In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: However , it is not certain as to the following : ( a ) what receptors were stimulated by the ACTH N - terminal fragments at the LC that resulted in this disorder ; ( b ) whether NE , released onto Purkinje cell synapses located at terminals of the ceruleo - cerebellar pathway , did indeed cause the long - term depression at Purkinje cell synapses ( previously described by others ) that resulted in the long duration of the movement disorder ; ( c ) whether the inhibition of inhibitory Purkinje cells resulted in disinhibition or increased excitability of the unilateral cerebellar fastigial or interpositus nuclei , the output targets of the Purkinje cell axons , that may have been an important contributing factor to this disorder .
### Output Text:
However , it is not certain as to the following : ( a ) what receptors were stimulated by the <span class="chemical">ACTH</span> N - terminal fragments at the LC that resulted in this disorder ; ( b ) whether NE , released onto Purkinje cell synapses located at terminals of the ceruleo - cerebellar pathway , did indeed cause the long - term depression at Purkinje cell synapses ( previously described by others ) that resulted in the long duration of the movement disorder ; ( c ) whether the inhibition of inhibitory Purkinje cells resulted in disinhibition or increased excitability of the unilateral cerebellar fastigial or interpositus nuclei , the output targets of the Purkinje cell axons , that may have been an important contributing factor to this disorder . 