= Metabotropic receptor
{wiki}
A <metabotropic receptor> couples <ligand> recognition to intracellular <signal transduction>, which can regulate a separate <ion channel>, alter <enzyme> activity or change <gene expression>. In the usual <neurotransmitter> context, <metabotropic receptors> are <G protein-coupled receptors>: receptor activation is relayed by a <G protein>. There need not be a change in <membrane potential> at all. <Neuroscience, 2nd edition edited by D. Purves and others/Two Families of Postsynaptic Receptors>{ref}
Two routes should be distinguished. An activated <G protein> can regulate an <ion channel> locally at the <cell membrane>, without an obligatory diffusible <second messenger>. Alternatively, it can regulate an <enzyme> that changes the availability of a <second messenger>, such as <cAMP> or <inositol trisphosphate>. Downstream <proteins> then change channel opening, <phosphorylation> or other cellular activities. The <receptor>, <G protein>, <second messenger> and final target are distinct components; an activated <G protein> is a relay <protein>, rather than itself a small-molecule <second messenger>.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others/General Principles of Cell Communication>[Figure 15-15]{ref}
Do not turn this useful <chemical synapse> classification into the claim that every <receptor> is a <GPCR>, or that every indirect response must pass through a <G protein>. For example, the <insulin> <receptor> is an <enzyme>-linked <cell-surface receptor> whose intracellular catalytic activity initiates <protein> <phosphorylation>. <Intracellular receptors> supply another category. “Metabotropic” commonly names the <GPCR> branch in <neuroscience>; the broader idea of biochemical coupling does not make all non-channel <receptors> structurally identical. <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others/Signaling through Enzyme-Linked Cell-Surface Receptors>{ref}
Separating recognition from execution allows <signal amplification> and branching. A persistently activated <receptor> can activate several relay <proteins>; a regulated <enzyme> can produce many <second messenger> <molecules>. As an explicitly invented count example, suppose one activated <receptor> activates four relay <proteins> during one interval, and each relay activates a distinct <enzyme> that produces fifteen <second messenger> <molecules> during the next interval. With sufficient substrates and no loss, this gives $4\times15=60$ <molecules>. Those counts do not specify a physiological gain or response duration. A directly gated <ion channel> also passes many <ions> while open, so amplification is not an exclusive property of <metabotropic receptors>: always name the input and output being compared. <The Cell: A Molecular Approach, 2nd edition by Geoffrey M. Cooper/Pathways of Intracellular Signal Transduction>{ref}
A particularly instructive electrophysiology experiment separated a chemical trigger from the <ion channel> carrying its eventual <electric current>. In 1987, Hirono, Ito and Sugiyama injected <messenger RNA> from rat brain into frog egg cells, then tested acquired responses to neurotensin and muscarinic <acetylcholine> stimulation. Under <voltage clamp> at $-60\,\mathrm{mV}$, neurotensin elicited transient, oscillatory inward <electric currents> after a delay of several tens of seconds. This delay refers to that response and preparation. <Reversal potential> measurements implicated increased permeability to chloride <anions>, rather than a pore specific to the applied <ligand>. Both neurotensin and <acetylcholine> responses were inhibited by pertussis toxin, which interferes with susceptible <G proteins>, and by intracellular EGTA, which binds free <calcium>. Injecting <inositol trisphosphate> mimicked the responses; a large prior dose of <inositol trisphosphate> reduced subsequent responses to either extracellular signal. These interventions implicate a shared intracellular relay and <calcium>-dependent channel opening.<Neurotensin and acetylcholine evoke common responses in frog oocytes injected with rat brain messenger ribonucleic acid by C. Hirono and others>[abstract]{ref}
The informative comparison with the nicotinic <acetylcholine> <receptor> experiment is mechanistic: an extracellular <ligand> can produce an inward <electric current> either by directly gating the pore or by mobilizing an intracellular <second messenger> and stored <calcium> that activate another pore. Intracellular <inositol trisphosphate> bypasses reception, whereas EGTA tests the need for free <calcium>. However, neither inhibition nor mimicry alone identifies every molecular connection: a perturbation can affect several targets, and an injected mediator can activate a pathway without proving that it is the only natural route. The egg cell's endogenous machinery and the mixed <messenger RNA> preparation also limit extrapolation to an intact <neuron>. The experiment establishes functional coupling in its preparation, rather than the exact molecular composition or timing of every <metabotropic receptor>.
Even <G protein>-dependent channel regulation need not involve a lengthy <second messenger> cascade. Pfaffinger and colleagues reported in 1985 that <acetylcholine>-evoked <potassium> channel activation in embryonic chick atrial cells required intracellular GTP, a <guanine>-containing <nucleotide>, and was abolished by pertussis-toxin pretreatment. Changes in cyclic <nucleotides>, including <cAMP>, did not account for the channel response. The report discusses intrinsic activation delays of roughly $30$–$100\,\mathrm{ms}$, illustrating relatively rapid <metabotropic receptor> action. These observations support a <G protein> coupling step without cyclic-nucleotide <second messengers>; they do not by themselves exclude every possible intermediary or prove the specific subunit-channel interaction.<GTP-binding proteins couple cardiac muscarinic receptors to a K channel by P. J. Pfaffinger and others>[abstract]{ref}
A small timing model makes the extra steps visible without inventing an experimental trace. Let $u(t)$ jump from zero to one at $t=0$ and remain on. For a direct-gating model output $d$, one relay variable $x$, and its downstream output $y$, take zero <initial conditions> and
$$
\tau_d\frac{dd}{dt}=u-d,\qquad \tau_x\frac{dx}{dt}=u-x,\qquad \tau_y\frac{dy}{dt}=x-y.
$$
All outputs are normalized <dimensionless variables>; they are neither recorded <electric currents> nor fitted <concentrations>. With illustrative <time constants> $\tau_d=2\,\mathrm{ms}$, $\tau_x=20\,\mathrm{ms}$ and $\tau_y=100\,\mathrm{ms}$, their <exponential function> solutions for $t\geq0$ are
$$
d(t)=1-e^{-t/\tau_d},\qquad x(t)=1-e^{-t/\tau_x},\qquad y(t)=1-\frac{\tau_y e^{-t/\tau_y}-\tau_x e^{-t/\tau_x}}{\tau_y-\tau_x}.
$$
Initially $d'(0)=1/\tau_d$, while $y'(0)=0$ because its relay must first build up. Both outputs eventually approach one. Equal normalized final values do not imply equal biological amplitudes or absence of <signal amplification>; normalization deliberately removes that question.
\Image[/ionotropic-metabotropic-receptors-timing.png]
{title=One-step and two-step responses to a sustained input: an illustrative timing model}
{description=Original analytic model curves with stipulated <time constants> of $2$, $20$ and $100\,\mathrm{ms}$ and zero <initial conditions>. Both panels show the same functions on different time windows; the dashed curve is the intermediate relay. Values are normalized and do not represent measured <electric currents>, physiological receptor-specific timings, or universal boundaries between <ionotropic receptors> and <metabotropic receptors>. The model omits <signaling feedback>, saturation and input removal.}
The model's useful feature is the delayed buildup created by sequential relaxation, not its arbitrary numbers. Actual kinetics also depend on <ligand> delivery, relay deactivation, <neurotransmitter clearance> and the final target. A local <G protein> route can act promptly, while an <ionotropic receptor> can remain active during continued exposure or feed a longer-lasting intracellular response. The beauty of <metabotropic receptor> coupling is its flexibility: the same recognition event can adjust electrical responsiveness and other cellular tasks through different targets. To classify an observed response, combine timing with molecular identity and controlled perturbations.
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