*1.2.1 Cholinergic projections*

The widely projecting and local circuits constitute the cholinergic system. The extended projections of cholinergic neurons originate in nuclei of basal forebrain and project throughout the brain. Acetylcholine is transmitted to the cerebral cortex via the "nucleus basalis of Meynert" and the "diagonal band of Broca". There are also cholinergic connections between the nucleus located in septal region and the hippocampus located deep into temporal lobe. The local circuitry of cholinergic system inside corpus striatum (important component of basal ganglia) interacts with neurons of GABA system and nigrostriatal dopamine system engaged in extra-pyramidal movement.

### *1.2.2 Cholinergic system: the role in cognition*

The cholinergic system has been linked to several cognitive functions such as memory, attention, memory, and processing of emotions. Research done on humans and animals indicated that sustained attentional performance has/had been driven by the cholinergic input from the basal forebrain [7]. In general, cholinergic system activation aids better focussed ability to retain the relevant stimuli and filter out the irrevelant ones, but particular projections to the medial prefrontal cortex regulate anxious reactions to contextual cues acetylcholine influences working memory and the attentional processes necessary for error detection by regulating arousal and attention. The cholinergic system is also tightly linked to emotional processing; moreover, the inputs of cholinergic neurons to the frontoparietal cortex modulate the direction of attention toward emotional expression. In view of the fact that the system is governing so many aspects of cognition, diminished cholinergic tone correlated to Alzheimer's disease and leads to poor intellectual execution that includes attentiveness, memory, attention, and executive functioning [8].

#### **1.3 Cholinergic receptors**

Nicotinic acetylcholine receptors are ion channels that are activated by a ligand. They are made up of five polypeptide subunits, including 2 α subunits plus β, γ and δ subunits with its further two subtypes viz. Muscular (N1) and Neuronal (N2). N1 consists of α1, β1, γ, δ (2:1:1:1) subunits in the case of embryo while α1, β1, δ, and ε (2:1:1:1) in case of adults. N2 possesses 2α, 3β subunits that can form twelve different combinations of nicotinic receptor subunits. When two acetylcholine molecules bind to the nicotinic acetylcholine receptor, there exists a change in the internal conformation of the pentameric structure, forming a trans-membrane aperture that allows the movement of Na<sup>+</sup> , K<sup>+</sup> (3 sodium ions out of cells while pumping 2 potassium ions into cells) and Ca2+ ions to pass through. Depending on the intensity of the initiating stimulation, the transport of these ions will result in cell depolarization [9].

mAchR, known as "Muscarinic acetylcholine receptors" belong to the subfamily of G-protein coupled receptor (GPCR) complexes, commonly found in brain, bladder, sweat glands, eye and gland, constitute a single polypeptide chain with 7 distinct sections organized in an alpha-helical structure. Hydrophobic residues in these alpha helices permit the polypeptide for spanning the neural membrane about 7 times. The 5th cytoplasmic facing loop along with the carboxy tail of this polypeptide communicates with G-proteins (made of α, β, and γ subunits), also known as secondary messengers. When acetylcholine binds to the muscarinic receptor, there is a change in its configuration, prompting the alpha sub-unit for releasing the naturally attached inactive metabolite (purine nucleotide diphosphate consisting of guanine and pyrophosphate) called guanosine di-phosphate, GDP and swap this for tri-phosphate molecule (guanosine triphosphate, GTP). α subunit dissociates from the distinct components (β and γ) upon binding of triphosphate molecule to alpha component. Further, GTP collaborate in accompany to other signaling proteins. An inherent activity of GTPase in α subunit metabolize GTP again to GDP, thus shutting off the second messenger system over time. Muscarinic receptors have numerous tissue-dependent activities in the physiological system of humans. These activities could be either of two: STIMU-LATORY or INHIBITORY due to their utilization of secondary messengers to achieve the desired results. M2 and M4, the subtypes of Muscarinic receptor, inhibits adenylate cyclase to function. There is an inhibition of adenylate cyclase by the alpha component of G protein upon binding of Ach to Muscarinic receptor subtypes (M2 or M4) that results in decrement of intra cellular cAMP and as there is an essential role of cAMP in activating/inhibiting the numerous down-stream elements of signaling pathway, it's decreased concentration leads to plethora of pitfalls. The actions of M type subreceptors (M1, M3 & M5) have been carried out via stimulating phospholipase C (PLC). The active complex of G protein communaticate to PLC and activates it that cause hydrolysation of phophatidyl-inositol to Inositol tri-phosphate (IP3) and diacyl-glycerol (DAG). The increase in intra cellular calcium concentration in the cytoplasm of the cell is due to the interaction of secondary messentar IP3 with its receptors located in SER (Smooth Endoplasmic Reticulum) [10].
