Effects of septal and hippocampal stimuli on paraventricular nucleus neurons

Cortisol alters firing rate and synaptic responses of limbic forebrain units

Single units antidromically identified as projecting to the hypothalamic paraventricular nucleus (PVN) from the lateral septum (LS) and preoptic area (POA) were recorded in adrenalectomized male rats. Eight of the 15 POA units identified as projecting to the PVN were affected by reciprocally projecting pathways also showing either orthodromic excitation or inhibition. A variety of responses from POA units following cortisol injection suggested a heterogeneity of PVN afferent projections arising from the POA and possibly involved in the regulation of adrenocortical secretion. Cortisol injection had variable effects on the reciprocal POA axon pathways demonstrated and also revealed the presence of reciprocal projections to four of the LS cells. Most of the 10 LS cells tested showed an increase in firing rate following the steroid injection. It is suggested that the reciprocal pathways demonstrated may provide a steroid-dependent, 'ultra-short loop' negative feedback circuit serving to regulate the neural control of adrenocortical secretion. Unidentified cells in both regions were also affected by cortisol injection, the majority of such cells tested being excited, some also showing steroid-sensitive synaptic effects following PVN stimulation. The latter results may be related to the involvement of the structures examined, and of adrenal glucocorticoids, in central mechanisms of arousal.

Hypothalamic afferent connections mediating adrenocortical responses that follow hippocampal stimulation

This study identified some neural pathways which mediate the adrenocortical responses that follow hippocampal stimulation. The increase in plasma corticosterone following dorsal hippocampus stimulation, in rats with electrodes chronically implanted under pentobarbital anesthesia, was blocked by dorsal fornix and lateral septal lesions and by small posterior hypothalamic deafferentation. Fimbria transection, lateral septal lesions, and posterior hypothalamic deafferentation, but not midbrain reticular formation lesions, also blocked the adrenocortical responses to ventral hippocampus stimulation. Our present and previous studies indicate that the dorsal and ventral hippocampal effects on the hypothalamus, which increase plasma corticosterone concentrations, are mediated by the dorsal fornix and fimbria, respectively, as well as by the lateral septum. A posterior hypothalamic input, which does not involve the medial forebrain bundle or the midbrain reticular formation is also essential for the activation of this response.