Who Needs Emotions? The Brain Meets the Robot

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functions. Niall (1982) notes that gene duplication is only one means of diversification;
another is development of a new or different function for a
peptide hormone. For example, he notes that prolactin enables fish to adapt
to varying salt concentrations; in mammals, it became involved in the control
of lactation. Moreover, many so-called pituitary hormones are made in
many brain and gut regions, possibly serving various functions in these different
structures. Medawar (1953) noted that “endocrine evolution is not
evolution of hormones but an evolution to the uses to which they were put.”
Thus, although there are many intriguing examples of conservation of function
across phyla, it is important to appreciate the diversity of signaling functions
as well.
NEUROCHEMISTRY AND PHARMACOLOGY
OF EMOTIONS
The above account provides an organizational framework for understanding
the hard-wiring of motivational circuits, how they are affected by sensory
stimuli, and how they have the ability both to effect behavioral responses via
direct motor outputs and to feed forward to influence higher cortical regions
and perhaps generate awareness. Communication between the billions of synapses
as well as general modulation of these systems is accomplished via chemical
signaling; but how and where do these substances act to produce changes
in emotion, mood, and behavioral state? Given the space limitation here, I
cannot possibly describe in detail the vast array of neurotransmitters and
neuromodulators that contribute to the functional role of these systems. Instead,
I have chosen several candidate systems that represent compelling examples
of chemical signaling systems that mediate motivation and emotion
and that have parallel links to related functions across phyla.
Dopamine: Reward and Plasticity
A great amount of attention has been given to the catecholamine DA in a
variety of species. In mammals, DA is proposed to play a major role in
motor activation, appetitive motivation, reward processing, and cellular plasticity
and certainly can be thought of as one candidate molecule that plays a
major role in emotion. Like the other catecholamines norepinephrine and
epinephrine, DA is synthesized from the amino acid tyrosine and involves
several biosynthetic steps employing the enzymes tyrosine hydroxylase and
dihydroxyphenylalanine (DOPA) decarboxylase. Receptors for DA exist in
two major classes: D 1-like (D 1 and D 5 receptors) and D 2-like (D 2, D 3, D 4).