RT-PCR detects Tk transcript expression in adult heads, but not in isolated antennae.

Tk transcript is first observed at embryonic stage 17, in 15 cells in the midgut, one of which is isolated from the other 14; a pair of medially located unidentified cells just posterior to the brain, and two lateral groups of cells that might be associated with the trachea. In larvae, Tk transcript is expressed in cells with endocrine cell-like morphology in the posterior midgut, just anterior to the Malpighian tubules. In the third instar larval brain, Tk transcript is strongly localized to 5 pairs of neuronal cell bodies in the larval protocerebrum, 2 pairs in the tritocerebrum, and 2 pairs of descending neurons. An additional 3 pairs of protocerebral and one pair of tritocerebral neurons have weak Tk expression. In the larval ventral ganglion, one pair of Tk-positive cells is found in the thoracic neuromere, and 8 pairs in the abdominal neuromere. In adult females, Tk transcript is observed in endocrine-like cells in the anterior and posterior thirds of midgut, but not in the central third.

Tk protein is expressed in a number of neurons in the larval brain which project into the ventral nerve cord.

Tk-protein is expressed in a subset of male adult protocerebral LPP2 Tachykinin neurons that co-express Scer\GAL4^Tk.

Tk protein is detected in most layers of the fan-shaped body, except for layer 3. Co-localisation with Scer\GAL4^ChAT.7.4 is observed in the central and dorsal layers of the fan-shaped body and 16-18 SMP neurons. Co-localisation with Scer\GAL4^c739 and Scer\GAL4^NP2320 is observed in the dorsal layers of the fan-shaped body and in a subset of SMP neurons.

Tk peptides were detected in extracts from adult and larval midguts.

Tk protein is expressed in 20 local neurons that innervate most adult antennal lobe glomeruli. These neurons have their cell bodies in two clusters, one dorsolateral and the other lateral to the antennal lobe. A subset of the neurons of the dorsolateral cluster co-express Scer\GAL4^GH298.

Approximately 24 Tk-immunoreactive neurons are found in the superior medial protocerebrum.

Tk protein is expressed in a subset of local interneurons (LNs) of the adult antennal lobe; 70% of Tk-expressing LNs also express Scer\GAL4^GH298.

Tk staining is observed in the posterior half of the posterior midgut and tend to be concentrated near the junction with the hindgut.

Tk immunoreactivity was found all along the length of the midugt.

Tk protein is expressed in two pairs of descending neurons in the larval subesophageal ganglion that project axons to the ventral nerve cord; two pairs of triterocerebral neurons that branch within the tritocerebral neuromere, and five pairs of protocerebral neurons that form processes within the protocerebral neuromere. Two pairs of Tk-positive neurons are observed in the metathoracic neuromere, and a few inconsistantly-labeled neurons are observed in the abdominal neuromeres. The adult brain has many Tk-expression neurons that form complex arborizations within the brain neuropils, as well as two pairs of descending neurons. Two large cell bodies in the metathoracic neuromere, as well as a pair in the abdominal neuromeres, also express Tk protein. Around 20 large Tk protein-expressing cells with morphology similar to endocrine cells are located in the larval posterior midgut, just anterior the Malpighian tubules. Many more Tk-positive endocrine cells, slightly smaller than the larval midgut cells, are found in the adult midgut; these cells have an elongated shape and often span the gut epithelium.