Thyrotropin‐releasing hormone (TRH) in the brain and pituitary of the teleost, Clarias batrachus and its role in regulation of hypophysiotropic dopamine neurons

Abstract

Thyrotropin‐releasing hormone (TRH) regulates the hypothalamic–pituitary–thyroid axis in mammals and also regulates prolactin secretion, directly or indirectly via tuberoinfundibular dopamine neurons. Although TRH is abundantly expressed in teleost brain and believed to mediate neuronal communication, empirical evidence is lacking. We analyzed pro‐TRH‐mRNA expression, mapped TRH‐immunoreactive elements in the brain and pituitary, and explored its role in regulation of hypophysiotropic dopamine (DA) neurons in the catfish, Clarias batrachus. Partial pro‐TRH transcript from C. batrachus transcriptome showed six TRH progenitors repeats. Quantitative real‐time polymerase chain reaction (qRT‐PCR) identified pro‐TRH transcript in a number of different brain regions and immunofluorescence showed TRH‐immunoreactive cells/fibers in the olfactory bulb, telencephalon, preoptic area (POA), hypothalamus, midbrain, hindbrain, and spinal cord. In the pituitary, TRH‐immunoreactive fibers were seen in the neurohypophysis, proximal pars distalis, and pars intermedia but not rostral pars distalis. In POA, distinct TRH‐immunoreactive cells/fibers were seen in nucleus preopticus periventricularis anterior (NPPa) that demonstrated a significant increase in TRH‐immunoreactivity when collected during preparatory and prespawning phases, reaching a peak in the spawning phase. Although tyrosine hydroxylase (TH)‐immunoreactive neurons in NPPa are hypophysiotropic, none of the TRH‐immunoreactive neurons in NPPa accumulated neuronal tracer DiI following implants into the pituitary. However, 87 ± 1.6% NPPa TH‐immunoreactive neurons were surrounded by TRH‐immunoreactive axons that were seen in close proximity to the somata. Superfused POA slices treated with TRH (0.5–2 μM) significantly reduced TH concentration in tissue homogenates and the percent TH‐immunoreactive area in the NPPa. We suggest that TRH in the brain of C. batrachus regulates a range of physiological functions but in particular, serves as a potential regulator of hypophysiotropic DA neurons and reproduction.

Concurrent changes in photoperiod-induced seasonal phenotypes and hypothalamic CART peptide-containing systems in night-migratory redheaded buntings

“…The rat midbrain sections containing the ventral tegmental area/substantia nigra were incubated in mouse anti-TH antiserum followed by Alexa-Fluor-594-conjugated anti-mouse IgG. To further establish the specificity of the mouse anti-TH antibodies in the brain of redheaded bunting, western blot analysis of the hypothalamic tissue was performed, as described previously (Singh et al 2019 ). Birds were anaesthetised as described above.…”

“…As previously described (Gutekunst et al 2010 ; Singh et al 2016a ), the number and density of CART-immunoreactive cells and fibers in the rostro-caudal series of sections through the POA and hypothalamic areas were analyzed using a qualitative scale (−, negative; + , occasional/isolated; + + , few; + + + , several; Table 2 ). Using computerized image analysis, the percent fluorescent area and number of CART-immunoreactive cells in the PONm, SCN, SOv, anterior hypothalamic nucleus (AN), LHN, PVN, DMN, IH, and IN were determined, as previously described (Singh et al 2016a , 2019 ). Briefly, we selected five hypothalamic sections encompassing rostro-caudal extent of each nucleus on either side of the brain.…”

Secretagogin in the brain and pituitary of the catfish, Clarias batrachus: Molecular characterization and regulation by insulin

“…In addition to that, a pair of primers from β-actin (baF and baR, Table 1) of C. batrachus (O. Singh et al, 2019) were used as reference. The quantification was carried out in 10 µl reaction volume containing 1× SYBR Green master mix and 0.4 µmol of respective primers.…”

“…As previously described in C. batrachus (O. Singh et al, 2019), the prosomeric model (Puelles & Rubenstein, 2003) was adapted for demarcating the telencephalon and hypothalamus as secondary prosencephalon. For the hindbrain, the nomenclatures of the neuronal groups in the brain of C. batrachus (O.…”

“…For the hindbrain, the nomenclatures of the neuronal groups in the brain of C. batrachus (O. Singh et al, 2019;Singru et al, 2007) and Ictalurus punctatus (Finger, 2009) were adapted. The line drawings of the brain and pituitary were prepared in CorelDrawX8 and the distribution of scgn-i elements depicted.…”