Digital filter

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https://en.wikipedia.org/wiki/Digital_filter

A general finite impulse response filter with n stages, each with an independent delay, d_i, and amplification gain, a_i.

In signal processing, a digital filter is a system that performs mathematical operations on a sampled, discrete-time signal to reduce or enhance certain aspects of that signal. This is in contrast to the other major type of electronic filter, the analog filter, which is typically an electronic circuit operating on continuous-time analog signals.

A digital filter system usually consists of an analog-to-digital converter (ADC) to sample the input signal, followed by a microprocessor and some peripheral components such as memory to store data and filter coefficients etc. Program Instructions (software) running on the microprocessor implement the digital filter by performing the necessary mathematical operations on the numbers received from the ADC. In some high performance applications, an FPGA or ASIC is used instead of a general purpose microprocessor, or a specialized digital signal processor (DSP) with specific paralleled architecture for expediting operations such as filtering.

Digital filters may be more expensive than an equivalent analog filter due to their increased complexity, but they make practical many designs that are impractical or impossible as analog filters. Digital filters can often be made very high order, and are often finite impulse response filters, which allows for linear phase response. When used in the context of real-time analog systems, digital filters sometimes have problematic latency (the difference in time between the input and the response) due to the associated analog-to-digital and digital-to-analog conversions and anti-aliasing filters, or due to other delays in their implementation.

Digital filters are commonplace and an essential element of everyday electronics such as radios, cellphones, and AV receivers.

Characterization

A digital filter is characterized by its transfer function, or equivalently, its difference equation. Mathematical analysis of the transfer function can describe how it will respond to any input. As such, designing a filter consists of developing specifications appropriate to the problem (for example, a second-order low pass filter with a specific cut-off frequency), and then producing a transfer function which meets the specifications.

The transfer function for a linear, time-invariant, digital filter can be expressed as a transfer function in the Z-domain; if it is causal, then it has the form:

${\displaystyle H(z)=\frac{B(z)}{A(z)}=\frac{b_0+b_1{z}^{-1}+b_2{z}^{-2}+\cdots +b_N{z}^{-N}}{1+a_1{z}^{-1}+a_2{z}^{-2}+\cdots +a_M{z}^{-M}}}$

where the order of the filter is the greater of N or M. See Z-transform's LCCD equation for further discussion of this transfer function.

This is the form for a recursive filter, which typically leads to an infinite impulse response (IIR) behaviour, but if the denominator is made equal to unity, i.e. no feedback, then this becomes a finite impulse response (FIR) filter.

Analysis techniques

A variety of mathematical techniques may be employed to analyze the behavior of a given digital filter. Many of these analysis techniques may also be employed in designs, and often form the basis of a filter specification.

Typically, one characterizes filters by calculating how they will respond to a simple input such as an impulse. One can then extend this information to compute the filter's response to more complex signals.

Impulse response

The impulse response, often denoted ${\displaystyle h[k]}$ or ${\displaystyle h_k}$, is a measurement of how a filter will respond to the Kronecker delta function. For example, given a difference equation, one would set ${\displaystyle x_0=1}$ and ${\displaystyle x_k=0}$ for ${\displaystyle k\ne 0}$ and evaluate. The impulse response is a characterization of the filter's behaviour. Digital filters are typically considered in two categories: infinite impulse response (IIR) and finite impulse response (FIR). In the case of linear time-invariant FIR filters, the impulse response is exactly equal to the sequence of filter coefficients, and thus:

${\displaystyle y_n=\sum _{k=0}^N{b}_k{x}_{n-k}=\sum _{k=0}^N{h}_k{x}_{n-k}}$

IIR filters on the other hand are recursive, with the output depending on both current and previous inputs as well as previous outputs. The general form of an IIR filter is thus:

${\displaystyle \sum _{m=0}^M{a}_m{y}_{n-m}=\sum _{k=0}^N{b}_k{x}_{n-k}}$

Plotting the impulse response reveals how a filter responds to a sudden, momentary disturbance. An IIR filter is always recursive. While it is possible for a recursive filter to have a finite impulse response, a non-recursive filter always has a finite impulse response. An example is the moving average (MA) filter, which can be implemented both recursively and non recursively.

