To what extent does local metabolism influence the action of

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					To what extent does local metabolism influence the action of steroids on the nervous
system and can this be exploited for drug development (JF).

         Steroids are a number of natural or synthetic substances that regulate the body’s
functions. Neurosteroids are endogenous steroids that accumulate in the brain, even in the
absence of steroidogenic glands (adrenals and gonads), as studied in rats. Neurosteroids
that are metabolised locally in the nervous system have the given advantage of being able
to cross the blood brain barrier quickly, and therefore being able to actively affect the
brain functions through, for example, neurotransmission. Metabolism of steroids in the
periphery may mean that the products of the metabolism are unable to meet their targets
within the CNS. Steroids within the nervous system need to act upon receptors in the
brain, through the local metabolism of steroids by endogenous enzymes synthesized by
surrounding neuronal cells. These enzymes are vital and control the metabolism of
steroids, and therefore control their steroidogenic actions within the nervous system.
         Neurosteroids have a critical role in the maintenance and mediation of many brain
functions. For steroids to be synthesized, specific enzymes are needed. It is well known
that there is extensive steroid metabolism in the brain, therefore meaning that there must
be regions of the brain that are well equipped with the correct enzymes needed for
‘steroid hormone biosynthesis’(1). Steroid hormones have a key role in growth,
development, maturation and differentiation of the brain.
         Local metabolism is really important in Steroidogenesis and steroid transmission
throughout the brain. Steroids are produced within specific tissues and their production is
dependent upon which enzymes are present, and whether these enzymes can metabolism
the precursor steroids. Local metabolism is favoured over systemic metabolism of
steroids, due to the delay of the latter metabolism. If steroids where to be metabolised at
remote areas in the periphery, e.g. liver, kidneys, problems may occur. The metabolites
produced would then have to be transported to other regions of the body. Also these
organs contain vast arrays of enzymes that may further metabolise the precursor to un-
wanted metabolites, which could be excreted due to homeostatic effects. In many cases,
metabolised (neuro-) steroids need to bind to receptors not in the periphery. Local
metabolism is needed to aid in the rapid actions of steroids, as well as maintaining the
ability to cross the brain barrier effectively, and efficiently. Local metabolism also
ensures that the metabolites produced, act upon receptors in the nervous system.
Metabolism is conducted by a series of steroidogenic enzymes. The synthesis of steroids
‘requires the coordinate expression and regulation of genes encoding the steroidogenic
enzymes in several different cell types (neurons and glia) at different locations in the
nervous system’ (2). Therefore localized metabolism can be restricted to cells expressing
the correct genes needed to express the equivalent enzyme; this would therefore confine
the actions of the steroids. Steroid metabolism, and in some ways their actions, can be
said to be determined by the enzymes present and competition between these enzymes in
the specific cell types. Local metabolism is reliant on the enzymes present.
         Most of the steroidogenic enzymes produced in the brain are located in the cortex,
hippocampus, olfactory bulb, basal ganglia, hypothalamus, thalamus, cerebellum, the
expression of some of these enzymes has also been identified in the tectum, pons,
medulla, pituitary and spinal chord, as well as in various regions of the peripheral
nervous system’. (1)

                                     Manpreet Sahni
                                    Neuropharmacology
                                        Essay 2007
        This shows a great importance in regards to rapid action as the steroids are
already within the brain area, where they will act upon various neurotransmitter channels
and ion-gates.
        Steroidogenic enzymes in the brain are not only expressed in different cell types
and areas within the brain, but also there is no tangible pattern of distribution of these
enzymes, in respect to time (development) or region of the brain.
What can be deduced is that the specific steroidogenic enzymes are needed for local
steroid metabolism will be produced by a particular tissue at a particular time through a
developmental and cell-specific expression of the enzyme genes (1).
        There is still further investigation into the ‘co-localization of steroidogenic
enzymes’ (1). The presence of some steroidogenic enzymes in glia cells, whilst others are
present in neuronal cells.
        These enzymes therefore and their presence or absence effect the actions of
steroids on the nervous system through the local metabolism of precursors.

