Introduction : In the late 1940s, when
I began my graduate studies, psychology and
physiology still had not met as scientific
disciplines. Animal behavior was studied in the
American laboratories of psychology but not in
its relation with physiological processes. Hull
2 and Skinner, 3 the leading behaviorists at the
time, both framed models of operant and
instrumental conditioning. Hull conceptualized
physiological processes as intervening variables
without making any attempts to define and
characterize them further. Skinner was equally
certain that concepts like intervening variable
were unjustified and even scientifically
unsound, specifying stimulus-response
relationships as all that was needed.
In Europe, the behavior of mammals was
studied systematically by the ethological school
with leaders like von Frisch, Lorentz, and
Tinbergen as towering figures. The ethologists
assumed inborn "instincts" as organizers of
behavior. By instincts 4 was meant inherent,
species-typical behavior patterns that were
assumed to have developed under the pressure of
evolution like the morphological and
physiological features of the species.
The behaviorists and the ethologists differed
in methods and outlook on science. The
behaviorists formulated problems that allowed
the use of animals to answer research questions
in behavioristic terms and gave a methodology
that would permit fine-grained analysis of the
behavior. A main contribution of the ethologists
was observation of animal behavior in the wild
as part of the animal's normal life and their
emphases on the evolutionary perspective. The
behaviorists considered individual experience as
the main organizer of behavior, whereas the
ethologists focused on species variations
determined by the genome. In one respect,
however, the approach to behavior of the two
schools was similar. Both thought that the
behavioral analysis was a prerequisite to
obtaining a complete explanation of the
behavior. None was concerned with the role of
the brain in the regulation of behavior, or, for
that matter, of the importance of any measures
relating molar and molecular event. None
anticipated the explosive rise of physiological
psychology, nor the development of behavioral
neuroscience soon to come.
I had from my early school days been curious
about animal behavior and its physiological
bases, but when I entered the university in the
midst of the 1940s, psychology did not even
exist as a separate subject in the Swedish
universities. I wanted to approach animal
behavior in a way that allowed me to study
behavioral and physiological variables in their
interaction. I happened to hear that in Norway
there was an anatomist, Alf Brodal, and a
neurophysiologist, Birger Kaada, who both tried
to relate anatomy, physiology, and behavior to
each other. So I went to Oslo. At the Anatomical
Institute in Oslo, where both these researchers
worked, I was given the task of studying the
female mouse estrous cycle. This study became a
revelation for me. Sitting at night in the
animal room and looking at the behavior of the
mice, I felt I was looking down directly into
nature itself. I observed each fourth day, how
the female mouse, within a matter of an hour,
entered a state of receptivity when her
rejection of the male was turned into
acceptance. The behavioral cyclicity was a
reflection of endocrine, morphological, and
neuronal changes, controlled by the brain.
Brodal, once a student of Judson Herrick, gave
me the attractive book of his teacher, The Brain
of Rats and Man, together with his own writings
on the limbic brain. Kaada, who had done his
Ph.D. studies during the war with John Fulton at
Yale university, gave me his work on electrical
recordings from the brain These experiences made
me decide to study reproductive behavior, which,
in such a wonderful way, twins hormonal, neural,
and behavioral influences.
Some years later, now with a doctoral degree
in male rat sexual behavior completed, I met
Lennart Heimer, who just had ended his medical
studies and had even written an introductory
textbook on neuroanatomy for medical students,
the first Swedish book of this kind. Joined by
our common interest in brain and behavior, we
began to work together looking for brain
correlates of sexual behavior. I will, to begin,
outline the main lines of this work. Questions
proposed during the course of these studies
prepared for later studies that were oriented
towards neurotransmitters possibly involved in
sexual behavior. These problems will be dealt
with in the second part of this report.
Central Neural Control of Male Rat Sexual
Behavior
As a background for the experiments to be
described below, Figure 1 depicts male rat
sexual behavior. When exposed to a receptive
female, the experienced male approaches her and
mounts her. After repeated mounts and
intromissions, ejaculatory behavior is elicited.
The ejaculation is followed by a refractory
period when the male is sexually unresponsive.
After 4-5 minutes, he resumes pursuance and
mounting of the female. Mounts, intromissions,
and ejaculations can be easily recognized,
counted, and expressed in terms of frequency and
latency of their first appearance. It is assumed
that the intromissions cause a rising sexual
excitation cumulating in ejaculation.
