreatitis model in rats, without spontaneous movement of the animals [81]. Pimozide, a dopamine D2 receptor antagonist known as a conventional antipsychotic, and bepridil, a multi-channel blocker used to treat arrhythmia and angina, both show significant T-channel blocking activity [82]. In male rat pain models, inhibiting T-type channels lowers the excitability of peripheral nociceptive sensory neurons and reverses pain hypersensitivity. Spinal excitability and pain hypersensitivity are reduced by Z944, a T-type calcium channel antagonist [83]. Cav3.3 function inhibition or attenuation could be a useful therapy for trigeminal neuropathic pain. The features of calcium channel blockers as a vasodilator and inhibitors of muscular contractions, such as pernio, anal fissures, face wrinkles, and painful leiomyoma, are the main reasons for their usage in dermatology [84].

Irrespective of side effects, opioids serve as a key drug for the treatment of any pain [85]. After binding with opioids, the G protein-coupled µ-opioid receptor gets activated, which in turn lessens neuronal excitability as well as the release of a transmitter by inhibiting the function of “N-type calcium channels” and/or stimulating numerous potassium channels [86]. The “N-type calcium channels” blockers, namely, ziconotide and N-triazole oxindole, are also employed for the treatment of pain. Another drug, namely, TROX-1, is reported to reverse the allodynia induced via inflammatory-induced hyperalgesia nerve injury [87]. One study reported that collapsing response mediator protein-2 (CRMP-2) modulates the function of “N-type calcium channels”. CRMP-2 overexpression enhances calcium currents and, hence, “N-type calcium channels” expression. On the contrary, CRMP-2 knockdown shows contradictory effects [88].

Disruption of the interaction between “N-type calcium channels” and CRMP-2 decreases neurotransmitter release, calcium current, as well as inflammatory & neuropathic hypersensitivity [88]. Another “N-type calcium channels” blocker, namely NP078585 (a derivative of lomerizine and flunarizine), is reported to reduce inflammatory and neuropathic pain in a rat model [89]. As drugs produced from natural resources have less or no side effects, several studies have also proposed the usage of natural resources as drugs in various diseases, including pain [87, 88]. For instance, in 2013, Adams and the team proposed that ω-conotoxins, small disulfide-rich peptides isolated from the venom of the marine cone snail, function as “N-type calcium channels” inhibitors and block nociception during neuropathic and inflammatory pain models [90] (Table 1).

Another dihydropyridine derivative, namely azelnidipine, is annul-type calcium channels” blocker as well as antihypertensive. It causes minimum sympathetic nervous system stimulation and has a protective role in inflammation during atherosclerosis [90]. Nifedipine, an“L-type Calcium channels” blocker, reduces the expression of inflammatory cytokines, for instance, TNF-α and MIP-2 [88]. In another study, authors reported that Diltiazem, an “L-type calcium channels” blocker inhibits the formation of aneurysm through blood pressure-independent anti-inflammatory effects [89]. “T-type calcium channels “inhibition via ethosuximide, mibefridl, KST5468, TTA-P2 TTA-A2 have the antinociceptive ability within the inflammatory models, which in turn decreases paw edema as well as immune cell number. TRPA1 channel blocker, namely HC-030031, also work effectively during chronic inflammatory or neurogenic pain [90].

However, as nothing is perfect in this world, earlier several studies have reported that calcium channel could have an adverse effect on human health. For instance, diltiazem and verapamil may have a negative impact on cardiac conduction. Verapamil may cause constipation and nausea. A high dose of verapamil, diltiazem and nifedipine inhibits insulin secretion. Calcium channel blockers may also be responsible for causing hypertension and cancer. However, none of the calcium channel blockers have been reported to affect protein or lipid metabolism, severely [90-94]. Thus, calcium channel blockers may be consumed under the supervision of medical practitioners. Information obtained in the present study will be highly useful for understanding and treatment of pain caused by inflammation due to calcium ion channels.

In conclusion, the origin of every pain syndrome is inflammation and inflammatory responses. Various voltage-gated ion channels that are permeable to calcium ions mediate a number of signaling functions, including, neurotransmitter release, calcium-dependent enzyme activation and calcium-dependent gene expression modification that is associated with the pain behavior. Hence, understanding calcium signaling is an important step towards understanding the neural network activity associated with pain management. Results obtained revealed that these channels play a significant role during inflammatory and neuropathic pain. Thus, inhibition of these channels through various analgesics serves as a therapeutic treatment against inflammatory and neuropathic pain.

However, few of these inhibitors have various side effects, including cancer. Hence, these inhibitors may be consumed under the supervision of medical practitioners. Additionally, since the last few decades, technology advancement has been made to recognize drugs from phytochemicals against various diseases, including inflammation. As there are only a few calcium channel blockers for treating inflammatory and neuropathic pain, there is still scope for identifying them from various sources, including natural resources, during the drug discovery process. Authors believe that, as the computational high throughput screening process requires less time and capital, these computational approaches when combined with wet-lab approaches may hasten the process of drug discovery. Many avenues can be used to raise awareness of ongoing advancements. Reviewing clinical studies, looking into newly sponsored initiatives, and connecting with peers in the sector are just a few examples. Because pain continues to have such a negative influence on individuals, their environment, and the healthcare system, the present surge in pain research offers an exciting chance for really transformative advancements in pain management in the future.