Rodent Testosterone 5-HT Interaction

Cross references:    Hormones in General   Steroids
 
Receptors in General  Receptor Evolution  Receptors Evolution Timeline  Intracellular Receptors   Testosterone Receptor  Testosterone Transcription Factor  Serotonin Gate (5-HT3R)   Serotonin Metabotropic Receptor  Serotonin Receptor Evolution 
Rodent 5-HT1R   Rodent 5-HT3R    SERT    Rodent Hormone-Nerve Interaction   
Human Serotonin  Reverse Engineering Amphioxus Serotonin    Serotonin References       



FINAL SUMMARY
OF FIVE REFERENCES
1998 - 2003


    These references specify three serotonin receptors that seem to be downregulated by testosterone.  The three are: 5-HT1AR, 5-HT2AR and 5-HT3R.      



INITIAL SUMMARIES
OF THE REFERENCES


Following are initial summaries of the references.  The references are designated by the first words of their title and their year of publication.   

The references themselves follow this list of their initial summaries. 

5HT1A-receptors ... 1998 
    "
The fact that both, 5HT1A-receptors and certain behaviors can be renormalized by the sex steroid ... ."  
    "
It would seem reasonable that the lower number of 5-HT1A receptors would indicate a reduced 5-HT turnover at 5-HT1ARs, but, at the same time, there may be an increased 5-HT turnover at other 5-HTRs which more than compensates for the lower turnover at the 5-HT1ARs."  

Ref: 90<161
Functional Antagonism ... 1998   
    "
Within the central nervous system, the 5-HT3 receptor is predominantly expressed in neurons in the area postrema and the mesolimbic system (3, 4), which receives a major dopaminergic input from the ventral tegmental area."  
    "
Although not explicitly stated, I get the impression that their use of the term 'allosterically' is intended to convey that the interaction is non-genomic, i.e., it does not involve the transcription of DNA in the genome to produce mRNA which is then translated into protein. "  

Ref: 77<161
Inhibition by steroids ... 1999   
    "
This is fairly technical, but, in plain English, steroids inhibited the effect of serotonin at the 5-HT3 receptor."

Ref: 76<161
Androgen actions ... 1999 
    "
Castration decreased while testosterone ... increased significantly the content of 5-HT(2A)R mRNA and SERT mRNA in the dorsal raphe nucleus"      

Neuroactive steroids ... 2003   
    "Steroids influence neuronal function through binding to cognate intracellular receptors which may act as transcription factors in the regulation of gene expression. In addition, certain so-called neuroactive steroids modulate ligand-gated ion channels via non-genomic mechanisms."    
    "
They seem to be using the term "neuroactive" to mean non-genomic."    

   



THE REFERENCES


5HT1A-receptors and behaviour under chronic stress: selective counteraction by testosterone (Goog)  - 1998   
Only abstract available online.  I got the PDF through the library
Unfortunately, I'm only able to copy-and-paste from the online abstract, not the PDF.  This is unfortunate because transcribing is much slower and laborious than copying. 
from the abstract   

    "Behaviour of chronically stressed male tree shrews is characterized by a reduction in scent marking, self-grooming and overall locomotor activity. It has been proposed that this subordination behaviour is related to the down-regulation of 5HT1A-receptors occurring in distinct brain regions of the animals. The high cortisol concentrations which accompany chronic stress are supposed to induce 5HT1A-receptor down-regulation. Because chronic stress in males also decreases androgen levels we investigated whether behaviour and 5HT1A-receptor expression could be renormalized by testosterone replacement.
    "
These data indicate that a balance between glucocorticoids and androgens is necessary to maintain ‘normal' numbers of the monoamine receptors. The fact that both, 5HT1A-receptors and certain behaviours can be renormalized by the sex steroid supports the view that 5HT1A-receptor are involved in the regulation of stress behaviour. However, the fact that overall locomotor activity was not returned to baseline indicates that different types of behaviour are distinctly regulated.

