Insect Hormones

Objectives

Know the major types of hormones and the types of tissues that produce them.
Be able to compare the roles of the nervous and hormonal systems in the regulation of insect physiology.
Be able to present an overview of the types of physiological functions that come under hormonal control.
you will know the difference between the polyphenism and polymorphism.

Topic Outline

Part 1: Introduction, Hormone production, hormone types and overview of hormone-modulated processes: moulting and metamorphosis, metabolic activiites, diapause, reproduction
Part 2: Polyphenisms, Chromatic Adaptations, Phase polyphenism in locusts

Activities

Minilecture 1: Hormone production systems
Minilecture 2: Hormones in Moulting and Metamorphosis
Go to the journal Science and read the following paper by Suzuki and Nijhout

Suzuki, Y. and Nidjout, H.F. 2006. Evolution of a polyphenism by genetic accommodation. Science vol. 311, no. 5761, pp 650-652.

Part 1: Introduction

Hormones play an important regulatory role in insect physiology.

The chemical signal (messenger) enters the circulatory system and is distributed throughout the body. By comparison with the nervous system this is slow and it produces a dispersed rather than localised effect.

It involves a single effector (a gland or group of glands) rather than a highly complex nervous response in which a similar response would have to be hardwired.

Products can be accumulated before distribution (though this doesn't happen in all cases)

Hormones have a coordinating role. Many behavioural and physiological processes can be coordinated by hormonal control. Moulting is an example.

The nervous system is the prime regulator of the hormonal system, thus sensory input—both internal and external—is integrated into the regulation of hormone release.

functions of hormones image

A general scheme for pathways of neuroendocrine regulation in insects. The central nervous system is the source of a large variety of neurosecretory hormones. These hormones are released from specialized neurohemal organs and may have an effect directly on a target tissue, or indirectly via the stimulation (+) or inhibition () of the secretory activity of conventional endocrine glands. Neurosecretory hormones can also feed back on the central nervous system and stimulate or inhibit its nervous and neuroendocrine activity. The central nervous system can also stimulate (or inhibit) neurosecretory cells and endocrine glands directly via conventional neurons.

From Nijhout, 1994.

Minilecture:

Hormones Production Systems

presented and prepared by

D. Merritt

Watch minilecture video (.m4v) 13MB

alternative format (.mov)

link to pdf file

Top

Hormone production

Endocrine glands

Two of the main endocrine glands are the prothoracic glands, located in the prothorax (the primary source of ecdysteroids) and the corpora allata (source of juvenile hormones).
Gonads: ovaries and testes
Secretory activity of endocrine glands is controlled via release of neurohormones from the CNS

Neurosecretory cells

Neurosecretory cells are neurons (found primarily in the CNS), that produce polypeptides instead of neurotransmitters.
parabiosis image The neurons usually have large cell bodies and widely dispersed axons.
Electron-dense granules in cells are usually densely distributed in the cell body and around periphery of nerves and ganglia. Frequently, the nerve terminals are concentrated in special “neurohaemal organs” which are sites of neurohormone release in the peripheral nerves or on the periphery of ganglia.
As integral parts of the nervous system, the neurosecretory cells are under nervous control. These cells form the link between the nervous system and endocrine system.

Techniques:

Ligation
Extirpation
Implantation
Parabiosis
Extract injections: bioassay
Immunocytochemistry

Mechanism of action of a steroid hormone. A steroid hormone passes through the cell membrane, enters the nucleus, and binds to a nuclear receptor protein. The receptor hormone complex then binds to specific regions of the DNA to control gene transcription. From Nijhout, 1994.

Top

Hormone types

Lipid hormones

Act on genes.

Pass through membranes and bind to receptors within the cell, to DNA for example, and thus can directly regulate transcription.

Ecdysone (a steroid) and juvenile hormone (JH) (a terpenoid) is a lipid hormone

Polytene cells of the Drosophila salivary glands were used to indicate gene transcription in response to ecdysteroids. Ecdysteroid injections cause “puffing” of the polytene chromosomes, indicative of localised transcription. There is a characteristic series of puffs in different locations indicating a cascading series of transcriptional events.

Polypeptides or amines (peptide hormones)

Bind to receptors in cell membrane of the receiving cell

The signal must be carried from the cell membrane where the receptor lies.

Act via a “second messenger” system to activate or depress enzymes or proteins and thus change the physiology of the cell. The second messengers are cAMP or cGMP. Involved in initiating a cascade of protein or enzyme activations that ultimately alter the cell’s physiology.

Hormone-receptor complexes can also act on Ca++ ion concentrations within the cell via a 2nd messenger system

Examples: Eclosion hormone and prothoracicotropic hormone

Peptide hormones usually produced by neurosecretory cells

Top

Brain-retrocerebral complex

Composed of:

brain neurosecretory cells: clustered cells
corpora cardiaca
corpora allata

Anatomy of the brain-retrocerebral neuroendocrine complex in a final instar larva of Manduca sexta, showing the anatomical relations of parts and their nervous interconnections.

Question marks indicate nerves with unknown destination. (From Nijhout, 1994.)

b, brain; CA, corpus allatum: CC. corpus cardiacum; cec, circumesophageal connective: dms, dilator muscles of stomodaeum; fg, frontal ganglion: hcg, hypocerebral ganglion; lm, labial muscles; ma, mandibular gland; mn, maxillary nerve; NCA, nervus corporis allati; NCCI+II, nervi corporis cardiaci I and II; NCCIII, nervus corporis cardiaci III; rn, recurrent nerve; s, sensilla; sea, subesophageal ganglion; tcc, tritocerebral commissure.

Brain neurosecretory cells
	Relatively large neurons that produce peptides instead of neurotransmitters. Viewed with the electron microscope the peptide “packets” are large electron-dense vesicles. The neurons are usually found in discrete clusters of a few neurons.

Corpora cardiaca
	2 functions: 1. the principle neurohaemal organ for brain neurosecretory cells 2. Intrinsic neurosecretory cells (e.g. those producing adipokinetic hormone) Get their name by being near the heart in many insects: allows the released products to be immediately distributed with the circulation of haemolymph.

Corpora allata
	Glands that are innervated from brain, via the corpora cardiaca Secretory cells synthesize juvenile hormone.

Prothoracic glands

Produce ecdysteroids
Found in prothorax
Cell structure: folded plasma membrane, typical of secretory cells
Innervated from suboesophageal ganglion or T1 or T2 ganglia
Ecdysone is not stored: it is released as it is synthesised
Prothoracic glands are not found in adults: programmed cell death during metamorphosis (Drosophila ring gland diagram)
In Diptera the RCC and prothoracic glands are fused to form the ring complex
Ring gland of Diptera is a fusion of CA, CC and PG

Ventral nerve cord
	Perivisceral organs: neurohaemal organs for segmental ganglionic neurosecretory cells

Distribution of neurosecretory cells (dots in the ganglia) and anatomical arrangement of neurohemal organs (black swellings on nerves) in a typical insect central nervous system. The neurohemal organs ot the brain are the corpora cardiaca (cc) and those of the abdominal ganglia are the perivisceral organs (po); abd, abdominal ganglia: sog, subesophageal ganglion; th, thoracic ganglia. (From Orchard and Loughton, 1985. Reprinted with kind permission of Pergamon Press Ltd.) From Nijhout, 1994.

Top

Overview of hormone-modulated processes