Difference equation

In discrete-time systems, the digital filter is often implemented by converting the transfer function to a linear constant-coefficient difference equation (LCCD) via the Z-transform. The discrete frequency-domain transfer function is written as the ratio of two polynomials. For example:

${\displaystyle H(z)=\frac{(z+1{)}^2}{(z-\frac{1}{2})(z+\frac{3}{4})}}$

This is expanded:

${\displaystyle H(z)=\frac{z^2+2z+1}{z^2+\frac{1}{4}z-\frac{3}{8}}}$

and to make the corresponding filter causal, the numerator and denominator are divided by the highest order of ${\displaystyle z}$:

${\displaystyle H(z)=\frac{1+2z^{-1}+z^{-2}}{1+\frac{1}{4}{z}^{-1}-\frac{3}{8}{z}^{-2}}=\frac{Y(z)}{X(z)}}$

The coefficients of the denominator, ${\displaystyle a_k}$, are the 'feed-backward' coefficients and the coefficients of the numerator are the 'feed-forward' coefficients, ${\displaystyle b_k}$. The resultant linear difference equation is:

${\displaystyle y[n]=-\sum _{k=1}^M{a}_{k}y[n-k]+\sum _{k=0}^N{b}_{k}x[n-k]}$

or, for the example above:

${\displaystyle \frac{Y(z)}{X(z)}=\frac{1+2z^{-1}+z^{-2}}{1+\frac{1}{4}{z}^{-1}-\frac{3}{8}{z}^{-2}}}$

rearranging terms:

${\displaystyle \Rightarrow (1+\frac{1}{4}{z}^{-1}-\frac{3}{8}{z}^{-2})Y(z)=(1+2z^{-1}+z^{-2})X(z)}$

then by taking the inverse z-transform:

${\displaystyle \Rightarrow y[n]+\frac{1}{4}y[n-1]-\frac{3}{8}y[n-2]=x[n]+2x[n-1]+x[n-2]}$

and finally, by solving for ${\displaystyle y[n]}$:

${\displaystyle y[n]=-\frac{1}{4}y[n-1]+\frac{3}{8}y[n-2]+x[n]+2x[n-1]+x[n-2]}$

This equation shows how to compute the next output sample, ${\displaystyle y[n]}$, in terms of the past outputs, ${\displaystyle y[n-p]}$, the present input, ${\displaystyle x[n]}$, and the past inputs, ${\displaystyle x[n-p]}$. Applying the filter to an input in this form is equivalent to a Direct Form I or II (see below) realization, depending on the exact order of evaluation.

In plain terms, for example, as used by a computer programmer implementing the above equation in code, it can be described as follows:

${\displaystyle y}$ = the output, or filtered value
${\displaystyle x}$ = the input, or incoming raw value
${\displaystyle n}$ = the sample number, iteration number, or time period number

and therefore:

${\displaystyle y[n]}$ = the current filtered (output) value
${\displaystyle y[n-1]}$ = the last filtered (output) value
${\displaystyle y[n-2]}$ = the 2nd-to-last filtered (output) value
${\displaystyle x[n]}$ = the current raw input value
${\displaystyle x[n-1]}$ = the last raw input value
${\displaystyle x[n-2]}$ = the 2nd-to-last raw input value

Filter design

Main article: Filter design

Although filters are easily understood and calculated, the practical challenges of their design and implementation are significant and are the subject of much advanced research.

There are two categories of digital filter: the recursive filter and the nonrecursive filter. These are often referred to as infinite impulse response (IIR) filters and finite impulse response (FIR) filters, respectively.

Filter realization

After a filter is designed, it must be realized by developing a signal flow diagram that describes the filter in terms of operations on sample sequences.

A given transfer function may be realized in many ways. Consider how a simple expression such as ${\displaystyle ax+bx+c}$ could be evaluated – one could also compute the equivalent ${\displaystyle x(a+b)+c}$. In the same way, all realizations may be seen as "factorizations" of the same transfer function, but different realizations will have different numerical properties. Specifically, some realizations are more efficient in terms of the number of operations or storage elements required for their implementation, and others provide advantages such as improved numerical stability and reduced round-off error. Some structures are better for fixed-point arithmetic and others may be better for floating-point arithmetic.

Direct form I

A straightforward approach for IIR filter realization is direct form I, where the difference equation is evaluated directly. This form is practical for small filters, but may be inefficient and impractical (numerically unstable) for complex designs. In general, this form requires 2N delay elements (for both input and output signals) for a filter of order N.

