Sphingomyelin Phosphodiesterase: An enzyme that catalyzes the hydrolysis of sphingomyelin to ceramide (N-acylsphingosine) plus choline phosphate. A defect in this enzyme leads to NIEMANN-PICK DISEASE. EC 188.8.131.52.Sphingomyelins: A class of sphingolipids found largely in the brain and other nervous tissue. They contain phosphocholine or phosphoethanolamine as their polar head group so therefore are the only sphingolipids classified as PHOSPHOLIPIDS.Phosphodiesterase Inhibitors: Compounds which inhibit or antagonize the biosynthesis or actions of phosphodiesterases.3',5'-Cyclic-AMP Phosphodiesterases: Enzymes that catalyze the hydrolysis of CYCLIC AMP to form adenosine 5'-phosphate. The enzymes are widely distributed in animal tissue and control the level of intracellular cyclic AMP. Many specific enzymes classified under this heading demonstrate additional spcificity for 3',5'-cyclic IMP and CYCLIC GMP.Phosphoric Diester Hydrolases: A class of enzymes that catalyze the hydrolysis of one of the two ester bonds in a phosphodiester compound. EC 184.108.40.206',5'-Cyclic-GMP Phosphodiesterases: Enzymes that catalyze the hydrolysis of cyclic GMP to yield guanosine-5'-phosphate.Phosphodiesterase 4 Inhibitors: Compounds that specifically inhibit PHOSPHODIESTERASE 4.Cyclic Nucleotide Phosphodiesterases, Type 4: A cyclic nucleotide phosphodiesterase subfamily that is found predominantly in inflammatory cells and may play a role in the regulation of CELL-MEDIATED IMMUNITY. The enzyme family includes over twenty different variants that occur due to multiple ALTERNATIVE SPLICING of the mRNA of at least four different genes.Ceramides: Members of the class of neutral glycosphingolipids. They are the basic units of SPHINGOLIPIDS. They are sphingoids attached via their amino groups to a long chain fatty acyl group. They abnormally accumulate in FABRY DISEASE.Phosphodiesterase 3 Inhibitors: Compounds that specifically inhibit PHOSPHODIESTERASE 3.Phosphodiesterase 5 Inhibitors: Compounds that specifically inhibit PHOSPHODIESTERASE 5.Phosphodiesterase I: A phosphoric diester hydrolase that removes 5'-nucleotides from the 3'-hydroxy termini of 3'-hydroxy-terminated OLIGONUCLEOTIDES. It has low activity towards POLYNUCLEOTIDES and the presence of 3'-phosphate terminus on the substrate may inhibit hydrolysis.Cyclic Nucleotide Phosphodiesterases, Type 3: A cyclic nucleotide phosphodiesterase subfamily that is inhibited by the binding of CYCLIC GMP to an allosteric domain found on the enzyme and through phosphorylation by regulatory kinases such as PROTEIN KINASE A and PROTEIN KINASE B. The two members of this family are referred to as type 3A, and type 3B, and are each product of a distinct gene. In addition multiple enzyme variants of each subtype can be produced due to multiple alternative mRNA splicing.Rolipram: A phosphodiesterase 4 inhibitor with antidepressant properties.