Opiates & Phagocytosis in Tetrahymena thermophila

What is the main effect of opiates on Tetrahymena phagocytosis?

Nanomolar concentrations of opiates inhibit phagocytosis.

Which drugs reverse the inhibitory effect of opiates?

Naloxone and naltrexone

What shape do the dose‑response curves for opiate inhibition have?

U‑shaped — no effect at very low or very high concentrations.

What does naloxone‑reversible inhibition suggest about Tetrahymena?

Presence of a primitive, receptor‑mediated opiate recognition system

Do Tetrahymena opiate responses match mammalian receptor subtypes (μ, δ, κ, ε)?

No — the recognition mechanism does not fit known mammalian subtypes.

Why is a receptor mechanism likely?

Effective at nanomolar concentrations + antagonists reverse inhibition + opiates are hydrophilic and unlikely to diffuse inside cells quickly

What is the optimal inhibitory concentration of met‑enkephalin?

2 nM.

What is the optimal inhibitory concentration range for leu‑enkephalin?

10–40 nM

Do all opiate agonists tested show similar maximal inhibition?

Yes — all produce comparable levels of inhibition

Which opiate classes were tested?

δ‑agonists, μ‑agonists, κ‑agonists, ε‑agonists.

What ions counteract met‑enkephalin’s inhibitory effect?

Calcium ions (Ca²⁺).

What neurotransmitter counteracts opiate inhibition?

Dopamine

What does dopamine’s effect suggest?

A dual regulatory system:

• Biogenic amines stimulate phagocytosis

• Opiates inhibit phagocytosis

What other unicellular organism shows similar opiate effects?

Amoeba proteus — opiates inhibit pinocytosis in a naloxone‑reversible, U‑shaped manner.

What mammalian cell type shows similar inhibition?

Macrophages — opiates inhibit receptor‑mediated phagocytosis.

What does this cross‑species similarity suggest?

Opiate regulation of endocytosis is evolutionarily ancient and conserved.

What particle was used to measure phagocytosis?

Tantalum particles (2 μm).

How was phagocytosis quantified?

% of cells containing tantalum vacuoles or a phagocytosis factor (experimental/control at 6 min).

How fast does phagocytosis reach a plateau in control cells?

About 6 minutes.

What is the proposed evolutionary significance of opiate inhibition?

Opiates may be general inhibitors of endocytic processes, a role conserved across evolution.

Why is receptor endocytosis unlikely to explain the U‑shaped curve?

Cells were exposed to opiates for only 3 minutes, too short for receptor internalization.

What is the overall conclusion of the study?

Opiates inhibit phagocytosis in Tetrahymena via a naloxone‑reversible, receptor‑like mechanism, modulated by Ca²⁺ and dopamine.