Direct form II

The alternate direct form II only needs N delay units, where N is the order of the filter – potentially half as much as direct form I. This structure is obtained by reversing the order of the numerator and denominator sections of Direct Form I, since they are in fact two linear systems, and the commutativity property applies. Then, one will notice that there are two columns of delays (${\displaystyle z^{-1}}$) that tap off the center net, and these can be combined since they are redundant, yielding the implementation as shown below.

The disadvantage is that direct form II increases the possibility of arithmetic overflow for filters of high Q or resonance. It has been shown that as Q increases, the round-off noise of both direct form topologies increases without bounds. This is because, conceptually, the signal is first passed through an all-pole filter (which normally boosts gain at the resonant frequencies) before the result of that is saturated, then passed through an all-zero filter (which often attenuates much of what the all-pole half amplifies).

Cascaded second-order sections

A common strategy is to realize a higher-order (greater than 2) digital filter as a cascaded series of second-order "biquadratric" (or "biquad") sections (see digital biquad filter). The advantage of this strategy is that the coefficient range is limited. Cascading direct form II sections results in N delay elements for filters of order N. Cascading direct form I sections results in N + 2 delay elements, since the delay elements of the input of any section (except the first section) are redundant with the delay elements of the output of the preceding section.

Other forms

Other forms include:

• Direct form I and II transpose
• Series/cascade lower (typical second) order subsections
• Parallel lower (typical second) order subsections
  • Continued fraction expansion
• Lattice and ladder
  • One, two and three-multiply lattice forms
  • Three and four-multiply normalized ladder forms
  • ARMA structures
• State-space structures:
  • optimal (in the minimum noise sense): ${\displaystyle (N+1{)}^2}$ parameters
  • block-optimal and section-optimal: ${\displaystyle 4N-1}$ parameters
  • input balanced with Givens rotation: ${\displaystyle 4N-1}$ parameters
• Coupled forms: Gold Rader (normal), State Variable (Chamberlin), Kingsbury, Modified State Variable, Zölzer, Modified Zölzer
• Wave Digital Filters (WDF)
• Agarwal–Burrus (1AB and 2AB)
• Harris–Brooking
• ND-TDL
• Multifeedback
• Analog-inspired forms such as Sallen-key and state variable filters
• Systolic arrays

Comparison of analog and digital filters

Digital filters are not subject to the component non-linearities that greatly complicate the design of analog filters. Analog filters consist of imperfect electronic components, whose values are specified to a limit tolerance (e.g. resistor values often have a tolerance of ±5%) and which may also change with temperature and drift with time. As the order of an analog filter increases, and thus its component count, the effect of variable component errors is greatly magnified. In digital filters, the coefficient values are stored in computer memory, making them far more stable and predictable.

Because the coefficients of digital filters are definite, they can be used to achieve much more complex and selective designs – specifically with digital filters, one can achieve a lower passband ripple, faster transition, and higher stopband attenuation than is practical with analog filters. Even if the design could be achieved using analog filters, the engineering cost of designing an equivalent digital filter would likely be much lower. Furthermore, one can readily modify the coefficients of a digital filter to make an adaptive filter or a user-controllable parametric filter. While these techniques are possible in an analog filter, they are again considerably more difficult.

Digital filters can be used in the design of finite impulse response filters. Equivalent analog filters are often more complicated, as these require delay elements.

Digital filters rely less on analog circuitry, potentially allowing for a better signal-to-noise ratio. A digital filter will introduce noise to a signal during analog low pass filtering, analog to digital conversion, digital to analog conversion and may introduce digital noise due to quantization. With analog filters, every component is a source of thermal noise (such as Johnson noise), so as the filter complexity grows, so does the noise.

However, digital filters do introduce a higher fundamental latency to the system. In an analog filter, latency is often negligible; strictly speaking it is the time for an electrical signal to propagate through the filter circuit. In digital systems, latency is introduced by delay elements in the digital signal path, and by analog-to-digital and digital-to-analog converters that enable the system to process analog signals.

In very simple cases, it is more cost effective to use an analog filter. Introducing a digital filter requires considerable overhead circuitry, as previously discussed, including two low pass analog filters.

Another argument for analog filters is low power consumption. Analog filters require substantially less power and are therefore the only solution when power requirements are tight.

When making an electrical circuit on a PCB it is generally easier to use a digital solution, because the processing units are highly optimized over the years. Making the same circuit with analog components would take up a lot more space when using discrete components. Two alternatives are FPAAs and ASICs, but they are expensive for low quantities.