My comments
1.  The table at  
Serotonin Metabotropic Receptor  shows that 5-HT1R is inhibitory, not excitatory.  Therefore, a first approximation interpretation would be that downregulation of 5-HT1R should be stimulatory, not inhibitory. 
2.  If, as proposed elsewhere in this paper, testosterone down regulates 5-HT, then the low testosterone levels observed should correlate with increased, not decreased, 5-HT activity.    
3.  I need to take another look at this paper to see on what basis they concluded that 5-HT1R activity was low when cortisol was high and testosterone was low.  It may be they were mistaken in their estimation of 5-HT1R activity.   
Further comments while rereading the paper
1. 
"Brain 5HT1A-receptors were quantified by in vitro receptor autoradiography."
  This seems reasonable, but it is different from the later methodology which measured 5-hydroxyindolacetic acid (5-HIAA) in the brain and cerebrospinal fluid.  The radiographic method measures the number of receptors of a specific type, while the 5-HIAA method measures the total turnover of 5-HT turnover at all receptors.  It would seem reasonable that the lower number of 5-HT1A receptors would indicate a reduced 5-HT turnover at 5-HT1ARs, but, at the same time, there may be an increased 5-HT turnover at other 5-HTRs which more than compensates for the lower turnover at the 5-HT1ARs.  
2.  The table at: 
    Serotonin Receptor Subtypes (Wiki)     
http://en.wikipedia.org/wiki/5-HT_receptor#Subtypes  
     lists 12, or possibly 13, different 5-HT receptor subtypes.  Four, or possibly 5, of these are members of the 5-HT1R family.  There are 3 members of the 5-HT2R family, and one each in the 5-Ht3R, 5-HT4R, 5-HT5R, 5-HT6R and 5-HT7R families. 
So there are plenty of other receptors where the activity of 5-HT may be increased even if it's decreased at 5-HT1AR.     


S
earching PubMed for "testosterone serotonin receptor" yielded first 161, then a few days later, 162 hits. 

Ref: 90<161
Functional Antagonism of Gonadal Steroids at the 5-Hydroxytryptamine Type 3 Receptor (PubMed)  - 1998   
Full length HTML and PDF available online for free. 
    "
Serotonin (5-hydroxytryptamine, 5-HT) is a major neurotransmitter within the central nervous system that acts through multiple receptor subtypes (1). The majority of these receptors are coupled to G proteins and mediate slow modulatory responses via second messenger signaling pathways (1).
    Only the 5-HT3 receptor constitutes a ligand-gated ion channel (2) that shares structural features with -aminobutyric acid type A (GABAA), glycine, and nicotinic acetylcholine receptors (2). Within the central nervous system, the 5-HT3 receptor is predominantly expressed in neurons in the area postrema and the mesolimbic system (3, 4), which receives a major dopaminergic input from the ventral tegmental area.
"
    "
Steroid hormone action involves binding to cognate intracellular receptors that, in turn, bind to respective response elements and thus modulate gene expression. The present study shows that the gonadal steroids, 17ß-estradiol and progesterone, may also act as functional antagonists at the 5-hydroxytryptamine type 3 (5-HT3) receptor in whole-cell voltage-clamp recordings of HEK 293 (human embryonic kidney
) cells stably expressing the 5-HT3 receptor.
    Functional antagonistic properties at this ligand-gated ion channel could also be shown for 17-estradiol, 17-ethinyl-17ß-estradiol, mestranol, R 5020, testosterone, and allopregnanolone ...
   The antagonistic action of steroids at the 5-HT3 receptor is not mediated via the serotonin binding site

    "
... gonadal steroids probably interact allosterically (see below) with the 5-HT3 receptor at the receptor-membrane interface

    Allosteric regulation (Wiki)   
http://en.wikipedia.org/wiki/Allosteric_regulation 
My comments
1.  It may not make any difference, but HEK 293 don't normally express 5-HT3R.  These cells "
had been stably transfected with an expression vector for the 5-HT3 receptor (5-HT3R-A) (2, 15)

2.  Although not explicitly stated, I get the impression that their use of the term 'allosterically' is intended to convey that the interaction is non-genomic, i.e., it does not involve the transcription of DNA in the genome to produce mRNA which is then translated into protein. 


Ref: 77<161
Inhibition by steroids of [14C]-guanidinium flux through the voltage-gated sodium channel and the cation channel of the 5-HT3 receptor of N1E-115 neuroblastoma cells.  (PubMed)  - 1999      
http://www.ncbi.nlm.nih.gov/pubmed/10543423   
Abstract only available online.  I got the PDF from the library.   

