Who Needs Emotions? The Brain Meets the Robot

50 brains
sensory neurons, interneurons, and motor neurons (Changeux et al., 1998).
Serotonin (5-hydroxytryptamine [5-HT]) is a further example of a substance
with an important role in various physiological and behavioral processes. In
the mammalian brain, over 15 different receptors for 5-HT have been cloned
and sequenced; some interact directly with ion channels and others with G
protein–coupled second-messenger systems (Peroutka & Howell, 1994). A
high degree of sequence homology exists between many of these and those
characterized for lower invertebrates such as Drosophila and Aplysia, as shown
in Figure 3.6.
Dopamine (DA) receptors are also widely studied, and five subtypes
have been cloned (Jackson & Westland-Danielsson, 1994; Missale et al.,
1998). Interestingly, there appears to be an insect homologue for the mammalian
dopamine D 1 receptor, which has been implicated in memory and
plasticity; a high degree of transmembrane domain homology exists between
the Drosophila Ddop-1 gene and the mammalian D1/D5 gene (Blenau
& Baumann, 2001). Much is now known about families of neuropeptide
genes and their receptors (Hoyle, 1999). For example, the nonapeptide
family, which includes vasopressin and oxytocin, peptides critical for neural
control of social communication, that is, territorial, reproductive, and
parenting behavior, provides a particularly good example of biochemical
evolution. This system in mammals has its ancestral roots in invertebrates,
with function in reproduction in some cases being conserved. For example,
oxytocin has multiple roles in maternal behavior in mammals, including
infant attachment (Insel & Young, 2000); a member of this family, conopressin,
regulates ejaculation and egg laying in the snail (Van Kesteren
et al., 1995); and the related vasotocin regulates birthing behavior and egg
laying in sea turtles (Figler et al., 1989). The neuropeptide Y (NPY) superfamily
is also widely distributed in evolution. This system is a good example
of peptide superfamilies where there is considerable sequence homology
for the presynaptic peptide across species but much greater diversity in
the evolution of its receptors (Hoyle, 1999). Since peptide receptors are
generally much larger than transmitter peptides, it is likely that there was
much greater chance for mutations and gene duplication in receptors with
receptor function being maintained. In mammals, NPY is involved in hypothalamic
feeding mechanisms; in a recent study of Caenorhabditis elegans,
one single-base mutation in the npr-1 gene, coding for a receptor structurally
related to the mammalian NPY receptor, was enough to dramatically
alter the feeding behavior of these worms (de Bono & Bargmann, 1998).
It is important to note that although I have emphasized interesting
sequence homologies coding for various chemical signaling molecules across
the evolution of species, there are many instances where a peptide or
protein has been conserved but evolves to serve multiple and often unrelated