Types of digital filters

There are various ways to characterize filters; for example:

• A linear filter is a linear transformation of input samples; other filters are nonlinear. Linear filters satisfy the superposition principle, i.e. if an input is a weighted linear combination of different signals, the output is a similarly weighted linear combination of the corresponding output signals.
• A causal filter uses only previous samples of the input or output signals; while a non-causal filter uses future input samples. A non-causal filter can usually be changed into a causal filter by adding a delay to it.
• A time-invariant filter has constant properties over time; other filters such as adaptive filters change in time.
• A stable filter produces an output that converges to a constant value with time, or remains bounded within a finite interval. An unstable filter can produce an output that grows without bounds, with bounded or even zero input.
• A finite impulse response (FIR) filter uses only the input signals, while an infinite impulse response (IIR) filter uses both the input signal and previous samples of the output signal. FIR filters are always stable, while IIR filters may be unstable.

A filter can be represented by a block diagram, which can then be used to derive a sample processing algorithm to implement the filter with hardware instructions. A filter may also be described as a difference equation, a collection of zeros and poles or an impulse response or step response.

Some digital filters are based on the fast Fourier transform, a mathematical algorithm that quickly extracts the frequency spectrum of a signal, allowing the spectrum to be manipulated (such as to create very high order band-pass filters) before converting the modified spectrum back into a time-series signal with an inverse FFT operation. These filters give O(n log n) computational costs whereas conventional digital filters tend to be O(n²).

Another form of a digital filter is that of a state-space model. A well used state-space filter is the Kalman filter published by Rudolf Kálmán in 1960.

Traditional linear filters are usually based on attenuation. Alternatively nonlinear filters can be designed, including energy transfer filters, which allow the user to move energy in a designed way so that unwanted noise or effects can be moved to new frequency bands either lower or higher in frequency, spread over a range of frequencies, split, or focused. Energy transfer filters complement traditional filter designs and introduce many more degrees of freedom in filter design. Digital energy transfer filters are relatively easy to design and to implement and exploit nonlinear dynamics.

γ-Aminobutyric acid

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https://en.wikipedia.org/wiki/%CE%93-Aminobutyric_acid

γ-Aminobutyric acid

γ-Aminobutyric acid (gamma-aminobutyric acid) /ˈɡæmə əˈmiːnoʊbjuːˈtɪrɪk ˈæsɪd/, or GABA /ˈɡæbə/, is the chief inhibitory neurotransmitter in the developmentally mature mammalian central nervous system. Its principal role is reducing neuronal excitability throughout the nervous system.

GABA is sold as a dietary supplement in many countries. It has been traditionally thought that exogenous GABA (i.e. taken as a supplement) does not cross the blood–brain barrier, but data obtained from more recent research in rats describes the notion as being unclear.

The carboxylate form of GABA is γ-aminobutyrate.

Function

Neurotransmitter

Two general classes of GABA receptor are known:

• GABA_A in which the receptor is part of a ligand-gated ion channel complex
• GABA_B metabotropic receptors, which are G protein-coupled receptors that open or close ion channels via intermediaries (G proteins)

Release, Reuptake, and Metabolism Cycle of GABA

Neurons that produce GABA as their output are called GABAergic neurons, and have chiefly inhibitory action at receptors in the adult vertebrate. Medium spiny cells are a typical example of inhibitory central nervous system GABAergic cells. In contrast, GABA exhibits both excitatory and inhibitory actions in insects, mediating muscle activation at synapses between nerves and muscle cells, and also the stimulation of certain glands. In mammals, some GABAergic neurons, such as chandelier cells, are also able to excite their glutamatergic counterparts.

GABA_A receptors are ligand-activated chloride channels: when activated by GABA, they allow the flow of chloride ions across the membrane of the cell. Whether this chloride flow is depolarizing (makes the voltage across the cell's membrane less negative), shunting (has no effect on the cell's membrane potential), or inhibitory/hyperpolarizing (makes the cell's membrane more negative) depends on the direction of the flow of chloride. When net chloride flows out of the cell, GABA is depolarising; when chloride flows into the cell, GABA is inhibitory or hyperpolarizing. When the net flow of chloride is close to zero, the action of GABA is shunting. Shunting inhibition has no direct effect on the membrane potential of the cell; however, it reduces the effect of any coincident synaptic input by reducing the electrical resistance of the cell's membrane. Shunting inhibition can "override" the excitatory effect of depolarising GABA, resulting in overall inhibition even if the membrane potential becomes less negative. It was thought that a developmental switch in the molecular machinery controlling the concentration of chloride inside the cell changes the functional role of GABA between neonatal and adult stages. As the brain develops into adulthood, GABA's role changes from excitatory to inhibitory.

