Neurobiology 303 -- Chapter 25 Outline
Hormones and the Nervous System

neuroendocrine system -- the interface between the nervous and endocrine
   systems, in which the two systems interact
    this interaction is mediated in part by direct connections of neurons
       onto endocrine cells
    however, the main interaction between the nervous and endocrine
       systems is mediated by molecules that serve both as
       neurotransmitters and as hormones
       some molecules that play both roles are given below:
                adrenaline
                oxytocin
                vasopressin
                dopamine

insect metamorphosis provides an excellent example of how hormones can
   affect the structure and function of the nervous system
    metamorphosis is the transformation of an immature insect from a
       larva to a pupa to an adult
    these changes are accompanied by dramatic changes in neural
       structure and function, as well as by the more familiar changes
       in shape and appearance of the animal
    two hormones especially important in metamorphosis are:
                ecdysone -- a steroid hormone
                eclosion hormone (EH) -- a peptide hormone
    steroid hormones are lipid soluble so they pass right through the cell
       membrane directly into the nucleus, but peptide hormones are
       water soluble and must interact with cell-surface receptors
    an environmental cue stimulates release of a trigger hormone that in
       turn causes ecdysone release -- ecdysone in turn directs
       metamorphosis and primes the insect to respond to EH
    another cue (like daybreak) stimulates release of EH into brain and
       blood from specialized neurosecretory cells -- EH in turn
       initiates molting

hormonal effects on insect neural structure during metamorphosis have been
   studied extensively in the tobacco hornworm, Manduca sexta
    three mechanisms of neural reorganization can be identified during
       metamorphosis:
             structural change in existing neurons
             death of larval neurons
             growth of new neurons

structural change is well illustrated by motoneuron MN-1
    in the larva, MN-1 innervates abdominal muscles on one side
       that causes the body to flex on that side -- MN-1 in the
       larva has one dendritic arbor contralateral to the soma
    in the adult, MN-1 innervates muscles on both sides that flex
       the abdomen dorsoventrally -- MN-1 in the adult has two
       dendritic arbors, one on each side
    the presence of ecdysone stimulates the growth of the second
       dendritic arbor in Manduca

other, selected neurons undergo programmed cell death during
   metamorphosis, and they die in a specific order
    for example, the selected interneurons die before the selected
       motoneurons, and the smaller selected cells die first
    programmed cell death is triggered in part by a sharp drop in
       ecdysone levels after a rise in ecdysone to a high level

still other neurons, left in a partially undifferentiated state in the larva,
   become fully differentiated and functional in the adult
    hormonal effects on insect neural function have been studied in the silkworm
       by James Truman
    in the molt from pupa to adult silkworms, EH has two functions
             activate eclosion behavior
             trigger the switch from pupa to adult behavior by turning off the
                former and turning on the latter
    pharate moth -- an adult moth that has not yet emerged from its pupal
       shell but is ready to do so
    EH causes the adult moth to wriggle free of its pupal shell, thus
       activating eclosion behavior in the pharate moth
    eclosion behavior, triggered by EH, is necessary not only to free the
       moth from the pupal shell, but to cause the moth to change its
       behavior from that of a pupa to that of an adult
    a pharate moth that has had its pupal shell peeled off experimentally
       will not exhibit adult behavior until release of EH has triggered
       eclosion behavior
    EH can produce eclosion behavior in a brainless moth, and can
       produce fictive eclosion behavior in an isolated nervous system
    note -- in all cases EH will not be effective before the moth has been
       prepared by ecdysone to respond to EH

molecular mechanisms of action of ecdysone
    as a steroid, ecdysone passes right through the membranes of cells and
       into the cell nucleus
    in the nucleus, ecdysone binds to ecdysone receptor proteins (EcR)
    the ecdysone/EcR complex then binds with a third protein called
       the ultraspiracle receptor for ecdysone (USP)
    next the ecdysone/EcR/USP complex binds with ecdysteroid response
      elements on the genome and promotes gene transcription
    the first genes to be activated, called early response genes, code for
       proteins that are themselves gene regulatory factors
    these proteins, alone or in combination with hormones, then activate
       late-response genes that code for the proteins that actually cause
       structural changes, differentiation, or programmed cell death
    late-response genes may also code for receptors for eclosion hormone

