Question 86#

Which of the following neurotransmitters act as a physiological antagonist for acetylcholine?

A. Serotonin

D. Norepinephrine acts as physiological antagonist for acetylcholine. Physiological antagonism is defined as the process wherein two chemical molecules that act through two different receptor systems result in opposing actions in the body that tend to negate each other’s physiological effect. Insulin and glucagon can be considered as physiological antagonists, to some extent.

Reference:

1. Johnstone EC et al., eds. Companion to Psychiatric Studies, 7th edn. Churchill Livingstone, 2004, p. 42.

Question 87#

Which of the following is not a ligand-gated ion channel?

A. Nicotinic cholinergic receptors

E. Muscarinic receptors act via the G protein-coupled second messenger system. Nicotinic cholinergic receptors operate via ligand gated channels that are permeable to Na⁺, K⁺ and sometimes Ca²⁺. GABA_A receptors are also ligand-gated ion channels that allow Cl^– ions to pass through, resulting in inhibitory activity. Glycine receptors are strychnine-sensitive ligand-gated ion channels with inhibitory activity. Metabotropic receptors aside, most glutamate receptors (NMDA, AMPA, and kainate) are ionotropic and allow Ca²⁺ transit (NMDA, AMPA) or Na⁺, K⁺ transit (kainate) via ligand-gated channels.

Reference:

1. Johnstone EC et al., eds. Companion to Psychiatric Studies, 7th edn. Churchill Livingstone, 2004, p. 43.

Question 88#

¹¹C raclopride is a commonly used PET ligand. Which of the following receptors can be identified by ¹¹C raclopride?

A. Serotonin receptors

D. ¹¹C raclopride is used to identify postsynaptic dopamine receptors (D2/D3). Raclopride is related to the antipsychotic sulpride and amisulpride (substituted benzamide); hence it acts as a D2/D3 inhibitor.

Reference:

1. Johnstone EC et al., eds. Companion to Psychiatric Studies, 7th edn. Churchill Livingstone, 2004, p. 79.

Question 89#

Leptin is a hormone involved in regulation of adiposity and meal intake.

Which of the following structures is the major source of leptin?

A. Hypothalamus

D. Adipose tissues are the major source of leptin. The amount of leptin in plasma is directly proportional to degree of adiposity in one’s body. Leptin enters the brain, by an unknown mechanism, to reduce appetite via a central mechanism. In obesity, a certain degree of leptin resistance exists, leading to high circulating levels of leptin.

Reference:

1. Mantzoros CS. Leptin and the hypothalamus: neuroendocrine regulation of food intake. Molecular Psychiatry 1999; 4: 8–12.

Question 90#

Which of the following substances does not interact with cell membrane receptors and acts directly on nuclear material to produce physiological effects?

A. Serotonin

E. Thyroid hormone acts in a similar manner to steroid hormone. Receptors for these hormones are not present in the cell membrane but, upon cellular entry, they bind to nuclear receptors to directly affect cellular processes. These nuclear receptors act as hormone-activated transcription factors that modulate gene expression.

Reference:

1. Brent GA. Mechanisms of disease: the molecular basis of thyroid hormone action. New England Journal of Medicine 1994; 331: 847–853.