Actions of Steroids: determined through enzymes
Studies de novo(8), have shown that steroids can have an effect on the brain via
‘intracellular receptors that regulate transcriptionally directed changes in protein
synthesis’ (1). These changes usually occur on a time scale of hours to days. These
steroids are usually involved in reproductive and neuroendocrine actions ‘Neurosteroids
usually act through mechanisms such as ion gated transmission (changes excitability of
GABA or NMDA, for example) receptors or through direct or indirect modulation of
other neurotransmitter receptor. These mechanisms of action bare great importance on
enzymes present, localization during development of individual, and the regulation of
their expression’ (1). These actions can occur on a time scale of milliseconds; this is
mainly due to the fast acting binding of the synthesised steroids to the voltage / ion –
gated channels. Steroid can also act on nerve cells through G-protein coupled receptors
interacting with an array of neuropeptide receptors.

        Here are a few enzymes that are essential in local metabolism of precursors and
their actions.
        The 1st, rate limiting step in the synthesis of all steroid hormones is the conversion
of cholesterol to pregnanolone. This reaction is catalyzed by an enzyme P450scc. This is
a mitochondrial enzyme, and acts by cleaving the side chains of the cholesterol
molecules. P450scc has a major regulatory role in the brain. This enzyme is encoded for
by a single gene on chromosome 15, the CYP11A1 gene. The expression of this gene was
further investigated (2). A difference in the degree to which it was expressed in various
steroid tissues was noted, especially in adrenal tissues which had higher levels of
expression.
CYP11A1 mRNA concentrations increases during development from childhood to adult
puberty levels. Findings also showed that the gene concentration was found to be higher
in women’s temporal and frontal neocortex as well as in the hippocampal area than these
areas of the brain of a male. Here a clear link between age and sex gene (CYP11A1)
expression dependency was noted in the human brain.
        Once pregnanolone has been synthesized by this pathway, it can then be further
metabolised by other enzymes present locally. Pregnanolone has been thought to be able

                                       Manpreet Sahni
                                      Neuropharmacology
                                          Essay 2007
to be produced in nervous system tissue, therefore metabolising enzymes must be
produced also within neuronal tissue.
        Pregnanolone enhances GABA- induced currents. It is a potent enhancer for
GABAA receptors; this act has many clinical implications. This action is shared with
benzodiazepines and barbiturates used in epilepsy (animal study evidence suggests
neurosteroids more favourable, due to their better tolerance and few issues with addition
and abuse. This is an example of an inhibitory steroid, which enhances the single
channel currents induced by GABA by increasing the frequency of channel openings (7).
        Drugs that can enhance the levels of pregnanolone in the brain synapse, by
inhibiting its metabolism are invaluable. A drug would have a target against the enzyme,
for example; drug targets against 3 B-HSD.
        Pregnanolone sulphate acts upon NMDA- and GABAA receptor mediated
currents. This is an excitatory steroid, which enhances the NMDA response and
suppressing the inhibition GABA, causing increased risk of convulsions and anxiety.
        In cases of depression there were indications of low levels of neurosteroids
(especially allopregnenolone) found in the plasma and cerebrospinal fluid, compared to
non-depression patients. The levels of neurosteroids were increased in patients by
administering them with SSRI’s (Selective Serotonin Re-uptake Inhibitors) (9) which act
locally, preventing serotonin metabolism. A hypothesis was put forward about SSRI’s
and their direct effects of increasing steroidogenic enzyme activity, and not through the
up-regulation of their genes.

Enzyme 3 Beta – HSD
       The main conversion is; pregnanolone (pre-steroid) into progesterone.
Progesterone is a well known steroid involved in the development of secondary sexual
characteristics. Progesterone and some of its metabolites have been described as having
sedative and anaesthetic properties at high concentrations in the brain, which has clinical
applications in anaesthesia.

