Frog Gonadotropin

Gonadotropin releasing hormone (GnRH) is the newer broader name covering both genders for what was originally called luteinizing hormone releasing hormone (LHRH). Yet even this name is to restrictive for it is also a neuropeptide acting as a neuromodulator for reproductive behaviors, specifically in amphibians, egg laying and fertilization behaviors. It was first characterized and synthesized in 1971 from pig and cow hypothalamuses.

Since then and up to 1993 a total of 7 different forms of GnRH have been identified as shown in figure 1 (Muske - 1993). The top graph of figure 7 shows the 7 forms of the GnRH peptide with boxes around the changed amino acids. The peptides are named for the taxa from which they were originally isolated. The bottom graph shows the taxa from which each of the different GnRH peptides were first found. Agnatha are jawless fish like lampreys. Chrondichthyes and Oseichthyes are normal fish. The significance of these various forms, if any, in a single animal remains unknown as of 1993.

Figure 1
Variations in the GnRH Chemical Among Different Taxa
M- mammalian, C1 - chicken 1, C2 - chicken 2, S - salmon, L - lamprey, D - dogfish, C - catfish, U - unidentified form (Muske - 1993)

Locations of the GnRH Neural Cells and Their Fiber Projections

The GnRH system in the salamander is shown in figure 2. It consists of two separate regions of cells shown as solid circles (the fibers are the lines). The forebrain region cells produce the so called mammalian GnRH while the midbrain region cells produce the so called chicken 2 GnRH (Muske - 1993). The so called mammalian GnRH cells in the forebrain of amphibians cluster around the terminal nerve which is the nerve which carries olfactory information to the septum. Significantly, these GnRH cells are clustered more towards the septum in anurans (frogs and toads) as shown in figure 3. This suggests that the reproductive behaviors they tigger in frogs is based on more than odor detection.

Figure 2
GnRH System in the Salamander *Taricha granulosa*.
CB - cerebellum, CH - optic chiasm, DP - dorsal pallium, HAB - habenula, HYP - hypothalamus, ME - median eminence, MED - medula, MP - medial pallium, MS - medial septum, OB - olfactory bulb, OT - optic tectum, POA - preoptic area, PT - posterior tubercle, PVO - paraventricular organ, TEG - tegmentum, TN - terminal nerve. (Muske - 1993)

Figure 3
Top View of the GnRH system in the Frog *Rana pipiens* (left) and the Salamander *Taricha granulosa* (right).
ME - median eminence, MS - medial septum, ON - olfactory nerve, OB - olfactory bulb, POA - preoptic area, OC - Optic Chiasm, (Muske - 1993)

The forebrain system projects mostly to the hypothalamus where the GnRH is released with an action potential signal into a group of non-blood carrying capillaries located in the median eminence. These capillaries transport the GnRH to the anterior pituitary. Most of the GnRH is stored in the neuron terminals in that region.

Electrical stimulation of the medial septum mimics that of GnRH. In the male bullfrog rana catesbeiana electrical stimulation significantly increased the blood levels of the gonadotropins with luteinizing hormone increasing more than follicle stimulating hormone. (This electrical stimulation consisted of biphasic 500 millisecond long square pulses at 10 Hz for 15 seconds at 50, 200, and 500 microamps. These pulses were entrained for 15 seconds on then off for 5 minutes). The blood levels peaked just at the end of the stimulation and then decreased such that luteinizing hormone was 50% lower after 30 minutes. The greater the current the greater the levels of blood concentration. (McCreery - 1984) Cross-section views of the GnRH cells and fibers in the frog are shown below in figures 4 and 5.

Figure 4
Cell Locations of the Forebrain GnRH System in the Frog *Rana pipiens*
DBB - diagonal band of Broca, LS - lateral septum, ME - median eminence, MFB - medial forebrain bundle, MS - medial septum, NA - nucleus accumbens, POA - preoptic area. (Alpert, et al. - 1976)

Figure 5
Fiber Locations of the Forebrain GnRH System in the Frog *Rana pipiens*
DBB - diagonal band of Broca, LS - lateral septum, ME - median eminence, MFB - medial forebrain bundle, MS - medial septum, NA - nucleus accumbens, POA - preoptic area. (Alpert, et al. - 1976)

References

McCreery, B.R. (1984). Pituitary Gonadotropin Release by Graded Electrical Stimulation of the Preoptic Area in the Male Bullfrog, Rana catesbeiana. General and Comparative Endocrinology 55:367-372

Muske, L.E. (1993). Evolution of Gonadotropin releasing Hormone (GnRH) Neuronal Systems. Brain, Behavior, & Evolution 42:215-230

Alpert, L.C., Brawer, J.R., Jackson, I.M.D., and Reichlen, S. (1976). Localization of LHRH in Neurons in Frog Brain (Rana pipiens and Rana catesbeiana ). Endocrinology 98:910-921

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