Connections of the hypothalamic paraventricular necleus with the neurohypophysis, median eminence, amygdala, lateral septum and midbrain periaqueductal gray: An electrophysiological study in the rat
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Abstract
Extracellular recordings were obtained from 555 paraventricular (PVN) nucleus neurons
in pentobarbital-anesthetized male rats. Cells were examined for their spontaneous
activity patterns and response to single 1-Hz electrical stimulation of the neurohypophysis,
median eminence, amygdala, lateral septum (LS) and midbrain periaqueductal gray (PAG).
Neurohypophyseal stimulation evoked antidromic activation from 109 neurons. Among
spontaneously active neurohypophyseal neurons, evidence of a recurrent inhibitory
pathway usually required pituitary stimulus intensities twice threshold for antidromic
activation. Orthodromic excitatory or inhibitory responses followed amygdala and LS
stimulation, but not PAG stimulation. The amygdala influence was predominantly inhibitory
to 'phasic' (putative vasopressin-secreting) PVN neurohypophyseal neurons. Neurohypophyseal
stimulation evoked orthodromic responses from 124 PVN cells; some of these neurons
were also responsive to stimulation in other sites. Median eminence stimulation evoked
antidromic responses from 37 PVN neurons; some of these cells also displayed phasic
activity but no evidence for recurrent inhibition. Twelve cells in this group were
also activated antidromically from both the median eminence and the neurohypophysis;
collision tests suggest that the median eminence innervation may be an axon collateral
of a neurohypophyseal pathway. Amygdala stimulation was inhibitory to some cells in
this category. Amygdala, LS and PAG stimulation evoked antidromic activation from
a small number of PVN cells, but none of these cells appeared to innervate more than
one area, including the neurohypophysis, and none displayed phasic activity. Orthodromic
responses were recorded among other PVN neurons after stimulation in these sites;
however, PAG stimulation was the least effective stimulation area. These observations
provide additional electrophysiological data that confirm efferent PVN connections
to all areas tested, afferent connections from amygdala and LS but not PAG, and the
possibility for coordinated activity among PVN neurons through local recurrent or
common afferent connections.