HORMONES BEFORE NERVES?

HORMONES TO NERVES?

Today, both interact in regulating functions of cells and organs, but which came first?

In many cases they interact to accomplish their job.  The same thing was probably true during the evolution that brought them to the way they work.  Hormones by definition operate on cells beyond the cells where they are produced.  But so do nerve cells.  The distinction is that hormones are chemical substances produced by cells but are transported to targets by the general circulation of the blood, with an exception for certain pituitary hormones noted below; neurosecretions are produced in nerve cells and transported along the axon to close proximity to the target cell.

HORMONES

Two methods are known by which hormones operate. 

In both methods the hormone is delivered to the target cell by the circulatory system and diffusion to the cell surface.  Peptide type hormones typically do not enter the cell.  They bind to a receptor that extends through the outer cell membrane and cause the inner side of the receptor to function as an enzyme that converts adenosine triphosphate (ATP) into cyclic adenosine monophosphate (cAMP).  The cAMP then activates the cell to do the function development had programmed it to do; thus the same hormone can perform different functions if particular cells with the receptors have different functions.

Steroid hormones typically have a different method of functioning.  They pass into the cell and attach to specific locations on a chromosome’s DNA.  The gene at that site is activated to produce the RNA that leaves the nucleus to interact in producing the product the gene specified.

NERVE CELLS

Nerves cells can control action of cells remote from the nerve cell body.  A long process, the axon, extends from the cell body to the target cell.  Proper stimulation of the cell body causes a transient depolarization of the membrane of the axon to rapidly progress to a termination closely applied to the target cell where neurotransmitters are released at the gap to activate the target.

The neurotransmitter can be ATP or some other molecule depending on the location and function of the cells.  In a few instances actual hormones can be released into the blood and be delivered by a special blood vessel network to a portion of the pituitary as part of the complex hormonal control complementing neuronal control of pituitary and its responsibility of secreting a variety of hormones controlling many other body systems.

EVOLUTION

The Final Step

Hormones generally have longer term, slower developing effects.  Nerves typically have a rapid response.  Sensory nerves and motor nerves enervating muscles have the fastest transmission. The response produced rapidly ceases because an enzyme in the gap between nerve ending and target cell contains an enzyme that rapidly degrades the chemical transmitting the stimulus. Nerves with less voluntary control transmit slowly to organs for less instantaneous responses.

An Intermediate Step

Some sponges have cells with short processes containing inclusions which have staining properties similar to neurosecretions.  Sponges do not have any rapid responses.  Some slow changes in the large opening, where water passing through the sponge leaves, have been reported.  Perhaps the closing is beneficial to the sponge during disruptive events when debris is settling into the sponge’s cavity.  A mechanism to send the distress signal from cells at the bottom of the cavity to cells around the opening could have selective value for survival if cells responded appropriately.  Elongated cells that had improved ability to deliver the message quicker would have suitable variations progressively selected until many generations and many new species later both hormones and nerves would provide the coordination we see today.

The First Step

It seemed to me that the one-celled animals (protozoans) might have chemicals analogous to hormones that control a portion of the cell but are produced within the cell.  The DNA does something like that via the RNA it produces.  But can the DNA be controlled?

The Experiment

Thirty-plus years ago I suggested a related study for a student looking for a research project.  During my master’s thesis topic, 60 years ago, I investigated some aspects of lipid chemistry in a protozoan, Tetrahymena.  I had used a technique others had described for synchronizing the divisions of Tetrahymena cells in their growth phase.  Heat shocks prevented completion of the division phase of the nucleus so the whole population was stalled at the same division stage.  Following return to normal temperatures most divided in synchrony about a half-hour later.

By ultra-sonically disrupting cultures during post shock periods and treating cultures that were not shocked with the disrupted cultures we hoped to find an induced peak of divisions within a half-hour.  There was no certain peak, although a few dividing cells in one culture was inconclusive evidence that was never followed up with more precise focus.  So the first protozoan “hormone” is yet to be discovered.

PHEROMONES

Chemical substances released by one animal in minute quantities that produce some specific behavioral response in others of the species are called pheromones.  A rock thrown to get your attention is not considered to be a pheromone.  Sex attractants are among the most widely studied pheromones.  Alluring perfumes are not pheromones either.  But some synthetic insect pheromones have been used to attract pest insects to traps.  Their release from many points in an infested field has also been shown to disrupt the male’s search for a female and make it less likely for reproduction to be successful.

The sex attractant pheromones of a closely related group of species from one insect family were found to consist of variations of the percentage of the same two or three volatile chemicals uniquely characteristic for each species.  That suggests one way new species could arise without geographic isolation.  Once success in reproduction correlated with a variant concentration, a new strain could evolve in its own particular direction.

Among other pheromones are alarm pheromones.  Trail marking substances could be used by the ant that placed them, and/or by others of the species.  Human pheromones are not well studied.  Two reasons may be significant.  Our ability to smell is worse than that of many other animals.  And the other is our reluctance to interfere during the private moments of others.  Could some of our intuitive decisions about others be based on otherwise unperceived pheromones?  Does a baby’s smell aide bonding or otherwise affect the mother?  Or vice versa?  Lots of possible pheromone responses may yet be found.

Joseph G. Engemann         May 10, 2014