Question

INFORMATION ABOUT IMAGE G FOR QUESTIONS 12-15:

GABA_B receptors (GABA_BRs) are G-protein-coupled receptors that can be found on both pre- and postsynaptic neurons. When they are activated by GABA, they can have a range of effects. For the synapse you are studying in the following questions, the effects of GABA_BRs are indicated in IMAGE G.

You are doing a series of experiments in which you are recording from the pre- and postsynaptic cell at a synapse. The experiments described below for each question are independent of one another (that is, they are performed on the same type of synapse, but in separate experiments).

In order to learn how this type of synapse functions, you are applying various drugs to the synapse. These drugs may be GABA_BR agonists or antagonists, which would be applied in the fluid bathing the synapse so they can activate or block the channels extracellularly, or they could be drugs that affect the function of G-proteins that are activated when GABAbinds to GABA_BRs, and these would either be applied using an inside-out patch clamp recording technique or injected into the cytoplasm of the cell. Assume the only effects you are seeing are due to activation of the GABA_BRs and the downstream effects of activating them. That is, there are no channels or GABA receptors other than the ones indicated that affect your results. Unless otherwise stated, assume all ion concentrations are normal and the reversal potentials are: E_Na = +60 mV, E_K = -80 mV, and E_Cl = -70 mV.

Key for IMAGE G: If a given G-protein dimer influences the activity of an effector, this is shown by a black line. When that G-protein dimer enhances/increases the activity of the effector, this is symbolized by an arrowhead at the end of the line. In contrast, if there is a perpendicular short, straight line at the end, this indicates that this G-protein dimer reduces/blocks the activity of the effector.

Use this information and IMAGE G to answer the following four questions (12-15).

Question 12:

Control data: stimulate presynaptic neuron and record the postsynaptic response

Experimental data: Apply GABA, then stimulate presynaptic neuron and record the postsynaptic response

Would application of GABA affect neurotransmitter release from the presynaptic neuron at this synapse, compared to when no GABA is present? If yes, how? For this question, assume activation of adenylyl cyclase does not affect neurotransmitter release, and assume the postsynaptic response you record is an accurate indicator of the amount of neurotransmitter release.

Presynaptic neuron I GABA Synaptic cleft Adenyly! cyclase GARAR VGCC GDP GDP Cytoplasm GTP Postsynaptic neuron Synaptic cleft Adenyly! cyclase VGCC NMDAR GDP GTP ATP CAMP Cytoplasm

1.

	a.	Yes; neurotransmitter release would be reduced
	b.	Yes; neurotransmitter release would be increased
	c.	No; neurotransmitter release would not be affected

Use the information in Question 12 and IMAGE G, as well as the information below, to answer Question 13:

Control data: apply GABA to the synapse and record the K⁺ current in the postsynaptic neuron when the membrane potential is held at -40 mV; synapse is in normal extracellular solution, with typical ion concentrations. Additionally, action potentials have been blocked with pharmacological agents.

Experimental data: change the extracellular solution to one in which there is no Mg²⁺. Then, apply GABA to the synapse and record the K⁺ current when the membrane potential is held at -40 mV. Action potentials are still blocked.

Question 2: How would the zero-Mg²⁺ extracellular solution affect the K⁺ current, compared to the control conditions with a normal level Mg²⁺?

	a.	The K+ current would be larger in the zero-Mg2+ extracellular solution
	b.	The K+ current would be smaller in the zero-Mg2+ extracellular solution
	c.	The K+ current would not be affected

Answer 1

Question 1

ans : (a) it wil decrease the neurotransmitter release.

• GABA :is Gamma Amino Butyric Acid.
• It is an inhibitory neurotransmitter
• often describesd as Amino acid neurotransmitter, synthesised from amino acid glutamine.
• it interacts with two types of receptors and hence 2 different inhibition pathways
• GABAA recptor: Ionotropic receptor , binding to this receptor causes
• openig of associated calcium (cl-), cl- ions flow into neuron, hypo polarise membrane potential of neuron and thereby making it less likely for the firing of action potential
• GABAB :Metabotrophic receptor, binding causes opening of pottasium (k+) channel , k+ flow out of the neurone, make neurone hypopolarise, reduce firing of action potential.

Question 2

• extra cellular Mg2+ can bind to specific site on receptor blocking passage of ions
• on post synaptic membrane GABA-B receptors induce sIPSCs by activating Kir 3 type K+ channels
• this hypopolarise the membrane.
• In normal post synaptic membrane Na flows onlt through AMPARs receptor beacause Mg2+ blocks the NMDA receptors , and Mg2+ is only removed with the more and more influx of Na+ ions when the neuro transmitter reaches the post synaptic membrane (released from pre synaptic membrane)
• GABA inhibits release of neurotransmitters, this favours the voltage sensitive  Mg2+ on the NMDA receptors and shunt any excitatory current .