**1. Introduction**

A calcium channel is an ion channel which displays selective permeability to calcium ion (Ca2+). Itis sometimes synonymous as voltage-dependent calcium channel(VDCC)[1], although there are also ligand-gated calcium channels (LGCCs) [2]. Voltage-gated calcium channels (VGCCs) are found in excitable cells (e.g., glial cells, muscle, neurons, etc.) [3–6]. There are at least six classes of VGCCs (L-, N-, P/Q-, R- and the T-type channels) that are distributed according to cell type andlocation, andthatmaybedistinguishedbyelectrophysiological,pharmacological, and structural characteristics. No small organic ligands are clinically available for other than the Ltype channels, although there are a number of experimental compounds for the T- and N-type channels [7].

It is known that VGCCs exert a regulatory control of CNS, cardiac, and muscularly activities and that their activity disorders can provide raise to physiopathological cases extending from cardiac and vascular disorders to central nervous system pathologies. Voltage-gated calcium channels inhibitors (VGCCIs) have been applied profitably to treat epilepsy and are arising as probable curative pathways as long as pathology, such as algesia, Parkinson's disease, anxiety, and addiction [8]. Therefore, calcium channels can be drug targets for nervous system diseases, and potential challenges and opportunities for the development of new clinically effective calcium channel inhibitors [8].

L-type VGCCs are located in neuronal cells, dendrites, spinal cord, adrenal gland, skeletal cardiac and smooth muscles, and many other locations [9–14]. L-type calcium currents typically require strong depolarization for their activation and are blocked by different antagonists (VGCCIs) including dihydropyridines (nifedipine), benzothiazepines (diltiazem), and phenylalkylamines (e.g., verapamil). VGCCIs are a class of drugs that disrupts the movement of calcium ions through calcium channels. These substances, by relaxing the smooth muscle tone, are commonly used to treat high blood pressure (hypertension), migraines, *angina pectoris*, Raynaud's disease, and also cluster headaches [9, 14]. In palliative medicine, they are used as analgesic drugs and in veterinary, they are used to treat experimental duodenal acute pain (colic) in sheep [15, 16]. Since high density of these channels are found in sinoatrial and atrioventricular nodes, VGCCIs decrease impulse conduction through these nodes and are used as antiarrhythmic agents.

The mode of action of verapamil similarly to diltiazem and nifedipine, is based on binding with the largest subunit α1 of Ca2+ channels. This subunit incorporates the conduction pore, voltage sensor, gating apparatus, and several regulation sites, e.g., by second messengers, drugs, and toxins. VGCCIs inhibit Ca2+ ions influx to the cells, which are the main Ca2+ currents in muscle and endocrine cells initiating many activities, such as gene expression, muscle contraction (excitation-contraction coupling), hormone secretion, neurotransmitter release, cell growth and regulation, neurons migration, cell damage, and death or finally cell survival [17].

Acute intestinal distension ("colic"), similarly as functional gastrointestinal disorders, inflammatory bowel disease or irritable bowel syndrome causes visceral hypersensitivity and may produce persistent pain [17, 18]. Visceral pain is described as pressure-like, intermittently squeezing or cramp, not well localized, vague in character, and difficult for patients to describe [19]. Visceral pain is frequently accompanied by nausea, sweating, defecation, vocalization, grinding, head movement, hyperventilation, hypertension, tachycardia, hypercortisolemia, and hypercatecholaminemia (**Table 1**).


1, Diltiazem + DD40; 2, Nifedipine + DD40; 3, Verapamil + DD40.

**Keywords:** duodenal distension, diltiazem, nifedipine, verapamil, behavioral signs, clinical symptoms, rumen motility, blood plasma cortisol, catecholamine concentra-

A calcium channel is an ion channel which displays selective permeability to calcium ion (Ca2+). Itis sometimes synonymous as voltage-dependent calcium channel(VDCC)[1], although there are also ligand-gated calcium channels (LGCCs) [2]. Voltage-gated calcium channels (VGCCs) are found in excitable cells (e.g., glial cells, muscle, neurons, etc.) [3–6]. There are at least six classes of VGCCs (L-, N-, P/Q-, R- and the T-type channels) that are distributed according to cell type andlocation, andthatmaybedistinguishedbyelectrophysiological,pharmacological, and structural characteristics. No small organic ligands are clinically available for other than the Ltype channels, although there are a number of experimental compounds for the T- and N-type

It is known that VGCCs exert a regulatory control of CNS, cardiac, and muscularly activities and that their activity disorders can provide raise to physiopathological cases extending from cardiac and vascular disorders to central nervous system pathologies. Voltage-gated calcium channels inhibitors (VGCCIs) have been applied profitably to treat epilepsy and are arising as probable curative pathways as long as pathology, such as algesia, Parkinson's disease, anxiety, and addiction [8]. Therefore, calcium channels can be drug targets for nervous system diseases, and potential challenges and opportunities for the development of new clinically effective