molecular mechanisms of action of eclosion hormone (EH)
    binding of EH with its receptor activates a second messenger cascade
       that causes an increase in the levels of cyclic guanosine
       monophosphate (cGMP)
    cGMP is a second-messenger trigger for eclosion -- injection of
       cGMP into a pharate moth triggers eclosion behavior just as
       readily as injection of EH itself does
    only four neurosecretory neurons actually produce EH and release it
       into the blood
    this causes a small increase in cGMP in many cells, which in turn
       leads, via some synaptic network mechanism, to the production
       of very high levels of cGMP in some 50 neurons
    the specific role of cGMP is to promote the synthesis and
       phosphorylation of certain proteins

juvenile hormone is also important for the transition to adulthood in insects
    juvenile hormone is a sesquiterpenoid
    juvenile hormone regulates the onset of behavior associated with
       sexual maturity
    it also turns on phonotaxis in female crickets toward the calls of male
       conspecifics
    juvenile hormone turns on phonotaxis by making the L1 pair of
       auditory interneurons more sensitive
    it does this by activating the transcription of genes that ultimately
       leads to the synthesis of more ACh receptors on L1 neurons

one recent technique that localizes peptides or small proteins in nervous
   tissue is called in situ hybridization
    in situ hybridization allows a researcher to localize a particular
       mRNA, such as that which codes for some peptide of interest
    cells containing that mRNA must synthesize the peptide of interest
    the technique works by synthesizing a strand of cRNA that is
       complementary to the mRNA, and tagging that cRNA with
       some radioactive or cytochemical marker
    the tagged cRNA is then placed on the target tissue so it can hybridize
       with its complementary mRNA
    after excess cRNA is washed away, precise localization of cells that
       synthesize the peptide or small protein of interest can be
       accomplished by localizing the marker on the remaining cRNA

sexual dimorphism and the vertebrate brain
    sexual dimorphism -- the behavioral and structural differences
       between the sexes
    behavioral differences between male and female animals are due to
       structural and functional differences in their brains
    these differences are brought about through the actions of sex
       hormones during development and in adult animals

steroids and mammalian sexual dimorphism
    the sizes of certain regions of the brain actually depend upon the sex
       of the animal
    some sexually dimorphic regions in rat brain are:
       sexually dimorphic nucleus of the preoptic area (SDN-POA) --
          two times larger in males than in females (i.e. has twice
          the number of neurons), it is necessary for expression of
          mounting behavior
       spinal nucleus of the bulbocavernosus (SNB) -- larger in males
          than in females, contains motoneurons of penile muscles
       anteroventral periventricular nucleus (APN) -- larger in females
          than in males, APN neurons secrete oxytocin, a hormone
          important in stimulating maternal behavior
    other mammals show similar patterns of sexual brain dimorphism

sexual dimorphism in males is dependent upon testosterone
    for example, injection of enough testosterone into a developing
       female rat can produce an adult female with an SDN-POA as
       large as that found in a normal adult male

there is a critical period in rat development, from a few days before
   birth to a week after birth, during which sex hormones can
   influence the formation of sexually dimorphic structures --
    hormones have lesser effects at other times

these sexually dimorphic regions are necessary for sexual behavior
    in adult males, destruction of the medial preoptic area eliminates
       copulation -- the males will still approach receptive females but
       they just won't mount and copulate
    similarly, destruction of specific areas of the brain eliminates sexual
       behavior in adult females
    a clear sign of sexual behavior in females is lordosis, a posture in
       which the hindquarters are raised to receive a mounting male

steroid effects on lordotic behavior in female rats
    once developed, sexually dimorphic regions of the brain can be
       activated by sex hormones to produce sexual behavior
    estrogens bind to many regions of the brain in female rats that are
       involved in sexual behavior
    the lordosis reflex in females is initiated by stimulation of Ruffini
       (touch) receptors near the hindquarters -- the sensory afferents
       relay the signal to interneurons, which in turn activate the
       motoneurons that elevate the hindquarters
    the lordosis reflex can only be initiated in the presence of the female
       sex hormones (especially estrogen but also progesterone)
    injection of estrogen and progesterone can cause the lordosis reflex to
       be initiated in a previously unresponsive female
    the sex hormones exert their effects by acting on neurons in the
       ventromedial hypothalamic region, causing them to become
       active by first promoting protein synthesis
    note -- the hypothalamic neurons cannot become activated by the
       hormones unless protein synthesis occurs -- for example,
       estrogen induces hypothalamic cells to synthesize protein
       receptors for progesterone
    modulation of the lordosis reflex by sex hormones involves a complex
       neural pathway -- steroids activate neurons in the
       hypothalamus, which activate neurons in the central gray
       matter and midbrain reticular formation, which activate neurons
       in the medullary reticular formation, which project to
       interneurons and motoneurons that control lordosis
    the modulatory pathway acts synergistically with sensory input from
       Ruffini endings to produce lordosis only when touch
       stimulation and estrogen are present together