Aromatase:
        This is the main steroidogenic enzyme that catalyses the conversion of
testosterone into estradiol (androgens to estrogens) in frontal and temporal areas of the
brain. This enzyme is encoded for by the gene CYP19. Aromatase activity was
investigated in few foetal brains. Activity of this steroid in a human brain was a lot lower
than in placental samples. The activity in the human brain was coherent with the activity
in other testosterone-containing tissues (e.g. adipose tissue and testicular tissues). This
enzyme evokes more male characteristics.

5 alpha – Reductase:
       5 a reductase- reduces its substrate progesterone to its ‘5 a reduced derivative’ (1),
5 a – Dihydro-progesterone (DHP) is a molecule of great interest in drug development.
This molecule can be further metabolized to a more potent steroid 3alpha, 5alpha -
tetrahydroprogesterone (3a, 5a – THP) (2).
In humans two isozymes of 5 a – reductase have been discovered. There is a predominant
expression of type 1 mRNA of this enzyme in patients suffering from chronic temporal
lobe epilepsy. Areas of the brain that have been highlighted as having high expression

                                      Manpreet Sahni
                                     Neuropharmacology
                                         Essay 2007
are; human temporal neocortex, subcortical white matter and hippocampal tissue (5, 6).
Little is known about the type2 enzyme and in human liver cells no type2 reductase is
expressed.
         In rat studies it was shown that 5 a – Reductase type1 was expressed in the brain
throughout development and into adulthood, with a slight increase at birth. Type2 studies
show only an expression late foetal and early postnatal development. There are
hypotheses detailing the modulation of type2 expression due to increased levels of
androgens in male life in early development. This has implications for sexual
differentiation of the brain. This enzymatic conversion within particular tissues of the
brain causes more male like characteristics.
         Drugs that maintain the ability to imitate the action of this enzyme are very
important clinically. Being able to convert progesterone to a less potent analogue is vital
in the brain, due to progesterone’s potent sedative and anaesthetic properties. Therefore, a
high concentration of this steroid in the brain is an un-wanted effect.

11 Beta – HSD:
       This is an enzyme that catalyses the formation of glucocorticoids in the nervous
system, through the local metabolism of cortisol to 11- dehydrocorticol.

          In the context to drug development, there are many areas in which local steroid
metabolism can be enhanced or delayed through enzyme alteration.
          The potencies of neuroactive steroid have been investigated for their clinical uses.
Through ‘biochemical and electrophysiological investigations’ (1), known steroids have
correlated data showing their sedative, anticonvulsant (7) and anxiolytics properties in
vivo. Alphaxalone is a synthetic, potent anaesthetic steroid. This compound is
progesterone/ pregnenolone derived, in 1941; Selye described the potential of
progesterone as an anaesthetic (1). It is quite a safe anaesthetic, as it is a steroid it easily
passes through the blood brain barrier to target receptors. Its main down- fall is its half
life. It has a long wear off period, metabolism of the steroid needs to be quicker.
          There are many synthetic glucocorticoids used clinically. Dexamethasone is a
synthetic steroid developed to utilise the ability of passing to the brain receptor targets
and further being metabolised by endogenous enzymes. This steroid is able to circulate
within the CNS. Its main use is to reduce brain inflammation. This steroid is preferred
over alternatives, here the ability for the drug steroid to be metabolised locally within the
CNS is essential. Prednisolone is also a synthetic steroid used clinically, but is
metabolised systemically, therefore influencing the action of the steroid.
          The detailed pathways of metabolism identified in rats are a great place for drug
development. Local metabolism of steroids is of the highest importance. Enzymes are
only synthesised in specific areas, of specific tissues. Steroids are under the control of
these enzymes; their actions are dictated by the enzymes. The enzymes direct ‘steroid
hormone biosynthesis’, through local production of enzymes that are able to act upon
steroids found within the nervous system, thereby commanding the steroids action upon
receptors.
          Local metabolism clearly effects the actions of steroids in the nervous system.
Steroids are metabolized locally by specific steroidogenic enzymes, which are
synthesised by particular tissues. If the specific enzyme needed to metabolise a steroid is

