Neurotransmitters are chemical messengers found in the synaptic vesicles that are characterized by the effect they have on the neurons and cells, once released (Ciccarelli and Meyer, 2006). A neurotransmitter can affect the post-synaptic cell activity through two methods- ionotropically and metabotropically (Purves, Augustine & Fitzpatrick, 2001). In this essay, metabotropic actions shall be discussed through dopamine pathways.
Specific neurotransmitters activate ionotropic and Metabotropic receptors. Ionotropic receptors, when activated, open channels that allow the flow of ions (Purves et al., 2001). On the other hand, Metabotropic receptors don’t have ion channels; they affect the activation of neurons through intermediate molecules called G-proteins that trigger intercellular events (Purves, Augustine & Fitzpatrick, 2001). As this process involves the activation of several molecules, the response of metabotropic receptors is slower than that of ionotropic receptors (Jessell, Siegelbaum & Hudspeth, 2000). Due to this, they remain open for longer durations ensuring that their effect is longer-lasting. The impact of metabotropic receptors can be seen as far spread in the cell (Marieb & Hoehn, 2007).
Metabotropic receptors have fine control over neurotransmitters’ activities by stimulating several pathways (Crupi, Impellizzeri &Cuzzocrea, 2019). Each neurotransmitter has specific pathways where its effects can be seen. Dopamine is one such neurotransmitter found in the mid-brain. It constitutes of less than 1% of the neuronal population but is understood to have influential effects on brain functions (Arias-Carrión, Stamelou, Murillo-Rodríguez, Menéndez-González & Pöppel, 2010).
Four main dopaminergic pathways synthesize and release dopamine (Speed, 2010):
Dopamine originates in the Ventral Tegmental Area (VTA) and travels through the amygdala, pyriform cortex, lateral septal nuclei and Nucleus Accumbens (NAc) (Ayano, 2016) and controls ‘reward and salience’. Mesolimbic dopamine arbitrates the feeling of reward and pleasure in the NAc (Bridges, 2016) and is released during pleasurable situations (food, sex, drugs). Due to this, the individual can experience and seek out pleasure. It is also involved in addictions. Specific cravings of substance activate mesolimbic pathways (Ayano, 2016). Dopamine is also released in salient stimuli that are not rewarding (tones, lights) (Ungless, 2016). Thus, it could be stated that consumption of substance increases pleasure while reducing the salience of an individual.
Mesocortical dopaminergic receptors arise from the VTA to Pre-frontal Cortex (PFC) and septohippocampal areas (Ayano, 2016) and control emotional and cognitive functions of behaviour (Speed, 2010). It is involved in areas of cognition, attention, working memory, decision making, social interactions and imbalance can lead to inattentiveness, indecisiveness, etc.(Ayano, 2016.; Bridges, 2016).
Dopamine originates in substantia nigra and project to basal ganglia (Ayano, 2016). This area is associated with movement, control, and learning new motor skills (Ayano, 2016.; Speed, 2010). 80% of dopamine in the brain is contained in this pathway (Bridges, 2016). Degeneration of this area is found to be a significant cause of Parkinson's disease characterized by loss of movement control, tremors, stiffness etc. (Barbeau, 1962). Recent studies also suggest nigrostriatal dopamine’s involvement in feeding behaviour. It is said to motivate us to seek fulfilment of our basic needs such as food (Dourish & Hutson, 1985.; Speed, 2010).
This pathway stems from arcuate and periventricular nucleus of hypothalamus and project to the pituitary gland (Bridges, 2016); also known as Hypothalamic-Pituitary Axis (HPA) (Ciccarelli & Meyer, 2006). Here, dopamine synthesizes hypothalamus and obstructs the release of prolactin from the anterior pituitary gland (Ayano, 2016). Prolactin enables lactation, sexual satisfaction etc. (Bridges, 2016). Thus, dopamine; which is the primary neuroendocrine inhibitor of prolactin, is also known as a prolactin-inhibiting factor (PIF) (Ayano, 2016).
Through the above explanation, it is evident that dopamine is an essential part of cognitive, behavioural and physiological processes. It has an impact on mood, motivation, reward-seeking behaviour, movement, attention etc. Imbalance in the dopaminergic systems can lead to several psychiatric disorders such as schizophrenia, attention deficit hyperactivity disorder, Parkinson’s disease, addiction (Speed, 2010.; Ayona, 2016). To ensure effective treatment, it is vital to understand the systems, their governing factors and the effects of dysregulation.
To summarise, dopaminergic system is an essential and influential part of the metabotropic neuronal communication. Through it, we can understand the notions of metabotropic communication and the behaviour pattern of metabotropic receptor neurons. Metabotropic neurons communicate through pathways, and each pathway has a significant role to play in the functioning of the brain.
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