There are a number of central neurotransmitters that are also
involved in the initiation of erection; one such crucial neurotransmitter is
dopamine.
Dopaminergic neurons have also been identified, traveling from the
caudal hypothalamus within the diencephalospinal dopamine pathway to
innervate the lumbosacral spinal cord (Skagerberg et al., 1982;
Skagerberg and Lindvall, 1985). Dopamine
receptors are distributed to various regions in the brain, with a high density
particularly in the basal ganglia. Dopamine is the main transmitter within the PVN, (Eaton et al., 1991). Thus,
dopamine may be expected to participate in the central regulation of
both the autonomic and somatic components of the
penile reflexes.
Dopamine receptors
are divided into two main families: D1- and D2-like
receptors which are in turn subdivided from D1 to D5. Apo morphine has a higher affinity for D2-like
receptors (Argiolas and Hedlund, 2001) that are thought to be the main site for
the induction of erections in the paraventricular nucleus (PVN) (Chen et al.,
1999). Apo morphine is therefore postulated to increase erectile responses by
acting as a conditioner in the PVN and to also increase the response to sexual
stimuli resulting in enhanced erections induced in the periphery (Rampin, 2001).