Who Needs Emotions? The Brain Meets the Robot

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heroin and other opiates, cannabinoids, nicotine, cocaine, amphetamine, and
caffeine. Animals will perform an operant response—say, pressing a lever—in
order to obtain an intravenous infusion of these compounds, in some cases
(such as cocaine) to the point of death, ignoring other essential rewards such
as food and water (Aigner & Balster, 1978). It is remarkable that even invertebrates
prefer stimuli that are associated with exposure to drugs; for example,
crayfish show positive place conditioning to psychostimulants (Panksepp &
Huber, 2004; see Fig. 3.7), and 5-day-old rat pups learn to prefer odors that
have been associated with morphine (Kehoe & Blass, 1986a). These behavioral
findings suggest that there are common chemical and molecular substrates
that rewarding drugs tap into across the animal kingdom.
Evidence supporting this hypothesis is mounting through the use of
powerful molecular biological and genetic techniques. For example, cocaine
acts primarily through its effects on the DA and 5-HT transporters, presynaptic
uptake membrane proteins that control the levels of these transmitters
in the synapse. These universal high-affinity monoamine transporters
are found in nearly every species studied, for example, in C. elegans (Jayanthi
et al., 1998). The DA transporter (DAT) protein has been characterized in
Drosophila and shown to be the target for cocaine-stimulated behaviors in
the fruit fly (Porzgen et al., 2001); DA is necessary for the activating effects
of cocaine, nicotine, and ethanol in the fly (Bainton et al., 2000). In a
notable study, it was found that, as for rodents, both D 1 and glutamate
N-methyl 1-D-as partate (NMDA) receptors are involved in the cocaine response
in the fruit fly; Torres and Horowitz (1998) comment that, “therefore
as in rats, the NMDA (and D-1) receptor pathways in this arthropod
represent obligatory targets for the behavioral effects of psychostimulants.”
This is remarkable given that the major substrates in cellular and behavioral
plasticity with regard to learning and memory are the D 1 and NMDA receptors.
A further example is provided by the protein DARPP-32. This intracellular
signal-transduction protein (DA-regulated phosphoprotein) is an
essential regulator of DA and glutamate signaling and plays a key role in
cellular plasticity, learning, and addiction in mammalian models (Greengard
et al., 1998). DARPP-32 immunoreactivity is also found in the lizard and
turtle brain (Smeets, Lopez, & Gonzalez, 2001, 2003). A further example
is the nicotinic receptor, an endogenous receptor for acetylcholine so named
for its high affinity for nicotine binding. Different functional subunits that
have been characterized in numerous species derive from primordial proteins
over 1000 million years old (Changeux et al., 1998).
Thus, findings are accumulating that identify conserved genomic substrates
and chemical pathways for psychoactive drug action across phyla. This
knowledge addresses proximate causations of behavior (Tinbergen, 1963),
or “how” drugs act in the brain to stimulate emotions. However, we are left