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DA is synthesized in the nerve terminal, The first step is the synthesis of dihydroxyphenylalanine (DOPA) from the amino acid, tyrosine. The DOPA is then decarboxylated to form DA. The DA is stored in synaptic vesicles: tc)protect it from metabolism. From the vesicle, DA is released into the synapse. Post-synaptic neurons have two kinds of DA receptors, known as D-1 and D-2. When D-1 receptors are activated by DA, adenyl cyclase activity in the cell is increased; adenyl cyclase produces an intracellular messenger molecule, cyclic AMP.

_ _- Z ........... ,_,,_ .............. 7_ Z - 7_Z - Z _ -- -Z ................. oo, -, - = = = -- .... _ - --_ _ _ -..... 7. 7 "-1 - - _! ;Iron fission of U-235 and Pu-239, Most of the neutrons have been shown to ortgtnate from the fission fragmentstnfltght, wtththeaveragenumberofneutrot_s per fragment being a rather strong functkm of tl_e fragment mass (Fig, 4), yield is about 1/6 the maximum), 1,0 _ e i_ lI 3 i l t T e ••Oil O• • ! a, ' 80 , ' ' 100 I j If" 1_0 lt0 160 A IO"i Fig, 4 Average number of prompt neutrons associated with fl'agments of different mass number formed in thermal neutron flsskm of U-235, ';, In addition to the prompt neutrons which are emitted in 10"_sec, much smaller numbers are en*fitted with time delays of up to nearly a minute, These delayed neutrons have been shown to originate from certain fission products formed by beta decay irt such highly excited states as to be unstable with respect to neutron emission, The preceding beta decay thus determines thehalf-li fe for neutron emission, Chemistry has been an important ingredient irt identifying the specific beta--decay precursors of these neutron emitters, Over 60 of them are known and they play a crucial role o "_ P u.

Ki behaved like radium, but the products of uraniurn bombardment behaved like bariurrl, Despite their corlvincing chemical evidence, Hahrl and Strassmarln were very cautious in announcing their conclusion IK,cause "it contradicts ali the experiences of nuclear physics to date," but their discovery was quickly confirmed by groups around tile world, As soon as the concept of fission was established, it became clear that ali the so-called transuranium, actinium, and barium t_topes that had been produced by neutron Irradiation of uranium were In fact Isotopes of elements tn the mtddle of the periodic table, (True transuranium elements were, wf course, dl,,_'ovei'ed sub_quently, but that is a separate story,) Over 4(X) fission products have Lx,,enidentified, rangh_g from zinc (Z=30) to europium (Z=63), Their isolation and characterization tnterrnsofmassnurnbers, half-ltves, fission yields, radiations, arm other properties has hwolved a vast amount of effort by radtochemtsts, Why is fission of such great importance and interest?