Brain development

GABA is an inhibitory transmitter in the mature brain; its actions were thought to be primarily excitatory in the developing brain. The gradient of chloride was reported to be reversed in immature neurons, with its reversal potential higher than the resting membrane potential of the cell; activation of a GABA-A receptor thus leads to efflux of Cl⁻ ions from the cell (that is, a depolarizing current). The differential gradient of chloride in immature neurons was shown to be primarily due to the higher concentration of NKCC1 co-transporters relative to KCC2 co-transporters in immature cells. GABAergic interneurons mature faster in the hippocampus and the GABA machinery appears earlier than glutamatergic transmission. Thus, GABA is considered the major excitatory neurotransmitter in many regions of the brain before the maturation of glutamatergic synapses.

In the developmental stages preceding the formation of synaptic contacts, GABA is synthesized by neurons and acts both as an autocrine (acting on the same cell) and paracrine (acting on nearby cells) signalling mediator. The ganglionic eminences also contribute greatly to building up the GABAergic cortical cell population.

GABA regulates the proliferation of neural progenitor cells, the migration and differentiation the elongation of neurites and the formation of synapses.

GABA also regulates the growth of embryonic and neural stem cells. GABA can influence the development of neural progenitor cells via brain-derived neurotrophic factor (BDNF) expression. GABA activates the GABA_A receptor, causing cell cycle arrest in the S-phase, limiting growth.

Beyond the nervous system

mRNA expression of the embryonic variant of the GABA-producing enzyme GAD67 in a coronal brain section of a one-day-old Wistar rat, with the highest expression in subventricular zone (svz)

Besides the nervous system, GABA is also produced at relatively high levels in the insulin-producing beta cells (β-cells) of the pancreas. The β-cells secrete GABA along with insulin and the GABA binds to GABA receptors on the neighboring islet alpha cells (α-cells) and inhibits them from secreting glucagon (which would counteract insulin's effects).

GABA can promote the replication and survival of β-cells and also promote the conversion of α-cells to β-cells, which may lead to new treatments for diabetes.

Alongside GABAergic mechanisms, GABA has also been detected in other peripheral tissues including intestines, stomach, Fallopian tubes, uterus, ovaries, testes, kidneys, urinary bladder, the lungs and liver, albeit at much lower levels than in neurons or β-cells.

Experiments on mice have shown that hypothyroidism induced by fluoride poisoning can be halted by administering GABA. The test also found that the thyroid recovered naturally without further assistance after the fluoride had been expelled by the GABA.

Immune cells express receptors for GABA and administration of GABA can suppress inflammatory immune responses and promote "regulatory" immune responses, such that GABA administration has been shown to inhibit autoimmune diseases in several animal models.

In 2018, GABA has shown to regulate secretion of a greater number of cytokines. In plasma of T1D patients, levels of 26 cytokines are increased and of those, 16 are inhibited by GABA in the cell assays.

In 2007, an excitatory GABAergic system was described in the airway epithelium. The system is activated by exposure to allergens and may participate in the mechanisms of asthma. GABAergic systems have also been found in the testis and in the eye lens.

Structure and conformation

GABA is found mostly as a zwitterion (i.e. with the carboxyl group deprotonated and the amino group protonated). Its conformation depends on its environment. In the gas phase, a highly folded conformation is strongly favored due to the electrostatic attraction between the two functional groups. The stabilization is about 50 kcal/mol, according to quantum chemistry calculations. In the solid state, an extended conformation is found, with a trans conformation at the amino end and a gauche conformation at the carboxyl end. This is due to the packing interactions with the neighboring molecules. In solution, five different conformations, some folded and some extended, are found as a result of solvation effects. The conformational flexibility of GABA is important for its biological function, as it has been found to bind to different receptors with different conformations. Many GABA analogues with pharmaceutical applications have more rigid structures in order to control the binding better.

History

In 1883, GABA was first synthesized, and it was first known only as a plant and microbe metabolic product.

