Gastrin-Releasing Peptide Signaling Plays a Limited and Subtle Role in Amygdala Physiology and Aversive Memory

Links between synaptic plasticity in the lateral amygdala (LA) and Pavlovian fear learning are well established. Neuropeptides including gastrin-releasing peptide (GRP) can modulate LA function. GRP increases inhibition in the LA and mice lacking the GRP receptor (GRPR KO) show more pronounced and persistent fear after single-trial associative learning. Here, we confirmed these initial findings and examined whether they extrapolate to more aspects of amygdala physiology and to other forms of aversive associative learning. GRP application in brain slices from wildtype but not GRPR KO mice increased spontaneous inhibitory activity in LA pyramidal neurons. In amygdala slices from GRPR KO mice, GRP did not increase inhibitory activity. In comparison to wildtype, short- but not long-term plasticity was increased in the cortico-lateral amygdala (LA) pathway of GRPR KO amygdala slices, whereas no changes were detected in the thalamo-LA pathway. In addition, GRPR KO mice showed enhanced fear evoked by single-trial conditioning and reduced spontaneous firing of neurons in the central nucleus of the amygdala (CeA). Altogether, these results are consistent with a potentially important modulatory role of GRP/GRPR signaling in the amygdala. However, administration of GRP or the GRPR antagonist (D-Phe6, Leu-NHEt13, des-Met14)-Bombesin (6–14) did not affect amygdala LTP in brain slices, nor did they affect the expression of conditioned fear following intra-amygdala administration. GRPR KO mice also failed to show differences in fear expression and extinction after multiple-trial fear conditioning, and there were no differences in conditioned taste aversion or gustatory neophobia. Collectively, our data indicate that GRP/GRPR signaling modulates amygdala physiology in a paradigm-specific fashion that likely is insufficient to generate therapeutic effects across amygdala-dependent disorders

Disrupted Functional Connectivity with Dopaminergic Midbrain in Cocaine Abusers

Background: Chronic cocaine use is associated with disrupted dopaminergic neurotransmission but how this disruption affects overall brain function (other than reward/motivation) is yet to be fully investigated. Here we test the hypothesis that cocaine addicted subjects will have disrupted functional connectivity between the midbrain (where dopamine neurons are located) and cortical and subcortical brain regions during the performance of a sustained attention task. Methodology/Principal Findings: We measured brain activation and functional connectivity with fMRI in 20 cocaine abusers and 20 matched controls. When compared to controls, cocaine abusers had lower positive functional connectivity of midbrain with thalamus, cerebellum, and rostral cingulate, and this was associated with decreased activation in thalamus and cerebellum and enhanced deactivation in rostral cingulate. Conclusions/Significance: These findings suggest that decreased functional connectivity of the midbrain interferes with the activation and deactivation signals associated with sustained attention in cocaine addicts

Responsiveness of locus ceruleus neurons in hypertensive rats to vasopressin

We studied the actions of vasopressin administered microiontophoretically onto neurons of the locus ceruleus in rats with deoxycorticosterone-acetate (DOCA)-salt hypertension and in control (normotensive) rats. Rats were studied at 3 days (prehypertensive stage) and 4 to 6 weeks after DOCA-salt treatment (chronic hypertensive stage). Experiments were performed in anesthetized rats using conventional microiontophoretic and single-cell recording techniques. Three days after DOCA-salt administration, the treated rats showed no rise in arterial pressure in comparison with control rats, but 4 to 6 weeks later, the treated rats had significantly greater pressures (p less than 0.01) than controls. Vasopressin administered with currents of 10 to 90 nA for 1 minute produced a current-dependent increase in the firing rate of noradrenergic neurons in all rats. Increases in the firing rate of noradrenergic neurons in DOCA-salt-treated rats, whether in the prehypertensive or the chronic stage, were significantly greater than increases in control rats. These findings indicate that 1) vasopressin can affect neuronal activity in the locus ceruleus and 2) noradrenergic neurons in the locus ceruleus of DOCA-salt-treated rats have an increased responsiveness to the excitatory effects of vasopressin in both prehypertensive and chronic stages of hypertension