L-type VGCCs are located in neuronal cells, dendrites, spinal cord, adrenal gland, skeletal cardiac and smooth muscles, and many other locations [9–14]. L-type calcium currents typically require strong depolarization for their activation and are blocked by different antagonists (VGCCIs) including dihydropyridines (nifedipine), benzothiazepines (diltiazem), and phenylalkylamines (e.g., verapamil). VGCCIs are a class of drugs that disrupts the movement of calcium ions through calcium channels. These substances, by relaxing the smooth muscle tone, are commonly used to treat high blood pressure (hypertension), migraines, *angina pectoris*, Raynaud's disease, and also cluster headaches [9, 14]. In palliative medicine, they are used as analgesic drugs and in veterinary, they are used to treat experimental duodenal acute pain (colic) in sheep [15, 16]. Since high density of these channels are found in sinoatrial and atrioventricular nodes, VGCCIs decrease impulse conduction through these nodes and are

The mode of action of verapamil similarly to diltiazem and nifedipine, is based on binding with the largest subunit α1 of Ca2+ channels. This subunit incorporates the conduction pore, voltage sensor, gating apparatus, and several regulation sites, e.g., by second messengers, drugs, and toxins. VGCCIs inhibit Ca2+ ions influx to the cells, which are the main Ca2+ currents

tion, sheep

226 Pain Relief - From Analgesics to Alternative Therapies

**1. Introduction**

channels [7].

calcium channel inhibitors [8].

used as antiarrhythmic agents.

**Table 1.** The effect of duodenal distension (DD40) on the ruminal motility (inhibition in % 5 min−1 in comparison to the control values) [10] and behavioral symptoms (number·5 min−1) in sheep before and after voltage-gated calcium channels inhibitor pretreatment at a dose of 1 or 2 mg *in toto* (i.e. 25 or 50 μg·kg−1 B.W.; n = 6).

Gastrointestinal sensory system consist intrinsic (enteric) sensory afferents and extrinsic (vagus, spinal cord, pelvic) afferents. Intrinsic sensory system functions independently of the CNS. Enterochromaffin cells within mucosa and enteroendocrine cells release 5-HT, CCK, orexin, and leptin which modulates and regulates motor activity of intestine [20, 21]. The submucosal enteric plexus and myenteric plexus have a high degree of synaptic interactions (enteric nervous system or a "gut brain"), which can be either inhibitory or stimulatory for the purpose of regulating gastrointestinal motility and peristalsis [22, 23].

Mechanisms of the reception, transduction, transformation and modulation of nociceptive stimulus, and reaction diminishing response on nociception are regulated by afferent systems to CNS and efferent systems from CNS "stimulating" reaction, but quenching (pain-gated).

Several data show antinociceptive/antistressoric effects of organic Ca2+ inhibitors of L-VGCCs in acute duodenal pain of sheep [16, 20]. These inhibitors potentiate the analgesic action of κopioidergic receptor agonists [15], as well as morphine by decreasing opioids' tolerance [24]. It was also shown by Bongianni et al. [25] that VGCCIs suppress not only metabolic but also behavioral expression of the morphine withdrawal syndrome. In experiments performed on mice, it was shown that verapamil blocked amphetamine and also physostigmine induced footshock-induced aggression [26]. It was postulated by Michaluk et al. [27] that VGCCIs show antinociceptive properties; but they also change the territorial behavior of animals [28] and conspecific aggression in fish [29]. Such effects were probably caused by the inhibition of Ca2+ entry into neurons, preventing the appearance of synaptic vesicles in axon terminal, and release of neurotransmitter into the synaptic cleft. Davis and Bauer [14] have shown in experiments performed on rats, that activation of L-VGCCs are necessary for the long-term retention of fear excitation.

A duodenal and/or colonic distension method, provoking jejunal pain, stimulates hypothalamic-pituitary-adrenal-cortical (HPA) and sympatico-adrenal system (SAS), pathways that revealed as an increase in cortisol and CA in blood plasma [18, 20]. A different role of L-type antagonists for VGCC has been previously identified in different types of experimental and clinical pain in man and animals. Present study examined comparative role of VGCC blockers from different chemical groups—diltiazem, nifedipine, and verapamil administered *i.c.v.* in the same four different doses (0.25, 0.5, 1.0 and/or 2.0 mg *in toto*)—to estimate the comparable effect on the development of pain-related symptoms, clinical signs, plasma cortisol and catecholamine level, and the inhibition of ruminal motor activity caused by 5 min lasting mechanical duodenal distension (DD) in the sheep.
