Release of endogenous dynorphin opioids in the prefrontal cortex disrupts cognition

Following repeated opioid use, some dependent individuals experience persistent cognitive deficits that contribute to relapse of drug-taking behaviors, and one component of this response may be mediated by the endogenous dynorphin/kappa opioid system in neocortex. In mice, we find that acute morphine withdrawal evokes dynorphin release in the medial prefrontal cortex (PFC) and disrupts cognitive function by activation of local kappa opioid receptors (KORs). Immunohistochemical analyses using a phospho-KOR antibody confirmed that both withdrawal-induced and optically evoked dynorphin release activated KOR in PFC. Using a genetically encoded sensor based on inert KOR (kLight1.2a), we revealed the in vivo dynamics of endogenous dynorphin release in the PFC. Local activation of KOR in PFC produced multi-phasic disruptions of memory processing in an operant delayed alternation behavioral task, which manifest as reductions in response number and accuracy during early and late phases of an operant session. Local pretreatment in PFC with the selective KOR antagonist norbinaltorphimine (norBNI) blocked the disruptive effect of systemic KOR activation during both early and late phases of the session. The early, but not late phase disruption was blocked by viral excision of PFC KORs, suggesting an anatomically dissociable contribution of pre- and postsynaptic KORs. Naloxone-precipitated withdrawal in morphine-dependent mice or optical stimulation of pdynCre neurons using Channelrhodopsin-2 (ChR2) disrupted delayed alternation performance, and the dynorphin-induced effect was blocked by local norBNI. Our findings describe a mechanism for control of cortical function during opioid dependence and suggest that KOR antagonism could promote abstinence.

Regulation of GnRH pulsatility in ewes

Early work in ewes provided a wealth of information on the physiological regulation of pulsatile gonadotropin-releasing hormone (GnRH) secretion by internal and external inputs. Identification of the neural systems involved, however, was limited by the lack of information on neural mechanisms underlying generation of GnRH pulses. Over the last decade, considerable evidence supported the hypothesis that a group of neurons in the arcuate nucleus that contain kisspeptin, neurokinin B and dynorphin (KNDy neurons) are responsible for synchronizing secretion of GnRH during each pulse in ewes. In this review, we describe our current understanding of the neural systems mediating the actions of ovarian steroids and three external inputs on GnRH pulsatility in light of the hypothesis that KNDy neurons play a key role in GnRH pulse generation. In breeding season adults, estradiol (E2) and progesterone decrease GnRH pulse amplitude and frequency, respectively, by actions on KNDy neurons, with E2decreasing kisspeptin and progesterone increasing dynorphin release onto GnRH neurons. In pre-pubertal lambs, E2inhibits GnRH pulse frequency by decreasing kisspeptin and increasing dynorphin release, actions that wane as the lamb matures to allow increased pulsatile GnRH secretion at puberty. Less is known about mediators of undernutrition and stress, although some evidence implicates kisspeptin and dynorphin, respectively, in the inhibition of GnRH pulse frequency by these factors. During the anoestrus, inhibitory photoperiod acting via melatonin activates A15 dopaminergic neurons that innervate KNDy neurons; E2increases dopamine release from these neurons to inhibit KNDy neurons and suppress the frequency of kisspeptin and GnRH release.

Endogenous Activation of Supraoptic Nucleus κ-Opioid Receptors Terminates Spontaneous Phasic Bursts in Rat Magnocellular Neurosecretory Cells

Phasic activity in magnocellular neurosecretory vasopressin cells is characterized by alternating periods of activity (bursts) and silence. During phasic bursts, action potentials (spikes) are superimposed on plateau potentials that are generated by summation of depolarizing after-potentials (DAPs). Burst termination is believed to result from autocrine feedback inhibition of plateau potentials by the κ-opioid peptide, dynorphin, which is copackaged in vasopressin neurosecretory vesicles and exocytosed from vasopressin cell dendrites during phasic bursts. Here we tested this hypothesis, using intracellular recording in vitro to show that κ-opioid receptor antagonist administration enhanced plateau potential amplitude to increase postspike excitability during spontaneous phasic activity. The antagonist also increased postburst DAP amplitude in vitro, indicating that endogenous dynorphin probably reduces plateau potential amplitude by inhibiting the DAP mechanism. However, the κ-opioid receptor antagonist did not affect the slow depolarization that follows burst termination, suggesting that recovery from endogenous κ-opioid inhibition does not contribute to the slow depolarization. We also show, by extracellular single-unit recording, that that there is a strong random element in the timing of burst initiation and termination in vivo. Administration of a κ-opioid receptor antagonist eliminated the random element of burst termination but did not alter the timing of burst initiation. We conclude that dendritic dynorphin release terminates phasic bursts by reducing the amplitude of plateau potentials to reduce the probability of spike firing as bursts progress. By contrast, dendritic dynorphin release does not greatly influence the membrane potential between bursts and evidently does not influence the timing of burst initiation.