For example, the inhibitory action of dopamine on PRL release mentioned above is accomplished through the binding of dopamine to receptors in the plasma membrane of the PRL-secreting cells. The response of neurosecretory neurons to specific neurotransmitters and neuromodulators or of pituitary cells to neurohormones is determined by the presence of specific receptors for these substances on the cell membranes of the neurosecretory neurons. It is important to point out, however, that while pharmacological identification of neurotransmitters regulating hypothalamic/POA neurosecretory centers is important and informative, knowledge of the neural circuits controlling neurosecretory centers is ultimately what must be determined to understand how various sensory signals relate changes in the environment to the hypothalamusвЂ“pituitary axes. Utilization of catecholamines and related drugs in vivo and in vitro also has contributed much to our understanding of neuronal regulation of hormone release. For example, application of dopamine to cultured pituitary cells with and without cocultured hypothalamic/POA tissue has made it possible to distinguish between the indirect stimulatory activity of dopamine on LH release via a neurotransmitter role in the hypothalamus/POA and its direct inhibitory action as a neurohormone on PRL release. Studies of this type have led to identification of neurons that regulate release of individual hypothalamic releasing hormones. Many pharmacological studies of nervous regulation of hypothalamic/POA neurosecretory centers have been conducted, employing neurotransmitters or drugs that either mimic (agonists) or block (antagonists) the activity of various known neurotransmitters (see Chapter 3). Abbreviations: ACTH, corticotropin ARC, arcuate nucleus CRH, corticotropin-releasing hormone DA, dopamine (prolactin release-inhibiting hormone) FSH, follicle-stimulating hormone GnRH, gonadotropin-releasing hormone LH, luteinizing hormone PRL, prolactin PVN, paraventricular nucleus TSH, thyrotropin TRH, thyrotropin-releasing hormone. Note that each hypothalamic hormone travels through the portal system and binds to receptors on different pars distalis cell types, evoking tropic hormone release. Origin and targets for some hypothalamic-releasing and release-inhibiting hormones. 1 Hypothalamic Hormones (Factors)ДЇIGURE 4-12. You will recall from Chapter 1 that most feedback is of the negative type but that enhancement of responsiveness or even positive feedback may occur, such as the estrogen induction of the midcycle GTH surge characteristic of female mammals (see Chapter 10). Feedback effects by tropic hormones and even by RHs and RIHs have been documented, but, for simplicity, we will focus our attention on the predominant feedback mechanism described above. Feedback may also alter the sensitivity of pituitary cells to hypothalamic RHs and RIHs or affect other neurons that innervate the hypothalamic neurosecretory neurons.

This feedback may affect neurosecretory neurons in the hypothalamus directly. The predominant feedback loop involves production of bloodborne hormones or metabolites resulting from the actions of tropic hormones on specific peripheral target cells ( Figure 4-1). The following accounts represent a generalized pattern of control in mammals, but individual species may vary significantly from this pattern.

Release of hypothalamic regulatory hormones is influenced primarily by neural activity and negative or positive feedback mechanisms of certain hormones. Carr Ph.D., in Vertebrate Endocrinology (Fifth Edition), 2013 C Control of Hypothalamic Hormone Release

Tropic hormones