In 1950, GABA was discovered as an integral part of the mammalian central nervous system.

In 1959, it was shown that at an inhibitory synapse on crayfish muscle fibers GABA acts like stimulation of the inhibitory nerve. Both inhibition by nerve stimulation and by applied GABA are blocked by picrotoxin.

Biosynthesis

GABAergic neurons which produce GABA

GABA is primarily synthesized from glutamate via the enzyme glutamate decarboxylase (GAD) with pyridoxal phosphate (the active form of vitamin B6) as a cofactor. This process converts glutamate (the principal excitatory neurotransmitter) into GABA (the principal inhibitory neurotransmitter).

GABA can also be synthesized from putrescine by diamine oxidase and aldehyde dehydrogenase.

Historically it was thought that exogenous GABA did not penetrate the blood–brain barrier, but more current research describes the notion as being unclear pending further research.

Metabolism

GABA transaminase enzymes catalyze the conversion of 4-aminobutanoic acid (GABA) and 2-oxoglutarate (α-ketoglutarate) into succinic semialdehyde and glutamate. Succinic semialdehyde is then oxidized into succinic acid by succinic semialdehyde dehydrogenase and as such enters the citric acid cycle as a usable source of energy.

Pharmacology

Drugs that act as allosteric modulators of GABA receptors (known as GABA analogues or GABAergic drugs), or increase the available amount of GABA, typically have relaxing, anti-anxiety, and anti-convulsive effects (with equivalent efficacy to lamotrigine based on studies of mice). Many of the substances below are known to cause anterograde amnesia and retrograde amnesia.

In general, GABA does not cross the blood–brain barrier, although certain areas of the brain that have no effective blood–brain barrier, such as the periventricular nucleus, can be reached by drugs such as systemically injected GABA. At least one study suggests that orally administered GABA increases the amount of human growth hormone (HGH). GABA directly injected to the brain has been reported to have both stimulatory and inhibitory effects on the production of growth hormone, depending on the physiology of the individual.

GABA enhances the catabolism of serotonin into N-acetylserotonin (the precursor of melatonin) in rats. It is thus suspected that GABA is involved in the synthesis of melatonin and thus might exert regulatory effects on sleep and reproductive functions.

Chemistry

Although in chemical terms, GABA is an amino acid (as it has both a primary amine and a carboxylic acid functional group), it is rarely referred to as such in the professional, scientific, or medical community. By convention the term "amino acid", when used without a qualifier, refers specifically to an alpha amino acid. GABA is not an alpha amino acid, meaning the amino group is not attached to the alpha carbon. Nor is it incorporated into proteins as are many alpha-amino acids.

GABAergic drugs

GABA_A receptor ligands are shown in the following table

Activity at GABAA	Ligand
Orthosteric Agonist	Muscimol, GABA, gaboxadol (THIP), isoguvacine, progabide, piperidine-4-sulfonic acid (partial agonist)
Positive allosteric modulators	Barbiturates, benzodiazepines, neuroactive steroids, niacin/niacinamide, nonbenzodiazepines (i.e. z-drugs, e.g. zolpidem, etomidate, alcohol (ethanol), methaqualone, propofol, stiripentol, and anaesthetics (including volatile anaesthetics)
Orthosteric (competitive) Antagonist	bicuculline, gabazine, thujone, flumazenil
Uncompetitive antagonist (e.g. channel blocker)	cicutoxin
Negative allosteric modulators	furosemide, oenanthotoxin, amentoflavone

GABAergic pro-drugs include chloral hydrate, which is metabolised to trichloroethanol, which then acts via the GABA_A receptor.

The plant kava contains GABAergic compounds, including kavain, dihydrokavain, methysticin, dihydromethysticin and yangonin.

Other GABAergic modulators include:

• GABA_B receptor ligands.
  • Agonists: baclofen, propofol, GHB, phenibut.
  • Antagonists: phaclofen, saclofen.
• GABA reuptake inhibitors: deramciclane, hyperforin, tiagabine.
• GABA transaminase inhibitors: gabaculine, phenelzine, valproate, vigabatrin, lemon balm (Melissa officinalis).
• GABA analogues: pregabalin, gabapentin, picamilon, progabide

In plants

GABA is also found in plants. It is the most abundant amino acid in the apoplast of tomatoes. Evidence also suggests a role in cell signalling in plants.