When Lennart and I begun our studies in the
beginning of the 1960s, little was known about
the role of the brain in the regulation of
sexual behavior. Removal of large parts of the
cerebral cortex, independently of their
localization, disrupted copulation in males of
several species. Later studies showed, however,
that lesions in the medial-frontal cortex
abolished mating in some rats, whereas lesions
in the more posterior regions, including the
cingulate gyrus, had little or no effects on
mating, suggesting different roles of frontal
and posterior parts of the rat cortex. It was
not until the end of the 1940s that stereotaxic
surgery was introduced into the laboratory,
allowing investigators to place discrete lesions
within the depths of the brain. It was reported
that electrolytic lesions in the hypothalamus
impaired male rat sexual behavior without
causing any apparent hormonal deficits. The
subcortical lesions performed in these studies
were, however, too large to admit localization
of the behavioral impairment to any specific
group of cells. Therefore, Lennart and I
decided, as a first task, to locate the neural
circuits essential for the sexual behavior. For
this we needed a stereotaxic instrument. We
built such an instrument, with the help of
Victor Kuikka, the skillful technician of the
anatomy department.
Not knowing where to begin, in the anterior
or the posterior end of the brain, we decided to
start in the rostral end of the brain stem to
continue forward. We made two interesting
findings. First, extensive lesions in the
junction of the diencephalon and mesencephalon
made them hypersexual. The males ejaculated
after only a few intromissions, within a very
short time, and showed abnormally shortened
PEIs. Within an hour, some males had ejaculated
a dozen times compared to three or four times
normally. Our second finding pertained to
lesions in the area of the medial preoptic
nucleus and the anterior hypothalamus (MPOA).
Extensive lesions in this area abolished sexual
behavior seemingly permanently; minor lesions in
this area, independent of location, caused only
a temporary impairment of the behavior. Both
these findings led us to suggest two neural
mechanisms regulating male rat sexual behavior,
one involving subcortical structures in the
caudal diencephalon and anterior mesencephalon,
exerting an inhibitory influence upon mating,
and another mechanism located in the MPOA,
mediating sexual arousal and controlling the
motoric aspects of mating.
The medial preoptic-anterior hypothalamic
continuum occupies a strategic position in the
limbic system. Its lateral zone is an
interstitial nucleus of the medial forebrain
bundle, a polysynaptic fiber system that
provides a major reciprocal link between the
medial basal telencephalon rostrally and the
midbrain tegmentum caudally. It is located
outside the main stream of the medial forebrain
bundle, but it is known to receive numerous
short fibers from the lateral zone, making its
functional state likely to be influenced by
impulses arriving in the hypothalamic region by
way of the medial forebrain bundle. Such
impulses are supposed to originate from visceral
and somatic sensory structures of the lower
brain stem, from the hippocampus, cingulate
cortex, septal area, and amygdaloid complex. Of
equal relevance should be axons to the medial
forebrain bundle arising from olfactory
structures, such as the piriform cortex, the
olfactory tubercle, and the olfactory lobes.
Besides its more diffuse afferent connections
from the lateral hypothalamic zones, the MPOA
receives a component of the stria terminalis, a
major fiber system originating in the amygdaloid
complex. The stria terminalis, in part at least,
originates from the cortico-medial subdivision
of the amygdala, a region known to receive
numerous fibers directly from the accessory
olfactory lobes. The position of this
intermediate area between what was then called
the limbic and olfactory telencephalon on the
one hand, the midbrain tegmentum on the other
hand, and bordering the gonadotrophic mechanisms
of the tuberal hypothalamus was compatible with
our finding that this region is important for
the regulation of mating. Considering the highly
heterogeneous, multimodal afferent
relationships, among which the olfactory
modality appeared to be particularly strongly
represented, we decided to focus on the role of
the olfactory system and moved, thereafter, in
the posterior direction, being interfered with
by lesions at various levels of the limbic
system. Failing any good anatomical guidance,
Lennart started to study the basal forebrain
using various silver-impregnation techniques.
Soon he had developed a technique of his own,
later to be published as the Fink-Heimer
technique.
Electrolytic lesions of the main olfactory
bulb, or surgical section of the lateral
olfactory tract, impaired, but did not prevent,
the occurrence of sexual behavior. Further work
was undertaken involving destruction of the main
olfactory bulb or sectioning of the lateral
olfactory tract. Again, we found a striking
variation of the behavioral effects obtained,
suggesting a role for factors other than
olfaction, as such (Fig. 3). We soon discovered
that sexual experience was one of these
disturbing factors: those males that were
sexually naive when made anosmic rarely ever
started to mate. The sexually experienced males,
by contrast, showed relatively small effects of
anosmia.
