## 1 Introduction  
Protozoan parasites can be subdivided into two major groups, i.e., the Excavates with flagellar structures and the group of Alveolates which contain membranous vesicles, so called alveoli, located beneath the plasma membrane. In the taxonomic rank the most important phylum with respect to disease in the group of Alveolates are the Apicomplexa comprising the genera _Plasmodium_, _Cryptosporidium_ and _Toxoplasma_. In contrast, the major group of Excavates consists of the phylum Euglenozoa comprising the genera _Leishmania_ and _Trypanosoma_ known as the kinetoplastids with large, massed DNA called the kinetoplast. The cAMP signaling pathway and multiple activated factors are involved in regulating numerous physiological processes, including growth, reproduction, differentiation and apoptosis. Disruption of this pathway can lead to treatment of the disease. The physiological functions depend on the targeted tissues, cells and organs. In mammals, for example, cAMP has multiple roles ranging from auditory function to mediating hormone action. cGMP is a central player in processes such as cardiac function and light detection in the eye. Various physiological functions are also attributed to G-proteins from knock-out studies in mice. G\({}_{\text{s}}\) and G\({}_{\text{i}}\) subunits contribute to cardiac functions such as contractility. Gq and G\({}_{\text{12}}\) have multiple functions like cerebral development, cardiomyocyte formation, craniofacial development and parathyroidism. GPCRs are the most intensively studied drug targets due to their involvement in pathophysiological processes. Research on G-protein coupled receptor (GPCR)-mediated signaling in protozoan parasites has been intensified during recent years. One widely used principle of signal transduction in eukaryotes is the signaling through GPCRs [1]. G-proteins represent a heterogenous group of proteins. In canonical GPCR-coupled pathways, binding of a ligand (agonist) to a receptor leads to a conformational change in the receptor protein which stimulates the binding of a heterotrimeric G-protein to the GPCR. Heterotrimeric G-proteins are composed of alpha, beta and gamma subunits. These subunits are triggered to interact with the receptor [2]. Once a receptor is activated (Figure 1) the GDP which is bound to the G\(\alpha\)-subunit is exchanged to GTP. The G\(\alpha\)-subunit dissociates from the G\(\beta\gamma\) dimer resulting in two functional subunits (G\(\alpha\) and G\(\beta\gamma\) dimer) signaling to downstream effectors like adenylyl cyclases or guanylcyclases which are responsible for cyclization of ATP/GTP to cAMP/ cGMP. Phosphodiesterases then hydrolize cAMP once a threshold has been reached. Finally, cAMP-dependent protein kinase A (PKA) or cGMP-dependent protein kinase G (PKG) is activated. Depending on the sequence and the functional similarity, G\(\alpha\)-subunits consist of four different subfamilies [3], i.e., (G\(\alpha_{\text{s}}\), G\(\alpha_{\text{i/o}}\), G\(\alpha_{\text{q11}}\) and G\(\alpha_{\text{12/13}}\)). Within the G\(\alpha_{\text{s}}\) subfamily there are two groups i.e., the G\(\alpha_{\text{s}}\) (stimulatory) and the G\(\alpha_{\text{ol}}\) (olfactory). G\(\alpha_{\text{s}}\) is expressed in all types of cells while G\(\alpha_{\text{ol}}\) is expressed in olfactory sensory neurons [4]. G\(\alpha_{\text{i}}\) is the largest and most diverse family. G\(\alpha_{\text{t1}}\) (t means transducin) is expressed in the rod cells in the eye, while G\(\alpha_{\text{t2}}\) is expressed in the cone cells of the eye [5]. In humans, G\(\alpha_{\text{q}}\) is ubiquitously expressed and G\(\alpha_{\text{12/13}}\) occurs in all types of cells. In contrast to the impressive progress in understanding cyclic nucleotide signaling in the mammalian host, research on this topic has been delayed in apicomplexan parasites and kinetoplastids. One reason might be the recently completed genomes of these parasites which revealed very different components of the pathways.