                                        Manpreet Sahni
                                       Neuropharmacology
                                           Essay 2007
not present, then no action can occur, thereby creating a build up of that particular
steroid. This can be advantageous in some cases. For examples with build up of
pregnanolone, a potent enhancer of GABAA R’s, in patients presenting with anxiety and
convulsions, this would be a positive action, reducing the symptoms displayed. But in
other cases, the absence of an enzyme can cause further problems. For example; if the 5
alpha- reductase enzyme was not synthesised in a neuronal cell containing higher
concentrations of progesterone, a steroid known to cause fatigue if present in excess in
the brain.




                                     Manpreet Sahni
                                    Neuropharmacology
                                        Essay 2007
References:

   (1) Neurosteroid Biosynthesis in the Human Brain and its Clinical Implications.
       Birgit Stoffel- Wagner. Ann. N. Y. Acad. Sci. 1007: 64-78 (2003).
       Neurosteroid metabolism in the human brain. Birgit Stoffel- Wagner. European
       Journal of Endocrinology (2001) 145 669 – 679.

   (2) Biosynthesis and Actions of Neurosteroids.
       Synthia H. Mellon, Lisa D. Griffin, Nathalie A. Compagnone. Brain Research
       Reviews 37 (2001) 3-12.

   (3) C. A. Frye, T. A. Reed, Androgenic Steroids; anti-seizure effects in an animal
       model of epilepsy. Psychoneuroendocrinology 23 (1998) 385-399.

   (4) Neurosteroids: from definition and biochemistry to physiopathologic function. In
       Neurosteroids. A new regulatory function in the nervous system. Robel, P,
       Schumacher M and Bailieu EE. Edition 1. Chapt 1, pg 1-25 Eds. E.E. Bailieu et
       al.

   (5) Expression of 5 alpha- reductase in the human temporal lobe of children and
       adults. B. Stoffel Wagner, M. Watcka et al. Journal of Clinical Endocrinology and
       Metabolism. (1998) 83 : 3636-3642

   (6) Expression of 5 alpha reductase and 3 alpha hydroxysteroid oxidorereductase in
       the hippocampus of patients with chronic temporal lobe epilepsy. B. Stoffel-
       Wagner, Beyenburg et al. Epilepsia 2000 41 140-147.

   (7) Anticonvulsant activity of neurosteroids; correlation with gamma- aminobutyric
       acid-evoked chloride current potentiation. T.G Kokate, B.E Svensson and M.A.
       Rogawski. Journal of Pharmacology and Experimental Therapeutics (1994) 270:
       1223-1229

   (8) Drug discrimination analysis of endogenous neuroactive steroid in rats. N.A Ator,
       K.A. Grant, et al. European Journal of Pharmacology (1993) 241: 237-243.

   (9) Selective Serotonin Re-uptake inhibitors directly alter activity of
       neurosteroidogenic enzymes. L.D Griffin, S.H. Mellon. Proc. Natl. Acad. Sci.
       USA 69 (1996) : 13512- 13517

   (10)       Biosynthesis and organizing action of neurosteroids in the developing
       Purkinje cell. Tsutsui K. Cerebellum. 2006; 5 (2): 89-96

   (11)      The Biochemical Basis of Neuropharmacology. Jack R. Copper, Floyd E.
       Bloom and Robert H. Roth. Seventh Edition, pg 156.

   (12)       Neuropharmacology Lecture Notes 2006/2007. Dr. J. Fry – Neurosteroids.

                                    Manpreet Sahni
                                   Neuropharmacology
                                       Essay 2007

				
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