The observation of the importance of
experience for sexual behavior came as a
complete surprise to us. From the considerable
literature on sex and olfaction existing at that
time, we were made to believe that odoiferous
signals were mainly coupled with preprogrammed
behaviors. Our observation, however, pointed at
a powerful role of olfactory memory. More recent
work has confirmed these observations, and, in
addition, indicated the importance of the
vomeronasal organ and the accessory olfactory
lobe in such a mechanism. It appears that the
sense of olfaction is particularly well suited
for storing emotionally important memories.
After these studies were concluded, Lennart
went to Walle Nauta at MIT to continue his
neuroanatomical work, which he had begun in
Sweden. This research was going to result in
this conceptual remodeling of the basal
forebrain that we are discussing here. As I have
tried to show you, this research program was
guided by a need to find ways to explore the
role of the brain in behavior, and reproductive
funtions, in particular. The work subsequently
performed by Lennart and his associates has
shown the role of a collection of structures,
including the nucleus accumbens, olfactory
tubercle, septum, diagonal band nuclei, and bed
nucleus of stria terminalis as well as the
extensive territory beneath the temporal limb of
the anterior commissure, which long was referred
to as the substantia innominata. Of special
importance to reproductive funtions is the
ventral striatopallidal system, the extended
amygdala, and the areas of transition between
these two systems. The concept of a critical
role of the MPOA in mammalian sexual behavior
has been confirmed in all mammals studied. The
MPOA, the bed nucleus of stria terminals, and
the medial nucleus of the amygdala are
reciprocally connected anatomically, and the
medial nucleus of the amygdala receives direct
projections from the main and accessory
olfactory lobes. An additional input from the
main olfactory lobe is received by afferents
from the cortical nucleus of the amygdala.
Pathways linking the cortiocomedial amygdala
with the bed nucleus of stria terminalis may
convey impulses generated by chemosensory
receptors of the olfactory systems promoting
sexual arousal. This applies in various degrees,
to all mammals, including humans. In view of the
major importance of gonadal hormones for sexual
behavior, it should be noted that all of these
areas are densely packed with gonadal hormone
receptors. These receptors are closely
associated with enzymatic systems that process
the prehormones to active agents.
THE MONOAMINERGIC PERSPECTIVE
Introduction
Returning to the early 1960s, Arvid Carlsson,
Åke Hillarp, and their students in the
pharmacology department close to our laboratory
in Göteborg were studying another aspect of
brain physiology, namely neurotransmitters.
Dopamine (DA) and 5-hydroxytryptamine (5-HT) had
just been discovered as having functions of
their own as neurotransmitters. The neurons
producing these substances could be visualized
by the histofluorescent methods, then newly
reported. We looked upon the pictures of the rat
brain offered to us, showing skies of neurons,
in deep green, transferring catecholamines, and,
in bright yellow, eurons using
5-hydroxytryptamine. The neurons were longer
than had ever been seen, stretching out between
the brain stem and the forebrain. Naturaturally,
we were eager to know more about the function of
these neuronal systems.
Dopamine
Dopamine (DA) is involved in all aspects of
sexual behavior, including sexual arousal,
copulation, and penile reflexes. Exposing the
male rat to a receptive female, we found a
selective increase in the synthesis of DA in the
nucleus accumbens. Further studies indicated
that this increase is characteristic of sexually
naive rats and does not occur in experienced
ones, suggesting a role of DA in the novelty
aspect of sexual stimulation rather than sexual
activity, as such.
Dopaminergic projections from the substantia
nigra and ventral tegmentum pass to the ventral
striatum and nucleus accumbens, respectively. 43
DA activity is reduced after localized lesions
are produced by treatments with a neurotoxin,
6-hydroxydopamine. Further, systemic treatment
with both DA D2 and mixed DA D1/2 receptor
antagonists in these areas reduces the level of
sexual arousal as assessed by prolonged mount
and intromission latencies and PEIs, without
accompanying alterations of the copulatory
activity.
The administration of a variety of DA
agonists enhances the copulatory activity, and
this effect is reversed by treatment with DA D2
receptor antagonists, as evidenced by a
reduction of the ejaculation latency. The MPOA
is the only site of action identified for the
stimulatory effect of DA in copulatory activity.
Such dopamine receptor agonists as apomorphine,
quinpirole, and lisuride stimulate copulation,
penile erection, and seminal emission; their
effects are reversed by DA receptor antagonists.
The stimulatory effect on penile erection
requires the presence of testosterone and
appears to be mediated postsynaptically. The
stimulation of penile erection originates in the
paraventricular nucleus, because injection of DA
agents in this nucleus induces penile erection
combined with yawning, 56,57 an effect probably
mediated by a release of oxytocin.
