On Sun. 8 Feb 2004 00:05:30 + 0000 (UTC) r norman wrote:
On Sat. 7 Feb 2004 0:52:56 + 000 (UTC), cncabej{at}aol.com (CNCabej) wrote:
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>>As  we well know, pineal cells do not receive the stimulus (day-night
cycles); they are invariably in darkness. That stimulus is received
by retinal neurons (they, nevertheless, do not produce 
melatonin, which code it in the form ofspecific electrical signals 
(this is the information on the nature of the stimulus). But this 
information is no "intelligible" to genes. This is why it is once more
processed  in the complex melatonin circuit, which releases a chemical signal
that via signal transduction pathways affects  expression of genes responsible
for melatonin synthesis in pineal cells<<
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>>It is not the external stimulus per se , but the chemical output
(=information) generated by processing of the stimulusin the neural circuit
that via the respective signal transduction pathways reaches the genes
responsible for the synthesis of melatonin, which is intelligible to (i.e.
contains informatio for) activating those genes. This information is
processing-dependent, hence epigenetic; it does not exist but is
computationally generated i response to external/internal stimuli. 

Let me try an analogy from linguistics (my hobby). When I spell Messapian (an
ancient indoeuropean language spoken in southern Italy more than 2 thoussand
years ago) the word "bila" it conveys no information to you as a receiver,
although it did for the Messapians. Now, I make it inteligible to you by
translating it into the  English "daughter". Similarly, the neural circuits
"translate" the external stimulus, which is "senseless"
to genes, into a
specific epigenetic information for their activation/inactivation. 
Do you agree?<<
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r n:

>I still think that "information translation" is not a proper
way of looking at
the situation.
Light entering the eye causes genes to be expressed in specific cells. There
are a lot of intermediate steps, but there is a demonstrable chain-and-effect
events that can be described linking stimulus to response. It is not necessary
to talk in terms of information generation, merely in terms of "event A causes
event B causes event C causes event D causes... causes eventZ. Light in a rod
or cone cell is translatedinto electrical potential, but that involves  a whole
series of intermediate steps. Changes in electrical potential inrod and cone
cells in the retina cause release of synaptic transmitter onto melatonin
secreting cells in the pineal, but that involves a whole series of intermediate
steps. Synaptic transmitter binding to receptors on melatonin secreting cells
in the pineal activate genes in the cell nucleus, but that involves a whole
series of intermediate steps.

The problem is one of cell physiology, not of information translation.<

If I got it right, you argue that the fact that "a whole series of intermediate
steps" are involved in the signal cascade from the external stimulus to genes,
this fact per se, excludes the possibility of any information being transmitted
via the cascade. Without the necessary explanation, this idea seems hardly
defensible to me.

Applying the same criterium to the process of gene expression, which also
involves many "intermediate steps", would lead us to the conclusion that no
genetic information is transmitted from genes to proteins, which is no less
difficult to defend.

You correctly describe those pathways as chains of events where "event A causes
event B causes event C causes event D causes....causes event Z." where the
effect of each element on the downstream element (secretion of hypothalamic
TRH, e.g. stimulates the  pituitary TSH, stimulates thyroid hormone, etc.) is
determined by stereochemical and thermodynamical properties of the interacting
molecules and of the environment. 

What takes place in the case of melatonin synthesis is different in an
essential respect. The external stimulus per se is neutral to any known pathway
in metazoans, it can't activate any specific gene. In order that it serve as a
cue, the input on the stimulus must first be processed in the retinal neurons
(a computational process), which codes it in the form of a specific pattern of
electrical signals. But the "electrically coded" stimulus is
nothing less than
information on the stimulus. It is different from the stimulus itself in the
same way that my name identifies me but is not identical with me. 

The information generated by processing the stimulus is not determined by the
stimulus itself but by computational properties in which the stimulus is
processed. That information is not stimulus-dependent but processing-dependent.


The information contained in the electrical code is still
"unintelligible" to
genes in the meaning that it can't activate the signal transduction pathway for
expression of melatonin genes. It must be further processed in the described
complex neural circuit, which ultimately transforms it in a chemical signal
that can activate that signal transduction pathway in all the "intermediate
steps" you talk of. This ultimate chemical form that makes possible  the
expression of melatonin genes, is different from the stimulus itself; it is
information on that stimulus that is generated (not preexisting) in the neural
circuits. No melatonin genes can be expressed without this information. 

This processing of stimuli in the CNS is necessary not only for external
stimuli, but for most of the internal stimuli, since because of the blood-brain
barrier most of the chemical signals (protein hormones, growth factors and
other secreted proteins) have no access to the CNS. It makes it possible for
them than in response to the same internal stimuli to activate genes that are
not expressed in other types of cells.
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