Hypothalamic input

Lesions of the lateral hypothalamic area (LHA) cause anorexia, whereas ablation of the paraventricular nucleus (PVN) cause a hyperphagic obesity syndrome. Consistent with these results, LHA neurons express the orexigenic neuropeptides MCH and orexin. PVN neurons produce several neuropeptides that are anorex-igenic when administered directly into the brain (CRH, TRH, oxytocin), in addition to their better known roles as endocrine regulators. LHA and PVN receive rich inputs from axons of NPY/AgRP and aMSH/CART-producing neurons in the arcuate nucleus. 

As discussed previously, the neurohypophysis has a direct anatomical connection to the hypothalamus. Therefore, the hypothalamus regulates the release of hormones from the neurohypophysis by way of neuronal signals. Action potentials generated by the neurosecretory cells originating in the hypothalamus are transmitted down the neuronal axons to the nerve terminals in the neurohypophysis and stimulate the release of the hormones into the blood. The tracts formed by these axons are referred to as hypothalamic-hypophyseal tracts (see Figure 10.2). The action potentials are initiated by various forms of sensory input to the hypothalamus. Specific forms of sensory input that regulate the release of ADH and oxytocin are described in subsequent sections in this chapter. 

Yoon, H., Enquist, L.W. and Dulac, C. (2005) Olfactory inputs to hypothalamic neurons controlling reproduction and fertility. Cell 123, 669-682. 

Petrovich, G.D., Canteras, N.S. and Swanson, L.W. (2001) Combinatorial amygdalar inputs to hippocampal domains and hypothalamic behavior systems. Brain Res. Rev. 38, 247-289... 

Another common mechanism for modulating hormonal response involves two (or more) hormonal inputs with both positive and negative effects (see fig. 24.21). The hypothalamic peptides, somatostatin and GRF, have opposite effects on GH synthesis and secretion. Similarly, glucagon and insulin have opposite effects on gluconeogenesis in the liver (see the discussion earlier in this chapter), and some of the effects of ecdysone on gene expression in insects are blocked by juvenile hormone (a terpene derivative fig. 24.22). 

The brain exerts an important modulatory influence over spinal reflex pathways that control penile function (Giuliano and Rampin, 2000). A variety of visual, auditory, olfactory, and imaginative stimuli elicit erectile responses that involve cortical, thalamic, rhinencephalic, and limbic input to the medial preoptic-anterior hypothalamic area, which acts as an integrating center. Other areas of the brain, such as the amygdaloid complex, may inhibit sexual function. 

Finnegan TF, Chen SR, Pan HL (2006) Mu opioid receptor activation inhibits GABAergic inputs to basolateral amygdala neurons through Kvl.1/1.2 channels. J Neurophysiol 95 2032-41 Fu LY, Acuna-Goycolea C, van den Pol (2004) Neuropeptide Y inhibits hypocretin/orexin neurons by multiple presynaptic and postsynaptic mechanisms tonic depression of the hypothalamic arousal system. J Neurosd 24 8741-51... 

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