steroid hormones can exert their effects peripherally as well as centrally
    for example, testosterone causes the penile muscles to enlarge in male
       rats -- the hypertrophied muscles then release a neurotrophic
       factor that causes enlargement of the dendritic fields of the
       spinal motoneurons that innervate them
    a similar mechanism causes the number of fast-twitch muscle fibers in
       the larynx of Xenopus frogs to be greater in males than in
       females -- this allows the males to make a trilling sound with
       rapid modulations that drive the females crazy!!!

steroid regulation of cyclic behavior
    levels of steroid sex hormones typically wax and wane with the
       reproductive cycles of animals
    songbirds have specialized brain regions that control singing:
             high vocal center (HVC)
             robust nucleus of the archistriatum (RA)
             X region
    these regions are lateralized in the birds cerebrum, and are located on
       the left side, as are the language centers in humans
    in birds line canaries and zebra finches, in which only the males sing,
       the song regions are much larger (i.e. have more neurons) and
       are more complex (i.e. have more synaptic interconnections)
       in males than in females
    enlargement of the HVC, RA, and X regions in males is due to
       testosterone -- gonadectomized males will not develop large
       singing regions and will not sing
    treatment with testosterone in gonadectomized animals (male or
       female) produces enlargement of these regions and singing
    singing in some songbirds like canaries is seasonal, and the sizes of
       the song regions increase and decrease as testosterone levels
       wax and wane with the seasonal mating cycle
    interestingly, the enlargement of the song regions in spring involves
       not only the expansion of the dendritic trees of existing
       neurons, but also of the birth of completely new neurons!

vertebrate peptide hormones
    animals actually synthesize and use many more glycoprotein and
       peptide hormones than they do steroids,
    many glycoprotein and peptide hormones play sexual roles
    furthermore, many glycoprotein and peptide hormones also play a role
       as neuromodulators in the brain
    two peptide hormones serve as useful examples
             vasopressin -- acts on the kidney to control the amount of water
                that is reabsorbed from the urine
             oxytocin -- stimulates uterine contraction after birth and elicits
                milk ejection in lactating females
    both vasopressin and oxytocin are synthesized by hypothalamic
       neurons that send their axons into the pituitary, and from there
       release their hormone directly into the blood
    these hormones also have effects on the brain

brain effects of vasopressin and oxytocin
    oxytocin helps to elicit maternal behavior by acting on neurons in the
       medial preoptic area of the hypothalamus (probably by
       inhibiting them, as the medial preoptic area is involved in
       mounting behavior in males)
    vasopressin actually inhibits the lordosis response

molecular mode of action of steroid hormones
    vertebrate steroids like testosterone and estrogen behave in the usual
       steroid fashion, by passing through the cell into the nucleus,
       and there binding with a steroid receptor -- the steroid/receptor
       complex then acts as an transcription factor, initiating synthesis
       of proteins that have various functions as additional
       transcription factors or as enzymes, receptors, channels, etc.
    however, steroids can also exert effects on neurons in under 2
       minutes, which is too fast for a genomic mechanism
    for example, estrogen increases the electrical excitability of specific
       neurons in the amygdala, even if protein synthesis is eliminated
    nongenomic steroid effects may be mediated by steroid receptors in
       the cytoplasm or even on the cell membrane

mode of action of peptide hormones
    peptide hormones are lipid insoluble, so they cannot pass into cells but
       must bind with receptors on the cell membrane
    all receptors for peptide hormones are coupled to specific G proteins
    binding of a peptide hormone to its receptor activates the associated G
       protein which then causes a biochemical cascade that has some
       effect on the neuron and/or its genome
    this mode of action is the same as that of neuromodulatory
       transmitters -- thus neuromodulators and peptide hormones are
       sometimes indistinguishable
    a bewildering array of hormonal receptor types, G proteins, and
       second messenger systems can be found in neurons
    the G proteins act through second messenger systems that include:
             activation of adenylyl cyclase and increase in cAMP
             inhibition of adenylyl cyclase and decrease in cAMP
             activation of the inositol triphosphate (IP3) pathway
                     and others

any hormone can bind with many different receptors
    in all cases, it is the nature and properties of the receptor that will
       determine the precise response that any neuron (or any cell) will
       have to a hormone