If realism is important in your novel, you should bear in mind that all the areas of the brain, although consisting of numerous ‘functions’ (for instance, the Area of Broca and Wernicke’s Area are implicated in the provision of speech, the hippocampus forms part of the emotional memory formation substrate, etc), by and large follow the same processing paradigm – receive inputs; do stuff; push outputs.
The elements of this paradigm also follow a predictable physiology – that is, the actual type of neurotransmitter may vary (dopamine, serotonin, GABA, etc), but the operation is the same: neurotransmitters (regardless of type) collect in vesicles and move towards the neural terminal in these vesicles, which merges with the terminal wall, thus causing the neurotransmitter content to be dumped into the synaptic gap. After an interval, a ‘pump’ sucks up the remaining transmitters (called re-uptake); the other transmitters of this collection bind with specific receptor channels on the dendrites of the recipient neuron. As an important note: this does not change for the different areas of the brain.
All controlled substances either act as agonists (accelerating this flow), or antagonists (interrupting the flow). Accordingly, your drug will not be able to isolate areas of function, (e.g. decision-making and memory formation), since neurotransmitters are widely shared by the neural functions. The closest we’d get is something similar to cocaine – which antagonises the re-uptake of dopamine, causing heightened states of alertness due to a dopaminergic hyper-flow in the synaptic gap.
As a further consideration: in order for the effect of your drug to exist, a substantial portion of the brain will need to be ‘knocked out’. The brain was not designed in a single architectural sitting. It is the product of 3.5 billion years of small, incremental changes. Consequently, there are ancient parts of the brain (including the misnomer ‘Lizard Brain’) and these are typically found at the core of the brain and close to the stem. The anatomically modern neural areas are usually located closer to the surface of the brain and on the extremities (for instance the inferior frontal gyrus, located at the tip off the pre-frontal cortex is implicated in the go\no-go phase of our decision-making process). For your drug to work, certain modern neural areas such as the primary visual cortex will have to remain operational (or the user will experience vivid visual effects of all sorts, resulting in not being a very capable ‘animal’).
The alternative to this knock-out, is to provide the ancient neural areas an unfair advantage (the 'how' remains elusive) – these areas are less complex than their modern equivalents and are therefore (as a rule of the thumb) capable of completing their less complex tasks in less time. That is why it is possible to ‘jump scare’ a human with something as preposterous as a ‘gorilla in the house’ type prank. The modern parts of the brain can compute that gorillas do not belong in your house and that it is a prank when one jumps out at you. However, the ancient pathways do not make such contextual calculations, and respond to ‘big, furry, moving very fast towards me’. The ancient parts of the brain are therefore able to complete their function (triggering the fight/flight reflex) earlier than the modern pathways, and… you utter a species-specific vocal alarm (aka a girly scream), hands fly up in defence, face contort in anticipation, upper body recoils from the threat – a classic early flight response.
Why would someone find this effect enjoyable? Some transmitters are associated with the ‘reward system’ (e.g. serotonin – implicated in enjoyable activities, such as food, sex, etc), others with heightened sense of awareness (in itself pleasurable). There are additional effects, such as endorphin (the brain’s own morphine) releases following certain stresses (causing inter alia the knees to go weak, and analgesia), and these too are quite pleasurable.
As the answer to your question is: construct a (realistic) reward to a chemical which causes medium term physical alterations to one type of ionic channel – that’ll cause the substance to be super addictive, requiring a substantial rehabilitation to kick (read up on the medium to long term effects of heroin).
For something this specialist, you will need further reading if you are to have any hope of staying between the lines. I suggest any of the works of V.S Ramachandran, Eric Kandel, Steven Pinker, Motoy Kuno (his work titled The Synapse), David Eagleman, etc. Ramachandran's Tell-tale Brain may provide an interesting (non-chemical) angle as a solution - think implants. Enjoy.