    "Effects of some naturally occurring steroids and synthetic analogues on the cation flux through the cation channel of the 5-HT3 receptor and the voltage-gated and tetrodotoxin-sensitive sodium channel were studied in N1E-115 mouse neuroblastoma cells by measuring the 2-min influx of the organic cation [14C]-guanidinium. The cation fluxes in intact cells were either induced by 2 min exposure of the cells to 5-hydroxytryptamine (5-HT, 100microM) or to veratridine (1 mM). Influx of [14C]-guanidinium through both channels was concentration-dependently inhibited by all compounds studied.
    "The rank order of potency for inhibition of the 5-HT3 receptor-induced cation flux was clomiphene approximately/= cyproterone acetate > estradiol > progesterone approximately/= allotetrahydrodeoxycorticosterone > alfaxalone approximately/= testosterone > aldosterone > dexamethasone.
"  
    "
The potencies of the steroids (and the synthetic analogues) for inhibition of the 5-HT3 receptor-induced [14C]-guanidinium influx were correlated with their lipophilicity (log P values). ... The relationship between most of the compounds in inhibiting both cation channels and lipophilicity is compatible with a common mechanistic principle in steroid-induced inhibition of the two channels, i.e. a nonspecific hydrophobic interaction with certain membrane lipids in the neighbourhood of the two channels.
"   
My comments:  
1.  This would imply that the inhibition of cation flux through the 5-HT3R channel by testosterone is non-genomic.  Unfortunately, the abstract doesn't give the time lag for the inhibition.  A short time lag would confirm that the effect is non-genomic while a long time lag would suggest a genomic effect. 
2. 
This is fairly technical, but, in plain English, steroids inhibited the effect of serotonin at the 5-HT3 receptor.  It also inhibited a voltage-gated ion channel with which I am unfamiliar. 
3. 
If the reduction of the sensitivity of serotonin receptors by steroids results in greater overall activity, as assumed in my paper Boys without Fathers , then the neurons bearing the 5-HT3 receptors must have an inhibitory effect on overall behavior even though the 5-HT3 receptors themselves are excitatory.  The simplest possibility would be for the neurons bearing the 5-HT3 receptors to be GABAergic, i.e. to be inhibitory and have a direct inhibitory effect on behavior.  If the neurons bearing the 5-HT3 receptors are excitatory but provide input to GABA interneurons, the end effect would again be inhibitory.  More extended chains of consequences are also possible.  
 

Ref: 76<161
Androgen actions on central serotonin neurotransmission: relevance for mood, mental state and memory  (PubMed)  - 1999   
Only abstract available online.   
    "
Castration decreased while testosterone ... increased significantly the content of 5-HT(2A)R mRNA and
SERT mRNA in the dorsal raphe nucleus
My comment #1
Although 5-HT2R is excitatory according to the table in  
Serotonin Metabotropic Receptor and testosterone appears from this to increase 5-HT2R induced excitation, this does not really qualify as "testosterone interferes with serotonin's inhibitory effect" as stated in the paper and for which this page is given as a reference. 
My comment #2
How do they measure "
the content of 5-HT(2A)R mRNA and SERT mRNA"?   Answer
Much to my astonishment, Google came up with the answer immediately.  See: 
Genomic Analysis   


Ref: 64<162 
Aggression: The Testosterone-Serotonin Link (PubMed) 
    Please see: 
Human Testosterone + Serotonin      


I apparently stoped searching PubMed for "testosterone serotonin receptor" at Ref: 64<162.  It might be worth my while to complete the search.   


Neuroactive steroids: mechanisms of action and neuropsychopharmacological properties (PubMed)  - 2003   
Only abstract available online. 
    "
Steroids influence neuronal function through binding to cognate intracellular receptors which may act as transcription factors in the regulation of gene expression. In addition, certain so-called neuroactive steroids modulate ligand-gated ion channels via non-genomic mechanisms.  ... classical steroid hormones such as 17beta-estradiol, testosterone and progesterone are neuroactive steroids because they may act as functional antagonists at the 5-hydroxytryptamine type 3 (5-HT(3)) receptor, a ligand-gated ion channel

My comment
They seem to be using the term "neuroactive" to mean non-genomic.   










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