Noradrenaline
The noradrenergic system originates in the
locus caeruleus and innervates the entire
forebrain. It stimulates sexual activity
probably in an indirect way but has an
inhibitory role on penile erection. Lesions of
the locus caeruleus, inhibition of noradrenaline
(NA) synthesis, and inhibition of NA release by
2-andrenoceptor agonists are all agents causing
a prolongation of ML, IL, and PEI. 62 Yohimbine,
an 2-adrenoceptor antagonist, has repeatedly
been reported to be effective in stimulating
sexual activity, presumably because the NA cell
bodies in the nucleus caeruleus are under tonic
2-adrenoceptor influence.
5-Hydroxytryptamine
Central brain serotonergic systems were long
considered to inhibit the neural mechanisms
regulating male and female sexual behavior. This
contention was based on (l) the observation that
a decrease in brain serotonin facilitates
ejaculation in rats, as evidenced by a decrease
in the number of intromissions to ejaculation
and a shortening of the ejaculation latency and
(2) the observation that an increase in
availability of synaptic 5-HT inhibited the
behavior, as evidenced by an increased number of
intromissions and a prolonged ejaculation
latency. A behavioral facilitation was produced
by the inhibition of tryptophan hydroxylase by
treatment with p-chlorophenylalanine, selective
destruction of brain serotonergic neurons by
5,7-dihydroxytryptamine, or electrolytic lesions
of serotonergic projections from the raphe
nucleas to the MPOA.
The major sources of serotonergic innervation
of the forebrain are the dorsal (DR) and median
(MR) raphe nuclei. A study was undertaken of the
effects on masculine sexual behavior of lesions
aimed specifically either at the median or the
dorsal raphe nuclei. In parallel experiments we
checked the specificity of the lesions by
examining the 5-HT decrease in target areas. The
MR lesions produced a relatively greater
decrease in septal than in neostriatal 5-HT
content: the DR lesions produced the opposite
pattern. The MR lesions caused a marked
facilitation of sexual behavior, as evidenced by
a decrease in the number of intromissions to
ejaculation and a shortening of the ejaculation
latency and of the postejaculatory intervals. No
changes in the mating behavior were observed
after DR lesions. These results receive further
support from pharmacological studies.
In our efforts to characterize the role of
the serotonergic transmitter systems in the
regulation of sexual behavior, we were given
access to a new substance,
8-OH-2-(di-n-propylamino)tetralin (8-OH-DPAT).
This substance was developed at the Department
of Pharmacology, University of Göteborg,
and at the Department of Organic Pharmaceutical
Chemistry, University of Uppsala, and was
characterized as a 5-HT receptor agonist. We
expected that this compound would inhibit the
behavior. Instead we found that 8-OH-DPAT
produced a drastic facilitation of the
ejaculation reflex. The number of intromissions
was lowered and the time to ejaculation
shortened. Sometimes the male ejaculated after
one single intromission, resulting in an
ejaculatio precox-like effect.
This highly specialized 5-HT receptor agonist
we had received in our hands was soon shown to
have high affinity to a subtype of receptor
named the 5-HT1A receptor. It had been known
earlier that certain ß-blocking agents,
like propranolol and pindolol, could antagonize
behaviors involving the 5-HT receptor. These
compounds, having selective affinities to 5-HT1A
receptors, antagonized the facilitation of the
ejaculation reflex induced by 8-OH-DPAT.
Recently another, more selective 5-HT1A receptor
blocker, WAY-100635, was found to have the same
effect. These and other observations suggest
that 8-OH-DPAT exerts its dramatic effect upon
male rat sexual behavior by stimulating 5-HT1A
receptors in the brain. Several other compounds
with high affinity for the 5-HT1A receptor,
including buspirone, flesinoxan, and FG 5893,
have later been shown to share the effects of
8-OH-DPAT on ejaculation behavior.
In order to examine a possible site of action
for these effects, we locally applied 8-OH-DPAT
to two sites in the ventral forebrain and also
onto cell bodies of origin in the DR and MR.
Infusion of 8-OH-DPAT in the MR accelerated the
rate of copulation, possibly by stimulating
5-HT1A somatodendritic autoreceptors, thereby
causing an inhibition of neuronal 5-HT activity.
No facilitation was produced by infusing
8-OH-DPAT into the DR. The median raphe
injections decreased 5-HT synthesis in the
nucleus accumbens, the ventromedial striatum,
and the amygdala, as well as in the hippocampus
and the septum. Also the DR injections of
8-OH-DPAT produced a decreased 5-HT synthesis in
the forebrain, but the effects were most
pronounced in the dorsolateral striatum and the
globus pallidus. Summarizing these observations,
we conclude that the MR belongs to a midbrain
neural system, inhibitory to sexual
behavior.