Alterations in responsiveness of noradrenergic neurons of the locus coeruleus in deoxycorticosterone acetate (DOCA)-salt hypertensive rats

Locus coeruleus may have a function in central blood pressure regulation and possibly in the pathogenesis of hypertension. In keeping with this notion, we have recently shown that deoxycorticosterone acetate (DOCA)-salt hypertensive rats demonstrate a greater increase in blood pressure induced by locus coeruleus stimulation than control animals. In an attempt to elucidate the underlying mechanisms leading to this alteration in responsiveness of the locus coeruleus, the sensitivity of noradrenergic neurons of the locus coeruleus to the transmitter candidates, epinephrine and glutamate, was investigated in DOCA-prehypertensive (3 days post-DOCA), DOCA chronic hypertensive (6-8 weeks post-DOCA) and control rats using conventional microiontophoretic and single cell recording techniques. Iontophoretically applied epinephrine produced a current-dependent decrease in spontaneous firing rate of all noradrenergic neurons in both DOCA-treated and control rats. Locus coeruleus neurons of DOCA-treated rats at 3 days and 6-8 weeks were less sensitive to epinephrine than those of control rats and the magnitude of the depression in spontaneous firing rate was less. By contrast, iontophoretic applications of glutamate produced an increase in activity of all noradrenergic locus coeruleus neurons. However, there was minimal difference in glutamate sensitivity between neurons of DOCA and control rats. Since the changes in epinephrine sensitivity are apparent not only in the chronic stage but also in the prehypertensive stage, our findings suggest a potential role of the adrenergic input to the locus coeruleus in the pathogenesis of DOCA-hypertension

Microinjection of vasopressin into the locus coeruleus of conscious rats

To determine whether vasopressin plays a role in central neural control of cardiovascular function by acting on the locus coeruleus we monitored arterial pressure and heart rate responses to graded injections of vasopressin (1-30 ng, 0.1-0.3 microliter) in the locus coeruleus of conscious, restrained rats. Cannulas were stereotaxically implanted in the locus coeruleus 2-5 days prior to experiment. Injections of vasopressin into the locus coeruleus produced dose-related increases in mean arterial pressure (12 +/- 2 to 57 +/- 6 mmHg) and heart rate (27 +/- 6 to 123 +/- 16 beats/min), which lasted over 1 h at the highest dose. Injection of the antipressor vasopressin antagonist d(CH2)5Tyr(Me) arginine vasopressin (10 ng) into the locus coeruleus blocked the cardiovascular responses to vasopressin. Administration of vasopressin into an area lateral to the locus coeruleus had no effect on mean arterial pressure but produced an increase in heart rate. Equivalent doses of saline, angiotensin II, and norepinephrine (NE) had minimal or opposite (NE) effects on arterial pressure and heart rate. Peripheral alpha-adrenergic blockade with phentolamine and beta-adrenergic blockade with propranolol blocked the cardiovascular responses to injection of vasopressin in the locus coeruleus. These results suggest that vasopressin may act in the region of the locus coeruleus to exert a central action on the cardiovascular system that is mediated by a stimulation of sympathetic outflow

Reduced activity of locus coeruleus neurons in hypertensive rats

The effect of blood pressure on the neuronal activity of the locus coeruleus (LC) was investigated by means of electrophysiological techniques in rats anaesthetized with chloral hydrate. The mean neuronal discharge rate of noradrenergic neurons of the LC was reduced by 19% in spontaneously hypertensive rats (SHR) and by 25% in deoxycorticosterone acetate (DOCA)-salt hypertensive rats compared with their corresponding controls. The cellular activity of 27 out of 40 neurons was reversibly suppressed by acute, peripherally induced blood pressure increases in normotensive rats. Five neurons were reversibly activated and eight neurons were not affected. Conversely, acute decreases in blood pressure stimulated the neuronal activity of the LC in 10 out of 16 neurons. These findings support the hypothesis of the LC having a role in both short and long term regulation of blood pressure