Also experiments with 5-HT clearly
demonstrated region-selective effects. Local
application of 5-HT into the nucleus accumbens
inhibited sexual behavior, whereas local
application into striatal areas ventral or
dorsal to this site produced no effects.The
local application of 5-HT onto serotonergic
somatodendritic autoreceptors in the DR and the
MR facilitated the behavior.
If activation of 5-HT1A receptors mediate a
facilitatory influence on sexual behavior, the
inhibitory influence obtained after treatment
with 5-HTP must be a consequence of stimulation
of other receptors. One of them may be the
5-HT1B receptor. Several pharmacological agents
selectively stimulate this receptor, resulting
in an increase in the number of intromissions
and prolongation of the response latencies. 76
Another class of 5-HT receptors is the 5-HT2
receptor. Treatment with DOI, a selective
5-HT2A/C receptor agonist, inhibits male sexual
behavior, an effect that is blocked by selective
5-HT2/5-HT1C antagonists, like ritanserin and
ketanserin. 77 Unlike the effects produced by
the various 5-HT1-selective receptors, this last
effect does not seem to be an effect specific to
the ejaculation behavior and may not even be
specifically associated with sexual behavior.
This raises the problem of possible differences
between 5-HT2A and 5-HT2C receptors. New and
selective pharmacological tools will probable
soon be available to clarify the role of
different 5-HT2 receptors in male sexual
behavior.
Treatment with 5-HTP presumably results in an
increased release of 5-HT at all serotonergic
synapses. In a series of recent studies, we
asked whether treatment with selective serotonin
antagonists even influences the inhibitory
effects produced by 5-HTP. By antagonizing the
effect of 5-HTP on the 5-HT1A, receptor we would
receive a potentiation of the inhibitory
influence produced by 5-HTP. Male rats were
injected with 5-HTP combined with benserazide,
and thereafter treated with either WAY-100635
(5-HT1A receptor antagonist), isamoltane (5-HT1B
receptor antagonist), or ritanserine (5-HT2A/C
receptor antagonist). We found that WAY-100635
potentiated the effects of 5-HTP, whereas
isamoltane blocked these effects. The
ejaculation pattern remained unaffected by
ritanserin. These results support the hypothesis
that 5-HTP stimulates as well 5-HT1A as 5-HT1B
receptors, the net effect of 5-HTP representing
a balance between activation of these two
receptor types.
It is worth noting that the drug effects on
male rat ejaculatory behavior reported here are
very different from their effects on penile
erections. Thus, 8-OH-DPAT, which facilitates
ejaculatory behavior, inhibits penile erections.
Furthermore, the nonselective 5-HT1B receptor
agonist, 1-(3'-chlorophenyl)-piperazine (mCPP),
induces penile erections, whereas the 5-HT1B
receptor agonist, anpirtoline, inhibits
ejaculatory behavior. Finally, stimulation of
5-HT2C receptors (formerly the 5-HT1C receptor)
induces penile erections.
CONCLUDING REMARKS
Let us trace our journey together. In the
beginning, the problem was to find an approach
to behavior that would reasonably well lend
itself to an analysis of underlying biological
mechanisms. Reproductive behavior, turned out to
be eminently suited for this purpose: essential
for the survival of the species, shaped along
with other morphological and physiological
features of the species, yet not of critical
importance for survival of the individual.
Furthermore, it is a behavior dependent upon the
senses and hormonal regulation, intricately
linked to brain functions. Few neuroanatomists
or psychologists in the late 1950s thought about
behavior in these terms. After the first
fumbling attempts to find the brain structures
involved in the control of male rat sexual
behavior, we discovered how little was known of
the neural organization of the brain, not least
of which were those circuits that controlled
reproduction. From a different perspective, this
surely was a challenge for Lennart Heimer to
devote himself to in-depth studies on the neural
organization of the basal forebrain. The new
conceptualization he has brought to this brain
territory has been a great gift to all
investigators in the fields of psychoactive and
emotional behavior, including behavioral
functions related to reproduction. The parallel
discoveries of monoaminergic neurons connecting
mesencephalic and lower brain stem structures
with all other parts of the nervous system was
revolutionary. Coupling these insights with the
wiring and neuroanatomical delineation of
structures in the basal forebrain not only shed
new light on relations between brain and
behavior, but also opened the possibility of
exploring brain functions with pharmacological
probes. Anatomy and pharmacology became two
sides of the same coin.
